基于动量源与精细多孔介质的堆芯特征组件域CFD计算分析技术研究

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

堆芯计算流体力学(Computational Fluid Dynamics,CFD)对确定堆芯薄弱环节、优化特征结构、提升安全及经济性等具有重大意义。典型压水堆燃料组件存在大量带交混翼片格架,导致存在网格数量激增、网格划分困难、求解难以收敛等问题。目前基于流固作用机理所建立的动量源模型未考虑交混翼背流侧低压区对流体的影响,预测交混翼下游域轴流分布误差较大,此外在识别交混翼片固体域、添加动量源项时较为困难。为此本文提出了一种基于精细化多孔介质和动量源的联合仿真方案来模拟5×5带交混翼片棒束通道冷却剂流动,该方案中的域识别技术可以精确定位流体域中交混翼片所在位置,并基于格架前端及交混翼片迎流与背流侧的流固作用机理,建立多孔介质和动量源模型。该方案在格架域,采用精细化多孔介质模型,提高原格架固体所在流域局部阻力系数,相较于在格架域施加动量源项,其数值稳定性更好。而在交混翼域,对动量源方案进行了优化,优化后的动量源模型考虑到了交混翼片背流侧低压区对流体的作用,其仿真交混翼下游的速度分布和传热更准确。该方案可以在网格生成的过程中无须考虑格架、交混翼片的几何特征,从而实现全结构化网格建模、CFD求解耗时短、对网格数量不敏感,鲁棒性较优。通过与实验及贴体网格仿真结果对比,验证了该方案仿真阻流、流动、交混、涡流、传热的有效性。同时仿真5×5棒束通道时可减少90%的网格数量和60%的计算耗时。此外,由于棒束通道交混翼在轴向和周向都是规则排列,因此本文开发的动量源方案通过调整坐标即可适用于更大流体域的燃料组件。

[Background]Reactor 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.[Purpose]This 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.[Methods]This 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.[Results]This 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.[Conclusions]This 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.