Research On The Performance Of Gravity Heat Pipes Used In Power Battery Cooling System

Due to their effective heat transfer capabilities,gravity heat pipes are frequently used in the aerospace,automotive,air conditioning and refrigeration,electronics,and other industries.The heat transfer coefficient of the heat pipe can be several times or even dozens of times higher than that of the conventional heat dissipation method,which can significantly increase the effectiveness and efficiency of the thermal management system.By dispersing metal or non-metallic nanoscale particles into water,ethylene glycol,and other liquids in a specific ratio,nanofluids are a new kind of fluid heat transfer medium.This paper establishes a numerical calculation model of gravity heat pipes to investigate the enhanced heat transfer characteristics,starting characteristics,steady-state characteristics,and working limits of nanofluid heat pipes in an effort to further enhance the heat transfer performance of gravity heat pipes and investigate the viability of using nanofluids as working fluids for heat pipes.In this paper,the geometric model of the gravity heat pipe is established,and the pipe diameter,length of each segment,minimum filling rate,and shell material are designed and calculated based on the carrying limit,boiling limit,and drying limit of the gravity heat pipe.The technical approach of VOF multiphase flow model coupled with UDF-controlled evaporative condensation model was then determined according to the two-phase flow and phase change heat and mass transfer process inside the heat pipe,and the numerical model was verified by combining pertinent experiments.As working fluids for heat pipes,four substances—pure water,GO nanofluid,water-based graphene nanofluid,and Al₂O₃nanofluid—were chosen.Under various heating powers,liquid filling rates,and nano-fluid concentrations,the time-varying vapor-liquid two-phase distribution,wall average temperature,thermal resistance,and equivalent convective heat transfer coefficient of heat pipes were obtained.It was compared and examined how well gravity heat pipes transferred heat.The viability and benefits of using nanofluids in heat pipes were investigated.The entire process from laminar film evaporation to mixed convection and nuclear boiling can be seen.The simulation calculated the full time-varying two-phase distribution cloud of gravity heat pipe.After the heat pipe has run for two seconds,the nanofluid heat pipe takes the lead to form a clearly visible continuous condensate film.The thickness of the liquid film increases uniformly and then gradually stabilizes,indicating that the nanofluid heat pipe has better starting characteristics.The condensed liquid can only be observed in the early stage of heat pipe operation to form intermittent small water droplets on the wall.The increase in liquid filling rate between 14.2%and 22.4%decreased the heat pipe’s corresponding speed,but after stable operation,the wall temperatures of each section tended to be consistent,and the overall thermal performance did not change significantly.The evaporation section’s thermal resistance increased by about 9.2%while the heat pipe’s overall thermal resistance decreased by about 42%as the heating power of pure water heat pipe increased from 10W to 40W and the temperature difference between the evaporation section and the condensing section increased.The equivalent convection heat transfer coefficient of the heat pipe is kept at 3300W/m²K after it has been operated steadily,which is about 51.4%higher than the 10W operating condition.The thermal resistance of the heat pipe evaporation section of graphene oxide nanofluids is significantly reduced by about 11.8%–13%compared to pure water heat pipes,and the total thermal resistance of heat pipes is reduced by about 4.2%–5.2%under the same heating power and liquid filling rate.Comparing graphene oxide nanofluid heat pipes to Al2O3nanofluid heat pipes,the thermal resistance is reduced by about 5.1%,and the equivalent convective heat transfer coefficient is increased by about 2.2%.The concentration of graphene oxide nanofluids is increased to 0.2%,and the equivalent convective heat transfer coefficient is increased by about 4.1%to 5%.The temperature difference between the evaporation section and the condensation section of the nanofluid heat pipe is smaller than that of pure water heat pipe,which improves the starting performance of the heat pipe and has better overall thermal performance.The high thermal conductivity of nanofluids allows them to absorb more heat from the wall to increase the temperature of the fluid and then phase change.The graphene oxide nanofluid heat pipe exhibits the best heat transfer performance when the thermal resistance and equivalent convective heat transfer coefficient of the four working fluids are compared under various working conditions.The thermal conductivity of Al2O3nanofluid is improved to some extent compared with water,but its viscosity increases greater,which has some bearing on the heat pipe’s overall thermal performance.The results demonstrate that when the heating power of the heat pipe exceeds 50W,the temperature of the evaporation section soars and the reflux condensate evaporates quickly on the high temperature wall,creating an updraft close to the wall of the evaporation section,which impedes the return flow of condensate in the tube and the rise of saturated steam,reaching the saturation point.The heat transfer limit of a heat pipe that uses graphene oxide nanofluids can be increased because these heat pipes have lower thermal resistance and a higher equivalent convective heat transfer coefficient at different heating powers.Additionally,these heat pipes require a higher heating power to create an unstable flow in the tube.