G,2 • FLOW THROUGH POROUS METAL 



powder with a certain amount of ammonium bicarbonate, compacting the 

 mixture at 80,000 lb/in. 2, and sintering in an atmosphere of pure hydrogen 

 for 4 hours at 2300°F. The variation in porosity is from about 18 to 

 approximately 52 per cent, for a variation of ammonium bicarbonate from 

 to 15 per cent. According to the results of experiments [1], the main 

 variable factor is the amount of ammonium bicarbonate which controls 

 porosity. The tensile strength of the porous specimens varies from 45,000 

 to 8000 lb/in. 2, with the variation in porosity from about 17 to 54 per 

 cent. From the viewpoint of strength it is advisable to use the maximum 

 practical compacting pressure and to adjust the amount of ammonium 

 bicarbonate in order to produce the required porosity. 



Recently a sintered-wire porous metal known as Poroloy was devel- 

 oped by Wheeler and Duwez [2] at the California Institute of Technology. 

 The wound-wire porous metal is made by wrapping a very thin and nar- 

 row ribbon of flattened wire (composed of any sinterable metal) around a 

 mandrel of any arbitrary cross section. After the wire has been wound on 

 the mandrel to the desired depth, the mandrel and wire, as a unit, are 

 placed in a controlled-atmosphere furnace and then sintered. Following 

 sintering the mandrel is removed and the metallic shape is processed into 

 a finished form. The porosity is formed by the space between the indi- 

 vidual strands of wire, and with proper control the pores, of predeter- 

 mined size, are interconnected and uniformly distributed, and form a 

 predetermined passage for fluid flow through the metal. 



The advantages of Poroloy over ordinary sintered powder-porous 

 metals are a higher strength for a given permeability because of its wire 

 construction and a higher ductility because of the continuous strands of 

 fine wire which bear a large portion of applied loads. In sintered porous 

 metals, on the other hand, the entire load must be carried by the indi- 

 vidual sinter bonds between the particles of powder. Furthermore, Poroloy 

 can be made like plywood, a nonisotropic material having greatest 

 strength in the direction of the bisection of the acute crossing angle, and 

 the lowest strength at right angles to this direction. When the wire strands 

 cross at right angles the material exhibits a uniform strength in all 

 directions. 



Permeahility of porous metal. An important problem in the design of 

 transpiration-cooled parts is the study of the flow of fluids through porous 

 metals. In other words, the permeability of the metal of which the parts 

 are to be made must be known. The permeability of the metal expresses 

 the capacity of a porous material to pass fluids when pressure differences 

 exist. As a result of the complexity of the structure of porous metals, a 

 complete analytical study of the problem of predicting the permeability 

 is precluded. The following discussion is based on the experimental study 

 of the flow of gas through porous metals [3,4]. 



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