管束相变储热器肋强化传热与二次流强化自然对流传热耦合机理

Heat transfer capacity of phase change material (PCM) is the critical factor for the efficiency of thermal storage of the shell and tube bundle phase change heat storage system (STBPCHS). The natural convection may be suppressed when the method of heat transfer enhancement only focus on the decreasing of conduction resistance. The main concern is to improve the performance of STBPCHS by changing this phenomenon. The coupling mechanism of heat conduction enhanced by the fins and natural convection enhanced by secondary flow in phase change domain will be studied in detail. The performance of STBPCHS can be improved by the active coupling action between natural convection heat transfer enhanced by secondary flow and heat conduction enhanced by the fins. The main contents include: (1) the effect of fin or tube wall on the performance of STBPCHS and on the natural convection heat transfer, including fin number, configuration parameters and liquid phase volume, interference mechanism of the enhancement of fin heat transfer and natural convection heat transfer; (2) the characteristics of secondary flow generated by vortex generator and the effect of it on the intensity of natural convection and heat transfer, the mechanism of the heat transfer enhancement related to the secondary flow of natural convection; (3) the coupling mechanism of natural convection enhanced by secondary flow and heat conduction enhanced by the fins, the effective coupling method not only for the enhancement of fin heat conduction but also for the enhancement of natural convection heat transfer, the coupling action criteria of natural convection enhanced by secondary flow and heat conduction enhanced by the fins. The prospective results include the new technique of increasing the heat transfer efficiency of STBPCHS and the theoretical basis for improving the performance of STBPCHS.

相变储热器相变侧传热能力是制约蓄/放热效率的关键因素。仅以拓展导热热阻较小路线的强化传热措施可能会抑制自然对流，而达不到强化传热效果。为了消除这一现象，提高相变储热器性能，拟开展相变区肋强化传热与二次流强化自然对流传热的耦合机理研究。目的是使肋强化传热及涡产生器产生二次流强化自然对流传热两者同向协调来优化储热器性能。以数值分析和实验为手段，研究内容包括：（1）储热区管外壁及外肋对自然对流强度及储热器性能的影响，肋结构参数、相变材料物性及液相容积对促进/抑制自然对流强度的影响规律及机理；（2）纵向涡产生器产生二次流的特性及其对自然对流强度和传热能力的贡献，自然对流传热的二次流强化传热特性及机理；（3）肋强化和自然对流强化传热的耦合机理，既强化肋传热与又强化自然对流传热的同向协调方法及判据。预期研究结果可获得提高管束相变储热器性能的新方法，为强化管束相变储热器传热性能提供基础数据和理论依据。

相变储热是提高能源利用效率的重要技术，而相变储热单元相变侧传热能力是制约蓄/放热效率的关键因素。为了提高相变储热单元性能，本项目以管壳式相变储热单元储热为研究背景，利用数值和实验的方法对储热单元内相变自然对流传热、肋片强化与自然对流强化耦合传热以及复杂区域相变传热等问题展开研究，所取得的主要研究成果包括：(1)获得了肋强化传热与自然对流强化传热的耦合关系，以拓展换热面积为主的肋强化传热手段，当肋片高度和数量的不断增加时，会导致液相流体的流动空间被分割，使得自然对流的发展受到限制，综合考虑肋强化传热与自然对流强化传热的相互制约关系，获得了肋片尺寸及数量的最优值；(2)提出了自然对流的评价参数r，利用该参数可有效地评价相变区液相自然对流传热的强度，当评价参数趋于1时，说明相变自然对流传热强度较弱，热量的主要传递方式为导热，而评价参数r的值越大则表明自然对流强化传热的效果越好；(3)考虑到自然对流对相变储热的重要影响，除加入肋片等拓展换热面积的强化传热手段外，还应考虑管壳相变储热单元的倾斜角度、热流体入口温差等对相变储热单元传热特性的影响，实验和数值结果均表明由于液相区自然对流的产生，会导致相变储热单元内部各区域传热速率不均匀，而这种传热速率的不均匀性是阻碍储热单元传热性能得到整体提升的重要因素。(4)获得了平衡相变储热单元内各区域传热速率以提升整体换热性能的方法与手段，对于多管管壳式相变储热单元储热过程而言，小幅提升下方换热流体的入口温度，即有利于提高下方区域的传热速率，也有利于创造热面朝上的液相流动空间，从而引导自然对流的产生和发展，缩小各区域传热速率的差异，提升整个相变储热单元的传热性能。综上所述，项目的实施揭示了自然对流强化传热与肋强化传热之间的相互制约关系，从改善自然对流及平衡各区域传热速率的角度，提出了强化储热单元传热特性的新方法，为提高管壳式相变储热器传热性能提供了基础数据和理论依据。