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abstract

Life safety is one of the objectives of fire engineering design for road tunnels. Fire engineering design requires maintaining a tenable condition for a period of time to allow occupants to evacuate to safety. This will be achieved by controlling the smoke under credible design fire scenarios in a tunnel. The critical location in a tunnel fire emergency condition is the tunnel region upstream of the fire, where occupants are most likely to reside as traffic jam can usually be created by the fire incident. Tenability for the downstream region of fire is not the main focus of this research because vehicles can generally drive out of the tunnel at a higher speed than that of the smoke flow, and local damper smoke extraction can help keep a tenable condition in the downstream region beyond the local fire zone, in case there is a congestion in the downstream region of the fire. To maintain a tenable condition in the upstream tunnel region from the fire incident, the required minimum longitudinal flow velocity to prevent smoke backlayering can be calculated based on NFPA 502 recommendations. This critical velocity takes no credit of the smoke extraction or active overhead fixed fire suppression effects. Smoke extraction with a dedicated smoke duct along the entire length of the tunnel is gaining popularity because of its efficiency and robustness in providing a tenable environment in the tunnel with unknown upstream and downstream traffic conditions. In this paper, a modified critical velocity to control smoke back-layering while smoke extraction and fire suppression systems are operating has been analyzed. This modified critical velocity is approximately 20% lower than the critical velocity that is recommended in NFPA 502. This allows significant savings on ventilation capacity for road tunnels which have a local smoke exhaust capability using a dedicated smoke duct. It is concluded that the smoke extraction performance is similar whether using ceiling dampers or vertical wall-mounted dampers for smoke capture to maintain tunnel tenability. However, tunnel gradients play a major role on the modified critical velocity for a nominated design fire and the required smoke extraction rate.

Conclusions

Based on a typical example 2 lane road tunnel with a fixed smoke extraction rate of 282 m3 /s, Computational Fluid Dynamics (CFD) analysis has been performed on selected cases to investigate the modified critical velocity considering specific smoke extraction configurations and other parameters. These parameters examined included the extraction damper locations, total number of operating dampers, tunnel gradient, fire location and the traffic blockages in the tunnel region that is upstream of the fire. This analysis has confirmed the following:  When the smoke exhaust system is operating, the required upstream ventilation velocity to prevent smoke backlayering can be lower than the standard critical velocity that is recommended in NFPA502. For example, for an uphill tunnel gradient of +1.6%, with the local smoke extraction near the fire, the critical velocity can be reduced from 3 m/s to 2 m/s. For a tunnel width of no more than 10 m, it has been confirmed that the design configuration with vertical side wall dampers and an alternative design configuration with horizontal roof mounted dampers develop equal capabilities to control smoke backlayering and to prevent smoke propagation downstream of the tunnel; the difference in required critical velocity is not significant for these different damper configurations.  Tunnel gradient plays an important role in establishing the modified critical velocity for a given design fire scenario. A tunnel segment with 4% gradient demands a critical velocity of 2.5 m/s, compared to 2.0 m/s for a tunnel with a gradient of +1.6%.  There is no significant impact on the critical velocity created by the number of operating dampers in this investigation. No difference was observed on the demand of critical velocity or the smoke propagation with four wall dampers or three wall dampers.  Fire location at the far side from the wall dampers requires a higher critical velocity. A critical velocity of 3.0 m/s would be required for a fire located near the wall and on the far side from the wall dampers. This is an increase compared to 2.5 m/s for the case with a fire located in the tunnel center for a tunnel segment with a downhill 4% gradient.