To investigate the effects of the diameter ratio on the mixing process and the thermal fluctuations of hot and cold fluids in a T-junction, an experimental setup was constructed that simultaneously measuring the temperatures of the outer wall surface, the near inner wall surface, and the fluid. Experiments were conducted for diameter ratios (D_m/D_b) of 1 and 2.5, corresponding to impinging jet and deflecting jet flow types, respectively, and the temperature data were normalized for analysis. The experimental results indicate that when the change in pipe diameter ratio alters the momentum ratio and thereby affects the flow type, impinging jets exhibit more pronounced thermal stratification and greater temperature fluctuations than deflecting jets. Power spectral density (PSD) analysis reveals that due to the thermal capacitance of the pipe wall, the main frequency of the wall energy shifts to a lower frequency than that of the fluid. Additionally, the PSD of both the pipe wall and the fluid in deflecting jets is generally lower than the corresponding values in impinging jets. Therefore, when the flow type in the T-junction is an impinging jet, reducing the branch diameter can effectively decrease thermal fluctuations in the pipeline, thereby preventing thermal fatigue failure.

In a liquid metal fast reactor, the mixing of coolant from the core outlet in the upper plenum induces temperature fluctuations in the adjacent wall temperature field, which can lead to thermal fatigue of structural components. This study uses computational fluid dynamics (CFD) software and employs large eddy simulation (LES) to numerically simulate temperature fluctuations in a parallel three-nozzle model. The accuracy of the simulation method is first validated against existing experimental data. A subsequent comparative analysis examines the frequency and amplitude of temperature fluctuations for three liquid metal coolants: lead-bismuth eutectic (LBE), liquid lead (Pb), and liquid sodium (Na). The results show that, in the parallel three-nozzle model, temperature fluctuations are more pronounced for lead-based coolants, increasing the risk of thermal fatigue in structural components.