对称/非对称冷却对制冷剂蒸气在微通道阵列内流动凝结换热影响的研究

Flow condensation heat transfer and vapor-liquid two-phase flow in microchannel arrays has become more and more important in recent years due to the increase in demand for thermal management of biochips, fuel cells, aerospace, integrated microchannel heat exchangers in energy and refrigeration engineering, etc. Most existing research on vapor condensation in microchannel arrays has focused on water vapor condensation with cooling on one side, with few studies on flow condensation of refrigerant vapor in microchannel arrays with symmetrical cooling. This proposal focuses on condensation heat transfer of refrigerants inside microchannel arrays fabricated by etching two identical borofloat glass wafers which will then be accurately bonded together to give symmetrical cooling. Experiments and numerical simulations will be conducted on the flow and heat transfer, including the two-phase flow regimes during condensation, the heat transfer characteristics, the non-uniform distribution of the vapor and liquid among the microchannels, and the microscale flow and heat transfer mechanisms based on the experimental visualization and heat transfer results. These investigations will describe the fluid flow and heat transfer characteristics from the experimental observations that define the microscale mechanisms. The research will also focus on the influences of the fluid flow and heat transfer non-uniformities, refrigerant physical properties, microchannel array configuration, microchannel dimensions and symmetrical versus unsymmetrical cooling on the heat transfer and two-phase flow characteristics. The research will identify the mechanisms controlling the micro flow condensation and will accumulate extensive experimental data with correlations and analysis methods for condensation heat transfer in microchannel arrays. The research achievements will extend analysis and design methods for this advanced technology which will serve as key methods for expanding condensation heat transfer research and utilization.

微通道阵列（Microchannel arrays）内凝结的流动与换热规律，是生物芯片、航天、能源、制冷及低温等领域的重要技术基础课题。目前相关研究多针对水蒸气单面冷却时的凝结，对有机制冷剂蒸气对称冷却条件下的流动凝结尚鲜有报导。本项目拟采用硼硅玻璃蚀刻、对称封装的矩形微通道阵列，研究上下表面对称/非对称冷却对微通道阵列内制冷剂蒸气流动凝结换热的影响。包括：冷却方式对凝结换热与气液两相流动规律的影响；凝结换热过程多个微通道内气液两相空间分布规律的差异；基于可视化研究的微观机理分析与宏观可测参量相结合的整体流动与换热特性描述。重点探析冷却方式、流动与传热不均匀、工质物性变化、微通道排布方式、尺寸效应等因素对流动凝结影响的微细观机理。通过研究，深化对微流动凝结换热机理的认识，积累和关联实验数据、发展分析方法。研究结果将为高技术应用奠定分析与设计的基础，对拓展凝结换热研究领域有重要学术意义。

高技术领域的发展对换热器提出了微型化、紧凑化的需求，推动了微通道换热器相变换热的研究。目前相关研究多针对水蒸气单面冷却时的凝结，对有机制冷剂蒸气对称冷却条件下的流动凝结尚少有报导。本项目采用硼硅玻璃和不锈钢蚀刻加工、对称封装的矩形微通道阵列，研究了上下表面对称/非对称冷却时有机制冷剂蒸气凝结的流动与换热规律。.对有机制冷剂蒸气在微通道阵列内流动凝结的气液两相流型、流型转变、两相流动压降与凝结换热进行了系统研究，揭示了微通道内制冷剂流动凝结的流型转变方式、机理以及对两相摩擦压降的影响。同时考虑了对称/非对称冷却方式对流型转变、两相压降的影响。通过对流型的拍摄，观测到了三种不同的连续流到间断流的转变形式，并给出了不同形式发生的判别条件。研究了双侧/底单侧冷却方式下方形微通道阵列内流动凝结换热和压降特性，发现底单侧冷却下的凝结换热系数显著高于双侧冷却下的凝结换热系数，对称冷却条件下的两相摩擦压降相比于非对称条件下更小，并揭示了相关的机理。.提出了近似椭圆形通道内流动凝结时的稳定环状流理论计算模型。基于线性不稳定性理论建立了计算微圆管内和近似椭圆形微通道内气液界面不稳定性的理论模型，并且与实验当中观察到的两种不同的波动实验结果进行了对比，验证了模型的可靠性。同时发现存在两种不稳定状态，这种状态改变可作为间断流发生的判据。.建立了模拟微圆管内流动凝结过程的二维瞬态数值模型。追踪气液界面演进过程，揭示了较小质量流率下喷射流形成机理。研究了质量流率、冷却壁温、饱和温度、管径对喷射流与间歇流的影响。揭示了喷射流与间歇流触发压力场振荡与界面波动的机理以及界面波动对换热的影响规律。分析了矩形宽高比、对称/非对称冷却热边界条件对方形微通道内流动凝结换热的影响，发现不同热边界条件下流型与速度场差异较大。