SAFETY/FIRE | TECHNICAL
The tunnel in the simulation has bi-directional traffic,
is 2km long, and has a moderate 1% gradient, downhill from left to right (chainage 0m – 2000m). There is no barometric pressure present. For the model, the truck fire is located at x = 1500m chainage. Airflow is modelled at locations 100m within the
tunnel from each portal. The locations are represented as u1 (closer to 0m chainage portal), and u2 (closer to the 2000m chainage portal). Positive flow is defined from left to right (i.e., with the chainage). It takes three minutes to detect the fire and to start
the smoke extraction (visible as u1 ≠ u2). With the extraction, u1 (left portal) becomes positive and u2 (right portal) negative, indicating airflow into the tunnel from both portals. Fresh air from the portals replaces the extracted smoke volume. In each scenario, the smoke exhaust capacity is 150m3
/s. Figure 2 shows the results of the three simulation
scenarios (Figure 2a-2c), represented on two graphs: on the left, the graph shows longitudinal airflow velocity versus time; and, the graph on the right shows smoke propagation along the tunnel chainage with time. On the latter graph, light grey shading indicates a potential for smoke stratification while dark grey shows smoke filling the tunnel cross-section. In scenario a), shown in Figure 2a, the tunnel has two
successive ventilation sections, each 1km long, totalling 2km. When the fire is detected, the extraction is started immediately. The extraction is equally distributed over a length of 1km, which is half of the tunnel length. In this scenario, the ventilation system is unable to limit the smoke propagation. It may slow down the smoke fronts, but after 18 minutes a major part of the tunnel is filled with smoke. As the smoke cools down, there is little chance of smoke stratification in 500m distance from the fire.
In scenario b), shown in Figure 2b, the tunnel is
equipped with smoke dampers in the false ceiling for local smoke extraction. When the extraction system is started, several smoke dampers are opened, limiting the extraction section to a length of 200m at the fire. While the same extraction capacity is applied, smoke control is significantly improved. The smoke propagation toward the left of the graph is slowed down allowing people more time to reach the emergency exits. The local extraction captures the smoke at a higher temperature, thereby limiting the buoyancy of the hot smoke and reducing the longitudinal flow in the tunnel. But such a concept is very vulnerable to external pressure from external wind or barometric pressure, or to buoyancy of hot smoke in steep tunnels. In scenario c), the smoke is fully controlled in the
extraction section. This is achieved by local extraction combined with controlled operation of jet fans in the tunnel, aiming at symmetrical flow from both sides towards the extraction section. Jet fans for flow control were introduced with the transition from distributed smoke exhaust to local smoke exhaust. In a real tunnel, the effect of an upgrade from
distributed extraction to local extraction is even more striking. A typical capacity for distributed extraction was 80m3
/s per km.1 /s to 200m3 This is about half the capacity
applied in scenario a). And, for such a tunnel, a typical capacity for local smoke extraction in scenario c) is 180m3
/s, which improves smoke control even further.
COMPLEX While the smoke control concept of using smoke dampers and controlled jet fan operation is proven much more effective, it comes at a price.
Above, figure 3: Smoke damper installation in 2004 August 2023 | 13
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