Investigation of heat transfer and flow characteristics in two-phase loop thermosyphon by visualization experiments and CFD simulations

Abstract

As an efficient heat transfer device, two-phase loop thermosyphon (TPLT) has a broad application prospect in many engineering fields, and its internal gas-liquid phase change and flow characteristics are the key factors affecting the heat transfer performance. In order to get a better thermal design and performance, it is necessary to clarify the heat transfer mechanism of the TPLT, especially under the condition of high filling ratio. In this study, the visualization experiment was combined with the CFD method to study the heat transfer and flow characteristics of TPLT. Due to the strong coupling relationship among the boiling process and the condensate process, the Lee model is optimized, and the relationship between saturation temperature and local pressure, and the parameter of subcooling degree in the Lee model were both introduced to accurately simulate the phase change process. Results shows that the optimized model has good prediction performance for the heat and mass transfer simulation of the loop thermosyphon. The two-phase flow in the loop thermosyphon changes from slug flow to churn flow with the increase of heating power at low filling ratio (50%), while the bubble size decreases and the number of bubbles increases with the increase of heating power (20W-80W) at high filling ratio (70%). The heat transfer mechanism before and after model optimization has changed, from saturated flow boiling to subcooled flow boiling at high filling ratios. At low filling ratio (50%), the fluid in heating section is saturated state, and the flow velocity at the bottom of the loop is low (0.018m/s). At high filling ratio (70%), the fluid in the heating section is supercooled state, and the fluid flow velocity at the bottom of the loop is high (0.171m/s). This indicates that the heat transfer mechanism of the loop thermosyphon at low filling ratio was saturated pool boiling, and the filmwise condensation occurs in the condensation section. However, the heat transfer mechanism is subcooled flow boiling at high filling ratio, and the gas-liquid two phases in the condensation section are in a mixed state.