F • CONVECTIVE HEAT TRANSFER IN GASES 



Now the temperature gradient at the wall may be steepened by either 

 increasing the mass flow external to a given boundary layer or inducing 

 transition from laminar to turbulent flow. At low speeds, the effect of 

 increasing the external flow is to thin the boundary layer by inertial force, 

 thereby steepening the temperature gradient. However, as the external 

 speed continues to increase, direct compression as at a stagnation point, 

 or dissipation of energy by internal friction, rapidly increases the temper- 

 ature of the fluid within the boundary layer, thereby also steepening the 

 wall temperature gradient; thus the effect of high speed is literally to 

 cover the wall with a layer of hot fluid, between which and the wall the 

 heat transfer then takes place. As a practical consequence, for example, 



Free stream 



■Ue 



y //y/yy 



Fig. F,2. Schematic of boundary layer in a compressible viscous fluid. 



increasing the external flow speed will first cool, and later heat, a surface 

 whose temperature is initially higher than the free stream temperature. 

 Transition from laminar to turbulent flow effectively thins out the inner 

 regions of the boundary layer by scouring action, thus greatly increasing 

 the temperature gradient and consequent heat transfer for low as well as 

 high speeds. 



Since, in high speed flow, the temperature of the boundary layer rises 

 because of compression or energy dissipation, it follows that for a given 

 speed there will be a certain wall temperature, above the free stream 

 temperature, at which no heat transfer will take place. It is proper to 

 consider the zero heat transfer temperature as the reference temperature. 

 Thus, at low speeds, the free stream temperature becomes the reference 

 temperature. When the wall is hotter than the reference temperature, 

 heat flows from the wall into the boundary layer, whereas, when the wall 

 is cooler than the reference temperature, the reverse is true (Fig. F,2). 



(340) 



