Tests to determine the effect of heat on the pressure drop through radiator tubes were made to establish the adequacy of previously developed theory. Tubes of various length-diameter ratios typical of .those in current practice were tested through a wide range of heat-input rate. The tube entrance Mach number was varied from 0.12 to the value for choking at which sonic velocity was attained at the tube exits. At usual radiator operating temperatures and Mach numbers the addition of heat produced large increases in the pressure drops required to induce given flows of cooling air through the tubes. These Increments in pressure drop were in good agreement with increments obtained from theoretical calculations of the heating effect. For very high tube temperatures corresponding, to high rates of heat input the experimental pressure-drop increments for tubes of large length-diameter ratios exceeded the theoretical values by an appreciable amount in the vicinity of the choking Mach numbers. The experimental choking Mach numbers, however, agreed with the values predicted by theory for all tubes tested.
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