The Liquid/Vapor Phase Boundary in a Porous Plug

Abstract

Porous plugs have been successfully used as phase separators for superfluid helium and its vapor in a number of space flight dewars. A rapid increase in the temperature and pressure drop across the porous plug has been seen above a critical flow rate by numerous experimenters. For this paper careful measurements of pressure and temperature differences as a function of flow rate and hydrostatic head have been made on two types of porous plugs. One porous plug is of sintered stainless steel, with larger pores but otherwise similar to that in the IRAS, IRT and COBE dewars. The other plug has pores of uniform cross-section within a glass matrix. The data were fit to a simple physical model which describes the pressure drop in terms of the thermomechanical pressure and the forces due to the hydrostatic head and surface tension. The results indicate that the liquid/vapor phase boundary exists at the outer surface of the porous plug for low flows or large hydrostatic heads. It retreats into the plug as the flow increases or the hydrostatic head decreases in excellent qualitative and good quantitative agreement with the simple model. The sintered stainless porous plug exhibits some hysteresis in the area near the phase boundary retreat. This hysteresis is probably explained by the difference in pore size near the surface of the plug.