Prediction of bubbly flow and flow regime development in a horizontal air-water pipe flow with a morphology-adaptive multifluid CFD model

Most multiphase gas-liquid flows of industrial and engineering interest often encompass multiple flow regimes and the transition between them. The wide range of interface scales involved is challenging to model, and this has so far limited the application of computational fluid dynamics to multi-regime flows and complex multiphase flow conditions. The morphology-adaptive GEneralized Multifluid Modelling Approach (GEMMA), developed in OpenFOAM, is designed to provide all-flow-regime modelling capabilities. The model implements in the multifluid modelling framework interface-resolving capabilities that are used to treat large-scale interfaces found in segregated flow regimes, while dispersed regimes remain modelled with the standard multifluid approach. In this paper, GEMMA is used to predict, for the horizontal pipe flow studied in the METERO experiment (Bottin et al., 2014), the development of the bubbly, plug, slug and stratified flow regimes starting from a homogeneous 1 mm bubble distribution at the inlet of the pipe. In the bubbly regime, the model predicts well the void fraction and bubble diameter distributions, but not the lower flow velocity when a bubble layer accumulates at the top of the pipe. Results also show that modelling closures developed mainly for vertical flow conditions, and which are a strong function of the relative velocity, may not be equipped to predict horizontal flows where relative velocities can be negligible. Beyond the bubbly regime, the model predicts the development of intermittent gas plugs, the increase in the length scale of the plugs approaching the transition to slug flow and the development of a stratified flow at the lowest water flow rate. The velocity of gas plugs is found to be in good agreement with literature models. Challenging to predict remains the transition region from bubbly to plug and from slug to stratified flow, where an anticipated transition to stratified flow is predicted in the slug regime. Overall, GEMMA provides a morphology-adaptive modelling framework that can achieve all-flow regime applicability, and the present work is a first demonstration of its capabilities for horizontal flow regimes. Short-term development needs are highlighted, such as additional validation and the improvement of bubbly flow closures, and the modelling of the dispersion and breaking-up of large interfaces to prevent excessive phase agglomeration.