Ventilation of Tunnels Fires, the Effect of Ventilation Velocity on the Burning Rate in Tunnel Fires - Literature review Example

Institution : xxxxxxxxxxx Title : Ventilation of Tunnels Fires Tutor : xxxxxxxxxxx Course : xxxxxxxxxxx @2012 Literature Review Introduction The occurrence of fire in tunnels has been a common incident that has always brought about devastating effects. Tunnel fire incidences such as the kings cross fire in London in 1985, in 1999, the Mont Blanc tunnel fire, in 2001, the St. Gotthard tunnel fire and the Channels tunnel fires in two years 1996 and 2008, have raised various question concerning the safety of tunnels as a means of transport. In addition the question concerning how to enhance safety in tunnels has also been raised. Ventilation of tunnels fires is one of the most sufficient methods of saving lives and also controlling smoke in event of fire occurrence. Many studies have therefore been conducted on the concept of ventilation of tunnel fires in order in order to provide analytic evidence that can be used in enhancing safety in tunnels in case a fire occurs. Hwang and Edwards (2005) highlight that ventilation is an efficient technique of controlling smoke when a fire occurs in a tunnel. When tunnel fires are ventilated, products of hot combustion and smoke create a layer close to the ceiling which further flows in an opposite direction to the ventilation stream. The presence of the stratified reverse flow is of great significance to fighting fires in tunnels. Hwang and Edwards (2005) assert that fire fighting in tunnels is enhanced by the critical ventilation velocity, which is the lowest velocity which prevents smoke from stretching against the longitudinal ventilation flow in the event of a fire. Hwang and Edwards (2005) examined a study undertaken by National Institute for Occupational Safety and Health. The study involved evaluating the role of the critical ventilation velocity in tunnel fires through the application of computer simulation. The method of the study involved the use of CFD program known as the fire dynamics simulator which is composed of a large eddy simulation (LES). The function of the large eddy simulations (LES) was to model the level of the floor in the ventilated tunnel. The critical ventilation velocity was stimulated on two tunnels of different sizes. The critical ventilation velocity that was applied was just adequate to stop the creation of a reverse stratified layer. The study then used the computer code in order to measure the profile of the compound velocity against the experimental measures. The findings of the CFD reveled that the leveling –off of the critical ventilation velocity occurred as the release of heat exceeded a certain value. At the critical ventilation point, the ceiling temperature attained its maximum level in both tunnels. The study concluded that ventilation of funnel fires is an essential aspect in fire fighting in tunnels. This is due to the fact that the presence of the critical ventilation leads to temperature stratification downstream away from the fire. As a result of this, smoke from tunnel fires can easily be reduced from spreading. (Hwang and Edwards, 2005) Palazzi et al (2005) also undertook a study to evaluate the role of the critical ventilation velocity during emergency fire scenarios. Unlike other studies that have been conducted using the CFD (computational fluid dynamics), Palazzi et al (2005), performed their study in both the real scale and the laboratory scale. The findings of the study indicated that the critical ventilation velocity varies depending on the cross sectional geometry of the tunnel. The findings clearly showed that there are two sides of the functioning of the critical velocity. When the heat release is of a low rate , the critical velocity differs as having a third power heat discharge rate .On the other hand when the heat release is of a higher rate , the critical ventilation then becomes independent from the rate of heat release from the fire . A similarity in findings of the two studies can be drawn from the study by undertaken by Palazzi et al (2005) and Hwang and Edwards (2005). Both researches reveal that the critical velocity greatly contributes greatly to preventing the spread of smoke that the fire produces in the upstream direction (this is referred to as back layering). As indicated by both studies, of the CFD analysis by Hwang and Edwards (2005) and Palazzi et al (2005) there is a similar trend of the leveling off of the critical ventilation velocity. This can be explained through the aspect of maximization of temperature above the source of fire. Jae Seong, et al (2008) studied the impact of the ventilation velocity on the burning rate of fires. The study involved an experiment that involved the conduction of Froude scaling in order to examine the effect or impact of the ventilation velocity on the rate of burning in tunnel fires. Jae Seong, et al (2008) used the pool fires of n- heptane in order to discharge heat from a range of 3.72 to 15.6Kw. Temperature distribution and the burning rate of fuel was then measured using a load cell. The findings of the study reveled that the increase in ventilation velocity results to an increase in the rate of burning of the N-heptanes fuel. The basic reason behind the increase in the ventilation velocity is that the supply of oxygen exists however it does not cause a cooling effect to the ventilation velocity but rather it increases the velocity. In making an analysis of the study conducted by Jae Seong, et al (2008) the relationship that exists between the burning rate and the critical velocity (Vc, m/s) can be termed as illegible. However an essential aspect to take note of is the fact that the ventilation velocity increases due to the existence of the supply of oxygen. According to Cheng, (2001) et al, ventilation of tunnels is a significant feature in facilitating a comfortable, functional and a safe tunnel environment. Cheng, (2001) et al, further highlights that the ventilation strategy depends on various aspects such as the length of the tunnel, the cross section and the alignment cross section. Usually for road tunnels that are 4 km long, transverse ventilation model is introduced. In the case of a railway system the ventilation system or model is predominantly influenced by the emergency incident in order to enhance smoke control when a fire occurs. Cheng,(2001) et al, undertook a study which utilized the longitudinal ventilation, with a push and pull ventilation model as a strategy of exhausting high temperatures of air and smoke out of underground tunnels effectively in case a fire breaks out. Cheng,(2001) et al, propose a model of the push – pull ventilation known as the reversed fire outbreak and evacuation simulation model, MFIRE. The method of study applied by Cheng, (2001) et al, involved setting up a fire simulation laboratory which was applied in a small physical tunnel with an objective of verifying the MFIRE model. The rate of temperature distribution and air flow in each were tunnel was evaluated against the simulated results attained by MFIRE. The findings of the study revealed that MFIRE can effectively be applied in simulation of tunnel fires. This is because simulation is usually designed in a way that investigates the rate and direction of airflow, emergency distribution resources and the distribution of temperature. These results therefore gave a confirmation that the push and pull ventilation model is indeed effective in exhausting smoke and high temperatures out of the underground tunnels effectively when a fire breaks out. From the study conducted by Cheng (2001), an essential aspect to be noted is that this particular model gives adequate information for establishing an emergency ventilation system, creating safety procedures and also reducing the damage caused in the network systems that exist in underground tunnels. Ballesteros, et al (2006) conducted a study to evaluate the semi- transversal approach of urban tunnel ventilation. The study method entailed experimenting on one of the biggest road tunnels in Spain Ronda del Mig”. The tunnel is 1535 m in length and is divided into two galleries which are parallel and independent of each other. Despite of the fact that the normal ventilation is attained through an interior passage which also extracts smoke, in the event of a fire, both sides of the passage usually open up. Therefore the ventilation system of the tunnel is semi-transversal ventilation system. The experiment entailed simulation of fire that has the thermal power of 30 MW with a simulation time of 15 munities. Emphasis was laid on the effect of the slope of the tunnel on the behavior of smoke in the gallery. The findings of the study reveled that a significant amount of smoke was exhausted through the two openings. In addition the rest of the smoke moved through the portal due to the existence of the slope in the tunnel. The opening at both sides of the fire , were useful because they were able to lower the aptitude of smoke due to clean air that come from the lower portal(Ballesteros, et al ,2006). The study by Ballesteros, et al ,(2006) is another significant example of the importance of ventilation of tunnels fires in the reduction of smoke. Although ventilation of tunnel fires is actually beneficial in dealing with emergency situations, essentially in the control of smoke, adverse effects can arise due to the ventilation of tunnel fires. Li et al (2011) reveal that ventilation tubes sometimes do rapture in the event of a fire and therefore leading to adverse effects of secondary catastrophes in a tunnel. Li, et al (2011) argue that the ventilation tube is basically very important equipment in tunnels. This is because it assists in air supply in the tunnel. Unfortunately although the ventilation tube is very useful, it is usually made of PVC materials that are non-combustible, as a result when very huge fires occur in a tunnel, the tube can be destroyed and further leading to adverse effects. Li et at (2011) highlight that the rapture of the ventilation tube brings about the mixture of methane and air. This may further result to the inducement of other secondary catastrophes such as gas expositions. Li, et al (2011) undertook a study to evaluate the influence of the ventilation tube rapture from fires on secondary catastrophes. The method of study involved setting up a real tunnel. Fire was then set on the tunnel. The influence of the rapture of the ventilation tube was then measured as the fire intensity was increases from 0, to 200 to 1200 Kw. The findings of the study revealed that when the intensity of the fire was increased progressively to 1200kW, other secondary catastrophes such as gas explosions occur. From the study, a negative aspect of ventilation of tunnels fires in presented. Although ventilation of tunnel fires is actually beneficial in dealing with emergency situations, essentially in the control of smoke, adverse effects can arise due to the ventilation of tunnel fires. This therefore results to the need for adopting the most effective ventilation designs that can prevent the occurrence of secondary catastrophes such as gas explosions especially when the intensity of fire is vey high. The study provides an explanation to the fact that many of the existing tunnel ventilation today are designed with a considerable level of redundancy and indeed they are have the ability to control smoke during emergency situations like the occurrence of fire. However there is need to improve the safety measures that can ensure that the ventilation systems do not result to other adverse effects such as secondary catastrophes like gas explosions which may lead to additional problems. Carvel, et al (2001), highlight that in the previous few years , the occurrence of fire on transport tunnels has been an issue of great concern , not only for the people who design the tunnels but also for regulators and the general public . The fires that have occurred on Austria’s Tauern Tunnel and Channel Tunnel that joins France and the U.K indicated just how devastating fire can be on tunnels. Carvel, et al (2001), highlight that many tunnels are usually equipped with the longitudinal ventilation system in order to control smoke in the event that a fire occurs in a tunnel. Never the less the influence of the adoption of such a ventilation system has in rare cases been examined. Carvel, et al (2001) highlights that the main objective of longitudinal ventilation is usually to pull all smoke emitted by a fire in one side, in order to develop a smoke free area during fire fighting. This is usually referred to as prevention of ‘backflow’ or back layering . Carvel, et al (2001) further argues that the influence of the longitudinal ventilation on fire spread or growth has not really been examined in despite of the fact that many experts do acknowledge that when air is directed to the fire, this results to a rapid growth of the fire. Carvel, et al (2001), therefore undertook a study to investigate the influence of the longitudinal ventilation on tunnels with vehicles that carry heavy goods. The study involved the application of probabilistic method in order to refine estimates that were created by a panel of ventilation experts. In addition estimates were also collected from an experimental set up on a series of wood cribs. The relationship between the flow of ventilation and the Heat Release rate (HHR), was then examined. The findings of the study revealed that longitudinal ventilation had a great influence on tunnels when they catch fire. The estimates from the ventilation experts revealed that the fire grew three times larger as opposed to a scenario whereby the tunnel used the natural ventilation. The growth in the rate of fire was a function of the ventilation velocity. The linear rate of fire growth ΔQ/ Δt was collected from the period when the rate of heat release was 20 kW to the time when the rate of heat was 100 kW. The results indicated that there was a linier increase in the rate of fire growth. Carvel, et al (2001), highlight that the basic reason behind the linear increase in the growth of fire is a function that is associated the ventilation rate which promotes the increase of flames along the array of fuel. From the study conducted by Carvel, et al (2001), what is evident that the longitudinal ventilation strategy is most cases found in many tunnels. However from the findings of the study, longitudinal ventilation can result to an increase in the fire as opposed to a scenario where natural ventilation (a tunnels that is not equipped with mechanical air controls or fans). The study therefore depicts that although the longitudinal ventilation strategy is assumed to be an effective method of ventilation of tunnel fires, technical problems do arise. As a result selecting the most effective tunnel ventilation strategy in required in order to prevent catastrophes such as the kings cross tunnel tragedy which was constructed using horizontal ventilation strategy. Damigos and Stefopoulos, (2007) highlight that the based on the fact that fire is one of the main problems for underground fires , there is an increased demand for the development of designs of underground tunnels that increase safety . Damigos and Stefopoulos, (2007) therefore undertook a study on the aspect of designing an effective ventilation system , which is able to facilitate effective control of smoke , reduce the spread of fire and also prevent the explosion of gases within the tunnel. The study by Damigos and Stefopoulos, (2007) involved evaluating recent research on the fire safety from ventilation practices and tunneling projects. Damigos and Stefopoulos, (2007) analyzed the longitudinal ventilation strategy and other ventilation strategies by evaluating their benefits and implications. The findings of the study reveled that the most effective approach of the ventilation systems is one that is characterized by two aspects which include adequate security and acceptability in economic terms. From the study by Damigos and Stefopoulos, (2007) much focus was directed to the longitudinal ventilation strategy. Damigos and Stefopoulos, (2007) argue that although various researches have published findings of the inadequacies of the longitudinal ventilation strategy, the benefits derived for the quite significant. One of the fundamental characteristics of longitudinal ventilation is that develops a longitudinal flow of air that is uniform. In addition the fans are accumulated on ceiling area of the tunnel. This therefore allows the entry of clean air into the tunnel. In the context of a fire the only practicable way in which smoke is driven out is through the portal of the tunnel. Another benefit of the longitudinal approach is that it is not expensive to install and also to maintain. Smoke therefore appears in only one side of the traffic (Damigos and Stefopoulos, 2007). Other mechanical approaches of ventilation such as the transverse ventilation approach are also beneficial. The transverse system for instance has two air flows and two ducts. The ducts are useful in the flow of fresh air into the tunnel. In the case of fire the smoke in the tunnel is removed through the ducts. The transverse ventilation is however the most expensive approach of ventilation in terms of construction and maintenance or operational costs. In the semi-transverse approach, fresh air is supplied by the ducts however no air extraction occurs. Fresh air is usually supplied transversely while on the other hand air that is polluted flows longitudinally. This therefore implies that in the semi-transverse approach it is actually difficult to control the longitudinal flow of air which is an essential aspect in dealing with emergencies fire situations in tunnels. The natural approach on the other hand is not costly however it can not be used in huge tunnels that are above 500(Damigos and Stefopoulos, 2007). The figures below indicate the approaches; Figure. 1 Natural ventilation Fig.2 Longitudinal ventilation after a fire Fig.3 Semi-transverse ventilation after a fire From the study undertaken by Damigos and Stefopoulos, (2007), what is evident is that the longitudinal ventilation approach is more effective in terms of dealing fire in tunnels and also in economic terms. This is because it is characterized by the two aspects which are; adequate security and acceptability in economic terms. Riess and Rudolf (2007) highlight that various ventilation strategies can be applied in case of a fire incident in tunnels. Riess and Rudolf (2007) undertook a study to evaluate the best ventilation strategy to be applied in the context of an underground road tunnel. The method of study involved carrying out a survey among ventilation experts in order to seek their opinions on ventilation systems that should be adopted in the case of a fire in underground tunnels. Riess and Rudolf (2007) set up a scenario question that would be given to the ventilation experts. The question entailed evaluating a 2km tunnel that was on fire . The presence of an emergency exist was not spell out. The volume of traffic was 600 vehicles from the left side to the right side. The speed for vehicles was 60km/h. The experiment involved dividing a tunnel into three equivalent sections using a fire detection system as indicated by figure (I below). The role of the fire detection system is to indicate the tunnel section that is affected by fire. The answers that were to be selected by the ventilation experts on how to ventilate include; a) Stratification preservation ,air flows absent , jet fans absent b) The use of longitudinal air flow, preventing flow reversal and maintaining stratification c) The use of higher velocity in order to keep smoke on one particular side d) Maintaining stratification e) Switching off all fans f) Lowering the critical velocity and switching off all fans Riess and Rudolf (2007) highlight that the findings arrived at from surveying the ventilation experts brought about various conclusions concerning ventilation strategies. Riess and Rudolf (2007) reveal from their survey that there are certain significant parameters that should be put into consideration when selecting a ventilation strategy. This include the density of traffic , the location of the fire , the number of vehicles in the tunnel , initial air flow, the number of existing fans and the turning cars . An essential aspect to take note of from this particular study is that; it is actually difficult to prescribe one specific approach of ventilation although many studies have been undertaken to evaluate various approaches. The difficulty in selecting a suitable approach arises due to the difficulty of ensuring that the velocity is low when a fire occurs and also estimation of the fire detection time is difficult. The study by Riess and Rudolf (2007) also brings about a concluding aspect that the critical velocity as an instrument of smoke control is not the only major component that should be considered when designing ventilation for tunnels or when selecting a ventilation strategy. Other components such as density of traffic, the location of the fire, the number of vehicles in the tunnel, initial air flow, the number of existing fans and the turning cars are also essential, this is the context of rod tunnels. Conclusion From the above analysis of literature what is evident is that ventilation of tunnel fires is basically a very significant safety measure. The paper has analyzed various studies that were centered on areas such as the role of the critical ventilation velocity, the significance of ventilation essentially the most common ventilation approach which is the longitudinal approach. The study has also evaluated some of the studies that indicate the insufficiencies of ventilation of tunnel fires. The review has also focused on the selection of an effective ventilation strategy. In conclusion, the analysis of various studies on ventilation of tunnels fires brings about a conclusive argument that ventilation of tunnel fires is indeed import. However it is essential to evaluate ways in which ventilation of tunnel fires can be improved in order to enhance safety in tunnels during fire emergencies . References Ballesteros, R, Carlos, S, Eduardo, B, 2006, Influence of the slope in the ventilation semi-transversal system of an urban tunnel , Tunnelling and Underground Space Technology, 21 (1), p21-28. Chow, W, 1998, On smoke control for tunnels by longitudinal ventilation, Fire Safety Journal, 13, (3), p 271-275. Carvel , R, Beard, A, Jowitt, P,2001, The influence of Longitudinal ventilation system on fires in tunnels , Fire safety Journal,16(1), p3-21. Cheng,H,Ueng,T.H,Liu,C.W,2001,Simulation of ventilation and fire in the underground facilities, Fire Safety Journal,36(6),p597-619 Retrieved Damigos, G and E.K. Stefopoulos, 2007, Design of emergency ventilation system for an underground storage facility . Tunnelling and Underground Space Technology, 22, ( 3), p 293-302. Retrieved From Hwang, C & Edwards, J, 2005, The critical ventilation velocity in tunnel fires—a computer simulation, Fire Safety Journal, Volume 40(3), P 213-244. Jae Seong , Seung Shin and Hong Sun, 2008, An experimental study on the effect of ventilation velocity on the burning rate in tunnel fires. Heptane pool fire case. Building and environment ,48(7) , p1225-1231. Li, Kun, Quan, Ding, Ciu, Chang-Fu. , 2011, Influence of ventilation Tube Rapture from Fires on secondary Catastrophes in Tunnels, Journal of combustion science and Technology,p75-83. Palazzi, F, Currò, B. Fabiano, 2005, A Study on Road Tunnel Fires Using Hazmat, with Emphasis on Critical Ventilation Velocity , Process Safety and Environmental Protection, Volume 83( 5), p 443-451. Riess, I and Bopp, R, Ventilation strategies in Case of fire in longitudinally ventilated tow way tunnes , Fire Safety Journal, Volume Read More