Large eddy simulation studies on the influence of turbulent inlet conditions on the flow behavior in a mixing tee

Abstract

Mixing of coolant streams at significant temperature differences in a mixing tee occurs in the cooling systems (e.g. Residual Heat Removal System) of a nuclear power plant resulting in cyclical thermal fluctuations that may lead to high cycle thermal fatigue (HCTF) in the structure. The present study aims to numerically investigate such a scenario using the large-eddy simulation (LES) method. The LES is based on an experiment conducted at the fluid–structure interaction (FSI) test facility, University of Stuttgart. Hot and cold water streams flow in the main and branch pipes with a temperature difference (ΔT) of 117K. The mass flow rate ratio (main/branch) is 4:1.Two LESs with different inlet conditions are performed to study and compare the effect of each inlet condition on the flow behavior downstream of the mixing tee. They are (i) unperturbed inlet conditions and (ii) inlets with synthetic turbulence. The experimental inflow conditions result in an incompletely mixed thermally stratified flow marked by significant buoyancy effects. Assessment of parameters like mean temperature and velocity predicted by both LESs exhibit very little deviation with each other indicating the insignificant effect of inflow turbulence on the bulk flow. But the inclusion of inflow turbulence is seen to have a non-negligible effect in the assessment of parameters like temperature fluctuations, turbulent kinetic energy and temperature–velocity correlation. The prevailing flow conditions are not seen to induce any dominant frequency of near-wall temperature fluctuations, an important factor in thermal fatigue analysis. Energy content of these fluctuations mainly lies in the frequency range of 0.1–2Hz.