Mechanistic modeling for ring-type BWR fuel spacer design 2. Local pressure drop and narrow channel effect

The fuel spacer is one of the components of a fuel rod bundle and its role is to maintain an appropriate rod-to-rod clearance. Since the fuel spacer influences liquid film flow on fuel rods in BWR core, its specification has a strong effect on thermal hydraulics in the core such as critical power and pressure drop. Spacers have been developed through empirical modifications, so that a large amount of test data is required for optimum design of the spacer. It is one of the important subjects to develop a mechanistic model of the spacer for future design of BWR fuel bundles. The authors proposed a liquid film flow limitation model (narrow channel effect) in which the momentum balance of the liquid film phase in the near field of the spacer is considered. Modeling of local pressure drop of the spacer is required for closing the momentum equation. In this paper, the effect of spacer geometry on the local pressure drop was, therefore, investigated experimentally and analytically in a vertical circular channel using air and water as test fluids.