G,7 • FILM COOLING 



in practice than in theory and the values of measured wall temperatures 

 are correspondingly higher for a given coolant flow. 



It has been found in the experiments that the velocity of a stream of 

 gas passing through a transpiration-cooled tube increases rapidly along 

 the length of the tube. The rate of increase depends on the mass flow ratio 

 Q/W and the density of the gas. The main stream velocity used in the 

 theory is independent of the length in the direction of flow. 



Fundamentally, the influence of the rate of coolant flow to the shear- 

 ing stress at the wall, to the main stream velocity profiles, and to the lami- 

 nar sublayer thickness must be thoroughly investigated experimentally 

 before an accurate theory in transpiration cooling can be realized. 



Different Physical Properties Between Coolant and Hot Gas. 

 When the physical properties of the coolant gas differ from those of the 

 main stream gas, Eq. 4-31 can also be used to approximate the relation 

 between the wall temperature and the coolant flow. It can be seen from 

 Eq. 4-31 that the specific heat is the most important property, with a 

 secondary effect due to the Prandtl number. The theoretical results com- 

 puted for nitrogen and hydrogen coolants give a fair agreement with 

 experimental data. 



The results of the experimental investigation of transpiration cooling 

 by injecting water as a coolant indicate that there is a critical value in 

 the amount of coolant, above which the surface temperature of the porous 

 material remains near or below the boiling point of water, and below 

 which the surface temperature increases very rapidly with decreased flow. 

 The instability of water coolant flow may be explained by the evapo- 

 ration of water inside the porous metal just below the hot surface. A film 

 of vapor rather than a film of water was formed on the surface exposed to 

 heat. It is undoubtedly true that this phenomenon would occur in the 

 use of other kinds of liquid coolant. Unless some means of controlling the 

 evaporation of the liquid coolant inside the porous metal is developed, 

 a liquid coolant cannot be successfully used in transpiration cooling. 



G,7. Film Cooling and Its Comparison with Transpiration 

 Cooling. 



General description. It was mentioned in the introduction that film 

 cooling is a method of protecting a surface from a high temperature gas 

 stream by separating the surface and the hot gas stream with a thin con- 

 tinuous film of a liquid or gaseous coolant. The coolant is discharged by 

 slots or orifices to the surface where the hot gas is flowing and is carried 

 downstream by the flowing hot gas. In this way an insulating film is 

 formed along the surface ; however, the film is gradually destroyed on its 

 way downstream by turbulent mixing. The coolant film has to be renewed 

 at a certain distance downstream by injecting a new coolant through 



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