G,l • INTRODUCTION 



materials so far developed for chamber wall coatings has made it im- 

 practical for rocket motors. Furthermore, there appears to be a limit to 

 the improvement in materials. It is generally accepted that, ultimately, 

 methods^ of cooling exposed surfaces must be used. A method of coating 

 the chamber wall with a layer of fluid would insulate the wall better than 

 a ceramic coating because of the much lower thermal conductivity of fluid. 



A promising means for controlling the heat flow to the wall of a rocket 

 motor (first used by Germans in the V-2) is the technique of introducing 

 small quantities of liquid at many points, distributed uniformly over the 

 interior surface of the combustion chamber. The liquid so introduced is 

 spread over the chamber wall in a thin film and eventually evaporates. 

 The essential advantage of this film-cooling method is that the screening 

 film of coolant fluid is permitted to vaporize, thus increasing its heat- 

 absorbing capacity many times over that of a system in which the fluid 

 remains in the liquid phase. It has a further advantage in that the fluid 

 will form a heat-resistant layer which separates the hot gases from the 

 chamber wall surface and in this way diminishes the heat transfer rate 

 from the hot gases to the wall. 



A logical extension of the film-cooling process is to increase the num- 

 ber of cooling orifices infinitely, i.e. to use a porous wall. The combustion 

 chamber walls to be cooled can be made porous by powder metallurgy or 

 the Poroloy process. A coolant in the form of a gas or liquid can be forced 

 through the pores. Such a technique is often referred to as sweat or tran- 

 spiration cooling. As the fluid passes through the porous wall in a direction 

 opposite to the heat flow, heat will be transmitted from the wall to the 

 fluid, the fluid forming a protective layer on the surface exposed to the 

 hot gases similar to the case of film cooling. In the method of film cooling, 

 the fluid film is gradually destroyed by turbulent mixing with the hot 

 gases so that the effectiveness of the film decreases in the downstream 

 direction from the point of injection. This disadvantage is eliminated in 

 the transpiration-cooling method where the coolant is continuously in- 

 jected along the entire chamber wall. In addition to this advantage, the 

 method of transpiration cooling provides much greater surface area for 

 heat transfer. It can be seen that the coolant absorbs heat as soon as it 

 enters the abundant region of the porous wall. Because of the great sur- 

 face area available for heat transfer, the method of transpiration cooling 

 is particularly desirable when nuclear energy is used as the power source 

 for rocket and jet motors. 



The purpose of this section is to present a critical review of the funda- 

 mental aspects of cooling by protective fluid films. It must be realized 

 that neither the theoretical nor the experimental aspects of this subject 

 have been sufficiently developed to permit a logical presentation, starting 

 from a basic assumption and progressing to the solution for engineering 

 applications. Instead, it is found necessary to review the progress of this 



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