Experimental study of the startup and heat-transfer performance of a novel two-phase closed thermosyphon with an internal phase-separation structure

Two-phase closed thermosyphons (TPCTs) are widely used as active cooling devices in permafrost engineering. Countercurrent transport of rising gas and returning liquid through a shared flow channel can impede working-medium circulation and constrain heat-transfer performance. This study developed a novel TPCT with an internal phase-separation structure comprising a liquid-guide tube in the evaporator section and a gas-guide tube in the condenser section. Comparative experiments evaluated the startup and steady-state performance of the novel TPCT and an otherwise identical conventional TPCT under constant heat inputs of 50 and 120 W and constant temperature differences of 3∼17 °C at inclination angles of 0∼90°. With the startup threshold defined as a mean evaporator heat-flux density of 1 W m −2 , the required wall-temperature differences were 0.67 and 1.09 °C for the novel and conventional TPCTs, respectively. At a heat input of 120 W, the novel TPCT exhibited a more uniform evaporator wall-temperature distribution, with a spatial wall-temperature difference of 0.87 °C compared with 2.37 °C for the conventional TPCT. The equivalent heat-transfer coefficient increased by up to 19.9%, and the total thermal resistance decreased by up to 16.1%. Under constant temperature difference conditions, the novel TPCT maintained a higher effective heat-transfer rate throughout the tested range. Both TPCTs achieved their best heat-transfer performance at an inclination angle of 30°. The results show that the internal phase-separation structure enables startup at smaller wall-temperature differences and improves evaporator temperature uniformity and steady-state heat transfer without auxiliary energy input. The observed improvements indicate the potential for more uniform thermal regulation in permafrost engineering.