Introduction to Combustion and Fire - Assignment Example

Combustion Order No. 282686 No. of pages: 9 1st 6530 Theoretical Questions (2) Chemical Elements and Compounds What are free atoms and radicals? What is the difference between an ion, free atom and radical? Free atoms and radicals are species whose electrons are unpaired in the outermost layer and may take part in chemical reactions. Ions are particles that are electrically charged whereas atoms that have either lost or gained electrons has an electrical change, but radicals are not charged. (4) Chemical Reactions and their Rates 1. What are the stoichiometric, fuel lean and fuel rich mixtures? Give an example. When fuel combines with the oxygen during combustion, the mixture gets balanced chemically and the Air- fuel ratio (AFR) is what is called the stoichiometric mixture. A fuel lean mixture is when the proportion of the mass of air to fuel is more than 14.7 to 1 and a fuel rich mixture is when the proportion of the mass of air to fuel is less than 14.7 to 1. E.g. Internal combustion engines. 2. What is the concentration and its units of measurements? What is a mole? The stoichiometric unit of measurement is 14.7 (gasoline) but this could fluctuate depending on the compounds that combine and alter the ratio. E.g. for a fuel that is MTBE (methyl tert – butyl ether) laden, the stoichiometric ratio could go down as much as 14:1:1. A mole is a measure of pure substance that contains an equal number of chemical units in 12 grams of carbon. 3. Explain temperature and concentration dependence of the chemical reaction rate and Arrhenius equation. The higher the temperature, the faster a chemical reaction will occur. The Arrhenius Equation determines the temperature. Molecules collide at higher temperatures. The greater the collision the greater the kinetic energy produced, thereby increasing the activation energy which in turn determines when a reaction takes place. The Arrhenius Equation k=A*exp(-Ea/R*T) K is the coefficient, A is the constant, the activation energy is Ea, R is the accepted universal gas constant and T stands for the temperature. (Kelvin) (5) Thermal Exploration 1. Analyze thermal explosion in adiabatic conditions and the mechanism of self-accelerating reaction. Define induction period. In adiabatic conditions, the temperature increases very rapidly at first, but then moves gradually and slowly for a period of time before finally creating a rapid acceleration leading to ignition. The induction period is the duration of a pre- burn lag during which the formation of an initial flare and combustion of fuel vapors takes place. (7) Ignition Describe the process of ignition of a solid combustible material by a hot plate. Explain evolution of the temperature field in the solid material. During the process of ignition on a hot plate, the solid combustible material gets ignited if there is a sufficiently high temperature and during the process there is considerable shrinkage because the material gets consumed gradually because of the heat. During the process of pyrolysis, the temperature field becomes more and more heated due to combustion of the solid material on the hot plate and when it reaches a certain point which is enough or ignition, it gets ignited. (8) Pre-mixed flame Why do flames propagate through a combustible mixture? Propagation of flames through any combustible material depends on the energy release rate which in turn is determined by the composition of the mixture, the environment and the source of ignition. Therefore the homogenous system is considered flammable when the flammability of the substance reaches a specific temperature and pressure which is then transmitted throughout the mixture. 2. What are the flame front and flame propagation velocity? Why a gas particle entering flame front is accelerated? Flame front and flame propagation velocity is the speed at which heat and mass transfer takes place. The most influential factor that alters this flame propagation and thereby the quality of combustion is turbulence. When entering the flame front, a gas particle is accelerated due to the turbulence that takes place through premixed reactants. The manner in which the propagation of the flame in a turbulent field takes place depends on the turbulent structures that a flame encounters. 3. What is adiabatic flame temperature? Adiabatic flame temperature is the maximum temperature that could be achieved by the given reactants. It is referred to as Adiabatic flame temperature, when the process of combustion occurs adiabatically without any shaft work. For any given combination of fuel and oxidizer, the maximum adiabatic flame temperature takes place using a stoichiometric mixture. (9) Detonation 1. Compare the main features of premixed flame and detonation. In a premixed flame the fuel gas is mixed along with an oxidizer before burning. The soot and carbon particles that are given off react with the oxidizer present in the flame, resulting in a lower mass output in addition to tiny soot particles. These particles together with the mass output grow when the content of oxygen in the fuel gas considerable reduces. Detonation causes severe damage to the engine due to the uncontrolled burning that takes place with the fuel air mix and tends to create very high pressure within the cylinder. To prevent detonation, richer mixtures (mixtures with a lower air- fuel ratio) are made use of in order to produce cooler products of combustion. 2. Describe internal structure of detonation wave. The structure of a detonation wave alternates between high pressure zones and chemical reaction zones. The theory of hydrodynamics depicts the shock front in the case of a high pressure zone and in the areas of chemical reaction it depicts chemical peaks in those areas. This is followed by areas of a relatively gradual change involving the other parameters during the effect of the unloading wave. (10) Diffusion Combustion – Fire Plume. 1. Compare a jet fire and buoyancy dominated fire, using the flame height-jet velocity diagram to explain flame shape. The diagram of the flame height jet velocity has a burner consisting of two co – axial tubes with the inner supplying the fuel gas and the annular gap that holds the air. Both stationary and vibratory combustion can be determined by varying their respective velocities. During a certain rate of gas flow, we can obtain a vibratory regime by an increase of the rate of air flow. When oscillations begin there is a substantial increase in the height of the flame. When the air flow is increased, a vibratory or stationary circle of flame is noticed over the border of the gas feed tube which leads into the formation of a jet. 2. Explain the low value of the Froude number for natural fires. Describe fluid-dynamic structure (air entrainment, buoyant flow, eddies) of a fire plume. In a fluid dynamic structure, the hot gases that arise by burning liquid fuel are driven upward through the action of buoyancy and is gradually made to cool down by the action of entrainment of ambient air. In the case of a diffusion flame that is non- luminous, the radiation loss was almost negligible, but in the case of a luminous flame, the energy that was radiated was intercepted by the surrounding fire came back to a buoyancy plume by the ingested air being heated. The interaction of the flame and fuel through solid fuel pyrolysis resulting in the generation of gas works according to the ambient conditions of the atmosphere. In the case of natural fires, there are a lot of fluctuations due to the shedding of huge eddies because of convection, strong and weak updrafts and downdrafts that contributed to the fire plume. (11) Combustible Liquids and Solids 1. Describe the flash point, fire point and auto-ignition temperature for combustible liquids. How these characteristic are measured in laboratories? Flash point – Flash point is the lowest temperature needed to produce sufficient vapor that forms an ignitable mixture. The greatest danger is from liquids that possess low flash points. Fire point – This is the point at which a liquid or solid ignites and forms a fire. This happens when the substance reaches its flash point. Liquids that are flammable possess a flashpoint that is below 100° Fahrenheit (37.8°C) Auto –ignition temperature – Auto- ignition temperature is the temperature at which a combustible liquid ignites devoid of any ignition source such a flame or spark. These characteristics are measured according to their flash points and other technical criteria, making use of the Workplace Hazardous Materials Information System (WHMIS) E.g. liquids that are flammable have a flash point that is below 37.8°C (100°F). Combustible liquids possess a flashpoint that is at or over 37.8°C (100°F) and below 93.3°C (200°F). 2. What is BLEVE? Explain possible effects of accidental liquid fuel releases on the surrounding. BLEVE is an acronym that stands for – boiling liquid expanding vapor explosion. A BLEVE occurs when a vessel that contains a liquid that exceeds its atmospheric boiling point ruptures or explodes. Accidental liquid fuel releases on the surroundings poses a serious fire hazard. Since both combustible and flammable liquids flow very easily, even a tiny spill spreads over a large surface and could burst into flames if an ignition source is available. When this happens the fire can spread very rapidly and give off vapors that are quite hazardous. 3. Discuss the main factors influencing flame spread over solid materials. Flames spread rapidly and easily because most solid materials are good conductors of heat. Materials such as wood, cloth, paper and cardboard are materials that easily absorb flammable substances thus spreading rapidly into other materials in the environment causing destruction and giving off poisonous vapors and gases. (12) Fire as a Combustion System 1. Define heat of combustion, heat release rate and combustion efficiency. Heat of combustion – When a substance burns completely, the amount of heat that is given off or released per unit mass or volume is the ‘Heat of combustion.’ Heat release rate – Heat release rate (HRR) is the rate at which heat is given off or released during combustion. The HRR is measured in Joules per second and is termed as Watts. It is rather convenient to measure the HRR either in kilowatts (1000 w) or Megawatts (1 million w) because fire gives off more than 1 watt. 2. Describe three zones in turbulent diffusion flame (fire plume). The three zones are – a pure fuel zone, residual gas zone involving air, and the mixed zone. The chief characteristics of a turbulent diffusion flame is that the oxidizer (air) and the fuel are separate in the beginning but combustion takes place in the zone or area where the gases mix. The next zone has the Laminar flames that are blue in color. This zone occurs because a little pre- mixing can take place close to the rim. During high flow rates, the flame stability gets lost due to an excess of air entrainment that occurs at the flame base and results in a turbulent flame zone. 3. Why is thermal radiation of importance in fire? Thermal burns occur due to sources of extreme heat that burns the skin tissues causing charring and death of cell tissues. E.g. flames, scalding, steam and hot metals. Thermal radiation is important because it equates emission and absorption in objects that are heated and thereby gives relief to people who get burnt. (13) Fire in Enclosures. 1. What is a positive thermal feedback for fires in enclosures? What is flashover and backdraft? Growth of fire in an enclosure develops in many ways dependent upon the geometry of the enclosure, type of fuel, flow of air, amount of fuel and the surface area. Fire growth in an enclosure depends on two main categories: factors dealing with the enclosure itself and those dealing with fuel. A flashback occurs in case the velocity near the walls fall much below the velocity of burning and a backdraft takes place when there is an excess of air. 2. Describe the conditions necessary for flashover to occur in terms of radiant heat flux at floor level, temperature of a hot upper layer, and minimum required heat release. The growth of fire depends upon the type of fuel as well as the amount of fuel. In building fires, the fuel is generally solid, while in certain industrial fires it could be a liquid. Furniture which is wood based does not burn very fast but burns for a long time, while modern materials like plastic burn for a shorter time but faster. The placement of the fuel package also affects fire development. If placed close to a wall, it takes in limited amounts of cold air, which creates high flames and high temperatures. 3. Explain fuel-controlled and oxygen-controlled regimes of fire in an enclosure. The growth of fire is determined by the temperature of the bounding surfaces on the upper part of the enclosure, since the layer of hot smoke and the upper bounding surface tend to move towards the combusting fuel. A small enclosure raises temperatures and causes faster Fire growth, while a large enclosure, with the same amount of fuel, has lower temperatures, slower Fire growth, takes longer for smoke to fill in and the amount of feedback to the fuel is also reduced. Buildings with high ceilings but large lower flat surfaces cause modest fuel feedback, because it takes a longer time for the flame to reach the upper limits of the enclosure while in a low ceilinged area, fuel feedback is greater, flames rise faster and Fire growth is accelerated. 4. Describe the main flow patterns associated with fire development in enclosures. Fire development is dependent upon supply of oxygen, and in spaces that are closed or small openings, availability of oxygen is reduced, so that the fire extinguishes by itself due to oxygen starvation, or may burn, but at very slow rates. The size, position and shape of ventilation is an important determinant in fire development. In its initial phase the opening serves as an exhaust that sucks out hot air from the enclosure, reducing the thermal feedback. The size of the opening is an important aspect in the primary stage of the fire development because this determines oxygen supply. 14. Fire in Enclosures: Fire modeling. 1. What are advantages and limitations of zone models? The various zone models are primarily used for rooms or buildings that are rectangular in shape and have ceilings that are smooth, flat and horizontal. It is advantageous because it helps us to understand the type of fire and gas build up when a fire occurs. It helps us understand the size of a fire in relation to the building and so helps to gauge the equations entrainment that could be successfully in a particular scenario. Besides this it helps one to understand the convection heat transfer, radiation and the conservation of mass as well as energy. 2. What is field modeling of fires? What are objectives of CFD fire modeling? 3. Describe verification and validation for field modeling? Why validation is necessary? References The Arrhenius Equation www.shodor.org/UNChem/advanced/kin/arrhenius.html Robert W. Van Dolah, Michael G. Zabetakis, David S. Burgess and George S. Scott (Feb. 1965) Fire Technology. “Ignition of the Flame- initiating Process.” United states Department of Interior, Bureau of Mines. E. J. Long, G.K. Hargrave, S. Jarvis, T. Justham, and N. Halliwell. (2006) Characteristics of the Interaction between Torodial Vortex Structures and Flame Front Propagation. Journal of Physics: Conference Series 45. institute of Physics Publishing. Wolfson School of Mechanical and Manufacturing Engineering, Loughborough. University, Loughborough, Leicestershire, LE11 3TU Read More