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عنوان فارسی مقاله:
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عنوان انگلیسی مقاله:
A modified critical velocity for road tunnel fire smoke management with dedicated smoke extraction configuration
سال انتشار : 2014
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مقدمه انگلیسی مقاله:
1. Introduction
Tunnel accidents involving a fire incident is a low frequency event. However, its consequence is serious if the fireemergency system is not properly designed and managed to cope with this special event.One of the design objectives of a tunnel fire life safety system based on smoke extraction is to maintain tenable conditionsin the tunnel and to contain the smoke within a manageable segment of the tunnel, allowing the occupants to be evacuatedthrough the exits or egress passages before developing fire hazards make the tunnel untenable.When a fire incident happens in a longitudinally ventilated tunnel, two zones will be developed. One is the tunnelzone in the upstream traffic location relative to the fire incident, the other is the downstream traffic location relativeto the fire location. In most cases, the downstream zone is of less concern because vehicles can continue to drive awayat a higher speed than that of the smoke flows, if there is no traffic congestion in the downstream zone. The local smokeextraction system can also help keep a tenable condition in the downstream section of the tunnel beyond the local firezone, which means there is less of a concern even if there is traffic congestion in the downstream. However, theupstream zone is the major concern because traffic will build up behind the fire because of the fire incident. Severalpublications have discussed smoke control to maintain tenable conditions upstream of the fire location[1–9]. However,the impact of smoke extraction and spray water fire suppression on the required critical velocity has not been includedin their investigations.In newly built road tunnels, local smoke exhaust systems with a dedicated smoke duct is gaining popularity because oftheir effectiveness in mitigating fire hazards developing in the tunnel. For example, the renovated Mont Blanc Tunnelbetween France and Italy, the Clem 7 tunnel and the Airport Link road tunnel in Brisbane Australia, and the Alaskan WayViaduct replacement tunnel in Seattle have adopted the concept of a dedicated smoke exhaust duct to ensure the smokein close proximity to fire incident can be extracted. Tunnel emergency ventilation system design to mitigate fire hazards nor-mally utilizes air flow momentum to effect smoke control with longitudinal flows that establish critical velocity as recom-mended in NFPA502 for vehicular tunnels[10]. However, this flow capacity does not take into account of the local smokeextraction effects.In some tunnels in the US, Japan and Australia, a sprinkler or deluge systems are being utilized to actively control the firespread and protect the tunnel structure. Gas cooling of the hot upper smoke layer is achieved through heat convection, masstransport and evaporative cooling effects as a result of sprinkler spray field created by overhead fixed fire suppression systemoperation. Unlike tunnels with longitudinal ventilation, when the smoke exhaust and water based sprinkler fire suppressionsystem are operating, this required critical velocity to protect the upstream zone can be reduced when the smoke extractionis enhanced with optimized local damper operation configuration to effectively limit the spread of smoke and untenable con-ditions within a local tunnel segment.This paper discusses a modified critical velocity for road tunnels, where a dedicated smoke extraction system and waterbased fire suppression system is provided. This modified critical velocity and the extraction rate will be determined througha performance based approach considering the specific tunnel ventilation and fire safety provisions of the tunnel. Severalcritical fire scenarios which should be considered have been highlighted. Two different tunnel gradients have also been ana-lyzed in this paper, and a methodology has been proposed on how to determine the modified critical velocity and the smokeexhaust capacity.Design parameters such as fire scenarios, fire sizes, tunnel gradient, fire location, smoke extraction location and totalnumber of open smoke extraction dampers are also analyzed to confirm the performance of this modified critical velocitywith an evaluation of system robustness of operating modes and configuration
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کلمات کلیدی:
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