Modelling and computation of cavitation and boiling bubbly flows with the NEPTUNE_CFD code

This paper focuses on the modelling and the numerical simulation with the NEPTUNE_CFD code of cavitation phenomena and boiling bubbly flows.Compressible, unsteady, turbulent 3D two-phase flow is computed by the NEPTUNE_CFD solver, developed jointly by EDF R&D and CEA. The numerical approach is based on a finite-volume co-located cell-centred approach and makes use of an original pressure-based multi-field coupling algorithm Mechitoua, N., et al., 2003. An unstructured finite volume solver for two-phase water/vapour flows modelling based on an elliptic oriented fractional step method. In: Proceedings of the 10th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH 10), Seoul, Korea].The cavitation nuclei come from wall nucleation or are pre-existing in the flow. Generated vapor bubbles are advected by the flow and expand in the regions where the local pressure is below the saturation with a tendency to agglomerate into slug bubbles.The model predictions compared with experimental data on enough selective local variables showed that satisfactory agreement could be obtained without any floating parameter to fit the data.The second part of the paper deals with boiling bubbly flow through a mixing device representing the effect of a fuel assembly spacer grid equipped with mixing blades (DEBORA-mixing experiment, CEA, Grenoble). Local measurements of the void fraction are provided downstream the mixing enhancer. The computations compare favourably with the experimental results; in particular, the global effect of the mixing blades was observed. A modification of the classical nucleate boiling model is proposed to overcome the strong model sensitivity with respect to near wall grid refinement.