The analysis of a mechanism of liquid replenishment and draining in horizontal two-phase flow

There is a regime of two-phase flow in which large waves or surges pass rapidly along a horizontal tube accompanied by splashing, wave-breaking and entrainment with the result that water is thrown to the upper surface of the tube. Between surges the film on the top surface is depleted by draining under gravity and by evaporation if the tube is heated. If the interval between surges is sufficiently long a dry patch may begin to form. In this paper, theory is given for the calculation of the film thickness left behind on the top surface and for the calculation of the time to dryout. The theory includes both the effect of the boundary layer developement during replenishment of the film and also the effect of the axial deceleration of the film at the point where the liquid replenishment ceases. Finally, the predicted variation of film thickness is compared with experimental film thickness traces obtained in this type of horizontal two-phase flow. The agreement is found to be very satisfactory. This analysis is of interest in connection with the prevention of permanent and intermittent dryout at low qualities in nuclear power station evaporators.