| Considering the serious energy shortage and environment pollution,the green energy power and clean vehicles have been drawn more attention all over the world.Electric vehicles(EVs)are popularly growing recently,which are more energy efficient and cleaner than conventional internal combustion vehicles.Power batteries,as the power sources,should meet the operational requirements of electric vehicles to supply long driving range and adequate acceleration,so that they are demanded to have high specific energy density,large capacity,good cycle life,durability and safety.However,power batteries are extremely sensitive to either excessive high or low temperatures or uneven temperature distribution,and then significantly result in short supply of energy,reduction of lifespan,fast degradation and even safety issues.Due to the thermal safety,stability and non-uniformity of power batteries,it is imperative to design an effective and well-designed battery thermal management system(BTMS)to prevent temperature extremes,help the battery function perfectly,extend longer cycle life and improve the performance of electric vehicles.Compared to traditional air,liquid,and phase change material(PCM)cooling/heating systems,heat pipe BTMSs possess the excellent thermal performance due to the advantages of high cooling efficiency,compact structure,flexible geometry and bidirectional characteristics.Considering the application of heat pipe in BTMS involving complex the phase change heat transfer of heat pipe coupled with power battery' dynamic heat generation and transfer,reliable thermal design and thermal analysis method is of great significance to design heat pipe BTMSs.In this paper,through combining the numerical simulation with experimental research method,taking a lithium-ion(Li-ion)power battery pack as the study object and the heat generation-transfer-dissipation-heating of battery as the main research route,the key technology of applying micro heat pipes in BTMS for EVs was studied.A three-dimensional(3D)thermal model of battery with time-varying heat source coupling with more internal heat sources was put forward.The calculation method of "heat-flow" coupled heat transfer numerical simulation for power battery was established.The BTMS based on ultra-thin micro heat pipe(UMHP)with phase-change heat transfer was designed.A ?segmented? thermal resistance model of a heat pipe is proposed to be integrated into the battery's thermal model for 3D computational fluid dynamics(CFD)simulation.The cooling/heating performance,dynamic temperature variation and temperature uniformity of heat pipe BTMS are analyzed under different influencing factors.The key issues of BTMS have been researched to design a high efficiency,low energy consumption,compact and lightweight heat pipe BTMS.The research content is as follows:1)Combining the heat generation,transfer and dissipation with the resistance characteristics of power battery,the time-varying heat source model was established.Then,a 3D thermal model of battery with time-varying heat source coupling with more internal heat sources was established by considering electrode heating effect.By CFD simulation,3D steady state and transient temperature field distribution and the dynamic variation of Li-ion power battery with different charge/discharge rate were analyzed.And the temperature variation and uniformity of battery under different cell spacing,natural/forced convection and thermo-electric inconsistent conditions were disscussed.And the corresponding experimental verification was carried out.2)On the basis of the coupled heat transfer theory,the calculation method of "heat-flow" coupled heat transfer numerical simulation for power battery pack was established.The "heat-flow" heat transfer boundary between battery and fluid was determined by establishing a coupled interface,the thermal parameters of which were obtained through the whole solving method for the "heat-flow" heat transfer simulation analysis of battery pack.Calculated by CFD simulation,taking a power battery pack as the study object,the dynamic characteristics of "heat-flow" field under different inlet and outlet layout and different natural/forced convection was simulated and analyzed.Based on the field synergy theory,the influencing factors and the relations between temperature field and flow field were analyzed,and the temperature uniformity was evaluated.Also,the calculation accuracy and efficiency of the model was verified.3)Based on the phase change heat transfer mechanism of heat pipe,the interaction between the phase change heat transfer limits and the size of the heat pipe was analyzed.The mathematical model between the largest heat transfer limit and the size of heat pipe was established for optimal design the length and steam chamber diameter of heat pipe.Considering the space limitations and the requirements of lightweight for EVs,UMHPs with thickness of 1 mm were proposed for BTMS.And the structure,materials,internal working medium and quantity of liquid of UMHP were determined and designed for power battery pack heat dissipation.Also,the fin size for UMHP cooling was optimally designed by analyzing the mathematical model between the pressure loss and heat transfer efficiency of fins and the fin size.4)Based on the phase-change heat transfer of heat pipe,a ?segmented? thermal resistance model of a heat pipe is proposed to express the phase-change heat transfer performance of different segments,the evaporator and condenser.The equivalent conductivity of each segment is used to determine its own thermal parameters and conditions integrated into the battery's thermal model for 3D CFD simulation,and then to analyze the temperature distribution and variation.The temperature distribution and variation of UMHP cooling were analyzed under different discharge rate,convection condition,arrangement,inclination and fin structure.And the heating performance of UMHP BTMS at different low temperatures was simulated.The results were compared with ?non-segmented? thermal resistance model of heat pipe,and the corresponding experimental verification was carried out.5)The experiment platform for cooling/heating temperature characteristic of power battery was established.The thermal characteristics of power battery were analyzed,including the variation of battery capacity,internal resistance,open circuit voltage with the temperature.The steady state and transient temperature distributions of power battery cell and module were studied under different charge/discharge rate.Also,the temperature characteristic experiment of BTMS with UMHP cooling/heating was carried out.The temperature variation of UMHP cooling was analyzed under different discharge rate,convection condition,layout structure and different road grade.Moreover,the heating effect of UMHP BTMS at different low temperature and heating power was evaluated by comparing with the direct heating method of battery through electric heating films as the heat source. |
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