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Article Excerpt
INTRODUCTION
Fires in tunnels pose major safety issues, especially with the increase in the number of tunnels, their length, and the number of people using them. The main fire safety issues include 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 (NFPA 2001; Lacroix 1998).
In a tunnel environment, life can be threatened in a number of ways: the inhalation of combustion products such as carbon monoxide and carbon dioxide, and the exposure to high temperatures and heat fluxes. Temperatures up to 1350[degrees]C and heat fluxes in excess of 300 kW/[m.sup.2] can be generated within a few minutes of ignition in certain types of fires. Furthermore, evacuation can be significantly hindered 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. For safe evacuation, viable temperatures, acceptable visibility, and air quality must be maintained in the tunnel.
A fire incident involves a sequence of events, which include fire ignition, fire development, self evacuation of the tunnel, and assisted evacuation after the arrival of the emergency services. Prior to the activation of ventilation systems, the fire must be detected and the detection be confirmed.
In the event of a fire, the airflow in a tunnel is modified due to the fire itself, the operation of the emergency ventilation system (EVS), and the change in the traffic flow in the tunnel. Airflow, heat release rate, natural ventilation, tunnel slope, and traffic flow are among the parameters that affect the smoke flow and its stratification (Heselden 1976). With no airflow in the fire zone, the buoyant smoke symmetrically rises and spreads along the ceiling on both sides of the fire source. Underneath the smoke layer, fresh air is drawn in toward the fire source in an opposite direction to the spreading smoke. This separation between the hot upper layers and cooler lower layers is termed "stratification." The combined effects of convective heat exchange with the tunnel walls and the mixing between the smoke and the fresh air layer causes the smoke to cool down and lose its stratification. After a period of typically 5 to 10 min (Lacroix 1998) both upstream and downstream sections of the tunnel can be completely filled with smoke. Thus, stratification is a temporary phenomenon. This time period is essential for tunnel users to rescue themselves. Hence, if stratification is part of the emergency operation strategy, then reliable and robust control of the longitudinal airflow velocity is essential.
During an emergency operation, EVS is needed to influence the flow of smoke and combustion products so as to create a safe environment for tunnel users to escape and for emergency services to intervene. Methods of controlling hot gases and smoke from a fire in a tunnel using EVS include longitudinal airflow, smoke extraction, and smoke dilution. Smoke can be extracted using localized extraction points, a transverse ventilation system, or a semi-transverse ventilation system.
Factors determining the requirements for EVS during emergency operations include tunnel length, traffic density, and the direction of traffic movement (bidirectional/unidirectional; with/without traffic congestion). The proper design of EVS and the optimal strategies for its operation requires the consideration of a real fire situation and its evolution with time and the determination of the potential fire and smoke threat (in terms of visibility, temperature, and toxicity effects).
The ultimate goal of "fire and smoke control" is the achievement of a system that is capable of producing the correct actions and instructions for all rescue services as a function of the actual fire scenario. However, optimal control is a challenging task due to the tremendous complexity and diversity of fire hazards.
Ventilation Systems
Ventilation of tunnels can be achieved using either natural or mechanical systems. Natural systems count on the piston effect of moving vehicles and the meteorological conditions (external wind, temperature, and pressure differentials between the portals) to produce airflow through the tunnel. Mechanical ventilation systems use fans to produce airflow and ducts and dampers to distribute this airflow. Regardless of mechanical ventilation equipment, the natural effects mentioned above are present in all tunnels to a varying extent.
A mechanical ventilation system is generally classified based on the direction of airflow in the roadway of the tunnel. Longitudinal ventilation systems produce longitudinal airflow in the direction of the tunnel axis, whereas semi-transverse and full transverse systems produce airflow that is perpendicular to the tunnel axis in the plane of a cross section. The choice of what ventilation system to use depends on several parameters, which include tunnel length, cross section, and grade; surrounding environment; traffic volume; and construction cost.
Backlayering
The backlayering phenomenon is defined as the...
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