The Effect of Building Geometry and Fire Location on Smoke Production - Assignment Example

THEORETICAL QUESTIONS 1. Review the main features of radiation and analyse the radiating gases produced in combustion. Explain the role of radiation in fire spread between neighbouring buildings and discuss the requirements for space separation. Theoretically, radiation is heat energy being transferred in the form of invisible waves (International Association of Fire Chiefs, 2007, p.126). Identical to the electromagnetic radiation coming from the sun is being absorbed by the earth; the human body also absorbs radiant heat from various sources such as an open fire. A fire produces substances which are commonly known as products of combustion. Smoke is an airborne type of combustion product consists of particles, vapours, and gases. The composition of gases from smoke depends on the substance being burned, temperature, and amount of oxygen available (International Association of Fire Chiefs 2007, p.125). Hot burning gases including methane and ethane rise with the thermal column and once the proper amount of oxygen is mixed with them; they begin to burn and radiate enormous heat to the surface producing more flammable gases. As the fire radiates out in all directions, the pyrolysis effect from the burning material generates tremendous amount of flammable gases while radiated heat raise their temperatures to its ignition point (Fire 1996, p.118). In a fire, radiation is from soot particles in luminous flames and from CO2 and H2O molecules that influences the rate of burning. Moreover, the spread of fire to other combustibles is often caused by radiative transfer. According to Friedman (1998, p.102) a 35kW/m2 radiative flux impose on a wooden vertical particle board takes roughly only 50 seconds to ignite. Generally, the stronger the radiation, the quicker the fire will spread and the more dangerous it will be for humans. For instance, a large fire can generate a radiation so intense that it can destroy human skin in just 10 seconds. As mentioned earlier, pyrolysis causes the fire to jump from one place to another. Radiation from fire ignites flammable gases that violently brakes and jumps from one building to another (Fire 1996, p.119). According to Furness & Muckett (2007, p.187), fire spread to another building through direct impingement or by radiated heat from the building on fire. For this reason, space separation is necessary to prevent fire spread across a certain boundary (Pulley 2008, p.83). Space separation requires provisions for fire resisting external walls and limitation on the size of openings particularly in the exposed face of the building (Furness & Muckett 2007, p.188). 2. Critically analyse the effect of enclosure ventilation on combustion and the composition of smoke. Fire in an enclosure mostly depends on the enclosure geometry and ventilation (Karlsson & Quintiere 2000, p.14). Initially, combustion in an enclosure is fuel-controlled while producing increasing amounts of energy, toxic and non-toxic gases, and solids. A fire burning at the centre of an enclosure will produce significant smoke and a fire plume is then produced due to buoyancy of hot gases. Cold air is entrained into the plume and mixture of combustion products and air will impinge on the ceiling of the enclosure generating a layer of hot gases. When the temperature in the enclosure reaches a certain limit between 500-600 degrees centigrade, flashover will occur and the fire will be fully developed. At this stage, combustion inside the enclosure depends on the availability of oxygen or ventilation-controlled where oxygen required for the combustion is coming from the openings or ventilations. As the fire decays or when all fuel is consumed, energy release rate and gas temperature are reduced and once again, combustion inside the enclosure will be fuel-controlled (Karlsson & Quintiere 2000, p.18). Although combustion inside an enclosure is limited to certain areas near the ignition source, it produces significant amount of smoke and toxic gases (Wang 2002, p.201). Moreover, temperature of smoke in an enclosure is higher and more hazardous to humans (Karlsson & Quintiere 2000, p.7) Smoke generally contains unburned carbon and toxic gases. The carbon particles are products of incomplete combustion which is carbon monoxide while the gases are hydrogen chloride (Mannan & Lees 2005, p.16). According to Hasegawa (p.122), smoke particulates and chemical compositions are dramatically affected by enclosure ventilation conditions particularly when the supply or air is restricted. Moreover, the changes in particulate properties caused by ventilation relate to smoke toxicity. 3. Analyse the effect of varying compartment/building geometry and fire location on the production of smoke. Compare various plume types including axisymmetric plumes, adhered plumes, spill plumes etc. Generally, smoke production in an enclosure depends on the properties of the fuel and ventilation conditions (Vinnem 2007, p.291). However, the geometry of an enclosure may have a significant effect since the enclosure is considered a combustion chamber where mass and heat balance affects temperatures and smoke production rates and concentration of combustion products and gases. Entrained air in the fire plume is the medium in which smoke is transported thus the rate of smoke production is similar to the rate of air entrainment which is influence by the fuel, fire size, and compartment geometry. The geometry includes the compartment volume, ceiling height, and the size and location of the ventilations. Similarly, if the fire location is near an opening or ventilation, smoke will be released to the outside as the opening exchange smoky effluent with new air (Fitzgerald 2004, p.331). An axisymmetric plume is generally identified with buoyancy caused by diffusion flame formed above the burning fuel. An axis of symmetry is believed to be inexistence along the vertical centreline of this plume type. More importantly, this fire plume entrained air horizontally from all directions. The difference between this category of plume and a “line” plumes is the fact the later only entrained air from two sides. In contrast, a “spill plume” is initially not affected by buoyancy as its flows horizontally before rising while an “adhered plume” adhered to the wall above the opening. The amount of air entrainment generally depends on the plume behaviour thus plumes that adhere to the wall near the openings are likely to entrain almost half of the air entrained by spill plume (Fitzgerald 2004, p.331). 4. What is the function of smoke control? When and where is it required? Critically analyse methods of smoke control. The function of smoke control is basically to reduce the deaths and property damage caused by smoke and provides continuity of operation with less smoke interference (Gondzik 2009, et al. p.1097). Smoke control is useful during evacuation particularly in large building since any significant build-up of smoke can affect visibility and may suffocate people trying to escape. Many reported fire incident’s casualties were either burned or died of smoke inhalation. Generally, smoke control is in part required to problems arising from deaths from toxic gases carried by smoke, and disorientation caused by lack of visibility (Purkiss 2007, p.7). The most common smoke control is force venting and smoke curtains to contain smoke in one area. However, the effectiveness of smoke barriers depends on how smoke leaks to it and the pressure difference on each side. In some buildings, smoke-stop doors were installed to prevent smoke penetration. However, most smoke control is aimed to confined smoke in one area such firewalls and smoke dampers that automatically detect smoke and closes the air duct. Some smoke controls are designed to dilute the smoke with outdoor air but this is not enough to control smoke and its toxic gases. For this reason, they are commonly combined with confinement systems and other fire intervention devices (Bingelli 2009, p.343). In large building or commercial complexes, automatic smoke ventilation is being use such as SHEVS or Smoke and Heat Exhaust Ventilation to ensure health and safety of occupants during a fire. It works by venting the heat and smoke through the roof (Department for Communities and Local Government 2006, p.50). 5. Critically analyse the use of standard fire curves for determining fire resistance. How and why does the approach vary for onshore and offshore applications? Generally, a standard fire curve uses actual fire temperatures to get accurate information on fire resistance of the construction (Franssen et al. 2009, p.5). The availability of a building component to withstand heat exposure in a certain period of time is its fire resistance. The standard fire curve involves performance criteria such as stability, insulation, and integrity of building components (Coutie & Davies 1993, p.105). However, although it provides a simpler means to determine the fire performance of building materials and components, it does not include considerations of physical parameters influencing fire growth and development (Moore & Wang 2007, p.20). The variation in its offshore applications comes from the differences in requirements. For instance, most countries require 90 to 120 minutes with a minimum 30 minutes fire resistance while others requires a minimum of 15 or 20 minutes (Coutie & Davies 1993, p.106). For instance, according to Armer & O’Dell (1997, p.80), fire resistance requirements exist in different countries for the same type of building. Moreover, type of test furnace and the radiation conditions inside them are not the same. REFERENCE LIST: ARMER G. & O’DELL T. 1997, Fire, static and dynamic tests of building structures, Taylor & Francis, UK BINGELLI C. 2009, Building Systems for Interior Designers, John Wiley and Sons, US COUTIE M. & DAVIES G. 1993, Tubular Structures V, Taylor & Francis, UK DEPARTMENT FOR COMMUNITIES AND LOCAL GOVERNMENT, 2006, Fire safety risk assessment: offices and shops, The Stationery Office, UK FIRE F. 1996. The common sense approach to hazardous materials. 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