Introduction to Combustion and Fire - Case Study Example

INTRODUCTION TO COMBUSTION AND FIRE Assignment work Matter. International system of units. Exercise Reduce the following dimension to its simplest form: (power/pressure)⅓ Dimensional analysis of a physical quantity involves expressing the quantity in terms of dimensional formulae M (mass), L (length), and T (time). Power: The rate at which energy is spent is known as power. The SI unit of power, Watt is the rate at which a joule (J) of energy is spent. A joule is the energy spent when a force of 1 Newton (N) pushes an object through a distance of 1 meter. A Newton is the force required to accelerate an object of 1 kg at the rate 1 ms-2 (Friedman 1998). 1 Watt = 1 J/s = 1 Nm/s = 1 kgms-2 m/s = 1 kgm2s-3 Pressure: Pressure is force per unit area and is measured in Pascal (Pa) (Friedman 1998). 1 Pa = 1N/m2 = 1 kgms-2/m2 = 1kgm-1s-2 In terms of M, L, and T, Physical Quantity Dimension Mass (kg) M Distance (m) L Time (s) T Power (Watt) ML2T-3 Pressure (Pa) ML-1T-2 Power/Pressure (ML2T-3)/( ML-1T-2) = L3T-1 (Power/Pressure)⅓ (L3T-1) ⅓ = L(T-1)1/3 = L/(T) ⅓ Distance/(Time) ⅓ L/T⅓ Therefore, the simplest form of (power/pressure)⅓ is L/(T) ⅓ or distance/(time) ⅓. Exercise 2. Convert in SI units a) 5cm/microsecond The SI unit of distance is m The SI unit of time is s 1cm = 10-2 m 1 microsecond = 10-6 s 1 cm/microsecond = (10-2 m/10-6 s) = 10(-2 –(-6)) m/s = 104 m/s Therefore, 5 cm/microsecond = 5×104 m/s b) 0.36×10-10 tons km/min2 The SI unit for mass is kg The SI unit for distance is m The SI unit for time is s 1 tonne = 103 kg 1 km = 103 m 1 min = 60 s 1 tonne km /min2 = (103 kg × 103m)/(60s × 60s) = (106 / 3600) kgm/s2 = (102/0.36) kgm/s2 Therefore, 0.36×10-10 tons km/min2 = (0.36×10-10) × (102/0.36) kgm/s2 =1×10-8 kgm/s2 2. Chemical elements and compounds Theoretical question What are free atoms and radicals? What is the difference between an ion, free atom, and radical? A free atom is defined as an atom that requires to combine with another atom to achieve stability, but is currently unattached, either because it has just been formed in a reaction, or is at a low pressure, or is frozen in an inert gas (Friedman 1998). For example, when chlorofluorocarbons (CFCs) are exposed to high energy sun rays, chlorine and bromine atoms are released. These chlorine/bromine atoms act as catalysts in the breaking up of ozone molecules. Free radicals are atoms or molecules possessing one or more unpaired electrons. Free radicals are formed as intermediaries of reactions. One of the most common free radicals is the hydroxyl free radical (HO∙). Ions, free atoms, and free radicals are reaction intermediaries. While ions are charged species, free radicals are groups containing unpaired electrons, and free atoms are single atoms without charge. Ions can exist in a stable equilibrium, but free atoms and free radicals are highly unstable and react with other atoms or molecules soon after formation. Exercise Explain chemical bonds in methane. The carbon in methane has an electronic structure of 1s22s22px12py1. During bond formation, an electron from 2s orbital is moved to 2pz orbital. This process requires a small amount of energy as the energy gap between 2s and 2p orbitals is less. So the new electronic structure is 1s22s12px12py12pz1 To form even length bonds, the electrons in 2s, 2px, 2py, and 2pz orbitals undergo sp3 hybridization. Electrons in four hydrogen atoms then bond with the four electrons in sp3 hybridized orbitals to from methane. In the process, four molecular orbitals are formed with an electron pair in each. To align the orbitals as far as possible (electron pair repulsion), they arrange themselves in a tetrahedron. 3. States of Matter: Fluids, Solids and Gases Calculate vapour densities (kg/m3) of pure C5H12 at 25°C and pressure of 1.013×105 Pa. Assume ideal gas behaviour Assuming ideal gas behaviour, the ideal gas equation holds true (Lewis & Evans, 2006): PV = nRT Where P is the pressure of the gas (Pa), V the volume (m3), n the number of moles, R the universal gas constant, and T the temperature (K). Volume of the gas can be calculated as : Here, P = 1.013×105 Pa T = 25 + 273.15 = 298.15K R = 8.3145Jmol-1K-1 n = 1 mol Therefore, Or V = 0.02447m3 = 2.447×10-2 m3 Vapour density is mass per unit volume of vapour. Pentane has a molar mass of 72.15 gmol-1. For 1 mole of pentane, mass is 72.15 g. As calculates earlier, at temperature 298.15K and pressure 1.013×105 Pa, volume of 1 mol of pentane is 2.447×10-2 m3. Mass = 72.15 g= 7.215×10-2kg Volume = 2.447×10-2 m3 Therefore, pentane has a vapour density of 2.95 kgm-3 at temperature 298.15K and pressure 1.013×105 Pa. 4. Chemical Reactions and Their Rates Theoretical questions 1. What are the stoichiometric, fuel lean and fuel rich mixtures? Give an example. A reaction where the products are in the most stable state is known as a complete chemical reaction. In fire, a complete chemical reaction with no fuel and oxygen left is known as a stoichiometric reaction. The reaction mixtures in such a state are stoichiometric mixtures. The stoichiometric oxygen to fuel mass ratio r is determined from the equation. The equivalence ratio () which describes the state of the reactant mixture, is defined as (Quintere, 2006): When a fire starts, =0. When all the fuel is burned and oxygen is left-over, 1. This is the fuel-rich state, and such a mixture fuel-rich mixture. Example: Fuel mixtures that undergo combustion in open environment is a fuel-lean mixture, while a fuel mixture burning in an enclosure without adequate oxygen supply is a fuel-rich mixture. 2. What is the concentration and its units of measurements? What is a mole? Concentration is a measure of the packaging of particles per unit volume and its unit is moles per dm3 or moldm-3. A mole is the unit to measure amount of substance. One mole of a substance contains 6.023×1023 atoms, molecules, or ions. 3. Explain temperature and concentration dependence of the chemical reaction rate and Arrhenius equation. Temperature dependence of chemical reaction In many reactions, rate constants increase with increasing temperatures. A plot of lnk vs (1/T) gives a straight line. The Arrhenius equation is used to explain this behaviour: A is the intercept of the line at (1/T=0), or pre-exponential factor, while Ea is the energy of activation given by slope of lnk vs (1/T). This shows the temperature dependence of rate constant. The greater the activation energy, the more the dependence of rate constant on temperature (Atkins & de Paula, 2006). Concentration dependence of chemical reaction: Concentrations of reactants and products form the basic data of chemical kinetics. In reactions where temperature is maintained constant, rates of chemical reactions are often measured by monitoring concentration of species in reaction mixture. In concentration-time graphs of a reaction, the slope of the tangent to the graph gives the rate constant at any given time. Exercise 1. Compare the chemical reaction rates at two temperatures T1 = 300 K and T2 = 600 K for Arrhenius activation energy E = 180 kJ/mole. The universal gas constant is 8.314 J/(mole K). If the rate of the reaction is k1 at temperature T1 and k2 at temperature T2, the combined Arrhenius equation for both temperatures is written as (Lewis & Evans, 2006): In the reaction, we have E = 180 kJ/mol T1 = 300 K T2 = 600 K R= 8.314 J/(mole K). Therefore, Therefore, Thus, as activation energy is large, rate constant changes dramatically with temperature. 5. Thermal Explosion Theoretical questions 1. Analyse thermal explosion in adiabatic conditions and the mechanism of self-accelerating reaction. Define induction period. In adiabatic conditions, a system cannot gain or lose heat energy. When combustion starts in a material in adiabatic conditions, thermal explosion could take place. Consider a bulk solid, where combustion starts as a smouldering reaction within and slowly propagates outwards. During the process, which is exothermic, heat energy is released. The rate of heat release () is calculated using an Arrhenius type equation (Drysdale, 1998): Eq. 1 Where is the heat of combustion, A the pre-exponential factor, n the order of reaction, V the volume, and Ci the concentration. In a non-adiabatic system, heat generated is lost to the surroundings, proportional to the change in temperature: Eq. 2 Where is the rate of heat loss, h the heat transfer coefficient, S the surface area of reaction volume, and the change in temperature. In a non adiabatic system, =. Eq. 3 However, as the solid is in an adiabatic system, heat energy is not removed from the system. Therefore, >. As this continues, the material continues to self heat, leading to self-acceleration of the system. Soon the system reaches induction temperature and thermal explosion takes place. The ignition process is slow and induction period may last for weeks. Induction period is the time delay between start of reaction and ignition. Exercises 1. Consider qualitatively 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. In Semenvovs model, temperature in the system remains constant and whatever heat is generated is lost, according to Eq. 2. Figure 1 is a Semenov plot of andagainst temperature for three values of ambient wall temperature. Critical ambient temperature is the temperature which gives a heat loss curve intersecting with the heat production curve. Figure 1 Semenov curve for thermal explosion. Source: Drysdale, 1998. At this temperature, = In the heat loss line where there is no intersection with heat production curve, >and temperature continues to rise at an increasing rate, leading to thermal explosion. 2. Calculate the induction period for adiabatic thermal explosion of flammable pyrotechnic mixture (polyvinilnitrate) upon various initial temperatures. Initial temperatures: T0 =300 K, T0 = 600 K, T0 = 900 K. 6. Forms of Heat Transfer Exercise 1. Calculate the rate of heat transfer through a 0.4 square meter of a plaster wall 3 cm thick. One side of the wall is at 600 °C, while other side is at 25°C. Thermal conductivity of plaster is 0.5 W/m x°C. Rate of heat transfer is calculated according to Fouriers equation, which states that the quantity of heat transferred per unit time across an area, A, is proportional to the area and temperature gradient. The equation is written as (SFPE Handbook, 2002): Where is the rate of heat transfer, k the thermal conductivity, A the cross-sectional surface area, T the change in temperature, and x the thickness of conducting surface (Quintere, 2006). Here, for the plaster wall, k = 0.5 Wm-1(°C)-1 A = 0.4m2 T = 600°C-25°C = 575°C x = 3 cm = 3×10-2 m Therefore, =(0.5 Wm-1(°C)-1×0.4m2) 575°C/(3×10-2 m) = 3.83×103W=3.83kJ/s The rate of heat transfer through the plaster wall is 3.83kJ/s. 7. Ignition Theoretical question 1. Describe the process of ignition of a solid combustible material by a hot plate. Explain evolution of the temperature field in the solid material. 8. Premixed Flame Theoretical questions 1. Why do flames propagate through a combustible mixture? Flames propagate through a combustible mixture because sufficient quantity of fuel and oxidant are available in the mixture to carry out the oxidation process required during burning. Hear the flames involved are lean flames that burn the fuel completely, while releasing burnt gasses to the outer atmosphere. 2. What are the flame front and flame propagation velocity? Why a gas particle entering flame front is accelerated? In premixed flames, the chemical reaction occurs within a narrow zone with a thickness of several microns. This combustion zone is called flame front (Encyclopaedia Britannica). Flame propagation velocity is the velocity with which a premixed flame moves normal to its surface through the adjacent unburnt mixture. Higher temperature at the flame front causes gas particles entering the zone to accelerate because of increased kinetic energy. 3. What is adiabatic flame temperature? The adiabatic flame temperature the maximum possible temperature in the combustion zone, where no heat is lost (Friedman, 1998). Exercise 1. The angle of a premixed flame front cone stabilized on the Bunsen burner is 45°. The combustible mixture velocity in the tube is 2 m/s. What is the flame propagation velocity? Figure 2 Flame propagation in Bunsen burner. Source: Quintere, 2006 Conical flame angle 2ø = 45° Unburned mixture velocity (vu) = combustible mixture velocity = 2 ms-1 Flame speed Su = vusin ø = (2 ms-1)sin(22.5)=0.76 ms-1 The flame propagation velocity of the Bunsen burner is 0.76 ms-1. 9. Detonation Theoretical questions 1. Compare the main features of premixed flame and detonation. Pressure and flame speeds differ considerably between premixed flame and detonation (Quintere, 2006): In premixed flames, flame speeds typically are in the ranges of less than 1ms-1 for fuel rich mixtures with air as oxidant, 1.55 ms-1 with acetylene, and 2.9 ms-1 for hydrogen; whereas, in detonation, flame speeds are in the range of 1.5-2.8 kms-1. Pressure increase across the flame front in premixed flames is quite low but uniform, while in detonation pressure rise is very high but not uniform. 2. Describe internal structure of detonation wave. Numerical exercise 1. Calculate 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. 10. Diffusion combustion. Fire Plume Theoretical questions 1. Compare a jet fire and buoyancy dominated fire, using the flame height-jet velocity diagram to explain flame shape. Figure 3 Flame shape with plume height and jet velocity. Source: SFPE Handbook of fire protection engineering, 2002. In the figure, initially, the flame is propelled by buoyancy and the height increases in a steady manner, as characteristic of buoyancy dominated fires. However, with increasing jet velocity, the flame shows increasing turbulence and flame height independent of flow rate. 2. Explain the low value of the Froude number for natural fires. Describe fluid-dynamic structure (air entrainment, buoyant flow, eddies) of a fire plume. Froude number is a measure of the relative importance of inertia and buoyancy in the system. Froude numbers are smaller in fires driven by buoyant forces. Natural fires are often propelled by their own buoyancy and hence have low Froude numbers. Fluid dynamic structure of fire plume Figure 4 Structure of fire plume. Source: SFPE Handbook of fire protection engineering, 2002 Figure 4 is a schematic diagram of a turbulent fire plume. The dotted line confines the entire buoyant flow of combustion products and entrained flow. Air entrainment is the turbulent mixing that brings air into the buoyant plume by large eddies along outside of the buoyant plume. This causes sharp, convoluted, and smoky fires (SFPE Handbook, 2002). 11. Combustible Liquids and Solids Theoretical questions 1. Describe the flash point, fire point and auto-ignition temperature for combustible liquids. How these characteristic are measured in laboratories? Flash point is the temperature at which the vapour pressure is adequate to give a lean limit mixture of fuel vapour in the air. At this point, there is a flashing propagation of flame, although the fuel does not ignite. Fire point is the temperature required to ignite fuel by burning gas above surface of the material. Auto-ignition temperature is the minimum temperature at which auto-ignition occurs. Laboratory measurements of these characteristics in a laboratory involve transient analysis of heat and mass transfer among gas-phase oxidative reactions above the liquid phase. Cups with the liquid surface at the bottom, cylindrical walls that allow heat transfer, and closed tops are used (SFPE Handbook, 2002). 2. What is BLEVE? Explain possible effects of accidental liquid fuel releases on the surrounding. BLEVE or boiling liquid expanding vapour explosion is the result of overheating of pressurized vessel by a primary fire. During overheating, internal pressure is increased, while the vessel shell is weakened up to the point of bursting, when contents are released as a large fireball. This is a delayed event that causes both near-field damage such as fatalities and remote damage such as injuries. 3. Discuss the main factors influencing flame spread over solid materials. Flame spread over solid materials involves flames in gas phase and evaporation or pyrolysis region in condensed phase. In the condensed phase, surface flame spread is measured in terms of flame spread velocity. Factors influencing flame spread over solids include: Presence of ambient wind. An opposed flow flame spread causes an upwind spread, while a wind-aided flame spread results in downwind spread. Source of wind could be an external factor such as meteorological factors or flame-induced eddies or by the spreading flame of an associated fire. Geometrical orientation of the solid. In solids, flame spread depends on whether the surface is horizontal or vertical, facing upwards or downwards. Numerical exercise 1. Calculate the average flame height for a pool gasoline fire. Diameter of pool is 4m. Average flame height Lf is calculated as: Lf=0.232/5-1.02D Where D is pool diameter and is the heat release, which is calculated as: , where is the mass burning rate Mass burning rate per unit area equals 0.011kgm-2s-1 for gasoline. Therefore, =0.011 kgm-2s-1 ×(3.14 ×(4m/2)2) = 0.138kg s-1 is heat of combustion (44.1kJ/g for gasoline). Therefore, = (0.138kg s-1×44.1 ×103 kJ/kg) = 6092.5 kJs-1 So average flame height = 0.23 (6092.5)2/5-1.02 (4m) = 3.4m 12. Fire as a Combustion System Theoretical questions 1. Define heat of combustion, heat release rate and combustion efficiency. Heat of combustion is the rate of energy released per unit loss of fuel mass. Heat release rate is the energy released for the loss of entire fuel mass. The ratio of total heat release rate to theoretical heat release rate is known as combustion efficiency. 2. Describe three zones in turbulent diffusion flame (fire plume). A turbulent diffusion flame has three zones: The combustion zone is the zone where fuel mixture and gas undergo oxidative reactions. On ignition, the fire starts in this zone and releases combustion products in a time dependant manner. The plume. Buoyant forces form a plume of upward moving high temperature gasses above the fire. Upper smoke layer. Along the plume axis, cooler and ambient air entrains with plume gasses and forms the upper smoke layer. 3. Why is thermal radiation of importance in fire? In fire, thermal radiation is important because (Friedman): Energy feedback from flames to burning material is often through radiation rather than convection. So rate of burning depends on flame radiation. Flame spreads to nearby combustibles by radiation transfer. The rapidity of fire spread directly depends on intensity of radiation. Radiation from the fire can be of such high intensity that firefighters may not approach the fire without protection. Exercises 1. Estimate the gas velocity in the fire plume. The gas velocity in a fire plume is calculated as: Where, u is local gas velocity at plume radius R, u0 the mean velocity, and the plume width. 2. Estimate the Froude number in fire plume. Froude number in a fire plume is estimated as: Where, is the total heat release rate, and are the ambient density and temperature, respectively, is the specific heat of air at constant pressure, g, the acceleration due to gravity, and D the diameter of fire source. 3. Calculate the heat release rate for PMMA (m=0.035 kg/m2 s, heat of combustion 23.0 MJ/kg, combustion efficiency 0.6. Mass burning rate = 0.035kgm-2s-1 Theoretical heat of combustion = 23.0 MJkg-1 Combustion efficiency = 0.6 Combustion efficiency is the ratio of effective heat of combustion to total heat of combustion. Total heat of combustion = Theoretical Heat of combustion × combustion efficiency =23.0 MJ/kg ×0.6 = 13.8 MJkg-1 Heat release rate = Total heat of combustion× Mass burning rate =13.8MJkg-1×0.035 kgm-2s-1 = 483 kJ m-2s-1 13. Fire in Enclosures Theoretical questions 1. What is a positive thermal feedback for fires in enclosures? What is flashover and backdraft? When a material burns in an enclosure, the heat released causes an increase in temperature of hot layer of gasses as well as enclosure walls and ceiling. Some of this radiated heat strikes the fire surface resulting in increase of mass loss rate compared with fires burning outside the enclosure. This increase in mass loss rate is called positive thermal feedback and is enhanced by increased heat release (SFPE Handbook, 2002). In a burning enclosure, flashover is a point of growth where flames are no longer confined to burning items, but also spread to fire effluents that are at a distance from the fire (SFPE Handbook, 2002). If a sudden supply of oxygen is allowed into a fire in enclosure, this mixes with the fuel-rich hot gases, causing a sudden increase in combustion and pressure. This could cause other windows and walls in the enclosure to fail. This phenomenon is called back draft. 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. Conditions necessary for flashover For many materials, the lowest heat fluxes required to cause piloted ignition are between 10 and 20 kWm-2. A heat flux of 20 kWm-2 to the floor can cause flashover. This flow of heat flux is associated with upper layer temperatures of 500-600°C. In burning enclosures, at lower heat release rates, the conditions are identical to open burning. However, at higher heat release rates of 1.5-1.75MW, flashover is reached. In fires reaching flashover, the interface between upper and lower hot layers is near the floor, and airflow reaches maximum for a given upper gas temperature. 3. Explain fuel-controlled and oxygen-controlled regimes of fire in an enclosure. In enclosures, fuel-controlled fires have sufficient amount of oxygen to react with all the fuel in the enclosure. So, only the fuel and not the environment affect the heat release rate and pyrolysis rate. In ventilation-controlled fires or oxygen controlled fires, the amount air in the enclosure is insufficient for burning the entire fuel. The rate of heat release then depends on inflow of air into the enclosure. As some of the pyrolysis products move out of the enclosure, the fuel no longer controls pyrolysis rate. 4. Describe the main flow patterns associated with fire development in enclosures. Figure 5 Flow patterns in enclosed fires. Source: Quintere, 2006. Figure 5 depicts the general flow pattern in an enclosed fire. In an enclosed fire, the flow field, which is stratified due to heat induced buoyancy, is primarily responsible for fire development. While the flow of fire depends on buoyancy, turbulence and pressure contribute to entrainment of fire plume. Thermal buoyancy and momentum cause a hot ceiling flow with thickness approximately one-tenth of the room height. Near the floor, a similar cold jet occurs. In between the two jet flows, recirculating flows form a four-layer flow pattern that occupies most of the space. Because of the cold lower layer flows and hot upper layer flows, a well-defined layer interface exists in the middle of the four-layer pattern (Quintere, 2006). 14. Fire in Enclosures. Fire Modelling Theoretical questions 1. What are advantages and limitations of zone models? Advantages: Zone models can predict the average macroscopic features of fires in enclosures. Zone models integrate the phenomena of fires with corresponding fire protection engineering components. Zone model is versatile to accommodate new phenomena, despite apparent inconsistencies with the uniform property layer assumption. Limitations: In a zone model, even the perfect entrainment relationship for an axisymmetric pool fire would not be perfect as a nonsymmetric airflows can bend a fire plume in an enclosure and change its entrainment rate. In the zone model, convective heat transfer data for floor and walls in a fire enclosure or for rooms beyond the enclosure have not been developed and are hence not computed explicitly. As fire source is treated as an input parameter, the zone model cannot model fire growth and spread, thus limiting the models simulation capacity. Fuel depletion and oxygen depletion are not considered in the model. 2. What is field modelling of fires? What are objectives of CFD fire modelling? Field modelling or computational fluid dynamics (CFD) modelling of fires involves dividing a volume under consideration into many subvolumes, to each of which laws of conservation of mass, energy, and momentum are applied. The objectives of CFD fire modelling are to solve partial differential conservation equations for an enclosed fire. 3. Describe verification and validation for field modelling? Why validation is necessary? Validation for field modelling is the process of determining the degree of accuracy of the model in representing real world in the users perspective. For example, validation of turbulence, combustion, and radiation is essential to investigate their representation of reality. Verification for field modelling is the process of determining the accuracy of implementation of a developers conceptual description of a model and its solution. For example, verification of turbulence, combustion, and radiation in the model helps in assuring the models accuracy in solving model equations. Validation involves comparing the models results with experimental data available. This is essential to determine a models applicability to a situation. For example, the turbulence models used in a CFD modelling software RANS were developed for turbulent shear flow rather than buoyant convective flow, which invalidates the assumptions of isotropic turbulence made in the model (SFPE Handbook, 2002). References Atkins, PW, & de Paula, J (2006). Physical Chemistry. Oxford: Oxford University Press. Drysdale, DD (1998). An introduction to fire dynamics, 2nd ed. Chichester: Wiley. Encyclopaedia Britannica. http://www.britannica.com/EBchecked/topic/209374/flame-front Friedman, R. (1998). Principles of fire protection chemistry and physics, 3rd ed. NFPA, Quincy MA. Lewis R. and E. (1997). Chemistry. Hampshire: Palgrave Macmillan. Quintiere, J. G. (1998). Principles of fire behaviour. Delmar Publishers, Albany, NY Quintiere, J. G. (2006). Fundamentals of fire phenomena. Chichester: Wiley. SFPE Handbook of Fire Protection Engineering, 3rd ed. (2002). Quincy MA: NFPA. Read More