Numerical Study On Solid-Liquid Phase Change In Paraffin As Phase Change Material For Battery Thermal Management

The continuous development of the national economy has caused a series of environmental pollution problems.The new energy vehicles have developed rapidly in recent years due to their advantages of zero emission.As one of the key technologies of new energy vehicles,the cycle life and thermal safety of power batteries have a significant impact on the performance of the vehicle.In order to improve the cycle life of batteries and thermal safety performance,this paper adopts the method of numerical simulation,for the battery thermal management system with phase change materials,used for paraffin solid-liquid phase change characteristics of battery thermal management system is studied.A two-dimensional square model was established in this paper,studied the thermal effect of heating wall`s location,paraffin thermal conductivity and heat flux of the heating wall on the paraffin phase change process,and focused on the analysis of the velocity magnitude and direction of the moving wall on the paraffin melting process and its reasons.The main conclusions are as follows:(1)The effect of the location of the heating wall on the phase change of paraffin wax was studied when heat flux was a constant.The results show that when the hot wall was at the bottom,paraffin`s melting speed is the fast;and when the hot wall is at the top,paraffin`s melting speed is the slowest;when the hot wall is located at the left and right,the paraffin melting rate is in the middle of the former two cases.(2)The effects of thermal conductivity on paraffin phase change process were studied when heat flux of the heating wall was a constant.It can be found that in the paraffin melting stage(about 0~500s),the thermal conductivity of paraffin did not produce significant effects on melting speed,and about 500 s later,thermal conductivity began to affect the phase change process,and it can be concluded that large thermal conductivity of paraffin melt faster.When the heating wall is at the bottom,the thermal conductivity has a significant effect on the phase change process at the beginning of paraffin melting.With the passage of time,in the late stage of paraffin melting,with the increase of the thermal conductivity of paraffin,paraffin wax was melted completely almost at the same time.(3)In view of the effect of heat flux on the phase change process of paraffin wax,the liquid fraction at different time were studied when different heat flux were applied to the heating wall.The results show that under the action of paraffin gravity,thesquare cavity of the paraffin was melted from the upper left corner,and the lower right corner part of the paraffin finally began to melt.the phase change interface morphology showed a positive slope,and the melting of paraffin on the whole in was a clockwise direction.Besides,with the increasing of the heat flux of the heating wall,the solid-liquid phase change of paraffin was faster.(4)In view of the influence of the moving wall on the phase change process of paraffin wax,the influence of the magnitude and direction of the velocity of the moving wall on the phase change process of paraffin wax was studied.The effects of the magnitude of the moving speed on the relationship between liquid fraction and time can be concluded that the faster the moving velocity,the faster the solid-liquid phase change.When the heating wall was the top wall,the phase change process was promoted because of the moving of the bottom wall.When the bottom wall moved in the same direction,the magnitude of the moving speed has an great effects on the gradient of the curve of liquid fraction and time.It can be found that the bigger moving speed,the bigger gradient.When the heating wall was the top wall,and the magnitude of the moving speed of the bottom wall were same,the moving direction of the bottom wall has no great effects on the phase change process of paraffin.When heat flux applied to the left wall without a moving wall,a clockwise vortex appeared in the fluid zone,and it split into two clockwise vortexes with the increase of time.With the increasing of the moving speed of the top wall,the speed of vortex splitting was faster.When the top wall moved along the negative direction of X axis,a vortex appears which is in the anticlockwise direction.It turned to be bigger with the increase of the slip velocity of the top wall.The anticlockwise vortex prevented the clockwise vortex moving from liquid phase zone to solid phase zone,which delayed the phase change process of paraffin.Similarly,the division of the clockwise vortex occurred in advance because of the slip wall moving in the clockwise direction.