Thermal Explosion, Deflagration, and Detonation - Assignment Example

FIRE ENCLOSURE DYNAMICS FIRE ENCLOSURE DYNAMICS Insert name: Insert course code: Instructor’s name: 16July, 2010. Question 1: thermal explosion A stoichiometric mixture exists when there is exactly the minimum amount of air required to completely react with the fuel. It is in these proportions that the reaction will be most efficient and the resulting flame temperatures will be highest. At the stoichiometric proportion, the energy to ignite the mixture will be lowest. The minimum ignition temperature is lowest at the stoichiometric concentration. Fire is a self-sustaining reaction. Once the chemical reaction begins, it can provide enough energy to continue the reaction between the fuel and oxidizer. It is not possible to keep oxygen liquid if the temperature rises above the critical temperature of 154.6K (-181.40F). As a result, if LOX is ensnared in a closed structure along with being permitted to warm, extreme pressure can over-pressurize the system. For instance, LOX ensnared between the valves can bust the connecting pipe. Certain kind of pressure relief ought to be provided where trapping might be common. Vent systems as well as relief should be sized to allow easy flow so that too much rear pressure may not take place. An instance where relief devices are used to avoid over-pressurization is in the case of cryogenic liquid storage vessels. The relief devices are designed to protect the inner vessel as well as the vacuum-insulated portion of the tank from failures resulting from inner and outer shell damage, overfilling or heat from insulation damage and fire. In particular scenarios, like when these containers are caught up by a fire which encroaches ahead of the ullage region of the container, container malfunction may possibly take place. In such cases, water should be directed onto the flame-impinged portion of the tank to allow tank to cool. The water should be enough so as to keep the tank wet (Beeson, Stewart and Woods, 1999 p. 46). A flammable gas mixture is a mixture of gases through which flames can propagate. The flame is initiated in the mixture through external means, and the limits of flammability are defined as the limiting composition of a combustible gas and air mixture beyond which the mixture will not ignite and continue to burn. The limits of flammability are determined by laboratory analysis as well as a number of environmental factors like pressure, temperature along with geometry and the size of the confining vessel can influence the outcome of these measurements. Flammable ranges tend to be broader at higher temperature; the lower limits decreases and the upper limit increases. As pressure increases, the lower limit remains constant while the upper limit rises. In narrow vessels, the flammability limits are narrowed due to substantial heat loss to the walls. Question 2: Semenov diagrams As the power of the cooling system reduces to levels lower than the heat production rate of a reaction, there is rise in temperature. If there is an increase in temperature, the rate of reaction also increases resulting into an increased rate of heat production. Since the heat production of the reaction may increase exponentially, whereas the cooling capacity of the reactor increases only linearly with the temperature, the cooling capacity may become insufficient resulting to temperature rise. A thermal explosion will thus develop. The heat balance is in equilibrium when the heat reproduction is equivalent to heat removal (Qrx =Qex). This occurs at the two intersections of the exponential heat release rate curve Qrx with the straight line of the heat removal curve Qex in the Semenov diagram. As temperature deviates from S to a higher value, the heat removal dominates and the temperature decreases until production and removal equalizes with removal. The system with cold walls resumes back to its stable equilibrium state. On the other hand, if temperature decreases, the heat production dominates and thus temperature rises until equilibrium is attained again. Thus the intersection at lower temperature corresponds to a stable operating point. For a small deviation to a lower temperature, cooling dominates and the temperature decreases until point (S) is reached again, while a small deviation to a higher temperature results in excess heat production, hence a thermal explosion develops (Stoessel, 2008 p. 51). The Semenov diagram shows the intersections S and I between the heat release rate of reaction and the heat removal by a cooling system which represent an equilibrated heat balance. At intersection S, we have a stable operating point, while at point I, we have an instable operating point. Point C represents the critical heat balance. Question 3: explosion, deflagration and detonation Deflagration: it is a combustion wave that propagates from an explosion at a subsonic speed relative to the un-burnt gas immediately ahead of the flame (flame front). It is also a substance effect of a material in which the reaction frontage proceeds into the un-reacted substance at a subsonic speed. An unconfined deflagration occurs when there is insignificant rise in pressure and where heat transfers as well as the mass transfer are the only factors affecting the velocity of advance of the reaction zone. Confined deflagration on the other hand occurs where there is significant rise in pressure and where heat transfers as well as the mass transfer are the only factors affecting the velocity of advance of the reaction zone. Explosive deflagration takes place is a system where there is a significant rise in pressure due to high velocity of advance of the reaction zone and where the main factor affecting the velocity of advance of the reaction is shock (Marshall and Ruhemann, 2001 p. 93). Explosion: this is an abrupt oxidation, or decomposition reaction resulting to an increased temperature or pressure or both. The term explosive atmosphere is used to refer to flammable substances that have the form of gases, vapors, dusts or mists mixed with air under atmospheric conditions where with occurrence of ignition, combustion spreads over to entire unburned mixture. Explosions may result from nuclear reactions, loss of containment in high pressure vessels, runaway reactions, high explosives or a combination of dust, gas or mist in air. A dust explosion takes place when a combustible material is dispersed within the air forming a flammable cloud allowing the flame to propagate through it (Furness and Muckett, 2007 p. 127). Detonation: it is an explosion resulting from a very rapid chemical effect of a material in which the effect front proceeds into the un-reacted substance at a supersonic speed. It is a combustion wave propagating from an explosion at supersonic speed relative to the un-burnt gas immediately ahead of the flame (flame front). A detonation has a heterogeneous reaction system with three zones; an un-reacted zone, a reaction zone and a reacted zone. Detonations develop more rapidly at initial pressures above ambient atmospheric pressure. They produce much higher pressures than an ordinary explosion. Detonation pressure becomes more destructive and severe when the initial pressure is high. Detonation can cause harm through two main forms of emissions: thermal radiation and pressure energy. Detonations are more dangerous because they give rise to higher peak positive overpressures and they emit their quantum of energy in a very short time thus possessing high levels of power (Marshall and Ruhemann, 2001 p. 92). Question 4: features of diffusion combustion Diffusion flames or combustion do not propagate: they are located where the fuel and oxidizer meet. This property is important for security reasons but it is accompanied by aftermaths on the chemistry/ interaction turbulence. This is because; failure to have propagation speed, a diffusion flame or non-premixed flame cannot impose its own dynamics on the flow field apart from being more sensitive to turbulence. Diffusion flames are also more sensitive to stretch than turbulent premixed flames: critical stretch values for extinction of diffusion flames are one order of magnitude smaller than for premixed flames. A diffusion flame is more likely to be quenched by turbulent fluctuations and flame-let assumptions are not justified as often as for turbulent premixed combustion. A diffusion flame is a flame that is long, undulating, yellow, very luminous and sooty. The diffusion flame has lower temperature than that of premixed flame, produces larger amounts of black smoke and some un-burnt fuel is lost. The over-all combustion efficiency is low. There are mainly two kind diffusion flames: over-ventilated flame and under-ventilated flame. In the case of an over-ventilated flame, excess air is available in the surrounding for complete combustion of fuel issued from the burner port. Consequently, the flame boundary converges towards the axis and presumes the shape of a closed conical flame. For an under-ventilated flame, the air is not sufficient for complete combustion, thus the flame surface expands towards outer wall forming an inverted conical flame (Mishra, 2008 p. 163). The Froude (Fr) number is given by the momentum of air induced by flame divided by buoyancy fore in the flame = U20/dg. Where Uo is the fuel exit velocity, d is the diameter of the burner port and g is the acceleration due to gravity. In Reynolds number (Re) and Froude (Fr) number- controlled laminar diffusion flames, the buoyancy forces acting on the hotter, less-dense gas in the region of the flame often distort the shape of the flame sheet. The other type is the Fr number-controlled turbulent diffusion flames where the velocity of the incoming fuel jet is relatively low compared to the velocity of hot gas accelerated in the turbulent flame zone by buoyancy (Johnson and Miyanishi, 2001 p. 36). Question 5: the burning rate of solids and mechanism of flame spread over solid surfaces When a condense-phase fuel, either liquid or solid, is burning, a diffusion flame is established at some distance on top of the free surface of the fuel. The fuel can either be in gaseous form or condensed form (either liquid or solid). Diffusion flames do not have primary characteristic properties that are measurable, like flame velocity. When a condense-phase fuel, either solid or liquid is burning, there is an establishment of a diffusion flame at certain distance on top of the free surface of the fuel. The gaseous fuel supply feeding the flame is produced by evaporation, pyrolysis and gasification of the condensed fuel. In solids, pyrolysis leads to charring of the fuel which is the chemical process of degradation as well as incomplete combustion resulting in char as a solid residue. At the diffusion flame, the chemical process can be assumed to be infinitely fast and thus the burning rate of condense fuels is a factor of heat and mass transfer rates only. The rate of conversion of condense fuel into gaseous form is normally determined by the rate of heat transfer from the flame to the fuel surface. A flame or heat can be spread through three main mechanisms; radiation, convection and conduction (Wilkie and Morgan, 2009 p. 48). The rate of productions of bulk combustion products (soot, smoke and other gases) is proportional to the heat release rate (HRR), as they are originated by the reaction rate at the total burning surface of the fuel. The hot smoke emitted from a fire will rise up due to buoyancy, forming the plume. As the plume rises, it entrains air from the surroundings, thus feeding the fire with oxygen. Also the diameter and mass flow rate of the plume increase with elevation as a result of air entrainment. Air entrainment has the effect of increasing the mass flow of gas in the plume, but also dilutes smoke concentration and lowers the smoke temperature. The heating mechanisms from an open fire are the radiation from the flames that transport energy in rays going in all directions originated at the fire, but is of relatively short range, and the heat convection of the hot smoke can travel significant distances. Heat transfer by conduction is important only very close to the flame rim on the surface of the fuel. Question 6: emissivity of opaque surface and grey body Bodies like plant, boxes and animal commodities are opaque and do not transmit incident irradiation. For opaque materials, transmissivity is equal to zero: τ = 0, and thus α + ρ = 1 where α is absorptivity and ρ is the reflectivity. When the surface of an opaque body is a perfect reflector from which all irradiation is reflected, ρ = 1 and the transmissivity and absorptivity is zero. The emissivity (ε) of a surface is the ratio of the energy emitted by a real surface to that of an equally sized and shaped blackbody emitting at the same temperature. At a given temperature, a blackbody emits the maximum possible radiation and thus the emissivity of a surface is always between zero and unity. Grey-bodies are surfaces that have monochromatic emissivities and absorptivities whose values are independent of wavelength. A material that attracts all incident radiation is known as a blackbody. The emission given off by a blackbody is only dependent on its temperature alone. For bodies at constant temperature, the emittance (absorption), the reflection and the transmittance of energy equals unity (Burg, 2004 p. 269). Question 7: the radiating gases produced in combustion, mean beam length and the use of emissivity charts The main products of combustion in building fires are the toxic gases produced, that is carbon monoxide and carbon dioxide. As the air supply to the fire becomes limited, the amount of carbon monoxide produced increases. Carbon monoxide has a density that is close to that of air and thus spreads much easily in air of the building in fire. It has no odor, hence making detection difficult. A concentration of 1.3 % is enough to cause unconsciousness after a few minutes. Carbon dioxide on the other hand is toxic but acts as asphyxiant in building fires. Unconsciousness is caused by only 9% of the concentration of carbon dioxide. Combustion of materials like rubber and plastics can produce more toxic gases such as hydrogen chloride, sulfur dioxide, phosgene and hydrogen cyanide (Mannan and Lees, 2005 p. 16-286). The mean beam length L is the radius of a gas hemisphere which will radiate to unit area at the center of its base the same as the average radiation over the area of interest from the actual gas mass (Mannan and Lees, 2005 p. 16-167). Emissivity charts are used to show correlations where total emissivity can be read directly instead of having to through the algebra of the wide band correlation and equations. Question8: effect of ventilation on the composition of smoke using equivalence ratio Fire has several stages ranging from smouldering combustion and early well-ventilated flaming to a fully developed under-ventilated flaming. Ventilation normally allows the mixing of fuel with oxygen. If there is good ventilation, fuel will mix properly with oxygen thus producing total combustion. If the amount of oxygen is equal to the amount of fuel, a stoichiometric condition is established with an equivalence value of one. The equivalence ratio is higher when the amount of air is less than stoichiometric and it is lower when the amount of air is more than the stoichiometric. In the case of an over-ventilated flame, excess air is available in the surrounding for complete combustion of fuel issued from the burner port. Consequently, the flame boundary converges towards the axis and presumes the shape of a closed conical flame. For an under-ventilated flame, the air is not sufficient for complete combustion, thus the flame surface expands towards outer wall forming an inverted conical flame (Hull and Kandola, 2009 p. 406). Question9: absorption, scattering and extinction of light Both absorption and scattering remove energy from a beam of light traversing the medium resulting to attenuation of the beam. This attenuation is called extinction and can be seen when we look directly at the light source. For instance, sun is fainter as well as redder at sunset than at noon. This is an indication of extinction in the long air path. Absorption is the conversion of energy of electromagnetic field into other forms like heat in a substance and coherent light scattering. Extinction = absorption + scattering. The extinguishment of a light beam over a given distance (1m), referred to as the extinction coefficient, is the optical density per meter. People become disoriented when this value exceeds approximately 0.30. Question 10: main sources of heat release and heat loss in a typical compartment fire The main source of heat loss is the effluent gases. It is also seen that as the area of the windows decreases, the percentage of heat transmitted through the walls of a standard concrete or brick compartment increases. The total temperature reached in a compartment is a function of the total internal area of the compartment. For highly ventilated areas, the rate of heat loss is greatest but this result to heat loss through windows, thus reducing temperatures. For lowly ventilated areas, there is lower heat loss but the rate of heat loss is also low thus giving rise to low temperatures (Purkiss, 1996 p. 56). Top of Form Bottom of Form References: Beeson, H. D., Stewart, W, F. and Woods, S. 1999. Safe use of oxygen and oxygen systems: guidelines for oxygen system design, materials selection, operations, storage, and transportation. PA, ASTM International. From http://books.google.com/books?id=t2WQFx4iYR8C&pg=PA46&dq=thermal+explosion+in+a+vessel+with+cold+walls&hl=en&ei=ZBg_TPKzN5GUjAe55LmfBw&sa=X&oi=book_result&ct=result&resnum=5&ved=0CDsQ6AEwBA#v=onepage&q&f=true (accessed July 15, 2010) Burg, S. P. 2004. Postharvest physiology and hypobaric storage of fresh produce. UK, CABI. From http://books.google.com/books?id=Q4C0Pxm3RvkC&pg=PA269&dq=emissivity+of+opaque+surface+and+grey+body&hl=en&ei=8vQ_TNCxFoTu0wT8zLShDw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCgQ6AEwAA#v=onepage&q&f=false (accessed July 16, 2010) Furness, A. and Muckett, M. 2007. Introduction to fire safety management. MA, Elsevier. From http://books.google.com/books?id=hbuJk7xjw1MC&pg=PA127&dq=Define+explosion,+deflagration+and+detonation&hl=en&ei=XUQ_TLHHMtKd4QaOxcTwCg&sa=X&oi=book_result&ct=result&resnum=3&ved=0CC8Q6AEwAg#v=onepage&q=Define%20explosion%2C%20deflagration%20and%20detonation&f=false (accessed July 15, 2010) Hull, T. R. and Kandola, B. K. 2009. Fire Retardancy of polymers: new strategies and mechanisms. Royal Society of Chemistry, 2009. From http://books.google.com/books?id=U02HtkqSCLYC&pg=PA406&dq=effect+of+ventilation+in+the+composition+of+smoke+using+equivalence+ratio&hl=en&ei=7ghATLH8Ncbb4Ab19PXcDg&sa=X&oi=book_result&ct=result&resnum=2&ved=0CC4Q6AEwAQ#v=onepage&q&f=false (accessed July 16, 2010) Johnson, E. A. and Miyanishi, K. 2001. Forest fires: behavior and ecological effects. Florida, Academic Press. From http://books.google.com/books?id=MXa8npbbahQC&pg=PA23&dq=Froude+number+for+different+types+of+diffusion+flames&hl=en&ei=61w_TOuiEIi24AaFzYWDCw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCoQ6AEwAA#v=onepage&q=Froude%20number%20for%20different%20types%20of%20diffusion%20flames&f=false (accessed July 15, 2010) Mannan, S. and Lees, F. P. 2005. Lee's loss prevention in the process industries: hazard identification, assessment, and control, Volume 1. MA, Elsevier. From http://books.google.com/books?id=UDAwZQO8ZGUC&pg=SA16-PA286&dq=the+radiating+gases+produced+in+combustion&hl=en&ei=hv4_TNe8DIye4Ab43PzYDg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCgQ6AEwAA#v=onepage&q=the%20radiating%20gases%20produced%20in%20combustion&f=false (accessed July 16, 2010) Marshall, V. C and Ruhemann, S. 2001. Fundamentals of process safety. Warwickshire, IChemE. From http://books.google.com/books?id=Sn4f_N_2rsMC&pg=PA92&dq=Define+explosion,+deflagration+and+detonation&hl=en&ei=XUQ_TLHHMtKd4QaOxcTwCg&sa=X&oi=book_result&ct=result&resnum=7&ved=0CEMQ6AEwBg#v=onepage&q&f=false (accessed July 15, 2010) Mishra. 2008. Fundamentals Of Combustion. New Delhi, PHI Learning Pvt. Ltd. From http://books.google.com/books?id=dnvZXCg5C0oC&pg=PA164&dq=types+of+diffusion+flames&hl=en&ei=WVs_TOvlOMb94AbMq83OCg&sa=X&oi=book_result&ct=result&resnum=7&ved=0CEgQ6AEwBg#v=onepage&q=types%20of%20diffusion%20flames&f=false (accessed July 15, 2010) Purkiss J. A. 1996. Fire safety engineering design of structures. MA, Elsevier. From http://books.google.com/books?id=65K2jXjm4WAC&pg=PA56&dq=sources+of+heat+release+and+heat+loss+in+a+fire+compatment&hl=en&ei=pB9ATM3XKt-W4gaTzNG2Dg&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDQQ6AEwAw#v=onepage&q&f=false (accessed July 16, 2010) Stoessel F. 2008. Thermal safety of chemical processes: risk assessment and process design. Switzerland, Wiley-VCH. From http://books.google.com/books?id=q4VkK7gLsI4C&pg=PA50&dq=Semenov+diagrams&hl=en&ei=5yI_TPqQGpmN4gaf5Ii6Cg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCgQ6AEwAA#v=onepage&q=Semenov%20diagrams&f=true (accessed July 15, 2010) Wilkie, C. A. and Morgan, A. B. 2009. Fire Retardancy of Polymeric Materials. NW, CRC Press. From http://books.google.com/books?id=eEqySGuTvgIC&pg=PA48&dq=the+burning+rate+of+solids+and+mechanism+of+flame+spread+over+solid+surfaces&as_brr=3&client=firefox-a&cd=1#v=onepage&q&f=false (accessed July 16, 2010) Read More