EXPERIMENTAL STUDIES OF SPACER GRID THERMAL HYDRAULICS IN THE DISPERSED FLOW FILM BOILING REGIME

摘要

During the reflood period of a hypothetical large-break loss of coolant (LOCA) accident the reactor core is expected to uncover. Simulations of large-break LOCAs that assume the partial failure of the emergency core cooling system (ECCS) often result in a slow recovery of the core, with steam cooling and dispersed droplet film boiling being the prevalent modes of heat transfer. Tests conducted at the Rod Bundle Heat Transfer (RBHT) facility at The Pennsylvania State University (PSU) have confirmed the significant effects that spacer grids can have on reflood thermal-hydraulics. The spacer grids have been found to enhance downstream convective heat transfer, and strongly influence droplet size distributions through early spacer grid rewet and droplet break up. Existing models for enhancement of heat transfer and droplet break up may not accurately account for these interactions between the coolant and the spacer grid. Data from two series of RBHT tests, low injection rate forced reflood tests and droplet injection tests, are presented in this paper to describe the effects of the spacer grids during dispersed flow film boiling. Heat transfer downstream of the spacer grids is clearly enhanced by presence of the droplets, while the downstream droplet size was found to depend on conditions of the spacer grid; dry or wetted. Results of this study demonstrate the needs to adequately account for the separate modes of dry and wet spacer grid heat transfer enhancement in predicting the thermal hydraulic behavior during reflood transients.