EVAPORATION OF SUPERFLUID-HELIUM IN A CAPILLARY

The steady-state evaporation of superfluid helium at a liquid-vapour i nterface near the exit of a cylindrical capillary is considered. In or der to maintain phase equilibrium, as the evaporation rate increases t he liquid-vapour interface will retreat into the capillary. Due to the very large density difference between the liquid and vapour, the retr eat of the interface into the capillary produces a sharp change in mas s flow. A simple capillary model has been applied to flow in porous pl ug phase separators by considering parallel capillaries of equal lengt h but with a distribution of diameters. In this case it is found that the change in mass flow is consistent with what has been observed for an increasing pressure drop across a porous plug. This suggests that t he equilibrium flow states occur as the liquid-vapour interface retrea ts into a porous plug in response to incremental increases in the pres sure drop across the plug.