Evaluation of Single-Phase CFD Applicability to Estimate Fluid Hot Spot Locations by Comparing Water DNB Data

For development of PWR spacer grids, it is necessary to confirm mechanical strength and thermal-hydraulic characteristics. Mitsubishi has applied Computational Fluid Dynamics (CFD) to thermal-hydraulic design of spacer grids. To satisfy the requirement of high thermal performance spacer grid, the compatibility of low-pressure loss and high Critical Heat Flux (CHF) performance is necessary; therefore, parametric CFD analyses have been carried out for new spacer grid designs. Related to spacer grid design, i.e., strap structures and mixing vane, the parametric analyses have been carried out to estimate pressure-loss of each spacer grid. Moreover, thermal analyses, where heat generation from fuel rods was taken into account, have been carried out to evaluate coolant mixing capability, which is assumed to relate the CHF performance, by comparing fluid peak temperature of each grid design. In our previous study (ICONE11-36087), the rod type Laser Doppler Velocimetry (rod LDV) and Particle Imaging Velocimetry (PIV) technique were applied to cross-flow and axial flow measurements in rod gaps and sub-channels to obtain reference data for verification of CFD estimation. Estimated velocity fields at the downstream of the grid were quantitatively compared with the measurements. As a result, it was confirmed that the CFD modeling estimated flow behavior in the rod bundle appropriately. In this study, CFD under single-phase condition, which took into account heat generation from rods, was performed to simulate flow conditions of water DNB test with the same design grid of the previous study. The correlation between estimated enthalpy distribution around fuel rods by the CFD and the CHF rods in the DNB test was examined. This study was performed in collaboration with Westinghouse Electric Company and Mitsubishi Heavy Industries, Ltd.