IT has been stated above that the rate of heat transfer is closely proportional to the temperature difference between the plate and the free air stream, and over the laminar portion it will also be proportional to the conductivity of the air. It remains to consider to what extent the actual temperature of the air in the boundary layer will influence the rate of heat transfer. The conductivity of air increases with temperature by reason of the increased molecular velocities, and we might expect, therefore, that the hotter the surface the greater will be the rate of heat transfer per unit of temperature difference above that of the air. This is, in fact, found to be the case.
Pye, D.R. (1933), "The Principles of Air Cooling: A Correlation of the Problem of Design with Researches on the Boundary Layer", Aircraft Engineering and Aerospace Technology, Vol. 5 No. 4, pp. 79-86. https://doi.org/10.1108/eb029671
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