SECTION F 



CONVECTIVE HEAT TRANSFER IN GASES 



E. R. VAN DRIEST 



F,l. Introduction. Any discussion of convective heat transfer in 

 gases is essentially a discussion of the characteristics of the boundary 

 layer in a compressible real fluid subjected to arbitrary wall temperature. 

 Since the density, viscosity, thermal capacity, and thermal conductivity 

 of gases vary considerably with the temperature in the boundary layer, 

 they thereby affect the rate at which heat may be transferred to or from 

 the adjacent surface. The state of the boundary layer is important; it 

 may be laminar, turbulent, or mixed, depending upon the Reynolds num- 

 ber, the Mach number, and the wall-to-free stream temperature ratio. 

 At high temperatures the gas may dissociate, or even ionize, the result 

 of which would be a change in the physical properties of the gas. Owing 

 to the variety of problems brought about by high temperature, a careful 

 analysis of heat transfer is paramount in the successful design of high 

 speed aircraft. Performance-wise, a knowledge of the temperature of the 

 skin of a high speed vehicle is necessary for accurate calculation of skin 

 friction. 



F,2. The Mechanism of Convective Heat Transfer. Convective 

 heat transfer is heat transfer to or from a flowing fluid. The region of the 

 flowing fluid which absorbs or gives up the heat is the boundary layer. 

 Strictly speaking, the heat transfer to or from the boundary layer takes 

 place by molecular conduction at the wall, whether the flow is laminar 

 or turbulent; thus the transfer of heat from a wall to a flowing fluid, or 

 vice versa, is the product of the thermal conductivity and temperature 

 gradient in the fluid at the surface of contact of fluid and wall. Since, 

 with fixed wall temperature, the heat transfer at the wall is proportional to 

 the temperature gradient in the fluid at the wall, any means of increasing 

 that gradient will increase the rate of heat transfer; and, of course, de- 

 creasing the temperature gradient will decrease the heat transfer. 



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