机械振动对微通道内凝结液膜不稳定性和换热特性的影响机制研究

Microchannel heat exchangers have been widely used in vehicle condensers and their vibration is unavoidable. This project aims to study the condensation flow and heat transfer characteristics in micro-channel under mechanical vibration conditions. Firstly, the visualized image and pressure fluctuation signal of condensate gas-liquid two-phase flow in microchannels are obtained by experiment. The characteristic information of flow instability is extracted by spectrum analysis and wavelet analysis, and then the influence of vibration on the instability of condensate film in microchannel is analyzed. Secondly, the mathematical model of condensation flow and heat transfer in the non-inertial reference system of vibration is established. The variation of flow pattern, velocity field, temperature field and Kelvin-Helmholtz instability with vibration is simulated and analyzed. The effect of vibration inertia force on mass and momentum exchange at the free interface of liquid film is studied, and the mechanism of vibration on the instability of liquid film is revealed. Finally, the variation of condensation heat transfer coefficient with vibration is analyzed. Based on the field synergy principle, the influence of vibration on the synergy of flow field and temperature field in condensate film is studied, and the mechanism of the influence of vibration on condensation heat transfer characteristics in microchannels is revealed. A quantitative prediction model of condensation heat transfer coefficient in microchannels under vibration conditions is constructed. The research results will lay a theoretical foundation for the enhancement of condensation heat transfer in microchannels by vibration and the optimization design of condensers under vibration conditions.

车载冷凝器常采用微通道换热器且不可避免会产生振动，本项目拟针对机械振动条件下微通道内凝结流动和换热特性开展研究。首先，通过实验获得微通道内凝结气液两相流的可视化图像和压差波动信号，通过频谱分析和小波分析来提取流动不稳定性的特征信息，进而分析振动对微通道内凝结液膜不稳定性的影响规律；其次，建立振动这一非惯性参考系中的凝结流动与换热数学模型，模拟分析流型、速度场、温度场和Kelvin-Helmholtz不稳定性等随振动的变化规律，研究振动惯性力对液膜自由界面处的质量和动量交换的影响，揭示振动对凝结液膜不稳定性的作用机制；最后，分析凝结换热系数随振动的变化规律，基于场协同原理研究振动对凝结液膜内流场和温度场协同性的影响，揭示振动对微通道内凝结换热特性的影响机制，构建振动条件下微通道内凝结换热系数的定量预测模型。研究成果将为利用振动来强化微通道内凝结换热以及振动工况下冷凝器的优化设计奠定理论基础。

车载冷凝器常采用微通道换热器，在车辆在行驶过程中，受路面不平和发动机振动的影响，冷凝换热不可避免会受振动的影响。因为振动增加了流体扰动，一般认为振动会强化换热，但实验发现对于冷凝还可能出现弱化换热的情况。本项目拟针对机械振动条件下微通道内凝结流动和换热特性开展研究。首先，通过实验获得微通道内凝结气液两相流的可视化图像和压差波动信号，通过频谱分析和小波分析来提取流动不稳定性的特征信息，进而分析振动对微通道内凝结液膜不稳定性的影响规律；其次，建立振动这一非惯性参考系中的凝结流动与换热数学模型，模拟分析流型、速度场、温度场和换热系数等随振动的变化规律，揭示振动对凝结液膜不稳定性、换热系数和摩擦压降的作用机制。.研究结果表明，与静止工况相比，振动并未产生新的气液两相流流型，仅仅是影响了流型的细节特性。施加振动在大部分情况下强化换热，最大强化了26%，但部分工况下也会削弱换热，削弱幅度为5%左右。整体来看强化和弱化的变化规律十分复杂，换热系数的增强主要取决于振幅，振动频率的影响较小。振动条件下的摩擦压降大部分工况大于静止条件下的结果，与换热系数的变化规律并不一致，表明振动对换热和压降的影响机理不同。机理分析认为当液膜厚度较大时，液膜湍动的影响占主导；当液膜厚度逐渐减小时，液膜厚度的影响逐渐占主导。振动会增加液膜波动，一方面增加液膜湍动能具有强化换热的作用，另一方面振动导致液膜挤占气相空间，使平均液膜厚度增大具有抑制凝结换热的作用，两种机制相互竞争，导致振动既有增强换热又有削弱换热的作用。对于摩擦压降，振动会增大液膜湍动能使得摩擦压降增大，也会增大液膜厚度导致液膜流速减小使得摩擦压降减小，在不同情况下两种机制的影响不同，导致振动条件下摩擦压降也呈现复杂的规律。.本项目研究发展了振动条件下冷凝换热和压降理论，研究成果将为利用振动来强化微通道内凝结换热以及振动工况下冷凝器的优化设计奠定理论基础。