Dynamics of droplet breakup through a grid spacer in a rod bundle

The dynamics of droplet breakup associated with the flow of a dispersed two-phase mixture through a rod bundle grid spacer during a reflood transient in a pressurized water reactor is studied theoretically. By considering the conservation of liquid mass and the kinetic as well as the surface energies of the droplets, an expression is derived for the ratio of the Sauter mean diameters of the droplets downstream and upstream of the grid. It is found that the Sauter mean diameter could decrease appreciably as a result of the shattering of the droplets when flowing through the grid spacer, thus increasing the interfacial heat transfer surface area. The decrease in the droplet size is dependent upon the Weber number of the incoming droplets, the blockage ratio of the grid spacer, and the fraction of the kinetic energy of the incoming droplets required to convert to the surface energy of the newly generated droplets during the breakup process. Comparisons of the theoretical results are made with the experimental data obtained at the rod bundle heat transfer test facility as well as with other relevant data in the literature and found to be good.