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113 TABLE 34 ology was verified by the observation of numerous large-scale INFLUENCE OF VENTILATION RATE ON FIRE tests and by CFD results (75). GROWTH RATE Ventilation Rate Growth Rate The slope of the tunnel has an important influence on the Less than 1 m/s About 5 MW/min dispersion of the flue gases. In general it can be said that owing About 3 m/s About 15 MW/min About 6 m/s About 10 MW/min to the chimney effect, the dispersion velocity of the flue gases Source: Ko and Hadjisophocleous (31). increases with the increase in tunnel slope. The longitudinal air velocity's increase will lead to changes of FHRR and fire growth rate, as was discussed in the previous chapter. INFLUENCE OF TUNNEL GEOMETRY ON FIRE HEAT RELEASE RATE INFLUENCE OF STRUCTURAL AND NONSTRUCTURAL COMPONENTS A tunnel is a confined space and presents one of the "worst ON FIRE HEAT RELEASE RATE case" geometrical shapes for fire development. The low ceiling and small cross section provide conditions that are conducive A tunnel will have a "fixed" and a "variable" fire load. The to high thermal loads to the tunnel structure. fire load resulting from fixed tunnel components, such as wall linings, and contents, such as cables, track, power sup- The Runehamar fire tests reached 200 MW (682 MBtu/hr) ply network, signaling system, lighting system, and radio using mockups of HGVs with high combustible loads in a tun- transmission equipment, can be assessed based on a statistical nel with a relatively small cross-section area and under longi- survey of typical tunnels. In a road tunnel, the variable fire load tudinal airflow. These test results may not be directly applied to consists of road vehicles and is more difficult to define because a tunnel with conventional cross-sections dimensions. Beard the density of the vehicles present in the tunnel is variable and and Carvel (35) have developed the approach to evaluate the a tunnel fire would not be expected to involve all of the vehi- impact of geometry on the FHRR. The research showed that for cles in the tunnel. a given combustible load the FHRR of a fire will vary depend- ing primarily on the relative width of the tunnel and the fire In a tunnel fire, it is unlikely that the fire will involve all of source (35). They concluded that fires that are small relative to the available fuel. In the growth stages, road vehicles are of the size of a tunnel will not be significantly influenced by the most interest. Later, elements of the tunnel, such as linings, tunnel geometry. Fires up to about half of the width of a tunnel might become involved. will be enhanced by the tunnel geometry, whereas fires with dimensions close to the width of the tunnel will be reduced. The size of the initiating fire and type of fuel is important because a relatively small fire source may not be capable of When compared, fires within narrow tunnels will generate igniting the material contents or the compartment lining mate- a larger HRR for the same fuel load than within wider tunnels, rials of the vehicle. Somewhat larger sources may be capable assuming sufficient air is available for burning in both cases. of igniting certain material contents, but not lead to flashover. (Tunnel height in those studies was much less a factor.) Larger or critical ignition sources result in flashover within the vehicle. The equation that best describes the relationship between fire HRR and the tunnel width Wtunnel and fire width Wfire is: Increasing the size of the initiating fire will increase the heat flux produced by the initiating fires. Increasing the HRR of the HRRtunnel = ( Btunnel BRunehamar ) HRRRunehamar fire may also increase the flame height, exposing larger areas (28 8) of material to high heat fluxes. where: Materials exposed to higher levels of heat will ignite more readily, release more heat, and potentially lead to the greater B = 24 (Wfire Wtunnel ) + 1 3 (29) spread of flame. The location of the initiating fire will also affect the heat fluxes produced by the fire. (The equation is valid for "enhancing regime" as identified by Beard and Carvel. The relationship between tunnel geom- The tunnel structure generally consists of a concrete lining. etry and fire size has yet to be established in the "diminishing The primary function of the tunnel lining is to bear the loads regime.") acting on the structure, especially in the event of fire. Different types of concrete are used in tunnels. Depending on the type of This equation allows one to estimate the design FHRR tunnel, generally normal strength or high strength concrete are against the values obtained in the Runehamar tests, consider- used. Different kinds of concrete will react differently to fires. ing that QRunehamar = 203; WfireR = 2.9 m WtunnelR = 7.3 m, or in any The goal is to have cost-effective, durable concrete that will other tests. Estimates show that for a 15 m (49.2 ft) tunnel, the have sustained load-bearing capacity during fire and eventu- design FHRR is about 100 MW (341 MBtu/hr). This method- ally without structural damage.