Influence of the diameter ratio on the mixing process and thermal fluctuations of hot and cold fluids in a T-junction

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.