Effect of non-condensable gas on the controllable loop thermosyphon

This study experimentally investigated the influence of non-condensable gas (NCG) on the performance of controllable loop thermosyphon (CLT) developed for refrigerators. The heat transfer of CLT can be intermittently stopped and started with a high frequency to control the heat transfer and loading temperature; thus, it has a wide range of potential applications, including the solar- and electric-powered cool-storage refrigerators. To evaluate the characteristics and start–stop performances for selected control modes, air was mixed in the working fluid R134a as NCG. The CLT with 4 different NCG ratios XNCG were tested under similar conditions. The more NCG exists in the CLT, the stronger deviation from the ideal working condition and the lower the heat transfer rate. Given inlet water-glycol mixture temperature Tg,in of -25 °C, the steady-state heat transfer rate of CLT decreases from 380.6 W to 304.2 W. While XNCG is less than 0.16%, the start-up time remains stable within 15-30 s, whereas it significantly increases with Tg,in when XNCG is higher than 0.31% because the heat transfer deterioration period appear. Particularly, start-up failures appear in both modes when XNCG and Tg,in are 0.47% and -17 °C and CLT could not automatically restore the normal heat transfer within 30 min. Besides, the stopping time generally decreases with increasing XNCG and Tg,in and remains lower than 100 s when the amount of NCG is less than 0.47%. Results demonstrate that a small quantity of NCG has a limited impact on the behavior of CLT, but its overall heat transfer could be low and its start-up performance will significantly reduce while the ratio of NCG reaches 0.47%.