Enclosure Fire Dynamics - Assignment Example

Enclosure Fire Dynamics T1. Analyse thermal explosion in a vessel with cold walls, including the mechanism of self-accelerating reaction and nature of induction period? Heat is the energy contained in a substance and measures the molecules motion that makes up that substance. In cold walls the molecules move very lower though it still heats. Cold walls normally have low heat content and the temperature is slower. For the cold walls of a vessel to gain heat, heat moves from a warm object. Cold is not transferred through the walls, it just means it is low heat being transferred. This is done through conduction where the vessel with cold walls is in contact with a warm one. The molecules which move very fast from a hot object bump into the cold walls whose molecules move very slow. The molecules that move fast give up some of its energy making them to slow down and lets the energy to speed up and thus heating up the slow molecules of the cold walls. Combustion in a vessel with cold walls takes place in a self-ignition or self-acceleration manner. The explosion that results is spontaneous because of the uniformly heated substance. Here the explosion process is where the reaction rate takes place as time progress and when there is a failure of relaxation of concentrations and temperature to hold reaction rate at level that is fixed. The self acceleration that takes place in cold walls is the thermal explosion. The limit of explosion means the totality of pressure, vessel diameter, composition and temperature which abrupt change to a fast explosion takes place from a slow reaction. In a closed system, there is correspondence of a transition region of conditions in the initial state to the explosive limit (American Institute of Chemical Engineers 1966). T2. Using Semenov diagrams, explain the effect of heat transfer on thermal heat balance in a vessel with cold walls, critical conditions of thermal explosion, and pre-explosion heating. With curve A, the system is at a low temperature and the reactants proceed to Tstable temperature because of the curve of the heat production above the curve of the heat loss. At the stable temperature, no more self-heating takes place. The temperature then rises if an external heat source comes in to heat the reactants. The temperature reaches T ignition, unstable point and becomes greater than then explosion occurs. The reactants begin to drop in temperature when the external source of heat is removed and it goes back to being stable again at temperature T stable. At curve B, the stable temperature, critical temperature and ignition temperature are at the same temperature. There is a slow rise of reactants to the critical temperature where temperature rapid acceleration takes place leading to thermal explosion. At curve C heat gain flux takes place which exceeds the flux of heat loss so that thermal explosion takes place at whatever the temperature the reactants are (American Institute of Chemical Engineers 1966). T3. Define explosion, deflagration and detonation. Explain the development and main features of detonation. Explosion is a sudden burst with loud noise caused by internal pressure, or it is a rapid energy release that cause pressure or shock wave to develop; like an explosion of a bomb, gun, et.c. Deflagration is a subsonic explosion or rapid combustion that spreads through thermal conduction where a hot material that is burning heats the next cold material layer and ignites it. In deflagration fires are resulted from flames to explosions and the shock wave moves at a speed that is less than the speed of the sound in the medium. Denotation on the other hand is supersonic combustion which spreads through rapid shockwave. The shock wave in denotation moves at higher speed than the sound speed in the medium. Denotation takes place in liquid explosives and conventional solid and gases that are reactive. Resolution of the wave system is very clearer in gaseous explosives than solid and liquid explosives because velocity of denotations in the latter area is very high. The denotation speed does not depend on the rates of chemical reaction that generate heat. This is because denotation is a rapid shock wave which when it heats a gas mixture that is reactive to a high temperature there is a release of heat combustion that takes place. It thus becomes easier to calculate the denotation velocity from the combustion heat and the physical properties of the mixture. A complete explosive denotation takes place at its highest velocity and is known as a high-order detonation. An incomplete denotation takes place at its lower velocity than the maximum and is known us a low-order denotation. Low-order denotations may take place when there are cracks in the explosives, explosives are deteriorated, and poor contact between the explosive and the initiator ( Glassman & Yetter 2008). T4. Review the main features of diffusion combustion. Explain the types of diffusion flames (momentum and buoyancy dominated fires). Compare the Froude number for these types of fire. A diffusion flame is a flame in which the oxidizer and the fuel are separated initially. This flame is represented by the forest fires, match flames, wood fires and candle flames. The fuel-oxidizer ratio in this flame varies throughout the flame. Diffusion flame can be classified as laminar, transitional and turbulent. Turbulent flames flow characteristics around and in the flame are irregular and disorganized. Laminar flames have characteristics that are steady and smooth in and around the flame. However, some flames are laminar on one section and turbulent in the other section. Classification of diffusion flames depends on the forces that control the mixing and flow processes. These forces are buoyancy, viscous and momentum forces. Turbulent are from a fire more hazardous than laminar flames. The two types of turbulent flames are momentum-dominated and buoyancy-dominated. Momentum-dominated turbulent flames are associated with high velocity jet issuing from an opening. Buoyancy-dominated is a lazy flame seen over a flammable liquid burning dish with upward flow that is driven by buoyancy. In laminar flame, the length of the flame is approximately proportion to the velocity while in turbulent flame; the length of the flame is independent of velocity. Turbulence spreads from the tip of the flame going downwards (Friedman 2008). The Froude number for turbulent flame is defined as Where ∆Tf is the temperature rise characteristics from combustion, Frf< is flames dominated by buoyancy and Frf> is flames dominated by momentum. The Froude number for laminar flame is defined as Where acr is critical centripetal acceleration and rcr is critical radius (Peters 2000) T5. Analyse the burning rate of solids and mechanism of flame spread over solid surfaces. There are two classes of flame spread in solid fuels which differ in the flame spread rates. They are the counter flow flame spread and the concurrent flame spread and they refer to the direction of the flame spread as related to the direction of the airflow. The flame spread in which the flame spreads in the same direction as the direction of gas flow is referred to as concurrent flame spread. The case where the flame spreads against the airflow is known us the counter flow flame spread. Concurrent flame spread is very effecting in unburnt fuel heating that later results in greater rate of flame spread. Much heat can be generated from counter flow flame spread because most of the flame is removed from the fuel that is not burnt (Drysdale 1999). T6. Analyse emissivity of opaque surface and grey body, and determine the total black body emissive power and total grey body emissive power at a given temperature. The emissivity of a surface is the ratio of radiation of energy from that surface to the radiation of energy from a blackbody at the same conditions, same wavelength and same temperature. The radiation efficiency of a real-world surface is compared to a blackbody (a body that absorbs all radiation which falls on its surface) radiator. The light or radiation an object reflects becomes the color of that object. There are some spectrum of sunlight that are absorbed while some are reflected when the spectrum hits an object. The color is that particular spectrum that is visible to the eye of a human being, but a body appears black when it absorbs all the radiation, thus reflecting no light. According to Kirchoff’s Law, ‘for an object at thermal equilibrium in a vacuum, the power radiated (e) must be equal to the power absorbed. A black body absorbs all radiation and re-emit it in a continuous spectrum. The objects then appear black when it is cold because no light is transmitted, though it emits spectrum of light that depends on the temperature. Higher frequency takes place when the wavelength is shorter and thus the temperature becomes higher. The colour of a hotter object is determined by the blue end of the spectrum that the object is closer to. On the other hand, the cooler object color is recognized when it is closer to the red spectrum. Black bodies emit infrared wavelengths mostly at a room temperature but they start emitting wavelengths that are visible as the temperature increases. This causes them to appear red, orange, yellow, white and blue as the temperature increases. The object becomes white when it emits ultraviolet radiation. An object is termed as a black body approaches emissivity is 1.0 (e =0.99 or more) of a source of infrared radiation Grey bodies are real objects which don’t behave as black bodies fully. They radiation they emit at a frequency given is just a fraction of what is needed as an ideal emission. Compared with a black body, the body emissivity specifies how well a real body radiates energy and depends on wavelength, emission angle and temperature. However, in grey bodies, it is assumed that absorption of spectrum and the emission of radiation on a surface doesn’t depend on wavelength, rendering the emissivity constant. An object becomes a grey body when is source of infrared radiation is less than 0.99 (Cverna & ASM International 2002). T7. Analyse the radiating gases produced in combustion, define the mean beam length and explain the use of emissivity charts. The 33% of the total heat released are carried by gases that radiate in the carbon dioxide, water and infrared. 63% of heat is contained in oxygen and nitrogen. This is the analysis of heat capacities of furnace gases. The introduction of the mean beam length is the length scale to geometry effect in radiative heat transfer evaluation between the volume of an isothermal gas and its boundaries. The mean beam length is the radius of a hemispherical gas mass where its emissivity is equivalent to that for the interest in geometry. The mean beam length is used to determine the radiative heat fluxes to furnace walls that are cold black and from an isothermal volume of hot combustion gases. The mean beam length of a gas geometry can be denoted as Le . By using mean beam length, gas emissivity is obtained which gives to a surface, radiant heat transfer because of emission from an adjoining gas (Cverna & ASM International 2002). T8. Critically analyse the effect of ventilation on the composition of smoke using equivalence ratio. Compare over ventilated and under ventilated combustion. A compartment fire can begin small by a dropped cigarette and spreads when ignited by a major fuel source like furniture. The fire exhausts the oxygen available and further spreads if additional air is found through doorway or window. Combustion products that are hot rise forming smoky upper layer below the ceiling and depend as the fire continues to burn. Smoke begins to spill out of the room on to the rest of the building when the hot layer extends down to another vent, an open window or top of a doorway. Windows and doorways are the ones that provide the path for combustion products and the air needed for combustion to continue. In under-ventilated combustion smoke builds up slowly due to inadequate air supply coming in. Smoke builds up so intensively that it can choke a victim to death in a very short time. In over-ventilated combustion there is a lot of air coming in leading to a lot of smoke building up to escaping to the open air. Less smoke is built up in the room; however, smoke can easily flow and build up in other rooms unlike in over-ventilated combustion. Smoke generated from under ventilated combustion has organic composition much higher than in over ventilated combustion (Janoffs, Royals & Gunaji 1995). T9. Analyse smoke optical properties: absorption, scattering and extinction of light. Examine the role of extensive (extinction coefficient, optical density per meter, optical density per meter) and specific characteristics (mass extinction coefficient, mass optical density per meter, smoke potential) and their relationships. The description of optical properties of smoke is the mass optical density and optical density. These properties are calculated is done for various sources in different size compartments. Concentration of carbon monoxide concentration addresses the smoke toxicity as substitute measure. Optical properties of smoke are more dependent on rate of burning and energy exposed. Smoke optical properties are described by volume optical extinction coefficients. Subscripts here indicate the form of extinction where e is for total extinction, s for scattering, and a for absorption. The extinction coefficients depend on the particles complex refractive index, their concentration and distribution of size. The scattering that is single and the specific extinction depend on the particles physical properties and fundamental chemical properties. Optical density and light extinction coefficient express the raw results when making optical smoke measurements. They relate incident light beam intensity to the transmitted light over a path length that is defined (Mehaffey &Mehaffey 1998). T10. Characterise the main sources of heat release and heat loss in a typical compartment fire. Analyse the difference between burning rates in open space and in a compartment. The rate of energy release of fire in a compartment occurs within the compartment and also outside because of flame extension that is normal or air supply that becomes insufficient to the compartment. There are conditions that permit burning outside and within the compartment. The smoke and the flame heat cause the vaporization of fuel at a mass flow rate. However, all the fuel may not burn completely in the compartment despite the fact that all of it may eventually burn because it is depended on the rate of air supply. It is either all the oxygen coming it or all the fuel that is burned, but whatever burns gives the heat release rate within the compartment. The burning rate of fire in a compartment increases when there is an opening that allows air to come in. When the ventilation is reduced, air that is used to feed the fire is reduced and thus the burning rate is also reduced. Air availability influences combustion and intensity of fire. Little carbon dioxide is formed when there is excess air and the fire is small. The burning rate of fire in an open space is very high due to the air supply through the openings or open air. However the heat is lost almost immediately because it is not trapped like in compartment fires. Products don’t burn intensely in an open fire as compared to the compartment fire. Oxygen need for combustion is readily available in open space than in compartment (Glassman & Yetter 2008). Referencing List American Institute of Chemical Engineers , 1966, Proceedings of the ... International Heat Transfer Conference, Vol. 2, American Institute of Chemical Engineers, p 133 Cverna, F. & ASTM International, 2002, ASM ready reference: Thermal properties of metals, ASTM International, p 524. Drysdale, D, 1999, An Introduction to Fire Dynamics, 2nd edition, Wiley, p 237 Friedman, R, 2008. Principles of Fire Protection Chemistry and Physics, 3rd edition, Jones & Bartlett Learning, p 184. Glassman, I. & Yetter, R. A, 2008, Combustion. Academic Press, 4th edition p, 261 Janoff, D. D., Royals, W. T. & Gunaji M, V, 1995, Flammability and sensitivity of materials in oxygen-enriched atmospheres, ASTM International, 1995 p 170. Mehaffey, J. R. & Mehaffey, J. R, 1988. Mathematical Modelling of Fires, ASTM International, p 84 Peters, N, 2000. Turbulent Combustion. Cambridge University Press, p 203. Read More