A generalised multiphase modelling approach with heat transfer and thermal phase change

The generalised multifluid modelling approach (GEMMA) was developed and implemented in OpenFOAM to address different interfacial scales in multiphase flows. The interface between two phases is tracked by adding an interface-resolving capability in computational cells occupied by large interfaces, while the model reverts to a standard multifluid formulation in regions of small/dispersed interfaces. In this work, the GEMMA model is further developed by adding the ability to predict heat transfer and thermal phase change processes. Additionally, the distinction between different interfacial scales is achieved by identifying appropriate heat transfer closures for each flow regime. The model is assessed against different test cases: predicting the condensation of single steam bubbles in subcooled water, condensation heat transfer at the interface in a steam-water co-current horizontal stratified flow, and direct contact condensation (DCC)-induced water hammer in a horizontal pipe. Comparison of simulation results with experimental data demonstrates the model successfully predicts the behaviour of condensing bubbles, heat transfer in turbulent stratified flow and effectively switches between large-interface and dispersed-interface flow regimes during the transition from a stratified into a slug flow in DCC-induced water hammer, which quantitatively captured the temperature drop due to the water hammering. Overall, the extended GEMMA model provides a comprehensive predictive tool for heat transfer and thermal phase change in multiphase flows, capable of simulating complex multiscale flows of industrial interest.