Experimental study on heat transfer characteristics of the large length-diameter ratio high-temperature heat pipe

Large length–diameter ratio high-temperature heat pipes (large l/d ratio HTHPs) demonstrate wide-ranging application potential in areas such as nuclear energy and solar energy development. Distinct in their geometric configurations, large l/d ratio HTHPs’ performance attributes significantly diverge from traditional high-temperature heat pipes (HTHPs). In this paper, a large l/d ratio HTHP is engineered with dimensions of Φ16 × 2400 mm, achieving an aspect ratio (l/d) of 150. The wick structure incorporates dual layers of 80-mesh 316 stainless steel wire mesh. Employing liquid sodium as the working fluid, experiments are conducted with filling ratios set at 15 %, 25 %, and 35 %, to evaluate its efficacy in heat transfer processes. This research delineates the analysis of evaporator thermal resistance and the effective thermal conductivity of large l/d ratio HTHPs across varying filling ratios and angles (0°, 15°, 30°, 45°, 60°, 75°, 90°). HTHPs with filling ratio of 25 % are selected to investigate the steady-state thermal transfer characteristics at different powers (2.0 kW, 2.5 kW, 3.0 kW), along with the variation patterns of the effective thermal conductivity and thermal resistance. The findings reveal that large l/d ratio HTHPs, specifically those with a 25 % filling ratio, demonstrate superior heat transfer capabilities at a tilt angle of 15°. With a heating power set at 3.0 kW, the thermal transfer efficiency progressively diminishes as the tilt angle is increased beyond this optimum point. It is observed that the increase in thermal resistance, which adversely affects heat transfer performance, primarily emanates from the evaporator section and intensifies with an increase in the tilt angle.