The Influence of Fire Area and Vent Factor on Enclosure Fire Development - Coursework Example

DESCRIPTION OF ENCLOSURE FIRES 1. INTRODUCTION TO FIRE GROWTH IN ENCLOSURE OR COMPARTMENT Development of fire in an enclosure depends on different factors such a geometry of the enclosure, available ventilation, the amount of fuel and it type, and area of the surface. There are various stages involved in fire development such as ignition, flashover, fully developed fire, and decay. Initially, the condition of the enclosure does not affect the behaviour or growth of fire inside it but as oxygen depletion occurs and while the temperature increase considerably, the growth and behaviour of fire gradually becoming dependent on enclosure condition. In a smaller enclosure for instance, a fire coming from fuel package will cause much higher temperature and fire growth than a bigger enclosure (Karlsson & Quintiere 2000, p.23). These conditions are due to the effect of ventilation and other factors mentioned above. According to Quintiere (1998, p.170), an enclosure fire include smoke containment and fire spread beyond the boundaries of the enclosure that indicates fire spreading through the openings or ventilations such as windows and opened doors. The following section describes how fire is developed inside a compartment or an enclosure. These include stages of fire development from ignition to decay. 2. STAGES IN ENCLOSURE FIRE DEVELOPMENT a. Ignition Ignition is generally viewed as a process resulting in exothermic reaction. Generally, ignition can be triggered by a flame coming from an already flaming source such as match, a spark from a lighter and other sources that can ignite a combustible material. Ignition results in a reaction that released energy or heat that increases the surrounding temperature beyond the ambient (Karlsson & Quintiere 2000, p14). There are actually two types of ignition – piloted and auto ignition. A piloted ignition often initiated and generated with a spark directed into a concentration of fuel. Piloted ignition occurs at the lower flammable limit as a minimum condition or when the fuel only needs a small spark to propagate. In contrast, auto ignition is where the fuel requires no sparks or source of flame to propagate. However, to enable ignition, the fuel must be within the ideal concentration range and the chemical kinetic processes that are involve must not go beyond the mixture’s ability to displace heat. The two types of ignition happens almost in the same manner in different types of fuel – solid fuels, decomposed fuel gases of liquid, etc. In liquid fuels for instance, sufficient rate of evaporation is required to generate an acceptable concentration for ignition. In this process, the evaporation rate depends on the temperature of the liquid. The higher the temperature, the more molecules break out from the liquid surface and mixed with the surrounding air. The mixture of air and evaporate liquid is influenced by temperature or humidity. When both humidity and liquid temperature reached 100%, liquid fuels will burn. This type ignition only requires fuel source to reach its lower flammable limit for flames to propagate above the liquid surface. In auto ignition, the mechanism of ignition for solid fuel requires it’s to reach a minimum temperature before it burns (Quintiere 1998, p.66). b. Growth After ignition and depending on the amount and type fuel, the fire will grow and generate more heat or energy as the flame spread. Although a fire in enclosure may be influence by different factors, fire growth in its initial stage is only dependent on fuel or fuelled controlled. This stage is commonly viewed as the pre-flashover stage where hot gases from the flame are being released and rising upward due to buoyancy creating a fire plume (Karlsson & Quintiere 2000, p.14). As the gases reached the ceiling of an enclosure, gases will spread across it and eventually reaches the wall moving downward. At this point, a layer of gases will be visible under the ceiling and the enclosure is now divided into two different layers – hot upper layer and cold lower layer. The hot upper layer constantly entrains air coming from the lower layer of the enclosure resulting to the increase of the hot upper layer volume as its move downward to the floor. As it descends, the hot temperature of the hot layer increase and heat is being transferred to ceiling, walls, and floor of the enclosure by radiation and convection. In addition, heat is also transferred to the fuel bed by radiation coming from not only with the hot layer of gases but heat from enclosure walls. Consequently, the burning rate of fuel increases and fire grows rapidly. If there is an opening somewhere in the lower part of the enclosure, the smoke from the hot layer with flow through when it reaches the top of this opening. As the fire continue to grow and other fuel packages reached its lower flammability limit caused by the resulting radiation from the upper hot layer, these secondary fuel packages will ignite resulting to a flashover (Karlsson & Quintiere 2000, p.14). c. Flashover A flashover is widely recognised as the sudden transition of a growing fire into a stage where it becomes fully developed (Karlsson & Quintiere 2000, p.18). Normally, the fire is in the flashover stage when the temperature in an enclosure reaches a certain point and flames flash over the entire surface of the area (Davletshina & Cheremisinoff 1998, p.236). Most experiment done with enclosure fire determines a flashover when the temperature reached 500-600 degrees centigrade. Another criterion is when the radiation to the floor of an enclosure is within 15 to 20 kW/m2. However, the simplest way to determine if the fire in an enclosure already reaches its flashover stage is when flames become visible in enclosure openings (Quitierre 1998, p.66). In other words, a flashover is a stage in enclosure fire development that causes the fire to reach its fully developed form as it starting to consume all the fuel inside the enclosure. As mentioned earlier a fire at this stage is in a momentary transition and demonstrates a remarkable increase in strength as it develops into a full grown fire. A flashover is affected by several factors which include heat flux increase resulting to rapid ignition and fire spread. It may be influenced by the fuel rich gases that have accumulated during the pre-flashover stage or significant increase in the burning rate and rapid spread of flame throughout the enclosure (Quintiere 1998, p.170). At this point, the condition of an enclosure affects fire behaviour such as the size of ventilation that limits the flow of air or ventilation-controlled fire growth. Opposed to fuel-controlled fire growth, this condition limits the production of smoke and CO. Moreover, the oxygen in the smoke layer becomes almost depleted and values of energy are in its peak (Quintiere 1998, p.170). In other words, as the size of the opening increases and supply of oxygen in the air coming from the enclosure increase, smoke and CO production increase. d. Fully developed fire As a result of the momentary flashover stage, a full developed now burns forcefully consuming or combusting all available fuels and oxygen in the enclosure. In the post-flashover stage, the energy release rate at this stage is faster and only limited by oxygen supply. In ventilation-controlled fire as discussed previously, oxygen is assumed to be coming from the enclosure openings. When the hot layer of smoke containing unburnt gases reaches the top of its opening, it flows out of the enclosure and burn while fresh air flows in below the opening. These hot gases have average temperature of 700 to 1200 degrees centigrade (Karlsson & Quintiere 2000, p.18). A fully developed fire in an enclosure can have a smoke layer almost just above the floor and if a lone opening is at the same height, the smoke and hot gases will still flow on top of it while allowing fresh air and oxygen to get in below (Quintiere 1998, p.173). The position, size, and shape of these openings are therefore important and must be given more consideration in an enclosure fire. This is because as opposed to fuel-controlled enclosure fire, these openings often times become an exhaust for hot gases and inlet for oxygen that helps fire growth. Placing these openings in a location that can effectively remove hot gases from enclosure can help reduce the rate of fire growth because thermal feedback to the fuel will be less. According to Karlsson & Quintiere (2000, p.18), many experiments concerning enclosure fire found that burning rates of fuel strongly depends on the ventilation. In other words, the rate in which a fire burns is controlled by the rate in which oxygen flows from the openings. This condition will only change when the burning becomes independent of the vent factor and becomes fuel controlled as the conditions of enclosure during the decay stage or when the fuel are consumed. e. Decay At this stage, the energy release rate becomes less along with the average temperature inside the enclosure. The fire at this stage is once again fuel-controlled. Enclosure fire even at its early stage can die down or decay depending on the availability of fuel or oxygen during the later part of fire growth. For instance, a fire started in an enclosure and begins to develop into a full-blown fire would die down if all oxygen sources are blocked. Similarly, when all potential secondary fuels inside the enclosure are removed, the fire would also die down due to the absence of fuel. In fire safety engineering, it is assumed the fire spread will occur if fresh air supply is sufficient to assist fire growth and for this reason fire in an enclosure at post-flashover stage depends on ventilations (Wang 2002, p.202). The limitations imposed on flow of air when the size of ventilation is considerably small can cause the energy release rate to decrease while temperature inside the enclosure drops. The flow of air at this stage governs the growth and spread of fire (Tan & Jaluria 2000, p.1). However, this does not necessarily lead to decay because the fuel can still release volatile gases but on lower rate resulting in slow fire growth. The decay stage is actually where the fully developed fire no longer has fuel to consume and although there may be sufficient supply of air from openings, the fire will eventually becomes fuel-controlled and finally die down In other words, the growth stage or the pre-flashover period of fire development and decay is related to fuel-controlled fire while the fully developed stage is associated ventilation-controlled fire. Decay can occur at the growth stage if the fuel is not sufficient enough to allow burning and fire spread. At the later part of a fully developed fire, decay occurs because all the fuels available inside the enclosure are already consumed which has the same effect as decay occurring at the growth stage (Karlsson & Quintiere 2000, p.21). 3. CONCLUSION Many factors affect fire development in an enclosure ranging from enclosure geometry, size and location of ventilation, availability and type of fuel, and the surface area. For this reason, fire develops in stages and normally starts with ignition of combustible materials from a spark or auto ignition. After ignition and during the initial growth stage, an enclosure is fuel-controlled where the conditions of the enclosure do not have any effect. However, when the fire reaches its flashover stage and the fire becomes full developed, the fire in an enclosure becomes ventilation-controlled. At this stage, the size and location of vents can affect the growth and behaviour of fire. A fully-developed fire depends on the availability of fuel and oxygen coming from the enclosure openings and consequently, the energy release rate is at its greatest. Smoke layer at this stage can reach a few inches just above the floor of the enclosure. Enclosure ventilations can serve as an exhaust for hot gases and inlet for fresh air. For this reason, the size and position of ventilations in an enclosure is importance since it can affect the behaviour and further growth of fire. During the decay stage, the fire is once again fuel-controlled as its growth depends on the remaining fuel inside the enclosure. The decay stage is not exclusively occurring after the post-flashover stage since fire can also die down and may not reach the flashover stage if the fuel and oxygen inside the enclosure is insufficient. REFERENCES: Davletshina T. & Cheremisinoff N. 1998, Fire and Explosion Hazards Handbook of Industrial Chemicals, Noyes Publications, US Karlsson B. & Quintiere J. 2000, Enclosure Fire Dynamics, CRC Press, US Quintiere J. 1998. Principles of Fire Behaviour, Cengage Learning, US Tan Q. & Jaluria Y. 2000, Mass Flow through a Horizontal Vent in an Enclosure due to Pressure and Density differences, International Journal of Heat and Mass Transfer 44 (2001) 1543-1553, Elsevier Science, US Wang Y. C. 2002, Steel and composite structures: behaviour and design for fire safety, Spoon Press US, OCLC Net Library via University of Central Lancashire Read More