This study adopts computational fluid dynamics (CFD) to examine fluid dynamics and heat transfer characteristics for turbulent flows around pressurized water reactor (PWR) fuel rod bundles with support grids. The NESTOR experiment performed by CEA-EDF-EPRI provided accurate data for various variables (mean and RMS axial velocities, pressure drops, and rod inner-surface temperatures) for a 5x5 PWR fuel rod assembly with and without complex split-type mixing vane grids (MVGs).
This study investigates isolated CFD methodological factors for rod bundles with complex split-type MVGs under isothermal and single-phase heated prototypic PWR thermal hydraulic conditions. Examined CFD methodological factors include; mesh size, isotropic/anisotropic turbulence models, axial domain considerations, and conjugate heat transfer considerations for the single-phase heated problem. Reynolds Averaged Navier-Stokes (RANS) turbulence models were considered because of their practical feasibility in application to the real PWR fuel rod bundle.
Under a high-quality mesh, results from isothermal steady calculations adopted steady RANS turbulence models with a wall function (high-y+ treatment) can produce; (i) accurate pressure losses across MVG and SSG spans, (ii) comparable mean axial velocity (MVG and simple support grid (SSG) spans), and (iii) comparable and RMS axial velocity fluctuation (MVG span) profiles with NESTOR experimental profiles for the MVG rod bundle with alternating SSGs in the downstream regions. Heated steady calculations over-predicted axial and azimuthal rod inner-surface temperature distributions at various axial elevations in the MVG and SSG span wake region.
