基于欧拉-欧拉两相流模型的核事故放射性物质扩散特性数值模拟

Numerical Simulation of Nuclear Accident Radioactive Materials Dispersion Characteristics Based on Eulerian Two-Phase Flow Model

为精细刻画核电厂事故后放射性物质扩散的时空动态特征,提出了一种基于欧拉-欧拉两相流模型的放射性物质扩散数值模拟方法,选择适用于描述放射性物质受浮升力驱动和环境风影响上升扩散过程的相间作用模型和湍流模型,并将多层嵌套正六面体网格划分方法与高精度、高计算效率数值离散方法相结合,实现了大空间内放射性物质扩散的快速、精确数值模拟。在验证了多层嵌套的正六面体网格可以更好地减小数值模拟过程中基于网格质心的格林-高斯梯度项离散格式带来的误差的基础上,对扩散上升过程进行动态分析,阐明了放射性物质在环境风场作用下上升扩散的一般规律。结果表明:在上升扩散前期,浮力起绝对主导作用,放射性物质上升速度较大,涡环外表面出现温度垂直分层,环境风对涡环运动过程影响可忽略不计;随着放射性物质上升,高海拔环境风使烟团整体沿下风向扩散,温度迅速下降,涡环结构不再明显。

To finely characterize the spatiotemporal dynamic features of the radioactive ma-terials dispersion following a nuclear power plant accident,a numerical simulation method based on the Eulerian two-phase flow model was proposed.First,the interphase interaction and turbu-lence models suitable for describing the buoyancy-driven and ambient wind-affected rise and dis-persion processes were selected as basic models.Furthermore,a multi-layer nested hexahedral grid partitioning method was combined with a high-precision,computationally efficient numerical discretization method to achieve rapid and accurate numerical simulations of radioactive materials dispersion over large spaces.Results of computation verified that the error caused by the Green-Gauss gradient term discretization scheme based on the grid centroid can be reduced using the nested hexahedral grid.Thereby,the four stages of the radioactive materials dispersion process—rapid vertical rise,vortex ring displacement,dispersive ascent,and trailing edge separation and sedientation—were dynamically analyzed.Accordingly,general laws of radioactive materials disper-sion under the influence of ambient wind fields were elucidated.Results demonstrated that in the initial stage of rise,buoyancy plays an absolutely dominant role,resulting in a relatively high ris-ing velocity.Simultaneously,a vertical temperature stratification appears on the outer surface of vortex rings and the influence of ambient wind on vortex rings can be neglected.As the radioactive materials rise,the high-altitude ambient wind makes the radioactive materials disperse in the dow-nwind direction,leading to a rapid temperature drop and less distinct structures of vortex rings.