54 Effects of Smooth Interface Assumption on the Accuracy of Simulation of Annular Two-Phase Flows

Two-phase flows, because of providing more heat transfer rate compared with single-phase flows, has drawn attention of many researchers. Two-phase flows commonly include flow regimes of different heat transfer coefficient, among those, annular flow regime usually experience maximum heat transfer coefficient. Flow management in this regime is usually a great deal because of probability of dry-out occurrence. Dry-out phenomenon considerably reduces heat transfer rate. Therefore, attempts should be made to prevent this phenomenon, to obtain as great heat transfer as possible. Entrainment is of parameters that highly affect probability of dry-out occurrence and its location. Entrainment in turn involves with some ripples of different frequencies that commonly form as a result of Kelvin-Helmholtz instabilities at liquid-steam interface in annular two phase flows. These ripples often grow to waves. Interfacial waves are the most significant potential of entrainment mass transfer. Most of two-phase flow simulations, simply neglect the effect of boundary waves. Such a simplifying assumption results in a considerable error in estimation of heat and mass transfer. In the present study, a theoretical approach is applied to simulate interfacial waves, in order to consider effects of these waves on heat and mass transfer phenomenon. In this regard, a ratio of evaporation contribution of mass transfer to entrainment contribution, due to existence of interfacial waves, is defined and computed to find whether it is rational to neglect these waves or not. The entrainment physiques and fundamental are studied through geometrical investigation of interfacial waves and the forces act on the waves. A general correlation for calculating entrainment is proposed based on the physical concept of entrainment phenomenon. The correlation is then confirmed to be consistent with empirical correlations.