Introduction to Combustion and Fire - Assignment Example

1 Matter International System of Units 1 - Reduce the following dimension to its simplest form: (Power/Pressure)1/3 Solution: Power = Work per unit time = W/t = Fd/t Pressure = Force per unit Area = F/d2 Power/Pressure = (Fd/t)/(F/d2) = ( Fd/t ) ( d2/F ) = d3/t (Power/Pressure)1/3 = (d3/t) 1/3 = d/(t) 1/3 2 – a) 5 cm / microsecond = b) 0.36 x 10-10 tons km/min2 = 2 Chemical Elements and Compounds 1 – Atoms are composed of negatively charged electrons, positively charged protons, and neutrally charged neutrons. It is the basic component of matter and it cannot be broken down by any chemical process. Radicals are molecular fragments that have free electrons which makes it highly reactive when in contact with other molecules. Ions on the other hand are charged particles that are formed when molecules that are bonded covalently loses or gains electrons. The excess or lack of electrons determines the charge of ions. The main difference among the atoms, radicals, and ions are their electrical charges. Atoms are electrically neutral whereas electrons are electrically charged. Free radicals on the other hand have free electrons but they are not necessarily electrically charged. 2 – The molecules of Methane (CH4) is covalently bonded using the induced dipole to dipole attraction or the London dispersion force. This force temporarily attracts electrons that occupy positions that allow the formation of temporary dipoles in atoms. 3 – States of Matter: Fluids, Solids, and Gases Solution: From PV = nRT, where n = mass/molar mass, 4 – Chemical Reaction and their Rates 1 – Stoichiometric mixture is the air-fuel ratio that occurs when fuel is combined with free oxygen and the resulting mixture is chemically balanced. The stoichiometric air-to-fuel ratio is approximately 14.7 to 1. Any other mixture whose air-to-fuel ratio is lower than the stoichiometric ratio is called fuel lean mixtures while mixtures that are more than the stoichiometric ratio is called fuel rich mixture. 2 – Concentration is the measure of how much of a given substance is mixed with other substances to form a mixture. A unit of measurement called mole is introduced to account for the amount of substances present in a system with elemental quantities. Below are some of the units of measurement for concentration. Concentration unit Definition Unit of measurement Molarity Number of moles of solute per liter of solution Molar (M) Molality Number of moles of solute per kilogram of solution Molal (m) Parts per million One part of solute per 1 million parts of solution ppm Normality The number of equivalent weight of solute per liter of solution Normal (N) Mass percent Number of grams of solute per 100 grams of solution % w/w Volume percent Number of milliliters of solute per 100 milliliters of solution %v/v Mass/Volume percent Number of grams of solute per 100 milliliters of solution %w/v Mole fraction The ratio of the amount of a particular substance in the system to the amount of all the substances in the system χ Mole percent One hundred times the mole fraction %x 3 – The Arrhenius equation k= Ae-Ea/RT shows that the reaction rate of substances is exponentially proportional to Temperature T and on concentration of the substance A. The collision theory suggests that the rate of reacting substances depends on the frequency of collisions which in turn is governed by the concentration A of the substance (i.e., more number of this substance in a given volume increases the frequency of collision) and on the temperature of the system (i.e., more energy due to high temperature speeds up the frequency of collisions). This is consistent with the Arrhenius equation where it says that increasing the concentration of the substance as well as the temperature of the system increases the rate of chemical reaction. Exercise 1 Direct computation shows that K 300 = Ae-180k/(8.314)(300) = 3.9x10-17 A K 600 = Ae-180k/(8.314)(600) = 1.7x10-11 A This means that the rate of reaction given the substance with the activation energy equal to 180kJ/mol, raising the temperature from 300K to 600K increases its reaction rate by up to 1,000,000 times! 5 – Thermal Explosion Thermal explosion happens when an unstable material rapidly decomposes producing heat and light (Cote & Bugbee, 1978). Adiabatic thermal explosions are explosions that occur when there is no heat transferred to or released from the system. This mechanism occurs when the mixture ignites in a very short time (called the induction period) and the resulting heat loss of the involved mechanism is incomparable to the thermal energy of the reaction. The induction period is the phase of the chemical reaction that occurs before the occurrence of an accelerated reaction (Terao, 2007). When the fuel mixture of a system reaches the condition where the temperature is instantaneously increased to some temperature T, self accelerating reaction occurs. Self accelerating reactions are governed by the release of heat of the system into the environment and the rate of reaction contributes to the increase in the temperature of the system (Liberman, 2008). Exercises 1 – According to Semenov diagram, thermal explosion in a vessel with cold walls depends on the initial temperature of the system. The temperature gradient of the system crosses the graphical behavior of the heat of release of the system q in two points I and S. The closer these two points are, the more unstable the thermal equilibrium becomes. As the temperature is increased, the gap between S and I gets closer and for very high temperatures, this gap no longer exist and thermal explosion occurs (Stoessel, 2008). 2 – The equation for the induction period for adiabatic thermal explosion is given by Direct calculation shows that tadd300 = 2.63e-14s tadd600 = 9.58e-14s tadd900 = 6.14e-15s This means that the higher the temperature is, the more rapid the explosion becomes. 6 – Forms of Heat Transfer 1 – According to the first law of thermodynamics, heat transfer from a region of higher concentration to a region of lower concentration. This means that the system of interest will give off heat to the environment. The rate of heat transfer can be solved using the equation Solving directly yields q = (0.5)(0.4)(575/0.03) = - 3,833 W. The negative sign is important to indicate that heat goes out from the system. 7- Ignition The formation of a hot spot is a necessary element for ignition to occur. A hot plate generates that hot spot by localizing the electrical energy in one spot and converts this energy into the hot spot. A solid material placed on this hot spot in the hot plate would not combust until the heat balance in the hot spot becomes more condensed to allow the critical condition of ignition to occur. 8 – Premixed Flame 1 - Three elements must be present in order for combustion to occur – fuel supply, source of ignition, and the presence of oxygen. These elements are present in combustible mixtures. Adding these elements to the mechanism of flame propagation which are heat conduction and diffusion, flames propagate through combustible mixtures. When heat flows from the flame to the inner core, it heats up the unburned mixture until it ignites. This ignition combusts the flame in front of the propagating flame and the cycle repeats. 2 – In order for flames to propagate, an ignition in the flame front must occur. Flame front is the area in the flame where combustion occurs. It is governed by the flame propagation phenomenon. Flame propagation phenomenon is the release of energy in the reaction zone which is then supplied to the flame front. One characteristic of flame propagation is flame velocity. Flame velocity is proportional to the ignition probability in the fresh mixture and is expressed as combustion velocity (Terao, 2007). It is the velocity of the flame relative to the initial fuel mixture normal to the direction of the surface of the flame. Because of the presence of high temperature in this area, a gas particle entering the flame front is accelerated backwards to the inner core of the flame for complete combustion. 3 – Adiabatic flame temperature is the highest achievable temperature during combustion. It is the resultant temperature of the combustion that preserves work, heat, and energy transformations in the system. Exercise 1 – The flame propagation velocity is given by SL = v sin  = (2) sin (450O) = 2m/s 9 – Detonation 1 – The distinct feature that separates a premixed flame from detonation is the speed by which flames are generated. Premixed flames have speeds from several centimeters to several meters. Detonation on the other hand generates waves at supersonic speeds. 2 – Detonation waves are initiated by the shock of compression and heating and are sustained by the energy of the chemical reactions. Waves in detonations propagate through highly combustible medium moving with a very thin flame front. Exercise 1 – 10 – Diffusion Combustion: Fire Plume Jet fire and buoyancy fire depends on the velocity of the fuel injection. When the fuel flow rate is high, it produces turbulent flows. The turbulence of jet flames is accounted to the presence of stagnant recirculation zones (Venkatesh et al, 1996). When the low rate of the fuel is low, the flame that is produced is a buoyancy flame. The upward laminar flow of buoyancy flames is brought about by the difference in the density of the hot gases and the surrounding colder gases (Karlsson & Quintiere, 2000). 2 – Froude numbers measures the inertia and the buoyancy of the system. . The numerator of the system denotes the momentum while the denominator. Natural fires typically have no momentum involved because of the absence of the fuel injected in the fire plume so the Froude numbers are typically low. A buoyant plume is the column of flame where the product of combustion rises above the surface. Air entrainment is the result of the interaction between a rising hot flame and the introduction of cold air in the column. The area where the cold air meets the rising hot air produces eddy currents. 11 – Combustible Liquids and Solids 1 – The lowest temperature at which the mixture of air and flammable vapor exist at any given surface without combusting is called flashpoint. The lowest temperature T in an open cup where ignition occurs followed by sustained burning is called fire point (consider Cleveland Open Cup). Auto ignition temperature t is the phenomena where the mixture of air and vapor self ignites. Flashpoint, fire point, and auto-ignition are measured by slowly heating a combustible fluid and a pilot flame is introduced in the vapor space at defined intervals. 2 – Boiling liquid expanding vapor explosion (BLEVE) occurs when a pressurized vessel containing pressurized liquid ruptures. The characteristic explosion associated with BLEVE is usually very destructive. If the pressurized vessel contains toxic liquid or harmful chemicals, the damage may cover wide area. 3 – As with any material, the factors that influence the spread of flame on solid materials are source of fuel, source of ignition, and presence of oxygen to sustain combustion. The direction of the spread is governed by the buoyancy force and the direction of the wind. Wind spreads in solid materials are opposed flow flame and wind aided flame (Tien, 2002). Exercise 1 – 12 – Fire as a Combustion System 1 - Heat of combustion is the amount of energy that is released when one mol of a compound undergoes complete combustion with oxygen. Heat release rate is the amount of heat released by a pool of fire. Combustion efficiency is the measure of how well the heating equipment converts fuel into useable heat energy. 2 – The three zones of turbulent diffusion layer in a fire plume are the viscous sublayer which is the closest to the burner surface, the buffer layer where the flame is highest and turbulent region which is the region above the visible flame. 3 – Thermal radiation is important in understanding fire because of the associated cause or origin of thermal radiation in burning flames. Generally speaking, the soot part of the flame is the biggest source of radiation. The higher the amount of soot there is, the more radiant the heat of the fire. Just by looking at the amount of soot produced in a fire, firefighters can get a good estimate of the radiant heat of the fire they have to fight. Exercise 1 - Fire columns are characterized by high velocity flames that serves as a jet fuel. The longer the fire plume is, the higher the gas velocity becomes. Fire plumes that are wider have low gas velocities. 2 – When there is an injection of fuel, fire plumes grow higher to become jet fires. Naturally, the Froude number of such fire is typically large. 3 – 13 – Fire in enclosures 1 - Positive thermal feedback is the mechanism by which the fuel supply rate of fire is controlled. Flashover on the other hand is the rapid transition of fire to enclose the total surface of the combustible material within the enclosure. A back drought is a smoke explosion typical in fire (Korhonen, 2000). 2 – For flashover to occur at floor level, gas temperatures must reach at least 600 degree Celsius at the ceiling level and that the heat flux of radiation at floor level must exceed the minimum heat release of 20kW/m2. 3 – Fuel controlled fire regimes are the part of the fire whose continuity is defined by the amount of fuel present in the fire. Oxygen-controlled regimes are the parts of the fire that whose burning rate is controlled by the amount of oxygen that are present. 4 – The flow pattern of fire in enclosures is defined by the following stages: ignition (process that produces an exothermic reaction characterized by an increase in temperature above the ambient); growth (the fire may grow quickly or slowly, depending on the type of combustion, fuel, oxygen access and interaction with the environment); flashover (rapid transition to a total surface involvement in a fire of combustible material within an enclosure); fully developed fire (the stage where energy released is at its greatest and is limited by the availability of oxygen); and decay (the energy release rate diminishes and thus the average gas temperature in the compartment declines). 14 – Fire Modelling 1 – Zone models are computational analysis that analyzes the compartment by dividing it into smaller zones. Its main advantage is its ability to model the correct structure of the compartment. Its major disadvantage is the computational platform it employs which is less comprehensive when it comes to taking in various factors that influence the fire in the enclosure ” (Vidmar & Petelin, 2001).. 2 – Computational fluid dynamics (CFD) divide the domain into three-dimensional cells or control volumes and solve the equations for conservation of heat, mass, momentum, and species for each control volume (Vidmar & Petelin, 2001). 3 – Verification checks the comprehensiveness of the data involved in the analysis whereas validation ensures that the analysis performed is consistent althroughout (Rasbash, n.d). Validation is necessary in order to see that the analysis is done objectively and the result obtained is consistent with the previous results. References Catoire, L. et al. (2004). Journal of Physical and Chemical Reference Data, 33: 1083-1111. Cetegen, B.M. (1982). Entrainment and flame geometry of fire plumes. Dissertation Abstracts International Part B: Science and Engineering, 43, 4: 189. Cote, A.E. and Bugbee, P. (1988). Principles of fire protection (2nd ed). Jones & Bartlett Publishers. Accessed on March 31, 2009 from http://books.google.com/books?id=a8jU-2xyw5EC&pg=PA55&dq=thermal+explosion#PPA56,M1 Dunn V. Backdraft and flashover, what is the difference? A Newsletter, F.D.N.Y. Friendman, R. and National Fire Protection Association. (2008) Principles of fire protection chemistry and physics (3rd ed). Jones and Bartlett Publishers. Grimwood P. (2003). Flashover, a firefighter’s worst nightmare. Firetactics. Harwood,W.S.and Petrucci,R.H. (1997). General chemistry: Principles and modern applications (7th ed). NJ: Prentice Hall. Interfire Online. Term of the Week: Heat Release Rate (HRR). Accessed on March 28, 2009. Accessed at http://www.interfire.org/termoftheweek.asp?term=2116 Johnson, E.A. and Miyanishi, K. (2001). Forest fires: Behavior and ecological effects. Elsevier Science. Karlsson, B. and Quintiere, J.G. (2000). Enclosure fire dynamics. Florida: CRC Press. Accessed on March 31, 2009 from http://books.google.com/books?id=lpOhUpqrD38C&pg=PA48&dq=%22fire+plume%22&lr=#PPA49,M1 Korhonen, E. (2000). Natural fire modeling of large spaces. Helsinki University of Technology. Accessed on March 31, 2009 from http://www.tkk.fi/Units/Civil/Steel/Publications/TKK_TER_series/EKORH.pdf Liberman, M. A. (2003). Flame, detonation, explosion—when, where, and how they occur. [Plenary Lecture: 3rd Int. Disposal Conf., Sweden]. Accessed on March 29, 2009 from http://www.ep.liu.se/ecp/009/001/ecp030901.pdf Liberman, M.A. (2008). Introduction to physics and chemistry of combustion. Berlin: Springer-Verlag. Accessed on March 31, 2009 from http://books.google.com/books?id=1ftfDwy-mAYC&printsec=frontcover&dq=introduction+to+physics+and+chemistry+of+combustion#PPA73,M1 Rasbash, D. et al. (n.d.) Evaluation of fire safety. Wiley. Accessed on March 31, 2009 from http://books.google.com/books?id=6htxtHDr0m8C&pg=PA360&dq=%22field+modelling%22+%2B+validation#PPR9,M1 Stoessel, F. (2008). Thermal safety of chemical processes: Risk assessment and process design. Wiley. Accessed on March 31, 2009 from http://books.google.com/books?id=q4VkK7gLsI4C&pg=PA50&dq=Semenov+diagram#PPA51,M1 Terao, K. (2007). Irreversible phenomena: Ignitions, combustion and detonation waves. Springer. Accessed on March 31, 2009 from http://books.google.com/books?id=yhcxFj0MvdkC&pg=PA14&dq=induction+period&lr= T'ien, J. (2002). Detailed modeling of flame spread processes over solid: progress and prospect. NIST. Accessed on March 29, 2009 from http://www.hull.ac.uk/php/331346/publications/CFD%20Modelling%20of%20Combustion%20and%20Heat%20Transfer%20in%20Compartment%20Fires.pdf Unknown Author. What is combustion efficiency. Accessed on March 28, 2009. Accessed at http://www.habmigern2003.info/2_combustion-efficiency.html Venkatesh, S., Ito, A., Saito, K., and Wichman, I.S. (1996). Twenty-sixth international symposium on combustion. [Symposium: The Combustion Institute]. Accessed on March 29, 2009 from http://www.fire.nist.gov/bfrlpubs/fire96/PDF/f96100.pdf Vidmar, P. and Petelin, S. (2001). Fire models for enclosed and open spaces in transport systems. ICTS. Accessed from http://mirage.fpp.edu/~pvidmar/My%20study/Articles/ICTS%202001.pdf on March 30, 2009. Whitten, K.W., Davis, R.E., Peck, L. & Stanley, G.G. (2007). Chemistry (8th ed). Thomson Brooks/Cole. http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-A/atom.html accessed 28 March 2009 http://www.chem.purdue.edu/gchelp/liquids/disperse.html accessed 28 March 2009. Read More