Simulation of heat and mass transfer process in a flat-plate loop heat pipe and experimental comparison

Abstract

A loop heat pipe is a passive heat transfer device with strong robustness, high efficiency, and good performance, and has been widely utilized in numerous thermal control systems. Herein, an intensive study of a flat-plate evaporator LHP from the perspective of experiment and simulation is presented. First, the vapor leakage problem, which was a common issue for most LHP systems, was solved by filling the assembly clearance with epoxy glue, and a flat-plate LHP system was fabricated to verify its operating performance. Test results indicated that the loop stably performed under various working conditions without temperature oscillation. The maximum heat flux was 11.25 W/cm2 and the minimum thermal resistance was 0.1340 °C/W. The heater surface temperature remained low except during the dry-out state in wick at the maximum heat load condition where a sharp increase was observed, and temperature hysteresis occurred during variable heat load tests owing to the transport lag in long liquid line. Based on the above experiment, an efficient 3-D CFD computational model was built to simulate the heat and mass transfer process in a flat-plate evaporator by the Volume of Fluid method. In addition, the flow regime with two-phase distribution at the scale of the entire evaporator was calculated by considering the structure effect. Boundary conditions were derived from the experiment and the mass flow rate at the evaporator inlet was deduced from the heat leakage effect in liquid line. Calculation results demonstrated that four symmetric vortices developed inside the compensation chamber and generated uneven subcooled liquid infiltration in wick, further leading to a slight offset of high-temperature zone to the evaporator inlet side. Parameter analysis indicated that the ribs conducted 84 % of the heat conduction using thermal paths formed between the heater surface and liquid–vapor interface. The vapor volume percentage in the vapor collector was above 80 %, and increasing the heat load reduced the vapor volume in wick as well as the percentage of heat adsorbed during evaporation. Experimental comparison illustrated that the model exhibited a high accuracy with an error less than 10.29 %.