A Second Order Turbulence Model Based on a Reynolds Stress Approach 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 and high critical heat flux (CHF) properties. Numerical investigations on the effect of angles and position of mixing vanes and to understand in more details the main physical phenomena (wall boiling, entrainment of bubbles in the wakes, recondensation) are required. 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), especially the standard ε − K model, while the use of Reynolds Stress Transport Models (RSTM) remain exceptional. But extensive testing and application over the past three decades have revealed a number of shortcomings and deficiencies in Eddy Viscosity Models. In fact, the ε − K model is totally blind to rotation effects and the swirling flows can be regarded as a special case of fluid rotation. 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 a swirl in the coolant water, to enhance the heat transfer from the rods to the coolant in the hot channels and to limit boiling. First, we started to evaluate computational fluid dynamics results against the AGATE-mixing experiment: single-phase liquid water tests, with Laser-Doppler liquid velocity measurements upstream and downstream of mixing blades. The comparison of computed and experimental azimuthal (circular component in a horizontal plane) liquid velocity downstream of a mixing vane for the AGATE-mixing test shows that the rotating flow is qualitatively well reproduced by NEPTUNE_CFD but azimuthal liquid velocity is underestimated with the ε − K model. Before comparing performance of EVM and RSTM models, we seek to apply the Best Practice Guidelines (BPG) to quantify the numerical errors. Due to the geometry and mesh size of cases featuring spacer grids, only few sensitivity tests can be performed. Therefore, we applied some recommendations of the BPG to two cases with very similar conditions but with a simpler geometry, the DEBORA-tube case and the ASU-annular channel case. Then, a geometry closer to actual fuel assemblies is considered. It consists of a rectangular test section in which a 2x2 rod bundle equipped with a simple spacer grid with mixing vanes is inserted. The influence of the turbulence model on target variables linked to CHF limitation will be discussed. Moreover, the sensitivity to the mesh refinement will be particularly examined. The study of this case is a further step towards the modelling of the two-phase boiling flow in real-life grids and rod bundles.