Estimation of Turbulent Mixing Factor and Study of Turbulent Flow Structures in PWR Sub Channel by DNS

Sub-channel analysis codes are presently a requirement for design and safety analysis of nuclear reactors. Among the crucial inputs for these codes, the turbulent mixing factor holds particular significance. However, acquiring this factor through experimental means proves to be a challenging endeavor, primarily due to the necessity for precise pressure equilibrium between sub-channels. Consequently, this requirement leads to the undertaking of expensive and intricate experiments for each new reactor or in cases where there are modifications in fuel bundle design. The need for Direct Numerical Simulation (DNS) stems from the challenges and costs involved in experimental techniques, and the uncertainties due to empiricism in Computational Fluid Dynamics (CFD) models. In this study, DNS has been conducted across six Reynolds numbers, ranging from 17640 to 1.176×105, in the geometry of a Pressurized Water Reactor (PWR) sub-channel. The resulting turbulent flow structures have been computed and their dynamics is examined. Furthermore, this paper presents a methodology for directly calculating the turbulent mixing factor from the fluctuating velocity field obtained from DNS data. The turbulent mixing process has been scrutinized in-depth, and a correlation for the turbulent mixing factor is developed. It is noted that most of the mixing occurs in the near-wall region. The study suggests different mixing factors for mass and momentum mixing. This paper aims to provide a comprehensive insight into the turbulent mixing phenomenon.