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This paper describes the computational fluid dynamics (CFD) methodology to simulate the boiling flow in a typical Pressurized Water Reactor (PWR) 5 ⨯ 5 rod bundle. The method includes the Eulerian-Eulerian two-fluid model coupled with the improved wall heat partitioning model. The NUPEC PWR Subchannel and Bundle Test (PSBT) International Benchmark are used for validation. The simulated surface averaged void fraction agree well with the experimental data, which indicate the promising application of the present method for modeling the boiling flow in the fuel rod bundle. The main emphasis of current research has been given to the analysis of the phase distribution around and downstream the spacer grid, the effect of the spacer grid structure, including the mixing vanes, the springs and the dimples on the void fraction distribution is investigated. The findings can contribute to a better understanding of three dimensional flow boiling characteristics and can be used to assist in optimizing the spacer grid.

Computational Fluid Dynamics (CFD) simulation has been increasingly used in Nuclear Reactor Safety (NRS) analysis to describe safety–relevant phenomena occurring in the reactor coolant system in greater detail. In this paper, the work about single-phase CFD simulation of rod bundles conducted in Shanghai Nuclear Engineering Research & Design Institute (SNERDI) is introduced. A single-phase methodology based on commercial software STAR-CCM+ is developed to simulate the flow field and temperature distribution in fuel rod bundles. Solid model is simply introduced at first. Mesh types, including tetrahedral, polyhedral and trimmer, are compared in order to select the most best one with both good accuracy and less cost. Several turbulence models available in STAR-CCM+, including standard k-epsilon model, realizable k-epsilon model (RKE), shear stress transport k-omega model (SST k-omega), and Reynolds stress model (RSM) are investigated. Trimmed mesh and RKE turbulence model with two-layer all y+ model are finally employed for following calculations. Vortex structures downstream of mixing vanes is qualitatively compared with Particle Image Velocity (PIV) results, and good agreement is achieved. The present method will be further refined in order to play significant role in future optimal design of fuel assembly (FA) grid.

Experiments of heat transfer at low flow rate are performed in a 5×5 square arrayed rod bundles. The diameter of the rod is 10mm with a pitch of 13.3mm, length of the test section is about 3 meters. Inlet Reynold number ranges from 2000 to 30000, Bo * ranges from 4×10−6 to 5×10−3. The rods are heated using a DC power, the heat flux ranges from 30 to 300 kW/m2. The experiment is aimed to investigate the buoyancy effect of mixed convection in rod bundles. The experimental data shows that similar with mixed convection in circular channels, buoyancy force has great effect on heat transfer at mixed convection regime in rod bundles. But the buoyancy effect appears at higher Bo* conditions. The spacer effect have also been investigated at both turbulent forced convection regime and mixed convection regime. The reconstruction of heat transfer downstream of spacers is different at different flow regimes, a reasonable explanation was provided.