ENGINEERING RESEARCH INSTITUTE UNIVERSITY OF MICHIGAN ANN ARBOR A FEW MEASUREMENTS OF THE TEMPERATURES ON UNHEATED SURFACES EXPOSED TO AIRPlANE ICING By H. HICKS and M. TRIBUS Project M992-D WRIGHT AIR DEVELOPMENT CENTER, U. S. AIR FORCE CONTRACT NO. AF 18(600)-51, E. O. NO. 462 Br-1 April, 1952

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN A FEW MEASUREMENTS OF THE TEMPERATURES ON UNtEATED SURFACES EXPOSED TO AIRPLANE ICING SUMMARY Six observations are reported on the heating of surfaces during ice formation. In each case good agreement with the predictions of Messingerl are found. INTRODUCTION During a visit to the summit of Mt. Washington, the authors performed a few experiments in which the temperatures of unheated surfaces exposed to icing were measured. The experiments were performed, for the most part, in a small 6-inch diameter wind tunnel designed, built, and Operated by the Navy personnel at Mt. Washington. More experiments were carried on than are reported here, but only in a few cases were we able to get reliable simultaneous meteorological data. In all icing runs, temperature increases were noted in the icing area except in very light icing. THE EXPERIMENTAL APPARATUS The measurements were made on a 15/16-inch diameter by 6 inches long cylinder of wood. A single iron-constantan thermocouple was imbedded

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN in the surface. The wires ran approximately 1/8 inch from the junction in the axial direction, then through holes in the surface to a hollow core, and thence to a Brown self-balancing potentiometer. The accuracy of reading was determined to be 1~F at 32'F by comparison with a Weather Bureau calibrated thermometer. The wooden cylinder was inserted radially into the 6-inch diameter wind tunnel. The tunnel was powered by an air ejector installed downstream of the test section.. Since the air was taken from an air compressor with a very small receiver, there was considerable variation in pressure and the air velocity in the tunnel did not remain steady. The results are presented in Table I. A typical calculation is given for run 15. Run 15. Liquid water content 0.26 grams/meter3 Drop size 10.6 microns in diameter Drop size distribution "A" Barometric pressure 798 millibar Velocity head in tunnel 13 inches alcohol (SG.= 0.827) Air velocity 231 ft/sec. Drop Reynolds modulus 45 Scale modulus 21.8 K 2.06 981 3o (from Langmuir-Blodgett, Ref.2) 0.565 EM (from Langmuir-Blodgett) 0.405 GM (from Langmuir-Blodgett) 54~ Rate -of water catch 7.6 lbs/(hr)(ft)2 at stagnation point

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Unit thermal conductance 56.9 BTU/(hr)(ft)2(OF) at stagnation point (Ref. 3) "b" (Messinger, Ref. 1) 0.134 Predicted surface temperature 22~F Measured surface tamperature 20~F Note: The ice accretion appeared as shown in Fig. 1. The water drops were concentrated near the tunnel centerline due to tunnel inlet effects. In addition to the tests described above, an attempt was made to measure the temperature of the interface during icing as a function of time, other conditions being fixed. The basic apparatus used consisted of a 1-inch diameter glass cylinder with a fine wire resistance thermometer attached along the stagnation line as in Fig. 2. TUA/IEL M WTI/AVE/. W4 LL AI FLOW I 1 —----— ~~~~ 3" —----—;~ —-I Fig. 1 Typical ice formation on 1-inch diameter cylinder in Navy 6-inch icing tunnel; 140 mph, 10.66 drop size, 0.26 g/m3. Suitable bridge and amplifier circuits were devised for measuring the temperature effects on the resistance wire. This arrangement had a temperature sensitivity of about 0.1~F and because of the small mass and proper location of the sensing element, the temperature of the interface could be followed closely. Unfortunately, the natural icing conditions were quite erratic and unreliable during the visit to Mt. Washington, and no adequate test could be given this latter apparatus. Several runs were made, however, 5

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN |'[: 1 ~1-inch diameter ______ thin-walled glass test tube 50 gauge nickel wire glued to stagnation line.. and covered with thin layer of "Krylon" Fig. 2. Second aparatus and in a qualitative way, the results are in agreement with what was expected. Additional runs will be made in order toprovide a better evaluation of the equipment, The experiments emphasized the desirability of using a recording instrument rather than an indicating instrument f6r the temperature measurement. The change of temperature isa quite rapid on exposure to icing, and it is difficult to take good data frOm an indicating instrument. CONCLUSIONS ore careful experiments are needed but these data do show that the effects of heat of fusion as described by Messinger are operative even through an ice layer 0.19 inch thick. — 4

TABLE I h Run Exposure U Btu Ru EM No. time,sec. t/. (Hr) ft)2(~F) comp. comp. comp. meas. meas. comp. 14 97 12 231 56.9 45 21.8 0.405 15 95 12 231 56.9 45 21.8 0.405 16 159 12 217 55.2 42 21.8 0.405 17 211 12 231 45 21.8 0.4o5 18 389 12 222 43 21.8 0.405 19 320 12 203 51.4 39.5 21.8 0.4o RW Run PO lb Ib ts ts W* oW* | I n in. n No. comp. (hr)(ft)21 P comp meas. comp. meas. comp. meas. comp. meas. _. comp. 14 0.565 7.6 0.134 22 20 15 0.565 7.6 0.134 22 21.5 1.27 0.915 16 0.565 7.14 0.129 21.8 18 2.00 1.06 54 70 0.080 0.0625 17 0.565 0 0 12 12 2.84 1.47 54 56 0.107 0.109 18 0.565 0 0 12 12.5 0.197 0.187 19 0.560 2.94 0.0574 16.7 20 3.80 2.29 See following page for explanation of symbols. 5

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Y = total temperature of stream R = rate of water catch at stagnation point U = tunnel air speed b = RwCp/h,dimensionless h = unit thermal conductance at (Cp = i for water) cylinder stagnation point ts= surface temperature ~F Ru= droplet Reynolds modulus W = weight of ice collected, / = scale modulus grams EM= percentage catch 9M= angle from stagnation point to farthest impingement | = "concentration" at stagnation point = ice thickness at stagnation point * The centrifuging effect of the bellmouth served to cause an ice formation such as show n in Fig. 1. Runs 14, 15, 16 had thermocouple at stagnation point Run 17 had thermocouple at 560 from stagnation Run 18 had thermocouple at 180~ from stagnation Run 19 had thermocouple at 300 from stagnation I1-

PEFEPENCES 1. Messinger, B. L.,^ tEquilibrium Temperature of an Unheated Icing Surface as a Function of Airspeed", I.A.S. paper S.M.F. Fund (IAS Preprint 342). 2. Langmuir, I., and Blodgett, K., "A Mathematical Investigation of Water Droplet Trajectories" AAF Tech. Rept. 5418, Feb. 1946. 3. Giedt, W. H,, "Effect of Turbulence Level of Incident Air Stream on Local Heat Transfer and Skin Friction on a Cylinder" Jour. Aero. Sci., Nov. 1951, p. 725. 7

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