Hybrid Multiphase CFD Model for Bubbly, Slug, Churn and Annular Flow Regimes in Vertical Pipes

Abstract

Abstract Prediction of erosion rate or flow-induced vibrations and fatigue assessment depends strongly on the way multiphase flow occurs through the fitting such as bend, tee, manifolds, etc. Therefore, it is important to have a better understanding of multiphase behavior to improve confidence in erosion and fatigue prediction. Multiphase flow behavior is more complex in vertical pipe compared to horizontal pipe because of the phenomena generated when the gravity vector aligns with the axis of the pipe. Transport mechanisms for gas-liquid multiphase flow in vertical pipe broadly consist of (i) entrance effects due to short unstable liquid slugs which form at the entrance, (ii) a flooding effect as liquid film drains downward along the wall, (iii) a wake effect from one slug that can destroy neighboring slugs and, (iv) coalescence of gas phase entrained in liquid slugs. This paper presents a hybrid multiphase modelling approach, which combines the Eulerian-Eulerian model and the Volume of Fluid (VOF) model to effectively capture all these transport mechanisms. A hybrid multiphase model allows selective use of surface tension and interface sharpening schemes in a Eulerian framework for any flow regimes. Experimental dataset on 67mm and 127mm diameter vertical pipe from Abdulahi ([1], [14], [15]) and Azzopardi ([16], [17], [18]) were used to benchmark CFD modelling for bubbly, slug, churn, and annular flow regimes in vertical straight pipe. These data contain multiphase flow characteristic for air-water and air-oil at both atmospheric pressure and elevated pressure. The predicted results obtained with our model show good agreement with the experimental work and clearly highlight the benefits of applying the hybrid multiphase modelling to predict the flow regimes and to get better understanding of complex flow behavior in gas-liquid vertical flows. Furthermore, sensitivity studies were performed to understand the effect of drag models, inlet boundary treatment, solution initialization, mesh resolution, and time step size on CFD predictions for multiphase flow in vertical pipes.