Investigation of Effectiveness of Emergency Ventilation Strategies in the Event of Fires in Road Tunnels
|Publication Date:||1 January 2005|
Fires in tunnels pose major safety issues and challenges to the designer, especially with the increase in the number of tunnels, their length, and number of people using them. The main fire safety issues include (NFPA 2001; Lacroix 1998): safe evacuation of people inside the tunnel, safe rescue operations, minimal effects on the environment due to the release of combustion gases, and a minimal loss of property.
The safety of tunnel users and rescuers is the main objective for the emergency ventilation system (EVS). Life can be threatened in a number of ways: the inhalation of combustion products, such as carbon monoxide and carbon dioxide, and exposure to high temperatures and heat fluxes. Evacuation can be significantly affected by poor visibility, power failure, blocked exits due to traffic jams or crashed vehicles, or obstruction resulting from a collapse or explosion in the tunnel. Temperatures up to 1350°C and heat fluxes in excess of 300 kW/m2 can be generated within a few minutes of ignition in certain types of fires. For safe evacuation, acceptable visibility and air quality must be maintained in the tunnel.
From the beginning of a fire, the airflow in a tunnel is modified and becomes highly unsteady. The modifications are due to the fire itself, the operation of the emergency ventilation system, and the change in the traffic flow in the tunnel. The smoke progress and its degree of stratification depend mainly on the airflow in the tunnel. With no airflow in the fire zone, the smoke moves symmetrically on both sides of the fire (Heselden 1976). The smoke remains stratified until it cools down due to the combined effects of the convective heat exchange with the tunnel walls and the mixing between the smoke and the fresh air layer. The other parameters that affect the smoke flow (Heselden 1976) and stratification are fire heat release rate, tunnel slope, and traffic flow.
In the event of a fire, the intent of the EVS is to provide tunnel users with a safe egress route that is free of smoke and hot gases. Tunnel operators must implement a plan of smoke clearing, which consists of selecting a sequence of fan operation with the objective of keeping the road upstream of the fire accident smoke free. This is done by limiting the upstream smoke flow and either venting it using fans or letting it escape through the downstream portal. When the fire department arrives on the fire scene, the operator must cooperate and modify, as needed, the fan operation in order to facilitate access to the site.
Establishing airflow requirements for roadway tunnels and, consequently, the capacity of the EVS is a challenging task due to the difficulty of controlling many variables. These include changes in traffic patterns and operations during the lifetime of the facility. Methods of controlling air contaminants and smoke from a fire in a tunnel using EVS include longitudinal airflow, smoke extraction, and smoke dilution.
To evaluate the effectiveness of the emergency ventilation strategies in the event of a fire in the two tunnels in Montreal, Quebec, NRC has undertaken a research project with the Ministry of Transportation of Quebec, Canada.
The first two stages of the project have been completed. An extensive literature review on vehicle tunnel ventilation for fire safety has been completed and provided a rational basis for choosing two CFD numerical models for the initial evaluation: namely, SOLVENT (Innovative Research, Inc./Parsons Brinkerhoff, Inc. 2000) and Fire Dynamic Simulator (FDS) (McGrattan et al. 2000). Based on comparisons with field test data, a model will be selected for use in the remainder of the project.