A Study on the Minimum Wetting Rate of Isothermal Films Flowing down on Outer Surface of Vertical Pipes

摘要

The minimum wetting rate (MWR) was investigated experimentally with an water film flowing down isothermally on the outer surface of test pipes arranged vertically. A dry patch was generated by blowing a small air jet onto the film temporary, and then observation was made to discriminate whether the dry patch was rewetted or not. The contact angle of the film at the top edge of the dry patch and the amplitude, length and velocity of large waves on the film were measured. The MWR decreased rapidly as the film flowed down and reached a nearly constant value at a position around 0.6 m down from the film inlet. The contact angle at the top edge of the dry patch varied periodically in a range synchronizing with the arrival of the waves. When the contact angle exceeded the maximum advancing contact angle, the rewetting of the dry patch was initiated. The existing correlations where the smooth surface film was assumed considerably over-predicted the MWR. The MWR was properly given by supposing that the dry patch is rewetted when the maximum of the fluctuating dynamic pressure of the film exceeds the upward component of the surface tension corresponding to the maximum advancing contact angle at the top of the dry patch.