CFD computational analysis techniques for core feature component domain based on momentum source and detailed porous media

Abstract

BackgroundReactor core computational fluid dynamics (CFD) plays a crucial role in identifying core vulnerabilities, optimizing feature structures, and improving safety and economic in nuclear reactors. However, conventional pressurized water reactor fuel assemblies often feature a multitude of spacer grids with mixing vanes, leading to challenges in mesh generation and numerical solution instability, excessive computational resource requirements. The current momentum source model established on the basis of the mechanism of fluid-structure interaction has not considered the effect of the low-pressure region on the fluid downstream of the mixing vanes, leading to significant errors in predicting the axial flow distribution downstream of the mixing vanes. Furthermore, it is challenging to identify the solid domain of the mixing vanes and to add momentum source terms.PurposeThis study aims to present a joint simulation scheme based on detailed porous media and momentum source modeling to simulate coolant flow in 5×5 rod bundle channels with mixing vanes, hence to reduce cells, lower mesh generation difficulty, and enhance numerical stability during the CFD solving process.MethodsThis scheme employed a detailed porous media approach in the spacer zone, while adopting Global Momentum Source Model in the vane zone. Simultaneously, a domain identification scheme was developed to determine the placement of momentum sources and detailed porous media models. The position of mixing vanes within the fluid domain was accurately located by this approach and established detailed porous media and momentum source models based on the fluid-structure interactions in the grid spacer zone and leeward side and windward side of mixing vanes. To simulate the flow field distribution within the spacer zone, a detailed porous media model was employed to enhance local flow resistance, thereby achieving an accurate simulation of the flow field distribution in the spacer zone. Finally, validation against experimental and body-fitted mesh simulations was performed to examine the effectiveness of this scheme in simulating flow blockage, fluid flow, mixing, and vortex shedding.ResultsThis scheme, compared to the momentum source scheme, exhibits stronger numerical stability. In the vane zone, the established momentum source model simultaneously considers the effects of the leeward side and the windward side of mixing vanes, leading to a more accurate prediction of axial flow velocity and heat transfer downstream of the mixing vanes. This approach allows for modeling without needing to consider the structure of the spacer grids with mixing vanes, thus greatly simplifying mesh generation. It achieves complete structured mesh modeling, significantly reducing the number of cells, and enhancing computational efficiency. Validation confirm the effectiveness of this scheme and results in a 90% reduction in cells and a 60% decrease in computational time for modeling and simulation of a 5×5 rod bundle channel with mixing vanes.ConclusionsThis scheme offers simplicity in modeling, reduces CFD computation time, insensitivity to mesh, and superior robustness. Furthermore, when identifying larger-scale components, the approach involves identifying the multi-span fuel components, since the mixing vanes form a regular array in both axial and transverse directions. Therefore, domain identification at a larger scale can be achieved by modifying coordinates, applying momentum source model developed in this paper.

Keywords

• rod bundles
• mixing vanes
• domain identification
• momentum source model
• detailed porous media