Thermal and hydrodynamic analysis of a self-purging hot reservoir variable conductance heat pipe

Abstract

A new design of a hot reservoir variable conductance heat pipe (VCHP) is proposed and investigated in this study. While a hot reservoir VCHP can provide much tighter thermal control performance in comparison to a traditional cold reservoir VCHP, it was seen that the hot reservoir design is prone to more common operating failures due to the accumulation of the working fluid inside the reservoir. Once accumulated, this excessive working fluid in the reservoir can only be removed by diffusion which takes a long time. In this study, we propose and investigate a new design for the hot reservoir VCHP that would perform an additional function of working fluid vapor purging in addition to operating as a normal heat pipe. In the proposed design, the non-condensable gas (NCG) section of the heat pipe is connected to the reservoir by an external tube, forming a NCG flow-inducing loop configuration, hence the name self-purging hot reservoir VCHP (SPHR-VCHP). With proper placement of the NCG pipe in the VCHP a pressure-induced flow is generated through the loop from the NCG section towards the reservoir. It is shown that this loop flow can remove the excess working fluid vapor in the reservoir significantly faster than by diffusion. A Fortran code to simulate the two-phase flow in the heat pipe section combined with an ANSYS Fluent CFD model of the external loop is employed to predict the fluid flow, heat transfer, and mass transfer behaviors inside the SPHR-VCHP. An experiment is set up for the SPHR-VCHP that is made of a 291 mm long titanium tube with an inner diameter of 14.7 mm. The working fluid and the NCG are acetone and helium, respectively. The CFD predictions of wall temperature are validated by the experimental data with mean absolute error equal to 2.68%. A thorough CFD study is then performed to optimize the loop design in order to enhance purging rates by changing the size of the NCG tube to operate under microgravity conditions for space applications. Based on this study an optimum NCG pipe diameter equal to 40% of the inner diameter and pipe length equal to the length of the evaporator section is suggested. The wall temperature results show that the SPHR-VCHP design does not impact the thermal performance of the original hot reservoir heat pipe, while also having a self-purging capability that makes the design safe from working fluid accumulations issues.