钛椭圆低肋横槽管外降膜蒸发流热特性

Heat flow characteristics of Titanium elliptical low rib groove tube with outer falling film evaporation

为提高钛椭圆低肋横槽管外降膜蒸发器传热效率,在试验验证基础上对椭圆系数E在1.0~2.0内管外液膜流热特性进行多相流数值模拟。结果表明:管外液膜分布均存在不同程度干斑,且液膜在临近干斑处较其他区域增厚明显,当E为1.2时液膜覆盖率最高;液膜沿轴/周向均减速铺展,当E为1.0和1.8时管上端液膜速度在无量纲轴向长度Z*为-0.25~-0.375及0.25~0.375之间骤减,液膜厚度沿轴向从喷淋口正下方位置向喷淋口远端变化趋势为先减小后增大;最大传热系数随着E增大而增大,管壁在液膜与干斑交界区域附近传热系数最高;平均传热系数先增大后减小,相较圆管,E为1.2和2.0时平均传热系数分别增大34.5%和15.7%;槽内汽含率分布与液膜分布呈相反规律,液膜整体汽含率随E的增大先增大后减小,槽内/外平均汽含率差值随E增加而增大。

In order to improve the heat transfer efficiency of the falling film evaporator with a titanium elliptical low rib transverse groove tube,the heat flow characteristics of the liquid film flow outside the tube were numerically simulated with the ellipse coefficient E being in the range of 1.0 to 2.0.The results show that,there are different degrees of the dry patches outside the tube.The liquid film is significantly thicker near the dry patches than in other areas,and the liquid film coverage is the highest when the value of E is 1.2.The liquid film spreads slower in both of the axial and circumferential directions.The liquid film's velocity at the upper of the tube decreases abruptly between Z*=-0.25~-0.375 and 0.25~0.375 when E is 1.0 and 1.8,and the changing trend of liquid film's thickness along the axial direction from the position directly below the spray nozzle to the far end of the spray nozzle decreases firstly and then increases.The maximum heat transfer coefficient increases with the increase of E.The tube wall has the highest heat transfer coefficient at the junction between the liquid film and the dry patches,and the average heat transfer coefficient increases and then decreases.Compared to the round tube,the average heat transfer coefficient increases by 34.5%and 15.7%when E is 1.2 and 2.0,respectively.The distribution of vapor-holdup in the tank is opposite to that of the liquid film.The overall vapor-holdup of the liquid film firstly increases and then decreases with the increase of E,and the difference between the average vapor-holdup inside and outside the tank increases with the increase of E.