新型低碳制冷剂R450A在折面螺旋折流板管束间冷凝机理及其传热强化的研究

At present, the shell and tube heat exchanger with segmental baffles is mainly used in water-cooled chiller as condenser and the heat transfer performance is weakened. The improved shell-and-tube condenser with fold helical baffles is proposed to be used in water-cooled chiller. The thickness of the liquid film can be effectively reduced and thus heat transfer can be enhanced. R450A was listed in new refrigerant replacement plan by the environment agency (EPA) in 2014, which is a promising candidate to R134a in a water-cooled chiller and other refrigeration systems. In the proposal, R134a and R450A are adopted as working fluids. The configuration of shell-and-tube heat exchanger with fold helical baffles will be optimized with an improved algorithm combing a Kriging response surface and the multi-objective genetic algorithm (MOGA).The condensation mechanism and heat transfer enhancement on complex wall of near-azeotropic mixture R450A will be theoretically studied. And the condensed phase transformation mechanism and condensate migration patterns between tube bundles with fold helical baffles will be studied through visualization experiments. The condensation heat transfer experimental study are performed in shell-and –tube heat exchangers with segmental baffles, sector helical baffles and optimum fold helical baffles, respectively. The experimental correlations of condensation heat transfer and pressure drop for different refrigerants in optimum shell-and –tube heat exchanger with fold helical baffles will be obtained. The conclusions of the investigation will provide theoretical guidance for the application of R450A and optimum shell-and-tube condenser with fold helical baffles in water-cooled chiller.

针对冷水机组常用的弓形折流板冷凝器传热恶化问题，提出采用改进型的折面螺旋折流板冷凝器来有效减薄管束液膜厚度而强化换热。美国环境署2014年提出新型低碳的替代制冷剂R450A，以期在冷水机组等制冷系统中对R134a等进行替代。申请人拟以R134a和R450A为工质，结合CFD多参数化数值模拟，对折面螺旋折流板换热器进行多目标遗传算法结构优化；并对近共沸混合物R450A在复杂表面的局部冷凝机理及其传热强化进行理论研究；通过可视化的冷凝流动换热实验，探索制冷剂在折面螺旋折流板管束间的冷凝相变传热机理以及凝液迁移流型的转变规律；在此基础上，分别对弓形折流板换热器、扇形螺旋折流板换热器和优化后的折面螺旋折流板换热器进行冷凝相变换热实验研究，并拟合制冷剂在折面螺旋折流板换热器中冷凝相变传热过程的压降和传热系数的分段实验关联式，为折面螺旋折流板冷凝器和R450A在冷水机组中的应用提供理论基础。

针对常用的弓形折流板冷凝器传热恶化问题，本研究提出螺旋折流板冷凝器来有效减薄管束液膜厚度而强化换热。研究从螺旋折流板换热器的结构优化和制冷工质的流动冷凝两方面开展，以数值模拟和实验研究为方法，最后以换热器管束间的流动冷凝可视化实验探究了工质冷凝相变传热机理以及凝液迁移的规律。.开展了螺旋折流板换热器折流板结构优化研究。利用流体力学软件对弓形、扇形和折面螺旋折流板换热器的流动换热应力性能进行了对比研究。结果表明相比弓形折流板，平面螺旋折流板的单位压降换热系数和单位功耗换热系数增加了62.2%-65.7%，折面螺旋折流板增加了73.9%-76.1%。结合流固耦合和遗传算法，得到了三组最优的螺旋折流板换热器结构。.进行了R134a、R1234ze(E)和R450A在微细通道内流动冷凝的数值计算研究。讨论了质量流速、管径和管型对冷凝换热系数和摩擦压降梯度的影响，分析了冷凝过程中的液膜厚度、气液相界面、液相速度和流线的分布特性，探讨了重力、表面张力和液相流态对冷凝换热特性的作用机制。对倾斜圆管内的流动冷凝进行了数值模拟，揭示了管倾斜角对冷凝换热和压降特性的影响机制。.开展了管束间冷凝相变的可视化实验研究。观察了传热管表面冷凝液膜的分布和蒸气在管束通道间冷凝相变传热的过程，研究了蒸气流速和几何结构变化过程中冷凝液在管束通道间形成和迁移规律的差别，讨论了不同位置换热管的冷凝传热特性。结果表明在整个实验系统达到热平衡后，测试件内的冷凝状态为膜状冷凝。管间距变化对冷凝过程的影响主要基于工质的流场变化。.进行了R450A和R134a、R1234ze(E)在水平和倾斜微细紫铜光管内流动冷凝换热对比实验。提出了一套可预测R1234ze(E)管内冷凝换热系数和压降梯度的冷凝换热模型以及压降模型。换热模型的平均绝对偏差和平均相对偏差分别为19.3%和-2%。压降模型的平均绝对偏差和平均相对偏差分别为20.8%和6.9%，为相变换热器在工业实践的应用中提供指导，补充了新型环保工质R1234ze(E)和R450A的实验数据库。