分液冷凝器的管程理论设计及热力性能评价

根据分液冷凝器强化换热思想对其管程理论设计方法进行了研究。依据质量流速和干度来判断每一流程中制冷剂的流型,并依此选取Cavallini换热模型公式的方法求其平均换热系数,同时采用Cavallini两相压降模型和Darcy-Weisbach单相压降模型分别确定冷凝区和过冷段的压降。针对一个案例计算了三种管程设计方案下冷凝器管内冷凝换热系数和端压值,并用惩罚因子PF对其综合热力性能进行了评价。计算结果表明:不同的管程设计方案中管内制冷剂的流量分配均匀性存在较大的差异,均匀性越好,其综合热力性能越优。在质量流速为1200~1500 kg/(m2.s)范围内,与同等换热面积的蛇形管冷凝器相比,其中最好的分液冷凝器的PF值减小了48.5%~54.1%,可见设计优良的分液冷凝器的综合热力性能明显优于蛇形管冷凝器。

The Theoretical Design of Tube-pass and Thermodynamic Performance Evaluation of Liquid-vapor Separation Condenser

In order to know the heat transfer enhancement mechanism of liquid-vapor separation condenser, the theoretical design method of tube pass in the condenser was investigated. The Cavallini heat transfer model was chosen to calculate the in-tube mean heat transfer coefficient by the flow pattern estimated according to the refrigerant mass flow rate and average quality in each tube. The two-phase pressure drop model of Cavallini and the single-phase pressure drop model of Darcy-Weisbach were also applied to calculate the pressure drop of condensation zone and sub-cooling zone, respectively. In addition, a case with three schemes was designed here to evaluate the mean heat transfer coefficient and pressure drop, and its thermodynamic performance was evaluated with the Penalty Factor. The result indicated that different schemes of tube pass resulted in the variable refrigerant mass flux rate in tubes, and the more even refrigerant distribution in the condenser, the better its thermodynamic performance. The Penalty Factors of best one of three liquid-vapor separation condensers is about 48.5%~54.1% less than the serpentine condenser with the same heat transfer area at mass velocity of 1200~1500kg/(m2.s). That is to say, the thermal and dynamic characteristics of the liquid-vapor separation condensers with good design scheme are superior to the serpentine condenser at the same conditions.