Influence of the Exhaust Volume on Boundary Layer Separation Under the Lateral Concentrated Smoke Exhaust Mode in Immersed Tunnel Fires

The boundary layer separation occurs when a fire breaks out, affecting the smoke exhaust effect in the immersed tunnel with the lateral concentrated smoke exhaust mode. The theoretical criterion of boundary layer separation is established according to the distribution of smoke flow velocity. And three boundary layer separation regions in the exhaust vents and exhaust duct are divided. The smoke flow velocity during the exhaust process in a standard three-lane immersed tunnel is simulated by FDS, and the area of each boundary layer region and its ratio under different exhaust volumes is analyzed. Results show that the order of blocking effect of each region from large to small is the region near the left sidewall of exhaust vents, the region near the right sidewall of exhaust vents, and the region near the inner sidewall of the exhaust duct. The area ratio of the boundary layer separation region in each group of exhaust vents is higher than that in the exhaust duct by 17.2%–22.7%. The increased exhaust volume is beneficial in reducing the negative effect on smoke exhaust by the boundary layer separation. The exhaust volume is 340 m³/s when the negative effect is minimum under 50 MW, at this time, the area of non-boundary layer separation regions on average in the exhaust vents and exhaust duct is 70.7% and 91.4%, respectively.