CFD simulation of an intermediate temperature, two-phase loop thermosiphon for use as a linear solar receiver

A two-phase, horizontal loop thermosiphon (HLTS) is reported here for its potential use as a receiver for commercial, line focus, solar thermal collectors. The design consists of a horizontal evaporator, a riser, a condenser and a downcomer with a U-turn. Dowtherm A, which is suitable for the temperature range of 473-673 K, was adopted as the working fluid in the HLTS. The heat and mass transfer mechanism and performance of the HLTS at different configurations, inclination angles, and with/without the U-turn was investigated across the heat flux range of real applications using the volume of fluid (VOF) model. The results show that the U-turn section effectively prevents bidirectional flow in the loop and improves the heat transfer performance of the device. Different flow phenomena, i.e. stratified fluid flow and drop-wise flow, were found in the horizontal and the inclined condensers, respectively. An inclination angle of 5° was found to significantly improve the heat transfer performance over the horizontal design. As might be expected, the performance continues to improve in larger inclinational angles and higher heat flux inputs. Overall, by providing understanding for the salient two-phase mechanisms, this work should enable researchers to optimize the design of the HLTS for linear focus solar thermal collectors.