G • COOLING BY PROTECTIVE FLUID FILMS 



the tube. For a given porous material and a given coolant, all the measure- 

 ments can be reduced to the ratio (Tg — T^)/{Ty, — To) as a function of 



Q/w. 



A comparison of the experimental data with the theoretical results of 

 [27] is given in Fig. G,6a in which (Tg - T^)/{T^ - To) is plotted against 

 Q/W. For a given porous material the measured values of {Tg — T^)/ 

 (Tg — To) and Q/W obtained within the range of temperatures and 

 velocities covered in the experiments fall more or less on a single curve 

 having the shape of the theoretical curves. But the theoretical curves 

 indicate there is small dependency on the Reynolds number. This can be 



Fig. G,6a. Comparison of theoretical and experimental results on 

 temperature difference ratio vs. mass flow ratio. (From [27].) 



seen in Fig. G,6a. Since the theoretical analysis does not include the con- 

 ductivity of the porous material, the result shows a fair agreement with 

 the measured values obtained with a porous ceramic specimen and the 

 poorest agreement with a porous copper specimen. 



Perhaps it should be mentioned that a very good agreement has been 

 obtained between the experimental results of a stainless steel specimen 

 and the theoretical curve computed from the laminar boundary layer 

 theory by Yuan [7] for a Reynolds number of 3 X 10*. This is given in 

 Fig. G,6b. Although the agreement is for a particular case, it is interesting 

 to note that qualitatively there is very good agreement between theory 

 and experiment in transpiration cooling. 



An experimental investigation of the isothermal laminar boundary 

 layer on a porous flat plate (injection begins at a distance parameter 



<476) 



