Large Eddy Simulation of Turbulent Penetration in a T-junction

摘要

Cyclic temperature fluctuations leading to the formation and propagation of cracks in industrial piping systems is an ongoing area of research and concern in the nuclear thermal hydraulics field. This paper focuses on turbulent penetration of a fast main pipe flow into the branch line of T-junctions, especially when the branch is not completely "dead, " as in a leaking valve scenario with very high main pipe/branch line velocity ratios. A large eddy simulation of turbulent penetration in a T-junction using the WALE (Wall-Adapting Local Eddy-viscosity) subgrid scale model has been performed. A highly resolved prismatic-polyhedral grid (～6 million cells) is utilized for the simulation of 13 seconds of flow time. A velocity ratio of 100 between the main pipe (D_m = 50 mm) and branch line (D_b = 26 mm) leads to turbulent penetration up to four branch diameters. Furthermore, a steady region of velocity shear leads to the formation of Kelvin Helmholtz Instabilities as the main flow boundary layer separates from the wall at the start of the branch line. The shearing of these waves is the likely cause of preferred scalar mixing frequencies in the branch. A comparison of the scalar mixing spectrum from the simulation with a broad peak in the mixing spectrum detected in experimental results, around 6 Hz, shows good qualitative agreement. Finally, the scalar average and RMS are found to be well reproduced by the LES simulations albeit with an underproduction of the mixing.