Experimental And Numerical Simulation Study On Heat Transfer Characteristics Of Loop Thermosyphon With High Heat Flux

The loop thermosyphon has the advantages of no power,compact structure,good isothermal property and high heat flux,so it has great potential in solving the heat transfer problem of electronic components at high heat flux.In the current research,heat flux is relatively small,generally within 10W/cm~2 and the filling ratio is less than 50%.Previous studies have explored that the loop thermosyphon can achieve the heat transfer more than 100W/cm~2 at a high filling ratio.However,there is a lack of research on the heat transfer law and heat transfer mechanism of the high filling ratio thermosyphon.Therefore,this paper systematically studied the heat transfer characteristics and two phase flow characteristics of the loop thermosyphon with filling ratios in the range of 35%-92.5%.Difference heat transfer process of the loop thermosyphon were compared under different filling ratios.The heat transfer mechanism of high filling ratio loop thermosyphon was revealed at the high heat flux.A loop thermosyphon heat transfer characteristic experiment platform was designed and built,the heat transfer characteristics of the loop thermosyphon at 80%,60%,and 35%filling ratios were studied.The results show that the 80%filling ratio loop thermosyphon have a special phenomenon that the evaporator temperature decreased as the heat flux increased,the loop temperature uniformity was greatly improved and the heat transfer capacity was greatly enhanced.However,loop thermosyphon did not appear this phenomenon at 60%and 35%filling ratios.The heat transfer characteristics of loop thermosyphons were compared at three different filling ratios.The results show that 35%filling ratio loop thermosyphon is suitable for medium and low heat flux density heat transfer.On the contrary,80%filling ratio loop thermosyphon achieved a high heat flux heat transfer above 200 W/cm~2 at a good heat transfer performance.In addition,it was also found that the heat transfer limit was reduced if the liquid filling ratio was too high,the condensation section was too long and the inclination angle was reduced.Reversely,increasing the flow mass of cooling water effectively improved the heat transfer limit,and the cooling water temperature had little effect on the heat transfer performance.The two-phase flow characteristics of high filling ratio loop thermosyphon was studied to revealed the heat transfer mechanism with visualization.It was found that bubbles were produced in the heating zone,and the bubbles rose through the liquid layer to the condensation section at a low heat flux.Due to the large interphase resistance,the pressure drop in the loop is large and the heat transfer resistance is increased.Under the condition of high heat flux,the number of bubbles and vapor volume produced in the heating zone increased,the bubbles promoted the flow of liquid working fluid to the condensation section,the flow rate of the working fluid increased and the working fluid formed a continuous and stable two-phase flow in the loop,and the heat transfer resistance was greatly reduced.The CFD numerical method was used to simulate the vapor-liquid two phase flow and heat transfer process of the loop thermosyphon at 80%,60%and 35%filling ratios.The simulation results show that there is no continuous circulation in the loop at 35%filling ratio.The working fluid near the evaporator was in the state of pool boiling,the bubble size was large and the working fluid in the condensing section condensed into a thin liquid film on the wall.A small amount of liquid above the evaporator flow across the top of the loop to the condensing section,the upper part of the condensing section condensed into a liquid film,and there were a large number of supercooled liquid in the lower half at 60%filling ratio.At 80%filling ratio,a continuous two-phase circulation was formed in the loop,and the bubbles at the evaporator was sheared by the liquid,and the departure diameter was small.The simulation results verified the experimental phenomenon and mechanism analysis of the heat transfer characteristics of the loop thermosyphon under different filling ratios in the second chapter.In this paper,the heat transfer law of the loop thermosyphon under different filling ratios were studied systematically,and the gas-liquid two phase flow characteristics and heat transfer mechanism in the loop were revealed.The heat transfer characteristics of the high filling ratio loop thermosyphon were studied through the experiment of heat transfer characteristics,and the heat transfer mechanism of the high heat flux was revealed through visualization experiment and CFD method.This study can provide theoretical guidance for the design and application of loop thermosyphon at a high heat flux.