Study On Heat Transfer Performance Of Micro-Channel Parallel Flow Loop Heat Pipe

Nowadays,the world is in the era of big data with information explosion.As the carrier of data computing and storage,the scale and number of data centers are gradually increasing,and the energy consumption is also increasing.Data center cooling system consumes a large proportion of energy.At the same time,the traditional heat dissipation methods are susceptible to dust and moisture interference,low heat dissipation efficiency,high maintenance costs and high energy consumption.There is an urgent need to study a new type of airtight cabinet heat dissipation method.In this paper,a micro-channel parallel flow heat exchanger is used,R134 a is used as the working fluid to design a micro-channel parallel flow loop heat pipe as a closed cabinet.The heat dissipation device achieves the goal of transferring the high heat generated by the unit to the outdoor through the flow and phase change of the working fluid in the evaporator and the condenser.The heat transfer performance and optimization method of the micro-channel parallel flow loop heat pipe is studied.The influence of design parameters such as liquid filling rate,height difference,heat exchanger structure,and operating parameters such as condenser end wind speed and system circulation flow on the heat transfer performance of the loop heat pipe system is revealed and the optimization method of the pump-assisted loop heat pipe is proposed.Based on the distributed parameter model,the steady-state heat transfer mathematical model of the micro-channel parallel flow loop heat pipe is established,which is programmed and calculated by the software Matlab 2020 b.The numerical results were compared with the experimental results to verify the feasibility of the model.The two-phase state,temperature distribution and heat transfer coefficient of each micro-element inside the flat tube are analyzed.The results show that when the liquid filling rate is low,the inside of the evaporator is easy to dry up,the two-phase heat exchange area is small,and the heat exchange performance of the system is poor.When the liquid rate is too high,there are many working fluids inside the system,and there is almost no single-phase gaseous zone inside the evaporator after steady state,which leads to poor circulation effect of the loop heat pipe system and hinders its heat exchange effect.Due to the different flow resistance of each branch of the heat exchanger,uneven distribution of working medium flow will occur.The closer the position of the flat tube is to the inlet of the evaporator,the smaller the flow of working medium.Compared with the inlet flat tube,the mass flow of the subsequent flat tube increases up 10.3%.Changing the structural parameters of the heat exchanger such as the height of the flat tube,the spacing of the fins,the height of the fins and the angle of the louvers can greatly improve the heat exchange performance of the loop heat pipe,but it will also affect the space structure and air side resistance of the heat exchanger.A micro-channel parallel flow loop heat pipe test bench was built to study the effects of different liquid filling rates,wind speeds in the condensation section and different height differences on the heat transfer performance of the loop heat pipe.From the point of view of heat transfer mechanism,the effect of liquid filling rate on heat transfer performance of heat pipe was analyzed.The results show that the optimal liquid filling rate of the system is in the range of 80%～105%,and the heat exchange when the liquid filling rate is 90.4%,which is about 1.47 times that when the liquid filling rate is 35.5%.With the increase of the liquid filling rate,it first increases and then decreases,and the minimum thermal resistance range and the minimum cavity temperature range basically correspond to the optimal liquid filling rate range.When the system operates at five condenser end wind speeds,as the wind speed increases,the cavity temperature first drops and then rises,reaching the minimum at 3.24m/s.Due to the increase of wind speed,the circulating working fluid obtains a better cooling effect,the overall temperature of the loop heat pipe system decreases,and the operating pressure also decreases gradually.The greater the height difference between the evaporator and the condenser,the greater the system circulation pressure,and the better the heat transfer performance of the loop heat pipe,however,the improvement effect is not significant.An effective optimization strategy to improve the heat transfer performance of the system is studied—the heat transfer performance of the pump-assisted loop heat pipe.A pump-assisted loop heat pipe experimental system was built to test the heat transfer performance of the loop heat pipe system under different circulation flows,and the start-up performance and variable heat load operation characteristics of the gravity-driven loop heat pipe were studied.The results show that when the filling rate of the gravity-driven loop heat pipe is 33.5%,the start-up fails.When the filling rate is 40.6%～65.4%,the starting method is overshoot startup.When the filling rate is 73.4%～119.5%,the starting method is gradual startup.No matter whether the heat load is increased or decreased step by step,the system can achieve steady-state operation within4 k W.The auxiliary pump can greatly increase the circulation flow of the loop heat pipe system,thereby improving the heat exchange performance of the system.When the mass flow rate is 0.15kg/s,the maximum value(wave peak)of the cavity temperature will not increase after reaching 52.1℃,and the temperature of each measuring point is in a state of periodic fluctuation.Comparing the influence of the auxiliary pump on the performance of the loop heat pipe system,it can be found that the auxiliary pump can quickly start the loop heat pipe system,and can also effectively improve the heat transfer performance of the loop heat pipe,and the maximum temperature of the cavity can be reduced by 11.1℃.