Large-eddy simulation of thermal striping in unsteady non-isothermal triple jet

Abstract

Unsteady temperature fluctuations of non-isothermal turbulent jets are encountered in many engineering applications including liquid metal cooled fast reactors (LMFR), and can cause thermal stresses on solid boundaries. An accurate prediction of the temperature fluctuations is important to assess potential thermal fatigue damage to components, and traditionally this has been done by RANS turbulence modelling calculations with limited success. In this study, a large eddy simulation (LES) technique was applied to predict the temperature fluctuations of thermal striping observed in a triple jet. The triple jet model was used as a mock-up of the outlet of fuel subassemblies in a nuclear fast reactor. The results show that LES predicted the highly oscillatory nature of unsteady thermal mixing of the triple jet. The LES results were in good agreement with the available experimental data in terms of mean, RMS, skewness and kurtosis. The large amplitude of the temperature fluctuations associated with the thermal striping was captured correctly, demonstrating that LES can be used to analyse unsteady characteristics of thermal striping. Instantaneous and time mean thermal fields were further analysed to assess the capability and accuracy of LES in the thermal striping study. The Spalart–Allmaras and realisable k − ε turbulence models were also considered along with LES. It is found that these turbulence models produced a very small amplitude of fluctuations, and failed to predict the correct magnitude of unsteady thermal fluctuations, highlighting the limitations of the RANS approach in unsteady heat transfer simulations.