Numerical investigations of turbulent single-phase and two-phase flows in a diffuser

Abstract

This study presents numerical investigations of turbulent single-phase (water) and two-phase (air-water) flows in a horizontal diverging channel (diffuser), extending our previous experimental work (Mansour et al., 2018a). The main target is to examine and discuss the prediction accuracy of available computational fluid dynamics (CFD) models under such turbulent two-phase flow conditions with separation, based on direct comparisons with detailed experimental data. After performing a mesh-independence test, the numerical results for single-phase flows have been validated against experimental data of the axial pressure change in the channel. Four different turbulence models, including the Realizable k−ϵ, the k−ω shear stress transport (SST), the Spalart-Allmaras, and the Reynolds Stress Model (RSM) were considered and compared. The results show that the Realizable k−ϵ and RSM models can predict the pressure change in single-phase flows more accurately, while only RSM could as well predict a velocity field close to the experiments. Accordingly, only Realizable k−ϵ and RSM have been used for further two-phase flow simulations, which were performed using a transient setup together with the Volume of Fluid (VOF) method to model the interaction between the two phases. It was observed that only RSM performed reasonably well concerning flow regimes and air accumulations. Finally, considering higher flow rates, even the two-phase flow regimes predicted by RSM start to deviate from the experiments. The present study underlines the limitations of existing CFD models when applied to such complex two-phase flows.