A GEneralized Multifluid Modelling Approach (GEMMA) : application to multiple flow regime phenomena in nuclear reactor thermal hydraulics

The generalized multifluid modelling approach (GEMMA) was developed to predict complex mixed multiphase gas-liquid regimes, where dispersed and large scale interfaces co-exist. In nuclear thermal hydraulics, the accurate modelling of these regimes is still an unresolved challenge that affects the prediction of phenomena such as pressurized thermal shocks, the counter-current flow limitation and boiling. The GEMMA solver addresses these issues by selectively applying a dispersed or a large interface multifluid formulation based on the local interface morphology. This is achieved using the local mesh resolution or the size of the dispersed phase, and without introducing an a-priori function of the local void fraction as available approaches often do. In this work, GEMMA is used to successfully predict a co-current stratified flow in a channel and the flow regime transition in a horizontal pipe, which provide some of the main challenges of key nuclear thermal hydraulic phenomena, but in simplified frameworks. The potential of the model in these areas is demonstrated and specific features validated. These include the suppression of turbulence at the interface that allows the prediction of stratified conditions and the coupling with a population balance model, which enables detection of the formation of gas plugs from bubble coalescence and the successive transition to the slug and stratified regimes in the horizontal pipe. Areas of improvement, such as improved modelling of the transition from continuous to dispersed interfaces, are also identified.