Experimental Study On Temperature Oscillation And Performance Comparison Of Loop Heat Pipe Enhanced By Pressure Head Of Evaporation

In the information age,high-performance chips are needed for data flow and computing processing,and the problem is that the power of chips and other electronic components is increasing.The high heat flux generated poses a great threat to the performance,efficiency and life of electronic chips.Therefore,an effective cooling device is needed to ensure the stability and safety of electronic chips.The loop heat pipe is gradually becoming the main means of cooling electronic components because of its high heat transfer efficiency,long heat transfer distance,simple and flexible layout.In this paper,the working principle and research status of loop heat pipe enhanced by pressure head of evaporation and driven by capillary force are introduced.It is found that there is not enough in-depth study on the temperature oscillation phenomenon in the operation of loop heat pipe enhanced by pressure head of evaporation.At the same time,the performance comparison of loop heat pipe under the two driving mechanisms is also lack of enough data support.Therefore,in this paper,the phenomenon of temperature oscillation during the operation of loop heat pipe is studied.By using the visualized loop heat pipe system and designing the corresponding experimental scheme,the temperature oscillation and two-phase flow pattern are combined to explore the causes and variation rules of temperature oscillation.At the same time,the performance of the loop heat pipe with different driving mechanisms is compared.Under the same experimental conditions,two kinds of loop heat pipe systems with enhanced by pressure head of evaporation and capillary force driving are studied under different heating power.The performance of the two kinds of heat pipes is compared through operating temperature,start-up time and system thermal resistance,The results are as follows:(1)When LHP-1(Loop heat pipe system with PU tube condenser)is used in the experiment,when the heat sink temperature is constant,with the increase of heating power,the amplitude of loop heat pipe temperature oscillation gradually decreases and the frequency increases.This is because higher heating power increases the mass flow rate of circulating working medium and shortens the length of liquid column in the condenser,while the flow pattern in the condenser gradually changes from slug flow to semi-annular flow;When LHP-2(Loop heat pipe system with copper tube condenser)is used in the experiment,the temperature oscillation amplitude of loop heat pipe is very small,because the high heat transfer efficiency of copper condenser makes the flow pattern in the condenser mainly plug flow,the liquid column is very long,and the gas-liquid interface is basically stable in the condenser.(2)When the flow pattern in the condenser is slug flow,the vibration amplitude of temperature oscillation is the largest,and the vibration amplitude is the smallest when the flow pattern is semi-annular flow.When the flow pattern is plug flow,the temperature oscillation amplitude is lower and the two-phase interface is stable in the condenser,so it is the most ideal flow pattern among the three flow patterns.(3)The experimental study on loop heat pipe system with different driving mechanisms and different power(55W,100W,140W,180W and 220W)is carried out.It is found that stable startup form is the most ideal start-up form in loop heat pipe operation.When overshoot start-up form occurs,the start-up time of LHP-A1(Loop heat pipe system with wick A1),LHP-B3(Loop heat pipe system with wick B3)and LHP-C2(Loop heat pipe system with wick C2)is 800s,1400s and 1200s respectively.The difference between the maximum temperature and the stable temperature under the three conditions is 2.5~oC,23.7~oC and 11.3~oC,respectively.It can be seen that LHP-A1 still has better operation performance and faster system regulation ability than the other two loop heat pipe systems in the form of overshoot start-up.(4)Compared with LHP-C2,LHP-A1’s operating temperature,start-up time and system thermal resistance are 11.1~oC/12.5~oC,19.1%/28.75%and 40.56%/38.7%lower at55W/220W heating power.It shows that the comprehensive performance of heat pipe can be greatly improved by making full use of pressure head of evaporation.It is also found that the wick structure with low thermal conductivity can effectively reduce the operating temperature and system thermal resistance,especially in the case of high heating power,but the start-up time will be prolonged.