Three-fluid model predictions of pressure changes in condensing vertical tubes

Pressure changes in annular downward flows of condensing steam in vertical tubes are predicted with a three-fluid model. Several available correlations for the steam–liquid film interfacial friction are applied in the three-fluid model. Since discrepancies are obtained between calculated and measured pressure changes, a new correlation for the interfacial friction coefficient is proposed by taking into account the influence of the steam and liquid density ratio. The new correlation provides good agreement. The influence of the gravitational, frictional and acceleration pressure changes on the total pressure change is analysed, as well as the droplets entrainment from the liquid film surface to the steam core.