Gas phase and liquid phase separation is necessary for one of two reasons. First, system-critical components are designed to specifically operate in a single phase mode only. Pumps, especially centrifugal pumps, lose their prime when gas bubbles accumulate in the impellor housing. Turbines and compressors suffer from erosion problems when exposed to vapor laden with liquid droplets. The second reason is that system performance can be significantly enhanced by operating in a single phase mode. The condensation heat transfer coefficient can be enhanced when the liquid of an entering two-phase stream is stripped thus permitting initial direct contact of the vapor with the cold walls of the condenser. High efficiency and low mass Environmental Control and Life Support Systems invariably require multiphase processes. These systems consist of water filtration and purification via bioreactors that encounter two phase flow at the inlets from drainage streams associated with the humidity condensate, urine, food processing, and with ullage bubble effluent from storage tanks. Entrained gases in the liquid feed, could have deleterious effects on the performance of many of these systems by cavitating pumps and poisoning catalytic packed bed bioreactors. Phase separation is required in thermal management and power systems whereby it is necessary to have all vapor entering the turbine and all liquid exiting the condenser and entering the pump in order to obtain the highest reliability and performance of these systems. Power systems which utilize Proton Exchange Membrane Fuel Cells generate a humidified oxygen exit stream whereby the water vapor needs to be condensed and removed to insure reliable and efficient system operation. Gas-liquid separation can be achieved by a variety of means in low gravity. Several active and passive techniques are examined and evaluated. Ideally, a system that functions well in all gravity environments that the system experiences is a requirement

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