Characteristics & Categorization of Tunnel Fires & Their Mitigation

Sisir Chakraborty, Director, C.Doctor & Co Pvt. Ltd.

A fire that develops in a tunnel interacts with the ventilation airflow and generates complicated air flow patterns and turbulence in the vicinity. The heat generated warms up the surrounding air, and in case of a slope inside the tunnel, buoyancy forces are created along the tunnel, which could govern the movement of the air flow inside the tunnel. If the resulting longitudinal flow velocity is not high enough, a reverse flow of hot gases in the ceiling will be created. This phenomenon is known as back-layering. In order to prevent any type of back-layering, the longitudinal velocity inside the tunnel has to be higher than a critical value.

Sincere efforts should be made to avail the natural ventilation available in the tunnel. The main problem with natural ventilation in tunnels is that not only the tunnel geometry, but the size and location of the fire govern the flow of hot gases in the tunnel, but also winds and atmospheric conditions outside the portals may have a strong influence on the ventilation system.


Effects of fire on natural ventilation inside the tunnel not only complicate fire-fighting procedures, but also present extreme hazards by rapidly propagating toxic fumes and gases far away from the fire. Sudden changes in the air flow could easily occur due to pressure changes inside and outside the tunnel portals. This situation can only be controlled when mechanical systems are applied.

Tunnels are often equipped with mechanical ventilation - sometimes termed as forced ventilation. Mechanical ventilation consists of supply/exhaust fans and/or jet fans in the ceiling. The consequence of using mechanical ventilation is mainly seen in terms of the combustion efficiency, spread of heat and smoke as well as the HRR in tunnels. Generally, the smoke flow descends to the floor level after travelling a certain distance. Then the smoke flow could approximately be considered as being fully mixed, but still there exists indistinct layers, that is: a lower layer with incoming fresh air (partly vitiated) and an upper layer with outgoing combustion products.

If a longitudinal ventilation system is activated, this stratified layer will gradually disperse. At first, on the upstream side of the fire, a smoke layer will still exist (back-layering). On the downstream side, the stratification of the smoke is gradually dispersed.

Flashover is defined as the rapid transition to a state such that all the surfaces of the combustible materials within a compartment are involved in the combustion.

Fires in compartments can easily grow to 'flashover' within a few minutes. Flashover is not expected to take place outside a confined space such as a compartment.

The volume of the compartment is very important as is the composition of the materials found in the compartment together with the opening sizes. Tunnel fires, in that sense meaning fires in a long space with two large portal openings, are, therefore, not likely to grow to a conventional flashover. The main reason is due to large heat losses from the fire to the surrounding walls, and lack of fuel in relation to the volume size and containment of hot fire gases.


Experiments and theoretical considerations show that flashover can easily occur in a train compartment or a truck cabin. This type of flashover will not occur inside a tunnel space. In the same way, the risk of secondary deflagration due to under-ventilated fires is much lower in tunnels than in building compartment fires. The main reason for this is the difference in the ventilation conditions (as explained above) and the geometry and heat losses to the surrounding tunnel walls. The amount of fuel load in relation to the tunnel volume also plays an important role.


Although flashover appears to be impossible in a tunnel fire, an under-ventilated fire in a tunnel is possible. This should be given special attention. In an under-ventilated fire, the consequences of the activation of a powerful ventilation system may be dramatic. The flame volume may suddenly increase in size and length, and the fire may easily spread forward due to the preheated vehicles' downstream of the fire - although this phenomenon cannot be defined as 'flashover' in the traditional sense of the word.

Fuel & Ventilation Controlled Fire

In fuel-controlled fire (or well-ventilated), the oxygen or the oxidant is in unlimited supply and the rate of combustion is independent of the oxygen supply rate or mass flow rate of air. The HRR is then determined by the fuel supply rate or mass flow rate of the vaporised fuel. A ventilation-controlled fire (or under-ventilated) is, in contrast, controlled by the oxygen supply and the rate of combustion or HRR becomes dependent on both air and fuel supply rates.


Mitigation Systems in Tunnels

A mitigation system is defined here as a technical system or a method to increase the safety during a fire. The main load is through heat flux from burning vehicles. The heat fluxes vary depending on the tunnel geometry, ventilation in the tunnel, and the type and shape of the fire load. Although the heat flux in kilowatts per square meter (kW/m2) should be used for describing the heat exposure onto a tunnel construction, it is seldom applied as input into models for calculating temperature rises inside the structure. Instead, different types of time–temperature curves are given and the boundary conditions are given as a lumped heat flow constant.

In well ventilated tunnels with relatively low ceiling height, the maximum gas temperatures can easily reach a level of 1350 °C, whereas in buildings this is usually in the range of 900–1100 °C. The main reason for this is the difference in the ventilation conditions and thereby the heat flux exposure. The understanding of heat fluxes and temperature development is of great importance.

The ventilation system is one of the most important safety features in tunnels. It makes it possible to control the smoke spread and thereby influence the outcome of a fire incident. The mechanical systems can be controlled automatically or by persons in a control center for a specific tunnel or tunnels.

In the early history of ventilation design (late 60s), the systems mainly consisted of smoke extraction systems, that is, the smoke was exhausted out of the tunnels. The terminology for these types of systems is 'semi-transverse' or 'fully transverse' systems. Semi-transverse means that they only exhaust the smoke, whereas fully transverse supply fresh air along the tunnel and exhaust the smoke. Today, these systems have been optimized in the design and are termed 'point extraction; systems.


Tunnels, especially those which have bi-directional traffic, are often equipped with point extraction systems. The smoke is not only controlled by extracting it, also by the longitudinal flow created inside the tunnel. Additional jet fans in the ceiling have also been applied to control the longitudinal flows. Transverse systems have been shifted to only using longitudinal ventilation by mounting jet fans in the ceiling. This is considerably easier to build and much less expensive. The conceptual idea in uni-directional tunnels is to create a smoke-free area upstream of the fire site. The main design parameters are the HRR in MW of a design fire and the critical velocity needed to prevent back-layering inside the tunnel.

One of the main risks with ventilation systems is the possible enhancing of the fire development and increased risk for fire spread between vehicles. Fire spread as governed by the flame length. and as governed by the heat flux. The more advanced methods can be combined with advance Computational Fluid Dynamic (CFD) calculations. Further, the smoke densities and temperature distribution are important. By calculating the walking speed, which is dependent on the visibility, and the hazardous environment, the evacuees are exposed to (toxic gases and temperatures), the evacuation time when (or not) they reach a safe region can be derived. The detection systems are necessary to alert tunnel users, fire services and the controller of the tunnel systems, to an incident. The most common system is based on line detectors where the convective heat from the fire indicates that there is a fire.

Depending on the fire size, tunnel height and ventilation rate, the systems can vary in response time. Other systems detect the smoke particles travelling inside the tunnel which requires that the smoke is lifted by the convective flow (buoyancy) to the location of the detectors. The common factor with all these systems is the dependence on the physics of the fire, which, therefore, requires a good basic knowledge of fire dynamics in tunnels when working with these systems.

The model scale technique is an important instrument in order to obtain useful and reliable information concerning fire dynamics in tunnels. The model scale technique is one of the most effective methods in gaining new knowledge and, therefore, it is important to present the theories behind it.