A Study on the Surface Tension Effect on Condensation Phenomena on the Thin Vertical Tubes

对垂直排列的细圆管外表面湿蒸汽凝结现象进行了观察和测量。凝结水的表面张力影响凝结水的流动模式，由于表面张力的不稳定性，凝结水表面出现了波纹状。对于外径为1.0 mm的管，观察到厚度均匀的光滑冷凝表面，而对于直径为0.5 mm的管，观察到厚度不均匀的波浪形状。随着管径的减小，波纹状变得明显。低凝结速率下的不稳定波长与Rayleigh分析自由液体射流的波长基本一致，但随着凝结速率和距顶部轴向长度的增加，不稳定波长也增加，而且采用运动图像分析法精确测量厚液峰之间的薄凝结膜厚度也是困难的。同时，将液体厚度的测量结果与VOF法的数值分析结果进行了比较，并在此基础上研究了细圆管外表面凝结水的特性。

Observation and measurement of steam condensation phenomena on the wetted outer surface of thin tubes arranged vertically were conducted. The condensate modes were affected by surface tension of the condensate, and the wavy shape of the condensate surface appeared due to surface tension instability on the thin tubes. While smooth condensate surface of uniform thickness was observed for tubes having an outer diameter of 1.0 mm, a wavy shape with non-uniform thickness was observed for tubes of 0.5 mm in diameter. The wavy shape became distinct with a decrease in tube diameter. Although the wavelengths of the instability at low condensation rates almost coincided with those obtained from analysis on free liquid jet by Rayleigh, they increased with increases in the condensation rate and axial length from the top.Furthermore, the precise measurement of thin condensate film thickness formed between the thick liquid humps by using the motion picture analysis. Also, the measuring results of liquid thickness were compared with the numerical analysis by using VOF method.On the basis of those results, the characteristics of condensate formed on the outer surface of thin tubes were investigated.