Experimental study on visualization of U-shaped array thermosyphon

U-shaped array thermosyphon has no capillary core structure, which can be used for power battery thermal management. In this paper, we performed a visualization experiment about U-shaped array thermosyphon. Its fluid flow and heat transfer mechanism and the influencing factors are studied. With input of heat power, the thermosyphon underwent stages of cold start-up, heat energy storage, transition and circulation flow stage. Before the start-up of the thermosyphon, there would be three kinds of vapor-liquid states of the working fluid, namely vapor tube, liquid tube and vapor plug liquid slug tube. After the start-up, the corresponding change were slug flow, annular flow or liquid cylinder oscillation. Slug flow and annular flow relative to liquid cylinder oscillation can enhance heat transfer. The main factors affecting the performance of the U-shaped array thermosyphon are the filling rate and working fluid. The liquid filling rate affects the stage of the working fluid at the stationary state and the flow pattern at the circulation flow state. Taking the temperature difference of start-up and the stability of circulation flow as the evaluation criterion, the performance of the thermosyphon of R134a is better than that of R113.