SECTION G 



m I m 



COOLING BT PROTECTIVE FLUID FILMS 



S. W. YUAN 



G,l. Introduction. One of the most important current problems in 

 aeronautical engineering is concerned with the flow of high energy gases. 

 Such flow has been experienced in engines of the turbine or ramjet type, 

 rocket motors, and nuclear reactors which use gases with high temper- 

 atures but relatively low velocities to develop power and/or thrust. The 

 combustion chambers, turbine blades, and afterburners are examples of 

 components exposed to high temperature gases. Recently, much atten- 

 tion is being given to the problem of aerodynamic heating in high speed 

 flight, in which exterior surfaces of aircraft and missiles are exposed to 

 gases with low temperatures but high relative velocities. In steady flight 

 at Mach numbers of four or higher, such surfaces become heated to tem- 

 peratures at which the strength properties of the strongest-known alloys 

 deteriorate markedly. Moreover, the pilot and such critical cargo as 

 instruments and explosives must be protectively cooled. 



In rocket motors, where combustion temperatures of 4000 to 5000°F 

 are easily reached, cooling has been used for some time. The conventional 

 method of cooling rocket motors is to use one of the propellants as a 

 regenerative coolant which circulates in ducts around the motor and is 

 then injected into the combustion chamber. This method limits the choice 

 of many high energy propellant combinations such as the hydrogen- 

 oxygen and hydrogen-fluorine systems, because they do not possess the 

 desired physical properties for a satisfactory regenerative coolant. Fur- 

 thermore, the inherent disadvantage of this method is that it is difficult to 

 increase the heat transfer coefficient of motor wall-to-coolant to a value 

 much higher than those values which exist in motors of current design. 

 This is so because, in order to increase the liquid film coefficient, the 

 velocity of the coolant must be increased at the sacrifice of increasing 

 the pressure drop through the motor. Since the allowable value of pres- 

 sure drop in the cooling jacket of a jet motor is limited, the coolant 

 velocity and the hquid film coefficient are also hmited. 



The heat transmitted from the combustion gas to the chamber wall 

 can be considerably reduced by placing some thermally insulating mate- 

 rial on the hot gas surface of the wall. Such an insulation has been tried 

 in the form of ceramic coating, but the limited lifetime of refractory 



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