NACA-TR-1020

Measurements of average heat-transfer and friction coefficients for subsonic flow of air in smooth tubes at high surface and fluid temperatures

Year: 1951

Abstract: An investigation of forced-convection heat transfer and associated pressure drops was conducted with air flowing through smooth tubes for an over-all range of surface temperature from 535° to 3050° R, inlet-air temperatures from 535° to 1500° R, Reynolds number up to 500,000, exit Mach number up to 1, heat flux up 150,000 Btu per hour per square foot, length-diameter ratio from 30 to 120, and three entrance configurations. Most of the data are for heat addition to the air; a few results are included for cooling of the air. The over-all range of surface-to-air temperature ratio was from 0.46 to 3.5.
Correlation of the measured average heat-transfer and friction coefficients with heat addition by conventional methods wherein the physical properties of the air were evaluated at the average air temperature resulted in considerable decreases in both the Nusselt number and the friction coefficient at constant Reynolds number in the turbulent region, as the ratio of surface temperature to air temperature was increased. The effect of surface-to-air temperature ratio was eliminated by evaluating the physical properties of air, including density, in the Reynolds number, the Nusselt number, and the friction coefficient at a temperature higher than the average air temperature.
Correlation of the heat-transfer coefficients was also affected by increases in the average air temperature, produced by increasing the inlet-air temperature, which resulted in a decrease in the Nusselt number. This effect of temperature level was reduced by using the smallest variation of thermal conductivity with temperature available in the literature and was eliminated when the thermal conductivity was arbitrarily assumed to vary as the square root of temperature.
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