Performance analysis of Dowtherm A heat pipe with internal vapor monitoring

Abstract

Medium-temperature heat pipes, operating in the 200–600 °C range, find widespread application in sectors such as nuclear microreactors, solar energy collectors, thermal energy storage, and space. Efficient, passive heat transfer devices, like heat pipes, are essential for power systems operating in this temperature range. Despite such a broad range, traditional working fluids for heat pipes in the medium-temperature regime frequently underperform, prompting the need for more research into these working fluids. Dowtherm A is attractive for its chemical compatibility with heat pipe materials, low toxicity, low flammability, and adequate thermal–hydraulic properties, things that cannot be said for most medium-temperature heat pipe working fluids. This experimental study investigates the performance of Dowtherm A as a medium-temperature heat pipe working fluid, using internal and external measurements to quantify the heat transport in the heat pipe. A 25.4 mm outer diameter, 316 stainless steel tube was used for the heat pipe testing. Ten wraps of 100 × 100 (100 openings per inch) 316 stainless steel screen mesh were used as the wick, with a sliding fit and no annular gap. A fill ratio of 103 % of the total wick void volume was used. An air jacket was attached to the condenser of the heat pipe for cooling. Internal and external temperature measurement was performed, utilizing optical fiber distributed temperature sensing and conventional thermocouples, respectively. All tests conducted were in the horizontal orientation. The test matrix consisted of three different cooling conditions, controlled by changing the flow rate of air in the jacket over the condenser, with multiple power levels for each cooling condition. It was found that the thermal resistance of the heat pipe is not influenced directly by the cooling flow rate but is instead linked to the operating temperature. A minimum thermal resistance of 0.58 °C/W was achieved at the highest operating temperature tested of 274 °C. This corresponds to a maximum effective thermal conductivity of 2300  W/m·K. This finding agrees with values from previous studies. Internal vapor temperature measurements determined the active condenser length, where vapor condenses—a useful tool in heat pipe design. The capillary limit, which governs power transport in heat pipes, was exceeded in all tests without dryout, suggesting Dowtherm A outperformed expectations. This finding questions the soundness of the commonly used theoretical capillary limit, as applied for organic fluids such as Dowtherm A. Collectively, these findings highlight Dowtherm A’s viability for use in medium-temperature heat pipes, offering improved efficiency and operational safety in diverse energy systems.