Thermal Performance Of Two-Phase Closed-Loop Thermosyphon With Liquid Heat Transfer Method

In this paper, it is the first time to introduce an experimental investigation on the thermal performance of a two-phase closed thermosyphon loop with a liquid heat transfer agent (TPCTLL), in which the liquid flow is accelerated by vapor bubbles that are continuously generated at the heated wall. TPCTLLs have shown interesting heat transfer characteristics, though it has not attracted very much attention from re-searchers. In order to understand the heat transfer method and thermal performance of TPCTLLs, repetitive experiments have been conducted on the designed closed thermosyphon loop with different filling ratios.First, the thermal performance of TPCTLL with filling ratio of 95% was studied and compared with that of single-phase closed thermosyphon loop (SPCTL). The ex-perimental results indicated three types of heat transfer phenomena, including natural convection around heating section at low heating power, quasi-steady-state flow with periodic fluctuations at medium heating power and stable flow with consecutively generated vapor bubbles at high heating power. In addition, the stable and outstanding operation of the TPCTLL appears at high input heat which exceeds the heat transfer limitation of the SPCTL with the same structure and working fluid, and the heat transfer coefficient of the TPCTLL becomes higher than that of the SPCTLIn order to improve the understanding of the heat transfer mechanism in TPCTLLs, a visualization experiment has been conducted to investigate the hydrody-namic flow in two loops with filling ratios of 90% and 95%, respectively. In this ex-periment, the bubble growth at the startup stage and the slug flow patterns at the steady stage have been observed by a digital high-speed camera. It is shown that the first generated bubble grew to the maximum size within 35 ms in tested loops; and the bubble rising velocity in the loop with the fill ratio of 90% was 0.35±0.05 m/s. In comparison with that of SPCTLs, the liquid circulation in TPCTLLs is accelerated by vapor bubbles. Thus TPCTLLs can achieve a better thermal performance than SPCTLs.Then, experiments have been carried out to investigate the thermal performance of a 6 mm-diameter TPCTLL at different filling ratio increasing from 0.60 to 0.95 and different heat flux increasing from 39.7 W/cm2 to 158.7 W/cm2. The experimental re-sults show that the thermal resistance of the TPCTLL is at the order of magnitude of 10-2℃/W, and the best filling ratio which provides the lowest thermal resistance var-ies with the heat load. In addition, the thermal performance of the TPCTLL is com-pared with that of the two-phase closed thermosyphon loop with vapor heat transfer agent (TPCTLV), which demonstrates that TPCTLL with lower thermal resistance may operate better than TPCTLV at high heat-flux.Based on the experimental results, a theoretical model was built to analyze the relationship between the heat transfer capability of the TPCTLL and the filling ratio. In comparison of the experimental results and theoretical results, it is found that the two results agree well only at low heating power. The influence of latent heat transfer through bubble generation at heating section is underestimated at high heating power. Thus, more experiments are required to be carried out to investigate the bubble gener-ation and distribution in TPCTLL at different filling ratios and heating power.Due to the large difference between the sensible heat and the latent heat of working fluid, the overall heat transfer capability of a TPCTLL is higher than that of a SPCTL and lower than that of a TPCTLV. Nevertheless, the heat transfer between the heating wall and the working fluid in a TPCTLL is realized by convective boiling, which can achieve higher heat transfer coefficient and critical heat-flux than the pool boiling method in a TPCTLV. Therefore, TPCTLV s are more applicable at heat ex-change conditions with high heat-flux and small heating-area.