Comparative study of operational characteristics for two-phase loop thermosyphons with and without pump assistant: Heat transfer performance and high-speed visualization observation

Abstract

The startup and heat transfer characteristics of pump-assisted two-phase loop thermosyphon (PTPLT) was experimentally investigated and compared with a TPLT using R245fa as the working fluid. To understand the coupling relationship between the operational performance and flow behaviors for the PTPLT, high-speed visualization observation was conducted and compared to the pump-free condition. The addition of pump ensured smooth startup and mitigated the temperature overshoot as compared with the TPLP. The local intermittent dry-out at the evaporator region could be suppressed with the increase of pump power, and a flow regime map was developed. The utilization of pump is beneficial to mitigating geyser boiling and transitioning to annular flow earlier at the evaporator region. Compared to the TPLT, the thermal resistance was decreased by 2.9–30.1 % within the power input range of 150–550 W for the PTPLT at the pump power range of 1.6–2.4 W. Additionally, the flow boiling heat transfer coefficient (FBHTC) within the evaporator region increased with the pump power. At the combined condition of 550 W power input and 2.4 W pump power, an FBHTC of 2972.6 W/(m²∙K) was achieved, 12.3 % higher than that of the TPLT. This study sheds light on the mechanism of performance improvement for the PTPLT from the perspective of flow visualization and can serve as a guideline for design and applications.