| In order to promote the achievement of the"double carbon goal"and the green transformation of the transport sector,the vigorous development of electric vehicles is one of the important means to save energy and reduce carbon emissions.However,lithium-ion batteries,as the main power source for electric vehicles,are subject to high or low temperatures that can harm their safe and efficient operation.The traditional battery thermal management system using air,liquid,phase change materials,and traditional heat pipes has the disadvantages of complex structure,large volume and mass,and high energy consumption.It is of great practical significance to develop a safe,efficient,and energy-saving battery thermal management system to promote the further development of electric vehicles.A novel passive battery thermal management system based on a two-phase thermosyphon loop is proposed for the above reasons.Moreover,a series of studies are conducted on the operational characteristics of the two-phase thermosyphon loop and the performance of the thermal management system under complex operating conditions,and the main work is as follows:（1）A two-phase thermosyphon loop with a horizontal evaporator was designed to meet the heat dissipation requirements of multiple heat sources in a small and confined space.A visualization test bench was established,and the effects of the filling ratio on the temperature,pressure,and heat transfer characteristics were investigated.The system can be started quickly at low filling ratios,but the upper tube is prone to local dry-out due to lack of liquid,reducing evaporator temperature uniformity and operating safety.Under high filling ratios,the startup is difficult,and an overshoot phenomenon is easy to occur,leading to large fluctuations in temperature and pressure.The system has the best heat transfer capacity at a filling ratio of30%,with a minimum thermal resistance of 0.12°C·W-1.（2）To address the problem of uneven heat generation by different cells in a battery pack,the effects of different non-uniform heat load distribution patterns on the overshoot phenomenon during the startup process were quantitatively analyzed.The coupling relationship between temperature and pressure was elucidated,the three stages of pressure change were summarized,and a method to inhibit the overshoot phenomenon during the startup process was proposed.The highest overshoot temperature during the startup process can be reduced by31.43%,the startup time can be shortened by 69.03%,and the total heat transfer coefficient can be increased by 28.34%to 3305 W·m-2·K-1 compared with the uniform heat load condition under the same total heat load.Different non-uniform heat load distribution patterns have little effect on the average evaporator temperature but significantly affect temperature uniformity.Increasing the heat load inhibits the overshoot phenomenon during startup,resulting in a smoother temperature curve and faster startup.However,this inhibitory effect on the overshoot phenomenon decreases with the increase in total heat load.（3）Based on the VOF model,the continuum surface force model,and the phase change model,and considering the change of the physical parameters of the working fluid and the influence of the pressure on the saturation temperature,the dynamic change process of the two-phase flow inside the two-phase thermosyphon loop is obtained by numerical method.The effects of different inclination angles on the flow characteristics,temperature uniformity,and heat transfer characteristics were visually analyzed.The results show that the flow pattern in the evaporator is dominated by slug flow.Moreover,the"accumulation of liquid"in the condenser at different inclination angles is the reason for the decrease of heat transfer capacity.The flow velocity in the evaporator increases as the inclination angle increases,and the difference in flow velocity between the two tubes in the evaporator almost disappears when the inclination angle reaches 30°.The inclination of any angle reduces the heat transfer capacity of the two-phase thermosyphon loop.Furthermore,the thermal resistance under the random condition is only 2.13%higher than that under the horizontal static condition,which fully demonstrates the applicability of the two-phase thermosyphon loop under complex and variable conditions.（4）A novel passive battery thermal management system was developed based on the two-phase thermosyphon loop.The temperature control performance of the battery thermal management system was comprehensively tested under constant current discharge conditions,dynamic stress test（DST）conditions,and cycle conditions.The safety and reliability of the two-phase thermosyphon loop in the field of battery thermal management were analyzed.The research results show that the novel battery thermal management system can control the maximum temperature below the safe temperature limit under different conditions,significantly improving the temperature uniformity of the battery modules and individual cells.The heat transfer inside the two-phase thermosyphon loop evaporator is always in the liquid phase or gas-liquid two-phase zone.There is no heat transfer deterioration due to local dryout,making the system safe and stable.In conclusion,the operation of the two-phase thermosyphon loop with a horizontal evaporator and its application in the field of battery thermal management are thoroughly and comprehensively studied by combining experimental and numerical simulation in this paper.The research results and methods can provide theoretical guidance and reference for the practical application of the novel battery thermal management system in electric vehicles. |
