Flow Characteristics of Fuel Bundle With New Mixing Vane Design

Abstract

The prevalent fuel assembly geometry of Pressurized Water Reactors (PWR) is constituted of open-lattice rod bundles arranged in a square configuration with grid spacers. In order to enhance the coolant mixing effects across the adjacent flow channels, mixing vanes are added to the grid. The presence of the spacer grids with mixing vanes imposes two opposite effects that would affect the thermal-hydraulic characteristics in the core. Grid spacers with optimized mixing vanes will generate more effective swirl flow across the neighboring subchannel to enhance the local heat transfer rate from the fuel rod cladding to the core coolant downstream the grids, however, it would slightly increase the local pressure drop that would render an negative effect, though it is small, on the core thermal margins. The key elements for mixing effects, such as mixing length, cross and swirl flows, hinged on the coolant flow that is redistributed by the mixing vanes. This paper presents a mixing vane grid design (or concept) that could increase the effectiveness of heat transfer. Its structure includes flow-oriented section and vane. Flow oriented channel section is set on the other side of the neighboring cell which is different from a conventional mixing vane design. For mixing effect, there exists a balance between the incoming flow area and the bending angle. It also effectively draws more coolant from the adjacent cells to the mixing vane. Thus, it is an effective way to create a stable swirl flow downstream. The flow mixing impact travels much longer than that caused by the most conventional mixing vanes.