An Enhanced Generalised Multiphase Modelling Approach for Slug Flow Boiling

Boiling flows are important in many industrial applications as they significantly enhance heat and mass transfer due to the large interfacial area between the different phases. However, these systems are complex, exhibiting various flow regimes based on phase distribution patterns. Recently, the use of computational fluid dynamics for predicting complex flows has increased, leading to advances in addressing thermal-hydraulic safety challenges. Among these, the Generalised Multifluid Modelling Approach (GEMMA) has been developed and implemented in OpenFOAM to allow prediction of the diverse interfacial scales encountered in multiphase boiling flows. This study enhances GEMMA to predict nucleate boiling flow behaviour across low and high volume fractions, with slug flow emerging near the wall in the latter. This advancement refines the Rensselaer Polytechnic Institute (RPI) boiling model, originally designed for small, spherical bubbles along heated surfaces, by incorporating a mechanistic force balance to capture bubble dynamics. This accounts for growth and detachment processes influenced by microlayer evaporation, heat transfer from superheated liquid, and condensation at the bubble cap due to subcooled liquid. Performance in the dispersed regime is evaluated by comparison with an Eulerian-Eulerian model, and validated against experimental data for a vertically upward subcooled boiling flow of refrigerant R12. Furthermore, the RPI framework is extended to encompass slug flow dynamics, where high gas volume fractions near the wall lead to slug formation from bubble coalescence on the heated surface. The accuracy of the enhanced GEMMA model is confirmed through comparisons with experimental studies of flow boiling on a downward--facing heated surface, featuring sliding slug bubbles along a channel under varying wall heat fluxes.