A STUDY OF SATURATED POOL BOILING POTASSIUM UP TO BURNOUT HEAT FLUXES Charles Phillip Colver.A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the University of Michigan 1963 Doctoral Committee: Associate Professor Richard E. Balzhiser, Chairman Professor John A. Clark Profe ssor E. E. Hucke -Professor D. L. Katz Professor E. H. Young

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ACKNOWLEDGEME1NTS The present study was carried out with support from Aeronautical Systems Division, United States Air Force under Contract AF33(616)-8277 under the direction of Professor R. E. Balzhiser. The author would like to express his gratitude to Professor Balzhiser, Chairman of the Doctoral Committee, for his constant advice and assistance throughout the course of this investigation. Special acknowledgement is also due to: Other members of the Doctoral Committee, Professors J. A. Clark, E. E. Hucke, D. L. Katz, and E. H. Young, for their many helpful suggestions. MSA Research Corporation for providing the potassium and the Computing Center at The University of Michigan for their donation of computing time. All who assisted in design, construction and operation of the experimental apparatus, and especially to "Fanny"' Bolen and John Wurster of the Chemical and Metallurgical Engineering Department Shops. Mrs. Elaine Fritz for diligent secretarial assistanceii

TABLE OF CONTENTS Page ACKNOW LDGEMENTS E.................................................... LIST OF TABLES................... *.. O....... d......... V LIST OF FIGURES...............................vi NOMENCLATURE....................................... ixi Chapter I. INTRODUCTION.......................................... 1 Object of Investigation.......................... 9 Liquid Metal Boiling Studies..........O..........O...0.. 2 Nucleate Boiling Correlations..................... 8 Burnout Correlations........................ 12 II. DESCRIPTION OF EQUIPMENT.................................... 15 Introduction.......................................... 15a a a Boiling Vessel and Filling Line......O................... 19 Condenser and Cooling Lines.............................0 22 Boiling Tube........000.................................. 23 Outer Pressure Vessel................................... 30 Potassium Charging System..4....................... o 30 Inert Gas Lines, Vacuum System and Knockout Drum........ 32 Power Supply, Power Leads, and Guard Heaters............ 34 Instrumentation................0.......................... 37 III. OPERATIONAL METHODS AND PROCEDURES......................... 4-1 Preliminary Preparations............................. 41 Startup Procedure........................................ 42 Operating Procedure for Nucleate Boiling Runs e......... o. 43 Operating Procedure for Burnout Determinations.......... 44 IV. RESULTS. o o o o o O L546 Nucleate' Boiling Water Data............................. 46 Nucleate Boiling Potassium Data......................... 4,9 Potassium Burnout Results o............ OO........Oo.... 57 Temperature Fluctuations 0o0eeo...................0..oeo 6 iii

TABZE OF CONTENTS (CONi3tD) V. DISCUSSION OF RESULTS, O O.. O O O O............ 0 0. 72 Nucleate Boiling Potassium Dataa... 72 Potassium Burnout Re.sults 0.....................O.. 76 Temperature Fluctuations................*.o 81 VIo CONICLUSIONS........ 8L4 Appendice s A. EXPERIMENTAL DATA........................................... 86 B. TREATMENT OF DATA..o.......................... 102 CO ESTIMATION OF HEAT FLUX AND TEMPERATURE PROFILE IN BOILING T3BE............ o......................... O 105 D. DETERMINATION OF POWER LOSSES IN BOILING TUBE CIRCUITRY....... 112 E. ESTIMATION OF ERRORS.............. O... O O O O 115 F. PLOTS OF NIUCIEATE BOILING POTASSIUM DATA,....... o..118 BIBLIOGRAPHY.........................o...... 126 00b00~b QO OO OO~0~~0b64008060

LIST OF TABILES Table Page I Experimental Data for Water..... 87 II Potassium Boiling Data f-or Run K-1 i...............0......o 89 III Potassium Boiling Data for Run K-2..........o.... 90 IV Potassium Boiling Data for Run K-3.................O 91 V Potassium Boiling Data for Run K-4.......... 92 VI Potassium Boiling Data for Run K-5.......................... 93 VII Potassium Boiling Data for Run K-6......................... 91 VIII Potassium Boiling Data for Run K-7........-....000........ 95 IX Potassium Boiling Data for Run K-8................O.e. 96 X Potassium Boiling Data for Run K-9........................ 99 XI Potassium Boiling Data for Run K-10......................... 100 XII Summary of Potassium Burnout Data.................. 101 V ~ ~ ~ O~~0~400600~

figure Page 1 Data of Boni2la and Co-workrers (6) t o.:003 Bil ing; Potass ium................................... a a a a.a a a. a a a.O 9 2 iGeneral Viev. of E xpe riment Ap aratus. o....e 17. 3 Schematic Diagram of ExperimentaL System.0 a., 0O000.......... i18 I-4 Sectionral Drawir ng of Eerimenrtal Apparatus.................. 20 5 View Showing Uninsulated Test >sel... O.... 0 O O 21 6 5View Shoring Bus Bar Asse-mb ly. a a aa a a o O 0 0 0 e O.... a.... O a.. O O O.... a 21 7 Cross Sectional Draewing of Boiling Tabe Assemblyoo....oo... 24 8 View:of Boilitg Tube Assembly........a a aa a a a a a a.... 25 9 Overall View of Bbilixng Tbe with The-rmcou.pIe Assemblies... 26 10 Views of Boiling 9ibe Cross Seet ona......................... a a a a. 27 11 ayout of Entry Holes i ter Ft:t::etion VO:l a a a O.... 3 12 Boilirng Tube Powe Supply a o a a o a. a a a a.a a a a a a a.,. a o o 35 13 Guard Heate3rs Circuities a a a a a a a a a a a a a a a a. a....a a a.. a a a 35 l4 Drawing oLf3 eCT 0trode 8 lao o- a a a a a a a a a a a a a a a a a a a a a a a a a a a a a. a a a 3a 14 Dratixig of Electrode Giands O O O O O O O O O.............. *.. O 36. 16 TIermt oupe Ciruitries0.... 0....:..,c.. 3-9 17 cmCaris: Boitilig Wate ta ith wo Pir ous Investigationsi, O.... o...o....o.o.....oOO..........O.......PO..o..-..,. 48 18 Boiling Data for Potassiuz Obtai'-ed at 12.2 psia Using Tube 2 1.9 Boiling Data fot Patassim- ObtainO in, the tg 134 -14a psia Usisg Tube 3 (Ru K-8 ) a ae aa...a.a. a o o o o o o o o a a o o ao o o. a. o. o o o a a a a a a a a 51 vi

LIST OF FIGURES (CONT'D) Figure Page 20 Boiling Data for Potassium Obtained in the Range 3.8 5.0 psia Using Tube 4 (Run K-10) K.......... o......,..... 52 21 Vertical Temperature Prrofile- Based on Free Surface Temperature (Runi K-8).................................... 54 22 Photographs Showing Three Views of Tubes 2 After Removal from Apparatus...................... 56 23 Comparison of Data from Tube 2 with Data of Bonilla (6)..... 58 24 Comparison of Data from Tube 3 with Data of Bonilla (6).... 59 25 Burnout Heat Flux Versus Saturation Pressure for Saturated Pool Boiling Potassium............................... 60 26 Photograph of Tube 1 After Removal from Apparatus........... 62 27 Photograph of Tube 4 After Removal from Apparatus.......... 64 28 Boiling Tube Thermocouple (Side) Trace for Pool Boiling Potassium at 5 psia.............................. 65 29 Boiling Tube Thermocouple (Bottom) Trace for Pool Boiling Potassium (Run K-4)............................... 67 30 Boiling Tube Thermocouple (Bottom). Trace for Pool Boiling Potassium (Run K-8).......,.......... 68 31 Boiling Tube Thermbcouple (Bottom) Trace for a Burnout Determination at Constant Press ure.. -.O. 0..........O eO... 70 32 Boiling Tube Thermocouple (Bottom) Trace for a Burnout Determination at Constant Heat Flux,....,............o...... 71 33 Comparison of Saturated Pool Boiling Potassium Results Near Atmospheric Pressure with Correlationso......e.... ooooo 77 34 Comparison of Saturated Pool Boiling Potassium:Burnout Data with Burnout Correlations.........o..........oOOO..OO. 79 vii

LIST 0F FIGUERS- (CONTD) Figure.Page 535 52sTemperature Proefoile of Graphite eater in:Boiling Tube, a e 111 36 Boiling Data for Potassium Obtained at 9.8 psia Using Tube 2 (Run K-2)..ooooooo.Oo.... o......Oo.ooo.ooooooooo0000 o* 119 37 Boiling:Data for Potassium Obtained at 0.7 psia Using Tube 2 (Run. K-35)o o o o.........ooo aeo o o. o o o o a. o e 120 38 Boiling Data for Potassium Obtained in the Range- 12. - 14.0 psia Using Tube 2 (Run K-4)o.........,....oo.,, 121 39 Boiling Data for Potassium Obtained at 13.7 psia Using Tube 3 RFun K-5........ 0 0...... 0 0 0 0 a 0 0 0 0 aO O O O O O O O O O O o 122 40 Boiling Data for Potassium. Obtained at 6 psia Using Tube 3 (Run K-6) 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 a0 0 0 0 0 0 0 a 0 0 0 O 0 a0 aa.. 0 0 0 a. 123 41 Boiling Data for Potassium Obtained at 0.9 psia Using Tube 3 (Run K-7)..ooooooooo.o..ooooo00o0.00........... 124 42 Boiling Data for Potassium Obtained in the Range 37 - 45 psia Using Tube 3 (Run SK-9)..........0. 0..ooo.0..ooooooo... 125 eri.

NOMENCLATURE a Thermal diffusivity (k/Cp p), acceleration A Area, term in Equation C-4 B Term in Equation C-4 BL Empirical coefficient in reference (31) L c Conversion factor from watts to Btu/hr, 3.415 Cp Heat capacity d Diameter g Acceleration of gravity gc Gravitational conversion constant, 32.17 lb ft/(sec2 )(lbf) h Heat transfer coefficient I Current k Thermal conductivity K Constant in Equation 10 Length Nu Nusselt number, hd/k p Pressure /Ap Pressure difference corresponding to superheat Pr Prandlt number, Cp 1/k q/A Heat flux R Bubble radius expressed by Equation 4, electrical resistance Re Reynolds number t Thickness of boiling tube end ix

T Temperature, (Temperature in the graphite) - (Temperature of boiling potassium) AT Temperature difference (Tw. - Ts) U Overall heat transfer coefficient V Voltage, Volume x Length dimension Ax Thermocouple depth from tube surface Latent heat of vaporization Viscosity it 73.14 p Density, electrical resistivity ~a Surface tension Subscripts ave Average b Bubble, bulk bo Burnout c Wall thermocouple g Graphite H-25 Haynes-25 2 Liquid s Saturation v Vapor w- Wall x

ABSTRACT Potassium pool boiling from the outer surface of a horizontal 3/8-in diameter by 1.25-in long Haynes-25 tube was studied at heat fluxes up to and including burnout. The scope of the investigation included operating at different pressures from a few millimeters of mercury to 4-5 psia. The experimental apparatus was designed to permit boiling at heat fluxes above ixlO6 Btu/(hr)(sq ft). It consisted of a 1.36-in ID by 24-in long Haynes-25 vertical boiling vessel containing an electrically heated bayonet tube. The boiling tube assembly consisted of a cylindrical graphite heating element, a boron nitride sleeve, and the Haynes-25 tube. It was horizontally positioned 2 1/2-in above the bottom of the boiling vessel. Direct current was passed through the graphite heater. The tube wall temperature was measured with three thermocouples positioned circumferentially in the tube. An adjustable thermocouple extending from the top of the test chamber measured liquid temperatures. Water was boiled in preliminary runs to provide a standard of comparison of results with those of similar systems. Measurements of heat flux and the temperature difference between the boiling surface and liquid saturation temperature were made for ten nucleate boiling potassium runs in the pressure range 1 to 45 psia. The data possessed considerable scatter; however, mean wall superheats were in the order of 200F for heat fluxes over 100,000 Btu/ (hr) (sq ft), Burnout heat flux determinations were made by two methods using four boiling tube assemblies in the pressure range 0,.15 to 22 psiao The fiArst xi

method involved holding-the heat flux constant and gradually decreasing the pressure, while in the other method the pressure was maintained constant and the heat flux increased, Burnout was indicated in either method by an instantaneous temperature rise measured by thermocouples inserted in the boiling tube wall., An empirical fit of the burnout results can be represented by the following equation: (q/A)bo = 4 X 105 pOl67 where (q/A)bo is expressed in Btu/(hr)(sq ft) and the saturation pressure, p, in psia.. Pronounced temperature fluctuations between 10 and 150OF existed in the heat transfer surface during the boiling of potassium. Below 250,000 Btu/ (hr)(sq ft) temperature fluctuations were commonr in the bulk liquid and were accompanied with noticeable pressure fluctuations. xii

CHAPTER I INTRODUCTION Heat transfer systems utilizing boiling liquid metals provide one method for efficiently exchanging large quantities of heat at relatively high temperature levels. The fact that the first three alkali metals, lithium, potassium, and sodium have densities less than water and possess high latent heats (approximately equal tq water for potassium while up to ten times that of water for lithium) makes them extremely attractive as heat carrier fluids in power cycles for space applications. In order to determine useful design information for such systems it becomes necessary to simulate actual conditions with relatively simple and yet similar experiments. Ultimately, it would be hoped to derive analytical or semi-empirical expressions, or use previously derived relationships substantiated by liquid metal boiling experiments, to provide reliable engineering design information. Some liquid metal boiling heat transfer data has been reported, but in only one investigation were burnout data obtained. Comparison of both the experimental heat transfer and burnout results with theoretical and semiempirical predictions has shown at best only fair agreement and indicated that more boiling data with different liquid metals is needed. Object of Investigation Any esxperiment yielding high flux boiling data would be extremely valuable, especially if burnout data could be obtained. Furthermore, if the investigation

2 covered a range of pressures it would be possible to show the influence of pressure on heat transfer as well as burnout. This investigation is concerned with the methods and results of pool boiling potassium at heat fluxes up to and including the burnout point. The information gathered is used to test existing correlations for nucleate boiling and for the burnout heat flux. The scope of the investigation included. operating at different.pressures in the range of. a few inches of mercury to over atmospheric pressure. Summary of Liquid Metal Boiling Studies The interest in boiling technology, and specifically boiling liquid metals, has developed at an increasing rate during recent years. Space does not permit discussion of the many scores of investigations covering all phases of the boiling field. The reader is- referred to any of the.several good overall summaries available in the literature for more detailed.discussions of the important variables,-e.g., liquids properties, types or orientations of heater surfaces, flow conditions, quality, etc. (3, 24, 28, 36, 51). A good concise survey of boiling burnout is also available (17)0 Several summaries exist which are devoted exclusively to liquid metal boiling. A literature survey on liquid metal boiling was conducted at The University of Michigan.and includes summaries and analyses of all liquid metal investigations through 1961 (3). Nearly 1200 references were included and pertained to all phases of the boiling field. A Russian publication (28) devoted entirely to problems associated with utilizing liquid metals as heat transfer media contains all important liquid metal heat transfer data collected in Russia, up through 1958o A recent paper by Gambill and Hoffman (18) summarizes the.. boiling metal heat transfer field up to mid-1961.

3 The first comprehensive boiling study with liquid metals was done by Lyon (34) who investigated boiling mercury, mercury with additives, sodium, NaK, and cadmium. The experimental apparatus consisted of a horizontally oriented stainless steel tube, 3/8-in OD by 5-in long, extending through a stainless steel vessel. Electrical heating gave fluxes up to 130,000 Btu/ (hr)(sq ft). All tests were performed at atmospheric pressure under conditions of saturated, natural convection boiling with nitrogen used as a blanket gas. With pure mercury and cadmium high temperature differences were obtained, and indicated that heat transfer was either by simple convection or film boiling. Temperature differences with mercury were as high as 1000oF whereas cadmium gave temperature differences up to 200~F for heat fluxes no greater than 13,000 Btu/(hr)(sq ft). Sodium, NaK, and mercury containing additives gave extremely good heat transfer characteristics with no evidence of the burnout heat fluxes being reached with any liquid at fluxes up to 130,000 Btu/(hr)(sq ft) at atmospheric pressure. Temperature differences of less than 100F were measured for sodium and NaK at these heat fluxes. Data for mercury containing 0.10 per cent sodium gave a temperature difference of 350F at 60,000 Btu/(hr)(sq ft), while mercury containing a trace of titanium gave a temperature difference of 120F at 100,000 Btu/(hr) (sq ft). The effects of additives in mercury gave rise to the speculation that these additives promoted wetting and improved heat transfer. Lyon noted rather large temperature fluctuations for boiling sodium, NaK, and mercury containing magnesium. For sodium at low heat fluxes, boiling occurred intermittently as indicated by system noise and occasional minor pressure fluctuations. At higher fluxes the amplitude and period of the observed temperature fluctuations were smaller, The qualitative behavior of the fluctuations was given by a sudden drop in wa~ll temperature followed by

4 recovery. He concluded that temperature fluctuations were due to local temperature differences found in the liquid.near the surface. As the heat flux increased the surface temperature more nearly approached steady-state conditions with fluctuations diminishing in intensity. Bonilla (5) and others (2, 25, 26, 32, 33, 34, 46, 47) have conducted boiling mercury investigations. For the most part, the data have not correlated particular:ly well, and the highest heat flux reached in each study gave no indication of having reached the burnout point. The pool boiling mercury study of Bonilla (5) was accomplished both with and without wetting agents in the heat flux range 4,000 to 200,000 Btu/(hr)(sq ft) at pressures of 4mm Hg to 45 psia. The boiling AT values covered the range of about 10'F at near atmospheric pressure to over 100"F at lower pressures, with increases in pressure tending to reduce the temperature-driving forces at the same heat flux. The apparatus consisted of a horizontal heating surface of low-carbon steel in a 3-in OD stainless steel vesselo Electrically heated Nichrome strips wrapped around copper fins extending from the bottom of the heating surface served as the heat source. Nitrogen was used as a blanket gas. It was concluded that prolonged boiling, at least on stainless steel, promoted wettingand increased the heat flux for the same temperature-driving force. A rather interesting study by Avery (2) was conducted to investigate the effect of surface geometry on boiling mercury and mercury with 071 per cent sodium. His apparatus was essentially idenrtical to the one used by Bonilla (5) and described above- Upon completion of each set of runs the vessel was disassembled and the boiling surface grooved further. The surface was grooved with parallel, 0O003-in wide by OoO04-in deep grooves, first 3/8-in then 1/8-in apart. Two boiling plates were used. The results demon

strated the significant effect surface- geometry has on boiling heat transfer to mercury both with and without additives. For example, with pure mercury at 60,000 Btu/(hr)(sq ft) using the second boiling plate, temperature differences without grooves were in the order of 6000F whereas for grooves 3/8-in apart the temperature difference was about 500~F. Despite efforts to reproduce surface conditions on each plate it was found that each gave considerably different heat transfer curves, The author suggests that differences in microscopic geometry may account for this disagreement. Romie and co-workers (46) boiled mercury containing traces of additives in a thermal-syphon-type heat transfer loop. Electrical power was supplied through the heating section giving heat fluxes as high as 600,000 Btu/(hr) (sq ft) with 5 per cent vapor qualities. The loop was fabricated from 7/16-in OD stainless steel and contained a 4-in long test section. Deposition of a thin, copper layer on the inside wall after it had been thoroughly cleaned was found.to promote wetting. It is pointed out that electrical current was passed.directly through the.surface from which boiling occurred. Since liquid mercury is:a reasonably good electrical conductor, the mercury coming in contact with the surface created a parallel circuit. This made it difficult to differentiate between heat generation occurring within the test section wall and that occurring directly in the liquid. The accuracy of results was estimated to be within - 20 per cent. There was no indication of burnout even at 600,000 Btu/(hr)(sq ft) and it was emphasized by the authors that the thermal and hydrodynamic performance of the loop gave every indication that even higher heat fluxes could be achieved before reaching burnout. Madsen and Bonilla (35) pool boiled NaK from a horizontally oriented 3-in diameter low-carbon nickel plate up to over s0,f000 Btu/(hr)(sq fe) in the- pressure range of 2nmm to 790mm Hg. Heat was furnished by molybdenum

resistance wire covered with alumina sleeves and wrapped around molybdenum fins brazed to the bottom of the heater plate. The entire system, including heater enclosure, was blanketed with helium gas. Correlation of the data by the method of least-squares showed variations of + 30 per cent or - 28 per cent. It was found that the boiling temperature difference was an order of magnitude larger than that reported by Lyon (34). The reason postulated for their larger AT values was the large temperature gradients that existed in the bulk liquid throughout all runs. It was also suggested that the larger vessel used by Lyon may have promoted. strong natural convection.currents, thus reducing temperature gradients in the pool. Madsen and Bonilla also reported that during the investigation, the temperature of the boiling plate near the surface fluctuated randomly, the amplitude of fluctuation changing slightly with different heat flux densities. The fluctuating behavior was frequently represented by sudden dips in the boiling surface temperature followed by recovery along a ragged. "saw tooth" line. Their data showed that sometimes a rapid temperature rise preceded the temperature drop, the rise and fall both taking place in less than 0.5 seconds. On the average, the fluctuations were in the order of 10'F at 135,000 Btu/(hr)(sq ft) although extreme variations of 25~F were noted. At low heat fluxes the sudden temperature drops were frequently accompanied by audible "bumps." Noyes (42) has studied pool boiling sodium up to the burnout point in a thermal-syphon pool boiler. A saturated pool was boiled from the surface of a horizontally oriented 3/8-in diameter by 5-in long electrically heated cylindrical test section inserted in a 6-in by 6-in cylindrical boiler. Measurements were made between 1200-15000F and at heat fluxes up to 800,000 Btu/(hr)(sq ft). Burnout points were determined in the pressure interval 0o5 to 1.5 psia. His'burnout data did. not correlate well with existing

7 burnout correlations and indicated that for liquid metals, the actual burnout point is considerably higher than that predicted by most burnout correlations. A non-dimensional burnout correlation was developed by alterations in previous predictions in order to bring sodium measurements into agreement with burnout values for other liquids. This correlation is discussed in a later section of this chapter. The nucleate boiling data of Noyes was obtained from five test sections at pressures between 1 and 8 psia. Variations in AT due to pressure variations were not observed. The predictions of Levy (31) and Forster and Zuber(16) were calculated for saturated sodium at 147 psia and 3.0 psia and compared to the nucleate boiling data. Although the correlations gave slopes which agree with the data, the AT values predicted by both correlations were higher than those observed. Comparing the data with the sodium results of Lyon (34) shows good agreement below 40,000 Btu/(hr)(sq ft). Both sets of data gave AT values of 8~F. At 100,000 Btu/(hr)(sq ft) Lyons' data gave AT values less than 10~F whereas Noyes found AT values just under 200F, a disagreement of approximately 10?F. Noyes suggested that the higher XT values in his experiment were probably due to generally lower pressures employed in his investigation. To obtain burnout data, Noyes operated at constant heat flux and gradually decreased the system pressure. He found that the beginning of the transition to film boiling was characterized by a marked increase in the amplitude of the test section wall temperature fluctuations. His explanation for these pronounced fluctuations was the formation and collapse of vapor patches in the region of the thermocouple. Actual burnout was avoided by cutting off cooling to the condenser, thus raising the system pressure. On two occasions, however, burnout occurred shortly after the pronounced fluctuations occurred, Seven burnout points were determined by this technique at pool pressures between 0.5 and 1l5 psiao

8 Concurrent with the present study are a number of liquid metal experimental programs in various stages of completion. A list of these is shown elsewhere (3)o Liquid metals receiving the most attention in recent studies have been potassium, sodium, NaK, and rubidium. The effects of pressure, velocity, subcooling, surface characteristics, quality, and fluid properties are all being considered. A current pool boiling investigation with potassium is being conducted by Bonilla and co-workers (6)~ The pool. boiler is very similar to that employed by Madsen and Bonilla (35) and consists of a 3-in diameter high purity (low carbon) nickel boiling plate. The data obtained to date were taken in the pressure ranges 2 to 20mm of mercury and 700 to 1500mm mercury with heat fluxes up to 100,000 Btu/(:hr)(sq ft)o Figure 1 is a plot of their data. It was found that the data from 700 to 1500mm mercury agreed satisfactorily with correlations (applied at 14000~F) of Forster and Zuber (16), Forster and Greif (15), Chang and Snyder (9), and Levy (31) in location of the curve. However, the data exhibits a somewhat steeper slope than the correlations. The low pressure data had unexpectedly high AT values (80-100~F). It was not believed that film boilling was taking place though no explanation of these high AT values was advanced. Nucleate Boiling Correlations Mny theoretical and semi-empirical expressions have been suggested for predicting heat flux in the nucleate boiling regime. For the most part, analyses have been based on postulated models which account for increased heat transfer coefficients over natural convection without surface boilingo A common practice has'been to group the influencing physical properties into pertinent dimensionless parameterso The various empirical constants in the expressions are evaluated.with actual data to produce a relationship for all boiling liquids~

700 to 1500 mm Hg 2-12mm Hg 100 7:0 0 ~0~ ~AT 0o F+ Figure 1. Data of Bonilla and Coworkers (6) for Pool Boiling Potassium. 0 + Natural Convection 10 20 40 60 80 100 200 AT, OF Figure 1. Data of Bonilla and Coworkers (6) for Pool Boiling Potassium.

10 Several correlations, such as the one derived by Rohsenow (43), necessitate knowing a constant for a particular heating surface and/or fluid combination. Since correlations of this type have limited value in the absence of previously obtained data, they are of little use for engineering purposes. For this reason, only those correlations capable of predicting the nucleate boiling curve under more general conditions will be discussed here. Forster and Zuber (16) derived analytical expressions for bubble radii and growth rates and then applied them to surface boiling. A Reynolds number was formulated for the liquid flow in the superheated boundary layer adjacent to the boiling surface. The analysis when applied to pool boiling gave the maximum heat transfer rates and yielded the following relationship: Nu = 0.0015 Re 062 Pr (1) where the Reynolds number, Reb, is given by: Reb AT CpR p2 RebV Pv (2) The Nusselt number, Nu,) for the system is: q/A R' (TW - T: ) a (3) and where R is given by: AT Cp p' 2abL p2) icui t st aP 6)P The relationship (Equation 1) has favorably predicted data of several liquids including recent studies with sodium (42) and potassium (6). Forster ard Grei~ (15) in their analysis deduced a correlation for nucleate boiling which gives the dependence of heat flux on the superheat

11 and system pressure. By judicious selection of the important parameters and dimensionless groups in the nucleate boiling process, these authors were able to develop an expression employing the same empirical constants for all liquid-surface combinations. It iso / aCp~ p~ Ts 1/4p_ 5/8 1/3 2 /A = 4.3 x 10 p nTs\ Pr AP JTi~-(xp,)5?/ ~~L} \~~~J(5) The correlation has satisfactorily predicted the nucleate boiling curves for different liquids varying from water (10) to mercury (), without necessitating any alteration in empirical constants. Recently the correlation was compared with pool boiling potassium data (6) with acceptable agreement. Chang and Snyder (9) in their analysis considered the concept of thermal eddy diffusivity as it might relate to nucleate boiling phenomena. Applying dimensional analysis to the fundamental equations of motion and energy and selected boundary conditions they derived a set of parameters characterizing boiling heat transfer. The following expression, good for vigorous boiling, results from their analysis: 4 4 To s AT q/A =4 x 10 CP Comparison with mercury data (() and potassium data (6) has shown satisfactory agreement for engineering design purposes. An interesting correlation by Levy (31) was derived under the premise that as the generated bubbles attain their maximum diameter, they carry all heat transferred from the boiling surface. The final expression is good for any liquid boiling under saturated conditions.o The expression is:

12 k1 Cp~ P AT q/A BL 7 T7 (P1 7 PY) (7) and is independent of pressure and heat transfer surface-liquid combinations. The constant BL is empirically determined and found to be well represented by plotting it against pv (see original article in reference 31). Comparison with sodium (42) and potassium (6) data shows fair agreement. It is not known which of the above correlations is superior for predicting all nucleate boiling heat transfer data. Mercury data has been equally well predicted by all correlationso For s.odium and potassium all correlations give good agreement in view of the fact that data to date has exhibited a great deal of scatter. The correlation of Levy, however, appears to predi.ct slightly high XT values, particularly when compared to sodium data of Noyes. Burnout Correlations According to a recent survey (17) of boiling burnout, some fifty equations have been proposed for predicting the critical or burnout heat flux under various conditions, i.e., free or' forced convection, with subcooling or quality, etc. Many of these are based on experimental data obtained from specific systems and may not be recognized as generalized correlations. Several theoretical treatments of the boiling process at burnout have generated theoretical or semi-empirical correlations. Although these correlations correctly predict burnout for several liquids, only one expression (developed using sodium burnout data) has shown even fair agreement when applied to liquid metals. No burnout data is available for liquid metals boiling under subcooled conditions. All burnout data for other liquids indicates that the burnout

13 point is raised with increased. subcooling at all pre.ssures. At l..L!ower'; pressures subcooling has been shown to have the greatest effect (3, 29,. 55). Several correlations are available for predicting burnout for different levels of subcooling (3, 23, 53). These correlations will not be considered here as the present investigation is concerned-with only saturated pool. boiling burnout measurements... Rohsenow and Griffith (44) developed an expression for the burnout point on the premise that at burnout, the active sites on.-the boiling surface become so numerous that growing bubbles on the surface coalesce to form a vapor blanketing layer. The final equation in their derivation contained empirical constants which when correlated with experimental data (1, 7,:10) gave: 0.6 (q/A bO t 143 wpt ba o Comparison of the above expression.with burnout data for water and organic liquids gave a deviation range of - 11 per cent. Poor agreement is given with sodium burnout data (42). Zuber and Tribus (53) derived an analytical expression which permits the prediction of pool boiling burnout. Their analysis considered the hydrodynamic instabilities and geometrical configurations of boiling systems. It was found that the maximum heat flux was limited by combined effects of Taylor and Helmholtz instabilities on the flow of the two-phase rmixture. Their expression is: (q/A)bo = 7 g(Pj-P /)g ( C P Althoughcomparisonwith some data, including that of Westwater and Santangelo (52) with methanol has shown agreement the prediction does not agree with sodium (42) burnout data.

14 Kutateladze (27) has derived. an expression very similar to that of - Zuber and Tribus, the principle difference being-the value for the constant. In his analysis. Kutateladze considered the basic equations of motion and. mechanical interaction.which apply to the hydrodynamics of a two-phase boundary layer adjacent to a boiling surface.. Employing dimensional analysis the following equation was derived: 2 1A )bo114 ~(q/A)bo f,pv )Pv When compared with data for water and some organic liquids for saturated pool boiling, the best value for K was found to be 0.16. Noyes (4-2) in a recent publication presented a modified dimensionless burnout correlation supported not only by published data for water and various organic liquids, but also his data for sodium. In essence the correlation was developed by modifying previous expressions to produce agreement with his burnout data. (q/A) = 0o.144 Xp r l/4 g 1/4 -0.245 (11) The major difference between this correlation and those developed from hydrocynamic theory, i.e., Zuber-Tribus and Kutateladze, is the inclusion of a,Prandtl number effect.

CHAPTER II DESCRIPTION OF EQUIPMENT Introducti on The scope of this study necessitated design considerations which would permit the pool boiling of potassium up to heat fluxes slightly over 1 x 106 Btu/(hr)(sq ft). The pressure range necessary extended from a few millimeters of mercury up to about 200 psia; the temperature range up to over 1800~F. Further, since the investigation included burnout measurements, it was imperative that a heat transfer assembly be designed which could be easily removed for inspection or completely replaced after it failed. To satisfy these requirements, high strength, high temperature materials'were selected when available and financially feasible. In addition, those materials in contact with potassium were selected because of their excellent corrosion resistant properties. The experimental apparatus consisted primarily of a vertical boiling vessel containing an electrically heated bayonet-type boiling tube, a water jacketed condensers an outer pressure vessel, guard heaters, an inert gas supply, a vacuum system, a potassium charge and dump system, and instrumentation. All process lines and equipment coming in contact with potassium or potassium vapor were made from either- Haynes-25 or stainless steel 304 or Also referred to as L-605 metal (composition: 10Ni - 48.9Co - 20Cr - 15W - 3Fe - Ol.C - 1Si - 2Mg). - 15

316. Welds involving Haynes-25 were made using Haynes-25 welding rods exclusively, with no less than two passes per weld. As an added precaution against leaks in the test vessel, all thermocouple wells and tubes extending into the boiling vessel were welded on both the outside and inside surface, with two passes per surface. A total of four passes were used when the top and bottom plates were welded to make up the test vessel. Connections made in the potassium system, where welding was impractical, used Swagelok (type 316) stainless steel compression fittings exclusively. Whenever possible, pressure gland sealants, gasket materials, insulation, brazing filler, etc.' were selected to withstand the highest temperatures possible. The thermocouple and tube pressure glands were Conax glands using lava as the sealant. Electrode glands located in outer vessel top flange utilized silicone rubber O-rings in lava seats. The upper temperature limit of these 0-rings was 650'F and represented the temperature limiting material in the experimental apparatus. A controlled atmosphere of helium was maintained around the boiling vessel. This served to reduce oxidation of the boiling vessel, and by balancing the helium pressure with the system pressure, permitted operation at elevated pressures (temperatures) with minimum pressure stresses in the test vessel. During normal operation and particularly at lower pressures, a slight positive pressure was maintained outside the boiling vessel. In this way, small leaks in the test vessel would not result in potassium contamination of outer vessel. A photograph showing the experimental apparatus and most of the instrumentation is shown in Figure 2. A schematic diagram of the experimental system is shown in Figure 3.

Figure 2. General View of Experimental Apparatus.

18 VACUUM PUMP CHARGE TANK KO- DRUM COLD TRAP PRESSURE POOL I REGULATORS BOILING VESSEL HELIUM CYLINDER Figure 3. Schematic Diagram of Experimental System.

19 Boiling Vessel and Filling Line A drawing of the experimental system is shown in Figure 4. Photographs of the test vessel are shown in Figures 5 and 6. Boiling took place in a vertical 2 foot section of thick walled Haynes-25 pipe, 2.15-in OD by 136-in ID. The vessel ends were formed by welding on 5/16-in thick by 2 1/4-in diameter Haynes-25 plateso Four holes were drilled in the top plate of the test vessel. A 3/4-in diameter, 16 BWG, type 304 stainless steel tube was welded into a 3/4-in diameter hole in the center of the plate, This tube in turn was welded through the outer pressure vessel and served as the vacuum and inert gas entry into the vessel. Three thermocouple wells equally spaced circumferentially around the center hole in the top plate extended into the test vessel 3, 13, and 16 inches. Each well was made from tubing 0.188-in OD by 0.032-in wall thickness and closed at one end with a'Heliarc welder. The bottom plate to the test vessel contained a 3/8-in diameter hole into which was welded a Haynes-25 tube 3/8-in OD by 0.057-in wall thickness. This tube served as the potassium charge line and was welded flush to the inside bottom plate to insure complete draining of potassium from the test chamber for tube replacement. Two Haynes-25 thermocouple wells, 2 and 4 inches, were welded through the bottom plate. They were made from the same tubing as those extending down from the top plate. A 9/16-in diameter hole was drilled 2 1/2-in from the bottom of the test vessel and threaded for a 3/8-in IPS male fitting. The hole was threaded completely through the 0.395-in vessel wall so that, when the boiling tube assembly was screwed. in place, the boiling tube would be completely inside the boiling chamber'The threaded fitting would then be exactly flush with the inside vessel wall. The liquid metal filling line was a Haynes-25 tube, 3/8-in diameter by 0.057-in wall thickness, welded to the bottom of the test vessel as mentioned

20 THERMOCOUPLE GLAND- HELIUM LINE TUBE GLAND / ELECTRODE GLAND FLANGE THERMOCOUPLE LEADS FILLING TUBE 7 ~ j lllf IiJ I 1 COPPER LEADS CONDENSER COILS CONDENSER PROTECTION VESSEL THERMOCOUPLE SHEATHS LIQUID LEVEL'% BUSBAR BOILING TUBE SWAGELOK FITTING Figure 4. Sectional Drawing of Experimental Apparatus.

................~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~:~:~~: ~~~~~~~~~~-~g:::::iiiie TX~:::::::'::iiii l:':''::::-:-::~: —:::' —:::::::'::: —:-::: Figure 5. View.Showing Uhinsulatea~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Test Vessel. Figure View Showing, Bus Bar Assom~~~~~~~::c::::::i.l::-:::::::_:-i: -:

22 above-. It extended through the protection vessel as seen in Figure 5 and was connected to the potassium charge system. A Conax (No. PG-4) stainless steel packing gland (using lava as the sealant) sealed the tube as it passed through the top flange. This gland permitted the tube to slide either way upon expansion and contraction of the boiling vessel. A 3/8-in Swagelok union (No. 600-6-316) connected the filling line to the charge system. Condenser and Cooling Lines The condenser consisted essentially of three parallel cooling systems welded around a stainless steel pipe press-fitted over the test section. As an added precaution against water leaks a welded steel protection vessel enclosed the condenser. The condenser was designed to allow for removal of about 4 x 10 Btu/(hr)(sq ft) while operating the system at 10000F. The section making up the condenser sleeve was a one foot by 2 1/2-in diameter, sch 60, type 304 stainless steel pipe. The inside surface was reamed to obtain a press-fit over the boiling vessel. An Atlas compound mandrel press was used to force the sleeve down over the boiling vessel. The cooling coils were 5/8-in diameter, type 304, 16 BWG stainless steel tubes bent around and welded to the sleeve to make up three separate and parallel cooling systems. The cooling systems were arranged in three, five and nine coils each, numbering from the bottom of -the condenser, and connected: in such a way that water entered each set of coils at the bottom. Tubes to and from the condenser were welded through the, top flange of the outer pressure vessel and connected to a water manifold system. This arrangement made it possible to operate all or any part of the condenser system and gave more flexibility to the operation of the equipment. Each cooling line contained a nominal 3/8-in pipe Crane gate valve. The valves were connected with 5/8-in diameter copper tubing. City water was used as the condenser coolant,

23 A one foot section of 6-in diameter black iron pipe was welded around the cooling coils; 1/4-in thick by 6-in in diameter cold rolled steel plates were welded to form each end. A 5/8-in diameter, type 304, stainless steel tube was welded through the top flange of the outer pressure vessel and welded to a 5/8-in diameter hole made near the top along the side of the condenser pressure vessel. This line was connected to the inert gas system. Boiling Tube At the outset of this investigation it was hoped that a compositeelectrically heated tube extending completely through the boiling vessel9 with a stabilizer gas flowing through the center, could be fabricated for use in this study. A tube of similar design was used by Ellion (13) in his study of boiling water under force convection. This design would have permitted determining the critical heat flux directly while minimizing the probability of burnout. Also, boiling could have been accomplished in the transition boiling regime. Unfortunately, this tube assembly design was found to be extremely difficult to fabricate and would in all probability not stand up for more than one heating cycle. It was finally decided that a tube design similar to that first used by Noyes (42) could satisfactorily yield high flux nucleate boiling data and permit burnout determinations. Although this design was a compromise from. the original design, nevertheless it could yield nearly as much information as originally planned an: in addition, would allow for rapid and easy replacement of the heater assembly. A cross sectional drawing of the bayonet heater assembly is shown in Figure 7. Photographs of the boiling tube and tube cross section are given in Figures 8 through 10. Figure 8 is a view of the boiling tube assembly showing the boron nitride sleeve and graphite heater before inst;allation,

1.281 SWAGELOK A FITTING. 1 25'- 3.5" I ~~~~~~0.25"" ITE ROD THERMOCOUPLE 0.0175".06875. WELLS 0;377" 0.234 SLEV'. \i MOLYBDENUM ROD 0.053 I3 HAYNES25 TUBING:: AND'PLUG BORON NITRIDE SLEEVE A, 0.035 SECTION A-A Figure 7. Cross Sectional Drawing. of Boiling.Tube Assembly.

rl bH co *rl..... ~~~~~~~r ~~~~~~~r ~~ R LaJ ~ ~ ~~~~~~~~~~~ H~~~P,0f.~'.~ C z.~~~~~~~~~~~~~~~~ U)~~~~~~~~~~r ~~..>,~~~~~~.., fi

Fiur veal iwfoligTuewihThrmcupeAseble.n Figure 9. Overall View of Boiling Tube with Thermocouple Assemblies.

:,~:''"xr - i:i::i:l:i:i::::i:::i:: i:i:i:::i:::::::-::::::::::::::::::::::::::::::::::-:::::::::::::::::::::::::::::::::::::::':'::':::~'::':::::::::::::::: s:~::~:~::::::~:::::::::::::::::::::::::::::':::: ~~:~-;~I:i::: ~:::~::-:;:::::::::-i:i:i:i:i-i::::::::: ii~:i:i:i:::::iii:_':ic:i:i:'_-_i::::i:::::::i:-i:i:::i:i:::i:::i::::-:_:-:-::-:::::::, ii::iiiiii~i:i:::::::i:i:::i:::':i:i:i: iili-::i:iiii:iiiiiii:::::i:iii-iiii::ii::::::-i::::'::::::-:'::::::':-:':-:,:::::::::::'::' PiFWBg:js--glr::r:::::_.:::.::i:::::_::::i::::::i(.:i: I::::II -- ili-i,;.i~j!' iiiii:iii.i:i:iiiiiiii::':':::::''iiiiii:i'::iiiiiii:iii:~-ii:': i:i:::i:::'::::-:::::::::::::: ::::~:::':-:::::-:::::::i iiii-iiiii-lii:ii,:::~::,:: ~:~:::l::i:::i:::i:::::::::::::::::::::::::~::::::::::::~:::::.:::::i:i:::::~:i:ji;iiiiii'iiiii'ii;iii:i:'::''::iiiiii:.::: ~:::i:-:'_:::':c:i:c:':i-ii'i:::::.::::::::::~:::::,:::: -:-:i —:ii -:'::::,:i:::i:::::':':'::::':::::::"::':::::':': —-:: -~i::.i~-i: -~''~-'''':' -::::: i.-::_ —--:::::::::::::::::-:i:::::i:::i:::i:i-:i::: iii:iii~i:iii-i:i::il.;ii-i-i I::i: ii~ i:i/-i::1~::.;'l;' -:::-':::::::::::::::~ _:-:_:-:_:~::::: iiliiiiiii.iii:ii-ii.iii::::':'::':: iiiii::::::::::::::::::::::i::::::~.::::::::::::::::,: ~i::::-.:i:::::::i~,::~:~:~:~:~~:~:::::.::;::-:-::::_:-:-:::i: —:i:i~_:-i:i::::-: —-—::-:-_:i::::i:~:::: sLEEVE:'' NR\DE BOROFI II IV.1~ Figure 10. Views of Boiling Tube Cross Sectionals.

28 Figure 9 is an overall view of the tube showing the thermocouple assemblies extending from the thermocouple wells. Figure 10 gives cross sectional photographs of the composite tube. The bayonet heater arrangement is made up of a cylindrical graphite heating element, a boron nitride insulating sleeve, and the boiling tube. Because of the importance of limiting large thermal gradients radially through the composite layers, it was imperative that each component be machined to the closest tolerances possible. To meet these requirements it was found that the following fabricating procedure gave quite satisfactory results: The boiling tube was a 0.377-in OD by 0.053-in wall thickness Haynes-25 tube. Three longitudinal grooves 2 3/8-in long by 0.040-in wide by 0.040-in deep were made 90~ apart in the tube with a surface grinder using a 0035-in. thick cut-off wheel. In tube 4 the three tube thermocouple grooves were placed at different locations longitudinally along the tube length. The three grooves in this tube instead were made 2 1/8, 2 3/8, and 2 7/8-inches long respectively. Stainless steel (type 304) hypodermic tubes 2 1/4-in long by 0.035-in OD by 0.022-in ID, closed with a Heliarc welder at one ends were then nicrobrazed into each groove. At the same time, the tube was nicrobrazed into a 3/8-in.(type 316) stainless steel bore-thru Swagelok male connector (No. 600-1-6-316BT). By brazing the tube into the fitting, it was not necessary to use ferrules with the Swagelok fitting to make the seal. The braze filler used was a nicrobraze paste type B (LM nicrobraze ANS 4777) purchased from Wall Calmonoy Company, Detroit, Michigan. The excess braze on the tube surface was machined off, the tube polished, and a Composition: 6.5Cr - 3.5B - 4o5Si - 2.5Fe -.015C BaLoi,

29 1/16-in thick by 0.277-in diameter Haynes-25 plug welded to close the end. of the boiling tube. To tgive the required smoothness- to- the- inside.e surface of the boiling tube, the tube was then reamedto 0..270-in diameter,.~ A 1/2-in diameter rod. of boron nitride, purchased from.the Carborundum Company, Latrobe, Pennsylvania, was drilled and:.reamed to 0.230-in ID. The sleeve was then put on a mandrel and finished to:'0.2695-in OD. The graphite used for the heating element was's ultra-purity graphite purchased from Ultro-Carbon Corporation, Bay City, Michigan. It was machined to a 1.25-in length by 0.230-in diameter. Extreme care was taken to assure that each end of the graphite element was cut off at a right angle in order that good electrical contact might be obtained at both ends of the tube, The small end of a stepped molybdenum,.rod, 2-in long by 0.25-in diameter and 1 3/4-in long by 0.23-in diameter, slipped in the b~oron nitride sleeve and was forced up against the graphite element. Along.the 0025-in diameter length of the rod the bus bar was clamped. The end of the molybdenum rod was then fitted into the spring mechanism support (discussedlater in this chapter). This mechanism forced the molybdenum rod against the graphite and also the graphite against the inside end of the boiling tube. Litharge (PbO) mixed with glycerine (to a consistency of thick paste) was used on the tube assembly threads. This mixture prevented the seepage of liquid potassium through the threaded joint. With the tube assembly tightened in the test vessel, the vessel was pressurized with helium and the threaded joint checked with a helium.leak detector, The detector was a type 24 —210 manufactured by Consolidated Electrod.ynamics Corporation, Pasadena, California. In no instance initially after tube installation, did the threaded.connection leak, even at the most sensitive setting on the detector.

30 Outer Pressure Vessel A photograplh of-the outer vessel is shown in Fi-gure 2. A drawing' of the vessel is given inFigure 4. This vessel was fabricated from a one fobt length of- sch 40, 14-in diameter, black iron pipe; three 400-lb, 14-in diameter, forged steell weld neck.flanges; one 400-lb, 14-in diameter, forged steel blind flange, and a 14-in diameter tube turn (Part No. 81XS) welding cap. Flange gaskets were standard, 14-in diameter, XH asbestos ring gaskets. Each flange contained: 20 bolt holes and used 1 1/4 x 8-in carbon steel machine bolts and hex head nuts. A block and tackle was used to lift the outer vessel in place for bolting. Two 1/4-in diameter steel cables with hooks, extended from a bar connected to the block and tackle. Steel arms (see Figure 2) welded to the outer vessel and lower flange extended out from the vessel to facilitate' hooking the block and tackle cables. All necessary entries into the pressure vessel were made through the blind flange. A layout drawing of these entry holes is shown in Figure 11. Thermocouples, voltage taps, guard heater leads, and the charging line entered the vessel through Conax glands while the water lines and gas lines were welded directly to the flange. The electrode glands were specially designed and are discussed in a later section of this chapter. Potassium Charging System All lines in the charge system were made from 3/8-in diameter, type 304, i6 BWG stainless steel tubing and were connected as shown in Figure 3, Valves were nominal 3/8-in vee-seat (No. 7VF6-316) Whitey valves with 0.312-in diameter orifices and high temperature (rated at ll0O1 F) asbestos packing.

400 LB., 14-IN. DIAMETER FORGED STEEL BLIND FLANGE 1~ ~_- --— THERMOCOUPLE GLAND ELECTRODE GLAND | FILLING TUBE HOLE HELIUM LINE HOLES FLANGE BOLT HOLES WATER LINE HOLES Figure 11. Layout of Entry Holes in Outer Protection Vessel.

52 Stainless steel, type 316, Swagelok male connectors (No. 600-1-6-316) were used with each valve. The charge vessel was a 2-in diameter by 14-in long section of sch 40 stainless steel pipe. Stainless steel discs, 1/4-in thick by 2-in diameter were welded to form each end. Two 3/8-in diameter drain holes were drilled in the bottom plate and welded to the system loading and test vessel charging lines. The two 3/8-in diameter holes in the top plate were welded to the inert gas line and an alternate potassium dump line. A 3/16-in diameter stainless steel thermocouple well extended six inches into the charge vessel. This well was made from 16 BWG (type 304) stainless steel and was closed at one end with a Heliarc welder. The inert gas line leading from the charge vessel used a 1/4-in (No. Y353H), type 316, stainless steel Hoke valve with asbestos packing. This valve was connected to the inert gas supply through a 1/4-in diameter copper line. When initially charging potassium to the charge system, a 6-in length of 3/8-in diameter stainless steel tubing connected the system loading valve to the potassium shipping container. A 3/8-in Cajon (No. 6SW-7-4-316) socket weld female connector was welded to the end of this line and could be screwed to a 1/4-in male pipe fitting on the shipping drum. An auxiliary access line to the charging loop was installed to be used as the feed line during the cleaning operation. A vessel containing methanol (or other cleaning fluid) could be attached to this line and methanol circulated in the charge system and boiling vessel. Inert Gas Lines, Vacuum System and Knockout Drum The inert gas lines are schematically shown in Figure 3. Helium from. a gas cylinder was supplied to a manifold system by a two-stage 0-3000 psig

and 0-200 psig Meco pressure regulator. Hoke 1/4-in:(No. Y353H) type 316 stainless steel valves, with.asbestos packing, regulated the gas supply -to each of the three systems, i.e., test vessel and potassium charge system, condenser protection vessel, and. outer pressure vessel. A fourth valve was used for exhausting to the atmosphere,. All. gas lines not coming in contact with potassium were of 1/4-in copper tubing. Standard Imperial compression fittings were used for all connecti ons. The vacuum system was connected to the manifold system as schematically shown in Figure 3, The valve in the vacuum line was a 1/4-in (No. Y353H) type 316 stainless steel Hoke valve. The vacuum pump was a (Cat. No. 91305) Cenco Hyvac 2 pump. A nitrogen cold trap of standard design was used to remove condensibles and avoid contamination of the vacuum pump oil. Calibrated bourdon pressure gauges (0-300 psig) measured pressures in each system. In addition, an open end mercury manometer was used in the test vessel system. Valves could cut out each pressure gauge and/or the manometer when operating outside the range of the gauge (manometer). The line above the test vessel contained a knockout drum to trap and condense any potassium vapor which migrated beyond the condenser, This drum was made from a 2 1/2-in diameter by one foot long section of sch 60 (type 304) stainless steel pipe, packed. with stainless steel lathe shavings. Discs 1/4-in thick were welded to form each end. A 3-in length of 3/4-in diameter, 16 BWG, (type 304) stainless steel tubing was welded through the bottom of the knockout drum and connected to a similar piece of stainless tubing extending down to the test vessel. A (No. 1210-6-316) Swagelok union made the junction. A 3/8-in diameter stainless steel tube extending from the top of the knockout

drum was connected to the inert gas line. A Hoke valve of the type used in' the inert gas manifold was installed-in the line leading from the knockout drum. In the top of the knockout drum a 3/16-in diameter (No. PG-2) Conax stainless steel tube gland was welded and fitted with a thermocouple well. This well extended through the knockout drum into the test vessel and could be adjusted to read temperatures vertically throughout the boiling vessel. It was approximately five feet long and made from 0.188-in diameter by 0.032-in wall thickness Haynes-25 tubing. Power Supply, Power Leads,. and Guard Heaters Power to the boiling tube was furnished from a 12KW, 3-phase, full wave Udylite rectifier with a rated output of 250 - 2000 amps at 18 - 6 volts DC respectively. It was made by the Udylite Corporation, Detroit, Michigan. The boiling tube electrical circuit is shown in Figure 12. Heavr power leads (300 AWG copper welding cables) carried current from the rectifier to the electrode glands passing through the top flange of the outer pressure vessel. The glands, shown in Figure 14, were made from 1 1/4-in diameter copper rods and used rubber silicone 0-rings in lava seats. The 0-rings were (No. GRC30-7 S1185) supplied by Gosh-en Rubber Company, Goshen, Indiana. Lava spacers (seats) were machined to size, cured at 1800~F, then slowly cooled to room temperature. Two 1/4-in thick by 2-in wide copper bus bars directed the current inside the outer pressure vessel to the boiling tube circuit. One bus bar was connected with heavy flexible copper- straps to the nickel-A bus bar. The second was connected with similar heavy flexible copper straps to the bus bar support (see Figure 6). The flexible straps allowed for any differential in expansion between the copper bus bars and the boiling vessel.

35 UDYLITE 208v SUPERIOR - RECTIFLER 50mv BOILING TUBE 3 PHASE 250-2000a SHUNT CIRCUITRY 17 KVA 6-18v SIMPSON WESTON 0-500 DC 0-30 AMMETER DC VOLTMETER Figure 12. Boiling Tube Power Supply TOP TEST VESSEL LOW POWER TRIPLETT HEATER, 100 WATTS SUPERIOR O- 25AC ('~-'-~~ F POWERSTAT AMMETER t LOWERTESTVESSEL LOWPOWER I10v 1 2.2 KW HEATER,, 200 WATTS I II LOWER TEST VESSEL HIGH POWER HEATER, 600 WATTS SIMPSON SUPERIOR - O-IOAC USE: POWERSTAT AMMETER FILL LINE ~~~~~~~~IlKv 1 10 I. ------- SIMPSON SUPERIOR O-0IOAC 8a POWERSTAT AMMETER CHARGE LINE FUSE IKW HEATER IlOv I -— oI I SIMPSON SUPERIOR I O- IOAC O'"'~C POWERSTAT AMMETER CHARGE VESSEL FU IKW HEATER IlOv 110vFigure 13. Guard Heaters Circuitries Figure 13. Guard H~eaters Cir~cuitries

BRASS NUT 9\\\ \r —- -\\\ POWER LEAD CONNECTOR LAVA A COPPER WASHER TOP FLANGE LAVA A COPPER RING O Figure 14. Drawing of Electrode Glands.

37 Figure 15 is a photograph of the nickel bus bar and spring loaded mechanism for the molybdenum shunt. Guard heaters were used around the test vessel, charge vessel, and all potassium lines. Their circuitries are shown in Figure 13. All heaters were 20 ga Chromel-A wire, electrically insulated from the containers and lines by 1/16-in thick asbestos sheeting. Copper (14 ga) leads from the guard heaters inside the protection vessel went through a (No. TG-14-A6) Conax bare-wire gland using a lava sealant. Power to the test vessel guard heaters utilized 1 KW 110v AC Powerstats. A 0-25 amp model 5331-S AC Triplett ammeter was used with the test vessel guard heater. Simpson (model 157) 0-10 amp AC ammeters were used with all other guard heater circuits. Instrumentation. A majority of the boiling vessel thermocouple locations are shown in Figure 4. Figure 16 gives the circuitry for all thermocouples. All temperatures, except those in the boiling tube, were measured using 20 and 28 ga Chromel-Alumel thermocouple wires (Hoskins 3G-178 grade). Fabrication of each thermocouple junction was accomplished by using the standard gas welding technique as recommended by the thermocouple supplier. Conax bare wire thermocouple glands (Noo TG series) were used to seal each thermocouple lead as it passed through the outer-pressure vessel. The three Chromel-Alumel boiling tube thermocouples (see Figure 9) were swaged assemblies with a 0.020 - 0.0005-in diameter Inconel sheaths. The matched thermocouple wires were 38 ga Chromel-Alumel (Hoskins 3G-178 grade) and were electrically insulated with magnesium oxide. Each thermocouple used a grounded junction. The thermocouples were calibrated and gave average deviations from standard tables of 0.32 per cent at 1L00 F. They were purchased from Pyro Electric Company, Wa:lkerton, Indiana,

RiG.M ECHANJSM. HOLD N CKEL A BUSBAR 0c.OLYBDENUM SHUNT f Boiling Tube Bus Bar Assembly.

39 LEEDS & NORTHRUP MICROMAX 16 POINT RECORDER I. LIQUID 4" FROM BOTTOM 2. LIQUID 2 FROM BOTTOM 3.VAPOR II FROM BOTTOM 4.VAPOR 21" FROM BOTTOM 5. VAPOR 8" FROM BOTTOM 6. OUTSIDE BOILING VESSEL ABOVE CONDENSER 7. OUTSIDE BOILING VESSEL,VAPOR SIDE 8. OUTSIDE BOILING VESSEL, LIQUID SIDE 9 OUTSIDE BOILING VESSEL, SPARE IO. OUTSIDE CHARGE LINE, LOWER THERMOCOUPLE I I. OUTSIDE CHARGE LINE, UPPER CONNECTOR 12. SHORTED BOARD 13.CHARGE VESSEL 14 - 16 SHORTED v ~~~~THERMOCOUPLE SWITCH I.LIOUID 4" FROM BOTTOM THERMOCOUPLE 2.LIQUID 2" FROM BOTTOM LEEDS a NORTHRUP LEADSPOR I FROMBOTTOM MODEL 8662 TO APPARATUS 4. VAPOR 21" FROM BOTTOM POTENTIOMETER 5. VAPOR 8' FROM BOTTOM 6. OPEN 7. OUTSIDE CHARGE UNE LOWER 8.0OUTSIDE CHARGE LINE UPPER 9.CHARGE VESSEL LEEDS a NORTHRUP MODEL 8662 POTENTIOMETER BROWN ELECTRONIK 0-5 mv PEN RECORDING POTENTIOMETER LEEDS a NORTHRUP MODEL 8662 ET POTENTIOMETER ATH ~_..: J t ~~~~~~WHEELCO O-lOmv PEN RECORDING POTENTIOMETER BROWN ELECTRONIK VARIABLE RANGE 0-10 mv PEN RECORDING LEEDS a NORTHRUP POENTIOMETER MODEL 8662 ICE POTENTIOMETER BATH,~- ~~~~LEEDS a NORTHRUP.O- 0 my PEN RECORDING POTENTIOMETER ICE Figure 16. Thermocouple Circuitries.

The vertically adjusting thermocouple measured temperatures at any desired location throughout the test vessel and was used as a liquid level finder when charging potassium. Three pen recorders were used to record the three boiling tube temperatures. Two of the recorders were O - 10mv full scale and used hand balancing Leeds and Northrup (model 8662) potentiometers in series to obtain the temperature ranges desired. One recorder was a L & N Speedomax while the other was a Brown: Electronik. The third recorder was a Brown Electronik with an adjustable scale of 0 - 1Omv span. A 0 - 5mv fast responding Brown Electronik pen recorder was used with the adjustable thermocouple. This recorder also used a standard L & N (model 8662) laboratory potentiometer in series with it to give the desired temperature ranges. Other temperatures were recorded on a L & N Micromax, 16 point, recorder with a range of 200 - 20000F. It was also possible to read a number of temperatures with another standard laboratory L & N (model 8662) potentiometer connected to an 8 point thermocouple switch. A pre-calibrated ammeter (O - 500 amps) and voltmeter (0 - 30 volts) were used to measure power to the boiling tube-. Voltage tap connections were made across the boiling tube circuit which included the molybdenum shunt, graphite heater, Haynes-25 tube end, potassium, and vessel wall.

CHAPPER III OPERATIONAL METHODS AND PROCEDURES Preliminary Procedures Upon completion of equipment installations the apparatus was thoroughly cleaned and tested. The boiling system and auxiliary lines were made helium tight up to 200 psig. All thermocouples and instrumentation were checked out. The system was then flushed several times and heated to boil. distilled water. The test vessel charging procedure;, method for determining liquid level, operation of condenser system, etc. were checked out and/or refined using water as the test fluid. As a means of determining what might be the best operational procedure with potassium, seven nucleate boiling runs were made with water. These runs covered a pressure range of one atmosphere to 150 psig with fluxes up to 0.5 million Btu/(hr)(sq ft). Operation of equipment and the method of taking data were essentially identical to that subsequently used with potassium (discussed later in this chapter). Upon completion of the water test, the water was withdrawn from the system, the test vessel and charging lines were heated to over 2000F and the experimental system outgassed for several days. This completely removed the last traces of water from the system. The apparatus was then heated up to 100O~F and held for several hours. Commercial grade potassium'was charged through the load valve from a standard 5-lb capacity MSA Research Corporatnion shipping container. The

shippLng container was connected.for filling as per.instructions (39) furnished by MNSA Research Corporation, Callery, Pennsylvania. A hot plate placed under.the. shipping container..melted the..:potassium for charging while a propane torch (small portable fuel cylinder torch) heated the connecting lines. All lines and vessel heaters were set at over 2500F. The liquid potassium was drawn up into the charge vessel by applying a slight pressure to the shipping container. The thermocouple in the charge well indicated when the potassium had reached the correct level by a sharp change in the thermocouple reading. At this point the loading valve was closed, the apparatus and shipping container allowed to cool to room temperature, and the container disconnected. The loading tube was then cleaned of residual potassium, using methanol. A cap was screwed on the loading valve connection. During the filling step a potassium leak developed at the threaded fitting between the shipping container and the filling line. In the course of filling, potassium completely solidified in the loading line and necessitated remelting with the propane torch. The first potassium to reliquify was blocked off on both sides in the line by solid potassium. This resulted in liquid potassium being forced through one fitting and causing a minor fire, Extreme care was exercised to prevent this from happening again. With the potassium in the apparatus all instrumentation, connections, etc. were checked out. Finally, the test vessel charging procedure was reviewed and carried out. Necessary refinements were made. Start-up, rocedure Before the initial run for each tube, the test vessel was charged to -the desired deptho This was accomplished simply by equalizingthe pressures in t'he test vessel and. charge vessel, then draining the liquid potassium into the test vessel to the' desired depth as indicated by the thermocouple

liquid level finder. The charge line test vessel guard heaters were set to give system temperatures over 2500F., Lowering the test-vessel level could be accomplished by applying slight positive pressure over the liquid in the test vessel. With the potassium at the correct depth in the test vessel, power to the test vessel and fill line guard heaters was increased to give the desired heat up rate. The normal rate for heating wasa:usually about 300~F/hro When the bulk temperature reached bulk saturation, the potassium was allowed to pool boil using only the guard heaters, for a sufficient time to.reach thermal equilibrium throughout the test system. Sometimes power to the boiling tube was turned on and the pool boiled for 30 to 60 minutes at fluxes approximating 200,000 Btu/(hr)(sq ft). The adjustable thermocouple served to measure bulk temperature and was normally positioned to read the temperature at a pool depth of approximately 4 to 6 inches. When system temperatures, at zero power to the boiling tube, remained constant over a period of time, the system was considered to be at steady state and ready for a boiling run. Operating Procedure for Nucleate Boiling Runs Following the- start up procedure, as described above, the system was ready to commence a boiling run. With the tube power source cut off, all boiling vessel liquid thermocouples, i.e., adjustable bulk thermocouple, and three boiling tube thermocouples, were read and recorded. The power to the boiling tube was then turned on and increased in steps up to the desired maximum heat flux. For each step the system was allowed to reach steady state, after which the following readings werye recorded (i) boiling tube amperage, (2) boiling tube voltage, (3) bulk thermocouple reading, and (4) the three boiling tube thermocouple readings. Since

temperatures in the bulk liquid- a-nd boiling tube generally fluctuated,- it wasnecessary to average the readings.'It should be"mentioned that' thrughout" each run, pen -recording potentiometers continuously recorded the bulk and the three boiling tube thermocouple readings.'The system pressure was read and: recorded about every third power setting. The three condenser temperatures were checked periodically, as these temperatures were used as an indication of when more condenser cooling was needed. It was-found that for the most part cooling water had to be turned on only periodically and then only long enough to refill the condenser coils with water. The condenser was operated by filling the top nine coils with water and then letting it boil out. This technique worked quite satisfactorily on all runs except the lowest pressure runs when it was necessary to let a very slight continuous flow of water go through the condenser. At all times only the top nine coils of the condenser were used. In all runs there was a slight increase in system pressure as-the heat flux was increased. This increase was generally due to the generation of increased amounts of potassium vapor and subsequent compression of helium cover gas. When a run was completed the power was returned to zero and all liquid bulk t~hermocouples read and recorded; the pressure,was also recorded. To start a run at a different pressure, the helium cover gas was regulated to the desired pressure and the system allowed to reach equilibrium saturation conditions. The procedure was then repeated. Operating,Procedure for Burnout Determinations The experimental. procedure for ob-taining a majority of the burnout points sas very similar to that used by CNoyes(42) ian his burnout measurements with sodium. In short, the method consisted of holding the heat flux constant

at the desired value and slowly decreasing the pressure until incipient burnout was reached. It was felt that measurements made in this manner were very reliable and permitted time for cutting off the tube power before actual tube failure occurred. In one nucleate boiling run, burnout was inadvertently reached by increasing the heat flux at a constant pressure. When burnout was reached, it was not possible to shut off the power supply before complete tube failure occurred. For this reason most subsequent burnout measurements were made by holding the flux constant while decreasing the system pressure. Usually the system pressure was set 30-in Hg above the anticipated burnout pressure. With the pressure regulated, the power was slowly increased to the desired heat flux. When the system reached steady state, the vacuum line (with vacuum pump operating) and boiling system valves were regulated to give a gradually (maintaining steady state conditions) decreasing system pressure. The rate of pressure decrease was usually controlled to less than 20-irn Hg per hour. Because -of the unsteadiness of the pressure decrease in some instances it was necessary at times to readjust the vacuum line valve. Incipient burnout was indicated by an instantaneous rise (at least 300~F) of the boiling tube- thermocouple. The rectifier was cut off immediately and the final thermocouple and pressure readings made. The procedure was repeated to obtain new burnout points.

CHAPTER IV RESULTS This chapter presents the experimental results-:obtained by the methods of the preceding chapter. All results were obtained through direct measurements or obseriations and are categorized under the general headings: Nucleate Boiling Water Data, Nucleate Boiling Potassium Data, Potassium Burnout Results, and Temperature Fluctuations. The nucleate boiling data consisted of measurements of heat flux,, system pressure, and'the temperature difference between the boiling surface'and liquid saturation temperature. The liquid saturation temperature here'denotes'the bulk temperature at the liquid-vapor inte:rface; frequently referred: tob as the free surface temperature. In the burnout determinations, only measurements of heat fl.ux and system pressure were necessary. Power i.lput to the boiling tube assembly was used to calculate the heat flux whi.e thermocouples in the liquid and tube all f.urnished temperatures. Determination of power losses in the boiling tube circuitry are discussed in Alppendix D vzhilee the er..ors associated wit-h each experimental measurement are estimated in Appendix. E Tabulations of all experimental data are given in Appendix A. A section covering the treatment of data is given in Appendix B. iv.ueiaate BoiliLng Wat+er Data Tnh- ohase cf th the inrestigation w-as undertasen to check the operability of the appaz-.atus and. to provide a standard for comparisoo n of results obtained i.n t ~is equipment with those from similar systems using the same test liquids.

Water was boiled at fluxes up to 500,000 Btu/(hr)(sq ft) in the pressure range from one atmosphere to approximately 150 psig. Boiling tube temperatures during the water runs were measured using only one thermocouple. It was located in the tube surface approximately 45~ from the vertical. The other two thermocouples in the surface had previously shorted out, thus a comparison between AT values measured around the tube was not possible. All data were taken by starting at zero heat flux and increasing the flux in steps to the desired levels. At each flux setting the system was allowed to reach a steady boiling condition before taking data. A summary of the data is shown in Figure 17 while a tabulation of all measurements is in Table I of Appendix A. The atmospheric data compares well with atmospheric water data obtained by Lyon (34-) who also obtained data from the outside of an electrically heated 3/8-in diameter tube. Atmospheric water data of Addoms (1), from a 0.024-in diameter platinum wire, gave lower AT values than the present investigation. It is seen in the present data, that with an increase of operating pressure from one atmosphere to 150 psig, the pressure effect on the AT is significant. At no time were pronounced temperature fluctuations observed. Since other investigators (22, 37, 38, and others) have reported fluctuating temperatures in a boiling surface while boiling water, it is felt that perhaps the response time of the temperature recorders used in this study was too slow to pick up such variations. The boiling runs with water were made using a boiling tube assembly with a slightly different thermocouple installation than those used for the potassium runs. Nevertheless, the thermocouple sensitivity should have been comparable in the two designs.

48 106 106 1.. i[[ I I [ ] 1 1 I I f /A 105- / - % / 0oPA Pressure / o Run W-1, oneatmosphere / 0o0 a Run W-2, one atmosphere / ~ Run W-3, one atmosphere 00/ r v Run W-4, one atmosphere 104/ Run W-6, 174 psia / 0 - Data of Lyon (34), one / atmosphere, from 318-in. dia. stainless steel tube -- Data of Addoms(1), one V 0 atmosphere, from 0.024-in. - dia. platinum wire 103!,l l I I I I l l! I i l l,, 1.0 10 100 1000 AT, OF Figure 17. Comparison of Pool Boiling Water Data with Two Previous Investigations.

In the development of a satisfactory boiling tube assembly several alternate designs were tested before arriving at the f:.nal design as described in Chapter II. One design which, at the time, was thought feasible called for brazing a 0.032-in diameter swaged thermocouple directly into longitudinal grooves made in the outer tube surface. The first tube fabricated in this manner proved satisfactory and was installed in the apparatus. All water runs were made using this tube. Later, upon charging the apparatus with potassium, the last thermocouple in the boiling tube assembly was shorted out on the first run attempted. Subsequent tubes fabricated in this manner had on the average 2 out of 3 thermocouples in each tube with open circuits. This fact necessitated a slight modification.of the tube design for the potassium runs. Nucleate Boiling Potassium Data Figures 18 through 20 present typical plots of nucleate boiling data for potassium taken on three, 0.377-in OD by 1.25-in long, Haynes-25 tubes. Ten runs were performed from.zero to burnout fluxes in the pressure range 1 to 45 psia. A complete tabulation of the original data is given in Tables II to XI in Appendix A. Plots of data for all individual runs not presented in this chapter are given in Appendix F. Each figure presents data.determined from the three boiling tube thermocouples, i.e., top, side, and bottom, for a single run. Scatter of the data prevented plotting data in the same figure for more than one run. No attempt was made to evaluate numerically the reproducibility of the data. An attempt was made to correlate the data using the least-square method to determine the best straight line log-log relationship. Curvature of the boiling curves at low fluxes prevented~ a good correlation, thus the method was discarded. A smooth curve was more representative of the data. The burnout level for each run is shown as a horizontal line in each figure.

5o0 106 I1 1 I 1 1 Burnout Level 105 cr 02 104 t- Run K-1 o Top Thermocouple o Side Thermocouple t ^ Bottom Thermocouple Tube: 2 Pressure: 12.2 psia 103 I I i I I I l i { [11 1,0 10 100 1000 AT,O F Figure 18. Boiling Data for Potassium Obtained at 12.2 psia using Tube 2 (Run K-1).

51 6 106_I I 1 I ] I I I'11 I I I I!Ii Burnout Level 105 10 Run K-8 o Top Thermocouple _ Side Thermocouple - A Bottom Thermocouple Tube: 3 Pressure: 13-14 psia 103 L l l l l l l I I I l I I I I I I I I 1 I 1.0 10 100 1000 AT,~F Figure 19. Boiling Data for Potassium Obtained -in the Range 13 - 14 psia using Tube 3 (Run K-8).

52 Burnout Level 105 - rA 0 0_ -0 Run K-10 o Top Thermocouple, 114-in. from free end o Side Thermocouple, center of tube a Bottom Thermocouple, 1/4-in. from vessel wall Solid Symbols Designate Up Cycle Tube: 4 Pressure: 3.8- 5.0 psia 103 I,,I,, l I10 I I 100 11000 Ill 1.0 10 100 1000 AT,~F Figure 20. Boiling Data for Potassium Obtained in the Range 3.8 - 5.0 psia using Tube 4 (Run K-10).

53 During run K-8 (Figure 19) the bulk temperature for each heat flux setting was measured at different depths in the liquid pool. Figure 21 is a graphical presentation of these measurements and shows that mixing in the potassium pool, at least in this run, was insufficient to maintain a constant pool temperature. In the plot, the temperature difference is the temperature above the liquid-vapor interface (free surface) temperature. Similar behavior was reported by Madsen and Bonilla (35) in their study of pool boiling NaK. As a result of these bulk temperature gradients, the temperature to be used as the bulk saturation was not so clearly defined. Madsen and Bonilla found that free surface temperatures equalled the equilibrium temperatures corresponding to saturation pressures. It also correlated better when used to determine the boiling AT. This fact was found to be true in the present investigation, thus the free surface (or liquid-vapor interface) temperature was used in AT determinations. In all runs except K-10 (tube 4), data were taken by increasing the flux in steps from zero to the highest value. No readings were taken for decreasing flux conditions. In each case an attempt was made to obtain data as near the burnout point as possible without actually reaching it. In run K-10 data were taken for stepped increased in flux from zero to over 300,000 Btu/(hr) (sq ft) and for stepped decreases back to zero. The data are shown in Figure 20. The apparent break (displacement) to the right on the down cycle is easily distinguishable. Also, recall from Chapter II that tube 4 was fabricated with the three thermocouples placed at different locations along the tube length to detect any longitudinal temperature gradients during boiling. One thermocouple was located directly in the middle of the tube length whilge the other two were placed l/4-in from each end. It can be seen from Figure 20 that the scatter

24 20 - 228,000 Btu I(hr) (sq ft) 75,600 Btu/ (hr) (sq ft) 16434,000 BtuI (hr) (sq ft) 9,634 Btu/ (hr) (sq ft) 0 Btu/ (hr) (sqft) UL- 4 a) -4 Center of Liquid y ~ I Boiling Tube Surface -8 -12 Tube: 3 Pressure: 13-14 psia -16 0 1 2 3 4 5 6 7 Pool Depth, Inches Figure 21. Vertical Temperature Profile Based on Free Surface Temperature (Run K-8).

55 of data is similar to other runs and does not show the existence of any distinguishable longitudinal temperature gradients. Pronounced temperature fluctuations were observed in the three boiling tube thermocouples during all runs. In several runs and particularly at fluxes below 100,000 Btu/(hr)(sq ft), temperature fluctuations were also significant in the bulk liquid and were usually accompanied with significant pressure fluctuations. The last section of this chapter is devoted to these temperature fluctuations, therefore, only their existence is noted here. Inconsistencies and scatter of the data prevented drawing any conclusions concerning the existence of characteristic variations in AT values for the top, side, or bottom tube surface. Also, the data showed no distinguishable pressure effect on AT over the intervals 0.7 - 14 psia and 0.9 - 45 psia. Upon completion of each set of runs the boiling tube assembly was removed from the apparatus and inspected to see what effect prolonged high flux boiling and incipient burnout had on the heat transfer surface. The tube (tube 2) used in runs K-l through K-4 appeared as it did initially with no noticeable surface changes except at the end of two of the brazed thermocouple grooves beyond the thermocouple junction. At this point a slight amount of braze in the last 3/16-in of each groove was eaten away giving the appearance that there may have been melting, erosion, or dissolution of the braze at these locations. Fortunately, the thermocouple junctions in each of the two cases were far enough removed (about 1/4-in) from these indentations that it was felt the thermocouple readings were not affected significantly. Photographs showing three views of the tube assembly after removal from the apparatus are shown in Figure 22. The dull finish and residue seen on portions of the tube surface resulted from the cleaning operation. Lead deposits coming from the litharge( PbO )-glycerin pipe dope mixture can be seen on the threads of thefitting.

:::i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~iiii~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~iiiiiii~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~i ~:~::~:~:~:~:~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~iiiiiii iiiiiiii~iiiii~iip:- i~i. i iii i iii iiii'ii iiiiiiiiii:iiii ii!i i ii. ii~i-iiiii.' i iiiii ii:: iiiiiiiiiiiiiii' ii: iiiii iiiii iiii'iiii iii::: iii.: ii:::iii iiiiii iiii: iiiiii-::i: iiiiiiiiiiiiiiiiiiiiiii:':::' iiii~~~~~~~~~~si~~~~~~iiii~~~~~i iiiiii -iii~~~~~~~~~~~~~~~~~~~~iii~~~~~~~iiii~~~~~~~~~ii~~~~~ii: —ii..:::.:.::::::::i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~i~~~~i:-i::.::.:.::~~~~~~~~~iii!111lii~i ~i~iiii~iiiiii~iiiiiii'.:'':: ii~i~- ~~~i:- i iiiiiii iiiiiiiiii'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~:i~;;iziiiiiziiiiiiiiiiiii~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~iiiiiiiii Figure 2:2. Photographs Showing Three Views of Tube 2 After Removal from Apparatus.

57 The tube (tube 3) used in runs K-5 through K-9 showed no effects of boiling and upon slight polishing of the surface it looked exactly as it did before installation. The tube (tube 4) used in run K-10 underwent destructive burnout on the last of six burnout determinations. A photograph of the tube is shown in Figure 27. A description of the burnout effects on the tube is given in the next section. The data from tubes 2 and 3 are compared with. the data of Bonilla (6) in Figures 23 and 24-. The present data shows more scatter at the same pressure than data of Bonilla (see Figure 1), however, it must be pointed out that Bonilla obtained data up to only 115,000 Btu/(hr)(sq ft) with only three runs in the range 700 - 1500mm Hg and one run at 2 - 12mm Hg. Potassium Burnout Results Figure 25 is a plot of the burnout data taken with four boiling tube assemblies in the pressure range 0.15 to 22 psia. A summary of the burnout measurements is given in Table XII, Appendix A. All tubes were 0.377-in diameter by 1.25-in length bayonet heaters made fr-,m Haynes-25 tubing. Heat fluxes were determined from a measurement of the power to the tube assembly. See Appendix B for method of calculation. The system pressure determined the liquid free surface temperature as discussed in the previous section. The most satisfactory method for making burnout determinations consisted of holding the heat flux constant and decreasing the system pressure until burnout was reached. This method is discussed in the preceding chapter. The other method consisted of holding the pressure constant and slowly increasing the power to the tube stepwise until a temperature excursion was observed in the tube thermocouples. Although the latter technique was more straightforward the probability of destroying the tube was much greater. Of the nine burnout points determined by this method, two ended in complete tube failure, while the other method gave nine values with no failures. Typical traces of

8 06F } -- r | |' 1I 700- 1500 mm Hg |.4 0) |T:P 2-12 mm Hg 105 " 0 4 99t "'-, P 104' Natural Convection + To.p Side Bottom Run Pressure 7- c $' K-1 12.2 psia _ P 9 40K-2 9-11.5 psia * I & K-3 0.7 psia' Q ~ K-4 1255- 14 psia Data of Bonilla (6) 31 0 l I I I I I I I II I l l l ll I I I I I I I 10O 10 100 1000 AT,~F Figure 23. Comparison of Data from Tube 2 with Data of Bonilla (6).

59 106'II I I 1 I I I I 1 I 1 1 I I 1_ 100- 1500 mm Hg *:- co 2-12 mm Hg 0 0 la 105 a. 104 iL ^ - > ~ Natural Convection 10 Top Side Bottom Run Pressure - ~9 9'-' 4 K-5 13.7 psia 9 9 4 K-6 6.0 psia 0o 0 K-7 0.9 psia * U *^A K-8 13 -14 psia -+ 4- - K-9 37-45 psia - Data of Bonilla (6) 103 1o0 10 100 1000 AT,OF Figure 24. Comparison of Data from Tube 3 with Data. of Bonilla (6).

60 107 =I.105 O Tube 1 by decreasing pressure * Tube 1 by increasing flux -o Tube2 by decreasing pressure C TuDe 3 by decreasing pressure A Tube 3 by increasing flux V Tube4 by increasing flux 10 0.1 1.0 10.0 100 Saturation Pressure, psia Figure 25.I Burnout Heat Flux Versus Saturation Pressure for Saturated Pool Boiling Potassium.

61 the boiling tube thermocouples during burnout runs by both methods are presented in the next section (Figures 31 and 32). Three burnout points were determined using tube 1, two at subatmospheric pressures (0.15 and 0.16 psia) and one at atmospheric pressure. The latter point was inadvertently obtained at the last power increase during a nucleate boiling run. When burnout was reached it was not possible to shut off the power supply before complete tube failure occurred. Since the power at burnout was not accurately read, the data point for this determination in Figure 25 represents the last stable heat flux measurement. This value should not be significantly different from the actual burnout flux since the \flux increase creating burnout was very small. Removal of tube 1 showed that it had completely melted at its midpoint. A photograph of the tube is shown in Figure 26. Only the half attached to the wall could be recovered for examination, thus the condition of the other half subsequent to burnout was unknown. Burnout determinations on tube 2 were all made at constant flux. Four burnout points were obtained from 1 to 14 psia. Photographs of the tube assembly after operation are shown in Figure 22. The only noticeable changes (discussed in previous section) in the tube surface after removal were the development of small indentations in the surface at the end of two thermocouple grooves. Five burnout determinations were made on tube 3 in the pressure range 0.7 to 22 psia. Two of these measurements were inadvertently made during nucleate boiling runs at constant pressure while the rest were determined by decreasing the pressure at constant flux. Fortunlately, the last increase in flux each time burnout was reached in the former determinations was small enough that the power source could be cut off before complete tube fairlure.

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The tube, after removal from the equipment, appeared as it did initially with no noticeable surface changes. Burnout measurements using tube 4 were made by increases of heat flux at constant pressure. Six determinations were made in the pressure range 0.7 to 7.4 psia. The tube failed on the last burnout determination (7,4 psia), making it necessary to shut down the equipment. A photograph of tube 4 is shown in Figure 27. It can be seen that a longitudinal crack, nearly 3/4-in long, had developed along the top thermocouple groove, thus exposing the graphite heating element to the potassium pool. In addition, the end of the tube extending into the pool had a hole of approximately 1/16-in diameter melted through it. Either of these failures would have caused the current to short around the graphite heating element and caused tube failure. Temperature Fluctuations Temperature fluctuations were observed in the boiling tube thermocouples during all runs. During several runs, pronounced bulk temperature fluctuations as well as pressure fluctuations were observed at low flux levels. Similar fluctuations have been reported in the literature (11, 22, 34, 35, 379 38). Figure 28 is a trace of the side tube thermocouple during a 5 psia nucleate boiling run using tube 1. Temperature fluctuations up to 1500F can be noted at flux levels below 250,000 Btu/(hr)(sq ft). In some cases the temperature increased rapidly to a point, then slowly (approximately 1~F/sec) to a maximum value. At this point it instantaneously dropped to the original value, Many times this drop exceeded the initial temperature rise and fell as much as 50~F below the original level. Similar behavior was observed on the other tube thermocouples and to a lesser degree in the liquid bulk. Each instantaneous drop in temperature was accompanied by a drop in the system.

iiiiiiiiiiii!i!ii iiiliiiii!ii!!!iiiiiiiiiii! iililiiiiii!i!ili!!iiiiiiiiii iiiii!iiiiiiill~ iiiiiiiii)i!i;il i!iiii!ii!iiiiiiii i! iiiii!i!!iiiii!!i ~i~i~i~i~i~iiiiii............................... i!iiiiii!iii:iiiiiiii!iiii iiiiii!111!i!iiiiil iiiiiii!iiiiiiiiiili!!ii!iiiii!i!! iiiiiiiiiiiiili!ii ii!iiii!iiiiiiii ii!!ii!iiii!iiii?!iii;iiiiili!iiiiiiili!iii~ii iiiiiiiiiiiiili! iiiiiiiiiiiiiiii ii!iiiiiii!iiI!i!iiiiii!!iiiI iiiiiiiiliiiiiii iiiiiiiiiiiiiiiii O ~ i!i!iiiiiiiii! iiiiiiiiiillii iiiiii!i!iiiii! ~iiiiiiiiiiiiiiii iiiiiiii!iiiii!ili!iiii!ililii i illiiiii!iiii!~ iiiiiii!ii!iii iiiiii!i!iiiii iliiiiiiiiiiiiiiiiii!iiiiiii?! ~!iiiiiiii!iii!!!iiiii!iiiii ii~iiii~11iiiii!!!~ili~iiiiii Figure 27. Photograph of Tube 4 After Removal from Apparatus.

! -'; — i —-r-~.:":- -..~..Oi_.~...o' 1. i ~i -?,-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~:" ~-:r-:-:-...[-:i- ~.-"''-:' —'-r-'-'r'i:;-7:-: —-rT-'.....~.................................'! I:~.i.. ~~~~~~:;.. I. i':'.:::~ i':.:,.;'ii. i'-!~:, i -,:~., z.l ~ ~tq ~-:;::~.: - - -:: i -- —: — ":';l:: 1';- 1 "._ _ _...l~ j;j I -i-i~ r~ ~- i1!:.1:: I:;/: I'i- L-:'II:~~~~~~~~~~~~~~~~~~~~~~~~~~''.'l —:'::- - 1 —Ii ——:.. I..i.',:-.-'::,~-:::,.:'.... ~~~-....1... t -~~..._._ 1-i —-- C: " -J...~. ~~__~- I'-r-.-' "i:-iiiI':i'Ji~iPreliminary Run -1 —!;-: ~~~~~/:''... i:,i.: i ~: ~. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~i.,.... i..:, — - _-~L.._.j.l..._I- - --; j:-......'....i......~J-....... ——:...r.....+.....: "''-::"..................... --— ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~....~ —i j.....i i - ~:-j.;~.........' —-.~-i"i-+ ~.L.-.~.' -: —: —'"'l +~ —-.......-.......................:-'':...-:..-:-.:... —-.. -1 I i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...-..: —- -i —— {ub 1 - -.il...i..i.-.:-: -/1.....i. — —:-:" i: i i I.:. II.:: I:!li i i~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I. —.I ~ ~ ~ -.........I. _ ~ I I i..... ~ ~/' ~''...................................... ~...................... +........,.........'-.......:-...............I- -:........ ~-' 1__!_..... _':.... i~~~~~~~~~~~.... i,,- ~r'...,....i_.spsi,. 51o _......i -_!~0.~~~~~~~ ~ ~~~~~~~~~~~, I -:::': ii...._.... x -'' —....I.: —-''i.._~...: —'":..l.._......- _.....~..... J..t —-'-~........' t"__, "Ti /I i::-c ——:-i —: —-i ~ J. — i i.J i'.i j i'j;::l! il'~,..'~ q/A Btlh)s i).!' I: i'!:':'. i i- ~!.'' i i: I I /;: i j / I i — i~~~~~~~~~l-:.C ":-'-;.....' i' J'''........'..:....j...:..~" — ~~~- ~~ --— ~ —'-1_-_ $ —-.-l —-t —-- L~, i ~ ~ I/: i i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~....-... L~~ ~ ~ ~ ~i 1 _~.:_.:_i.~'i'~.,,....:-.i i..~...i!:~!1 I I ~~~~~~~~~~~~ri: il~'......................i...i.' --- iv.i I ~ ~~:. 3 6,0.:_... -—..4200../..47..0_.'-'._... ~~~~~-Ilii......... —- iIIi i-i —i._ - i_-:.:._ju L:..:~i~~.......:....- -.... I: c...... ~~ ~ ~ ~~i-i: i iI i.i -i;ItiiiIi; ~t~-~i;~i. -~ -; — ~ -i- -...-I-Lj...~ ~-'. —' —-............[~~~~~~~~~~~~~1..........'ii'.......i i:: i ~ i: f ~ ~ ~ ~ ~' I. j.- J i~ ~ - l j~.i'! 1 y~~~ t..L.i i_,:: I -i-.jj-li-i:~~.!~l; —: -:- j... ~ ~~~~~~~~~~~~~~~~~~I -1:''-j r-i-:-:-i - - --.-r-:, --.-.-i —:'-:- —, i.!''.'.! i,~~: — I1: _~. 1I..~ __.~j~-.-~-` —-—.:-~-.-..:- 1 —,I:.i.-Iir' i i~~~~~~~~~~~ —.i ~ ii —:.m~:. -.: " I-I ~~~ ~ ~~:...'- " -',......... i I i ii: i....'~~i i-"" — ~: - ~- ~...............:.......................::. ~ ~ ~ ~ -lti-~!ii i, J!'.l, Ii ii-! — I i.:.... _..:.'~.. — ii.. —.-~ -.~..!..._l.i..:.... ~-11':~:i~l~ j:i i i...1..i: ii:Iii-~.._...: _._.IL.~I _/.. __.i..i...:.~._.I1..^~J_ I-..,',...... _"':'~':i~ J'i~: i~!!!l~ ~ ~ ~ ~ ~ ~ ~~~~~I. l''!!'!,:... ~. i'i.:...........ue ~........ hemcule(ie)Taefo ol oln Potassium at ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ psia.~~~~~~~~~~~~~!

66 pressure and an audible bump. The largest bulk variation, measured 1/2-in below the liquid surface(total depth 6-in), was 70~F. This coincided with a tube temperature fluctuation of 150'F shown in Figure 28 at 184,000 Btu/(hr)(sq ft), Above 250,000 Btu/(hr)(sq ft) the amplitude of fluctuations in the boiling tube decreased significantly. Although upward spikes as great as 50OF were frequently observed, no significant downward drops in temperature were observed. Above 250,000 Btu/(hr)(sq ft), the bulk temperature and system pressure remained essentially constant. The only significant changes in pressure were gradual increases that followed an increase in flux. This resulted from an increased generation of potassium vapor and subsequent compression of the helium cover gas. Figure 29 shows a trace of the bottom tube thermocouple for run K-4. A plot of the complete data for this run is shown in Figure 38. Fourteen flux settings are shown on the plot with no level maintained for more than one minute duration. It can be observed that at no time at low flux levels were pronounced fluctuations observed as in the previous figure. Similar fluctuations were recorded during this run for the other boiling tube thermocouples while the bulk temperature remained constant throughout the run. No fluctuations in the system pressure- were observed. The trace for the bottom tube thermocouple in run K-8i s given in Figure 30. Each flux setting in this run was held for at least 7 minutes. The fast response of the system after each flux increase is apparent. Fluctuations at low flux levels seem more pronounced but with a loner frequency than at higher levels. An occasional downward spike can be observed at the lowest flux whereas the only significant spikes at the higher fluxes occur upward. The other tube thermlocouples showed similar behavior hereas ithe buk temperatnure or pressure did no't fluctuate at any flux level.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~, —-O_. f ~~~~../..7..... 1.'i t~~.~: j-l- — lj —--- ji.i.:.i... I.. 11.1.....:..~~~~~~~~.....,...._I...1._..._._.. ~..:..-...-...... ~.-' i - i........ ~!'i'~- ~ i, i i ":..:::'. j.r! —'-J?-....~ r -..:;' r;....: i~- —:,: -:~'!::1'~ ~: i: -i ~:-: i:; ~-~~~ —;- ~~~~~ — I:'~::~~ t: i —; i~; 2 - - -"T-'-jL-. -~ Lr'~ ~,.-i —::t3~:; —2L -'~L.c-iLL'rL. 2Z_'... ZLL. Ii. /L. L.. —_. ~..L.!:/.. i.L_:.LL-~dLL..._L-L..:...L...~......i._.: z L.;I — -......._'.-_:: i~___LL ___:'-"'':'-~ -'T"'-i"I-;'Ii:'i........i'i".....- r"",-...._...,- I''':; "'i", l.';' ~ ~' i'..... "I'..... i.............. —r-....~.-. + —-1 —,,...............: ~ ~-i~ j~~-~ —........................?..... i.... -i —~-~ —~ -..........'............ — ~j-'.~-!4.-I:{-i —i —-~-... ~i —!:...!....:':... i_.F I i;' 1.: ~~.-i-i.1- ii-iL" -i..... i...... i:: I~ i —-~ -i~ —; ~ ~ ~ ~~-:..;l...li.. i;II."i:...... ~~~~~~~.~~~ ~ ~ ii —.-. i:- i.i. 1-:-I-,'-;,.:'!:~ii —- — 6-I -. ~ ~.-..t.~.. ~ —~ —. —,:. i:.I,.I'-~~ -~~~1; ----- ~~~~~~~~, 1i-.Ili' t-[ 1' [,~', F' i - i —i -- i -,..- I ii-;-!..i.... l l —i-i........1. [!: i: i'-I-:~:': 1'':-l-:: i-i i. i.:i:..': ~ ~-:' I —:!;..:...I _..__.!..."~:..~.~_..!i.......I._'.....-~...-... —' —:- i-....:.-'!-I-~..1..._...:' —' ~..~...j_., I -:-:-' ~.~..i... I:;.!~i -....~... t..~I:- i -:- ~ -: * ~'-.._...I:!.';~I- ~._..I'.'__, -'..Ii._. i xi..LL__ -_ —.......: _.:._._u-....LL.L~.: 2_ I... ~ —... i.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ L...i.........:- ~;l~.~i.;~ii..' F —-' —-~..............' i.i.; L........._/ _............. t —........:-~l —L~~~~~ ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. i ~ j._.:........_.............:._i i~-i.....' -................-..... K- I..I...!~ ~ ~ ~;1._I.. jIj... ~i.i...:'r.:...i~r ~,'' ~~~~ Iii- l_'_!~-:_! i:::.' -.... i::, i.'~.!!: ~ ~i'.. —:~ I, |TIt- i —.i. i'i: ~':i''!...'. i Iub.' -~...~....~", —?....-' I....., ~..:-,....-,~ ~~i I~.;: ~ ~ ~!':........i t. i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ i. i'; ~ -::!-~ t i -:..'.-.- i. -i i i~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~!: _ -~-: —~?.;..:......-i... ~,i~,, i..i.............: i':::.:lA Btl r,.....::, RunK-4.... ~- ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~,:~-., ~~1.....,' ~.!', _..I. ~.i'- I'.- I,,''.i-~'-; ~_: _.i...'-ri-'_:':...I'' j~':..... _~.... ~:'''.;..I.t.....~..:.................'.......................'......... 1~~~~~~~~~~~~~~~~~~~~~~~~~~......;f- I:;''.'-'-?."-!'":..T'.;i.':. -i-;.i.:...::I: -:.''~.!."- ~ - *:.'; —': -.!:' i -~ 1.. i':,::~....i.~ i.......'" — n-" —,....i; - --.....-" "-.-!................... i........:'.:I.' ~ —~~~~~~~~~~~~~~~~~~~~~~~~~~i- --— ~' —-'- -.-......:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~..... ~i-....................-:................. ~ I.......',..:-.... ~! —i-......:':,:0 i...;! ~... _....j I\-.~~~~ ~~-I —- -— r-t — ~~~~-',. ~: i'."'.i,.: -......_.I_.:.,...I.'.........'.!...I'.. ~"';::...,"-"'i-':*''., ii..... ~" ~'.t —..........:. —-.......................................;....i i- ~.. iI~..i.. i f I i I~~~~~~~~~~~~-''.... ~~ ~~~~~~~~~~~ -*-.~.-.-~.~...'.^..... ~..... ---: ~- ~ ~i- 1- - ~~~~~~~~~~:-"-~ —"..i............. — ~i-~ —- --.i_-L-T` ~~~~~i-: —.:-.i..~]........_-:(...j....... — ----- -" ~ ~..... t —i::':.......... —::....~'* F;'.... ~,iln -'~,.~Fmcopl (btm;?'c i_'-% Po'..... Potassium (Run~~~~~~~~~~~~~~~~~~'-c.:

- -,...I. a. I...',,-.-' -,.x.. -''' -~..'-............................................ x -'-'' ~'" -':~~~~~~~~~~-. i - -,-. - - -..- -~..! ~~T -"-,:: a..I... ~.:',-.4........... -,- I-.-....- —.-'..- 7-..' ~..........i-i-._x::L,"'-,..-.~'.- i~'....'-'u. -I~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!.tI., I-i r- r1I iI,-' -...: - -~!:-.. - ----`, -1- -, —,,-; -I~~~~~~~~~~~ ~~ ~~~~~~~~.. -..,.1.-.i.....-,F.: fi~.. i..-... -. -..-. I 1 -,I — - ".I:.-.:!..-..;..-..1....................................................... -, I - q. r I I; 6:...L; I! -'-:.. ~- - - T.-, i..... I -': -.- -_II.77......-. L;- I-i -. - -'.4-l'_~- L-LL r'i.-~2'' --' —t?-Z -1-i'f... I.! I:;~ ~ ~ ~ ~~:-;..-..... - ---......:-,jr':....'....I~.... -~~~~~~~~ ~ ~ ~'.........-i- _-. —'.. -,.;F...... I -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~-:I:, 4.. I,V; ~:.- I I. o: -;..:.....:..+-. - L- i. — L.... 4 --... I. -....-' 4.'.' -.'-.,- - --- _ -4-_ —. — I -'.-'.'..l-._..-.L_..I ~-i.;.. -:-i-:-;- - -...-I —i -- - --- -i —;-r-:~'-+4 4..................-.l-:::;: ~:i' -- —..._i! -~.. -,... —.- -.... -~ ~ ~ ~~~~~~~~~ —~'~ i'',' —-- -, —-!_-'.tz......':-ii'ti —~?~ [-,F ~:~[,-: —— ~,i!~:.:-~- -:~ -:~~'"'" —~-. ~'.,-~L,_~.._...I..I..I............-.': ~ - -.........: L... 1~ - 1.~-,..I- -.-.....i —-_i _~_.-_ I, +..I............'a',-!,.I;1I'-:..I.."-:..I.."-:".....,,,.-.-.. —....,.....-T......-...- - -... -. II..-......"-"'''',,,.-;~ -—: —: i.. -.-...i:: —-_II'i?,T-~ — Y —-i-. —~ - -t - -..-. — I? —-— t... —i- -, —------— t —' +- —?........-:-..." —l-:~ —-r... il I:.-+ 4. I. 11~~~~~~~~~~~~~~~~~~~~~~~~_ i':!,II!'- -;... I!.. f;;,_!,. I, i, r I.; i.. I I= I.....:;~~~ ~.??]-:tT1. -, — I —.:;x.~..;.-;!.;;.::;;_ - ~1 _.'Z:z;. ]_:.:iz:.,i?..T:..!__':. ~: { -':- _! = t- -!q-:-:-i —-~ —i-.!-' i —h —- -!Ji —--— i —N -., —i4-i —. -!- -., —+ —', —!.;' —'::!..'-_L__...___i_,.!__ Z'Z-i "': ~2-IO I- -. T_ -. —: — - I: -.....r-,-,_ ~ —'..-..I.-' - +: -I""!-+''4.... -I 1. - L..'i —!.' —-. -"[.- 1'"';,, -. I.:. ~~ ~~~~~~-. E --- 4-i;4.~. —t-p- I,:-4-i —!-Fhh-.-"U1 —:-'~,k -..... -.-.-..'... --:.....t —T -.i-1... -;- -h.I.l-' —f"............... "~............1-,,-; — -T; —.e —...... m..~ ~~ ~~~~ ~~-I,;.. -... - -.-.. i'1 —'.'-' " ~- -':-....... —'..... Z. -....-..T;..1.... -~~-.. i- -, —. —-__;._ - IL -- — T~-'"' -ri'-;'-, -' -— _-.- ] —. I r"'I-I-.-!!..... -.~ -—. —.-.-? —.-:.. -I.1. -... I'-t' -~A7;::: ~.47~Z;'p~-xi —t''!.-f...: -':' - -- -~...-i-. —';..-.,.I. i ——. —— I...-. -I -.. ~~~~~~~~~~~~~~~~~~' -.._ —I-.-F ~ -~.., -1... - -...... -~-.. _:-....!_~......i i i. -' —.- - - ~...:...,_ ~_:....... -- ]._-. u.-'. -,.. _. 4-i...-i —" —!-+4...__._,.._.'_ _U.... -:'.:: —— t —---—'-!,-' —L,-+_- r'-i-.-:_... — L. -,.-I -i-t- — e-.i ~ ~ -.-..~~~~~~~~ ~~~~~~ ~ ~ ~~~~ ~~~ ~ ~~~~~~~~~~~~~~~~ "-,":- - -I-'-.;;.I " II-T....... -,H:, t,',,.-,.. —._''-4_'-, —H —-, ——.: - — ~~ t --'; I.: I:. ~;I, m-i: — ~~I. ~.-. I.. - - -...~-L -~- ~J —. —L4-..;..........................-. L... —, —~-.,~~;-.,-.......... - II. 1 —- i ]~.... _.....!....-.;J -..-,-~ -i ~-'T-''' —-. ~.;...-F'l"'~:-,. -. r-..... - l —— I- Im.I.I..I- —'.... I. —-':-' 4...-.-.-'-.u..,.L_.__:......i.......-L. ~I —:!-' -~"' — L-,- -- -t..!.u._-....:,,;I~~~~~~~~~~~~~~~~~~~; 4-. rI..I — —. —.,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~;..,z-.....-.1......'~-;,-I.........-....... ---'...... ~,....~..~..1...... -t' -~u, - -?: 4,.....;~,-+ —-{-Ikf- ~-. —~! ~~~~~, —.-'~ —I -. -I- -; H.,-! —i- -,-H-+,-. —

69 Figure 31 is the bottom tube thermocouple trace showing a burnout determination at a constant pressure of l14.6 psia using tube 5. The burnout flux was 559,000 Btu/(hr)(sq~ ft). Traces for-the- other tube thermocouples were.essentially identical. The emf rise at burnout represents a temperature increase of over 3000F. The sharp drop -to bulk temperature following burnout occurred whe-n the boiling tube-power supply was turned off. Subsequent temperature rise resulted when the power supply was turned-back on and raised to a flux of 434t,0O0 Btu,/(hr)(scl ft). This was done to check for possible tube or thermocouple failures. At no time during this run were bulk fluctuations observed. A trace of the bottom-tube thermocouple showiing a burnout determination.at a constant heat flux of 599,000 Btu/(hr-)(sq. ft) is shown in Figr 52. At burnout the system pressure was 1.0 psia and was decreasing at the rate-of about.02 psila/min. The temperature rise after burnout was over LI-00'F before.the power supply could be shut off. The-temperature, then dropped to the bulk.saturation temperature. Similar-behavior at-burnout was observed on the other tube thermocouples.

_: —:.j., ~ ~ ~ ~.::.1.L - -. ~~,.!-. I ~ I: ~.'.i-.! - - -:.- 2.. _:.! —.-.:,..~ -!" i'., ~~~~~~~~~~~~~~~~~~~~~:......;:..,~~ ~ ~~~~~~~ ~ ~~~~~~~~~~-.........:.!,.,-..~~~~~~~~~~~ ~ —k; I-..LF.I'::~ ~~~~~~~~~~~~~~~~~~~:- i~.i,,j ii:,. ".l, —.i:.~~~~~~~~~~~~~~~~~~~~~''-"" 1. II-... — ~~~~~~~~~~~~~~~~~~~~~~~~~~....;- ]i[... -..:-......... -~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~ _:.1:i.I. m:7..I.....7.. I — ~%.......... II'~~~~~~~~~~~~~~~~~~~:__.I:. ~...1-.:. I 1~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~~~~~~~~,~_,-..i.. -.~-...:: _,~_.,.14.,.+_, —-.Li.I.!. -., -........... _r.p..;.. -— r-~~ —-ii — —'t-"......!-t....... -~~~~~~~~~~~~~~~ —i~~~~~~~~~~~~~~~~~~~~ -I -I ~ —'i.......,~~~~~~~ -- f:-i-!-::-:-:-:~.. - i -~ - -- I:'. f.....1:.-_::.. ~~~~~~~~~~~J ~ ~ ~ ~ ~ ~ ~:-.,~:-.-:~'::iII~: - ~~~~~7~~~ -L-:. —'-, -,....':':~''~i:,!::...,.I;!!iI~~4:: —-[~-'::-' —',LI —-~~~~~~~~~~.-~~,.4 ~......,~~~~~~~~~~~~~~ -.v 1..'']: _I.:~I —1...-, i,:....I... —--.-. F-..-. -- ~~~~~:,:j.. -L.- 1, -.I i,I;.i [." I. — i... -.-.J...:;-"T ":. —.....,, ~ ~ ~ ~ ~-~.... ---— i ~~~~~~~~~ —,.- ~ ~ ~ ~~-:' —:'; —.,...:- ";'::" _ ~_'~.. -;..:~ ~... -1-! I:~~~~~~~~~~~~~~~~~~~~~~"_I:...1.'_:,. I ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~,I,.~.I,,...:'":,-::-::i:- ~-F-~:...~~~~~~~..~~-;'';.. —...- J.I..p...I?- W:, -i-::-: I:.- I -.....'.. -... -: —; -..-~.-..... - I ~~~~~~~~~~~~~~~~~~~. —-- -,:-. —,'..::....... -i —.T.-I.:-'I.. I~~~~I;, —-II -~i::...~-......-1.1.~I- -I- -~-.. - - --:- -;. —;~,::-!:::I I:. I:!;.;. 1..!-:-.-'7~..,..I.I..... -~.:.;; -.1,w-,..., -__.I-~- -'"7'~-:-':..::. " ~-i-.I-I:,~Z-... —.- -~.-I —T.:... ~ - ~~~~~~~~~~~~~~~~:.7-i. I...I I ~~~~~1....:.::,,-'~'!..I...!.:. -~~~~~~~~~~~~ - ~:I ~-!i ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~'..I...1 ~ ~ ~~~~~~~~~~~~~~.....;. ~ ~-....I-ii.: -r.~~'~ I ~-~..i-:4,......-... - -----,-..- I -~~~~~~~7.. —— ~~~~~,. --—.I ~~~~.....,-:I -~ ~ ~ ~ ~ ~ ~~:'- 1.-.... I I.......

I F l: X X~~~~~~~~~~~~- - X:U ILWt 11;urnou t Excursion Cq[A }-ri4,i 388X,000 Btu I(hr) (sq F-1 igXe 52. Boiling Thbe Thermocouple(bottom) Burnacout Presse a Bro Deera Cons n ==t $X0<~~~~~~~~~~~~~~~i-1-x Standardization of - ig ~~~~~~~~~~~~~~~~~~~~~T Reor.J Decr>ea1sing Prssr Adjustment E lx

II)ISCUSSIO0N OF RE~SIM2S "te potass-iumr resolts- ~e sentled in~ the- previou)s Ch~apter have provided dai~a, whi_-ch, should'be very va-Luab"'e to the understanding and prediction of-t-aimeta-l boilingheat tLansf er,'1this -chapter 111lI discssthese results and attempt to eXPlain some of the. phenomsna e ncounte'red. Also,9 comparisons W ill'be; mr-i ad f or botlh the nucleate boiliog data and burnout data with e x s tL'ig for. —1lations. N~u _-Leate ELl~gPtasv t The sri tanceto teperature gradien+_:ft, i the potassiur-ri bulk Ii uid wa not u'Irzpe: ted. N-sc,:m- an2id Bonill a (p35) in t — eir stiudy of, NaK pool'boiling,hm zozatal surf ace found a simila-. behavior- vhen they me-asuared tempera-'t'i-''es as a fu n ction of Pool dept.; up Lo haeat flxs of 100,000 B~tu,/(hr-)(sq. ft). Th~.. 0. that l"th h,:aat:flu~es r angin,,b fr~om 40, 000 to 100,'.000 Btu/(hr ) (s-q ft) th>~ h E-'1yeofb 1quid -,metal ii tepool dnres i pth to between'5-120(YF-i base con thev f-ree surxface tm rar. pr~ofi lat cha gad -with eech- j~~ry i;v~.N correlatlab_"L be&h-ia r Could be, detected from I a ~~arrt ~ cib cow d in Lth presk-ent data fo i, uKI-8 sho' n in Fl g, L~o <a; a~ ~empeoat ~ra dff orer. cc.-s wrr neer1 gater ha 22 0F. At Zero powernpu to tetub'athme i:1qjuid po,~ol wqas niot at a ost~at -te-mperature, rn,V sed r4 i 4e oo -oQLchsfrmtepo boto he fell

73 nearly 220F from a depth of 2 inches to the bottom of the liquid poo-l.' This superheat, around 2 inches depth, was-attributed to-h~eat input from the guard heaters, whereas the filling inlet extending from the bottom of the test. vessel was enough to subcool the potassium at the bottom of the vessel. As the flux was increased it was noticed that the' -gradients similarly increased to a pool superheat of 150F at 228,000 Btu'/(hr)(,sq ft). Subcooling on the pool bottom continually decreased for each increasei flux. Above 75,0 Btu/(hr)(sq ft) the level of subcooling at the bottom was )t0F or less with the pool becoming superheated above one- half an inch from the bottom. The decrease-in superheat at the highest flux was probably due to increased circulation of the liquid promoted by vapor generation. The nucleate boiling potassium data in this study displayed considerable scatter. By examination of the complete data of tubes 2 and 3 (Figures 23 and 24-) it is seen that the AT values, when. operating-at heat fluxes less than 10,000 Btu/(hr)(sq ft), varied as much as 8-.F and were in the range of l.50F to nearly 90F. At fluxes in the order of 500,000 Btu/(hr)(sq ft) the AT values varied as much as 4i00F and were in the range from less than 10'F to, in one case, over 500F. Nevertheless, the general location of the data agrees with data of Bonilla (6) in the range;700 -1500mm Hg. The uncertainty in calculating the boiling AT values probably accounted for much of the scatter in the processed da-ta.. Appendix B treats this subject in detail, therefore it will be only briefly discussed:here.' The boiling surface temperatures were dete-rmined by averaging thermocouple va ri ations in the tube wall and'then extrapolating. to the surface, The cal-culation to

most cases were never larger than 30'F- -even at the highest fluxes- -it i-s,. seen that even th-e slightest error-in any of the- ab~ove vabltes-would significantly distort the- boiling AT values. For example, at 300,000 Btu/(hr)(sq ft) a 10 per cent error in any of the above values, singularly,.ould account for an error of over 50F in the AT. Although there were undoubtedly errors associated with both the heat, flux determinations and the method for measuring the depth of thermocouple junctions., it was felt that the uncertainty in the value-for the thermal.conductivity, k, of the tube- wall was the major source of error. The considerable mass of Nicrobraze, of different-thermal conductivity than Haynes.-25., surrounding each thermocouple made it very difficult to estimate a good value of thermal conductivity. Using the value of Haynes-25 alone gave AT values (in nearly all runs at high fluxes) that de-creased markedly with each increase in flux. This suggests that the mass of the braze was indeed significant and the- effective thermal conductivity should be an intermediate value betwveen, the braze and HaYnes-825. Calculations using the individual thermal con — ductivities gave AT values at 500,000 Btu/(hr)(sq ft) -which differed by 250F. Using the two-dimensional re-laxation-method for heat conduction, and using a heat flux of 500,,000 Btu/(hr-)(sqj ft), the boiling AT values werecalculated and found to be-on the order of 100F higher than those using the thermal conductivity of Haynes-25 alone. Even these were frequently still1 as much as 50F less than those required to give non-de~creasing boiling AT, values. For this reason it was felt that the best value to use for the effective wall thermal conductivity should be that value which gave a curv(.D

75 to the scatter of the boiling data. Inconsistencie i the temperatur profiles for run K-8 (Figure 21) gave reasons to believe that during other boiling runs slightly different gradients could possibly have existed. These, of course, would have been directly related to the boiling AT. Another factor which could have affected boiling AT values was the fact that burnout measurements were made between many of the boiling runs. High flux and highnAT values inherent with these determinations may have s ignifi-~ cantly changed surface proper-ties which in turn could have affected the/-AT. For example, it is known that such factors as surface aging and surface roughness alter the average surface. temperature- and hence -the boiling AT (36). The one boiling run (run.K-b0) in which measurements were made on both the increasing and decreasing flux cycle indicated the possibility of significant hysteresis effects with boiling potassium. Although one run -is hardly sufficient to substantiate this effect,, the apparent break of the boiling curve on the downward cycle'is easily distinguished in Figure 20, Behavior of the type suggested by this run is certainly not uncommon. Several investigators have reported displacements or breaks in the boiling curve on both the increasing and decreasing cycle. The general phenomena has been labeled as hysteresis effects of boiling. Several investigators (f,- 12, )i49) have found a temperature overshoot on the increasing cycle just before the boiling process transforms from totally convective to nucleate boiling. This is shown by an extension of the convection boiling curve beyond the point'where it normally bends upward due to contribution of nucleate boiling., More will be said about this behavior in the last section of this chapter. Some invesigatos (2, 7 ) have fond for waehto tedce in yl h

76 possessed a greater slope, by a factor of over two, than that of the in-....... creasing cycle. The present data on the down-cycle for all thermocouples first appears to be displaced to the left and then changes in slope and gradually tends back to the right. It seems probable that the low AT of the first heat flux setting on the decreasing cycle resulted from the retention of most of the nucleating sites of the previous setting. It is possible that with further stepped decreas-es in flux, a disproportional amount of 8sites became inactiveE thus the AT dontinually increased. The boiling data for tubes 2 and 3 near atmospheric pressure were averaged and compared to the nucleate-boiling correlations presented in Chapter I. This eomparison is summarized in Figure 33. The correlations were evaluated using the properties for saturated potassium at 1400~Fo The prediction of Levy appears to give somewhat high values for AT even in light of the above discussion on the errors associated with the AT. The other predictions give good agreement not only in the location of the curve, but also in the prediction of the slope of the curve within the uncertainty limits of the experiment. The fact that the correlations were developed f rom the boiling behavior with non-metals indicates the significance of the agreement. Potassium Burnout Results The burnout data obtained from either decreases of pressure at constant heat flux or increases in heat flux at constant pressure proved exceptionally reproducible. In no instances did individual data points vary more than 15 per cent from the average curve of Figure 25. Comparison of the determinations made by the two methods gave very favorable agreement and indicated the reliability of measurements by either method. An empirical fit of the pool boiling burnoutt results is represented by the following equation4

77 106 - I I I I I Illj I I I I I 1'1 I I I i IIl Chang and Snyder (9) Forster and Zuber (16) Levy (31) Forster and Grief( 15) 105 103 I lT l l l il l l llllube3 10 Tube 2 10/ 10 10 100 1000 AT,~F Figure 33. Comparison of Saturated Pool Boiling Potassium Results Near Atmospheric Pressure with Correlations.

78 q/A = 4LxlO5 p0.167 where q/A is expressed in Btu/(hr)(sq ft) and the saturation pressure, p, in psia. Figure 34 compares burnout data obtained in this study to several burnout predictions. The predictions are based on theoretical considerations and have in most cases been substantiated with burnout data. Discussions of all predictions except Addomds are given in Chapter I.'The prediction of Addoms was obtained from reference (36) from a correlation of (q/A)bo/Xpv(gk/pUp)L /3 versus (p~ - p )/pv for water and organic liquids. None of the relationships successfully predict the experimental data although Noyes' successfully estimates the magnitude of the burnout level. Recall the fact that Noyes developed his equation by alterations in previous predictions in order to make them compare with his sodium burnout data. It would therefore be expected that his equation would best predict the correct magnitude of the potassium burnout curve. The theory of Zuber and Tribus (53) for subcooled boiling predicts burnout values (with only a few degrees subcoolinrg) of the same magnitude and slope of the present data. Since this suggests that significant subcooling may have existed, the burnout results and experimental procedures were thoroughly reviewed. Examination of pool temperatures recorded in run K-8 (Figure 21) for fluxes well below burnout indicated that the pool was actually superheated except on the pool bottom where subcooling existed. Since this region of subcooling extended less than one-half an inch into the pool (or two inches below the tube), and superheating existed beyond that point, it -as concluded that this would not have had a significant effect on burnout measurements. Also, recall that a majority of burnout determinations were made by gradually lowerring the pool saturation temperature (pressure).

79 7 107, I'I,,'I I I l 1 I I I 1,11, 1 1 106 " LT Investigation coi F Noyes (42) Rohsenow and 105 Griffith(44) Kutateladze (27) Zuber and Tribus (53)!04 0.1 1.0 10.0 100 Pressure, psia Figure 34. Comparison of Saturated Pool Boiling Potassium Burnout Data with Burnout Correlations.

80 Obviously, this procedure would have completely eliminated any tendency for subcooling that might have temporarily existed. The question may also be raised whether the boiling tube end losses had a significant effect on burnout determinations, particularly since tube 1 underwent destructive failure by completely melting at its midpoint. Although an analytical estimate (Appendix C) indicated that these end effects were minor and did not contribute significantly to the boiling results, it was felt that a definitive check should be made. In order to do this, the last boiling tube assembly was fabricated with the three thermocouples located at different locations along the tube length and, in addition, had a thermocouple fitted into a hole drilled into the Swagelok fitting. This latter thermocouple was intended to indicate the significance of heat losses out the end of the graphite heater contacting the molybdenum. Actually, before any runs could be performed using this tube, the main guard heater surrounding the test vessel shorted out. Thus the temperatures indicated by this thermocouple were lower than would normally be expected and indicated adverse values for the heat losses. Measurements with this latter thermocouple indicated that even under- this adverse condition the heat losses out the end of the tube ranged from a high of 7.4 per cent at a heat flux of 22,600 Btu/(hr)(sq. ft) to a low of 0.13 per cent at a heat flux of 333,000 Btu/(hr)(sq ft). The results gathered by the three tube thermocouples were presented in Chapter IV and are shown in Figure 20. As previously mentioned, it was found that at any flux level, axial temperature gradients could not be detected with the wall thermocouples. Considering these findings, the conclusion therefore was that end effects were not significant to the burnout behavior in this study.

Temperature Fluctuations Temperature fluctuations seem to betypical of all boiling systems although with liquid metals the magnitude of fluctuation appears to be greater and more easily identified. Boiling studies of non-liquid metals, which have used conventional temperature recording equipment, have found that temperature fluctuations were usually no greater than about 50F and were difficult to characterize. The general temperature behavior observed in the present study -was in qualitative agreement with previous liquid metal boiling studies. The rather large temperature fluctuations encountered in the tube wall were also observed by Lyon (341) in boiling sodium, NaK and to lesser degree with mercury containing magnesium and by Madsen and Bonilla (35) in boiling NaK. For sodium boiling at 23,600 Btu/(hr)(sq ft) Lyon frequently observed sudden drops (2 sec. duration) in the wall temperatures followed by recovery. The total cycle covered 15 seconds. Similarly Madsen and Bonilla observed a fluctuating behavior which was also represented by sudden dips in the boiling surface followed by slow recovery. Their data sometimes showed a rapid temperature rise preceding the temperature drop, the rise and fall both taking place in less than 0.5 seconds. Note similar temperature behaviors of the present data at low flux levels in Figures 28, 29, and 30. The pronounced variations of Figure 28 at low flux levels occurred in nearly all runs early in the study, although throughout the experiment fluctuations of this type were generally experienced at the onset of operation with each new tube. It is interesting to note that fluctuations very similar to e-,e,;e -v-re~ measured by Moore and Mesler (38) in a study of pool boiling water. By measuring thermocouple voltage signals with an oscilloscope, they were also able to observe characteristic behaviors in regular time intervals. However, instead of cycle periods measured by seconds they found the total

82 time required for a cycle to be in the order of 20 milleseconds. They - further found that at higher fluxes the behavior was identical but the temperature dips occurred more frequently. Careful scrutiny of the fluctuating behavior at low heat flux levels in Figure 28 certainly gives an interesting contrast to the fluctuations at higher fluxes. Recalling the behavior recorded by Moore and Mesler at high fluxes, it is seen by inspection of the present data that although it was not possible to discern any characteristic behavior at the higher flux levels, other than the existence of an occasional upward spike, their significance may become more apparent by examination of the variation encountered at the lower heat fluxes in Figure 28. Consider a system boiling in the convective region such that the bulk liquid is superheated and actual vaporization occurs only at the liquid surface. As the heat flux is increased, it is obvious that a point will be reached when the temperature difference between the heating surface and the liquid bulk is great enough so that the heat transfer surface instantaneously breaks into nucleate boiling. Corty and Foust (12) found that it was possible to retain convection boiling heat transfer to a much higher AT than would be normally required for vigorous nucleate boiling. This is a type of boiling hysteresis discussed earlier in this chapter. Using water Corty and Foust found that superheats as great as 40-50Fabove the saturation temperature were possible even though stable nucleate boiling normally began when the AT reached only 25~F. When the flux was further increased to the point that it spontaneously broke into nucleate boiling, the ATD then decreased to the normal expected value. Similar behavior was observed by Bankoff, et al (4) in his study of boiling methanol. Examination of the temperature behavior shown in Figure 28, at heat fluxes below 250,000 Btu/(hr)(sci ft), indicates that the same behavior may

83 have existed. However, for a given heat flux instead of the nucleate boiling process remaining stable, it was periodically lost with the system reverting back to convection boiling. The upward spikes then may represent a superheatirng of the system necessitated by a dying out of active nucleation sites. When the superheat again became great enough, the system broke into vigorous boiling with a resultant drop in temperature. Many times this drop exceeded the initial temperature rise and approached the liquid saturation temperature. This behavior agrees with previous investigations of both liquid. metals and non-liquid metals discussed earlier in this section. Immediately after this temperature drop the temperature- fluctuations indicated stable boiling, After a. time —dependent on heat flux level —stable boiling was lost and the process repeated. A.s the flux level was increased, these upward temperature. spikes occurred more frequent and with shorter- duration. Finally, the upward spikes were of such short duration that the subsequent drop in temperature was almost instantaneous. At the higher fluxes if the temperature of the surface dropped below the normal nucleate boiling value, the rapid recovery was nearly always faster than the response time of the recorder- thus dips were generally not detected. This would explain why downward spikes were markedly less frequent than those occurring upward.

C:UIAPTER Vi CONCLUSIONS The following conclusions may be drawn as a result of this investigation: 1. The agreement of the water data with a previous investigation (34) demonstrated the reliability of the results obtained with the experimental apparatus. 2. The nucleate boiling curve for potassium in a saturated pool may be predicted within the uncertainties of this study by the equations of Chang-Snyder (9), Forster-Grief (15), and ForsterZuber (16). In addition, the data compared favorably with the potassium results of Bonilla (6). Mean wall superheats were in the order of 200F for heat fluxes over 100,000 Btu/(hr)(sq. ft). 53 Measurementa made on both increasing and decreasing heat flux cycles indicated the possibility of significant hysteresis effects with boiling potassium. 4. Mixing of the potassium pool in this study was insufficient to maintain a constant pool temperature. The temperature gradients which existed throughout the liquid potassium pool were associated with liquid superheat. In general,. increases in heat flux gave similar increases in pool superheat. Saturated pool boiling potassium burnout data can be represented by a straight line when plotting the burnout heat flux versus the saturation pressure. An empirical equation representing the data is: 84

(q/A)b = 4x 10 p0.167 Furthermore, burnout measurements obtained at constant flux by decreasing the pressure or at constant pressure by- increasing the heat flux give consistent and reliable results. 6. Existing burnout correlations based on liquid thermal transport properties as well as hydrodynamic properties of a two-phase system do not successfully predict the experimental potassium burnout data obtained in this study. 7. Pronounced temperature fluctuations between 10 and 150~F exist in the heat transfer surface during the boiling of potassium. Below 250,000 Btu/(hr)(sq ft) temperature fluctuations in the bulk liquid are common and are usually accompanied with noticeable pressure fluctuations.

APPEiDIX A EXPERIMENTAL DATA

TABLE I NUCLEATE BOILING WATER DATA HEAT FLUX TEMPERATURES, ~F' AMPS VOLTS BTU (Hr))(Sq Ft) Bulk Tube AT Run: W-.1 Pressure: 1 Atmosphere 19 0.3 1900 211 218 6.5 37 0.6 7200 - 228 15.1 50 0.8 13100 - 236 21.5 68 1.1 24200 - 241 23.5 88 1.5 40700 - 250 28.1 104 1.7 57000 - 257 30 7 124 2.1 80600 - 262 29. 3 138 2.3 100000 - 270 32.1 150 2.5 118000 - 275 32.3 Run: W-2 Pressure: 1 Atmosphere 20 0.3 2100 211 218 6.4 89 1.5 41600 - 256 33 8 121 2.0 76800 - 274 42.3 154 2.5 124000 - 291 46.6 183 3.1 176000 - 310 51.7 Run: W-3 Pressure,: 1 Atmosphere 25 0.4 3280 211 224 12.1 46 0.8 11100 - 232 18,0 69 1.2 25000 212 244 26.3 91 1.5 43500 212 254 31.3 126 2.1 83300 211 270 36.6 155 2.7 126000 211 286 41, 1 188 3.1 186000 212 305 44~ Run: W-4 Pressure: 1 Atmosphere 23 0.5 3440 211! 219 7.1 52 1.1 18700 212 238 22 0 72 1.5 34400 211 245 24 7 93 1.9 55700 - 253 27.0 123 2.4 92000 - 267 31o3 145 2.8 126000 - 279 34o1 166 2.9 154000 - 292 39o6 201 3.4 216000 212 313 43 9

88 TABLE I (continued) NUCLEATE BOILING'WATER DATA HEAT FLUX -TEMPERATURES, "F AMPS'VOLTS BTU (Hr)(Sq Ft) Bulk Tube AT Run - W-5 Pressure: 169 psia 31 0.4 4340 368 373 3 9 64 0.9 18500 368 384 11 4 103 1.5 48000 368 400 20.0 139 2.0 87400 370 424- 34.2 176 2.5 140000 368 442 39.0 217 3.1 212000 369 469 48.0 237 3.4 253000 367 478 46 8 269 4.0 327000 367 497 47.53 288 4.1 374000 367 506 44,5 313 4.5 443000 368 528 49~ 3 Run: W-6 Pressure: 174 psia 187 2.7 158000 369 440 30o6 308 4.4 429000 376 506 28 8 308 4.4 429000 372 509 31o 8 62 0.9 17400 370- 393 18,o 7

TABLE II NUCLEATE BOILING POTASSIUM DATA Ruvn K-i Tube 2 - Pool Depth 4ff inches April 2, 1963 Pressure Amps Volts Heat Flux TE14PERATUIRES Btu Bulk Tube Top Thbe Side Tube Bottom psia (hr)(sq ft) EMB OF EMF OF AITOF EMl OF AT,0F EMF OF AT,0F 14.2 62 0.8 15 600 30.8 1364 31.1 1377 11.5 31.15 1379 14.0 31.15 1379 14.0 87 1.2 32 900 30.8 1364 31.4 1389 21.8 31.35 1387 20.8 31.2 1381 15.0 115 1.6 58 000 30.82 135 31.55 1396 25.4 31.4 1389 20.1 31.3 1385 16.5 150 2.0 94 500 30.9 1368 31.7 1402 24.8 31.55 1396 21.7 31.5 1394 20.3 186 2.5 146 400 30.9 1368 31.98 1415 32.8 31.75 1405 27.3 31.7 1402 25.1 219 2.9 200 000 30.85 1366 32.25 1426 40.6 31.9 1411 31.7 31.9 1411 32.9 263 3.5 290 000 31.0 1372 32.5 1437 36.9 32.0 1415 23.7 31.95 1413 23.5 303 4.0 382 000 31.05 1374 32.85 1453 41.0 32.2 1424 24.6 32.2 1424 26.9 359 4.8 543 000 31.05 1374 33.3 1472 45.3 32.6 1442 31.9 32.4 1433 26.1 0 0 0 31.0 1372 30.95 1370 31.0 1372 - 31.0 1372

TABLE III NUCLEATE BOILING POTASSIUM DATA Run K-2 Tube 2 - Pool Depth 41 inches April 2, 13 Pressure Amps Volts Heat Flux TENPERATURES Btu Bulk Thbe Top Tube Side Tube I (hr)(sq. ft) EFT 0 F EMF OF AT,0F EMF OF AT,0F EMF 0F AT,FF 9.8 62 0.75 14 600 29.48 1301 29.50 1308 - 29.60 1312 4.0 29.4 1303 113 1.4 49 800 29.48 1307 29.8 1321 9.2 29.80 1321 10.7 29.65 1314 4.0 153 2.0 96 300 29.48 1307 30.07 1332 15.7 29.95 1327 13.6 29.8 1321 8.2 197 2.6 162 000 29.48 1307 30.35 1344 21.3 30.15 1336 18.3 29.9 1325 8.2 258 3.4 276 000 29.48 1307 30.75 1361 27.2 30.3 1342 16.6 30.1 1334 10.3'8 288 3.8 345 000 29.48 1307 31.05 1374 33.6 30.4 1346 16.1 30.2 1338 334 4.4 463 000 29.48 1307 31.45 1392 40.1 30.65 1357 19.2 30.35 1344 9.0

TABLE IV NUCLEATE BOILING POTASSIUM DATA Run K-3 April 2, 1963 Tube 2 Pool Depth 42- inches Pressure Amps Volts Heat Flux TEMPERATURES Btu Bulk Tube Top Tube Side Tube Bottom zpsia |(hr) ( sq ft) EMF ~F EMF ~F AT, OF EMF OF AT, F EMF F AT, ~F 0.7 0 0 0 20.85 941 20.85 941 20.85 941 20.90 943 64 o.8 16 100 20.80 939 21.0 947 6.4 20.95 945 4.9 20.95 945 5.0 96 1.2 36 200 20.80 939 21.2 955 12.5 21.1 951 9.6 21.05 949 7.8 142 1.8 80 300 20.80 939 21.6 972 25.2 21.35 962 17.7 21.18 955 11.1 174 2.2 121 000 20.85 941 21.87 984 31.3 21.45 966 17.0 21.25 95 9.7 197 2.4 149 000 20.85 941 22.15 996 40.5 21.50 972 21.1 21.3 960 10.0 0.6 229 2.8 202 000 20.95 945 22.45 1008 43.4 21.8 981 22.6 21.5 970 12.8 0 0 0 20.85 941 20.85 941 20.85 941 20.85 941

TABLE V NUCLEATE BOILING POTASSIUM DATA Run K-4 Tube 2 - Pool Depth 4~- inches April 7, 1953 Pressure Amps Volts Heat Flux TEMPERATURES Bpsiatu Bulk Tube Top Tube Side Tube Bottom (hr)-(sq ft) EMF AT,~F EMF OF TT,F EM F ~F T, 12.5 41 0.5 6 450 30.75 1361 30.9 1368 4.4 30.9 1368 4.6 30.95 1370 6.6 72 1.0 22 700 30.75 1361 31.1 1377 11.8 31.1 1377 12.5 31.05 1374 9.5 93 1.2 35 100 30.75 1361 31.2 1381 14.6 31.2 1381 15.7 31.1 1377 11.9 113 1.5 53 500 30.83 1365 31.3 1385 14.8 31.3 1385 16.4 31.2 1381 12.8 143 1.9 85 60o 30.85 1366 31.5 1394 19.7 31.4 1389 17.3 31.35 1387 15.8 165 2.2 114 000 30.85 1355 31.7 1402 24.9 31.5 1394 20.4 31.5 1394 21.1 196 2.6 161 000 30.95 1370 31.9 1411 25.4 31.65 1400 19.3 31.6 1398 18.3 217 2.9 198 000 31.05 1374 32.0 1415 21.8 31.75 1404 15.8 31.55 1400 14.0 247 3.3 257 000 31.10 1377 32.3 1429 27.1 31.95 1413 18.9 31.8 1407 14.5 260 3.6 294 000 31.19 1380 32.5 1437 28.4 32.1 1420 20.4 32.0 1415 17.2 283 3.8 338 000 31.35 1387 32.65 1444 23.7 32.25 1426 16.5 32.1 1420 12.5 307 4.05 391 000 31.45 1392 32.85 1452 23.1 32.45 1435 17.5 32.3 1429 13.8 14.0 339 4.5 482 000 31.55 1396 33.1 1464 21.3'32.65 1444 15.5 32.5 1437 11.9 0 0 0 31.2 1381 31.2 1381 - 31.2 1381 31.2 1381

TABLE VI NUCLEATE BOILING POTAISSIUM DATA Run K-5 Tube 3 - Pool Depth 6 inches April 20, 1963 Pressure Amps Volts Heat Flux TEMPERATURES pBtu Bulk Tube Top Tube Side Tube Bottom psia (hr)(sq ft) EMF 0F EMF OF iAT, F EMP OF AT,0F ENE F F ATOF 13.7 0 0 0 31.0 1372 31.0 1372 31.0 1372 31.0 1372 103 1.4 45 400 31.0 1372 31.2 1381 5.4 31.2 1381 6.3 31.4 1389 12.6 145 2.0 91 300 31.05 1374 31.25 1383 1.8 31.25 1383 3.5 31.6 1398 15.1 196 2.65 164 000 31.0 1372 31.45 1392 7.1 31.4 1389 7.1 31.9 1411 23.1 227 3.01 215 000 31.0 1372 31.6 1398 9.0 31.5 1394 9.0 32.17 1423 270 3.7 315 000 31.0 1372 31.7 1402 5.2 31.7 1403 12.0 32.55 1439 36.5 309 4.2 409 000 31.02 1373 32.15 1422 16.7 31.85 1409 11.2 32.9 1455 42.3 0 0 0 31.0 1372 31.0 1372 - 31.05 1374 31.0 1372 -

TABLE VII NUCLEATE BOILING POTASSIUM DATA Run K-6 Tube 3 - Pool Depth 6 inches April 20, 1963 Pressure Amps Volts Heat Flux TENPERATURES psia Btu Bulk Tube Top Tube Side Tube Bottom (hr)(sq ft) EMF OF EMF OF AT,0F ENF OF AT,OF EMF OF AT,0F 6.06 0 0 0 27.65 1229 27.65 1229 - 2 7.65 1229 27.65 1229 40 0.45 5 670 27.7 1231 27.70 1231 27.75 1233 1.7 27.75 1233 1.5 103 1.40 45 400 27.75 1233 27.90 1239 2.4 27.90 1239 3.3 28.05 1246 8.6 6.05 156 2.05 101 000 27.6 1226 28.03 1245 11.1 27.85 1237 4.9 28.20 1252 16.2 198 2.6 162 000 27.7 1231 28.15 1250 6.2 28.05 1246 5.2 28.55 1267 20.3 248 3.3 258 000 27.7 1231 28.50 1265 13.7 28.25 1254 7.4 28.95 1284 28.0 290 3.9 356 000 27.7 1231 28.75 1276 16.9 28.45 1263 10.5 29.45 1306 40.5 0 * 0 0 27.65 1229 27.80 1235 - 27.70 1231 -- 27.70 1231

TABLE VIII NUCLEATE BOILING POTASSIUM DATA Run K-7 April 20, 1963 Tube 3 - Pool Depth 6 inches Pressure Amps Volts Heat Flux TEMPERATURES Btu Bulk Tube Top Tube Side Tube Bottom psia (hr)(sq ft) EMF ~F EMF ~F AT, ~F EMF OF TF~F /F T, F 0.9 0 0 0 21.65 975 21.65 975 21.75 978 21.80 981 36 0.4 4 530 21.6 972 21.65 975 2.6 21.75 978 5.7 21.80 981 8.5 103 1.4 45 300 21.6 972 21.75 979 3.4 21.85 983 8.3 22.07 992 15.6 147 1.9 87 800 21.55 970 21.90 985 8.1 21.95 987 11.7 22.35 1004 25.5 169 2.2 117 000 21.4 964 21.90 985 11.8 21.90 985 13.9 22.45 1008 32.6 198 2.6 162 000 21.4 964 21.95 987 10.2 21.92 986 12.2 22.6 1015 35.3 232 3.0 219 000 21.4 964 22.05 991 9.7 22.0 989 11.7 22.85 1025 39.7 0 0 0 21.35 962 21.35 962 21.35 962 21.40 9654

TABLE IX NUCLEATE BOILING POTASSIUM DATA ~,cun K.-8 K-8 ~~~~~~~~~~~~~April 20., 1963 Tube 3 Pool Depth 6 inches Pres- Amps Volts Heat Flux Depth TEMPERATURES sure Btu in Bulk Tube Top Tube Side Tube Bottom psia (hr)(sq ft) EMF "F EMF "F AT,"F EME "F A, OF EME' "F AT,"F 13 0 0 0 6 30.9 1368 30.85 1366 30.8 1364 30.7 1359 5 30.9 1368 31.0 1372 31.0 1372 30.9 1368 4 30.9 1368 31.0 1372 31.0 1372 -30.9 1368 3 30.9 1368 31.0 1372 31.0 1372 30.9 1368 2 31.0 1372 31.0 1372 31.0 1372 30.9 1368 1 30.9 1368 31.0 1372 31.0 1372 30.9 1368 0 30.5 1351 31.0 1372 31.0 1372 30.9 1368 51 0.6 9 630 6 30.8 1364 30.95 1370 5.2 30.9 1368 3.4 30.9 1368 3.1 5 30.85 1366 30.85 1366 30.9 1368 30.9 1368 4 30.95 1370 30.85 1366 30.9 1368 30.9 1368 3 31.0 1372 30.85 1366 30.8 1364 30.8 1364 2 31.05 1374 30.85 1366 30.8 1364 30.8 1364 1 30.85 1366 30.85 1366 30.9 1368 30.9 1368 0 30.5 1351 30.85 1366 30.9 1368 31.0 1372

TABLE IX (continued) NUCLEATE BOILING POTASSIUM DATA Run K-8 April 20, 1963 Pres- Amps Volts Heat Flux Depth TEMPERATURES sure Btu Bulk Tube Top Tube Side Tube Botton psia (hr)(sq ft) In EM F OF EMF "F AT F EMF "F T, F EMF "F T, ~F 137 1.75 75 600 6 30.9 1368 31.35 1387 13.0 31.3 1385 12.4 31.6 1398 22.7 5 30.8 1364 31.35 1387 31.3 1385 31.6 1398 4 31.0 1372 31.35 1387 31.3 1385 31.6 1398 3 31.1 1377 31.30 1385 31.2 1381 31.6 1398 2 31.2 1381 31.30 1385 31.2 1381 31.56 1398 o 1 30.6 1355 31.30 1385 31.2 1381 31.6 1398 0 30.8 1364 31.30 1385 31.2 1381 31.6 1398 237 3.05 228 000 6 30.9 1368 31.70 1402 16.0 31.5 1394 12.2 32.3 1428 37.9 5 31.0 1372 31.70 1402 31.5 1394 32.3 1428 4 31.15 1379 31.70 1402 31.5 1394 32.3 1428 3 31.25 1383 31.70 1402 31.5 1394 32.2 1424 2 31.25 1383 31.70 1402 31.5 1394 32.2 1424 1 31.0 1372 31.70 1402 31.5 1394 32.2 1424 0 30.75 1362 31.70 1402 31.5 1394 32.2 1424

TABLE IX (continued) NUCLEATE BOILING POTASSIUM DATA Run K-8 April 20, 1963 Pres- Amps Volts Heat Flux Depth TEMPERATURES sure Btu Bulk Tube Top Tube Side Tube Bottom psia (hr)(sq ft) E*n F EM F ATF EM F AT~F XEMF F,F 328 4.2 434 000 6 31.1 1377 32.3 1428 16.8 31.87 1410 6.8 33.2 1468 48.9 5 31.1 1377 32.3 1428 31.87 1410 33.2 1468 4 31.25 1383 32.3 1428 31.87 1410 33.2 14658 3 31.40 1389 32.3 1428 31.87 1410 33.2 1458 2 31.35 1387 32.3 1428 31.87 1410 33.2 1458 1 31.25 1383 32.3 1428 31.87 1410 33.2 1458 0 31.05 1374 32.3 1428 31.87 1410 33.2 1458

TABLE X NUCLEATE BOILING POTASSIUM DATA Run K-9 Tube 3 - Pool Depth 6 inches April 20, 1953 Pressure Amps Volts Heat Flux TIEMPERATURES Btu Bulk Tube Top Tube Side Tube Bottom psia (hr)(sq ft) F AF EMF OF ~F, 0F EMF 0F t FTF 37 72 0.9 20 400 36.0 1592 36.0 1591 36.2 1601 7.8 36.25 15603 8.0 113 1.4 49 800 365.53 1615 36.7 1623- 4.1 36.95 15634 16.0 37.1 1641 21.2 155 2.0 97 600 36.63 1619 365.95 1634 7.3 37.1 1641 16.1 37.4 1554 25.5 207 2.6 169 000 365.78 1626 37.30 1650 10.7 37.3 1650 13.8 37.8 1672 29.6 258 3.3 268 000 36.88 1631 37.650 1663 10.9 37.7 166557 19.8 38.5 1703 46.0 45 308 4.0 387 000 37.08 1639 37.85 1674 5.5 37.9 1676 13.6 38.9 1722 45.4

TABLE XI NUCLEATE BOILING POTASSIUM DATA Run K-10 Rune K-10 1July 11, 1953 Tube 4 - Pool Depth 62 inches July Pressure Amps Volts Heat Flux TEMPERATURES Btu Bulk Tube Top Tube Side Tube Bottom (hr)(sq ft) EMF ~F EMF OF AT,~F /TF F AT, F EMF ~F T,F 3.8 0 0 0 26.1 1163 26.1 1163 26.1 1163 26.1 1163 81 1.1 28 000 26.0 1158 26.45 1177 16.3 26.4 1175 14.6 265.35 1173 13.3 160 2.2 111 000 26.3 1171 27.05 1203 21.3 26.9 1197 10.6 265.95 1199 15.3 212 2.8 187 000 26.65 1186 27.7 1231 26.9 27.4 1218 16.1 27.4 1218 20.7 247 3.3 257 000 26.9 1197 28.1 1248 26.1 27.75 1233 14.2 27.7 1231 18.5 8 278 3.8 333 000 27.05 1203 28.5 1265 29.7 28.1 1248 16.7 27.95 1241 17.8 202 2.7 172 000 27.15 1207 28.0 1244 20.4 27.7 1231 9.4 27.65 1229 11.5 155 2.1 103 000 27.1 1205 27.95 1241 26.0 27.55 1224 10.3 27.6 1225 14.8 5.0 115 1.6 57 900 27.05 1203 27.75 1233 24.4 27.5 1222 14.1 27.55 1220 13.5 72 1.0 22 700 265.95 1199 27.6 1226 24.8 27.35 1216 15.1 27.4 1218 17.6

'101 TABLE XII SUMMARY OF POTASSIUM BURNOUT MEASUREMENTS METHOD OF HEAT FLUX PRESSURE TUBE DETER- AMP VOLTS.BTU MINATION (Hr)(Sq Ft) 1 DP 0.164 258 3.5 285,000 DP 0.152 258 3.5 285,000 CP 15.0 395 5.3 660,0002 2 DP 3.72 323 4.75 484,000 DP 1.0 308 4 0 388 000 DP 3.94 348 4.7 515,000 DP 11.35 369 5.0 580,000 3 DP 14.6 359 4.85 550,000 DP 0.715 307 4.2 407, 000 DP 1.42 330 4.5 468, 00 CP 22.2 400 5.3 669,000 CP 13.9 379 5.0 598,000 4 CP 1.24 312 4.2 413,000 CP 1.80 307 4.2 405,000 CP 0.50 288 4.0 363,000 CP O 0.70 278 3.7 324, 000 CP 3.5 342 4.65 500 000 CP 7.4 364 5.0 573 0003 1DP = Decreasing pressure at constant heat flux CP = Increasing heat flux at constant pressure 2Destructive burnout, tube shown in Figure 26 3Destructive burnout, tube shown in Figure 27

APPENDIX B TREATMENT..OF DATA In order to save a great deal of work in.processing data, a computer program was written for the IBM 7090.digital.computer at The University of Michigan Computing Center. For a particular boiling tube power setting, the program calculated the boiling tube heat flux, q/A, and temperature difference, AT, between the boiling surface and liquid saturation (or freesurface) temperature. A separate curve was determined for each of the three boiling tube thermocouples." In addition, the data was correlated using a least-square subroutine to determine the best straight line log-log relationship for each run. However, curvature of the boiling curves at low fluxes (probably due to the influence of convection boiling) prevented a representative correlation. Thus, the least-square correlations were later discarded and are not present herein. The boiling heat flux was determined by dividing the known tube surface area into the total power input to the tube, less any predetermined. losses. The expression.is: q/A c=r where I and. V are the DC cirrent and DC voltage drop, respectively,, across the graphite heating element,.A the boiling tube OD area in sq. ft.g. and c the factor to convert watts to Btu/hr:. The voltage drop across. the heater element was determined from a measurement of the total.voltage drop 102

103 across the boiling tube assembly, less the lead losses. See Appendix E for a discussion.of these losses. Boiling surface temperatures were determined by averaging thermocouple readings in the tube wall and extrapolating to the surface. The boiling AT was calculated from the following: AT = ATc - qA where ATc represents the total temperature difference between the thermocouple and bulk temperatures, q/A the heat flux, Ax the depth of the thermocouple junction in the tube surface, and k the effective thermal conductivity of the boiling tube material. The value for ATc was determined from thermocouple measurements while Ax was determined by sectioning the boiling tube and visually measuring the depth to the center of the thermocouple junction. This was accomplished to an accuracy of + 0.0005-in using a measuring microscope. The existence of a considerable mass of Nicrobraze surrounding each thermocouple and possessing a markedly different thermal conductivity than Haynes-25, made it very difficult to determine the effective value for the tube wall thermal conductivity in the vicinity of the thermocouple junctions. Reference (20) lists the thermal conductivity of Haynes-25 at a value of 120 Btu/ (hr)(sq ft)(OF/in) at 8000F to 165 Btu/(hr)(sq ft)(OF/in) at 1400~F whereas the only available source (14) for Nicrobraze AMS 4777 gives a value k = 377 Btu/(hr)(sq ft)(OF/in),: assumed at ambient temperature. For Ax = 0.016-in, the numerical value of Ax/k at 14000F was bounded in the limits (0.0000428 using kNicrobraze) <AQx/k< (0.000097 using kHaynes25) assuming kNicrobraze was constant with temperature. This represents a ST variation at 500,000 Btu/(hr)(sq ft) of approximately 250F.

-104 In using k for Haynes-25 it was found in nearly all runs that at high"fluxes, the AT values decreased markedly with each incremental increase ini fluxo Since this was not believed to be possible, it"indicated that the mass of the braze was indeed significant and -:::served to distort the isothermns in the tube wall near each brazed thermocouple grooveo. The value for the effective k near a thermocouple groove should have been, as a minimum, that value in which AT remained constant with increasing flux. Calculations showed that this minimum k was. between the Haynes-25 and the Nicrobraze values and gave on the average AT values about 15'F higher-at a flux- of - 500,000 Btu/(hr)(sq ft) using the k for Haynes-25 alone. Considering a boiling tube heat flux of 500,000 Btu/(hr)(sq ft), the two dimensional relaxation method for heat conduction was used to determine isotherms in the tube wall at the location of a braze filled thermocouple groove. From these calculations the temperature at the center of the brazed groove was about 10'F less than a similar wall location in the Haynes-25. Recall from above that this value was still 50F less than' required to yield non-decreasing AT values with increases in heat flux. The conclusion therefore was that the best value for the -effective k should be that value (for a particular tube thermocouple) which gave a boiling curve with no reversal in slope.

APPENDIX C.ESTIMATION. OF HEAT FLUX AND THE-TEMPERATURE PROFILE IN THEI BOILING TUBE It might be expected that end effects in.the boiling tube were significant inasmuch as the L/D ratio for the graphite heating element was only 5.45. Calculation of these effects was carried out by formulation of the differential equation for heat transfer from a cylinder with internal generation. Several simplifying assumptions.were employed to permit an analytical solution. Also, adverse boundary conditions were specified in order to obtain an estimate.of the maximum heat losses. The losses at the ends were considered to be approximately equal and therefore the longitudinal temperature profile was considered. to be symmetrical about the midpoint of the tube. The analysis applied only to half of the tube, for which x = O to x = f/2, where x = O corresponds to the midpoint and I the total length of the tube extending into the potassium pool. The general one-dimentional problem is given in Carslaw and Jaeger (8). For specific boundary conditions,. Lyon (34) solved the problem and determined errors in. calculating the radial heat flux near the center of a Globar heating element. The present problem assumes no radial temperature gradients in the graphite (one-dimensional) and that all physical properties are constant with temperature. The differential equation is: d2T 4UT - cI2R dxr kd k c-1 g g 105

106 where T = (temperature in the graphite) - (temperature of boiling potassium) x = length dimension, O<x </2 k = thermal conductivity of graphite U = overall heat transfer coefficient: between graphite and boiling potassium I = current R = resistance of graphite V = 1/2 volume of graphite c = conversion factor from watts to Btu/hr This equation includes a term for the radial surface heat transferr, and was formulated using an overall coefficient, U, which'includes all- interfacial and material resistances. The BN, Haynes-25, and interfacial resistances are contributors. The boundary conditions for the graphite rod heater are~ dTx =0 0 (center of tube) C-2 -k T dTx _ H25k x = ~/2 (free end of tube) -C-3 dx - k t g where k = thermal conductivity of Haynes-25 H-25 I = total length of graphite t = thickness of Haynes-25 tube end. The first boundary condition assumes equal heat transfer out each end of the graphite heater while the second condition is a heat balance at the interface between the graphite heater and the end of the Haynes-25 bayonet tube. This latter condition assumes infinite contact conductance between the graphite heater and Haynes-25 disk. It should be pointed out that although symmetry was assumed in the rod, i.e., no heat transfer across the plane x = 0, the end of the rod which

107 contacted molybdenum (x = -1/2) possessed a considerably different condition. However, since I R dissipation in the molybdenum tended to reduce the magnitude of the graphite end losses, it was felt the largest loss occurred at the free end. of the tube. Also, the I R dissipation in the Haynes disk was neglected so that the actual loss would be somewhat less than that predicted here. Applying the two boundary conditions gives the following particular solution: B/A k t — cosh Al B k tA 2 g sinh Al - cosh Al 2 A kH_ g 2 2 c-4 H-25 in the region O<x </2 and where A2 = 4U/k d and B = cI2R/k gV g g This expression represents the temperature along the graphite heater and can be used to determine the heat loss out either end or the radial heat flux at any point along the boiling tube. Using the following values: thermal conductivity of the graphite, k = 20 Btu/(hr)(sq ft)(~F/ft), the thermal conductivity of Haynes-25, kH-5 = 20 Btu/(hr)(sq ft)(OF/ft), the diameter of the graphite heater, d = 0.23-in, and the total length of the graphite heater, ~ = 1.25-in, gives:.I2R T = 545 U cosh(Il0.hT x); 545I2R tVlO.4U sinhlO.4 1.25 cos( l.U1o22 U ~:24 24 for O<x <1.25/24-ft. To obtain the most adverse value for heat loss, the first term on the right hand side should be a maximum. This is accomplished by setting the thickness of the Haynes-25 end to zero or t = 0, then: I2R T = 545I 2R 545 "U cosh(lO'i.-u x) U cosh. VU Fl.25 6

108 and UT 54 5IR 545R R cosh(110.4u x) C-7 os El0 o.4 1 e25 24 - This expression is equivalent to: radial heat transfer' rtotal heat loss) from boiling surface input (end loss)' C-8 The heat flux error at the center of the tube, where the thermocouples are embedded., is given by the following expression~ 545I2R cosh(VlU x Percent Error - cosh(V0: 1 254: - 24 - Jo00 545I R -9 Choosing the total heat transfer coefficients for the graphite-boron nitride contact and. boron nitride-Haynes-25 contact (54), h(contacts) = 2000 Btu/(hr)(sq ft), the boiling heat transfer coefficient, h(boiling) 10,000 Btu/(hr)(sq ft) and finally the thermal conductivity of the boron nitride sleeve (55) k(BN) = 15 Btu/(hr)(sq ft)(OF/ft), the overall heat transfer coefficient is calculated to be U a 750 Btu/(hr)(sq-ft), and the heat flux error at the center of the tube is then: 100 Percent Error = 2 cosh X.4(750) 1. 52 24 gx = 0 C-10 The average heat flux over the middle half of the boiling tube is determined from the following equation: 7/4 dim ((q/A ) dx q/A)ave x ~/4 -0 0-11 The value for the heat flux is given by Equation C-7, thus:

1.209 4(12) 452R - 5452R cosh(4 x) d cosh(\!i 1 0 2' q/A )ave = 1.25/4(12) C-12 for U = 750 q/A)ave = 545I R(1 - 0.0432) = 545I R(0o956) C-13 Since the term 5451I2R represents the theoretical radial heat flux from the graphite rod, the average flux over the middle half of the tube is then 95.6% of this theoretical value. In order to check these heat loss calculations against actual conditions, tube 4 was fabricated with the three thermocouples placed at different locations along the tube length.~'One thermocouple was located directly in the middle of the tube length while the other two were placed 1/4-in from each end. With this arrangement longitudinal temperature gradients were not detected up to fluxes over 300,000 Btu/(hr)(sq ft). This demonstrated that the heat flux was essentially uniform along the middle 60% of the tube and substantiated the calculations above. In addition, the tube 4 assembly had a hole, 1/16-in diameter by approximately 1/4-in deep, drilled into the Swagelok fitting. The hole was fitted with a thermocouple such that the junction was approximately 1/2-in away from the end of the graphite heater contacting the molybdenum rod. The main guard heater surrounding the vessel shorted out during the heat-up cycle using this tube, so that the temperatures recorded by this thermocouple were actually much lower than would be- normally expected and indicated heat losses from the tube, while operating without the use of the main guard heater. Even under this most adverse condition, the heat losses out the end of the tube contacting the molybdenum were as low as 0135% at 333,000

110 Btu/(hr)(sq ft) to a high of 7.4% at 22,600 Btu/(hr)(sq ft). The lower value at the high flux was, at least in part, due to an increase in heat generation in the molybdenum. The temperature profiles in the graphite were calculated using Equation C-6 at heat fluxes based on the boiling tube OD, of lxlO, 5x105, and lxlO5 Btu/(ihr)(sq ft). These are shown in Figure 35. The heat flux at any point along the graphite rod is determined by simply multiplying the temperature by 750.

LL rx t-Thermocouple Location 22= 2.5 k rVessel Wall 6'~ -_2.5 - I I x 10 8Btu /(hr )(sqft) X2 2. I0 EE 0- E 1.5':::-'~ 5 x 105 IBtu/(hr) (sq ft).6 -0.4 -0.2 0 0.2 0.4 0.610 Btuhr) sq ft -0.6 -0.4 -0.2 0 0.2 0.4 0.6 Length Along Tube, in. Figure 35. Temperature Profile of Graphite Heater in Boiling Tube.

APPENDIX D DETERMINATION OF POWER LOSSES IN BOILING TUBE CIRCUITRY Voltage measurements, made across the nickel bus bar and the boiling vessel wall, were used with the current to determine the total power to the boiling tube circuit. Included in the circuit were the nickel bus bar, the molybdenum rod, the graphite element, the end of the Haynes-25 boiling tube, the liquid metal bulk (which was in parallel with the Haynes-25 portion of the boiling tube), and finally the Haynes-25 boiling vessel. One method used for obtaining lead losses involves experimentally measuring the resistance of the circuit with the graphite excluded. After tube 1 failed by melting in half, the liquid potassium was able to come in direct contact with the molybdenum rod. Thus, the power measurement included only lead losses in the circuit, The only losses not in this measurement were the contact losses between the graphite and molybdenum rod and the graphite and Haynes-25 tube end and the losses in the Haynes-25 tube end. The tube end loss was negligible. Contact losses were estimated from actual contact resistance data for graphite(41). Using a very conservative value for the contact pressure (400 psia —the calculated pressure was 1480 psia) the total loss for both contacts was 0.97 per cent of the total power input, From these findings the value determined by this method for the lead losses was considered quite reliable. The actual power loss was measured for saturated potassium boiling at one atmosphere pressure and found to be 10.6 per cent of the total input. 112

113 No measurements were made at other temperatures. However, since the total measured resistance of the boiling circuit remained very nearly constant over the operating temperature range, it was assumed that the lead resistances (thus power losses) and graphite heater resistance would maintain a constant value. In order to obtain a definitive check of the above measurement, individual losses in the circuitry were computed from resistivity data. The total resistance of the tube circuit was represented as a series of resistances. E R I measured total I nickel bus bar + Rmoly rod graphite + RH-25 end + RKH-25 tube R RH+R25 tube +H-25 vessel contacts The fifth term on the right handside of the equation represents the parallel circuit of the bulk potassium and Haynes-25 tube and the last term represents the sum of the electrical contact resistances at the various contact points. Knowing the- electrical resistivity, p,7 for each substance and using the relationship R = ps/A, where ~ is the length and A the area, the individual resistances of the circuit were calculated. Resistances through the nickel bus bar, boiling tube end, and vessel wall were assumed negligible. Using values of electrical resistivities from references (20, 21, 40, 41, 50) the individual resistances were calculated to be Rmoy rod 0.0008 ohms, moly rod R = 0..0004 ohms, R 0.0122 ohms, and contacts graphiteRK+RH-25 tube The sum of these resistances is 0.0134 ohms with the lead losses accounting for 9 per cent of the total. This total calculated resistance

114 compared to an average value of 0.0135 ohms for the total resistance measured experimentally. The lead losses calculated above gave a disagreement of only 1.6 per cent from the aforementioned experimentally measured value. The percentage value ultimately used for lead losses in all data processing was 10 per cent. Comparison to the values above gave uncertainty limits of - 1 per cent.

APPENDIX E ESTIMATION OF ERRORS Heat Flux The energy dissipated in the graphite heater was determined from a measurement of the total power to boiling tube4circuit less power losses in electrical leads. The heat flux was then calculated by dividing the corrected power by the outside area of the boiling tube. A calibrated ammeter and voltmeter were used to obtain total input power to the tube circuit. The uncertainty in reading the ammeter was about - 0.5 amps whereas the voltmeter could be read to about - 0~05 volt. The error in total input power could then be as much as - 3.8 per cent at 50,000 Btu/(hr)(sq ft) and - 1.3 per cent at 500,000 Btu/(hr)(sq ft). Electrical lead losses are determined in Appendix D. The uncertainty in the value for these losses was thought to be - 1 per cent of the total power input. The effects of neglecting end losses in the graphite heater are analyzed in Appendix C. Neglecting longitudinal heat conduction in the tube wall.or boron nitride sleeve, the total heat flux error at the center of the tube was calculated to be + 2 per cent, Over the middle half of the tube the calculated error was + 44.4 per cent. The outside surface area.was determined from the tube diameter and length. They were measured to a precision of 0O0005-in and 0,0156-in respectively. These variations correspond to a possible error in the tube 115

116 area of - 0.7 per cent. Overall errors at the center of the tube were therefore - 7.5 per cent at a heat flux of 50,:000 Btu/(hr)(sq ft) and - 5 per cent at 500,000 Btu/(hr)(sq ft). Temperatures The three thermocouples used in tube 1 were calibrated against a Pt-Pt, 10o Rh thermocouple which wsas calibrated by the National Bureau of Standards, The average deviation from standard tables was + 0.32 per cent (+ 3o.5F) at 1100~F. All other thermocouples used in subsequent boiling tubes were made from the same lot of swaged assemblies. The liquid bulk thermocouple —was calibrated against those of tube 1 at zero heat flux. Principle errors in this study were caused from results involving temperature measurements. The temperature fluctuations in the tube wall were averaged and then extrapolated to the boiling tube surface. Since the temperature fluctuations were not always clearly defined with maximum and minimum deflections, some error in reading the mean value was inherent. This possible error was probably in the order of - 35F. The best value for the calculated. boiling tube wall temperature is discussed in Appendix B. Considering the results, the accuracy in the method is considered to be +- 50F at 500,000 Btu/(hr)(sq ft). Precision laboratory potentiometers (L&N No. 8662) and pen recorders (see Chapter II) were used to record emf for all thermocouples. Since the + laboratory potentiometers and recorders could be read to within - 0o05mv any error introduced by these instruments were completely overshadowed by errors aboveo The error in boiling AT values, therefore, would be - 80F at 500,000 Btu/(hr)(sq ft),o

117 Pressure The Bourdon pressure gauges were calibrated by a dead weight tester and could be accurately read to within - 0.5 psia. The mercury manometer could be read to within 0.1 in Hg (0.049 psia).

APPENDIX F PLOTS OF NUCLEATE BOILING POTASSIUM DATA 1.18

119 106 6 1t1 i I I II I I I I I i ~ I I I I I I IL. Burnout Level 0 105 _, Cr 104 Run K-2 o Top Thermocouple o Side Thermocouple _ z Bottom Thermocouple Tube: 2 Pressure: 9.8 psia 103 I I I J 11 I I I ll I 1oO 10 100 1000 ATO F Figure 36. Boiling Data for Potassium Obtained at 9.8 psia using Tube 2 (Run K-2).

120 106 _6i i I ~ II 1 1 111111 II ItI I I I I ll0 Burnout Level.1050; rA 104- Run K-3 o Top Thermocouple _ ~ Side Thermocouple _ Bottom Thermocouple Tube: 2 Pressure: 0.7 psia 101.0 10 100 1000 AT, OF Figure 37. Boiling Data for Potassium Obtained at 0.7 psia using Tube 2 (Run K-3).

121 106 6I 111111 1 1 1111 Burnout Level A 0O o o 0 o5 0 100 C104_ — Run K-4 o Top Thermocouple o Side Thermocouple A_ ^ Bottom Thermocouple Tube: 2 Pressure: 12.5-14.0 psia 10I I I I I 11 I I I 1 1.0 10 100 1000 AT,~ F Figure 38. Boiling Data for Potassium Obtained in the Rlange 12.5 - 14-.0 psia using Tube 2 (Run K-4).

122 10 Burnout Level 10 10U4- Run K-5 0 Top Thermocouple o Side Thermocouple A Bottom Thermocouple Tube: 3 Pressure: 13.7 psia 3! I I I I IJII I I I I I II - I I I I I I I 1010 100 1000 AT, F Figure 39. Boiling Data for Potassium Obtained at 13.7 psia using Tube 3 (Run K-5).

123..10 11.0 I I I I I I I I [ l I I [ I ][L Burnout Level 105 104 / Run K-6 o Top Thermocouple,_ / o Side Thermocouple _ ^ Bottom Thermocouple Tube: 3 Pressure: 6 psia 103 L I l l l l 1111 1 l l I l [ ill! l l I l ]! [ 310 1.0 10 100 1000 AT O F Figure 40. Boiling Data for Potassium Obtained at 6 psia using Tube 3 (Run K-6).

124 6 106 I! I I I I I I I I I I'. I I1 f! I Burnout Level 5 0 0 A ~~~10 ~!o0 Run K-7 o Top Thermocouple 0 A / c/ //~0 Side Thermocouple a Bottom Thermocouple Tube: 3 Pressure: 0.9 psia 103 ~ ~I I I I illl I I X i I[ill 1 Il 10 1.0 10 100 1000 AT, OF Figure 41. Boiling Data for Potassium Obtained at 0.9 psia using Tube 3 (Run K-7).

125 -6 Burnout Level 100 A 00 A Cr 10 104 Run K-9 o Top Thermocouple o Side Thermocouple A- Bottom Thermocouple Tube:3 Pressure: 37-45 psia 103 I I I I I I Il l I I I I ll I I III 1.0 10 100 1000 AT OF Figure 42, Boiling Data for Potassium Obtained in the Range 37 - 45 psia using Tube 3 (Run K-9).

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