CFD ANALYSIS OF TUNNEL FIRES

Abstract

Majority of transport modes use tunnels, whether it is rail, road or water. Millions of people travel through these tunnels everyday. Recent disasters have shown that consequences of fire are very severe in tunnels. Today, the safety of tunnels has acquired international importance. As the number and length of road and rail tunnels are increasing and more people are using them, an enhanced emphasis is being laid on taking justifiably appropriate fire safety measures. The safety obviously calls for better detection and fighting systems, and also efficient ventilation and smoke control systems. In order, first to understand various traditional safety features of tunnels, a number of important long distance road transport tunnels of Europe have been studied extensively with respect to their technical specifications including fire safety measures. It is found that the tunnels have a wide variety of features. It is also found that most of the countries either do not have or have minimal formal guidelines of fire safety in tunnels. NFPA - 502, 'Standard for Road Tunnels, Bridges, and Other Limited Access Highways' evolved into a code, only in 1998. Earlier it was, at best, merely a recommended practice. In December 2002, the European Commission however proposed a new directive on 'Safety in European Road Tunnels'. The safety features of the studied tunnels are compared with those of NFPA 502 and with the proposed directive of European Commission. It is found that there are substantial differences in some provisions as given in NFPA 502 and proposed directive of European Commission. In order to provide better safety features inside tunnels, the most stringent measures are proposed in the form of general fire safety guidelines. It is suggested that in future tunnels these observations / recommendation should be complied. The higher safety standards will ensure less fatalities and less damage to tunnel structure in case of fire. The aforesaid study of safety components shows that ventilation system is one of very important safety measures inside tunnels used for controlling and extracting smoke in case of fire emergency. In long tunnels, where ventilation is provided by mechanical means, two types of ventilation lay outs exist - longitudinal and transverse. The longitudinal ventilation is provided through jet fans located axially below the ceiling or through jet injection system where the fans are located in a fan room and air is supplied through ventilation shafts. In longitudinally ventilated tunnel fires, smoke and hot gases form a layer below the ceiling and flow in the direction opposite to the ventilation stream. This phenomenon is called back layering. The ventilation velocity just sufficient to prevent back layering of smoke over the stalled vehicles is the minimum velocity needed for smoke control and is known as the critical velocity. The ability of the longitudinal ventilation system to prevent back layering is the current industry standard to measure the adequacy of the system for smoke control. The ventilation velocity depends on number of parameters such as heat release rate (HRR), tunnel geometry, slope etc. This implies that ventilation system has to be designed for each individual tunnel. The ventilation system can though be designed and evaluated through experimental studies of each tunnel, but that is impractical and expensive. Alternative method is to use mathematical modeling which when coupled with flow visualization techniques provides an excellent means to study the environment inside a tunnel. This should help in designing appropriate ventilation system effectively without the need to conduct experiments. There exist mainly two types of mathematical models - simple zone models and complex three dimensional field, also called CFD models. To assess the capability and utility of simpler and time efficient zone models for predicting thermal environment inside the tunnel in case of a fire, a comparative study of a zone model and a CFD model has been carried out. For this, a fire scenario inside a naturally ventilated tunnel has been simulated using multi room zone model, CFAST and field model, CFX. For simulation, a tunnel of length 150 m having a rectangular cross section of 80 m2 has been considered. The temperature and velocity profiles generated by fire, placed at a distance of 20 m from one end of portal have been predicted. The simulation by CFAST has been carried out by dividing the tunnel into 1,2,5,8,10,12 and15 compartments of equal size, where these compartments are joined by openings or vents having same cross section as that of the tunnel. In case of tunnel divided into 15 compartments the fire source position lies at the position of vent; CFAST predicted very high temperatures. The simulations have also been carried out by dividing tunnel into unequal sized compartments such that position of fire is at the center of the compartment. It was found that for accuracy of results, location of fire source inside compartment is an important factor. Computational difficulty is experienced when tunnel is divided into more than fifteen compartments. The CFX and CFAST predictions show that smoke temperature changes with a pattern roughly similar to that of heat release rate. The temperature profiles at selected positions cannot be predicted by CFAST unlike CFX. The detailed features like flame tilt, flow field can only be observed from CFX predictions. It is concluded that zone models alone can not be used for studying fires inside tunnels. On the other hand, the CFD models can be a in powerful tool in analyzing problems involving far field smoke flow, impact of fixed ventilation flows etc. Therefore CFD model has been used to evaluate ventilation strategies in a transport tunnel in case of fire emergency. The aim is to study the smoke movement inside tunnels, and determination of critical ventilation velocity for smoke control in longitudinally ventilated tunnels which are similar to tunnel sections of Delhi Metro Rail corridor, India. The tunnel sections considered have different modes of longitudinal ventilation - ventilation through jet fans and through jet injection system. Both these modes of longitudinal ventilation are evaluated. The CFD program, CFX is used to study the effectiveness of smoke ventilation system to control smoke spread in the event of fire inside the tunnel. For the first study, where axially mounted jet fans located below the ceiling provides necessary ventilation, the tunnel section considered for analysis is 100 m long, 6 m wide and 9 m high. It is assumed that a fire source producing a constant heat release rate of 4 MW is located at the center of the tunnel. The numerical model used is first verified using experimental results available in the literature. The model is then used to simulate the fire environment inside the tunnel. The results of CFD simulations are compared with those of empirical correlations available in the literature. The effectiveness of smoke ventilation system is then studied. For this the effect of ventilation flow rate, both uniform and non-uniform airflow in tunnel, on thermal environment inside the tunnel is studied. The critical velocity necessary to prevent back layering for the two scenarios are determined. A ventilation scenario where both inlet and exhaust fans are activated is also studied. iv It is found that under natural ventilation conditions inside a tunnel, the smoke moves symmetrically along the crown in both directions, and cool entrained air from bottom of tunnel portals move towards the fire source. For uniform induced air flow, it is found that CFD predicted higher critical ventilation velocity (2.18 m/s) than predicted by empirical relations developed by Wu and Bakar (1.52 m/s). For non uniform induced air flow the required critical axial fan velocity is found to be much higher and lies between 8 m/s and 10 m/s. It is also found that the stratified layer of smoke in downstream region is not formed, and no escape can take place from the downstream direction. In the second study, where air is supplied through jet injection system, the section of tunnel considered is 400 m long, 5.5 m wide and 6 m high. The analysis has been carried out by assuming a variable fire source with a peak heat release rate (HRR) of 16MW, located at the center of the tunnel. Ventilation ducts are located in the ceiling near the tunnel portals and inclined at 10° to the plane of the ceiling through which fans discharge air. The influence of the fire HRR curve slope on the smoke flow dynamics in this realistic tunnel model fitted with inclined fans is investigated. The physical models used are same as those used in previous study. In case of fire two scenarios are studied: (i) fans activated immediately and achieve its full speed after detection of fire, (ii) fans activated at delayed times to take into account the response time of the fans to achieve its maximum speed. The velocity of supply and exhaust fans necessary to remove smoke in 30 sec from the upstream direction is determined. It is found that the smoke moves symmetrically along the crown in both directions and reaches tunnel portals in about 3 min. It is also found that for this type of tunnel configuration higher supply and exhaust velocities are required to produce the desired critical velocity. The velocities of fan required to produce different desired axial velocity inside the tunnel is determined and is represented in the form of a graph. The exhaust fans do not influence the velocity in upstream area but are necessary for smoke removal in the downstream direction. It is also necessary that fans are activated to full speed within three minutes of starting of fire in order for the ventilation system to be effective for desired smoke removal.