Mechanistic modeling for ring-type BWR fuel spacer design: 1. Drift flow model Original Research Article

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 are required for optimum design of the spacer. It is, therefore, important to develop a mechanistic model of the spacer for future design of BWR fuel bundles. The authors considered that enhancement of droplet deposition is induced downstream of the spacer where velocity profiles split by a spacer are recovered into one velocity profile; higher velocity in the core and lower velocity in a gap clearance between the rod surface and the spacer. In this paper, the effect of spacer geometry on the droplet deposition rates downstream of it was investigated experimentally in a open rectangular channel using air and water as test fluids. A Phase Doppler Anemometer (PDA) was used for this measurement. In addition, velocity profile equations were obtained from the drift flow model. From these results, a mechanistic model of enhancing droplet deposition rate downstream of the spacer was proposed.