Large eddy simulation study on the mixing mechanism of non-isothermal lead-bismuth eutectic in T-junctions

T-junctions are widely used in nuclear reactor piping to merge and redistribute flows. Frequent mixing of non-isothermal fluid within these fittings leads to thermal fatigue in the piping system. Liquid metal cooled fast reactors (LFRs) have developed rapidly and are attracting growing international attention, owing to their prospective advantages for Generation-IV nuclear systems and potential roles in future fusion-energy applications. This study employs large-eddy simulation (LES) to analyze the temperature and velocity fields of non-isothermal LBE mixing in the T-junction under various momentum ratios ( M _R), revealing the vortex-structure evolution and near-wall thermal-pulsation mechanisms in the main pipe for three typical flow regimes (impinging jet, deflected jet and wall jet). Compared to the wall jet case ( M _R= 4.51), the impinging jet case ( M _R= 0.25) exhibits an approximately 11.3% higher dimensionless radial RMS temperature peak, with its peak location shifting from y / r _m= 0.175 to y / r _m=-0.685. In all three regimes, maximum circumferential near-wall temperature fluctuations at 1D_m section occur at θ =±65°∼±75° A deeper mixing-mechanism analysis of the impinging jet shows that vortices shed from the leading edges of the main and branch pipes at the same frequency, propagate downstream along the main–branch interface, and induce vigorous mixing. As the measurement section moves downstream from 0.5D_m to 4D_m from the branch inlet, the dimensionless RMS temperature peak gradually decreases and the region of strongest pulsation shifts from the upper to the lower portion of the pipe. Across all sections, the dominant near-wall temperature-fluctuation frequency band lies between 1.4 and 1.8 Hz. The findings will serve as a reference for the thermal-hydraulic design of future liquid-metal-cooled fast reactors.