| The electric vehicle has been developed rapidly in recent years due to their dual advantages--low energy consumption and zero emission.The power battery is the core component of electric vehicle.When the heat produced in extreme working conditions(such as high-rate discharge)cannot be dissipated in time,the reliability of battery will be reduced,and fire and explosion will occur when the situation is severe.When the composite Phase Change Material(PCM)is applied in the battery pack,it can effectively control the temperature of the battery pack and improve the temperature distribution's uniformity of the battery pack.Comparing with many other composite PCM,the Expanded Graphite(EG)PCM have many advantages as follows: the improvement effect of thermal conductivity is obvious;the structure does not change macroscopically when the phase change occurs,etc.This thesis mainly adopts the numerical simulation method to research the heat dissipation performance of the expanded graphite's PCM battery pack.Moreover,the heat management mode of the battery is optimized to improve the stability of the battery pack during the course of the long-term application.The main findings are summarized as follows:(1)The heat-generation mechanism and heat-transfer characteristics of lithium-ion battery were analyzed,and the relationship between the internal charge-discharge resistance and SOC of lithium-ion battery at different temperature was researched by establishing the charge-discharge experimental platform;the three-dimensional thermal effect model of single lithium battery was established,and the velocity field and temperature field of single lithium battery in the case of natural cooling under different operating conditions were conducted a simulated analysis.(2)Under different discharge rates,the effect laws of EG mass fraction on the peak temperature of the expanded graphite's PCM battery pack,the maximum temperature difference between batteries,and the liquid phase rate of composite PCM,were explored.The simulation results show that the liquid phase rate of composite PCM and the peak temperature of battery pack will increase with the increase of EG mass fraction at the same discharge rate.When the battery pack discharge is at high rate and the mass fraction of EG is too high,the peak temperature of the battery pack will exceed the ideal operating temperature.At this moment,there is the risk of thermal runaway in the battery pack.(3)In order to improve the heat dissipation performance of the battery pack at high discharge rate,the influence of the surface convection heat-transfer coefficient on the heat dissipation performance of the battery pack under the discharge rate of 3C was researched.The simulation results show that when the mass fraction of EG is high,the increase of convective heat-transfer coefficient will inhibit the increase of peak temperature as well as the composite PCM liquid phase rate.However,it has little effect on the temperature difference between batteries.When the mass fraction of EG is low,the increase of convective heat-transfer coefficient will make the liquid phase rate of composite PCM increase.However,it has slight effect on the peak temperature and maximum temperature difference.(4)Through the previous researches,it is found that the improvement of the convection heat-transfer coefficient on the surface of the battery pack can improve the heat dissipation performance of the battery pack.In order to further enhance the heattransfer efficiency of the composite PCM and the external,the cooling duct was added to the composite PCM.Through orthogonal experiment,the effect laws of four factors(air duct number,ventilation mode,flow rate of cooling medium,and initial temperature of cooling medium)on the peak temperature of battery pack,the maximum temperature difference between batteries,and the liquid phase ratio of composite PCM,were explored.The optimal combination was obtained by using the comprehensive balance method.The results after optimization was compared with those without cooling air duct,which confirmed that adding cooling duct in the expanded graphite PCM battery pack improved the heat-transfer efficiency and alleviated the thermal runaway problem of the battery pack under high rate discharge. |
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