A Critical Role of Radiation in Combustion Reactions - Assignment Example

T1. The term radiation refers to energy that travels through space such as heat, sound, light, and ionizing radiation (Friedman 2006, p.25). In combustion processes the radiation of interest is thermal radiation. This form of radiation, like other electromagnetic radiation, does not require a medium to propagate. It can easily pass through a vacuum. It travels by means of electromagnetic waves forms, and can also be characterized by its wavelength, energy and frequency. It is also important to note that radiation energy from the source is emitted outwards in straight lines and travels in all directions. The radiating gases produced during a complete combustion are compounds of elements derived from both the fuel and the oxidizer. For instance, a combustion reaction with methane as the fuel and oxygen as the oxidizer produces carbon dioxide and water vapour. Both carbon dioxide and water vapour are gaseous compounds derived from methane and oxygen. In another example involving hydrogen and oxygen the resultant gas is water vapour. In most real world cases of combustion reaction, the oxidizing agent is oxygen from the ambient air, which means that the resultant gases include carbon dioxide, nitrogen and water vapour. Ambient air contains high percentage of nitrogen gas. The truth is that combustion reactions are often not complete. In this case, radiating gases may not be the same as those produced in a complete combustion process. For instance, carbon-based fuels always produce carbon dioxide besides soot (or unburned carbon). In light of the above, radiation is important and plays a critical role in fire spread between neighbouring buildings. We have noted that radiation allows transfer of heat energy from one place to another irrespective of whether there is a medium present. Therefore, radiation would cause temperatures rise in adjacent buildings that were initially not affected by fire, and consequently, result in ignition of available fuel or combustible materials. However, radiation becomes important when The requirements for space separation, is nonetheless, important and should be reasonable to reduce fire spread between adjacent buildings. It is known that the amount of radiant heat reduces as it moves further away from the source (Lin 2000, p.496). This makes buildings that are spaced far apart more secure in terms of fire spread than those that are closely spaced. The type of hazard also determines the space separation. Thus, it would be required that a building housing flammable fuel is spaced far away from other building because such fuel produce high temperatures to facilitate longer travel of radiation. In addition, space separation avoids fire spread through convection, which is considered the most dangerous form of fire spread. T2. Enclosure ventilation plays a critical role on combustion reactions. Ventilation increases the amount of oxygen (the oxidizing agent) available for combustion. Thus, good ventilation allows sufficient oxygen needed to have a complete combustion reaction. In this condition, the resultant fire has little smoke and produces high temperature. On the other hand, poorly ventilated enclosures mean there is little oxygen available for combustion. This results in incomplete combustion that usually produces low temperatures and a lot of smoke. The rate of combustion is also dependent on the amount of oxidant (or oxygen in this case) available for the reaction, and this is determine by the amount of ventilation. So, highly ventilated enclosure will experience faster combustion than poorly ventilated enclosures. In fact, there would be absolutely no combustion if there was no oxygen or any other oxidizing agent available in the enclosure. Enclosure ventilation partly determines the composition of smoke. This is because its composition is dependent on the amount of oxygen available for composition. Thus, highly ventilated enclosures will experience completely burned fires and hence less smoke. This also means that the smoke has little constituents. However, due to high temperatures involved with completely burned fuel, the smoke produced is likely to have vapours and aerosols comprised of various particulate matter and compounds. This is however also dependent on the nature of fuel and compounds or elements available during the combustion. Thus, presence of sulphur will result in smoke containing sulphur dioxide, while carbon and hydrogen fuel will results in water vapour. Nitrogen oxides are also found in smoke involving high temperature fires. Poorly ventilated enclosures have insufficient oxygen required for combustion and therefore result in a lot of smoke. Combustion of, for instance, organic materials in such condition would result in smoke comprised of soot (or unburned carbon). Incomplete combustion also results in production of smoke with hydrogen sulphide. T3. The compartment/building geometry and fire location have an effect of the on the production of smoke. The size of a compartment determines how air mixes with fuel during combustion. A large fire in a small room, for instance, would not experience effective mixing of fuel and air because of space restriction. On the contrary a large space would allow air to reach all side of the fire and fuel to allow effective mixing of the fuel and oxidant. The fuel/air mixture determines the nature of smoke produced. The location of fire also determines the availability of air for combustion, and in turn, dictates the production of smoke. Fire located at the corner may receive air through one side of the flame. This means that the side where air is restricted would produce more smoke due to incomplete combustion. Fire plumes refer to the buoyant flow of gases above a burning fuel source whereby by the hot less dense mass of gases rises upwards over the colder denser mass due to the density difference. The axisymmetric plume is caused by a diffusion flame developed above the burning fuel so that air is horizontally entrained from all directions. Line plumes or adhered plumes are developed above a narrow and long burner so that air is entrained only from two sides. Spill plumes are also known as free plumes and entrained of air is distorted. T4. The main function of smoke control is to save lives, and in this regard, it aims at preventing smoke from coming into contact with the occupants of the building (Spengler, Samet & McCarthy 2001, p.14.1). It keeps smoke within the area of origin and away from exits routes to allow safe evacuation of building occupants. It also attempts to direct smoke in a particular direction where smoke would have little or no risk to people, or building occupants. Smoke control is required when there is a fire outbreak in building and enclosures. It is required especially in enclosures and buildings where smoke movement would pose a risk to lives of the occupants as well as obstruct the evacuation efforts. Some areas where smoke control is required include egress, high-rise building, smoke-proof enclosures, underground buildings, stages and platforms, covered mall buildings, assembly seating, windowless buildings, and building atrium (Patterson 2006; Anonymous, n.d). There are a number of smoke control systems that are in use today. These include smoke venting, compartmentation, dilution, bouyancy, pressurization, and airflow (Spengler, Samet & McCarthy 2001, p.14.1). Smoke venting is where vents are installed on the roof of a building to actively control smoke. Compartmentation involves subdividing building floors into recommended sizes. Smoke dilution refers to smoke removal from non-fire areas to maintain levels of particulate or gas within acceptable standards. It involves providing air into affected spaces to dilute the smoke. Pressurization works by creating a pressure difference across a barrier. The side with high-pressure can be egress route or where people take refuge, while smoke is exposed to the low-pressure side. Airflow is more or less like pressurization, only that airflow involves air flowing through huge openings. Buoyancy is venting smoke through passive and fan-powered vents usually situated in ceiling of big, open areas such as enclosed shopping mall and atria. T5. The use of standard fire curves in determining fire resistance is of importance in regards to fire control and protection. Fire curves are used to illustrate fire performance of certain materials under certain fire conditions. With standard fire curves, these conditions are determined by laboratory set ups or conditions. Therefore, the representation may not reflect the outcome in a real life fire incident. It is worth to note that most of the standard fire curves are close to the ISO 834 standard, which was determined in specific conditions that may not match actual conditions. The difference between the actual fire curve and the standard fire is critical as far as accuracy is concerned. A standard fire curve continues to increase with time, while an actual fire curve decreases on reaching a particular maximum (Franssen, Zaharia and Kodur, 2009, p.5). Thus, use of standard fire curves is bound to give less accurate information in regard to fire performance of constructions materials. Franssen, Zaharia and Kodur (2009, p.5) notes that an actual fire curve would provide more reliable information than standard fire curves. . The approaches for determining fire resistance of onshore and offshore application vary. The main difference of the two approaches is in the temperature intensity. Thus, for onshore application, a less cellulosic fire curve is used, while in offshore application, a hydrocarbon fire curve, which is more intense, is used. This is because offshore application involves hydrocarbons while on shore application mostly involves cellulosic fuel. Reference list Franssen, JM, Zaharia, R & Kodur, V 2009, Designing Steel Structures for Fire Safety, CRC Press, Australia. Friedman, D 2006, Surface Transportation Security: Volume 10, A Guide to Transportation's Role in Public Health Disasters. Transportation Research Board, United States. Lin, C 2000, Study of exposure fire spread between buildings by radiation. Journal of the Chinese Institute of Engineers, Vo. 23. No. 4. pp. 493 - 504. Patterson, TL 2006, Illustrated 2006 building code handbook. McGraw-Hill Professional, New York. Smoke control, 13 March 2010, . Spengler, JD, Samet, JM & McCarthy, JF 2001, Indoor air quality handbook. McGraw-Hill Professional, New York. Read More