Abstract: Jet ventilation is widely used in the ventilation design of highway and railway tunnels as an important air supply method during tunnel operation and disaster periods. This ventilation method has also been applied for fire control in immersed tunnels. We conduct numerical simulations using computational fluid dynamics (CFD) to study positive ventilation in the upstream and reverse ventilation in the downstream (P-R) for an extra-wide immersed tunnel. The effects of fire source location and jet fan air velocity response strategy on the ceiling temperature decay, carbon monoxide (CO) distribution, and smoke exhaust efficiency were investigated for varying fire source locations. The results show that flames will be tilted to the side of the jet fan with a smaller air velocity. Additionally, the jet fan air velocity should be adjusted based on the relative distance between the fire source and the smoke vent. Among the studied scenarios, the most effective outcome was achieved when the air velocity was adjusted to 25 m/s on the side near the smoke vent. Also in this scenario, the phenomenon of smoke deposition was effectively mitigated and the average smoke exhaust efficiency reached 87%. Moreover, we found that the temperature decay of the tunnel follows an exponential decay law. The temperature decay rate is significantly higher on the side closest to the smoke vent compared to the farther side. This research provides a theoretical basis for smoke control strategies for fires that occur in immersed tunnels.

Key words: Immersed tunnel; Longitudinal temperature decay; Longitudinal carbon monoxide distribution; Tunnel fire; Reversible jet fan air velocity

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