G • COOLING BY PROTECTIVE FLUID FILMS 



additional slots, because otherwise the wall temperature would eventually 

 approach the temperature of the hot gas. The insulating effect of liquid 

 film layer is better than the gas film layer due to the fact that the liquid 

 coolant is evaporated by the heat from the flowing hot gas. The heat re- 

 quired for evaporation keeps the temperatures of the liquid film and the 

 wall to the evaporation temperature, until a point downstream from the 

 slot is reached where the liquid is completely evaporated. 



In early work in Germany certain scientists employed the method of 

 film cooling in rocket motors by injecting a coolant through small holes 

 into the chamber and nozzle walls of a regeneratively cooled motor to 

 cool the predetermined "hot spots." A series of holes generally uniformly 

 spaced around the circumference of the wall has been employed if a com- 

 plete wall is needed to be film-cooled. A different injection method is to 

 introduce the coolant through inclined holes along the motor walls. This 

 method has the advantage over the injection through radial holes in that 

 it retains most of the coolant along the wall that is to be cooled, while 

 injection through radial holes cannot avoid discharging a portion of the 

 coolant directly into the main gas stream. Tangential holes which inject 

 the coolant around the circumference of the motor, thereby taking ad- 

 vantage of centrifugal force to hold the coolant on the wall, have also 

 been employed. 



An investigation of film cooling based on the injection of the coolant 

 through a slot around the circumference of the combustion chamber or 

 nozzle is presented in [45]. The advantages of the radial injection of the 

 coolant through a slot are: (1) the coolant flow rate may be accurately 

 predicted in any section of the motor, and (2) a uniform film can be 

 established at all sections of the motor. 



Film-cooling prohlems. The heat transfer process in film cooling is 

 essentially the same as the process in transpiration cooling. One of the 

 most important problems in film cooling is to determine the length of the 

 fluid film along the wall. This depends on the quantity of fluid injected 

 and the stability of the fluid film. Up to the present time there are neither 

 theoretical analyses nor experimental investigations which yield a definite 

 and general answer in regard to this important phenomenon. Hence it is 

 limited only to particular results which will be presented here. In Fig. 

 G,7a the wall temperature of a particular liquid-film-cooled tube is shown. 

 The liquid-film-cooled length is indicated between the points A and C. At 

 point C the wall temperature reaches the boiling temperature of the liquid 

 and beyond the point C the liquid is completely evaporated, while the 

 wall temperature increases rapidly to a value approaching the gas tem- 

 perature. If a gas coolant is used instead of a liquid the length of the film 

 would only exist for a short distance between B and C. This makes the 

 use of a gas coolant in film cooling inadequate since numerous slots would 

 be required. 



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