Large eddy simulation of thermal turbulent mixing and reduction of temperature fluctuations with swirl in T-junction

Abstract

Turbulent mixing of two fluids having a temperature difference can lead to temperature fluctuations in the T-junctions, particularly in the primary cooling circuit of nuclear power plants. Temperature changes produce thermal stresses causing thermal fatigue. This is a key topic related to nuclear security. This article presents the numerical results of two thermal turbulent mixing simulations, applied to a T-junction (OECD-NEA-Vattenfall T-junction). These simulations apply, respectively, to the case of a flow with and without rotational movements into the cold side. The numerical predictions were made using the large eddy simulation with the commercial code Fluent. The numerical results obtained in the first case were compared with the available experimental data and were found in an agreement satisfactory to the profiles of average variables and the spectrum of the temperature near the wall. The comparison with RMS profiles was less satisfactory in the center of the main tube, but was of good quality in the near-wall region. In the second simulation, the main movement of the fluid is affected by the spiral paths created by the swirl velocity caused at the cold inlet, which improves the mixing of hot and cold fluids downstream of the T-junction. From the analysis of these results, it clearly appears that the temperature fluctuations decrease near the wall in this case which leads to less high thermal loads on the tube, and hence a lesser thermal fatigue.