Abstract In the dispersed flow film boiling (DFFB) regime, which exists under post loss-of-coolant accident (LOCA) conditions in PWRs, droplet dynamics play a critical role in heat transfer. The DFFB regime is characterized by a very high void fraction, such that the flow regime is akin to a mist type flow. Consequently, the dominant heat transfer pathway under these conditions is to the entrained droplets, which is a strong function of the surface area of the droplets. The major heat transfer mechanisms include radiation from rods to droplet surface and convective heat transfer from bulk vapor to droplets. Evaporation of droplets also results in an increase in the local vapor velocity, further enhancing the heat transfer coefficients. Spacer grids and mixing vane structures play an especially critical role in the DFFB regime. Collision of the droplets with these structures results in an increase in their surface area, which causes a sharp increase in heat transfer immediately downstream of the mixing vanes. In this study, we present detailed interface-resolved simulations of multiple droplet dynamics in a prototypal reactor sub-channel with spacer grid and mixing vanes using PHASTA, a massively parallel finite element-based flow solver. The droplet-vapor interface is implicitly captured using the level-set method, allowing for resolution of inherent complexities of the regime including, droplet-structure collision and droplet break-up and coalescence. The simulations provide the evolution of interfacial area, volume and Sauter mean diameter of droplets along the axial length of the sub-channel domain. Further, a comparative study of the upstream and downstream mean velocity and Reynolds stress tensor profile is presented to emphasize on the effect of spacer-grid and mixing vane structures on the bulk flow, with and without the presence of droplets.

中文翻译：

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DFFB 条件下液滴与间隔网格相互作用的界面捕获模拟

摘要 在压水堆冷却剂损失后 (LOCA) 条件下存在的分散流膜沸腾 (DFFB) 状态中，液滴动力学在传热中起着关键作用。DFFB 流态的特点是空隙率非常高，因此流态类似于雾状流。因此，在这些条件下，主要的传热途径是夹带的液滴，这是液滴表面积的强函数。主要的传热机制包括从棒到液滴表面的辐射和从本体蒸汽到液滴的对流传热。液滴的蒸发也会导致局部蒸汽速度的增加，从而进一步提高传热系数。间隔网格和混合叶片结构在 DFFB 制度中起着特别重要的作用。液滴与这些结构的碰撞会导致其表面积增加，从而导致紧邻混合叶片下游的热传递急剧增加。在这项研究中，我们使用 PHASTA（一种基于大规模并行有限元的流动求解器）对带有间隔网格和混合叶片的原型反应器子通道中的多液滴动力学进行了详细的界面解析模拟。使用水平集方法隐式捕获液滴 - 蒸汽界面，允许解决方案的固有复杂性，包括液滴 - 结构碰撞和液滴破裂和合并。模拟提供了沿子通道域轴向长度的液滴的界面面积、体积和 Sauter 平均直径的演变。更多，