Heat transfer and two-phase flow of a metal foam enhanced horizontal loop thermosyphon for high power solar thermal applications

Loop thermosyphon, as an efficient heat transfer device, is considered as a potential receiver for high power solar thermal applications. A horizontal loop thermosyphon with metal foam (MF-HLTS) was proposed for further improve the thermal stability when operated in horizontal position, and experimentally investigated compared to that without metal foam (S-HLTS) in this work. The experimental results show that the filling ratio of 57.5% produced the lowest total thermal resistance of 0.056–0.093 K/W, with a maximum increase of 836% in terms of thermal performance at steady-state. Meanwhile, the start-up performance of MF-HLTS with the progressive start-up is superior to that of the S-HLTS and EF-HLTS with the overshoot start-up in terms of start-up stability and time under various filling ratios and heat fluxes. Additionally, the bidirection-oscillation flow is easy to occur inside S-HLTS, resulting in temperature and pressure fluctuations with amplitudes reaching 12.53 kPa and 2.67 °C. For MF-HLTS, the internal flow pattern exhibits stable unidirectional flow, with a maximum improvement of 78.5 times. All results demonstrated that the horizontal loop thermosyphon with the auxiliary of a metal foam in evaporator and straight section can greatly improve thermal performance and flow stability, which is promising in the energy utilization applications.