Experimental studies of spacer grid thermal hydraulics in the dispersed flow film boiling regime

Spacer grids have been found to enhance downstream convective heat transfer and to strongly influence droplet size distributions through early spacer grid rewet and droplet breakup. Existing models for enhancement of heat transfer and droplet breakup, however, do not appear to accurately account for these interactions between the coolant and the spacer grid. Data from two series of rod bundle heat transfer 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 the presence of the droplets, while the downstream droplet size was found to depend on the condition of the spacer grid: dry or wetted. Results of this study demonstrate the need 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.