The minimum wetting rate (MWR) of an isothermal R-113 falling film on an outer surface of a vertical 25 mm OD pipe was examined. The fundamental characteristics of waves (the amplitude, frequency, velocity and wave length) on the falling film were also investigated in the range of the film Reynolds number Ref = 60 ∼ 600. The MWR was not obtained even at extremely small flow rate of 0.38 cc/min (Ref = 0.70). That flow rate was much smaller than the value expected for the smooth surface film although the surface observed was flat. It was pointed out that another factor such as the surface tension or the wettability might need to be included for the MWR expression. The mean film thickness was well represented by the Nusselt film thickness equation although the film surface was covered with many waves. In Ref ≤ 130, the waves were symmetrical and did not deform as the waves proceeded to downstream. When Ref ≥ 300, the waves were like roll waves and the amplitude grew as the waves traveled. The wave amplitude, frequency, velocity and length all showed similar variation tendency that each value changed the trend of variation for Ref across Ref ≅ 300. The waves behaved like theoretical small-perturbation waves in Ref ≤ ∼ 300 and the characteristics of the waves deviated from that when Ref > 300.

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