Abstract: The flow and heat transfer characteristics of falling water film outside horizontal tubes in falling film evaporators in seawater desalination were investigated with two-dimensional numerical simulation. Two types of tubes,including a smooth circular tube and six oval-shaped tubes were employed in the simulation.The physical model and mathematical equations for the circular tube and oval-shaped tubes are represented separately.Volume of fluid method is employed to investigate the influence of sectional shape of tube on distribution of film thickness and heat transfer performance.The numerical results indicate that the flow velocity of falling film on the oval-shaped tubes is larger and the film around the oval-shaped tube is more uniform and thinner than that around the circular tube.The film thickness grows with increasing semi-axis ratio,ε,and decreases gradually at first and then increases rapidly along the circumference.The minimum value of the film thickness appears approximately at the circumferential dimensionless position of 0.69 and 0.70-0.84 for the smooth circular tube and oval-shaped tubes,respectively.Compared with the circular tube,the oval-shaped tubes have lower dimensionless temperature and thinner thermal boundary layer,so that heat transfer performance is better.The oval-shaped tube with ε of 2.4 presents the best heat transfer performance with Nu of 0.32,which is 12.68% higher than that of the circular tube.The numerical results correlate well with the experimental and predicted data in literature.

Key words: oval-shaped tube, falling film, film thickness, Nusselt number

摘要： 针对海水淡化系统的水平管降膜蒸发,建立了二维数值模型,分析了光滑圆管和6种不同截面形状的蛋形管外降膜流动及传热特性。采用VOF方法考察了不同管型对管外液膜分布和传热特性的影响。数值结果表明:蛋形管外液体沿周向流动较圆管稍快,且可获得更均匀更薄的液膜;液膜厚度随半轴比ε增大而减小,随周向先逐渐减小后迅速增加,圆管和蛋形管的液膜最小值分别出现在周向相对坐标0.69和0.70~0.84附近。蛋形管的膜内量纲1温度较圆管的小,其热边界层厚度较薄,具有更好的传热性能。拟合数据得到,ε为2.4的蛋形管具有最好的传热性能,其Nusselt数可达0.32,较圆管的高出12.68%。最后,将数值模拟结果与文献中的数据进行了对比,验证了数值模型的可行性和合理性。

关键词: 蛋形管, 降膜流动, 膜厚, Nusselt数