电动汽车在节能环保领域有着巨大的优势而受到广泛关注，而锂离子电池作为纯电动汽车的唯一动力源，其性能的好坏会直接影响到电动汽车的整体性能。本文主要采用数值模拟的方式，针对电动大巴在高温高倍率下的散热问题进行了研究。结合电动大巴的特点初步设计出相变材料结合水冷的热管理方案。并采用Solidworks软件建立三维电池模型，利用CFX软件对电池温度场进行分析，对比了在增加相变材料（PCM）和水冷后的散热效果，初步验证了方案的可行性。在此基础上对多种影响因素进行分析，最后对热管理方案进行优化分析，以保证电池在极端条件下能正常工作。主要工作如下：（1） 对锂离子电池单体在25°C和40°C的环境温度下进行放电，在相同倍率放电时，其温度变化基本相同，温升最大偏差为0.3°C。锂离子电池在40°C条件下进行1C和1.5C放电仿真曲线与与实验值大致吻合，其最大偏差为0.9°C；（2） 对不同配比的膨胀石墨/石蜡复合相变材料进行制备。结果显示相变材料的潜热值随石蜡含量的增加而线性升高，而相变的范围与石蜡含量无明显联系。热导率随石蜡含量的增加而降低，在低导热方向无明显变化；（3） 利用CFX软件对电池组结构进行温度场模拟，在一次充放电循环后，电池温度升高33°C，在增加PCM后，电池组温度降低为66.8°C，但随着充放电循环次数的增加，电池组温度不断升高。在PCM的基础上增加间歇性水冷，只在2C充电时开启，在一次充放电，电池温度进一步降低，为62.6°C，在进行充放电循环时，电池温度在增加到79.7°C后就不再继续升高，说明方案的初步可行性；（4） 对散热设计中不同石蜡含量的PCM、PCM厚度、管径形状、流速和管径大小对电池组散热效果的影响进行了分析。在此基础上利用正交试验得到单面PCM最佳方案：PCM厚度为25 mm，长径比为2，流速为0.254 kg/s。增加双面PCM可使得电池最高温度进一步降低，同时电池温度均匀性也有较大程度的提升。在进行位置优化后，可得到在水管偏移距离为60 mm时电池组温度最低，且温度一致性最好。将电池组中的空气改为PCM，可进一步降低电池组最高温度及温度差，分别为38.5°C和9.3°C。
Electric vehicles have great advantages in the field of energy conservation and environmental protection, and have attracted wide attention. As the only power source of pure electric vehicle, the performance of lithium ion battery will directly affect the overall performance of electric vehicle. In this paper, the heat dissipation problem of electric buses under high temperature and high current is studied by means of numerical simulation. Combined with the characteristics of electric buses, the thermal management system of phase change material(PCM) combined with water cooling was initially designed. The three dimensional battery model was established by Solidworks, and temperature field was analyzed by CFX. The heat dissipation effect was compared with the increase of PCM and water cooling, the feasibility of the scheme is preliminarily verified. At the same time, the influence of various factors are analyzed. Finally, the thermal management design is optimized to ensure that the battery can work normally under extreme conditions. The main works are as follows:(1) The temperature changes of the lithium ion battery are basically the same at the ambient temperature of 25 and 40 °C, the maximum deviation of the temperature rise is 0.3 °C. The simulation curves of 1C and 1.5C discharge for Li ion batteries at 40 are roughly in agreement with the experimental values, with a maximum deviation of 0.9 °C.(2) The expanded graphite / paraffin composite phase change materials with different proportions were prepared. The results show that the latent heat of PCM increases linearly with the increase of paraffin content, while the extent of phase change is not significantly related to the content of paraffin. The thermal conductivity decreases with the increase of wax, but has no obvious change in the direction of low heat conduction.(3) CFX software was used to simulate the temperature field of the battery structure. After a charge discharge cycle, the cell temperature increased by 33°C. After increasing PCM, the temperature of the battery group decreased to 66.8°C, but the temperature of the battery group increased with the increase of the charge discharge cycle times. On the basis of PCM, intermittent water cooling is added only when 2C is charged. At one charge and discharge, the battery temperature is further reduced, the charge and discharge cycle is carried out at 62.6 °C, and the battery temperature will not continue to rise after the increase of the temperature to 79.7 °C, which explain the preliminary of the scheme.(4) The effects of PCM, PCM thickness, pipe diameter, water speed and diameter on the heat dissipation of the battery were analyzed. The best solution of single side PCM is obtained by orthogonal test. The thickness of PCM is 25mm, the ratio of length to diameter is 2, and the water speed is 0.254kg/s. The maximum temperature of the battery can be further reduced by adding double side PCM. At the same time, the cell temperature uniformity also has a greater degree of improvement. The temperature of the battery group is the lowest when the displacement distance of the pipe is 60 mm, and the temperature consistency is best. Changing the air between batteries to PCM can further reduce the maximum temperature and temperature difference of the battery pack, which are 38.5 and 9.3 °C respectively.
