An Experimental Investigation of Fire Behaviour in Compartment Fires - Literature review Example

Running header: An experimental investigation of fire behavior in compartment fires with ceiling vent An experimental investigation of fire behavior in compartment fires with ceiling vent. Course Name Professor’s Name Institutional Affiliation City and State Where Institution is Located Date An experimental investigation of fire behavior in compartment fires with ceiling vent Literature review Introduction Ship compartments, nuclear power plants and underground structures have only horizontal openings or ceiling vents but not vertical openings. Such compartment fires with horizontal ceiling vents usually have different behaviors from other compartments owing to the fact that the flow exchange at the openings and vents is of vital importance in determining the growth and spread of the compartment fires. Thus, ceiling vented compartments are considered a typical kind of under ventilated fire scenarios which are characterized by both the unsteady air exchange with the environment as well as quickly falls of the smoke layer. It is worth noting that air mixed with combustion product in such fires is entrained by the fire which results in oxidation of the fuel under ventilated condition. The elevated temperature as well as reduced oxygen content of the reaction area makes the fire behavior quite different from free burning fires. The incomplete combustion within ceiling vented compartment may for instance lead to unsteady burning phenomenon including flame oscillation, ghosting flame and even self-extinction. Owing to their nature, compartment fires with ceiling vents have in the past attracted much attention with researchers conducting numerous experiments on the fire behavior. Researchers have identified major factors of combustion characteristics in a compartment with ceiling vents to include heat release rate and the ventilation condition. Thus, this chapter investigates the available literature regarding fire behavior in compartment fires with ceiling vents. The literature mainly concerns itself with the factors outlined above, how such factors as the size of the vent and its height, the type of fuel used impact on the characteristics of the fire as well as the relevant calculations. Many studies have been carried out to investigate the fire behavior in compartment fires with ceiling vent. According to Qize etal (2015), the impact of stratification is that the hot smoke impedes the steady flow of air at the ceiling vent when dealing with compartment fires under ceiling ventilation. According to him, the main difference between such fires and fires in a vertically vented enclosure is the presence of a stratified two way flow where combustion products flow out at the upper part of the vent while fresh air flows in from the lower part of the vertical vent. The two way flow is not present in compartment fires with ceiling ventilation and hence the combustion products impede the steady inflow of air at the ceiling vent. It is to be noted that when the vent is at the ceiling in most cases with fire, both density and pressure differences will exist across the vent leading to both buoyancy and pressure effects according to Jaluria et al (1995). According to Venkatasubbaiah et al (2012), the fire behavior is strongly affected by the vent location and size and the flow turbulence which makes the ceiling vent flow rate to be nearly 10% of the normal fires for comparable density differences and vent sizes. Qize (2015) investigated ethanol pool fires and found that when the area of the ceiling vent size increases, the fire behavior transitions from initially being chocked and then extinguished to becoming erratic pulsating and finally to becoming strong steady burning state. According to Jaluria (1995), ventilation limited extinction occurs in the first regime of fires. There have been many studies that on fire behavior that have focused on the ghosting flame analysis, flame pulsation behavior as well as vent flow oscillation with ventilation limited extinction occurring in many cases. These studies indicate that ceiling vented compartment fires are usually under ventilated according to Zhang (2013). Qize etal (2013) conducted an experiment to investigate the vent size effect on self-extinction of pool fire in a ceiling vented compartment that had interesting findings. The findings were similar to the once by Tu (1991) who conducted a series of tests in a 0.43m3ceiling vented compartment with ethanol pool fire where the vent was varied between 0 and 0.3048 * 0.3048m. The results indicated that increasing the ceiling vent size made the fire phenomena to range from chocked and then extinguished pool fires to erratic pulsating pool fire and finally to strong steady pool fires. However, self-extinction only occurred in the chocked and then extinguished pool fires. A similar experiment by Wakatsuki (2001) but using heptane as the fuel arrived at similar conclusions thus indicating that the ceiling vent size has the key influence on fire burning owing to the limitation of oxygen. Numerous research have been carried out regarding self-extinction based on the limiting oxygen index (LOI) including the ones conducted by Beyler (1991) with no ventilation. The findings were that fire would extinguish when the local oxygen concentration reach the LOI. Further findings were that the time of self-extinction for the fire has a linear relationship to the ratio of the compartment volume to heat release rate and as such proposed a method of predicting the self-extinction time for the closed conditions. On the other hand, Bailey (1993) conducted a similar experiment and found that the mass loss rate decreases with oxygen depletion from the volume fraction of 21% to 13.5% with the fire being self-extinguished when the oxygen fraction drops to 12%. Thus, it can be concluded that in ceiling vented conditions, the oxygen supply from the vent has significant influence on self-extinction. However, according to Qize etal (2013), the mechanism of how unsteady burning with self-extinction transforms to steady burning under the influence of ceiling vent is yet to be known. In investigating fire behavior in compartment fires with ceiling vents, the heat release rate is also studied in a bid to quantify the fire hazard. The heat release rate largely depends on the conditions of the openings and this is easily determined in typical building fires with vertical openings. However, for compartment fires in which the only opening is a ceiling vent, there are no factors capable of describing ventilation conditions and hence it is difficult to determine heat release rates according to Zhang et al (2013) furthermore, the burning in such compartments is unsteady since there are complex interactions between the bi-directional vent flows and the inside environment. According to James (2002). According to Yuan et al (2013), the heat release rate depends on the fuel mass loss rate and the combustion efficiencies. Combustion efficiencies is dependent on fuel type, oxygen concentration and pool size for fire in compartments with ceiling vents. On the other hand, the heat release rate, ventilation system and the thermal and geometrical characteristics of the compartment greatly affects the gas pressure generated by the fire according to Pretel (2005). When the vents are large enough, the pressure variation will be weak and hence such a fire will not be a major hazard. Fires in well confined and ventilated enclosures such as in ships may however induce substantial pressure variations resulting into a number of consequences For instance the loss of the dynamic confinement occurring when the increase in internal pressure exceeds the pressure outside the compartment according to Zhang (2013). According to Pretel (2005), the other effect is the possible mechanical damage to safety devices installed within the ventilation network. There has also been numerous research on the fire behavior in compartment fires with ceiling vent when different types of fuel are used in order to determine whether the behavior is dependent on the type of fuel used. According to Pretel (2005), most of the research has concentrated on alcohol and hydrocarbon fuel with a single component with the most widely fuel in use being blended fuel that has more complex combustion characteristics than the ones of single components in real scenarios that use petroleum products and bioenergy fuels are used. According to Zhang (2013), using blended fuels involves a special phenomenon known as azeotrophism which changes the boiling point of fuel and could result in an enormous impact on the burning process. For open spaces, the phenomenon divides the combustion process into four stages namely the initial growth stage, the azeotropic burning stage, the single component burning stage and the decay stage. However, when the fire is confined in a compartment with a ceiling vent, the thermal performances and fire hazards are different and are associated with a number of problems according to Poulsen (2012). For instance, it is not clear how the mixing proportions affect combustion parameters including the burning rate, fuel temperature as well as gas temperature. It is also not clear what special thermal phenomenon would occur in such a compartment. Whether ghosting flame appears and its critical threshold for such ghosting flame occurrence is also not clear according to Zhang (2013). In an experiment conducted by Yanming et al (2015) to investigate fire behaviors of blended fuel in the ceiling vented compartments. It was found that after ignition, the fire spreads to the whole pan and the flame height increases gradually. A lot of bubbles appear near the vessel and then covers the whole pool surface. As the volume fraction of n-heptane in the blended fuel increases, two distinct phenomenon do occur according to Nasr (2005). These include steady burning and self-extinction. When n is less than AP- azeotropic proportion, the burning is stable and the flame is attached to the pan during the whole combustion process which is considered steady burning. However, when n>AP, the combustion is relatively complex and changeable. Ghosting flame also occurs as stated by He et al (2015) with the flame being detached from the pan by going from place to place and oscillating violently and is blue in color. This is followed by self-extinction. Poulsen (2012) also investigated the burning rate of blended fuel in the ceiling compartment. He observed that when n< AP, the burning rates increase slightly but when n-AP, the burning rate increases significantly. According to Yanming et al (2017), the phenomenon is attributed to bulk boiling of azeotrope. When the temperature of lowest thermocouple reaches boiling point, the bulk boiling burning stage appears and the burning rate increases dramatically. When n >AP, a sharp rise occurs along with ghosting flame. At this point, the maximum burning rate is nearly three times that of when n Read More

Qize etal (2013) conducted an experiment to investigate the vent size effect on self-extinction of pool fire in a ceiling vented compartment that had interesting findings. The findings were similar to the once by Tu (1991) who conducted a series of tests in a 0.43m3ceiling vented compartment with ethanol pool fire where the vent was varied between 0 and 0.3048 * 0.3048m. The results indicated that increasing the ceiling vent size made the fire phenomena to range from chocked and then extinguished pool fires to erratic pulsating pool fire and finally to strong steady pool fires.

However, self-extinction only occurred in the chocked and then extinguished pool fires. A similar experiment by Wakatsuki (2001) but using heptane as the fuel arrived at similar conclusions thus indicating that the ceiling vent size has the key influence on fire burning owing to the limitation of oxygen. Numerous research have been carried out regarding self-extinction based on the limiting oxygen index (LOI) including the ones conducted by Beyler (1991) with no ventilation. The findings were that fire would extinguish when the local oxygen concentration reach the LOI.

Further findings were that the time of self-extinction for the fire has a linear relationship to the ratio of the compartment volume to heat release rate and as such proposed a method of predicting the self-extinction time for the closed conditions. On the other hand, Bailey (1993) conducted a similar experiment and found that the mass loss rate decreases with oxygen depletion from the volume fraction of 21% to 13.5% with the fire being self-extinguished when the oxygen fraction drops to 12%.

Thus, it can be concluded that in ceiling vented conditions, the oxygen supply from the vent has significant influence on self-extinction. However, according to Qize etal (2013), the mechanism of how unsteady burning with self-extinction transforms to steady burning under the influence of ceiling vent is yet to be known. In investigating fire behavior in compartment fires with ceiling vents, the heat release rate is also studied in a bid to quantify the fire hazard. The heat release rate largely depends on the conditions of the openings and this is easily determined in typical building fires with vertical openings.

However, for compartment fires in which the only opening is a ceiling vent, there are no factors capable of describing ventilation conditions and hence it is difficult to determine heat release rates according to Zhang et al (2013) furthermore, the burning in such compartments is unsteady since there are complex interactions between the bi-directional vent flows and the inside environment. According to James (2002). According to Yuan et al (2013), the heat release rate depends on the fuel mass loss rate and the combustion efficiencies.

Combustion efficiencies is dependent on fuel type, oxygen concentration and pool size for fire in compartments with ceiling vents. On the other hand, the heat release rate, ventilation system and the thermal and geometrical characteristics of the compartment greatly affects the gas pressure generated by the fire according to Pretel (2005). When the vents are large enough, the pressure variation will be weak and hence such a fire will not be a major hazard. Fires in well confined and ventilated enclosures such as in ships may however induce substantial pressure variations resulting into a number of consequences For instance the loss of the dynamic confinement occurring when the increase in internal pressure exceeds the pressure outside the compartment according to Zhang (2013).

According to Pretel (2005), the other effect is the possible mechanical damage to safety devices installed within the ventilation network. There has also been numerous research on the fire behavior in compartment fires with ceiling vent when different types of fuel are used in order to determine whether the behavior is dependent on the type of fuel used. According to Pretel (2005), most of the research has concentrated on alcohol and hydrocarbon fuel with a single component with the most widely fuel in use being blended fuel that has more complex combustion characteristics than the ones of single components in real scenarios that use petroleum products and bioenergy fuels are used.

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