G,7 • FILM COOLING 



Tsien [46] has indicated that, when the friction drag of a gas passing 

 over a liquid film is of appreciable magnitude, the flow of a film coolant 

 on a solid surface can be compared to the flow of the liquid near the wall 

 of a pipe completely filled with a turbulent flowing liquid. Furthermore, 

 when the film is of sufficient thickness to include a turbulent layer of 

 coolant on the gaseous side of the film, instability occurs and some of 

 the coolant breaks away in the form of droplets. The criterion of the insta- 

 bility of the coolant film can be completely described by ?/*, a parameter 

 indicating flow conditions near the wall. The above phenomenon was also 

 interpreted by Rannie [47]. Knuth [45] has calculated some NACA experi- 

 mental results on film cooling by Sloop and Kinney [45] and found that a 

 definite value oiy* exists for the criterion of the instability of coolant film. 



Distance downstream of coolant 

 injection 



Fig. G,7a. Liquid-film-cooled length in smooth duct 

 (injection begins at A). (From [49].) 



An experimental study of the stability of the liquid film under various 

 flow conditions is given in [45]. In this experiment, visual observation was 

 made by injecting the liquid coolant through a slot into the test section 

 where the air was blown through. At very low flow rates of the coolant, 

 it entered the test section smoothly and flowed in a uniformly thin layer 

 along the surface of the test section downstream from the injection slot. 

 As the flow of liquid was increased, a point was reached where small air 

 bubbles were formed immediately downstream of the slot. The mean ve- 

 locity of the liquid in the slot corresponding to the condition where the 

 bubbles first started to form was taken as the critical velocity of injection. 

 If the liquid flow was increased beyond this critical velocity, the major 

 portion of it separated from the wall of the test section. 



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