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

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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.