The effect of pulsating liquid sodium coolant inlets on flow structures and thermal mixing in a T-junction

Mixing of two pipe streams at different temperatures features in the secondary circuit of compact sodium-cooled fast reactors. This study presents the numerical simulation of flow features and mixing characteristics in the T-junction with the inlets subjected to co-pulsating and out-of-phase pulsating flows over a 4-s flow cycle. The main and branch pipe inlets are subjected to sine-sine and sine-cosine velocity variations. The momentum ratio is constant (3.57) in the first case, but varies between 1.73 and 7.38 in the second. The velocity magnitude profile in both cases gradually recovers to its pre-mixing shape after the T-junction; however, the profiles differ between the two half-cycles in Case 2. Turbulence intensity varied more in the upper part than in the lower part of the main pipe. The 90 o out-of-phase pulsating flow at the inlet resulted in increased temperature and turbulence intensity fluctuations along the flow axis. Momentum interactions dominate thermal mixing in the upper part of the pipe, while the lower half is dominated by gradual convection in the shear layers. The lower temperature mixing degree in 75% of the flow cross-section and 20% reduction in the temperature difference at the outlet show that more thermal mixing was achieved in Case 2. The 90° out-of-phase difference between the mixing inlets also causes the position of the thermal fluctuation to be momentarily away from the wall, which has implications for the extent of damage expected at the junction.