Lateral flow fields in four subchannels of a model rod bundle fuel assembly are experimentally measured using particle image velocimetry. Vanes (split-vane pairs) are located on the downstream edge of the support grids in the rod bundle fuel assembly and generate swirling flow. Measurements are acquired at a nominal Reynolds number of 28,000 and for seven streamwise locations ranging from 1.4 to 17.0 hydraulic diameters downstream of the grid. The streamwise development of the lateral flow field is divided into two regions based on the lateral flow structure. In Region I, multiple vortices are present in the flow field and vortex interactions occur. Either a single circular vortex or a hairpin shaped flow structure is formed in Region II. Lateral kinetic energy, maximum lateral velocity, centroid of vorticity, radial profiles of azimuthal velocity, and angular momentum are employed as measures of the streamwise development of the lateral flow field. The particle image velocimetry measurements of the present study are compared with laser Doppler velocimetry measurements taken for the identical support grids and flow condition.

The present study demonstrates a process used to develop confidence in Computational Fluid Dynamics (CFD) as a tool to investigate flow and temperature distributions in a PWR fuel bundle. The velocity and temperature fields produced by a mixing spacer grid of a PWR fuel assembly are quite complex. Before using CFD to evaluate these flow fields, a rigorous benchmarking effort should be performed to ensure that reasonable results are obtained. Westinghouse has developed a method to quantitatively benchmark CFD tools against data at conditions representative of the PWR. Several measurements in a 5×5 rod bundle were performed. Lateral flowfield testing employed visualization techniques and Particle Image Velocimetry (PIV). Heat transfer testing involved measurements of the single-phase heat transfer coefficient downstream of the spacer grid. These test results were used to compare with CFD predictions. Among the parameters optimized in the CFD models based on this comparison with data include computational mesh, turbulence model, and boundary conditions. As an outcome of this effort, a methodology was developed for CFD modeling that provides confidence in the numerical results.

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Current Pressurized Water Reactors (PWR) fuel assembly thermal-hydraulic (T/H) analyses are performed on a subchannel basis that neglects detailed heat transfer and flow distributions surrounding fuel rods. Subchannel codes such as VIPREW require input of thermal mixing and hydraulic loss coefficients that are obtained from costly experiments. Fuel thermal margin or performance is quantified in terms of Departure from Nuclear Boiling Ratio (DNBR) for PWR applications or Critical Power Ratio (CPR) for Boiling Water Reactors. DNBR and CPR predictions for reactor design and safety analysis rely on empirical correlations that are developed and qualified from costly rod bundle water DNB tests. Demands for extended power uprate, high fuel burnup, zero fuel failure, and new nuclear plant designs require a revolutionary advancement in T/H capability for better understanding of coolant behavior and more accurate predictions of thermal margin of the Light Water Reactor (LWR) core and fuel designs under normal operation and postulated accident conditions. Computational Fluid Dynamics (CFD) has been used in many aspects of PWR fuel designs in Westinghouse. Significant advancement in 2-phase flow modeling has been made in the recent years. This paper will illustrate CFD–based DNB modeling development in Westinghouse Nuclear Fuel. A 5×5 test bundle PWR experiment from the ODEN DNB test facility was modeled in CFD using a relatively new 2-phase boiling model. The model geometry included the details of the mixing vane spacer grids. When compared to the test data, the CFD model demonstrated that the DNB power was reasonably predicted. The CFD model also revealed the detailed flow behavior and the 2-phase flow distribution, both of which will be beneficial for the development of new grid spacers.