Abstract:

The ternary lithium battery is increasing being used in the field of electric vehicles. Studying the jet velocity field of the jet valve during thermal runaway of the battery is crucial for deducing critical pressure and pressure changes inside the battery. This information holds significant importance in optimizing battery design and accurately predicting flame propagation. However, lithium batteries often have two ejections during the thermal runaway process. The ejections comprise chemical reaction gas production, electrolyte evaporation, as well as particles and fragments from various battery components. In addition, the evaporated electrolyte condenses after cooling, resulting in a gas-liquid-solid three-phase mixed state within the jet valve jet process. This complexity increases the difficulty of identifying and testing the velocity field. Therefore, in order to realize the visualization of the jet flow of the jet valve and the identification and calculation of the velocity field under high temperature conditions, an experimental bench composed of a high-speed camera, a laser system and a battery explosion-proof box was constructed in this study. The high-speed camera was used to capture and process the thermal runaway jet process of the battery. Firstly, a preliminary analysis of the flow field flow pattern was carried out. Then, the adaptive filtering algorithm effectively reduced imaging noise and enhanced droplet edge detail information. Histogram equalization was then used to further improve image contrast. Finally, the cross-correlation algorithm based on sub-pixel precision interpolation was used to analyze and calculate the velocity field of the jet flow field. This approach yielded velocity field data on a two-dimensional plane at different times intervals. The velocity characteristics of different regions at different times were further analyzed, which provides a foundation for the subsequent research on the mechanism of battery thermal runaway and the improvement of battery safety performance.

Key words: imaging, electrolyte, two-phase flow, image preprocessing, velocity field

摘要：

三元锂电池在电动汽车领域得到了越来越广泛的应用。对电池发生热失控时的喷阀射流速度场开展研究，将有助于反推热失控过程中电池内部的临界压力及压力变化，对辅助优化电池设计、准确预测火焰传播等具有重要意义。但是，锂电池在热失控过程中往往存在两次喷射，喷射产物不仅包括电池内的化学反应产气、电解液蒸发等，还包含部分颗粒和电池内部各部件的碎片等；此外，蒸发后的电解液也会在遇冷后凝结，使整个喷阀射流过程呈现气-液-固三相混合状态，提高了速度场辨识与测试的难度。因此，为实现在高温情况下喷阀射流的可视化及速度场辨识计算，本研究搭建了主要由高速相机、激光系统和电池防爆箱组成的实验台架，采用高速相机对电池热失控射流过程进行拍摄及处理。首先对流场流态开展初步分析；然后通过自适应滤波算法，有效降低成像时噪声的影响，并增强液滴的边缘细节信息；同时利用直方图均衡化进一步提高图像的对比度；最后，采用基于亚像素精度插值的互相关算法对喷射流场的速度场进行分析计算，得到了不同时刻的二维平面下的速度场数据，并进一步对不同时刻不同区域的速度特性进行了分析，为后续电池热失控机理研究和电池安全性能提升工作提供了基础。

关键词: 成像, 电解液, 两相流, 图像预处理, 速度场