Liquid Flooding From an Evaporator Upon Compressor Start-up in Microgravity

Vapor compression cycles are not an established technology for microgravity applications yet. One reason for the low technology readiness level in microgravity is the concern of liquid flooding from the evaporator, which could reduce the lifetime of the compressor. Liquid refrigerant can accumulate in the evaporator while the cycle is turned off and slug into the suction line upon the next compressor start-up. This paper describes experiments on parabolic flights, which investigated this issue. A test stand was designed to observe liquid flooding from the evaporator upon compressor start-up. The time from compressor start-up until flooding first occurred and the duration of flooding were recorded and used as quantifiers. They showed that the occurrence of liquid flooding is mainly a function of the refrigerant mass in the evaporator before start-up and that there is a mass threshold below which flooding does not occur. Test were conducted with an off the shelf and one similarly sized transparent evaporator. The hypothesis that the threshold would be significantly smaller in microgravity was not confirmed with the data collected on the parabolic flights. Instead, flooding typically occurred at the same charge level for both evaporators and both testing environments. Flow visualizations showed that mostly stratified flow during start-up in ground testing became mostly annular flow in microgravity. Both flow regimes keep liquid close to the wall so that vapor generated during the start-up process can escape through a void tunnel.