In the dispersed flow film boiling regime the dominant path of heat transfer from the fuel rods is to the entrained droplets in the reactor sub-channels. The heat transfer coefficient strongly correlates to the surface area of the droplets, which is effectively characterized by the Sauter mean diameter. Owing to the interaction of droplets with spacer grids and mixing vanes sharp increase in heat transfer coefficients are reported immediately downstream of spacer grids by prior experiments.

In this study, using state of the art computing facilities and the massively parallel PHASTA code, we present high resolution simulations of droplet-spacer grid interactions under conditions similar to DFFB flow regime. Level-set based interface tracking method is used to resolve the interface between the two phases. Fully developed turbulent flow field is obtained from single-phase steam flow adiabatic simulations. Two-phase simulations are performed by superimposing the level set contour over the obtained single phase velocity field. The results from twophase simulations demonstrate the capability of PHASTA code to capture the interface during droplet spacer-grid collision events.

The objective of the present work is to collect numerical data on the Sauter mean diameter of droplets downstream of spacer grids. The data will be compared with the experiments and existing mechanistic correlations in the literature for Sauter mean diameter modification due to spacer grids. The results from the simulations will serve to improve the correlations in thermal hydraulic codes and can also serve as training data for reduced order twophase flow modeling.