G • COOLING BY PROTECTIVE FLUID FILMS 



demanding subject made along several independent lines of attack which 

 may serve as basic references for further research and exploration. Since 

 the basic theories on heat transfer and fluid dynamics problems are 

 treated at length in other sections of this volume, only the application 

 of such theories to heat transfer in transpiration cooling is discussed in 

 detail in the present section. 



G,2. Flow through Porous Metal. 



Porous metal. The porosity of a specimen may be defined by 



Porosity (per cent) 



_ specific gravity of the alloy — specific gravity of the specimen 

 ~ specific gravity of the alloy 



The specific gravity of the specimen can be determined by weighing and 

 measuring the specimen after sintering. An ideal porous medium is a 

 medium which is composed of innumerable voids of varying sizes and 

 shapes termed pore spaces. Pores are interconnected to one another by 

 constricted channels through which the contained fluid may flow under 

 the influence of a driving pressure. A clear way to comprehend the porous 

 medium is to visualize a body of ordinary unconsolidated sand. 



Porous metals can be produced by the powder metallurgy process. 

 A method adopted by German scientists is to sinter the metal powder 

 in a refractory container without any previous compacting pressure. The 

 advantage of this method is that porous parts having complicated shapes 

 which would be difficult to press in dies can be produced. However, the 

 porosity of the finished product is very difficult to control due to the fact 

 that only one variable, namely the particle size of the powder, seems to 

 have a great influence on the porosity after sintering. 



An alternative method of preparing porous metals was developed by 

 Duwez [1] at the Jet Propulsion Laboratory of the California Institute 

 of Technology early in 1945. This method consists of mixing the metal 

 powder with a certain amount of porosity-producing agent which is com- 

 pacted at high pressure and then sintered at a high temperature. The 

 formation of pores in the compact, interconnected by constricted chan- 

 nels, is due to the fact that the porosity-producing agent decomposes into 

 a gaseous state and must escape through the grain of the metal powder. 

 Because of the shrinkage of the compact during sintering, some of these 

 channels may close. However, a sufficient number of channels remain 

 open to make the metal permeable. 



The technical details of the methods of preparation of porous metal 

 have been described in [1]. As an illustration, the technique used for pre- 

 paring porous stainless steel by the use of ammonium bicarbonate may be 

 briefly reviewed. The porous specimens were prepared by mixing the metal 



(430 ) 



