DNS analysis of flow and heat transfer of SCO2 in a square subchannel-Effect of gap width

The heat transfer and flow characteristics of supercritical carbon dioxide (SCO2) in a tight lattice are problematic in the thermal-hydraulic design of reactor fuel assemblies. Herein, direct numerical simulation (DNS) was performed to investigate the heat transfer and flow behavior of SCO2 in a vertically upward square subchannel. Emphasis was placed on clarifying the effect of the pitch-to-diameter (p/d) ratio, defined as the subchannel pitch over the rod diameter, on the time-averaged heat transfer and instantaneous velocity/temperature fluctuation. It was found that the wall temperature exhibits a "V-shaped " profile along the rod circumference, which is caused by the combined effects of buoyancy and an uneven flow area. The wall temperature is lower for subchannels with smaller p/d ratios; meanwhile, the circumferential non-uniform heat transfer becomes stronger, which also requires to be considered. The time-averaged streamwise velocity and fluid temperature have similar distributions on the subchannel cross-section, that is, high velocity and temperature occur near the rod gap, while regions with low velocity and temperature are identified in the subchannel interior. With an increase in the p/d ratio, both the temperature and velocity gradients are enlarged. Large-scale flow pulsations were identified in the subchannels. The instantaneous velocity fluctuation is weakened with increasing gap width; however, the fluctuation in the wall temperature exhibits an opposite trend. The effects of the p/d ratio on the dominant frequency of velocity/temperature fluctuation are not universal. Among the three selected cases, a subchannel with a small p/d ratio had a higher dominant frequency of instantaneous fluctuations. The turbulence structure was studied using a quadrant and anisotropy analysis. It was found that the outward and inward interactions included the main turbulence structures in the subchannel interior. In addition, with an increase in the p/d ratio, the transformation from disk-like turbulence to rod-like turbulence can be identified. (C) 2022 Elsevier Ltd. All rights reserved.