A Study of the Spatially Developing Laminar-Turbulent Transition in Rod Bundles

Rod bundles play a crucial role in various nuclear reactor core designs, including light water reactors, liquid-metal reactors, heavy water reactors, and some gas fast reactors. In this study, a series of high-resolution direct numerical simulations of laminar-turbulent transition in square rod bundles is performed. The goal is to determine the transition onset as a function of the pitch-to-diameter (P/D) ratio and Reynolds number in infinite square lattices. Very long domains are investigated in a first-of-its-kind study using the graphics processing unit–based spectral element code NekRS on the supercomputer Summit. Unique to this study is an in-depth analysis of the spatially developing transition and its impact on mixing properties. It is demonstrated that laminar-turbulent transition occurs even at fairly large P/D ratios at very low Reynolds numbers and primarily through a gap instability, at least in the initial phases of the transients. Frequency analysis is also introduced.