Fire and Explosion Investigations - Assignment Example

1. Matter. International Systems of units (SI) Exercises: 1. reduction of the following dimension to its simplest form: (Power/Pressure)1/3 Lets say X = (Power/Pressure)1/3 log X = Log (Power/Pressure)1/3 log X = 1/3 log (Power/Pressure) X = 10 * [ 1/3 log (Power/Pressure) ] 2. Conversion in the SI units of the following values: a) 5cm/microsecond 5 cm = (5 * 10 )mm = 500 mm and 1 microsecond = 0.000001 seconds So 5cm/microsecond = 500 mm / 0.000001 second = 5 * 108 mm/seconds b) 0.36 * 10 - 10 tons km/min2 1 tonne = 1000kg = 1000000 g 1 km = 1000 meters 1min2 = (60 seconds) 2 = 3600 seconds [ (0.36 * 10 – 10) * 1000000 * 1000 ] / 3600 = [(3.6 * 10 – 11) * 109] / 3600 =(3.6 * 10 – 2) / 3.6 *10 – 3 = 1 * 10 – 5 grams meters /second 2. Chemical Elements and Compounds Theoretical question Free atoms are atoms that are not bound together and exist as a basic unit of nucleus and electrons. Free radicals are atoms, ions, or molecules that have unpaired electrons on the outer shell, thus making them unstable and reactive. The difference between an ion, free atom and radical is that an ion is either an atom or a molecule that has gained or lost one or more electrons, thus making it negatively or positively charged; while free atoms are whole atoms that are not bound together; a radical is an unstable ion, atom, or molecule; or they have reactive electrons at the outer shell. Exercise: explanation of the chemical bonds in the molecule of methane. A molecule of methane is composed of four hydrogen atoms and one carbon atom. The hydrogen atoms share electrons equally with the carbon atom forming covalent bonds between hydrogen and carbon. 3. States of Matter: Fluids, Solids and Gases Exercise: calculation of the vapour densities (kg/m3) of pure C5H12 at 25oC and 1 atm = 1.013 105 Pa (Assuming ideal gas behaviour). vapour density of C5H12 = mass of C5H12 / mass of atmospheric air Atomic weight of carbon = 12.011 and atomic weight of hydrogen = 1.00794 mass of C5H12 = (12.011* 5) + (1.00794+12) = 22.15028 vapour density of C5H12 =22.15028 / 1.00794 = 21.8638 4. Chemical Reactions and Their Rates Theoretical questions: 1. Definition of stoichiometric, fuel lean and fuel rich mixtures with an example. Stoichiometric, which is also known as theoretical combustion is a process of ideal combustion where fuel is completely burnt; that is all carbon is burned to carbon dioxide (CO2), all sulphur (S) to sulphur dioxide (SO2) , and all Hydrogen (H) is burned to water (H2O). Fuel lean mixtures are mixture whose content of air is higher than its Stoichiometric ratio, while fuel rich mixtures are those whose content of air is lower than its Stoichiometric ratio Example of a stoichiometric (theoretical) combustion of Methane: CH4 + 2 (O2 + 3.76N2) = CO2 + 2H2O + 7.52N2 2. What is the concentration and its units of measurements? What is a mole? Generally, concentration is the amount of a particular substance in another substance. However, it is applied in homogenous chemical solutions to mean the amount of solute that has dissolved in a given solvent. Its units of measurements are mole fraction, percentage mass composition, molarity, Molarity, and normality. A mole is the amount of a substance that has the same atoms, electrons, ions, molecule, other particles, or group of particles (or rather chemical units) as the number of atoms in 12 grams of carbon-12. 3. Explanation of temperature and concentration dependence of the chemical reaction rate and Arrhenius equation. The chemical reactions rate is dependent on concentration and temperature. Therefore, the rate of chemical reaction is directly proportional to concentration and temperature. That is an increase in temperature or concentration increases the rate of reaction. Nonetheless, at certain a temperature or concentration the rate of chemical reaction does change. Arrhenius equation is a formula that shows the dependence of a chemical reaction’s rate constant under certain temperature. That is, k= Ae –Ea/RT, where k is the rate constant, T is absolute temperature, Ea is activation energy, A is the pre-factor (or pre-exponential factor), and R is the gas constant. Exercise: Comparison of the chemical reaction rates at two temperatures T1 = 300 K and T2 = 600 K for activation energy E = 180 kJ/mole. The universal gas constant is 8.314 J/(mole K). k= Ae –Ea/RT K1 = 1/ A*e [-180kj/mole / [0.008314 kj / (mole K) * 300K] = A*e-72.16 K2 = 1 / A*e [- 180kj/mole / [0.008314 kj / (mole K) * 600K ] = A*e-36.08 K1 / K 2 = A*e-72.16 / A*e-36.08 = e / 72.16 * 36.08/e = 1/2 Therefore, the rate of chemical reaction at temperature T2 is faster than at temperature T1 5. Thermal Explosion Theoretical questions: Analysis of thermal explosion in adiabatic conditions and the mechanism of self accelerating reaction and definition of induction period. Thermal explosion features flame combustion occurring in a closed vessel where exponential rate of reaction increase and pressure explodes; additionally, there is no heat transfer. Self acceleration in a chemical reaction occurs when the rate of energy release exceed the rate by which heat transfer processes transport the energy to the container wall. Temperature in the system then rise, and in effect, reaction rate rises. This leads to more rise in temperature resulting ultimately to an explosion (Griffiths, Barnard and Bradley, p. 3). Induction period is the initial phase during a chemical reaction when the rate of reaction is slow but later accelerates (Law, 307). Exercises 1. Qualitative consideration of Semenov diagram for thermal explosion in a vessel with cold walls. Explain the effect of initial temperature and size of the vessel on the critical conditions for thermal explosion to be possible. Altering the wall temperature or rather the ambient temperature in an rising manor heat loss line goes further away from heat gain curves resulting in a bigger range in which thermal explosion would happen. The initial temperature increases causing pressure to increase violently such that it causes thermal ignition. The size of the container should not increase otherwise the pressure would decrease and divert an explosion occurence. 2. Calculate the induction period for adiabatic thermal explosion of flammable pyrotechnic mixture (polyvinilnitrate) upon various initial temperatures. Initial temperatures: T0 = 300K, T0 = 600K, T0 = 900K. Log (IP) = a – t/36 Where IP is induction period in hours, “a” is a constant and t is temperature a = 4.333 at 300K, log (IP) = 4.333 – (26.85 oC /36) = 4.333 – 0.746 = 3.587 therefore IP = 3954 hours at 600K, log (IP) = 4.333 – (326.85 oC /36) = 4.333 – 9.079 = - 4.746 therefore IP = 55720 hours at 900K, log (IP) = 4.333 – (626.85 oC /36) = 4.333 – 17.413 = -13.08 therefore IP = 1.202 * 1012 hours 6. Forms of Heat Transfer Exercise: calculation of the rate of heat transfer through a 0.4 squire meter of a plaster wall 3 cm thick. One side of the wall is at 600 0C, while other side is at 25 0C. Thermal conductivity of plaster is 0.5 W/m x 0C. Rate of heat transfer = ∆Q / ∆t = k * A* (∆T / x), where k is thermal conductivity, A is cross sectional area, ∆T is change in temperature, and x is thickness. ∆T = 600 0C - 25 0C = 575 0C ∆Q / ∆t = 0.5 W/m x 0C * 0.4m2 * (5750C / 0.03m) = 3,833.33 W 0C 7. Ignition Theoretical question 1. Description of the process of ignition of a solid combustible material by a hot plate and explanation evolution of the temperature field in the solid material. The process involve transfer of energy to the combustible material through conduction. The heat energy excites the atomic elements of the combustible material, and if it adequate, it causes a thermal ignition or explosion. 8. Premixed Flame Theoretical questions 1. Why flames propagate through a combustible mixture. Waves of exothermic chemical reactions may spread through a homogenous mixture either as subsonic deflagration or supersonic detonation. The detonation wave occur due to “rapid compression of the substance in a shockwave, which rapidly compresses and heats the substance so the reaction can proceed at a higher rate.” In addition, flame propagation may occur due to “molecular thermal conductivity and diffusion (Liberman, p. 89). 2. What are the flame front and flame propagation velocity? Why a gas particle entering flame front is accelerated? Flame front is the leading perimeter of a flame that is propagating via a mixture of gases or across the surface of a liquid or solid. Flame propagation velocity is the speed at which the flame is spreading or propagating. A gas particle entering flame front is accelerated because the temperature increases causing the particles to collide with others more rapidly. 3. What is adiabatic flame temperature? Is the temperature that is achieved in a reaction with constant pressure and without transfer of heat. Exercise 1. The angle of a premixed flame front cone stabilised on the Bunsen burner is 450. The combustible mixture velocity in the tube is 2 m/s. What is the flame propagation velocity? 9. Detonation Theoretical questions 1. Comparison of the main features of premixed flame and detonation. Unlike a premixed flame (deflagration) that is subsonic, a detonation is a supersonic combustion wave; meaning that its propagation velocity is higher than that of sound in front of the wave. Moreover, detonation may propagate at high speed even without obstruction or confinement unlike a premixed flame; detonation propagate by shock compression. 2. Description of the internal structure of detonation wave. A detonation is a three dimensional shock wave preceding a zone of reaction. The preceding shock wave comprise of “curved shock segments”. However, between the shock wave segments, at the detachment line, the wave interact through a “Mach stem configuration” (Wingerden, Bjerketvedt and Bakke, p. 2). Numerical exercise: calculation of the velocity of steady-state freely propagating strong detonation if the ratio of specific heat capacities is 1.26 and heat of combustion 750 kJ/kg. Velocity = 1.26 * 750 kJ/kg = 945 kJ/kg 10. Diffusion combustion. Fire Plume Theoretical questions 1. Comparison of a jet fire and buoyancy dominated fire, using the flame height-jet velocity diagram to explain flame shape. 2. Explanation of the low value of the Froude number for natural fires and description of fluid-dynamic structure (air entrainment, buoyant flow, eddies) of a fire plume. Low Froude number means that the rate of fire spread is low; the natural fire does not spread very fast. 11. Combustible Liquids and Solids Theoretical questions 1. Description of the flash point, fire point and auto-ignition temperature for combustible liquids and how these characteristic are measured in laboratories. The flash point of combustible liquids is at or above 37.8oC or 100oF. This represents class II and III of National Fire Protection Association classification. Class II have flash points at or above 37.8oC (100 oF) but below 60.0oC (140 oF). Class IIIA have flash point at or above 60.0o C (140oF) but below 93.4 oC (200 oF). Class IIIB have point at or above 93.4 oC (200 oF). Fire point for most combustible liquids is “only a few degrees above the flash point” (Cheremisinoff, p.185). The auto-ignition of most of combustible liquids is between 300oC (572oF) and 550oC (1022oF). Nonetheless, some liquids have auto-ignition temperature that are low as 160 auto-ignition 160oC (356oF) (Canadian Centre for Occupational Health and Safety). Flashpoint is measured through the open cup or closed cup method. Open cup method is characterized by placing the sample in an open cup that is then heated and the flame is taken over the surface of the cup. In closed cup method the cups are sealed with a lid where source of ignition may be introduced. Fire point is determined through the same methods use for measuring flash points. Auto-ignition temperature are usually measured by use of a 500 ml flask placed in an oven with controlled temperature. Small sample portions are then squirted inside the flask at different temperatures until auto-ignition of the vapour. 2. Meaning of BLEVE and explanation of possible effects of accidental liquid fuel releases on the surrounding. BLEVE refers to boiling liquid expanding vapour explosion. It is a kind of explosion that results from rupture of a container with pressurized liquid. Accidental liquid fuel release may result in an explosion. In situation where a container with pressurized liquid rupture, for instance, the vapour release may be rapid hence lowering the pressure in the container. In effect, this causes the liquid in the vessel to boil violently and then to liberate very huge amounts of vapour. The high vapour pressure cause substantial overpressure waves that can cause destruction to the surrounding. 3. Discussion of the main factors influencing flame spread over solid materials. Flame spread is influenced by the fuel and the oxidant. Availability of the oxidant which is usually oxygen cause flame to spread faster while deficiency slows down the spread. The more combustible the fuel is the faster the spread of the flame. Numerical exercise: calculation of the average flame height for a pool gasoline fire where diameter of pool is 4m. Flame height (Hf) = 0.235 Q2/5 – 1.02D Where Q is heat release rate of pool gasoline fire in kw, D is diameter of pool fire in m. Assuming heat release rate of gasoline is 5 * 103 kw, Hf = [0.235 * (5 * 103) 2/5] – [1.02 * 4] Log [5 * 103) 2/5 ] = 2/5 log 5000 = 2/5 * 3.69897 = 1.4796 Antilog 1.4796 = 30.13 Hf = 0.235 * 30.13 – 4.08 = 3.0006 12. Fire as a Combustion System Theoretical questions 1. Define heat of combustion, heat release rate and combustion efficiency. Heat of combustion is the amount of energy that is released as heat when a mole of a substance goes through a complete oxygen combustion. Heat release rate is the rate at which fire generates heat. Combustion efficiency is a measure of how well the conversion a particular fuel into useable heat energy occurs at a particular time period by a heating equipment. 2. Description of the three zones in turbulent diffusion flame (fire plume). In turbulent diffusion flames three zones are apparent: the inner zone contains gas that is unburned; the reducing or the reduction zone, which is at the middle of the flame, is characterized by deficiency of oxygen; while the oxidizing zone or the outer zone there is adequate oxygen. 3. Why thermal radiation is of importance in fire. Thermal radiation is of importance in fire because it contributes to spread of fire or ignition of fire in areas that are not on fire. moreover, thermal may be cause burning of things including people even when they are not in contact with the fire flame. Exercises 1. Estimation of the gas velocity in the fire plume. Gas velocity (u0) = (2/Π) ve, Where ve is in meter per seconds (m/s) and is “the velocity of the fire plume front at the end of the rise phase” (Gheorghe, p. 183). 2. Estimation of the Froude number in fire plume. Froude number (Fr) = V / √gD, where V is velocity, g is gravity, and D is cross sectional area divided by top width. 3. Calculation of the heat release rate for PMMA (m''=0.035 kg/m2 s, heat of combustion 23.0 MJ/kg, combustion efficiency 0.6. Heat release rate (q') = Hc * m'" * A, where Hc is heat of combustion, m" is rate of mass loss per unit area, and A is the area. q' = 23.0 MJ/kg * 0.035 kg/m2 s * 1 = 0.105MJ/s ± 0.6 13. Fire in Enclosures Theoretical questions 1. What is a positive thermal feedback for fires in enclosures? What is flashover and backdraft? Thermal feedback refers to radiation of fire energy back to the room contents from the sides (walls, ceiling and floor) of the enclosure. Flashover is “a transition phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperature more or less simultaneously and fire spreads rapidly throughout the space, resulting in full room involvement or total involvement of the compartment or enclosed space” (National Fire Protection Association). Backdraft is the situation where a fire has insufficient oxygen thereby combustion ceasing although the temperature of smoke and fuel gases remain high. 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. All the exposed surfaces must ignite; that is “all the combustible surfaces existing in the lower layer of the enclosed space and exposed to the upper layer radiant flux must become ignited” (Kennedy and Kennedy, p. 24). The minimum required heat release is 400oF. 3. Explain fuel-controlled and oxygen-controlled regimes of fire in an enclosure. Fuel-controlled regimes of fire in an enclosures are those where the growth of the fire is determines by fuel since there is adequate oxygen. Oxygen-controlled do not have sufficient oxygen and oxygen or ventilation determines fire growth (Klaene and Sanders, p. 196; Karlsson and Quintiere, p. 18). 4. Describe the main flow patterns associated with fire development in enclosures. Fire in enclosures begin with an item being ignited. If the energy release rate is adequate, fire spread to other fuel before influencing the environment of the enclosure. 14. Fire in Enclosures. Fire Modelling Theoretical questions 1. What are advantages and limitations of zone models? The advantages include few number of control zones to solve the energy and mass equations, hence less time consumption and less cost as compared to CFD models, and there is availability of user-friendly computer programs for these models. The limitations include less accuracy especially for complex geometries, and because of the numerous assumptions that are made, and there is a requirement of the user to have some training in dynamics of enclosure fire prior to using the model. 2. What is field modelling of fires? What are objectives of CFD fire modelling? Field modelling of fires refers to use of computational fluid dynamic (CFD) models to simulate compartment fires. The objectives of CFD fire modelling are to split the area of interest into several small and discrete three-dimensional cubes and to solve for every control volume the basic equations governing the conservation of mass, temperature, density, species, and velocity. The modelling also aims to yield the Navier-Stokes equation (Quintiere and Karlsson, p. 3). 3. Description of verification and validation for field modelling and why validation is necessary. Validation is scientific and entails comprehensive measurement of somewhat simple flow. Boundary conditions are completely detailed and the modeller tries “to reproduce the flow” out of these boundary conditions. Verification, on the other hand involves model application to full-scale tests. However, these tests lack the essential information for specification of the primary and “boundary condition with complete accuracy and detail ( Carvel and Beard, p. 275). Validation is important to help in knowing whether the results of a model lead to design decisions that are acceptable. Since model check-lists do not reveal if a model gives the correct answers or if results mirror real flow behaviour, validation becomes vital to create “confidence in the ability to predict flows” through use of a model in fire scenarios for which there is data for comparison ( Carvel and Beard, p. 274). Works cited Canadian Centre for Occupational Health and Safety, Flammable and combustible liquids - Hazards, 1997, April 28, 2009, Carvel, Richard and Beard Alan, The handbook of tunnel fire safety, Thomas Telford, United Kingdom, 2005. Cheremisinoff, Nicholas P. , Handbook of industrial toxicology and hazardous materials, CRC Press, 1999 Ernest Orlando Lawrence Berkely National Library, Control procedures for flammable and combustible liquids, Chemical Hygiene and Safety Plan, 2008, April 29, 2009, . Gheorghe, Adrian V., Integrated risk and vulnerability management assisted by decision support systems: relevance and impact on governance, Springer, 2005. Griffiths, J. F., Barnard, J. A. and BradleyJohn N., Flame and combustion, CRC Press, 1995 Karlsson, Björn and Quintiere James G., Enclosure fire dynamics, CRC Press, United Kingdom, 2000. Kennedy, Patrick M. And Kennedy, Kathryn C., Flashover and fire analysis - a discussion of the practical use of flashover analysis in fire, Fire and Explosion Analysis Experts, Sarasota, Florida, 2003, April 28, 2009, . Klaene, Bernard, Sanders, Russell E. and National Fire Protection Association, Structural fire-fighting: strategies and tactics, (edn2), Jones & Bartlett Publishers, United Kingdom, 2007. Knox, Joseph, Physico-Chemical Calculations, Read Book, United States, 2008. Liberman, Michael A., Introduction to Physics and Chemistry of Combustion: Explosion, Flame, Detonation, Springer, Australia, 2008. Morris, Ting, Rosewarne, Graham, Mole, Black Rabbit Books, New York, 2005. National Fire Protection Association, NFPA 921 – Guide for Fire and Explosion Investigations, NFPA, Quincy, MA, 2004. Quintiere, James G., Fundamentals of fire phenomena, John Wiley, Michigan, 2006. Wingerden, Kees V., Bjerketvedt, Dag and Bakke, Jan R., Detonations in pipes and in the open, Christian Michelsen Research, Bergen, Norway. Read More

Thermal Explosion Theoretical questions: Analysis of thermal explosion in adiabatic conditions and the mechanism of self accelerating reaction and definition of induction period. Thermal explosion features flame combustion occurring in a closed vessel where exponential rate of reaction increase and pressure explodes; additionally, there is no heat transfer. Self acceleration in a chemical reaction occurs when the rate of energy release exceed the rate by which heat transfer processes transport the energy to the container wall.

Temperature in the system then rise, and in effect, reaction rate rises. This leads to more rise in temperature resulting ultimately to an explosion (Griffiths, Barnard and Bradley, p. 3). Induction period is the initial phase during a chemical reaction when the rate of reaction is slow but later accelerates (Law, 307). Exercises 1. Qualitative consideration of Semenov diagram for thermal explosion in a vessel with cold walls. Explain the effect of initial temperature and size of the vessel on the critical conditions for thermal explosion to be possible.

Altering the wall temperature or rather the ambient temperature in an rising manor heat loss line goes further away from heat gain curves resulting in a bigger range in which thermal explosion would happen. The initial temperature increases causing pressure to increase violently such that it causes thermal ignition. The size of the container should not increase otherwise the pressure would decrease and divert an explosion occurence. 2. Calculate the induction period for adiabatic thermal explosion of flammable pyrotechnic mixture (polyvinilnitrate) upon various initial temperatures.

Initial temperatures: T0 = 300K, T0 = 600K, T0 = 900K. Log (IP) = a – t/36 Where IP is induction period in hours, “a” is a constant and t is temperature a = 4.333 at 300K, log (IP) = 4.333 – (26.85 oC /36) = 4.333 – 0.746 = 3.587 therefore IP = 3954 hours at 600K, log (IP) = 4.333 – (326.85 oC /36) = 4.333 – 9.079 = - 4.746 therefore IP = 55720 hours at 900K, log (IP) = 4.333 – (626.85 oC /36) = 4.333 – 17.413 = -13.08 therefore IP = 1.202 * 1012 hours 6. Forms of Heat Transfer Exercise: calculation of the rate of heat transfer through a 0.

4 squire meter of a plaster wall 3 cm thick. One side of the wall is at 600 0C, while other side is at 25 0C. Thermal conductivity of plaster is 0.5 W/m x 0C. Rate of heat transfer = ∆Q / ∆t = k * A* (∆T / x), where k is thermal conductivity, A is cross sectional area, ∆T is change in temperature, and x is thickness. ∆T = 600 0C - 25 0C = 575 0C ∆Q / ∆t = 0.5 W/m x 0C * 0.4m2 * (5750C / 0.03m) = 3,833.33 W 0C 7. Ignition Theoretical question 1. Description of the process of ignition of a solid combustible material by a hot plate and explanation evolution of the temperature field in the solid material.

The process involve transfer of energy to the combustible material through conduction. The heat energy excites the atomic elements of the combustible material, and if it adequate, it causes a thermal ignition or explosion. 8. Premixed Flame Theoretical questions 1. Why flames propagate through a combustible mixture. Waves of exothermic chemical reactions may spread through a homogenous mixture either as subsonic deflagration or supersonic detonation. The detonation wave occur due to “rapid compression of the substance in a shockwave, which rapidly compresses and heats the substance so the reaction can proceed at a higher rate.

” In addition, flame propagation may occur due to “molecular thermal conductivity and diffusion (Liberman, p. 89). 2. What are the flame front and flame propagation velocity? Why a gas particle entering flame front is accelerated? Flame front is the leading perimeter of a flame that is propagating via a mixture of gases or across the surface of a liquid or solid. Flame propagation velocity is the speed at which the flame is spreading or propagating. A gas particle entering flame front is accelerated because the temperature increases causing the particles to collide with others more rapidly. 3.

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