Investigation of flow phenomena to prevent thermal fatigue in a mixing tee of hot and cold water (Utilization of dynamic mode decomposition for numerical simulation results)

A numerical simulation of conjugate heat transfer was conducted to analyze the thermal mixing phenomenon at a T-junction, with a specific focus on the reverse flow from the main pipe into the branch pipe. To investigate the causes of temperature fluctuations on the inner surface of the branch pipe, dynamic mode decomposition was applied to the simulated temperature field. The analysis revealed a dominant 66 Hz temperature fluctuation, with a corresponding Strouhal number of approximately 1, at the point where the main stream and branch pipe flow collide. A lower frequency fluctuation of approximately 30 Hz was observed at the right corner. In the reverse flow region along the right wall of the branch pipe, fluctuations of 5.5 Hz and 0.29 Hz were also detected. These findings suggest that the vortex roll-up generated in the central part of the branch pipe collides with the right corner, leading to relatively low-frequency temperature fluctuations on the inner surface. Based on this analysis of temperature fluctuation factors, the possibility of reducing the risk of thermal fatigue damage in the branch pipe was investigated by altering the merging angle of the T-junction. When the merging angle was changed to 45 degrees, the root mean square values of both the inner surface temperature and the fluid temperature near the branch pipe wall decreased compared to the 90-degree case. This change also led to a reduction in the maximum range of temperature fluctuation on the inner surface of the branch pipe, a crucial indicator for fatigue assessment. The results suggest that the amplitude of stress at the upper end of the thermal sleeve installed in the branch pipe can be mitigated by changing the merging angle of the T-junction.