快速失压流体跨临界区瞬态传热机理及预测模型构建

The expander in the supercritical organic Rankine cycle system is prone to leakage of working fluid, and the amount of leakage directly affects the performance of the circulation system. However, the rapid depressurization process of supercritical fluids involves the migration of transcritical regions and the complex thermal properties and transport properties, resulting in the heat transfer characteristics and mechanism of which have not yet been clarified. Based on this, the project uses a combination of mechanism experiments and theoretical modeling to carry out the study of transient heat transfer of supercritical R134a in the transcritical phase during pressure dips. Firstly, through visualization experiments and dynamic analysis techniques, the effects of various parameters on the morphological changes of fluids during the transcritical migration process are analyzed, and the influence of morphological changes on transient heat transfer characteristics is explored. Furthermore, the characteristic parameters are collected at high frequency, and the oscillation characteristics of temperature and pressure signals are characterized. The heat transfer and mass transfer coefficient correlations of multi-parameters are established and the transient heat transfer mechanism in the transcritical phase is revealed. Finally, based on the momentum transfer between gas and liquid, a mathematical prediction model considering the gas-liquid phase interface is constructed to obtain transient heat transfer characteristics, and the influence mechanism of gas-liquid interface on non-equilibrium heat transfer is elucidated. The research results will enrich the basic laws of transient phase change heat transfer, which can provide an important theoretical basis for the optimization design and thermal efficiency of the organic Rankine cycle system.

超临界有机朗肯循环系统中的膨胀机易发生有机工质泄漏，泄漏量的大小直接影响着循环系统的性能。然而，超临界流体快速失压过程涉及跨临界区的迁移以及复杂的热物性和输运特性，但其传热特性及机理尚未明晰。基于此，本项目采用机理实验和理论分析相结合的方法，开展压力突降时超临界R134a在跨临界阶段的瞬态传热研究。首先，通过可视化实验和动态分析技术，剖析跨临界迁移过程中各参数对流体形态变化特征的作用，探求形态变化对瞬态传热特性的影响规律。然后，高频采集温度和压力信号，并对其振荡特性进行表征，建立适用跨临界区且考虑多参数的传热传质系数关联式，揭示跨临界阶段的瞬态传热机理。最后，在气液间动量传递的基础上，构建考虑气液相界面的数学预测模型，获得瞬态传热变化特性，阐明气液相界面对非平衡传热的影响机制。研究结果可丰富瞬态相变传热的基本规律，为有机朗肯循环系统的优化设计及热效率的提高提供重要的理论依据。

超临界有机朗肯循环发电技术采用低沸点有机物作为工质，在中低温热源驱动下可获得较高的能量利用效率，具有更好的经济性。膨胀机是有机朗肯循环系统中的重要设备，然而由于其结构部件很小、工况复杂、制造工艺极高等特点，易造成有机工质的泄漏，对发电系统循环性能乃至环境的影响亟待解决。因此，本项目以超临界有机工质压力突降过程中的热动力学变化特性为主线，采用可视化实验和理论分析相结合的方法，深入研究高压及超临界流体的瞬态传热特性。具体研究内容包括：①快速失压流体形态变化特征及瞬态传热特性实验研究；②微小通道内高压流体传热特性研究；③快速失压流体瞬态传热数值模拟研究。获得的主要研究成果为：①可视化观测了高压流体在快速失压过程的形态变化特征，动态监测了温度和压力的振荡特性，分析了热动力学变化变化规律，发现在跨临界区迁移闪蒸阶段变化趋势与亚临界流体相反；②实验研究了高压流体在微小通道内的流动和传热特性，并分析了不同因素下的变化规律，建立了传热系数关联式，计算值与实验值吻合良好，误差均在±10%以内；③针对高压有机工质溶液压力突降过程中的传热特性建立了数学模型，模型考虑了降压过程引起的气流运动、液体表面的蒸发换热和对流传热、以及液体内部的有效热传导。对比了实验数据和计算结果，验证了模型的可靠性。降压蒸发过程中，在相同初始温度下，初始压力越高，溶液的蒸发速率越快，温度下降的也越快。该研究有助于非平衡热力学传热规律及多相流瞬态传热机理的进一步发展，对超临界有机朗肯循环系统的优化设计及热效率的提高具有重要的理论指导意义，是具有实际应用价值的基础性研究工作。