Comparative study of two-phase natural circulation and gas-side mechanically driven loop used in air-conditioning systems

Abstract

A two-phase natural loop (TPNL) has been widely used in air-conditioning systems for energy conservation. Under some extreme conditions, a liquid pump is usually added to be a two-phase liquid-side mechanically-driven loop (TPLMDL), but it is not always satisfactory because it enlarges the pressure drop along the gas line. Then, gas-side driving becomes an option, but the performance of the two-phase gas-side mechanically-driven loop (TPGMDL) is unclear, due to a lack of relevant investigations. In the study, an experimental setup including both TPNL and TPGMDL was established and measured comparatively in 5 typical cases. The results indicate that: (1) for a well-functioning TPNL approaching the “ideal cycle” (Zhang et al., 2018), the gas-side driving cannot further improve the performance by increasing flowrate, and the proposed “ideal cycle” for TPNL is the boundary of the enhancement; (2) when the TPNL deviates significantly from the “ideal cycle”, the gas-side driving can improve the performance noticeably to approach the “ideal cycle”, by eliminating overheating and flooding; (3) contrary to liquid-side driving, gas-side driving increases the condensing temperature and decreases the evaporating temperature, which improves the performance; (4) when the flow resistance in the liquid line is considerable, the capacity can exceed the “ideal cycle”, but the cycle becomes essentially a “heat pump” cycle (Tc > Te), rather than a “heat pipe” cycle (Tc ≤ Te) any more.