Numerical simulation and mechanism analysis of turbulent mixing in shaped T-junctions with inclined branch pipe

Simulation and mechanism analysis of the mixing process between hot and cold fluids in T-junctions can help prevent pipe cracking and failure caused by thermal fatigue in the pipe wall. Most studies are based on configurations involving different incident angles of branch pipe. However, the thermal mixing performance of T-junctions with inclined branch pipe also requires thorough evaluation due to their widespread use in piping systems and the related investigation is quite limited. Here, “inclination” specifically refers to the circumferential position (azimuthal angle) of the branch pipe on the main pipe, which differs from the common upstream or downstream tilting configurations. This paper simulates the thermal mixing process in T-junctions with inclined branch pipe based on the Large Eddy Simulation, and investigates the effect of different branch inclination angles on the mixing performance and thermal fatigue risk under the inlet condition of large temperature differences and turn jets. Under the conditions of hot branch inflow and cold main inflow, increased branch inclination angle negligibly affects flow patterns, but intensifies the buoyancy-driven rotation of the thermal stratified interface, thereby changing the uniformity of downstream thermal mixing and reaching the best uniformity with inclination angle of 90°. The related mechanism analysis is innovatively performed through the correlation of the spatial temperature gradient and the vorticity. Vorticity probability density distribution analysis confirms that increased branch inclination angle elevates mean vorticity at specific temperature gradients intervals, which enhances heat transfer and improves mixing uniformity. Additionally, due to the combination of thermal mixing and buoyancy effect, the locations of near-wall high-temperature fluctuations exhibit significant asymmetry. Therefore, a fitting equation is proposed to predict the circumferential position of near-wall high-temperature fluctuations, aiming to improving thermal fatigue risk assessment in piping systems.