Interface capturing simulations of droplet interaction with spacer grids under DFFB conditions

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.