Experimental Study of the Heat Transfer Performance of a Two-Phase Closed Thermosyphon under Ultralow Ambient Temperature

Two-phase closed thermosyphon (TPCT) demonstrates excellent performance in chilling the stratum of permafrost regions. While existing studies on the TPCT have mainly concentrated on its heat transfer performance under moderate negative ambient temperature conditions, its performance at ultralow air temperature conditions is rarely reported. This research carried out an array of experiments at ultralow ambient temperature to explore the heat transfer performance of the horizontal and vertical TPCTs. The results indicate that for the horizontal TPCT (HTPCT), the heat transfer rate increases in direct proportion to the outer-wall temperature difference when it ranges from 0.16°C to 24.39°C as the condenser temperature drops from −2.41°C to −30.02°C. Therefore, the equivalent thermal resistance of the HTPCT remains constant at 0.377°C/W. However, for the vertical TPCT (VTPCT), a threshold temperature difference exists for the equivalent thermal resistance due to the difference in the working fluid flow pattern. The equivalent thermal resistance is 0.504°C/W when the temperature difference of outer-wall is below the threshold temperature difference. It decreases to be 0.266°C/W once the outer-wall temperature difference exceeds the threshold value. When the ambient temperature of the HTPCT condenser section drops below −5°C, the outer-wall temperature fluctuates significantly due to partial circulations of the working fluid. For the VTPCT, it shows an increasing trend. The study can provide new insights for the optimization of TPCT under ultralow ambient temperature conditions.