Studies relevant to this research project

3.3.1

Tunnel geometry and ventilation condition

Forced ventilation systems can affect fires in tunnels in many ways. In some situations, increasing the airflow in tunnels may cause a reduction in the severity of the fire; however, in other situations, increasing the airflow will cause the fire to engulf a vehicle more rapidly, resulting in a substantial increase in the heat release rate of the fire (Jagger and Grant 2005).

At Heriot-Watt University, research projects were carried out to estimate the influence of longitudinal ventilation on fire size in tunnels. The estimates were based upon experimental data using a Bayesian methodology producing a probability distribution of fire size for an HGV, car and pool fire (Carvel et al 2004a).

The influence of tunnel geometry and ventilation condition on fire in tunnels can be described as: open vent k Q Q = ψ Equation 3.1

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According to Carvel et al (2004a), in a situation where airflow is not restricted, a wider tunnel is safer as the HRR of the fire is less in a wider tunnel as compared with a narrow tunnel of the same height. However, it is important to note that the ventilation condition in the tunnel may have a bigger influence on the heat release rate than the tunnel geometry. It is important to consider both geometry and ventilation condition when making a design fire estimate.

From the fire tests mentioned above, it has been observed that the ventilation condition in the tunnel does affect the heat release rate. The large-scale fire tests conducted in Norway (refer to section 3.1.2) using wood cribs with (Figure 3.4) and without ventilation (Figure 3.3) illustrated that there is a substantial difference in heat release rate when the ventilation condition in the tunnel varies.

The fire test conducted in the EUREKA 499 on a Leyland DAF 310ATi type tractor with a trailer is another example of the influence of ventilation on heat release rate (refer to section 3.1.2). From the fire test measurements shown in Figure 3.9, there is a decrease in heat release rate when the ventilation was cut off between 13.5 to 16.5 minutes (EUREKA 1995). The result from this test showed that there is a significant difference in maximum heat release rate when the ventilation conditions in the tunnel are varied.

A fire test with truck loads consisting of 800 kg of wooden pallets, four tyres placed on top and a metal frame with a tarpaulin to simulate a small truck was performed in the Second Benelux Tunnel with various tunnel velocities (refer to section 3.1.3). Observations by Lemaire and Kenyon (2006) indicated that the fire growth with ventilation is about 2 to 3 times faster than the fire development without ventilation and the peak heat output for ventilated fire is about 1.2 to 1.5 times higher than a non-ventilated fire (Figure 3.11).

Another two tests were performed in the same tunnel as mentioned above; a car fire test with no ventilation and another with 6 m/s ventilation were conducted. It was found that at 6 m/s, the fire spread is not accelerated compared to the test without ventilation. The heat release rate is about 1 to 2 MW for half an hour before a peak of 5 MW occurs followed by

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(Figure 3.10), fire spread to the rear of the car is delayed due to the ventilation in the driving direction resulting in lower heat release rate at the first half an hour of the fire development (Figure 3.37). From these tests, it is worthwhile to note that the location of fire ignition will affect the behaviour of the fire development as ventilation could cause an accelerated fire spread if the fire originates in the rear of a vehicle and delay the fire spread if fire occurred at the front of the vehicle (direction of airflow from rear to front of the vehicle).

Figure 3.37: Fire development of a car fire after 10 minutes. (reproduced from (Lemaire and Kenyon 2006))

3.3.2 Fuel load

One of the factors to consider when designing a road tunnel is the unknown nature of the potential fire load. In other transportation tunnels such as rail tunnels, the potential fire load can be reasonably estimated in view that the train and the passenger load is controlled by the operating agency. This is not the case for the typical road tunnel as an innocent looking truck may not necessarily be classified as hazardous cargo but may be carrying a load that is capable of supporting a fast-growing fire (Bendelius 2003).

A compartment fire is usually defined as either fuel-controlled or ventilation controlled. In the growth phase (pre-flashover stage) where there is sufficient oxygen for combustion, the fire is dependent on the flammability and amount of fuel. The fire is defined as fuel controlled. As burning progresses, the fire will continue to develop up to a point at which

Air Flow Direction

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increase of heat release rate and temperature. The period after flashover is called the fully developed phase, during this period the heat release rate is dictated by the oxygen flow through the openings and the fire is said to be ventilation controlled. The mode of combustion (whether the fire is fuel-controlled or ventilation control) can be estimated using the equation as shown in Equation 3.2 (Ingason 2005).

. . 3000 Q m_a = φ Equation 3.2 where:

= the mass flow rate of air supply (kg/s) .

Q = the heat release rate (kW).

If ∅ _{(air-to-fuel mass ratio) is greater than 1 the fire is considered fuel-controlled. The}

last phase is the decay phase where the fire has consumed most of the fuel and the heat release rate will diminish. The different phase of a typical compartment fire is shown in Figure 3.38.

Figure 3.38: Different phases of a compartment fire (reproduced from Ingason (2005))

According to Ingason (2005) tunnels are often equipped with mechanical ventilation and the majority of fire in tunnels are often fuel controlled. Tunnel fires are not likely to grow

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radiation to the floor of the compartment is 15 to 25 kW/m2) due to large heat losses from the fire to the surrounding walls and the lack of containment of hot fire gases. Thus their burning rate would not be controlled by the rate of air supply to the fire. For such fires the burning rate and heat output is dependent on the amount of fuel in the tunnel.

A recent full scale fire test conducted by the Swedish National Testing and Research Institute SP in a road tunnel (Runehamar) in Norway showed that a fire in a heavy goods vehicle loaded with ordinary mixed goods can create a heat release rate between 66.4 to 201.9 MW. If a heavy goods vehicle carrying loads with substantial energy content catches fire in a tunnel, the fire can be extremely intense and burn for a long period of time. A discussion of this fire test taken from Ingason and Lonnermark (2003) is presented in section 3.1.5