Study On Flow And Heat Transfer Characteristics Of Pulsating Heat Pipe With Zeotropic Mixtures

With the increasing integration of electronic chips,the cooling of high heat flux electronic devices has become one of the core technologies in the development of information technology.The heat transfer capacity of pulsating heat pipe(PHP)can reach 100W/cm~2,which is a potential cooling method.Based on the analysis of heat and mass transfer mechanism of PHP,zeotropic mixture was applied to PHP,especially immiscible mixed refrigerant,which overcomes the two bottlenecks of traditional refrigerant pulsating heat pipe,such as large temperature difference in start-up operation and easy dry-out in stable operation.Five working fluids with different boiling points(water,HFE-7100,ethanol,R141b,R123)were used in the experiment,and binary mixtures were composed according to different mixing ratios(1:0,4:1,2:1,1:1,1:2,1:4,0:1).Visualization and performance experiments were carried out under different experimental conditions(heating power,filling rate,inclination angle).Visual experiments were recorded and analyzed in the tube.The flow pattern changes of gaseous and liquid refrigerants in start-up and stable operation stages were studied.The heat and mass transfer mechanism of non-azeotropic mixed refrigerants in start-up and stable operation stages under different operating conditions was analyzed in detail.A two-dimensional mathematical model of single-loop PHP was established.The flow pattern,pressure,temperature and velocity vectors of working fluid in the pipe were obtained by simulation calculation.The main contents and conclusions are as follows:(1)A simultaneous test rig for visualization and heat transfer performance of plate PHP was built,and heat leakage analysis,uncertainty analysis and repeatability experiments were carried out to ensure the accuracy of the data.(2)Through visualization experiments,the flow patterns of zeotropic mixtures were analyzed and studied.The results showed that bubbly flow,plug flow,annular flow or mixing flow would occur with the increase of heating power in the operation of PHPs.Compared with pure refrigerants,when zeotropic and immiscible mixtures started up,bubbles at the bottom of the mixtures gathered at the interface of the two refrigerants,which made the temperature and pressure of the evaporation section increase rapidly and accelerated the start-up.In the stable operation stage of PHP,zeotropic immiscible mixtures formed emulsion when they oscillated,which made it easier to form a one-way pulsating circulating annular flow in the pipe without local drying.(3)The start-up and stable operation characteristics of PHPs were studied experimentally.The results showed that zeotropic immiscible mixtures could shorten the start-up time of PHPs,and the existence of high boiling point refrigerants could shorten the length of PHPs.The start-up of miscible mixtures was influenced by the hydrogen bond association grid structure formed between the two working substances,and more hydrogen bonds would increase the start-up time.In the stable operation stage,the immiscible mixed refrigerants could form an emulsion that promoted one-way pulsating circulation flow,which made the evaporation section not easy to be burned out.For the intermiscible mixed refrigerants,the boiling point,specific calorific value and vaporization latent calorific value of R123 and R141b were relatively small at high heating power,which made the mixed refrigerants prone to local drying.But at low heating power,the evaporation section was not easy to be burned out.The heat transfer resistance was relatively small,and the system ran well.(4)A two-dimensional mathematical model of single-loop pulsating heat pipe was established,and numerical simulation was carried out under different boundary conditions(50W/m~2,10W/m~2,1000W/m~2,5000W/m~2).The results showed that there were tension,convergence and tearing phenomena in the gas and liquid working fluids.With the increase of heat flux in the evaporation section,the gas plug in the tube became more fragmented.At the same time,combined with the pressure and temperature distribution nephogram,it could be seen that circulating flow could be formed in the pipe.