Combined evaluation of second order turbulence model and polydispersion model for two-phase boiling flow and application to fuel assembly analysis

► High-thermal performance PWR (pressurized water reactor) spacer grids require both low pressure loss, high wall heat transfer coefficient and high critical heat flux (CHF) properties. ► Reynolds Stress Transport Models are substituted to K − ɛ turbulence model because it is totally blind to flows rotation occurring in spacer grids. ► Calculations in a geometry closer to actual fuel assemblies in PWR thermal-hydraulics conditions show that the K − ɛ and R ij − ɛ models may predict globally the same void fraction level because of errors compensations with the K − ɛ model downstream of mixing vanes. ► The effect of the angle of the mixing vanes which is of relevant interest to optimise the CHF, has been addressed in the paper with both turbulence models: a value of 30° seems to be as an optimum with the R ij − ɛ model, in good agreement with the value determined experimentally. In contrast, we were not able to show an optimal angle from the calculations performed with the K − ɛ turbulence model. ► The improvement of the bubbles population description is recommended in order to well predict the reactor thermal margin and safety. High-thermal performance PWR (pressurized water reactor) spacer grids require both low pressure loss, high wall heat transfer coefficient and high critical heat flux (CHF) properties. A further detailed understanding of the main physical phenomena (wall boiling, entrainment of bubbles in the wakes, recondensation) is needed and can be reached by numerical simulation. In the field of fuel assembly analysis or design by means of CFD codes, the overwhelming majority of the studies are carried out using two-equation eddy viscosity models (EVM) as turbulence model, especially the standard K − ɛ model, while the use of Reynolds Stress Transport Models (RSTMs) remains exceptional. In contrast, extensive testing and application over the past three decades have revealed a number of shortcomings and deficiencies in eddy viscosity models. Indeed, the K − ɛ model is totally blind to flow rotation, e.g., in the presence of swirls. This aspect is crucial for the simulation of a hot channel in a fuel assembly. In fact, the mixing vanes of the spacer grids generate swirls in the coolant water, which enhances the heat transfer from the rods to the coolant in the hot channels and thus limits boiling. In this work, all the models are implemented in NEPTUNE_CFD, a three dimensional multi-fluid code developed especially for nuclear reactor applications,