Turbulence statistics analysis of cross flow and heat transfer over an inline tube bundle using DNS

Helical tube bundles are preferably utilized in high -temperature gas-cooled reactors (HTGRs) for steam generators (SGs) and intermediate heat exchangers (IHXs). However, to enhance their design, it is essential to understand the turbulent flow and heat transfer characteristics of cross flow over tube bundles. In this work, the convection of cross flow over an inline tube bundle with P / D = 1.875 is simulated using the Direct Numerical Simulations (DNS) method. Findings from the velocity spectrum and the drag/lift force variations show that the turbulent wakes swing at a main frequency corresponding to a Strouhal number (Sr) of 0.114. Based on the timeaveraged velocity contour, the flow field has been divided into three parts: the main flow region, the recirculation region and the shear layer between the main flow and recirculation region. The flow field around the tube is symmetrical along the streamwise direction. Each half -tube surface ( theta = 0 degrees -180 degrees ) could be divided into three parts by the impinging point ( theta = 40.5 degrees ), separation point ( theta = 109.8 degrees ) and reattachment point (180 degrees ). Analysis of the Reynolds stress fields shows that large values of u ' u ' and u ' v ' are present in the shear layer, whereas v ' v ' dominate at the end of the recirculation region. Large w ' w ' mainly locates at the front side of the tube, where the main flow impinges on the tube wall. The convection terms are found to be non -zero while evaluating the transports of the Reynolds stress. The production and dissipation terms of turbulent kinetic energy (TKE) are found to be imbalanced, with pressure and turbulent diffusion terms playing key roles in balancing the TKE budgets. Similar to the flow field, the temperature field is also characterized by three regions: the main flow region (low temperature), the recirculation region (high temperature) and the shear layer (strong temperature gradient). The u ' T ' exhibits a clear distinction between positive and negative values near the shear layer. Positive values are concentrated within the recirculation region, while negative values are found outside this region. Both v ' T ' and u ' v ' show the same distribution patterns with larger magnitude present in the shear layer. The convection term of temperature fluctuation variance and turbulent heat flux budgets are found to be non -zero. For T ' T ' /2 budgets, the convection term is dominated by molecular diffusion in the near -wall region while it is dominated by turbulent diffusion in the main flow region. The turbulent diffusion has a significant influence on the balance of u ' T ' budget.