Large eddy simulation on thermal mixing of fluids in a T-junction with conjugate heat transfer

• LES of fluid mixing in a T-junction at ΔT = 117 K and 123 K is performed.
• Dynamical thermal stratification flow behavior downstream of T-junction.
• Temperature fluctuations have maximum amplitudes of about 3.4–5.6% of ΔT.
• High amplitude fluctuations occur near stratification layer in the mixing region.
• Energy of temperature fluctuations mainly contained in the range 0.1–3 Hz.

High cycle thermal fatigue failure in a nuclear power plant T-junction piping system may be caused by near-wall temperature fluctuations due to thermal mixing of hot and cold fluid streams. In the present study, thermal mixing at temperature differences (ΔT) of 117 K and 123 K between the mixing fluids is numerically investigated using Large Eddy Simulation (LES) method with the commercial Computational Fluid Dynamics (CFD) software ANSYS CFX 14.0. LES results from the study are validated with experimental data obtained from Fluid–Structure Interaction (FSI) test facility at the Materials Testing Institute (MPA), University of Stuttgart. Mass flow rate ratios (main/branch) in both cases are 4 and 6, respectively. LES results in both cases show that there is incomplete mixing of fluids and within three diameters downstream of T-junction, the mixing results in a dynamical thermal stratification flow behavior, which is maintained throughout the computational domain. Mean temperature predictions by LES show good agreement with the experimental data, whereas the root mean square (RMS) temperature fluctuations are over or understated at a few positions. The temperature fluctuations have amplitudes ranging from 0.09 to 5.6% of ΔT between the mixing fluids. Incomplete mixing of fluids and relatively lower amplitude of temperature fluctuations are mainly due to lower Reynolds number of 3670 in the cold fluid coming from the branch pipe along with buoyancy effects in the flow due to higher inflow temperature in the main pipe.