水平管外水膜流型判据与特征长度

建立了蒸发式冷凝器水平管外水膜流动测试装置。观测发现水膜流动指示剂扩张角小于30°时,水膜湍流明显;扩张角大于50°时,水膜层流明显。研究了水平管外水膜流型随管径及喷淋量的变化,归纳出管外水膜层流临界Reynolds数阈值为15,湍流临界Reynolds数阈值为25。喷淋量增加的过程中,粗管比细管更容易达到湍流。管外水膜流动Reynolds数中的特征长度与水膜厚度一个量级。水膜越厚,特征长度越大;且管径越大,特征长度越大。

Flow pattern criterion and characteristic length of water film outside horizontal tubes

The test device of water film flow outside the horizontal tubes for evaporative condenser was set up to investigate the water film flow pattern outside the horizontal tubes and the characteristic length of water film flowing.Experiments were performed on a vertical row of horizontal tubes with stainless steel sheet between them.It is discovered by observations that water film gets into turbulent flow strikingly when the expansion angle of indicating liquid is less than 30° and the film goes into laminar flow obviously when the angle is over 50°.The changes of water film flow pattern outside the horizontal tube with the diameter and flow rate were studied experimentally.The results show that the critical Reynolds number threshold of water film outside the tube is 15 for laminar flow，and 25 for turbulent flow.With the increase of flow rate，the flow pattern of the film outside the larger diameter tube gets into the turbulent state earlier than the smaller one.The characteristic length of water film flow Reynolds number is the same order of magnitude as the water film thickness.The thicker the water film，the greater the characteristic length, and the larger diameter the tube，the greater the characteristic length.