Development of a droplet entrainment rate model for a vertical adiabatic two-phase flow

Abstract

The authors have assessed some well-known droplet entrainment and deposition rate models against a wide range of vertical annular-mist flow experiments. The results of assessment showed that the Hewitt model predicts quite well for low-pressure air-water experiments and the Hewitt and the Okawa models are reasonably good for high-pressure steam-water experiments. However, the models still have large errors and significant deviations. Especially, it is known that their entrainment rate models are inappropriate to the developing region of an annular-mist flow because they were developed using experimental data estimated with the equilibrium assumption that the entrainment rate is equal to the deposition rate. To solve these problems, a new entrainment rate model is proposed here.Key variables for droplet entrainment are deduced from the analysis of experimental data and then the basic form of the new model is suggested as an empirical correlation. For the least-square fitting of the new correlation, the experimental data set of entrainment rate versus flow condition were needed. However, the entrainment rate cannot be measured in experiments but should be estimated. In this study, the droplet entrainment rates of 349 experiments were realistically estimated by using the continuity equation of the liquid film in an annular-mist flow, where the measured axial change of the liquid film mass flux can be taken into account, that is, the equilibrium assumption is not always needed. The new model was fitted using the data set and, then, assessed against 470 annular-mist flow experiments. The results showed that the new entrainment rate model in combination with the Okawa’s deposition rate model is definitely better than the existing models in terms of both accuracy and precision.