Simulation of droplet entrainment in annular flow with a morphology adaptive two-fluid model

The CFD modelling of annular flow is a challenge since it should take into account the continuous liquid film, the continuous gas core, and the dispersed droplets simultaneously. Recently, the CFD department in Helmholtz-Zentrum Dresden - Rossendorf (HZDR) are developing a morphology adaptive multifield two-fluid model that has great potential to simulate the three structures in a single computational domain. To complete the annular flow simulation in the morphology adaptive framework, a new droplet entrainment model is proposed in this work, which describes the mass transfer from the continuous liquid film to the dispersed droplets. The new entrainment model is developed based on the shear-off mechanism on the interfacial wave, implying that the entrainment behavior is dominated by the shear force and surface tension on the gas and liquid interface. Contrarily to previous entrainment models, the new model is correlated to the local flow parameters on the interface, so that it is suitable for the CFD framework. The properties of the new model are discussed and the model performance is verified by laboratory experiments. Qualitatively, the proposed model can simulate the generation of the interfacial wave from the inlet to the downstream. The droplets are mainly entrained at the front tip of the disturbance wave, where the shear force effect is remarkable. These phenomena are identical to the physical understanding of the droplet entrainment. Quantitatively, the characteristics of the droplet fraction matches the experimental results. The liquid film fraction obtained with the new model is analyzed and compared with the experiment. It turns out that the statistical parameters of film fraction's average value, standard deviation, possibility density function, and power spectral density match well with the experiment. Finally, the model comparisons show that the new model outperforms previous models with regard to droplet generation.