Numerical Evaluation of Thermal Performance of Two-Phased Closed Thermosyphon for Solar Applications

Abstract A heat pipe is an effective passive heat transfer device with a highly efficient heat transfer rate and thermal conductivity through evaporating and condensing a working fluid circulating in a vacuum-sealed container. For solar applications, heat pipes are usually used in evacuated tube collectors (ETCs). However, the use of heat pipes in solar photovoltaic/thermal (PVT) collector is limited. Generally, a heat pipe composes of three sections: the evaporator section, the condenser section, and the adiabatic section. A wickless heat pipe, also known as a closed thermosyphon or gravitationally assisted heat pipe, relies on gravity to return the working fluid to the evaporator. Various parameters could greatly influence the thermal performance of wickless heat pipes such as initial volume filling ratio (VFR), inclination angle, inner diameter of the heat pipes, ratio of evaporator/diameter and type of working fluid used. In this study, 2D transient computational fluid dynamics (CFD) simulations were performed using ANSYS Fluent to simulate the two-phase flow (water/steam) and to study the effect of different parameters on the heat transfer processes of a two-phase closed thermosyphon (TPCT) to increase its thermal performance. The 2D CFD simulations using multiphase volume of fluid (VOF) with two Eulerian phases could replicate the mass and heat transfer processes in comparison with published experimental data. A good agreement can be observed between the present CFD study with the published data in comparing the temperature profiles and thermal performance of the heat pipe. The numerical simulation of the evaporation and condensation process indicated that the thermal performance and characteristics of the TPCT are influenced by certain key parameters. In the future, full-scale 3D CFD simulations are planned to be performed to evaluate the thermal performance of heat pipes for solar photovoltaic/thermal (PVT) applications.