Three Dimensional Modelling of the Hydrodynamics of Oblique Droplet-Hot Wall Interactions During the Reflood Phase After a LOCA

During the reflood phase, following a LOCA, the main mechanism for the precursory cooling of the fuel is by convective heat transfer to the vapour, with the vapour being cooled by the evaporation of the entrained saturated droplets. However, it is believed that the droplets that reach the rod could have an effect on this cooling process. Despite the fact that those droplets do not actually wet the fuel rod due to the formation of a vapour film that sustains them and prevents them from touching the wall, the temperature drop caused by the impingement of such water droplets on a very hot solid surface (whose temperature is beyond the Leidenfrost temperature [1]) is of the order of 30–150 C, [2],[3]. The associated heat flux is of the order of 10⁵–10⁷W/m² and the heat extracted is in the range of 0.05 J over the time period of the interaction (a few ms) [2],[3]. The hydrodynamic behaviour of the droplets upon impingement is reported to affect the heat transfer effectiveness of the droplets. In the dispersed flow regime the droplets are more likely to impinge on the hot surface at a very small angle sliding along the solid wall, still without actually touching it, and remaining in a close proximity for a much larger time period. This changes the heat transfer behaviour of the droplet. Here, we investigate numerically the hydrodynamics of the impingement of such droplets on a hot solid surface at various incident angles and various velocities of approach. For our simulations we use a CFD, finite volume computational algorithm (TransAT^©). The Level Set method is used for the tracking of the interface. We present three-dimensional results of those impinging droplets. Validation of our simulation is done against experimental data already available in the literature. Then, we compare the findings of those results with previous correlations.