Estimation of turbulent energy mixing factor in PWR sub-channel by DNS

Turbulent mixing within sub-channels plays a crucial role in understanding the thermal hydraulics of reactor channels. It serves as an empirical parameter in sub-channel analysis and has long been a challenge in the nuclear industry. Conducting experiments in this context is challenging due to the stringent requirement of maintaining pressure balance among sub-channels to prevent convection effects. Fortunately, direct numerical simulation (DNS) is emerging as an invaluable tool for addressing this persistent issue. DNS enables the direct computation of turbulent mixing by analyzing fluctuating lateral velocities, offering a more profound understanding of the underlying phenomena. In this study, DNS was conducted at six Reynolds numbers ranging from 17,640 to 1.5 × 10⁵ in pressurized water reactor (PWR) geometry to investigate the lateral mixing driven by turbulence. By studying intricate mechanisms governing the turbulent mixing, the valuable insights into reactor thermal performance and safety are provided. Furthermore, a correlation for turbulent mixing of energy based on the DNS data has been derived, enhancing our ability to model and predict this critical aspect of reactor behaviour. Additionally, this paper explores temperature fluctuations occurring at the fuel rod surface due to turbulence. A probabilistic distribution for temperature fluctuation under specific reactor conditions is presented. (author)