i K RESRCH INSUITTE Final iopor B5aESIGATION OF tILATING OF AIm Ev f S f i A WMihD TUNEL BY HXA IS OF Asl' A ELECTRICALt DISCHARGE Haldo Lo S&4mth Harold C~ EFrly T t LA BORA.TO. A ES ABkcE~lEfRM PZOV.XIG GROUND ART ORtDNA.MCE CORPS ODA~204.Ol 8~PBD-1307 fa} ~ PROJECT 2Oo TB 30108

ST ST I? o o 9 s o a 0 4 e 0 C,t 0 v0 I ^ ' (I NT":' J 0 -) " TTl, TiT.:D FO?.1 STAB BIX IZNG A de C 'TN A TR$TRSI 4i cM IXjL ^:.i?.-^^ a? ^) p t? o < c t s <3 4^) t $'j 0 5U. 44 ~'( C o 4) 0Ii t 0 ~, G 1 Q 1 < ^ f. a a - o, o~, W,,, o 43 0 16 Ct 6 vo pEAPi4O ) rAHCEA 71 OFJ A &4 I G T l VI TtI TO X PT DIFF USIVITY AND STABIL T RF Pi R IJ4 1OMNTOU ZfiF s YYEOSTROVDE TUWaNEL o C. o 20 U Behvrimr efts, 4 a Ga D ansitie 29 Hjroqren AS'terglo onsidT &rafti raa U 6 $) 34 icperij tgr to Control Orie n atin of ArAc 5 t " 36 &.h 1.r o,. a a thA Htt: Eleetpeodet^rlX2Xe eT n el t^(.^^j s oe Q 3 53 52 I;X o nES pi U D.iSGARGSS o 9, ~ o a ~ o;'; Q 9 e i o? i S 57 \WT~sfy.OfU41IC, ~ JL~a g~~ ~3js ~I fl!4 ban O tCC 04)0 a c;

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LIST OF UJ:.LU&SRATIONS 1o Are in an AiSf re& Witheout a manetic Field s o? 7 2o ind. T i-mi BeteE M etO Pole P ieces o o o o 12 3 "tContour ElectrodeWl Tinnel 2 Photograph 0 o o 13 4 Sh gaph of 7 Wdge n M 4 Flow o o o o o 19 fe-ductiro. of lI& ti Field in OCGmtowu lMectrode~" TMaSe 0 a a o 00 0 0. O OOo 30 6o Firging a netie Fie3l in "Contour Elect1rode T'M:LS O a o o o O o o o o o o 4 o a 40O B10o, &.Hetod E cadeW n Tumuel.PhecV aph,> a 0 o. o 0 o 3A "Hxt Eleetr cde)a o ~ o o o o o o 0 0 o 0o Q 12~ Sce&ce Di a of iyprsoi Tmel it S3 M BNab i o oo o o o o o o 73 132 Emo i r Hr Ratio vs Mah of E o twoosoa toReat o 0 0000 00 0 78

P&.KOIgS ML Harol Cg Early Haldon Lo Smith James L. Amick Jamm E' ErcEadnoll.llicam Go DurvM Vnmund Kayser Donald LH Baumgart.ner Betty S, Slcheider Project Engineer Research Associate Research Associate Wind Tunnel Facility Willow Run Research Center.Assistart Professor of Aeronautical EngineeriLng Professor of Electrical Engineering Machinist and Technician.T chnician Editorial Assistant Consu.nt. Consultant Consulitat Part time Part time Part tiee

I r5 AW.ST RACT Expsriments have been made in a 1s x 1a "blow dowa" iTnd tumnel to test the feasibility of using an electric arc for tdding heat to a.ach 4 airv4raame The arc coltm was tabilisedt tran vere to the flow by ean of a strong agnetic fieldi and up to 5 kilowatts of electrical po r twere dissipatedo However umniform heating was not obtainedD because t had a strong tendency t conentrate in the boundzy7 layer and appeared to fill only about 1/2 to 2/3 of te tmmenl cross sectiono, As a m eos o irvcuwenting these difficultiesp ssveral altemnative eIactrical heating methods are proposed0 It is befieved that these proposed methods hold much more promise thn~ the system which tas testedo

IOTESTIGATION OF HEATING OF RSTRE A WI TUNNEL BY MEANS OF AN ELECRICAL DISCH E I - INTRODUCTION A wind tunnel capable of producing hypersonic air flow at mruch higher static temeratures than can be achieved with present techniques would be possible if a satisfactory ueaus could be found for adding heat to a supersonic flwro An investigation of the use of an electrical discharge for this puvTose has been conducted as a University of Michigan Research Project sponsored by the Exterior Ballistics Labora~ tory of the Any Ordnance Corps Aberdeen ProTing Grounde

This project was admtAistered through the Engineerim7g Research institute of the University, and utilizeedthe facilities of the Department of Electrical Engineering. The first part of this report describes an experi, ental investigation of a particular system of electrical alr heating0o Dring the course of this laboratory program several alternative methods for using an electrical diScharge wrere proposedo Section X of this report outLines a variety of these alternatives~ many of wlich appear to be much superior to the method eplcyed, in this investigation0 This research was an outgrowth of a previous project at the University of Michiganu sponsored by the Office of Ordnance eesearche In the earlier works dealing iLfth elec trical wind phenomena. it was shown that a diffuse discharge capable of generating a high speed air flow could be obtained by the use of a strong transverse magnetic field at pressures of the order of 0ol mm of Hg and lowero (4)^ (ne of the.ms of the present project Ts to evaluate the use of these Refe rences given in parentheses can be foiun i the bibliographyo

techro ques for heating a supersonic flow in a wid t niel at ast.atic prsessures of several mllimeters of H-go For the e xperimental phases of this projecte a smai "'bl csw dowrn wind tunnel wae used and provided a ach. flow at stat'.ic pressureos of about nm of Hg5 The test @ection area was of the order of one square inchk. Up to 5'$ %o'watts of electrical power wer e put into a c dis.harg thich was sAbilzaed by a magnetic fieM t3an.. to the ar trea ie the discharge want, fom o: side tof e tv-xnel across tte flow to the opposite idOde The b$ihavfi,.or of this t 2pe of Si scharge has beenr inr'st'igated for a variety of magnetic field configurations an d tr den~'tt:''eo.!erimentsbs with a "pual ed" diischarge were a&so Made In Twhicth pewal cEuren;ts of 1500 amperes w-rere passed thr'iough thb fl"oh The order in u'h.ich, various t opics a ppear in this re:por corresponds roughly- i ltLh le c'. ronolo order in vhldch the tTork was donee The results of th'is investigat.ion are presented in a rather descriptitxve fashion since the type of behavior thiich. ca observed iid not app$ar t$o iwararb a more anlytBfcic study.

A number of considerations are involved in the choice of the air density which offers the most advantk geous conditions for heat addition~ At msfficiently low gas densities the heat addition is mich more diffuse and unifarmo however, an appreciable fraction of the energy which is added to the gas does not appear immediately as themial energy, but persists as excited and iorDised states of the gas molecules This effect becomes more prononlmed at low1 densitieso The seriousness of this aituation for a practical xWind tunnel application has not yet been deteir iined Appendix II is a copy of a letter from Dro We Bo Kunkel of the University of California in which he describes asme of the constituents to be encountered in these afterglow s Appendix I is an analysis of the compressor powmr requirements for a wind tIursel with supersonic heatingo Ths analysis was made by Mro Jm s L. Aaick under the direction of Prof, Janms. Broadwell0 Mro Aminck is associated t'ith the University of eMchigan Wind Taunnel at billow Iutnr and Prof> Broadw el is a member of the faculty of the Aeronautical Engineering Departmento The analysis shows that the required compressor power is leas if the h-'at i'.s added to the airstream at low values of Mach number~

ioeo, near the front end of the tunnel where the gas density is highere At these densitiesg howeverv the problem of obtaining a diffuse discharge becomes more difficulto

<Bfc 6 o2 l T0I - 1 THOD FOR STIILIZING A e ARC IN AN AISTEAM At gas densities corresponng to pres&rs of the order of tena mili]mters of Hg and higher, apprmte theal eq i - riu exists between the electrons and ions and the zieutral gas particles in an electrical dischargeo (This situation is true onty for low values of E/po(l)) ) The teperatu of the plas is hig enough to produce thermal ionization in accordance 40th Saha9 ecuationo The power input to the discharge is ust enough to balince the loss of heat from the arc colm and to maintain a high enough t erxatire to p rovide the required degree of ionizs Under these conditioLns, a rc e ole m e'btablit. thed %rct rrs to an airtream wil move do ream at the sme velocity as e flwo The air nAtion relativ t e to te electrodes cries t heated gas w th t, d the ar colum wtra length s iU tzated in Fig la There is no mechanim by which the are can move fror the region of heated gas into the colder air lathout the action of a mgnetic field. When the arc gets too long it il extinguish tw se of losses resulting frn its greater legtho If the open circuit voltage across the electrodes is aifficient to break don% the gap, thle arc Tdfl xe-ignite and will again blow out downstreMan re8ilting in the unstable behavior inCdi..ated by Figo lao x) Reference (1) po Q J& tferences given in parentheaSes can be founi in -the bibliogr=hyo

Airstream AIrstfream ai c. /e. - / ' I^ // K s. // ) i c. I Iou '\ '1 \J /., at, A cA t/ It l /lgetc Fel

An exception to this situation aists when the spacixtg beten. the electrodes is about the ae as the cross section of the arco This occurs at low densities where te size of the dt&charge becoms very 3larges Under such conditionas illu trated i FIg 1b., the voltg gr adient along the leading edge of the ar is sufficient, to produce th required amut of ionization for the &r. eto e cist in a stable fa Si.on transverse 'o the air flowo Whea a magnetic field is establiahed at right angles to tha cuent ~flow fjx ean electrical discharges a force is aerted on the cuharged particles0 The mgnttude and direction of thia force &ee given by: Force per mit volum J x B here J is the current density a4d 1B is the magnetic fli x denisity'o This iagnetic force causes a d-c ' to move in a drection pe.rendic2lar to both the mguetic field amd the current flowo With a sufficienAtly strong field, a arc can be made to travel art peede of several thoumsmds of feet per second0 It can be shownm tha.t u-nadr these conditions the air which is intercepte d by the are col.xrn is nort hekated to the temperature required by the Saha relation (even at preszsures as high as one atmosphere) Thus, the iagnmetic field forces the arc to move through essentially cold airy afrd in so doirg raises the air temperature only a few hundxed d 4gre( ein -siteai of 5000 or 6000 degre.s which wtuld esalif tierta ecli1 tUbrn wereestablisheda

One of the objectives of hthis resea- rch program was to inrrestigate the use of a magnetic force to stabilize the position of a d-c ac across a supersonic air stream~ In the wfnd tunnel applicationf an arc would be established transverse to the air flow and a magnetic field weold be oriented to produce a force on the charged partiles directed upstrem,. In this way the aerodynamc forces tending to move the discharge dowmstream could be appraid mately balnced by the magnetic forceo For this situation the magnetic force imparts a component of upsbrem drift velocity to the electrons and the positite ions in the discharge. This momentum is cormunicated to t he netral gas molecules, and has a tendency to slow doiwn the air flow, Howevers at several m of Hg static pressre this upstream force is vexy smafl and the mobentum of the flow is reduced by a neglinc^ gible amount The ionization is a fraction of one perceaut and only one-third of the directed kinetic energy of the ions is t ransferred as directed kinetic energy to the gas particlesa (4) The other two-thirds of the ion energy appears as random thermal energy in the g-aso At etremely low gas densities, however, the situation is quite different and the magnetic force on the ions can be effective as a ans for generating a rindo (2)(3)(4)

sPo IS).Ig III APP LUS The major items of equipment and the instraaentation?oeyed by this project were available in the Deparbient of Eltrical Engineering of the Universityo They consisted essemtiaily of vacuum equip t to operate the blow down wind tunel and electrical apparatus for producing the air-heating discharge, A steel tank measuri.ng 28" z 30" x 96 (a volums of about i47 cubic feet) provided the vacuum reservoir A Kinney type CVT 556 mechanical pump was ued to evacuate the tzako It had a capacity of about 15 cubic feet mper ute and could pump the t'ank donm between mms to a pressure of less than a ititera in abor ut 20 minuteso Dxy nitrogen as used in the wnd. tunnel f~or wst of the high power heating erez':i nts; although room air wats eployed extensively for preliminary t'ests Nitrogen wms used since an eletreical discharge air mc v caruos odes of nitrogen which will quickly deteriorate vacuum pm oilo For most of the tests the m.trogen -as supplied at a-ospheric pressure from a large soprene balloon supported at the top of the brass tube which can be se n on the right hand side of Fig. 2o Pressure meas^i eets conrstituted the prija mnthod of instr umentationo For pressures of a few centimeters of Hg and less. an Aiphatron ionization gage made by the National Research Coo was used- For higher preamres, mercury tahometers were employe the close end v ariety beig quite satisfactory for total

r'.t..1 - head pr*esstUrte iaswureentso The other mjor pier es of equipaent consisted of a mag> net, the maget poer sppy and two high voltage power smpplies The magnet Tras designed for intenmittent operation anad could prcw dace about 6000 assin a four inch air gap (eight inch diameter pole pieces), Tapered pole pieces were also used and re sulted' t:A a =mbstantially higher field strangt over a three inch diameter area Figs 2 is a photograph of a typiald experiment&J. set'ip with a tunnel in place between the pole pieces of th.e magneto The power suppl}y for thxs mignaet could deliver any dt esiXed currext up to about 300 amFpres (at about 75 volts)0 lOe of the high voltage pow? asupplies vas a 10 kiloitt to 30 amperes or short tntervalso Varying amonnts of series resiatanee and inductance wHera used wih ese pmer supplies to provide a steady arc current of any desired valueo The iind tmrnels used for air heating tests were coxw strutted in the machine shops of the Electrical ngineering Depto They were made as simple as possible and yet provided for a wxidde variety of tunnel and electrode configurations The flow in the~ test section of the tunels had a cross section of the order of one square inch,, A variety of different tests was made at Mach nszbers vryiiLng from Mach 3 to Mach 5o For the tUmnel design used most extensiveWy9 the tunnel

I f 4./ Fig. 2

I t,*', s ii: "I Fig. 3

-havd cna rctangX mar cross seotion tilt tw 8straigb,% pwarallel sidztes The other two sides diverged at about 30 intc3ded i angle fontiang a adege-sha-ped nozzleo. Au exaple of this type of tunnml is shownm i Figs8 2 and 3c, Strai t sides were used in the noszless resumling in radial flow at the eaito As far as the air heating experiments ere concerned the,adial flOw appeared to be oZf n consequence and the simplicity of straight side's was a great cosm venience in constnctiono

IV T "CONEOUR gEECTRODE" TUN.tEL Description A simple arrangement for establishing a dt- ure in a supersonic flow is to use the two diverging contours of a wedge nozzle as the arc electrodeso This arrangement has proved reasonably satisfactoy and was used extensively in this investigationo The detailor of a tU.mnel of this type ca be seen in Figo 3 -nhich is the same tunnel sw:an in Fig4 2 with the top coer r&v voedo It was desined s o that a wide variety of icontour electrode" geoametries could be tested without having to alter the basic tunnel structtwre The structural members consisted estsBe t'ially of two side rails runnsing the lengch of the tamnel, They were braced at thbe front end by ea brass crossaviece which supported. the inlet tube9 amd -re secured to the vacuum fltai.ge on the domL stream end by means of two brass pl3.ates One id rail was made of a dielectxic material (phenolic laminate) to siwptiLy the ele.-~ trical insulation problem0 ThLi rubber gaskets on these members provided a vacuum seal wein the top and bottom cover plates were bolted together as shown in Figo 2o The tm parallel interior wa ls of the trnnel-'sre made of a dielectric.material so as not to short circuit the contour electrodes0 Pyrex or Vyor glass (Vycor contains a high percent%age of fused quar ere uead for these walils in order to withstand the heating and also provide visual observation of the discharge The separation between these parallel glass w

16 was usually one incht ho-ever, it was cut to oneAhalf inch in raany instances in order to reduce the mass flow and provide longer running timeso When the oneha1S inch spacing was us~ed as shown in Figo 3s the inner glass plates were not fitted in a vacuumOtight mannero E-perimen'tally, it wa foimd more conveniLeAt to use additional cover plates of Plexiglass outside of the Pyr"e or Vyor liners and to make the vacuum seal between the Plei glass covers and the aide railso In this type of tmnel the heat adition. took place L a, the nozzle exit iich in all cases was about one inch wideo Downstream from the nozzle arT the discharge region a wide variety of eonfiguration was ua ed These contours ranged from a two inch constant sarea section and adjLustable second throat to a dra- stic&ally divergentm section0 For thie tunnel shown' in Figa 3 a divergent geometry w-as used followed by a slight cons.triction to aid in presanre recoery nd incre ae runn ig tvimeo Aerod^tic easuremuients Presswe Peas aentss Static pressure measurements wre used ext3ensivel to deter.mine the flow conditions in the tunnelo When different contours were first tested several preasure checka wee made to insure that the tunnel wBas functioning properly from a aerodyamic standpoint before the heating diLe eharge '~mis operatseds

,.-L If "..* sor sivera&;l c&:rr os of otati p t r.ae ndW I. t vri o s vatoint a 0 long the nozlie,U, All 4,xf,Theors;a~~d:>~as agr5emed ^PIw s 1w5l viac u vmies ca](alcsIta-ed frm theg aea ratioe a &Sag imvtg Mrxil t l,acbco:,.Ivo tefit a tlwoa%4 paoieng osf )C080 inchers 'the c.culatu.. x~~m~ eoar a-t:he rx n a2 o. t, ^f~t 2ile a l otf I>a'a:i. hi idicatg thte fiv'atiof h ~ mt.oderai.ec amt, in ih.xt of ~tdar layaoers ~;tat:lets 'l ':h inaw;,EOs o saicd o.t,.n taed: K eae of t.s~hem, 'lim~ b~ior. oss of pribes lotCated aes.', 1 u/2 ls2, 1Zma -from the e^ x>e aus i1how Fig. 3 Th ptbz e ie ade.e ftosts m thCee o aife,- ttr p ix- t ]el The hides oCf the(-, ttc pem Ore p3rW w3 a a ebraetas need3,e abnut 3At. inh lg a 0,6 C6tes o ne i nch '*.r4 p ie es i f d. m qth ali t ) opsal le ate g la&fY *oia lgooh dT e Vopical iea d of static i eAi.^&t cjxaer 40 v e39 mih of g respectT yr Vo9 be i t en E~ar ge o5h wat3t s te:Uin.a ' f id dtvalue fo a" Lteah o t f ithd a 81 —ouga uhs tS' Slaxado-gi e h ee miiqee e ye tri d aL a Sfn ' ia atrw x tra n- of th, iLL J I ttial x tirn do m el ri^. In WUraze, tes t h 'a separat vLon IIt.een the parallel Ides of the tuii oael ox>9 x an piec tt IL- oit f uty optical t gp>2ass hrtg cxlscd ozat Mhe audra rr33. s iris&tea~ d of the Pelt aS 7 DLy> —tes & -xOT - nX Fig" 2c A; 25 tt Verait t concae.nitrated, are. lgt t ith a

.w 18 - Si);ot jem^ o0029 4rvZsI^ as aX,,,'t us ligfaf 4 mrc no TC, he beat rdita Cd^i e";th dvergent lighit ct ad treobr3 ^ nt 8ouceWM lo atd lf, th ree f1.eet.mr. 19r o ol Se;e1a- dl uteits anando to6-0 use a gooS,1d c3kemA s fo-r objtai"ig arS l ligh%,o Figo 4 ts a hm-ea -,wiah tf the sh ock m a 7~.Ma,,..r Ixc,, T1m ht:~se 1 hh doapz;.-.-. halS"nIfr e doub: ble udgee The<` 'M Iage ajppted P. 0xt 3st' 9esmi by a OPd]2 i.~c;ch,diah-e~t.er.rdo The w..dge span Twueas aboiu% 3 o o. a... m? Xd thns.e d.d not xteed na43l the bz 'y: to '> t.cp..botto f t)e tIem n these. ts of th-,et, segtt".ion:mif? a con':nia'bu;zn of the a'braight nozzle9 z,^ll,. hi, tbhi e coM os,toe cnc;oimr t uz ar edl thbe mnimsle xi t $ s a o s ]oa allel to the straigkh smidt, tbiwa formlfring a cons'tmantb area me.t-r.ion w.-ith a.,, x S css section. The fTroie the: dlagngi... to the para:lteLtl es."t Sc"ton coa Cour a be isenk t t" e k iupn,:ez left of!i"a l:.~ mk-.,:g tl.she,e, Conseqs,,,lt^ the. ' (vo t, rast.atioe) Othe iaA II the p,3,,las xe..a.e d'ua, and t'he presence of tiay distB.t ~ j ho@t.mveIr l.5t m,%rg o:TIT. lde^talas whir.ch cua'smoot lbe< sel vi. sruallyo Thi3s 3 buod i;,,p,,rani. Ill ',Yus rates several features of the,lowz -w", k.ch we,, aE enm-;untAeSre d in a tu.mnel of thiB a,ze a, t-.tese

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.t 2.0.. Nieh 3n.iuMlerPS A str.ong bw ~ave from the wdge c.an be observed emich eids abruptly a short dbistance rom the wallE of the tunnel, probably due to the presence of a moderately thick 'bounda-ry4yer0o On thre straigh't aide of the turnel, noticeable bouda4tayr thic eBning ca, be seeno This may be due to a combination of the bouncna y-, ker sahc'ck w xave inter.-Xaction from the bottom of the wrdge and a etpres$ion wave from th tuming of the flow jut ahead of- ito A maesurement of the shock angle indicated a Mach nwber of 3olo Statie pressure matar.ewents Just ahead of the wdge indicated a Macai nsaer of a little less tha 4 TIas disagrsemient probably Jtdicatet a xieduction ijm ach number in this region dute to two sets of compression w ves: those originating from the change % in tunel contour at the end of the diverging noassle and those caused by rthe bound.ary;,layer thickezningo Second Toat Thhe fi ixst. tunnel designs had a two inch long consteat area section follo-wed bW an adjustablo second throato In preliurintary tests placig even a small wedge r probe in the constant area test section caused a very large increa8e in the static presm,.reo Since this appeared to be a consequence of overconstrio.tion of the second throat meamsrements were xade on the effect of varying this spacingo With the pressuwe ratios obtainable, supersonic flow could not be establijshed wAth a second throat spacing less tha Oo inches W.en the acing was o.h5 inchess the rrning tim inas 20 seeo ne however arty

^ 21 B wedge or probe in the test section would break up the 2ltmo WhE t the second throat was opened up all the way (1.0 inciches) s that a straight section of about 6 inches followed the noszle the running time was reduced to 5o5 seconds, but sufficiently a4imCi obstacles could be tolerated without upsetting the lowo In al.I of these testsa the static pressure, measured at the nozzle exit. was very consistent and varied only one pr two tentth1 s of a rMJL — meter for different second throat spacings0 Since a few seconds of' mrumnig time was considered sufficient, for most of the electri cal experimants, the second throat was eliminated in a13. subsequent testso

22 'P M~l V -PWP.EN. 1 AND BEATVIO0 OF A d-c AlC.I TIHE QCONTOUQ R Q EigtCTRoDE] TUNtL With a magaetica& r atabilized arc in the contoxr electrode tunnel, several general observations itsmiat'ely beeoa i apparent0 The most obvious characteristic is that kthe discharge has a strong tendency to orient itself slantwise across the tuir nelo The cathode end of the arc alwIys tends to be dcwistre-ea from the ntoueo ends, The upstreaP m boundry of t he d1iscarge is ait, an mngle of ~rom 20~ to 70~ to the direction of the air flowi d-, pending upon experimental conditionsa Another more important observation is tl'w Mt the c.ts a very strong preference for the boundai y layer regi~ont The cross section of the arc column is not round, but is t0iha~ or our times greater Tha the dimension parallelt to the flo thta1n t he tratsrver^e direction It appears as a uniforziy bright sheet ltocated adjacentt to one of the dielectric wmalls of the timnelo The Cdi8scharge is probably Jsiitiated in the boundary layer and then growe out ijto Uie hit~her velocity flow as the power and dlifusivrity arPe in reate&> Slight c eh e in the thickness of the top or bot toJ. boundary layei or ~in the orientation of the magnetic field mk the i? dcischrge juwq fro one dielectric wall to the other, There are tio reasons Way the arc has a strong preference for the boundary 2ayero First, the gas density in the bound ary layer is less tamn the free stream density by a factor of two or three0 Secondlyr the heat loss from the arc eolmmn and consequentl the f en lergy frequireeLrtsos are les in the boundary layer because of the lower velocity flo.<o

The most obvious effect of the magnetic field is the force theat is exe'rted upon the charged particles, Thin enables them to counteract the wind forces and when these two forces are made to balnce approximately, a stable arc can be established across the stream0 With a sufficiently strong magetic field, on the oher hand, the discharge can be made to m.ove upstzream and right through the tkroat0 Strengthening the magnetic f.ield tends to straighten out the skewed orient'ation of the arc column' hov ever, this action can be carried only so far before the greater field strength dr:ives the discharge too far upstream0 A strong magne tio field also tends to incre'tuse the dif fusix-rty of the dtq,-il charge, thus improving the uniformity of heating0. It appears highly desirable to use as stronLg a magnetic field an possible consaistent with stable operation0 It should be noted that ~i-tjhout a magnzetic field the discharge is completely unable to withstand the action of the wind0 It ine adiately blows oiut dolmstream and will prefer a path a foot or xaore in length rather than cross one inch of high velocity flow0o The magnetic field can also be oriented parallel to teh applied electric field instead of -transversc to it as in the sitUS'a tion described above0 In this case the arc column tends to be collimated along magnetic flux lines; however, the aerodynamic forces on the discharge cause it to angle off in a slantwise

n ab qM mmaner and h t r hu c across the magnetic flux lies, nioen bthbi8 hk.appens there is a cormpontent ofC cuirrewt flong at MrigEht angles tt the magnetic field which gives rise to a magnetic force on the ta in the J x B direction in the same mann.er as describecd* aboveo Thkst, rmaes the arc def lect towards one wall of the tmunnel cr4d re&alts *n an isnerently skaeewed orienrtation,. The parallel r:ag etic field doms not appear to have any sgnificmaxt> merit as far as ast:biliij.Xg a discharge in a high velocity flow is concernsedo Increas'ing the cdischarge current tends to mtake the are eolwmmn grown in siLe and thus fil, more of the ~tunel @. Xeos ect It is difficult to obtain a good estimrzte of the size of the ate becausee of V skewed oriientation which. it assumeV s, o.ne 01 Oase a Sfowr kilowatut discharge cazi g! 10 ap2ees appeared to fil. about three-fourths of -the tunnel in the Iech h region, and wms crossing the tumnel at vaboxut a U.o angle. The separatAion tbetween the parallel trnel i3LrLs was 1/2 xnch and the drownirstream co'nt)utr was highly divergernt a t s4hoxmn in Figo 3. The a tparert size of the discharge columm was probably mialeading siuce high 8peed mot-ion pictures (to be described 3zater) indicate that the positi.toJnil of the are column was fluctuating Srapidly creating the impressic of greater diffusivity thhan actually existedo lncreaosing the current b ong a certa_ n -evels how'efier, appears to increase the' current densityo This gives rise to maore intense heatingl t als3o.inreases the effective magnetic fotce ing,,Bt, It _,r9~;~f,,r9,ete'Bc -force~f~G~BSP~~t Z fX;C

,a 25 f... on the~ avc thus causing it to rnc nppsteeaon Soie compenst;ation can be made by decreasing the mazgnAetic field; hoevere if the field is decreased too cmch, the dj scharge is apt to blow out do.streamo If the power is increased beyond the o oi a kilowatt level, in tshe contour electznde design an u.nstable condition arises vfere the discharge eithez' rune up into Wthe higfher density flow jaust b-elow the throat (T t ere it constrictB xjto:a very wis l channel) or blows out d oasmtrealm up the tnel conbtoprs d the hetateninfg ecltion in oxdored to prevent tcbe tunne)l f rom GchokingUo In one design a 12 x 1V constant ares section a bout one tc h long followed the nozz1ei The flow in this section was about P.ich an d the masts f~lw rwas approximately 3-0 gr3im/seCo When h.,o8 kw of powxr ere pu't i.ntEo the arc, a uniform riischarge was obtained z.L/ich fiLLed aboxt ha32X of the timnel cross section and did not appear tuo upseEt the flowo Hovrever whein the power was increatsed bsyonyd ts lelo the dif ^fcharge bscaw a-ch inore brill anf an lutoi.oue an, i's ChTacsteriatfei of arcs at high pressure axd temperatpr'e. The.abatic 'pressu're i; t he@ constant area section also jm2Wzed from about 55s of Hg to about 35 -mt of Hgo These effects appeared to be the revult of excessive heat addition in tIhe constant aea section ad ai resai: ant choki ig of te flo t In AppendZ i III t1he am% of heat reqtuied to choke t'0'e

flo unader tese conditions is calculated based on certai' idealied asEsnptionS It is shozm that in a constant area sect-ion the addiA. tion of 2,16 kiloTatts of heat to the air stream will bring 1Ythe flow from Mach h to Mach l1 The experimental observation that 'perroni flow T~-as Still present even though -the amount of' electrical power into the a:rc r as theore tically more they an sufficient to chokeQ the flow appears to indicate that a substantial amount of the added energy was going to the tunnel wills or being carried dowmst.ream in the fo:m. of excited and ionized,moleouleso: Ho.-ve,, r it is significant that adding a sufficient, amount of energy to the ~low in 'the constant area section did choke the tunnel o In order to avoid this condition, the tunnel cross section -as opened up by a considerable amount downstrueax from the heat addition section, as shown iinFig~ 3o When -bhe highi:r dirergent contowrs wiBre used at the nozzle exit th choking tendency did not,ppear to be present; hoe^rerp the running tire was re.duced to about. for or five second3 -ln generalt pressure msaswenents mide while the hezat gf discharge was operating ere less consistent and gave highexr Tre ings than tfhout the discharge0 The amount of the Dpressur increas e ows of the order d wa-s roughly proporitonal to the power itxputo Measurements made in the heating region were quite erratic, since the dsh e f the chge id notil the ole crss section of the streami d the increase in preaure. $? usually

t,-1 ig t-rher t '"an::"'2 elseeaeo A n cra *c8. m:sS a B. s iner thhe at tic pressre mieatsnred at a po.': -.igy:,. apstreama from t dhie dsc*8arge Thit: wavi.ral prbaly caus... ed b'y a.,.tet ion si.m Al'la-ur to bounlaxy l.a-yer sh ock v-fvave inAeato.n - rf.ch t1Al arc p,?!ced d 4uc ad5-,. oB,:i.. &ty ai the ~floo *oete in3g tteg ti l rinrolve.i~ Small3- hole i adxbalat;on.3 h..ad ta o be ubesed: an.d at these. low poreaaures f"ile tio e uired.:,,i,.,i for t hew yEstwn to reaeh ameqil3 in vas Cthe 4sanm a's tS lhes lgIof the rmUn, ]Sort reea.dings e.e only, a rowug indicJ, i'ion of the~:p:,?Are4e,^situ mv.s nevrirer actiie advi rexl ec.. -id I -gvly thxe pl' eaO:p mare -prdolrs) tl cthe a-S.h _. ie a..r.. b.:Li bat ten.Ced to be mo..,,,:.:,,t: than, aea eni,96ts xmaade- *in71 tc3e dxS3cbalrge pregion $he c t sta~tic ar d riina. ct presaurs both 8hsod tehe clT ge-exal inc5reasr ls- exned4 h tr the rati f zEtwa t to '^;fctlc vznti cx to ime ecrea..s6,,li-gh ly..,. Bsed upon the 3E yleigh. 'ralt, i. bbal i;, ieabiS..5 atted a v odee.:: L.s.,er,,ng of th.e Ia.-h miber dtOltrS frm t h 'c~a Fi.g:.?~~[y diwrergex:i ecn tour:m~t. a~s om. 'in Figcc 3c, 'were a'sc:a$d inl.se testao This a-dditional eqs',i, on pw'det':.ed " ".......irease I-D Mach 'viludch wmild h-ual ben b peeeed hadai at l ecxrs t t Q,, a ecta. otn bsen.s..edo Preestire re.adbaigs taken,,ile n the 6dischargek puas,w;re:ratfi:tg Il e.:e considE ':red to be of, Dldt-ad ' vla.-e Withoi: o:tX hueaft ing t.e:e,:as, mdtubtedly cons8iderable iturbulence in thle lo.- '5,

a~pears to be iabstantiated by the exratic preasr..e Va. ations Thich were observed exper:6 entally The ratio of ilIpact to stat t:ic pressure provides an indication of JIach number. Howeveri in order to determine the other flow parameters a third ig s-der m 7ma;werment is necessary, since the total energy of the air stream has been increased by the heat aId%.ton).

~. 29 -' VI EXPERIMENTS TO IMPROVE DIFFUSIVITY AND STAsIL:TY OF ARC IN "COCNTOUR EL.ESTRODE" TUNNEL An important requirement of any air heating system is to add the heat uniformly throughout the cross section of the flow in order to minimize inhomogeneities and turbulence The maJr o bjetive of this investigation was to obtain a dischaLrge of stefiiet diffusivity to meet tlis requirement of unofroxm heatingo A fairtoer goal was to devrise a mans for stabilising the position of the ai ro and correcting the skewed orientation0 Most of the expewtnrienteQ work on this contract was directed towards a solution of these two problems0 imeats at Reduced Gas Densities Previous work at the University of Michigan has shown that a diffuse discharge can be maintained in a high-peed airstream at densities corresponding to a static pressure of ~a frac. tion of a silimeter of Hg and at temperatzures of mawy hundrned of degrees Centigradea In the present wind tunnel srtuation,: tLe very low stream temperature makes the density greater thea would seem to be indicated by the static pressureo This density corresponds to a room temperature pressuare of Yl&ny mtL.meters, and vI dor these conditions the discharge tends to be far from diffuset c An obvious approach to this problem wab to decrease e the tream

-" 30 - density in the discharge region to obtain the desired diffuav3trit Several different experiments were conductedt with thia goal in mind. The parameters which were varied wre the Mach nm'tber (area ratio), the initial pressure (po), end the initial terperature (To)o Te c itis n he flow conditions i nthe twnel were determined from static pressure measurements made without the discharge running. Increase in Mach Number The simplest procedure for obtaining a loer stream density to reas e t ho ine Mach number by narrowing the throat openingo Tests were made with the t-hroat spacing varied between 0,1O00O and 0022;" with the mtunel shown in Fig 3o Table A serves to smmarize the results of these ewperiments0 It gives values for a calculated Mach number and a mea-a ured Mach number The calculated ach number as as ba^ed upon the measured area ratio wand the a8supxtion of isentropic -inviscid exrpansion0 The so-called measmaed Mach number was derived from the measuraed static pressure at a point just forward of t.he nozzle eit,. also assI8mig isentropic expansion0 TABLE A Throat Calculated Measured Width Mach Number Mach Number 0al00" 309 308 Oo080" 1 3o9 OoO60" 65 1o2 OoO4O 45o0 4o.5 0o022" 5oT h09

An xanation of these data shows that the discrepancy betwaeen measured and calculated -Mach number was appreciable when the throat spacing was decreased belowi 0.06O% Excess-ive bound&y7 layer -thickening at the higher Mach numbers was believed to be the explanation of this discrepancy. The effect of this boundary7 layer growth was probably accentuatoed by the elonagated roct&Qangular cross section of the naozzle exit (l's x 1/21"). H1easurements of the static and impact pressure with the pressure probes;just downstreamkfom the nozzle also confirmed this general.1 behavioro An attempt was made to reduce this boundary layer forma,tion by incereasing the Reynolds number. This. was doaie by mncreasing the i'ni'tial pressure (po) riand expanding to an even higher Mach. number. With the pressure regulator available, it was possible approximately -to double po) using the same thcroat wi~dths as lioted in Table A. These experiments were even less fruitful than the tests in which po was atmospheric., and when only the area rati.o was varied. The few electrical tests that were imade in a higher rnh number flow appeared to confirm the, assumption of excessive bai~ndary layer thickeningo Although the discharge wias noticeably more diffuse, it had a very strong tendency to remain close to the dielectric boundaries of the tunnelo

"w-~ 3 2 al On the basis of these tests no further attevpts we-re miade to obtain a lower density by increasing the Mach rnumber. For thcise experiments, it did not appear feasible to expand beyond a Mach number of about 4 since the boundary layer begins to occupy en objectionably large percentage of the flow. A-thick boundary laWyer is very undesirable in this application since a d&-c discharge has such a strong tendency to localize in this regionio furtherq the decrease in gas density -which is obtained by increasing the Mach number becomes less pronouncedp since the rate of change of density with Mach number becomes lo-wer for Mach numbers above abouto 4*, Decrease in Initial Density Tests i&,%re made in which -the gas density in the working section of the tunnel was reduced both by decreasing the initial pressure (pa) and increasing the initial temperature (TO). Pressure measurxemAentss, without, the dicharge runi=.ng, again were used for determining the flow coneeit-lonsp and a throat wridth of OoO8O"1 was employed in most cases0 The initial& pressure in the isett.Ltng chamber could, be reduced by throttling the inlet valve of the tunnel0 It could be lowered to about half an atmosphere before 'the r~unning time be~came excessively shorto A small electric furnace was used to heat the air ne ing the tunnel. It was constructed from heating elements; taken from an electric range and placed in a large ceraveic cyrlinder

.IO33 917 about 10 inches in diammter and two feet longo Smal bricks &axd pieces of brass w ere also placed in the furnace to help increase the thermal capacity and to boaffle the air around the heaterso A sml ron~-constantan thermocouple was used to measure the teRmoera — ture in the settling chamber of the tunnel0 Very fine wire (3~-il) and a sensitive mi'llivoltmeter provided a rapid respoi-,ise. Thi's arrangement wams calibrated with a conventional potentiometer bridge 'in order to compensate for the current drawn, by-, the mli voltmaetear0 W~ith this arrangement it was possible to double appro~imately the absolute temperature of the gas in the -settling chambero The essential results of these tests are summarized in Table B. It ia interesting to note that, based upon pressure measurements, the measured Mach nunber was not significantly affected by these changes in density. Throat width = 0*080"' Calculated Mach Not, = * Initial Conditions Pressure Teerature HevrdCalculated Maclh Roo Stream Density 7410 300 309 3o5 x l0~ 300 300 3,7 147 x UlO74j0 553* 8 I 8 x 10"1 -

- 3. )4 At these reduced densities, the discharge was notic'eabC more diffusees and appeared to occupy more of the tunnel cress s^ction for the same currents HoweverF thee arc always clung to the dielectric boundaries and spread upstream and downstream to a much greater extent than at the higher densities. It still hJlsowd wwry little tendency to move into regions of higher velocity flowo At the lower densities the arc column was not as bright. and apparently had a much lower current densityo It zaes possibl.,f to use a stronger magnetic field and there was less tendency to localize near sall imperfections.i the waltls The voltage drop 8cross the arc was also lowero Thus$ the energy inpuit per units volume was considerably leas th.an at higher densities. This was desirable, since it tended to produce a less abrzIpt change in tho stream properties and thereby reduced turbulence effects and the tendency toward flow-choking. Just as at higher dena.itiesa the are became unstable as the current was increased beyond a certa5i. value The arc column also had the same slantwdse o:ientation; however, this was less noticeable since the arc extended fa'rther along the length of the tunnel0 Ni'ren Afterglow Considerations An important factor to be considered when operating an arc in low density air or nitrogen is the formation of excited or iond.zed molecules that persiAt for long interva.s of timei,

At a fairly7 high pressure (a substantial part of. an atmosphere,) the percentage of ionized and excited atome, for a giLven gass is a function of temperature and pressure only,. as given by $aha 't~s equation,, There 'is no significant persistance of ionized or excited states after the temperature has deareasedv while fit lower densities a discharge inntrogen results in energy sibsor tion:J a various metastable states0 These excited molecules can pers~is for objectionably long intervals before the energy appears au tiherinal energy, or is radiated as afterglow. Appendix, II is a copy of a letter froas Dr,, W. B.Im~e of the University of California which gi.ves Sooe ideas of -'(-he com plex situation encountered in connection with afterglowO effects. Dr. Kunkel has studied activated low pressure gas streams as a means of "flow visualization"1 at very low densitieso In the air heating tests made on. this project tle're Tv,.1 a pronounced increase in the amount of aftergloww; vixiich could. be noticed at the lower densities. Under some conditions a t rai of~ reddish-brown -afterglow could be seen blowing out dowrastr ean from the discharge. The ease with which it could be oboe)rved even though nextkf to a highly- lwuinous plasma was a good" indication. of itt; strong _intensity0 On one occasion., the laboratory wa~,s &ckwk ened awl 'the interior of the vacuum tank was observed tbxough -thc~s plsate glass window which covered one end oli it. U'lith the

Wr 36 I di~scharge -running, a jet of grreenish-rown afterglow could be MCOD enterig the tank and diffusing throughout the interioro The effects of an activated gas in a wind tunnel S~ituat~ion have yet to be evaluated0 Experiments by Rayleigh and others haive shown that the energy, which an excited gas can deliver to PmatC1 surface may be. as high as 22 calorie s per gram of the gazo (5)(6) It is not known how serious this delayed heating may be or how long i1t Will persist In the airstreamo.2eim-ents to Control Orientation of Arc Dif ferent techniques were employed to impar ove the stcability, and to correct the skewed oriLentation of the dr-c arc Jin the contour electrode tunnela0 From Visual observations., the discharge appeaired to cross the tunnel at an angle of 20'D to 700,p and the problem of establishingr the correct orientation at first seemed to be one of controlling the locationis of t he cathode, and anode end,,- of the a rec colvtrgn0 The best way to accomplish this was byj proper enaipulzt.tion of tbe magnetic forces on the arc,, although dielectric" barriers and other devices wore also used. In a nuamber of experiwents the magnetic field was madeu mvore than twice as strong on the cathode si~de of the tunnel. E~ven,With this increased 'magnetic force on the cathode e —nd of them arcq there wans virtually no change in the slantiiiee prs-itieon of the. di scharg.e.

-~ 3? ar In other epriritents the magnetic f ield adjacent, to then anode mirace was decreased by means of an iron insert9 fitted into the electrode as can be seen in the lower corutour in Figr 3o The iron had the effect of by'jpassing mgetic flux which otherwise would bepresent near the anode surface0 Although the iron produced ony a moderate improvement in the skewed orientation of the arc, it was quite effective in preventing the discharge from rnigtoo fare upstream0 Thus, it was possible to use a stronger magnet~ic field and higher currentss, and to fill a larger cross -section of tha tunnel without having the di~scharge move up into the Vhret. Larger inserts than shown in Fig0 3 were also umd but there was noc noticeable difference between the various sizeso In gener.l, a stronger mgetic field tends to increase the diffusivity of the arc, and als helps to straighten out the cngle at which the arc crosses the tbunne1o Since the reduction in mgetic field due to the Iron insertse tended to limit the are froim running too far upstra,q it apperdthat a properly tapered mgnetic field would stabil-ize the arc in the nozzle exto This approac was tested with the &rrangement illustpratedl in Figo 5, Iarge iron inserts were placed in the, brass contour blocks so 'that forwad of a point about 1 1/2, inches frovm the nozzle exit the contours were essentially f3 rono Thi& iron shortbcircuited the magnetic field around the tunnel openingand tUhus effectively removed thre nia, etic force from the arc coltm wnen it maotred upstrbeam into the region of the iron contours. Fig0 5 (b), is a. sketch of the

Jrn s. ert 0nco~~ 0f ~ t t oir Uj~~ v; L watu 'd's t JEn e. d tATcrs, An4 a -a AMA / i n Co t or I Et3h e, tade TL4 ne/

a 39 a section through the tunnel at A-A and shows the relation of the iron inserts to the tunnel opening and magnet pole pieces0 The approximte configuration of the magnetic flux lines is si lso showm in redo For thee the tests the separation between the parallel glass tunnel walls was 1/2 incho Electrical testas wth the tumnel showm in Fig, 5T ere very unsatisfactoryo The discharge pressed very hard against the dielectric walls of the tunnel and formed an exceed:ingly intense hot channelo For the sae power input it tended to burn the Pyrex glass much more quicklys and as the current increasedD it showed no tendency to extend into the flowo Even at lower gaa. densities the discharge was very bright and hugged the wallso At no time did it occupy more than. about 1/h to 1/3 of the tunnel cross sections even with currents up to 15 ampereso Any effectiveness of the magnetic barrier was lost in this strong tendency to cling to the sides of the tunnel A probable explanation of this behavior can be described most easily by comparing it to the situation sketched in Figo 60 In this case, the contour electrodes were made entirely from brassa and iron rods 1" x 1" x 4" were placed above and elow the tunnel as shownI The approximate shape of flux linies is sketched in red in Figo 6 (b)o A coxnparison of Figo 5 (b) and Figo 6 (b) shows tha e fn g f e gntic fiepl inie of th ee tc neleoe is essentially opposite for these two caseso

*/-~"~,_lpg(~*00'10.11_~ ear-Psnp k ~ M~~t, I~~ (ta Tor Vi;ew O0rP r$s Ccootor _BAi3g HFptrion Aabn^^G' FT e l d Ls F rC I nV a M IP-, toPel ^C in t,., n, r A & I I d e, Ct el Is:

The behavior of the discharge An the tuaidel sketched in Fig, 6 was soderately goodo Most iaportant, it did not p; t ews a;gi~atnat the glass Twall and in jcme instances it evrn appeared to be a ishorl distance out into the tm,elo With 5 apieres the arc column appeared to be abeut t 1/l J^ch in diamrtetr Band had a smo 4lthy cy drieal peari11 <mceo It ms not excessively bright and appeared very steady.Cw uhtiorm A. ltiugh it w as always iear th e top or bottom^ it appeared to be pusi',bng out away fr'om the bo adary layer wud into>> the center of the flow.o The position of the arc collumzi along th:e length of the tBnnel, ho Hieverv,as cruite senaitTve to small Clh; ige3 in the magawtic field,) This situaction made it inpossible to incrascv the curierA beyonad the 10 ampere lvel and cornsaquent3y a th: a as never =0e than 2/3 filledt The difference in the behavior of these two arangeme.ntps is a consequence of the shape of the maEgetic fleld.E o or the tunnel sketched in Figo 59 the field is generally weakz but has its3 great'et value next to the two glass walk because of the fringing effetsf at the edges of the iron inaert&o For tsh tunnel in Fkig 63 how-er>~) the field is slightly etronger at the center than at the top or botto% of the tnnel sinLce the field is fringtig in the opposite directiono TSh Figo 5, the agnetic force on the discharge i s si trcongeat, next to the walls0 If the colum1 i nwves toward the center of the tun the t agnetic force ao be mes les and the:.a ty to ci 3;nt.erc the wind decraatses, The most stable position t ia Figo, 5 t, reforem.

imjediately adjacent to the dielectric wallso In Figo 6, on the other hands the magnetic force on the plasma is slightly stronger in the center of the tunnel, so that the wind forces can be counteracted more effectively in this region than near the top or bottomo This simple explanation does not account for the variation of stream velocity and gas density through the boundary layer. If these factors are taken into consideration, the situation in Figo 5 is made even worse, while in Figo 6 the stronger magnetic force at the center of the tunnel ia offset by both the higher stream velocity and increased gas densityo Thus, for the situation in Figo 6, the discharge probably initiates in the boundary layer and grows out toward the center of the stream, but is only able to push out a short distance before the factors of increased density and wind dominate over the slight increase of magnetic field strengtho This explanation is only approximate, but appears to provide at least a qualitative interpretation of the observed behavioro Another method which was used for correcting the orientation of the discharge was to place a dielectric section in the ele-^ trodeo An insert of silicon carbide was placed in the anode contour in the same position as the iron insert shown in Figo 3o It preventled the anode end of the arc from moving upstream and when combined with a strong magnetic field was moderately effective tl straightening out the orientation of the arco The difficulty with any such dielectric barrier is that the arc presses very hard against the

1... 4;3 - junction between the metal and dielectric, axid form a, ver hot concentrated spot which will quickly erode even the aw st refraco x~y materials0 This hot spot also forms a highly concentrated reagion in the discharge which detracts from the uniformaity of the heating~, A third device which wase used in an attempt. to conrect the,-kswdposition of the arc was to place either iob'ts or n in the cathode contour. From early obseriration$ it appeaxrd that one. of the reasons for the slantwise orientation was tbat the processes at the cathode wemre more sensitive to the winad thm. those; at the anode. It was reasoned that a suitabl.y p.laced slot onr the cathode contour would provide a sheltered place in whafich the> cathode spot could locate0 A variety of both. Yk1ts and fincs, -as used without any apparent success0 In most casets ao coneluszif.Mn" could be drawn since the slot or fin was always plAaced miLdwey between the two parallel glass wallsp, while the dischatrge was 9.u-an s. aadJacantj to one or the other of the glass Imls0

VII -HIGH SPEED MOTION PICTURE S High speed motion pictures were taken of the disch-zz%,e in the contour electrode tunnel, using an eight rll~irneter Faat~ur camezra which had a maimmspeed of about 8,gO frames per second.. Fig0a 8 is an enlrgement made from a selected section of these fi ns This figure is a negative print; th-erefore, the arc appears black ag int a white background0 Fea consecutiv;e fraemes are, show~no, This enlagement was printed so as to baring out the details in the arc and thus is considerably underexposed, and consequentlyr, the outline of the -tunnel canot be seen in this print0 Fig0 7 is a dgraiwamtic sketch of Ihe tunnel asa viewed from the swmpo sition as the Fastax camerea and should help to orient the reader as to -what is portrayed0 I that is vis:ible in -the enlargement in Fig0 0 is the arc and parte of the reflection in the brass cathode cont-our0 These high speed pix'tures brought out several aspects of the discharge which could not be observied otherwise. The general shape and appearance of the arc and the formation of a pron~ounced. cathode spot can- clearly be seen. Alth ough it isL not evident in the four frames shown in -the enlargement in Fig0 8, the ar spt was dacing rapidly over the cathode surface0 Under some cond-itions it see-sa ved back and forth in a. periodlic fashiong gradualv working dowustream -and then 'muring upstreaa abruptly bfor.oe workig oak again. From the previous Visual obser-va-tionas thil

I ^eff, tion of rt in 2rassctour 5c i. a. 7 r s p e ~/2 0 i a,.I-. I e, i'r U.". atz

--..I...:..: Fig. g 8

- 47 M rapid motion of the cathode spot had given the irTression of a &heet of uniform width extending straight across the tunnel in a slantwise direction0 In these pictures the leading edge of the are colum has a distinct curvature and is somewhat brighter than the traiing portionso Under sa ae conditions ths curvature ceZe so great that a new cathode spot would form upstream and the arc would progress stepwise up the tunnel. Al1 of these observwetions of the high speed motion pictures only served to brinxg out the nomn- nom ature os the discharge under the present eieri mean conditions0

" 48 h VIII - ROD ELECTRODE TUN All of the experi ments in the contour electrode tunnel strongly indicated that some form of restricted electrode was necessary in order to anchor the arc in the nozzle exito In order to accomplish this, a new tunnel as constnrcted in which two copper posts served as the electrodeso The nozzle in this tunnel discharged directly into a large chmber forming an open jeto The electrode posts which were isolated electricall from the rest of the tunnel were located i.mmedtely downstream from -the nozzle exit, just at the edge of the supersonic Jet from the nozzleo Fig 9 is a sketch which illustrates the essential features of this design and Fig. 10 is a photograph of this tunnel vdth the top cover removedo. The settling chaar and forward portion of the nozzle are hidden under a lacoid slab in these figures (Laacoid is a phenolic laminate which was used for the structural mbers) Thin Teflon gaskets were used between the amcoid covers and the brass nozzle blockso The whole arrangement was clmed together very tightly by several se. is (hidden under the black wx in the photograph). This arrangement provided an excellent seal aroud the throat of the tmunel and elmnated the leak problems which had been encouni tered in the contour electrode tunmel designis AM 1~ T edge s szzle was again xwed, and with a Oo065 inach thro3at a Mach 4

r - --- -- - -- -~ — I I I %, T\ (D I \ 0 % w — all \ GY \.. - - -- -- " I B Brasss Co."tou rs Am(4 0ie~ jh J ' No zzht..A.rc., C h a Intocr F i~0 k - I a-.,o %% I 007 MV1 II. 1 A CjrJ~i~rp 3 J. Ir ft A.. r 1

~~ii: a:I:i::i:iiiiii~i:j:i:~:~:~:~. ',

flow with a I" x 10 cross section wa obtaked in the region o.' the electrodes0 Vycor or yrxglass liners were used as the. paa~lb wal of the tunnel over the last two inches of the nozzle and for the chamber containint the discharge electroes. These butted against the Teflon gaskets and formed a smooth continu&tion of the tunel wal Vhich could withstand the heat; of the disocharge-and provide vimmlobservation of the arc0 In order to obtana vacuma ight structure, Plexiglass and lMycalex cover plates war-e fitted over the chamer contaiig the glass linerso These wre cluped together outside 1,f the tunel in munch the same mn neor a s in Fig0 2P "O rn stock was embedded, in the lacoid mbrs as shown in Figap 9 andIO to provde agood seal with the covers., This arranernent, made it veryr esy to open the tinnelfo.r modifications and yet insued a vacuum-ight structur at all tine s0 For the photograph in Fig. 10s, the, top cover plate an d glass liner were r~emoved to show the interior of the tunnel more clearly,, The copper rod electrodes projected through holes drilled;, in the bottom glass liner and were located about, M inch do tnrrewm from the end of the nozzle blocks0 In Fig0 10 the large circla bases of the electrodes are easily visible, but, were located belo w the bottom glass liner. Al!so Visibles, through the glass., r short p~eces of rubber tubing which Iwovided electrical insuam~a. tion between the bases of "he electrodes0

Behavior of a d-e Arc in the "Rod Me ctrcdell Tunnel A &-a arc was maintained between the rod elcectrodes by the action of a magnetic field in the sme nner as in the contour electrode tunnel0 The are remied in the bounar layer region and clung very close to the glass linxers just as in the contour electrode t-amelo There appeaze to be evtn less tendency to spread out into the mrain flow; however, this may have been due to a thicker bounda7layerp Most interestingly of aUp the arc columna had the same slantwilse orientation across the airstrewan even thoughn this rc,~cjuired a considerably longer discharge path. The ext4ent to 14h,10h the arc went upstream depended upon the magnetic field, but the anode side was always farth,3r upstream than the cathode &wide e~ en though the twio electrodes wexre directly across fr~om each othero In prel.Iminary tests the contours were made entirely from brass.~ If the magnetic field or current were made, too strong,,, the dif-r charge would go from one electrode to the bottom. of the brass ontour just upstream from it,, 'then it would enross the tunnel to the other contour upstream under the lacoid cover plaUej and ~tini1-V go to the other electrode,t maig three separate arcs in a~ll, Ceramic inserts wrje-re later placed 'in -the downmstreami portions of the contours as seen in Fig0 10 in order to e ir-xnte this behavior0 uven Ath the ceramic insartstelvcwn fo h cathode post slantwise across the tunnel to a po:int o n the anodT

contour which was upstream from the anode post., and then it pa~sse along h~e surface of the contour to the anode post,0 bJith this pricular configuation the magnetic force drove the arc very tight~ly against the surface of t he anode contour; therefore, h insert had to bemade of a highly refractory material in order to withs ad this intense heating0 Zirconiu bonded s lcon car'bide, was used,9 but even this shoed some erosion after several second~i total rumnn tL. The insert on the cathode si&des on the others 4ad Could be made of almost any convenient dielectric since it wt~as not subjected to direct contact with the arc0 The, rod electrodes appeared to be very Clesircable in one respect, however0 Under some conditions a 1u do B cyclindrical sheath surrounded the cathocb post. This is characterintics ofa low density glow-type discharge in a strong magnetic field.. "Magnetic trapping" of electrons In a cylindrical cathode sheatL tends to replace the concentrated cathode spot at low enough densities. Further, when the discharge was observed to go to the brass contours, clearly visibl e arc tracks were formed on the flat ends of the contors,~ but no tracks cAnld be not iced on Irhle

- 44 W Several exeriments were made in an effort to force arc out of the boundary layer and into the center of the fLow. The most successful approach was to use short "horn elect~rodez" which projected into the strea as shown in Figr, 3110 In this arrangement the active portion of the electrode was restricted to the central on -hird of the tunnel midway botween the top and bottom0 Thus., the discharge path through the boundary layer was about three t-imes longer than the direct path across the tunnel between the horn tips0 With the horn electrodes in Fig.0. the discharge tend~d to fill much more of the tunnel.ta when the post electrodes of' Figs0. 9 and 10 were used0 With a power input ofC about two ki)LO. watts the discharge appeared roughly ais shown inl Fig, ll1. Ln some cases it was on the bttom of' the tunnel instead of' on th e. top, but 'it always avoided the center and al.ways skewed upstreamn on the anode side0 It was interesting to note, that -when the ~~cli went subsonic., the arc -Iimmediately localized on the tips of. the horns and went straight across the center of' -the tunnele Fi.-h.c~g in the subsonic situation, the arc as viewed froma the top head a 'IS" shape with the anode end again upstreama, The strong tendency for the arc to as&n a iqlatwise orientation across the tunnel is an inerentb ch-,actaristic of ths type of dischage. These experiments indicated quite conclusively tht ths elvior did not niecessarily depeand upon the processes at the anode or cathodes but wsa co sequence of th

Anoe T oeLoo View ~~P~~'rCKEga~i~pac^aa'iL i maT~t Giass Sa.eeves t> r Eacdtr* i$I 4nsulston Ei d View / u9e 1 X ~ 09 7 HIr n I t::a:0 r/or (9e P /i

momentum transfer mechanism in the arc column itselfo A very satisfactory explntion of the skewing tendency can be postulated, based upon the complex maobilities of ions and electrons in a magnetic field0 However, a worthwhile method for correcting this condition has not yet been worked outo

IX - PULSED DISCHARGES An entirely different approach was tried for obtaining an electrical dischae chge which ould ompletely fill the tunnel cross section~ It appeared reasonable that if a pulse of sufficiently high current were passed through the discharge it would at some point fill the whole flowo If this method were to produce approximately uniform heating the repetition rate of the pulses would have to be high eh ouh so that each volue element of the stream would be exposed to several pulseso The duration of each pulse would be very short in order to maintaSi a reasonable value of average power and still provide peak currents of a very high levelo Most types of electrical discharges have a tendency to produce relaxation oscillations when a capacitor is placed across the electrodes When relaxations occur, the arc current flows in very short pulses, and in between these the arc extinguishes until the voltage across the capacitor becomes high enough to break dowm the gap. Oscillograxa were made of the voltage across the contour electrode tunnel to investigate relaxation effectso With no capacitor arnd using the series resistance and inductance in the power supply, e loyed with most of the tests, the voltage was reasonably steady at a value between 350 and 500 volts, depending upon experimental conditionso "Noise" fluctuations of about 50

to 100 volts were superimposed upon this cd-c valueo As the amouint of capacitance across the tunnel was increased, the amplitude of these fluctuations became greater until with 0o05 o ifd of capacitance definite relaxations were present0 When relaxations occurred in the contour electrode tunno)., the arc was much brighter and hotter and had a stronger tearr dency to erode the glass walls0 As the value of capacitance was increased, the arc moved upstream where it constricted into an intense, fine channel0 Because of this behavior it was impossible to use a capacitor greater than Oa.l ~fdso The peak current under these conditions was about 100 amperes. The pulsed discharge technique was used more successf3y in the rod electrode tunnel2 shown in Figso 9 and 10. With the discharge colum anchored at the nozzle exits, rmch higher pulse currents could be used, In order to take full advantage of this characteristic,. an additional modification was ixntroduced into the circuit, An a ir-iry -blown spark gap was p._ In. l serios with- the capacitor and the tunnel electrodes0 This a dliarry gap could. be adjusted to break down at an desired voltage so that for a given value of capaciotance the macimum voltage and thusthe peak currents could be greatly increasedo The gap between the tLmnel electrodes broke downm at about 1,~000 volts; however, with the external gap it was possible to charge the capacitor to as high as 10 kv before discharging it across the h.airstream0

In all instances, the pulsed discharge was a very bright hot spark with an apparent diameter of between 116 and 1/ incho Various values of capacitance and voltage were used and the stored energy wa a a ig as raed as high as 15 wattseconds per pulseo In one case a Oo04 ifd capacitor was charge to 10 kvo The peak current was calculated from oscillogram of the voltage waveforms to be 1,500 ampereas Even with this extremely large value of currents the discharge ppr aared a an intensely bright channel about 1/h inch in diametero With the auxiliary spark gap the discharges were oscillatoryo (The stray lead inductance in the discharge circuit was about O07 pho) Thus, the magnetic field ad the e park columm move upstream on the first half cycle and downstream the next half cycleo With a large value of capacitance, the "ringing frequency" was lofp and the discharge appeared as a thin sheet extending an inch of so both upstream and dosnstreamo With a smaller capacitor, the ringing frequency was higher and the columns did not move as far during each half ycle*, and the discharge went straight across the tu elo, Just as with the magnetica ly stabilized d,e are the pulsed discharge occurred in the boundary layer next to the 1lass linerso A series of experints was. therefores conducted in which a pulsed discharge was initiated directly across the middle of the flowo This ws -accomplished ith small wire tips or hornso Two of these tips can be seen in Figo 10 projecting out into the

60 l. tunnel fron the middle of the electrode postso Another initiation arrangement consisted of a glass thread coated with Aquadag and stretched across the middle of the tunnel between the two electrodeso The sa pulsing techniques were used with these "initiation" devicesa and the same general behavior was observedo However, the discharge spread across the stream to a slightly greater extent than when it took place in the boundary layer0 The oT spark through the center of the tunnel also was less oscillatory$ indicating that there was greater daing here probaby as ater d the result of greater heat losso When the spacing between the wire tips shown in Figo 10 wasmore than /8 inch, the discharge took place through the boundary layer rather than across the supersonic flow between the tipso The spacing between the rod electrodes was 11, inches~ Based upon spark breakdown considerations, this evidence indicated that the gas denwity in the stream as about twice the mean boundary layer densityo In all of these eperimentas the ule etition rate was so low that each discha e occurred as an isolated pse That is, there was more than sufficient tiem between sparks for the residual ionisation to be blown downstream before the succeeding spask was initiatedo The calculated strean velocity was 2100 ft/seco Thus, a minium repetition rate of one every 10 to 20 h seco would be required even to approximate uniform heating0 The maXimUm repetition rate which could be obtained mith an ai lo ark gap required at

-~ 61 - least 100 to 200 seco betweiren piules in order for the gap to de ionise sufficiently for consistent fir~ngo If the repetition rate were made sufficien ly high some reidual ionization would remin in the flow from the previous breakdo- and this ould td tend to increase the diffusivity of the succeeding pulsee, The etiin e rmtepetion rate l tati of 'e air-blowm spark gap was not fully appreciated when these experiments were undertakeno In order to obtain the desired pulse an elaboraete set-up involving several hydrogen thyratrons and switching. circuits would have been necesary, and this appeared to be too extensive a program to undertake on the present contractO

62 X - ALTENATIVE AIR HEATING METHOiS The experiments described in this report were limited to a specific type of electrical discharge and to a relatively small range of experimental conditions, This method of air heating was particularly attractive because of its simplicityo Based upon the results of this investigation, it does not appear likely that a simple system can be devised which could readily be incorporated into an existing wind tunnel, It is probably more realistic to think of designing a tunnel which would be compatible with the requirements of the heating systemo Several alternative schemes are outlined below which are somewhat more complicated than the magnetically stabilized arc, but they offer the promise of overcoming some of the difficulties which have been encountered, and indicate a variety of possible approaches. Hig FreE ene Corona A corona discharge from small wires or points is much more diffuse than an arc column at the same pressureo The formation of space charge in a corona stremer tends to prevent it from growing too large. New streamers continually form giving rise to a region which is more or less uniformly filled idth a myriad of tiny discharge paths.

The rate of power diissipation (and heat gene-ration) of a d-~c corona is very low and is limited by the spaking voltage between the electrodes0 For &-c or 60 cycle power and a small wire in air, the mXwa corona Curr.ent ammits to a fraction of a rnilliazpere per inch of wire mid the power dissirpated is of the order of a few watts per cubic 'Inch. Howeverp when a higW..frequency voltage is8 used, the situation isB th~eoretically much more favorcable0 It has been reported' th up to a frequency of 100 kep the corona power is proportiona to (f +25), where f is the frequency of the applied field0 (1-1) This relertion m~ay laso hold at higher frequencies and the corona power could become quite largae uinder the proper conditionsC, In the presence of a supersonic airstreamp the propex-n ties of a corona discharge would be modified considerablya- If x-f power were usedV the streaamea s whiuch form duiring each half cycle would tend to be broken up by the airstream before the next half cy cle 5 and thus they would never grow to an cappreciable Aize Even if some streamers did bridge the gap betwee~n the, e' lectrodc-,ss the current probably would not grow to a significant v~,alue dur.I-ng one half cycle because of the high ci-rcuit reactance and~ the hirgh initi.al resistance of the breakdown streamra

C" 64ea Ina ordr to heat the whol tunnel cros section unifoml~ it pebably ould be ncesar to s polyphae m per and a nlutiplicity of corma ~ires meted -in such a 2menr that at any instant the discharge would tend to take place bea twoee wires ao opposite sides of the flo It ay be necessary to wis in the center. of the sairtreaml howeverg there is a possibility that by properly phasing the rxf pwer the us of mWam wires located along the tunel malls and parallel to the f m igh$t provie the requied miformity odf heatgo T re is considerable theoretical basis for predicting that the diffusivity of a disin an aitrem could be grat3y ee d y ne s of an ternal source of ioni g radiation such a a high intensity beam of beta particeso If the airstream is uniformly ionized by _u. a meanss it should be possible to pass an cf ourre.n throuh the gas nt generae heato With an exteral so e of ionization g usconduction could take place in relatively cool gas There would be no need for the high gas temporatura required to produce thesal ionization. The Usual type of high teeratre are eolm would ot be preet 3 the electron temperature ere kept low enogh that essentialy a of the ionizatioan as d the beta bem Under ach conditions, the distribution of r-f current density and heating would be deteriad by the distibution of the ionig radiationo

- 65 - Rough calculations as to the order of magnitude of the quantities involved indicate that such a process looks quite feasible An assumed ion density of 10s per ca3 would provid enough electrical conductivity to asaure an effective rate of heat generation0 For conditions comparable to those in the present eperimental work, the volume recombination rate is slow enough so that 90 microseconds are required for the ion density to drop from 10'2 to 10-1 ions per c3, This corresponds to about two to three inches of stream motiono A high intensity electron beam could be produced by n "opn o window" type of electro n gun suh as that used by Karlovitz at Westinghouseo It would be desirable to use a magnetic field to curl up the beta beam into a spiral inside the ind tunnel in order to increase its ionising effectivenesso A thorough evaluation of this scheme would involve a fairly elaborate research program0 However, if the principles are soundf it proably has greater potentialities than the other methods discussed in this section0 i pi "with an Ultr afast Movin Are Colm By the ctio acgt icon off a magntic field a d arc colmm be driven at very high thgh see ho h essentially cold airxo If one or more such arcs were made to traverse contiulously a section of a wind tVnnel at sufficiently rapid rates, it might be possible

to aq-roxite uniiform heating., provided each volum elewent of the flow were "seemred" many timso As ment1ioned in Sectio 11 of this report,% the gas is heated only a relatively small amount~ on each pass under these conditionso Radical ohanesp thereforep are not produced in the flow by a rapidly moving arc The diffueivi-ty of the discharge colm would not be an important factor, n thus this method could be used at alot aydesired gas density0 Experiments at atmospheric pressure have shown that a pulsed arc running along two parallel rod electrodes under the action of a magnetic field can be made to travel at 2000 feet per second. At lower pressures this speed could be increased con — siderably. High speed motion pictures have also been made of an aeat atmo peric pressure, revolving in the annulr pce between a central post and an outer ring electrode0 The arc appeardas a spiral Ispoke" and rotated at speeds uop to 800 rpsa At lower pressures andA higher currents, t,.ie velocity of thi-Ls spoke woul also be increased by a cons. erable amunt0 (2) There are maqpossible ways to mae an arc scan an airstream. For instance,, if a central electrode could betolerated in the flows a radial discharge could be used in a circularly symmetrical tunnel0 With ane axial magnetic fields, the arc colum would spiral out from the central electrode to the walls of. the tunnel., and revolve at very high speeds0 Other possible ar'rangements might use a transerse arc column scanning back and forth across

w 67 - the flow9 or a logi arc chee which would move thregh the wfl in a lateral fashion. This type of scan ing might be best hieed wth ah d- mgnetc field end an f arc current. The major problem encountered with scanning techniques is to obtain the desired uniformity of heating without creating excessive turb nce in the ai reo The amunt of turbulence which would be generated by a scanning arc and also the degree of turbulence which could be tolerated in a particular applica tion would have to be determinedo If these values were cc atiblep the sca mthod of air heating would have my desirable features A Hige En nw Mectrodele s Diecharge Under the proper conditionsp more diffuse heating could be obtained with a high frequency discharge than with a magneticaly stabilized d ar Previous xperience of project personnel has shown that an electrodeless discharge at 500 mc ad atmospheric pressure is not collimateds but tends to be ba apedo If an electrodeless discharge were wed, the wall of the tunnel would be made of a dielectric material9 and the discharge current would flow through the dielectric as a capacitance or displace6ent current If the capcitive rea tance of this dielectric wall were sufficiently high, the are could not concentrate at oe spot (a it does on a metallic wall) but would sprea out over all of the avaiable areao

W68S Further, the inductive reactance of a M l concentrated ac colmn in much higher than for a column of larger cross section, At microwave frequencies, this results in a "skin effect" which causes the current to flow near the surface of the ar colum and thereby tends to enarg the. dia ro Calclatio show that this effect would be very significant at 1000 megacycleso The use of a high frequency discharge appeared suffiv eifnty promising to warrant urther conaideration, and a earch was made of the literature on the subject0 It can be shown that the codeucs'ivity of an ionised gas at high fequencies is compla (both real and imana3y coponent are present(9) ) (10) A other things it is proportio^ l to (.) Jo)/(p + s2) where a is the electron collision isnqnyg and w is the frequeny of the applied electric field. Wm the colision frequey is high (at high pressres) or for low frequency fieldss >W and the condu&tivity is essentially the sam as in the d- taseo On the other hands for very low resures or high frequency fields < e and very little real poer is tranferred into the dischasrgeo In ordr to evaluate the r-f heating method sple cAculatow$ were made bas7d upon certain appraszmte waumptionso For conditins cotarable with those existing in the tunnels describd in this reports the collision frequency, is of the order of 1011^ If frequaenies of a reasonable value fro a

patical stanpo~ints r considered the electroswould eprience a getmWcofLi~s drigeach MUcycle adthe mcni for roucing ionization would be essentx ll thesmeas n, d- dichage Thleis plies that anobjeetio&ab3ky high tmeature amc colum wudbe necessaryT to swtain the dischagos Baed on this arguet,9 vry short wave —lmg microm pwrwould be requ d to obtana sit mation aprm ciabW~j different frmthe d-c ae A Ver Low Denasi Wind w l The evidence gained from the present investigation and fro previous work at the University of Michigan indicates that at gas densities corresponding to a static pressure of OJl mn of Hg, a magnetically stabilized &c.r arc would be sufficiently diffuse to prvide uniform heatingo In order to conduct tests at these low pressures., a fairly elaborate research program would be required0 A very low density wind tumel would have to be constructed, n a high speed vacuum system 'with enough capacity to provide continuous operating would be required since a "blow down"l type of tunnel wol not be feasible at these low densities* There would also be a variety of problems associated withr the operation and instr ntation of a wdtunne in the alipz-flow region0 (A suitable tunnel would be compaable to Tunel #2 at the University of Calif ornaat Berice za)

-q0 ma The problem of air afterglows would become much more severe at low densities For certain applications the presence of these excited molecules might not be obJectionableo However, their effect upon a spersonmc flow wold have to be evaluated before the usefuness of this method could be detemineio A_ e in Throsu An are colum can be maintained in a. statble fashion in an axial air flowo For certai processes in ch.al engineering a lotgit dinal are ia ued ia a gta eae= to supply large a n e ts of energy for endothermic reactions0 Also certa i n types of European coressed air ciuit breakers use air at sonic velity passing thrmugh a nozzle e rrounding t hie column In these applications, the are centers itself with respect to the air flow and constricts into a small dia r with a very high current density0 (1) In a similar mmner a longitudinal arc might be used in the throat of a wind tunnel to heat the air passing througho The clischarg e wouM canter itself in the flow in a stable fashiono It would undoubtedly be very intense right in the throat9 but as the flow capanded downstre a, the gas density would become considerably reduced, due parly to the higher maperature9 and the arc diaaeter would increase considerably0 One of the lmitation of this method would be the necessity of having an electrode in the center of the airstreamo

An electrical discharge coud be used to heat the air entering the throat of a conventioml wind tunmel9 At the present tie such a system does not offer anr advantage over dexting methods, If the severe problems of heat transfer in the thzrat can be solved and. f very high temperatures are desired, an electrical ezthod might ultimately be superior to present techniques0

PENDMIX I: TlIMROETIOAL WIND TUNNB4L POWER li RXEIJENDTS WITH SUPER-SO IC HEING 10 A preliminary analysis has been made of the effect of heat additon ai supersonic speeds on the power requiremens of a hypersonic wind tunnel. The analysis utilizes the onefl dimenstional flow equations and. numerical tables of Reference 7c In addition to the assumption of onemdimiensional flows it is assumed that the flow is inviscid9 with isentropic expansion between reservoir and test section except in the region of heat additono A schematic diagram of a hypersonic wind tunnel with supersonic heating Is shown in F igure 12,9 on wibioh also is illustrated the system of subscripts used0 2o The work done by an ideal compressor in compressing isentropocally a volume of gas V at the final diffuser eoit pressure Pf up to the initial upstream stagnaPt')-ion pressuer pol, is This ideal compression process will be uzed. as a standard for comparing the tunnel power requirements for various programs of heat addition0 The power for continuous operation of the widtunnel writh. supersonic heating, (exclusive of the electrical power used in actual heating of the air by the electric discharge.- 9I the ptheu -newr used to tn maintain nn

. p 4 96 - d - v 4z 0 "t -C a~ -c zc 0.U) t IY-" gj ~IC ci~ _aa- a 0-%E~ 1C Cs ''., 9~$ gp% Q a Ld v I c ^~i~~ u~ *0 -4 --0 4fl (n 0 0

magnetic field that may be required to control the discharge) is then ~A -,where m is the mass flow per second through the tunnels T02 is the stagnation temperature downstream of the region of supersonic heating, and R is the gas constant (al quantities are in the ftom1bo0msee0 vunits)o Now with m~~~3 v~ the horsepower is given by woe OOP ' F7 30 The comparison of. various types of supersoni1c h-eating is aided by use of the ratio of Reynolds number base on tPmel height to horsepower required for continuous, operation. The Reynolds number is defined as Where/istemsdeitvs the flow velocity, I ~s the chaacteristic length, and is the viscosity0 Lot the chaacteristGic length be the tunnel height, h9 and exopress the variation of /,with temperature by thwe empirica relation (for air) A ~0(L )0 7

Thaen~q The Reynolds number~mhorsepowe ratio can now be exres sed as wthere the test section is assumed squaea In order to determine how supersonic heating affe~cts the above ratios, two specific cases will be treated:o (1) heat added in a diverging portion of the nozzle at such a rate that the Mlach number remains oonstant, and (2) heat added in a constanyt area portion of the nozzle0 In both cases an isentropic ex"~ pansion occurs between the heating zone and the test sectiono ~o For constant M-ach number heating9 the alysis of Reference 7 gives f7o wihich integrates to (It is of interest to note that the increase of area in the region of heating is9 for. this case~ IT?.M

XThfe Reynolds numbershorsepower equation becomes The results for this case are plotted in Fige 13 as a that the diffuser pressure reeovery, 9 is twite that ZNBy uorder of the tables of Reference 7 the loss of stagnation o For econstant area heating RefereneQ 7 gives pressure in the heating zones an then the Reynolds number so, horsepower ratio, were caleb.ated for the se oonditions as the constant Mach number case The curowve for constant area heating is terminated at L1 = 4ol since for lower val ues of 1 the required temperat'ure ratio cannot be obtained before M2 becomes unity0 "

* 7?7 C; 60 Shown on?igLwe 13together with the above results is the value of 'RS for the case in which sufficient heat is added in the settling chamber (at M a 0) to bring the test section static temperature T3to o1000 R (representing approximtely the heating required to prevent condensations of air)0 It is seen by comparing this point to the point where 1 a 0 on the constant Iach number curve~ tlhat a large portion of the inc inease the power requirements with heating comes just from the fact that the test section stagnation temperature is increased The -increase in power required due to lotis in stagnation pressure is shown by the slope of the two curves in Figure 13o The loss in stagnation pressure increases with the Mach number at which the heat addition occurs It is proved in Reference 8 that the rise in stagnation pressure loss with Mach number is a general result independent of the programs of heat addition and/or area changeo

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- 79 - APPENDIX II (Copy) Low Presae Reseah Engineering Field Station 301 6Ith Street Richmond, California April 9, 1954 Dro Ho Co Ear3y Research Engineer University of Michigan Ann Arbor, Michigan Dear Dro Early: I am sorry I have postponed answering your letter for so longo. The reason for mry hesitation is the fear that what I have to say will not be of too much help to you, Your problem seems to be exactly the opposite of ourso We are dissatisfied because much of the energy put into he air stream goes to heat rather than potential. energya while you would like to suppress all forms of potential energy in order to get a iaxm yield of heato At the present time it is our belief that the principal long lasting componenets carrid bed by the activated low pressure gas streams are oxygen atoms in the case of air and nitrogen atoms in the case of relatively pure nitrogeno A powerful discharge may very well produce several percent dissociation~ All other constituents like ions, electrons and metastable molecules decay to rather low concentrations within a few millisecondso The nitrogen atoms produced in an air discharg ae a re rapid rmoved in reactions resulting in oxides of nitrogen, probably mostly nitric oxideo At higher pressures, perhaps above 100 mm Hg, the atomic oxygen tends to attach to oxygen molecules resulting in the well-known formation of ozoneo The nitric oxide formed is not stable in the presence of air, being easily oxidized to yield nitrogen peroxideo If atomic oxygen is present simultaneously with the nitric oxide the resulting oxidation is accompanied by the emission of a greenish glowo This characteristic light emission is utilized as a test for the presence of free oxygen atoms (c~fo Gaydon "The Spectroscopy of Combustion "), The rate at which the atomic constituents disappear depends very much on the gas pressure and the relative abundance of the various componentso Polyatomic impurities in general "quench" the activity rather rapidly; eogo water or organic vapors will reduce the duration or even completely inhibit the production of the

Saterglows, But wether this eans that most of the energy is transformed into heat rather than into some other fora of inert potential energy (endothermic reactions) is not really knowna It will certainly depend on the details of the processes involvedo Using merely a higher pressure to speed up the reactions, on the other hand, may not be aufficient since even at 50 mm Hg pressures the air afterglow lasts at least 100 milliseconds and the excess free oxygen probably outlives the glow several timeso In nitrogen the lifetimes are even longero What is actually needed is a catalyst which, for full energy recovery, returns the gas to its original chemical composition before the flow enters the nozzle throat. Unfortunately no such gaseous substance is known to me; you would have to institute your own search for it Partial recovery can, of course, be obtained easily if an adequate fraotion of impurity may be added and if sufficient time can elapse between excitation and expansion of the gaso The nearest thin tto a catalyst is an inert metal surfaceo If it is possible to insert some sort of a filter, consisting perhaps of a tube section filled with platinum shavings, practically all energy will be converted into heat0 The trouble here is, that primarily the temperature of the metal is raised so that the stream is heated again largely by heat transfer from the surfaceso In the last analysis, one wonders then whether it would not have been simpler to heat the metal parts directly by some electrical meanso This, of course, you will be qualified to judge better than Io I am sorry to have so few constructive suggestions~ Your problem is not an easy one I would be glad to help you if I couldo If there is anything else in which I could be of assistance feel free to let me knowo I wish you luck with your researcho Very sincerely, Wo Bo Kunkel Physicist WBKsmr

AP PENDIII THEORETICAL HEAT ADDITION IN A CONSTANT ARASECTION TO CHOKE FO Assume that heat is; added in a constant area section of a wind tunnel to a gas having constant specifc heat and compositiono For a "simnple To change" the relation between the t otal temperature and Mach number is given by: (7) / 2 M1 ~~g2 Y. a2 N12 1+ 'a 2 /2+~&1 If enough heat is added to bring the Mach number to uinitryy the flow will choke* Assume T,02 a 3000K, N *3 h,P 6', lah, N2 1*ls then T0,2 = 5150K. Thuss, a change in total temperature of 2150 Centigrade reduces the flow from Mach 4& to Mach Io If the mass flow is 10 gm/sec., and if Co= 0o214 cal/gm/deg, the energy in-. put to just choke the flow is 516 cal/sec or 2o16 kwo

VERY RECE? DEVELOPTMW The higWfrequency corona tyTe of discharge, discussed on page 62 has been briefly testeds, and the results look very enouraging. The corona was produced at at atmospheric pressure, using r4 poer from a I Iw 10 megacycle dielectric heating urmitt In the presence of a stream of air from a s0 psi air hoses the discharge was not constricted or blown away by the wind. Instead, the region between the electrodies bmras fil.led with yria.ds of closely packed hair-l~ike streamerst, The corona power dissipation at r-?f frequniicies was two or three orders of magnitude larger th3an in a previous test here do power was used. Further tests are planmed in the near future using a 5 kw, 400 ke induction heating oscillator which is availableo

aL 0 5 CO3 BIBLIOGIPHY (1) Cobine, Jo Doa "Gaseous Conductors," New YorkD McGraWill Book Coo Ineo 19.4E91 (2) Earlk y Ho Co and Dow, Wo Go, "Supersonic tind at Low Pressures Produced by Arc in Magnetic Field," The Phs l Review Vol 799 Noo 1, po 186, July 1 1950, (3) McBe~e, W Do and Dow, Wo G3a, "The Influence of a Transverse Magnetic Field on an Unconfined Glow Discharge9" Transactions of the American Institute of Electrical Egieers, July l953o (4) Earfly, Ho Cof Smitha H0 Lo, and Lue Daniel CO "IElectrical Wind Phenomena" Summay Report Noo 1 Project M989 Engineering Research Institute, University of Michs igan, Novo 1952o (5) Lord Rayleigh Procee Ro Soci (London) Vol 176 ppo 1A-16 1940 o (6) Benson, James M Jourl of J lied Ph s~sc V ol 23, P. 757S 1952 o (7) Shapiro, Ao Ho, Hawthorne, Wo Ro, and Edelman, Go Mos "The Mechanics and Therodynamics of Steady One-Dimensional Gas Flow," Journal of Alied Mechanics ol 1 Noo 4, Deco 1947o also: Shapiro, Ao He0 "*The Dyrmics and Thermodynamics of Co pressible Fluid 1Flow_" Vol IZ New York, The Rona.d Pres$ Coo. 1953o

ms^- ag.,e. (8) Wagener, Po. No0 h) NAVORD and Lob Ro Results II2 Report 2376, Kc s ~tOJL Hypersonic T umT l Diffuser Investigatiton" Marchp 1952o (9) (10) Margenaun Ho, at High Noo 10, "Conduction and Dispersion of Ioniaed Gases Frequencies, The P sicl Revi ew, Jolo 699 po 5038 Nay 1a5,.19h6o verhar s Eo:d BarO r S o C ~~he Admit tance of Iti.g Freusency Gas Disc.harges.,".Ph. Sei.ev V&o. 76. Noo 6, po 8399 S&pto 15 19490 Peskp F. Wo, Jr.,. ieectic TPhmena in Hig Voltge Ei r New 'fot, Me Graw HAIt B oo k o co 1929

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