[en] It has been demonstrated that phase separation in helium-2 space cooling systems can be achieved by active phase separators with a micron-sized flow channel of adjustable length and well-defined geometry. This paper describes an experiment aimed at understanding the helium flow characteristics and the cooling mechanism in such phase separators. It is found that the adjustability of the gap length provides greater sensitivity, and a more extended range of variability in the mass flow rate than can be achieved by variation of the pressure drop alone. Direct feedback of the gap length on the bath temperature is of special importance at low temperatures and correspondingly low values of pressure drop. Highly sensitive open-close operation is attainable by variation of the gap length without the requirement of additional heat input