Study on Laminar-Turbulent Transition in Square Arrayed Rod Bundles

TL;DR

This paper investigates the laminar-turbulent transition in square arrayed rod bundles relevant to nuclear reactor coolant flow, using DNS simulations to analyze flow stability, onset, and growth of instabilities across various parameters.

Contribution

It provides a detailed DNS-based analysis of flow stability and transition mechanisms in simplified rod bundle geometries, highlighting the role of linear instability and vortex street formation.

Findings

Identification of flow instability onset locations

Analysis of spatial growth of turbulence

Impact of Reynolds number and geometry on transition

Abstract

The study of coolant flow behavior in rod bundles is of relevance to the design of nuclear reactors. Although laminar and turbulent flows have been researched extensively, there are still gaps in understanding the process of laminar-turbulent transition. Such a process may involve the formation of a gap vortex street as the consequence of a related linear instability. In the present work, a parametric study was performed to analyze the spatially developing turbulence in a simplified geometry setting. The geometry includes two square arrayed rod bundle subchannels with periodic boundary conditions in the cross-section. The pitch-to-diameter ratios range from 1.05 to 1.20, and the length of the domain was selected to be 100 diameters. No-slip condition at the wall, and inlet-outlet configuration were employed. Then, to investigate the stability of the flow, the Reynolds number was varied from 250 to 3000. The simulations were carried out using the spectral-element code Nek5000, with a Direct Numerical Simulation (DNS) approach. Data were analyzed to examine this Spatio-temporal developing instability. In particular, we evaluate the location of onset and spatial growth of the instability.