全干度区间内微小通道流动沸腾传热的均温特性研究

Direct refrigerant cooling technology for battery thermal management on EV shows the advantages of compact structure and higher heat transfer efficiency, but there are still some problems such as nonuniform mass flow distribution and overheating at high vapor quality. In this project, focusing on the micro-channel flow boiling under the condition of low heat flux in this technology, the flow pattern at vapor quality range from 0 to 1, the laws of droplets entrainment and the mechanism of dryout in annular flow will be studied systematically. Experimentally, Micro-PIV particle image technology and high speed micro-photography will be comprehensively used to observe the flow pattern and flow field distribution in a single tube. And the heat transfer coefficient at different flow patterns will also be explored. Combining numerical simulation, capacitance tomography and laser focus displacement meter, the laws of droplets entrainment in the transition from plug flow to annular flow, the liquid film dryout and the deposition of droplets in annular flow will be clarified. Theoretically, through establishing or modifying the current physical model of annular flow and comparing with the experimental results, the relationship between structures of the tube and local vapor quality when partial dryout occur can be disclosed. The research results will enrich current two-phase flow theory, and then provide theoretical basis for optimizing the design of multiple micro-channel cold plates that can avoid overheating at high vapor quality, so as to make the direct cooling technology of battery thermal management mature as soon as possible.

新能源汽车电池包的冷媒直冷技术具有结构紧凑、换热效率高等优势，但是尚存在如流量分配不均、高干度下过热等问题。本项目针对直冷技术中的低热流密度微小通道流动沸腾，对全干度区间内的流型分布、高干度下环状流的液滴夹带规律和干涸机理进行深入研究。实验方面，结合超高速显微摄像、Micro-PIV粒子图像测速技术研究单通道内全干度区间的两相流型变化以及流场分布，并研究不同流型下换热系数的差异。结合数值仿真、电容层析成像和激光共聚焦位移测量技术，研究塞状流向环状流转变时液滴夹带规律，以及环状流中液膜的干涸和夹带液滴在液膜上的沉积规律。理论方面，通过建立或修正现有的环状流物理数学模型，揭示环状流中通道结构对管壁液膜局部干涸的影响机理。上述研究成果将丰富和发展现有的两相流动理论，进而为避免高干度下过热的多重微小通道冷板的优化设计提供理论依据，推进电池热管理直冷技术走向成熟。

电动车电池冷媒直冷技术具有系统简单重量轻等多种优势，但两相流动的不稳定性所致的均温性问题一直是此技术的瓶颈。此外电池的热流密度较低，高出口干度下质量流速较小。本项目针对低质量流速、低热流密度、扁平通道（冷板重量轻，流道分配数目小）有机工质两相流动流型、传热均温性以及流动分配问题进行了深入研究。建立了两相流可视化实验平台，获得了流型图和换热系数，结果表明低质量流速下流型以环状流为主体，没有雾状流，换热以通道中间液膜蒸发两侧液体补充为主，而高干度下存在部分干烧导致换热系数骤降。通过数值仿真方法进一步揭示了该类型两相流换热的机制。为解决高干度下干烧问题拓展研究了表面铺设有多孔介质通道的两相流换热特性，结果表明由于多孔介质的毛细作用使得通道表面液膜分配更均匀，改善了高干度下均温特性。此外用VOF仿真研究了多排通道两相流动能提供分配特性。本项目的研究为冷媒直冷技术冷板的设计提供了实验和理论依据。