Effect of heat leakage on the operational characteristics of two-phase loop thermosyphon with a thermal insulation pipe

Abstract

In this paper, a two-phase loop thermosyphon (TPLT) with a hollow polytetrafluoroethylene (PTFE) thermal insulation pipe (TIP) installed below the evaporator section was fabricated to experimentally investigate the effect of heat leakage on the TPLT performance. The startup and heat transfer characteristics of TPLT were compared and evaluated using HFE-7100 as a working fluid at volumetric filling ratios of 35–55%. Two startup modes, with and without temperature/pressure overshoot, were evidenced for the TPLT, and they were heat load dependent. The startup time of TPLT increased with the filling ratio and decreased with the power input. Owing to the hot-cold fluid interaction and mixing at the lower part of the evaporator characterized by larger temperature differences for them, it caused the evaporator temperature fluctuations, and the temperature oscillation amplitudes aggravated with the increase of filling ratio at lower power inputs. The utilization of a TIP into the TPLT increased both the thermal efficiency and thermal performance at filling ratios of 35 and 45%, while opposite results were obtained at the filling ratio of 55%. At the filling ratio of 35%, a highest thermal efficiency of 87% was achieved for the TPLT with a TIP at the power input of 90 W, which was 7.8% higher than that without a TIP. Accordingly, a maximum effective thermal conductivity of 1.5 × 105 W/(m·K) was obtained and 8.2% higher than that without a TIP. The addition of TIP lowered the heat leakage towards the liquid line, and it provides a cost-effective option to improve energy utilization and conversion efficiency for prospective applications.