Pressure loss in confluence flow with small branch angles and variable mainline geometry

This work identifies a special confluence flow emerging at the confluence sections between the mainline and the on-ramp of bifurcated tunnels, which is characterized by small branch angles and variable mainline cross sections and hence is distinct from conventional confluence flow in T-shaped pipes in terms of its flow pattern and pressure loss mechanism. A numerical model was constructed and validated against experiments and then utilized to investigate the pressure loss characteristics. Results showed that because of the small branch angle of bifurcated tunnels (usually no more than 30°), flow separation at the junction of conventional T-shaped pipes no longer occurs. Instead, airflow from the branch with a larger velocity (either the mainline or the on-ramp) enters the confluence section as a jet, and the turbulence dissipation on the jet boundaries contributes a large part (sometimes over 80%) to the pressure loss. Subsequently, effects of the variation of cross section under different lane configurations and also the velocity difference between the mainline and the ramp were discussed quantitatively. Finally, a novel semi-theoretical model embracing the turbulence dissipation under different lane configurations was developed accordingly to predict the confluence loss coefficient of both the mainline and the ramp, and then validated against experimental data in the literature to demonstrate its applicability.