Wind Effect on Smoke Movement in Compartments - Report Example

Institution : xxxxxxxxxxx Title proposal Tutor : xxxxxxxxxxx Course : xxxxxxxxxxx @2013 WIND ЕFFЕСT ОN SMОKЕ MОVЕMЕNT IN СОMРАRTMЕNTS Chapter one: Introduction Fire accidents have increased in a high rate over the recent past, therefore, causing alarm across the globe. Indeed, there has been increased number of building fire accidents hence drawing a lot of building users’ attentions as well as increasing their concern on high rise building fire safety problems. Tremendous harm and destruction is a accompanied by fire accidents where lives are lost, people are injured causing partial or permanent disability as well as destruction multibillion dollar valuables and properties as observed by Campbell (2008). Additionally, forest fires accidents affects severely the biodiversity causing the migration and death of wild life. Following this, fire accidents severely hurts economy of a particular state. For instance forest fire tends to cause detrimental effects on the ecotourism of a particular state as well as the environment degradation (Corbett, 2009). While the world have devised different mechanisms of handling fires accidents, several factors such as wind have been made this efforts more complex as they accelerates the rate of fire spread therefore causing more harm. In fact, wind is categorized as a major threat to fire accident fighting and in most cases fire fighters apparatus have been overcome by it. Indeed, wind according to Campbell (2008) has an influence on the dynamics of smoke dispersion in a compartment. Aim: In this regard the major aim of this proposal is to conduct a short study of the wind effect on smoke movement in compartment applying one method learned to gain some insight. Objectives: In conducting this study the project aims at achieving the following objectives Working with CFD program Exploring simple simulations with a wind effect and how do result differ Design different scenarios by use of CFD Including Literature review after looking into several case studies, books and journals Analyzing the results Providing recommendations for better design for fire engineers In conclusion will clearly explain how results were achieved Method: In the effort of realizing the aim of this proposal the CFD method will be used to explore different scenarios and trends. Chapter two Literature Review When fire is unconfined, much of the heat produced by the burning fuel is escapes via radiation and convection. This is not the case in a compartment as there are materials in the compartment which include the walls ceiling floor and other accessories as well as equipments which absorb the radiant heat produced by the fire. Further, the radiant heat not absorbed is reflected back hence accelerating the temperature of the fuel and the rate or combustion. In this regard, hot smoke and air consequently heated by the fire become more buoyant and rise. On contact with cooler materials such as the walls and the ceiling of the compartment, the heat is conducted to the cooler materials hence raising the temperatures further. As such, this heat transfer system raises the temperatures of all materials and components in the compartments and consequently as nearby fuel is heated it begins to pyrolize as argued by Karlsson & Quintiere (2000). With time the rate of pyrolysis can accelerate to a point where flaming combustion can be supported and fire extended. In additional to the heat energy in the compartment, the speed with which the fire develops, peaks heat release rate and duration of burning is always dependant of different factors which according to Karlsson & Quintiere (2000) they includes the compartment geometry, ventilation fuel type, amount of fuel, and surface area of the room. At the same time, compartment fire develops in stages which can be categorized into incipient, growth, fully developed and lastly the decay stage. As such, the development forms a curve such that it is low during the incipient stages and increase in intensity and complexity subsequently (Karlsson & Quintiere, 2000). In details incipient stage of fire development is the simplest fire that has not impacted significantly on the environment (heat toxicity and visibility) inside the compartment. Fire in this stage has reduced risk and can be controlled or put off through the use of portable fire extinguishers or small hose line without the need for protective gears or breathing apparatus as Grant & et al (2000) observed. The basic requirements of a fire are heat, fuel, and oxygen and as such further development of an incipient depends on the distinctiveness and configuration fuel involved. The air in the compartment provides adequate oxygen to maintain and carry on the fire development. The radiant heat, on the other hand, warms adjacent fuel and continues pyrolysis process. Consequently, due to reduction in density a plume hot gases as well as flame rises from the fire to the and mixes with the cooler air in the compartment. Eventually the transfer of energy increases the overall temperatures in the room. At the peak of the stage, the plumes/flames of hot air starts reach the ceiling and begin to spread horizontally across the ceiling. Beyond the peak point the fire becomes dangerous to life and health threat according to Grant & et al (2000). Given more oxygen the fire enters the growth stage and therefore the rate of energy released by the burning fuel continues to increase hence making the fire considerably more complex (Grant & et al, 2000). At this point gases in the compartment may be described to exist in two layers. That is a hot film of air extending downward from the ceiling and a cooler layer toward the floor. Indeed, in addition to the transfer of heat by radiation and convention, radiation from the hot gases layer also heats the surfaces of the compartment and its stuffing. Through flame spread or ignition of other fuels within the compartment the fire can continue to grow intensely. Consequently as the flames reaches the ceiling they bend and begin to spread horizontally. As such, as observed by Welch & et al (2007), the pyrolysis products and flammable byproducts of imperfect burning in the hot gas layer continuously ignite hence more horizontal spreading across the entire ceiling. In fact, this situation of the fire is known as rollover and it is an indicator of flashover (Welch & et al. 2007). As Kui Au & et al (2007) claimed flashover therefore is a transition period from the growing stage to full grown (developed) fire. Although it may not always occurs, when it does flashover is always characterized by a rapid transition to a state of total surface involvement of every combustible material within the compartment (Kui Au, & et al, 2007). The litmus of knowing whether the fire has reached flashover stage is when the temperatures read 5000-6000 C (9320-11120 F). At the point of flashover burning gases will actually push out openings in compartment at a substantial velocity. So as to cross over from this stage to full develop stage the fuel must have sufficient energy to develop flashover conditions (Kui Au, & et al, 2007). In addition, the fire burns the contents of the room be it newspapers, couches, or anything else in the room. Ventilation can further act as another factor that accelerates the combustion given that fire must have sufficient oxygen to reach the flashover. If enough ventilation is available the fire must reach full developed stage but may not continue in it complexity later. Fully developed stage therefore is the point post flashover stage where the energy release is at its peak (Drysdale, 2011). Although it is usually limited by ventilation, the fire at this point is very destructive and very risky. Unburned gases accumulate at the ceiling level and frequently ignited as they leave the compartment resulting in flames showing at the doors or windows. The gas temperatures at this stage range from 7000- 12000 (12920-21920 F) and as such it requires high specialized protection apparatus and breathing gears (Drysdale, 2011). Finally the fire enters at decay stage where it goes down as the available fuel is consumed. The heat release rate progressive declines the fire may return to a fuel controlled state as the available oxygen supply becomes adequate for of combustion (Drysdale, 2011). On the other hand, wind which is a fire catalyst always affects fire dynamics and smoke dispersion behavior in a built space (Drysdale, 2000). It has been mostly observed experimentally that ambient wind has antagonistic impact on the compartment fire: accelerating fire severely and cooling the fire by blowing away and dilution of combustion gases. At the same time, wind pressure also impact on the external flames/plumes ejected from a compartment ventilations. Strong wind may greatly influence fire spread and smoke movement tendency in a room as argued by Butcher & Parnell (1979). In a compartment without the influence of wind, fire produces smoke normally just like the normal air. Indeed smoke produced by fire in a closed compartment rises up to the ceiling due to the lowered density thus forming a smoke blanket at the ceiling and as well increasing the air pressure due to the expansion aspect. Following further combustion and growth of the fire the hot smoke and other gases may exact a lot of pressure in kilo Pascal which has the ability to break closed windows (Butcher & Parnell, 1979). At this level the fire is said to be at the flashover (peak of growth stage) as seen earlier. The location and number of opening of the compartment as well as presence of wind determine smoke motion in the compartment. In this case Butcher & Parnell (1979) observed that when wind is weak there will be no effect on the movement of smoke in the compartment. There will be a normal spreading of smoke due to “smoke’s own mobility” which is due to the fact the smoke consists hot gases which are less dense that the air in the compartment. The normal circulation of air inside the compartment is capable of carrying the smoke around the compartment (Karlsson & Quintiere, 2000). On the other hand, when wind is strong there will be a huge effect. The wind might blow smoke back into the compartment, and the accumulation of smoke inside the compartment will increase. Strong wind will assist fire to spread faster (Karlsson & Quintiere, 2000). Indeed, a leakage at the top of the room such as a mechanical vent and another leakage near the bottom on the opposite side results to movement of air from the higher opening to the lower opening when there is no temperature difference between the inside and the outside of the compartment and ambient wind blowing from left to right as Cochran (2012) observed. The direction in the compartment becomes downwards in this case. If there is fire in the compartment on the other hand, the buoyancy caused by difference in density drives smoke and air to move upwards. As such, the ambient wind has opposite effect on the thermal buoyancy hence accumulating smoke inside and thus creating the need for a vent at the top most level of the compartment (Cochran, 2012. When the opening in the windward side is lower than the leeward side then it creates an assisting wind scenario that is blowing wind reinforces the buoyancy and entrainment of the fire. On the other hand, rooms with large ventilation such as windows doors reduces the gaseous pressure and allows wind to cool down the layer of heated smoke and gases at the ceiling through removing the heated gases which facilitate to the combustion as seen earlier (Cochran, 2012). This helps to reduce smoke which actually kills individuals through toxic gases contained in it way before the fire burn the body. Chapter three: Conclusion In the literature review it is apparent that fire in apartment under goes different stages and in subsequent stages there in increases in complexity and risk of the fire except in decay stage where the fire complexity and intensity reduces progressively. Among many factors wind also accelerates the speed of smoke movement thus causing more harm to the inhabitant of a compartment through emission of toxic gases such as carbon monoxide. Equally, it is evident that positions of vents as leakages are very fundamental in maintaining the mass movement of smoke and other hot gases due to pressure gradient between the internal space of the compartment and the atmospheric pressure. Chapter five: Recommendation Following the above knowledge on the stages of the fire in a compartment and the dynamics cased by the wind as well as vents, the proposal therefore recommends that architectures should always design the floor of building ensuring there is enough and large ventilation as well as smoke vents to the upper parts of the compartments. This will therefore facilitate the safety of building inhabitants and reducing the extent of harm during fire disasters since smoke movement is regulated and controlled such that the harm facilitated by it is reduced. Chapter six: References References Butcher, G, & Parnell A,(1979), Smoke Control in Fire Safety Design. Spon Campbell, B, (2008), Disasters, Accidents, and Crises in American History: A Reference Guide to the Nation's Most Catastrophic Events. New York: Infobase Publishing Cochran L, (20120, Wind Issues in the Design of Buildings. ASCE Publications publishers Corbett, G, (2009), Fire engineering's handbook for firefighter I & II. Tulsa, Okla: PennWell Drysdale, D, (2011), An Introduction to Fire Dynamics, Edition 3. New York: John Wiley & Sons Drysdale, D, (2000), An introduction to fire dynamics. New York: John Wiley & Sons. Grant G, & et al, (20000, Fire suppression by water sprays. Progress in Energy and Combustion Science. Vol. 26, Issue 2, Pp 79–130 Karlsson, B, & Quintiere, J, (2000), Enclosure Fire Dynamics. London: CRC Press LLC Kui Au, S, (2007), Compartment fire risk analysis by advanced Monte Carlo simulation. Engineering Structures. Vol. 29, Issue 9, pp 2381–2390 Welch S, & et al, (2007), BRE large compartment fire tests-characterizing post-flashover fires for model validation. Fire Safety Journal. Vol. 42, Issue 8, pp 548–567 Read More