Resolution-Adaptive Modelling in Nuclear Safety: Free Surfaces and Bubbles

Process engineering and energy production systems often feature gas-liquid flows with coexisting two-phase flow regimes and a broad range of interfacial and turbulent scales. Suitable simulation methods can be found for each particular morphology, such as Volume-of-Fluid for larger, resolvable and continuous features of stratified flows, and the two-fluid model for unresolved dispersed bubbles or droplets. A morphology-adaptive multifield two-fluid model (MultiMorph) presented here is developed based on the OpenFOAM Foundation Release, with the aim to handle different coexisting dispersed and continuous flow structures for a wide range of spatial resolutions within a common computational tool.

The present work highlights specifically the following aspects of the model: a resolution-adaptive momentum transfer for under-resolved flow structures on coarse meshes, interface turbulence damping in strong shear flow near a gas-liquid surface with high density ratios between the phases, and morphology transfer models. This enables both transitions, disintegration and accumulation, between dispersed and continuous phase morphologies of the same fluid. Application of the MultiMorph model is presented on the following selected set of safety related test cases: a stratified counter-current flow case with partial flow reversal and liquid waves, and a plunging jet case, with entrainment of gas bubbles. Results are evaluated with measurements from the corresponding experiments.