Numerical study on the turbulent mixing in channel with Large Eddy Simulation (LES) using spectral element method

Turbulent mixing is an important thermal hydraulic phenomenon in the reactor core rod bundles and it leads to strong momentum and energy transfer among adjacent subchannels in fuel assembly. Generally, turbulent mixing coefficient is usually referred to describe turbulent effect, which commonly sets to a constant or calculated utilizing Reynolds number dependent fitting correlations based on experimental data. In this paper, a high-scalable and high-performance spectral element method combining with high passed filtered Large Eddy Simulation (LES) model has been proposed to simulate the turbulent flow through parallel wall channel and square channel with a cylindrical rod for single-phase flow. In the parallel wall channel case, mesh sensitivity analysis and model validation were accomplished by comparing the simulation data with DNS references utilizing key parameters such as velocity profile and stresses in the near wall region. Meanwhile, for the square channel with a cylindrical rod case, instantaneous lateral velocity monitored in the gap center has been compared with the experimental data with different Reynolds numbers and geometric conditions. From the simulation approach, both the absolute oscillation amplitude and the oscillation frequency increase with the pitch diameter ratio. Finally, the simulation results of turbulent mixing coefficient dependent on Reynolds number were validated with both experimental and theoretic references to demonstrate the feasibility of spectral element method combining with LES model. The root mean square (RMS) value of lateral fluctuating velocity was adopted to reflect the effective mixing capability, predicting the turbulent mixing phenomena with a reasonable degree of accuracy from the specific calculation strategy. Fast Fourier Transform (FFT) also performed and got an agreement with reference data.