Introduction to Physics and Chemistry of Combustion: Explosion, Flame, Detonation - Assignment Example

T1. Analyse thermal explosion in a vessel with cold walls, including the mechanism of self-accelerating reaction and nature of induction period? Thermal explosion is the rapid or sudden inflammation of uniformly heated materials. In order for thermal explosion to occur, two conditions must be met. First, the temperature of the reacting medium must rise above the ambient temperature past the critical temperature (To) defined by the Semenov Diagram during self heating (Glassman, 1996). This rise of the temperature of the reacting medium relative to the ambient temperature is due to the self accelerating reaction of the medium. Second, this increase in temperature must occur faster than the heat transfer between the reacting medium and its surroundings as a result of the consumption of reactants; otherwise, no explosion could occur. When the thermal equilibrium between the system and its environment is disrupted, the reaction produces an explosion after a given time, called the induction period. Self accelerating reaction occurs because of the formation of active particles which may result from two things: first, from the thermal motion of these particles and second from the branching of the chain reaction due to the excess energy (Liberman, 2008). When the rate of chain branching exceeds the rate of chain termination, an explosion occurs. Induction period is the time it takes for the reacting substances to explode from the time they react. It is typically seen as the slow phase of chemical reaction of reacting substances (Ayeni, 1982). More precisely, it is the time it takes for the buildup of active particles to reach measureable values for them to continue rising without limit (Liberman, 2008). 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. There are three critical conditions for thermal explosion determined by the Semenov diagram. First, the heat production rate would exceed the heat loss rate in which explosion occurs after some time. If the heat loss exceeds heat production after a given induction period, the temperature rises to a peak and then gradually decays as the heat production and speed of reaction is slowed by the consumption of reactants. This is the mechanism of the second type of self-accelerating reaction. The last case of self accelerating reaction is the zero order reaction which occurs when the final temperature of the reactants is in equilibrium with the ambient temperature and stabilizes over the period of time; hence no explosion occurs (Ficket & Davis, 1979; Glassman, 1996). For an explosion to occur in a vessel with cold walls, the heat transfer between the reacting medium and its surrounding must not reach thermal equilibrium, as seen in the Semenov Diagram’s first critical condition (Semenov, 1959). That is, the temperature of the reacting medium must be higher than the temperature of the walls of the cold vessel. This suggests that the heat transfer of the reacting medium and its environment must be faster than the heat transfer between the environment and the reacting medium in order to obtain a temperature higher than the critical temperature and thus obtain an explosion. Pre-explosion heating is the maximum thermal condition experienced by the reacting medium before explosion occurs. The temperature of the system is dictated by the difference in the rate of formation of active particles and the rate of chain termination (Liberman, 2008). T3. Define explosion, deflagration and detonation. Explain the development and main features of detonation. Thermal explosion is the rapid buildup and sudden, often violent release of energy in systems which are subject to exothermic reactions (Strehlow, 1984). This is usually accompanied by the release of gases at high temperature. The rate of this buildup and release is dependent upon pressure and temperature. Thermal explosion can be deflagration or detonation. Deflagration is the extremely rapid burning of materials. The oxidation process occurring in deflagration involves extremely sudden evolution of flame and vapor (Glassman, 1996). Deflagration is much faster than normal combustion but is considerably slow compared to detonation. Detonation on the other hand is the rapid exothermic conversion of solid to gas, usually accompanied by a shockwave and a reaction zone trailing behind it. The propagation of the energy front in detonation is typically faster than the propagation speed of sound in the medium (thus the supersonic shockwave) whereas the propagation of energy front in deflagration is slower than the propagation speed of sound in the medium. The detonation of material is dependent on the value of its Maximum Experimental Safe Gap (MESG). The lower the MESG value of a material (like gas), the more likely it is to detonate (Bull, 1979). Steady detonations are characterized by pressure over time history. During the onset of detonation, a rapid increase in pressure (called the Von Neumann spike) is observed which is followed by the smooth expansion (of pressure) until it approaches a constant value. This leading shock experienced by the detonating material usually has a higher pressure compared to the overall pressure of detonation. The initial rise in pressure approaches a maximum which is very close to stoichiometric concentration which decreases as the detonation reaches its limit. 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. Diffusion combustion results from the combination of fuel and oxidizing agent through diffusion. Initially, the fuel and the oxidizing agent are separate (as opposed to premixed flame. In a diffusion combustion, combustion takes place when the stoichiometric conditions are met, often leading to high-temperature combustion (Simmons & Wolfhard, 1957). Typical characteristics for diffusion combustion is slow burning process that produces comparably more soot than premised flames due to the insufficient amount of oxidizer for complete combustion to occur. There are two types of diffusion flames; these are momentum-driven flame and buoyancy-dominated flame. Buoyancy-dominated flames occur when the hot gas is surrounded by cooler gas in a flame and the temperature gradient between the two masses forces the hotter mass of gas upward due to buoyancy. The buoyancy force determines the upward velocity of the flame especially if the speed of injection of fuel is not particularly high. Momentum-driven flames on the other hand are flames that propagate when the injection rate of the fuel is sufficiently high pressured to produce a jet. The Froude number in flame dynamics is related to the mean flame height. The Froude number is the ratio between the flow velocity and the acceleration due to gravity multiplied by the diameter of the flow source. Buoyancy-dominated flames have smaller Froude numbers compared to momentum-driven flames due to their respective flow velocities (Delichatsios, 1992). T5. Analyse the burning rate of solids and mechanism of flame spread over solid surfaces. The burning rate of solid is the rate by which a certain mass of solid is burned or vaporized (Glassman, 1996). The burning rate of any solid substance depends on other factors such as the energy release rate, combustion efficiency of the materials, and the pool diameter of the fuel. Though there are materials that exhibit a fairly constant burning rate, the burning rates of solid materials generally vary over time which typically undergo through three stages –ignition, growth, and decay (Simmons & Wolfhard, 1957). As the material is ignited, the burning rate increases until a maximum value of burning rate is achieved, then the burning rate decays until most of the combustible material is burnt. Flame spread is the growth of the perimeter of the fire after ignition. There are two general mechanisms that contribute to flame spread – wind and gravity. The natural flow, the buoyancy of the flame is governed by gravity alone and the growth of the flame across solid surfaces depends largely on the inherent characteristics of the fuel and the materials in the flame. In a forced flow, however, the presence of wind determines the propagation of the flame across the surface (opposed and wind-aided) (Glassman, 1996; Strehlow, 1984). The thickness of the solid material also contributes to the behavior of the flame spread since the heat transfer of flames can only penetrate certain thickness at any given time. In other words, flames take more time to spread on thicker solids than on thinner ones (Simmons & Wolfhard, 1957). 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. T7. Analyse the radiating gases produced in combustion, define the mean beam length and explain the use of emissivity charts. T8. Critically analyse the effect of ventilation on the composition of smoke using equivalence ratio. Compare over ventilated and under ventilated combustion. 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. 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. References Ayeni, R.O., (1982). On the explosion of Chain thermal reactions. Journal of Australian Mathematics Society. (Series B),24: 194-202. Bull, D. (1979). “Concentration limits to the initiation of unconfined detonations in fuel/air mixtures”. Chem E 57:219-227 Delichatsios, M. (1992). Transition from momentum to buoyancy-controlled turbulent jet diffusion flames and flame height relationships. Combustion and Flame. 92(4). 349-364 Fickett W, Davis WC (1979) Detonation. University of California Press. Glassman I (1996) Combustion. 3rd edition, Academic Press. Liberman, M. (2008). Introduction to Physics and Chemistry of Combustion: Explosion, Flame, Detonation. Springer-Verlag Berlin Heidelberg. Quintere, J. (1997). Principles of Fire Behavior. Delmar Publishers. Semenov, N.N., (1959). Some Problems in Chemical Kinetics and Reactivity Part 1 and 2, Pergamon Press, London. Simmons, R. & Wolfhard, H. (1957). Combustion. Flame 1, 155-161 Strehlow RA (1984) Combustion fundamentals. McGraw-Hill. Read More