Flow obstructions in nuclear reactors can be a large source of pressure loss and may have a significant effect on local distributions of flow, void fraction and heat transfer. The objective of this work is to develop and test a mechanistic modeling framework for multidimensional predictions of flow and heat transfer around spacer grids in reactor fuel assemblies. Both single-phase and two-phase flows have been modeled A state-of-the-art computational multiphase fluid dynamics solver code, NPHASE-CMFD, has been used to simulate several cases of flow around obstructions. NPHASE-CMFD is a RANS-based computational multiphase fluid dynamics (CMFD) code which uses the ensemble average concept. It is capable of simulating non-adiabatic flows of multiple fluid components in complex geometries. The overall goal of the current work is to develop a physically consistent and numerically accurate computational methodology for the analysis of both single-phase and two-phase flow phenomena around spacer grids in nuclear reactors. The spacers are modeled as thin flow obstructions placed very close to the heated walls. The current approach capitalized on the recent advancements in the development of multidimensional modeling concepts of subcooled boiling in heated channels. The results shown in the paper illustrate the underlying modeling and computational issues which must be addressed and resolved to successfully apply the RANS-level CFD approach to studying flow and heat transfer phenomena in nuclear reactor fuel assemblies.
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