Analysis of heat transfer to axial dispersed flow between rod bundles under reactor emergency cooling conditions

Analysis is carried out for dispersed flow heat transfer under reactor emergency cooling conditions. The present formulation explicitly reveals an extra dependence of the heat transfer coefficient and Nusselt number on the mean vapor temperature for droplet dispersed flow which is not found in single phase flow heat transfer. The heat transfer results obtained from three different geometries: an infinite square array of cylindrical rods, an annulus and a circular pipe: are compared; all have the same hydraulic diameter. It is found that, within the framework of the present analysis, results for annulus and the rod bundles agree well when the pitch-to-diameter ratio is 1.5 or greater. The circular pipe is in general a poor approximation for rod bundle geometries except at a pitch-to-diameter ratio of about 1.3 which is typical of present day light water reactor fuel assemblies