Chapter 13
Conclusions
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13. Conclusion Backdraft can develop from fires of either ordinary combustibles or ignitable liquids that become oxygen starved yet continue to generate a fuel-rich environment in a building with limited ventilation. If fresh air is allowed to flow into the vitiated space, such as by opening a door or breaking a window, a gravity current of air will flow into the compartment while the hot fuel-rich gases flow out through the top of the opening. Once a localized flammable mixture is formed and is in contact with an ignition source, the fuel-rich gases will combust acutely, the temperature will rise rapidly and the fire will develop into a deflagration. The deflagration will cause the gases to heat and expand within the compartment, thus forcing unburnt gases out of the opening ahead of the flame front. These gases will mix with additional air outside the fire compartment. As the flame crosses through the building and penetrates the doorway, it ignites the gases outside the space resulting in a fireball and a blast wave. 90% of the studies carried out on backdraft have been carried out in the experimental area. The remaining 10% are numerical simulations that do not really take into account the fire evolution in the compartment. In this thesis, the first two-zone model that allows us to predict and analyse backdraft in a compartment fire as a result of the fire’s evolution has been developed. Answers have been provided throughout the thesis to questions like: what the influence of the partitions is on the backdraft phenomenon, with which kind of fuels the risk of backdraft is higher or what the effect of the size of the openings is on the backdraft deflagration, which parameters influence the size of the fireball or for example, how the available energy for backdraft is distributed inside and outside the compartment when deflagration occurs. Based on the answers to these questions, graphical methods and tables have been developed that allow us to: • • • •
Predict the maximum accumulation of unburnt gas in the compartment, the duration of the fire, and the maximum pyrolysis rate. Obtain the fireball’s diameter as a function of the compartment volume and vent area. Obtain the time at which glass submitted to fire could crack. Obtain the average velocity of the gravity current as a function of opening geometry, compartment ratio and presence or not of obstacles.
Essentially, these predictive tools have aimed at providing useful and fast information that a fire fighter can use to try to avoid or decrease the risk of backdraft when facing a fire. A brief summary of the major conclusions is given below. It is concluded that for backdraft to occur a great accumulation of unburnt gas is needed. The unburnt gas may consist of gas species such as hydrocarbons, 322
13. Conclusion carbon dioxide, hydrogen or any other combustible fuels. In the literature, it is often found that the most important parameter leading to backdraft is the hydrocarbon content. In light of the present research, on slight modification to this parameter must be given here: hydrocarbon must be understood as the combustible fuel. For each different unburnt-burnt-air mixture exists a lower concentration of unburnt gas that can lead to backdraft. This lower concentration depends on the amount and type of the gas species present not only in the unburnt mixture but in the mixture of the compartment. More detailed studies on the influence of the type of unburnt gases, carried out in Chapter 8, show that increasing the amount of inert species such as nitrogen, water or carbon dioxide reduces the flammable range of the mixture and, therefore, the risk of having backdraft. The opposite effect is found when increasing the unburnt gases. Furthermore, these studies reveal that having gaseous fuels inside the compartment with a higher number of carbon reduces the flammable range but also reduces the lower flammability limit of this mixture. That means that the risk of backdraft when creating an opening increases. It is also concluded that a much higher quantity of unburnt products is found in fire compartments with smaller heights, larger fire areas and partitions with lower thermal inertia. The ventilation is a parameter that must be looked into carefully because, for example for big openings, more quantity of unburnt gas may escape, but more oxygen is used for combustion and, therefore, more heat is available for heating, thereby enhancing the pyrolysis rate of the fuel. After twelve CFX-ANSYS simulations of the gravity current prior to backdraft, it is shown that the mixing of the gravity current behaves as a function of the opening geometry, compartment ratio and presence or not of obstacles. The degree of mixing is obtained according to the Froude number, which ranges from 0.5 (no mixing) to 0.0 (well-stirred situation). Furthermore, with the Froude number, the velocity of the gravity current can be obtained providing the fire fighters the position of the gravity current. Assuming that the inner mixture is within or above the flammable limits just before creating the opening, it can be concluded that there will always be a localized flammable region along the interface of the two streams that can be ignited when it comes into contact with a hot element. This reveals that backdraft could happen at any moment from the time of opening. The results obtained by using the simple model correspond well to the backdraft experiments (Fleischmann, 1993; Weng, 2000 and Weng, 2002). However, no backdraft in real buildings have been simulated. At this point, the author would like to comment that these backdraft experiments and others found in the literature have been carried more or less in the same way. Therefore, it is important to keep in mind the following
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13. Conclusion differences between backdraft produced in a real fire and those produced in containers: •
• •
The situation in a backdraft experiment is well controlled. The layout of the compartment, location of flammable vapour, sources of ignition, positions of vents, etc. are known. In a real backdraft situation, this is not the case. Therefore, backdraft can occur a long time after a vent has opened, particularly where there may be the possibility that flammable vapours have been trapped at a high level. Backdraft experiments are carried out with simple gases such as methane or propane. In a real backdraft situation, this is not the case and this influence must be quantified. In backdraft experiments the flow of fuel is cut a few seconds before the opening time. In a real backdraft situation, pyrolysis will continue after the door is opened. This fact will influence the gravity current structure, mixing and, consequently, the deflagration.
The severity of backdraft in a scenario can be obtained as a function of the energy released inside and outside the compartment and of the size of the fireball. This energy is provided at every time step from the opening time allowing the fire fighter to know which period is the most dangerous for backdraft to occur. In the author’s opinion and those fire experts (Dominguez, C., Elorza, J., and Beyler, C.) who have been interviewed (see Annex VII) all this data and information provided by the program could be useful for fire fighter training or even for building design. For example: • •
•
Simulating and showing the results of several backdraft cases to the fire fighter brigade can provide a deeper knowledge about the backdraft phenomenon that can be useful when facing a fire. By knowing the parameters that lead to backdraft, the building can be designed in such a way that the risk of occurrence decreases (e.g, knowing that fuels with a higher number of carbon increase the risk of backdraft, the materials of the partitions will be chosen taken into consideration this effect). By knowing that in a compartment the risk of backdraft is high, one or other criterion could be chosen (e.g. sensible smoke and/or temperature detectors) in order to try to avoid or decrease this risk.
Further research must focus on: • • •
Backdrafts in real compartments (no containers) with common fuels found in real fire scenarios such as tables, sofa, etc. The extinction of flames in under-ventilated compartment fires. The re-ignition of flames as a result of a supply of air in under-ventilated compartment fires.
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13. Conclusion • • •
Extended validation of Le Chatelier’s rule for gases other than methane. Deflagration of non-stoichiometric mixtures in fire compartments which are poorly stirred. Application of this model to quantify the mitigation effect of water mist.
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