Numerical Simulation Of Working Fluid On The Operation Characteristics Of Pulsating Heat Pipe

High heating flux is the main reason for the aging of equipment.Pulsating Heat Pipe(PHP)has great potential in the field of heat dissipation of high heating flux electronic devices due to its high efficiency of heat exchange and strong adaptability in structure.According to the physical process and mechanism of heat and mass transfer of PHP,the numerical model and experimental research system of PHP are established respectively.The factors affecting the number of loops(single and double loop),diameter(1mm and 2mm),water bath heating temperature(40 ~ 100?),working fluid(water,ethanol,acetone,R141 b,R123,R134 a,R290)of PHP are studied.Through numerical simulation and experimental study,the influence of the thermal properties of the working fluid on the heat transfer performance of the PHP is analyzed.The main contents and conclusions are as follows:(1)A 2D numerical model of PHP is established,and the influences of the number of loops,diameter,heating temperature of water bath and working fluids on the start-up and stable operation characteristics of PHP are studied.In the start-up process,the flow pattern of working fluid in the tube experienced the formation of vaporization core,the growth and growth of bubble,the formation of gas plug and liquid plug,thus forming plug flow;while in the start-up process of double circuit PHP,it showed sudden boiling.In stable operation,it is easier for the working fluid to form stable plug flow in the double loop PHP.(2)The starting process of PHP is different when the diameter of working fluid is different.When the diameter is 1mm,the start-up of PHP is slow,and the bubble evolution is clear during the start-up process;when the diameter is 2mm,the working fluid will suddenly boil.With the increase of water bath heating temperature from 40? to 100?,the start-up time of PHP becomes shorter,and the flow pattern of working fluid in the pipe changes into bubble flow,block flow(plug flow)and agitation flow.When the temperature of water bath is 60?,the distribution of gas-liquid plug is uniform and the pulsation is good.The interface between gas plug and liquid plug is clear,and the distribution of gas plug and liquid plug is more uniform,and the overall heat transfer temperature difference is small.(3)A three-dimensional numerical model of plate type PHP was built to study the start-up and stable operation characteristics of PHP with different working fluids(water,ethanol,acetone,R141 b,R134a,R290).The results show that when the heating temperature of water bath is 50-80?,the deviation between simulation and experiment is small,and the result is reliable.The study of start-up characteristics shows that the boiling point,latent heat of vaporization and surface tension of the working fluid are the main factors affecting the heat transfer characteristics of the PHP.With low boiling point,low latent heat of vaporization,PHP is easier to start.However,with the start-up process,the influence of boiling point will gradually weaken,and accordingly,the influence of latent heat of vaporization and surface tension will be more prominent.In the process of stable operation,surface tension,latent heat of vaporization and viscosity are the main factors affecting the operation characteristics of PHP.(4)Based on the results of numerical simulation,considering the influence of viscosity,surface tension,latent heat of vaporization,thermal conductivity,specific heat and density on the heat transfer performance of PHP,the dimensionless analysis of the thermal physical properties is carried out,and the order of magnitude analysis of each dimension is carried out,and the heat transfer correlation of PHP based on dimensionless physical properties is obtained.(5)The experimental platform of heat transfer performance of plate type PHP is designed and built,and the accuracy of the experimental system is verified by uncertainty analysis and repeatability experiment.Water,ethanol,HFE-7100,HFE-7200 and FC-40 were used as experimental objects.The effects of boiling point and latent heat of vaporization on the start-up time and temperature of PHP were analyzed.The results show that the start-up effect of PHP is better under the condition of low boiling point and latent heat working fluid.However,the starting performance of PHP is not only determined by boiling point and latent heat of vaporization,but also by thermal conductivity,viscosity and water bath heating temperature.