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

Modeling of annular flow with the computational fluid dynamics (CFD) is challenging as one has to consider several, rather different, phenomena simultaneously: the continuous liquid film, continuous gas core, and dispersed droplets. A morphology-adaptive multifield two-fluid model (MultiMorph) developed by Meller et al. ["Basic verification of a numerical framework applied to a morphology adaptive multifield two-fluid model considering bubble motions," Int. J. Numer. Methods Fluids 93(3), 748-773 (2021)], with three numerical phase fields, is well suited to simulate such multiple flow structures. Droplet formation plays an important role in annular flow, and a new droplet entrainment model is proposed, expressed as a phase morphology transfer term from the continuous liquid film to dispersed droplets phase field. The new model is developed based on the shear-off entrainment mechanism on the interfacial wave, implying that the droplet formation is dominated by the balance between the shear forces and the surface tension forces at the gas-liquid interface. In contrast to the existing entrainment models, the new model considers the flow parameters locally at the interface, and it is suitable for phase-resolving CFD frameworks without input of global parameters such as a pipe diameter. The proposed model is implemented in the MultiMorph framework based on the OpenFOAM Foundation release open-source CFD library. The performance of the new model is evaluated by conducting own annular flow experiments with void fraction measurements using electrical resistance tomography, as well as with comparison to published models from the literature. Qualitatively, the model can adequately resolve the formation of interfacial waves on the liquid film downstream from the inlet. The simulated droplets are primarily generated at the tip of such waves, which is consistent with the physical understanding and experimental observations of droplet entrainment. Quantitatively, the modeled entrained droplet fraction matches well the experimental observation in the developing entrainment region. The liquid film fraction obtained with the new model is analyzed and compared with the experimental data. Good agreement between measured and simulated statistics of the liquid film fraction, i.e., the mean, standard deviation, probability density function, and power spectral density, is demonstrated.