近饱和液体工质自激振荡强化传热机理及尺度效应研究

Novel heat transfer device with high heat flux, simple structure, without external driving work can be achieved by utilizing the technique of self-excited oscillation in near-saturation liquid, which has wide application prospect in such fields as high heat flux electronics cooling, heat recovery, heat dissipation in hypergravity and microgravity environment. However, the mechanism of energy conversion and transport is still not clear. In this project, based on numerous new experimental phenomena of the research team, the effect of enhancing heat diffusion and convection by internal oscillating flow is considered as the main mechanism of enhancing heat transfer, while the internal instability of the near-saturation liquid in a Constant Volume System is the internal cause for self-excited and self-sustained oscillation. Mico-PIV visualization technique is to be used to observe and analyze the dynamic evolution of the flow field from the startup to the stable operation, to accurately measure and analyze the variation of waveform, amplitude and frequency of the pressure wave, and to reveal the mechanism of enhancing heat transfer. Non-linear mathematical model and quantitative prediction method are to be created based on the conservation equations of the compressible fluid and the special thermophysical properties of the near-saturation liquid, so as to acquire accurate correlations in dimensionless form among the oscillating flow characteristics, the heat transfer characteristics, the geometrical parameters, the thermophysical properties and the thermal boundary conditions. The micro-scale effects on the starting performance, the oscillating flow characteristics and the heat transfer characteristics are to be thoroughly investigated, in order to provide theoretical basis for the realization of miniaturized, flexible heat transfer device with even higher heat flux and longer transport distance.

利用近饱和液体自激振荡技术可形成结构简单且无需外功驱动的新型高强度传热元件，在高热流密度电子器件冷却、余热回收、超重力和微重力环境下散热等领域有着广阔应用前景，但目前对其能量转换与传递机理尚不明晰。本课题基于研究团队大量最新实验现象，认为内部振荡流强化热扩散和对流作用是强化传热的主要机制，而定容系统内近饱和液体自身不稳定性是自激、自持振荡的内因。拟借助于micro-PIV可视化观测技术，分析从启动到稳定工作过程中流场的动态演化，精确测量和分析压力波波形、振幅和频率的变化规律，揭示其强化传热机理。从可压缩流体守恒方程组和近饱和液体特殊热物性出发，建立非线性数学模型和定量预测方法，获得振荡流特性、传热特性与几何参数、热物性及热边界条件之间精确可靠的无量纲特征参数关联式；深入探讨尺度微细化对启动特性、振荡流特性及传热特性的影响规律，为实现微型化、柔性化、更高热流密度、更远距离热传输提供理论依据。

本项目以高热流电子器件冷却、高温器件热防护及热能利用过程强化传热为背景，研究了一种闭式两相环路传热器件。我们从可压缩流体质量、动量和能量守恒方程组出发，采用NIST的真实流体热物性方程（包括近临界和超临界区域），对刚性封闭环路中靠近饱和线和临界温度（0.7Tr < T < 1.1Tr）的液态R134a工质建立了非线性数学模型，并进行了一系列数值模拟，再现了压力波和速度场非线性自激振荡的动态演化过程，初步摸清了传热器件内部压力波的形成条件，并分析了其强化传热的物理机制。我们制作了R134a工质传热环路，以氧炔焰为热源，测试验证了两相传热环路的超强传热能力。我们搭建了流场可视化实验观测平台，实现了与压力波和温度场的高速同步测量，开展了不同充液率、不同热流密度条件下传热环路中两相流流型、压力和温度波动、传热特性实验研究。我们利用功率谱密度（PSD）和标准差（SD）分析了不同工况下的波动频率和振幅，研究了热流负荷、充装率、倾斜角对于热阻、温度均匀性及传热能力的影响规律，分析了高充液率条件下的气泡泵效应，以及不同工况下各自的传热机制和运行特性。我们研究了R134a、无水乙醇、纯水等工质的间歇沸腾不稳定现象，探究两相流不稳定性（波动特性）及其对传热特性的影响规律，获得了间歇沸腾的流型动态变化规律、发生条件和影响因素，并绘出了不稳定性图谱。我们还制作了多件NaK合金工质的高温两相传热环路实验件，进行了加工、充装和实验测量验证，并建立了流动与传热数学模型及数值计算程序。项目实施过程中，参加国际、国内学术会议各2次，截止目前已发表论文12篇（有标注9篇），其中SCI检索6篇（有标注4篇），获发明专利授权3项，省部级二等奖1项，培养硕博连读生2名。通过4年的努力，基本上解决了设定的关键科学问题，实现了预期目标，完成了设定研究任务，为高性能传热器件的优化设计奠定了可靠的理论依据和关键数据支持。