G,5 • TRANSPIRATION-COOLED PIPE FLOW 



One of the essential parameters in the present investigation is the skin 

 friction at the wall. In Poiseuille's flow the skin friction coefficient at the 

 wall Cf has a constant value of ^/Re. The wall friction coefficient as calcu- 

 lated from this analysis indicates that the effect of injection in a pipe flow 

 is to increase the wall friction coefficient. In a boundary layer flow on a 



0.008 



0.007 



0.006 



0.005 



Cf 0.004 



0.003 



0.002 



0.001 



0.002 



0.004 0.006 



Vw/Ui 



0.008 



Fig. G,5c. Variation of local wall frictional coefficient 

 with fluid injection. (From \S1\.) 



porous flat plate the effect of fluid injection at the wall is to increase the 

 thickness of the boundary layer and to decrease the velocity gradient at 

 the wall. Hence the wall friction decreases in this case. On the other hand, 

 in a pipe flow the effect of fluid injection at the wall is to accelerate the 

 main stream velocity. Hence the velocity gradient at the wall, which 

 determines the wall friction, increases. For a fluid injection ratio Vw/^i = 



( 465 > 



