SIMULATIONS OF TWO-PHASE FLOWS WITH A MULTIFIELD APPROACH
Résumé
Safety issues in nuclear power plant involve complex bubbly flows. To predict the behavior of these flows, the two-fluid approach is often used. Nevertheless, this model induces a natural diffusion of interfaces, which results in a poor accuracy in the calculation of the local parameters. Therefore, to simulate large interfaces such as slugs or free surfaces, located methods have been developed using the single-fluid model. In this paper, the two approaches have been coupled in the CMFD code NEPTUNE_CFD to simulate adiabatic separated flows. The averaged momentum balance equations are solved for each field and are followed by an artificial compression step, which fixes the interface thickness and ensures mass conservation. Moreover, since the two-fluid model allows the existence of relative velocities at the interface, a drag force is used to cancel them. This article proposes also a new formulation for this force, to take into account the physical properties of the flow. To validate this approach, an analytical test case with a static bubble has been simulated with a mesh refinement test. Then, the simulation of the Kelvin-Helmholtz instability has been performed to highlight the effect of the modification of the drag force. The sheared interface is particularly sensible to this force, which has an important influence on the flow parameters such as the interface velocity. Finally, these developments have been compared to three other codes by simulating the Rayleigh-Taylor instability.
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