管间距对水平管降膜蒸发流动形态和传热的影响

摘要

为了探讨管间距对水平管降膜蒸发的影响，以水为工质，研究了19 mm直径的铝黄铜水平管上降膜蒸发的传热与流动状况。通过对3种管间距在Re范围35～347下液膜流动形态的观测，归纳出水平管降膜流动流态转变时的临界Re数；测量了管壁表面温度和饱和蒸汽温度，计算得到管子表面的平均传热系数和局部传热系数，分析了管间距对局部和平均传热系数的影响。实验结果表明：在较大管间距情况下，不同流态间转变的临界Re数没有明显区别，但是在小管间距情况下，滴柱状向柱状以及柱状向柱帘状转变的Re数会变小；平均传热系数会随着管间距变小而降低，且当管间距为0．3倍管径时，喷淋密度的变化对平均传热系数的影响将变得不明显。此外，沿管周方向顶部的局部传热系数也随管间距变小而明显下降。%In order to investigate the influence of tube spacing on horizontal tube falling film evaporation, an exper-imental study is conducted on the heat transfer and flow pattern with 19mm out-diameter Al-brass tubes by using water as working fluid.This study focuses on the effect of three different tube spacing on the flow pattern and heat transfer characteristics.The variations of flow patterns with different tube spacing in the range of Re from 35 to 347 were observed and the critical Re numbers of flow pattern transition of the horizontal tube were obtained.The aver-age and local heat transfer coefficients were obtained from the measured temperatures of tube surface and saturated vapor by thermocouples.The local and average heat transfer coefficients are analyzed with the variation of tube spac-ing.The results showed that there is no obvious difference of critical Re number for pattern transition at larger tube spacing, but the critical Re number at small tube spacing will be small for patterns transition from droplet-column to column and column to column-sheet.The average heat transfer coefficient decreases with decreasing of the tube spacing.As the tube spacing is 0.3 times of tube diameter, the average heat transfer coefficient will be not related with the spray density.In addition, the local heat transfer coefficient on the top of the tube peripherally decreases with the decreasing of tube spacing either.