Introduction to Combustion and Fire - Assignment Example

Question 1 Theoretical I: The units of measurements which form the base as physical standard based on physical objects with physical descriptions are known as fundamental units of measurement. The most widely used fundamental units of measurement is International System of Units or SI which has seven base units in all and can be enumerated as follows: meter (m), kilogram (kg), second (s), ampere (A), Kelvin (K), mole (mol) and candela (cd). Derived units are those which are defined as combination of fundamental units under the SI and are 22 in all. Theoretical II: Prefixes which are derived from the words for numbers in several languages especially Latin and Greek are known as numerical prefixes. They denote integers, negative powers of 10 and several vulgar fractions. Exercise I: (a) Pressure/Power = N/m2 / J/s = kg.m-1.s-2/ kg. m2.s-3 = m.s = meter second (b) Work/Velocity = N.m/ m.s-1 = kg.m2.s-2 / m. s-1 = kg.m.s-1 Exercise II: (a) 250 cm/microsecond = 250 * 10-2 m / 10-6 second = 250 * 104 m/s (b) 100000 gigagrams cm/minutes = 105 * 106 kilograms * 10-2 m / 60 seconds = 1.67 * 107 kg m s-1 (c) 150 miles/minutes = 150 * 1609 m / 60 s = 3.88 * 108 m/s Question 2 Theoretical I: Chemical bond is the physical attractive force binding the atoms and molecules together and provides stability to the chemical compounds. Covalent, Ionic, hydrogen and van der Waals’ are the types of covalent bond. For example, H2—C==C—H2 has chemical bonds between Carbon (C) and Hydrogen (H). Valence electrons refer to the outermost electron of an atom which determines the chemical reactions of that atom with other atoms. For example, the figure shows one valence electron in the Lithium (Li) atoms in the outermost energy level. Lithium Atom model Theoretical II: Free atoms are atoms which have a complete outermost shell and therefore do not need to gain or lose electrons to gain stability. They therefore exist in an independent free state and under rare conditions take part in chemical reactions. Radicals or free radicals are atoms or molecules or ions with unpaired electrons which are highly reactive and readily take part in chemical reactions. Ions are groups of atoms which have an overall positive or negative charge due to unequal number of protons and electrons after loss or addition of electrons. The positive ions are known as Cations and negative ions are known as Anions. Exercise I: Carbon monoxide has a co-ordinate or dative bonding which occurs between lone pair of electrons of one of the bonding atoms. Carbon has 4 electrons in its outermost shell and oxygen has six. Out of these two covalent bonds are formed between the two pair of electrons of carbon and oxygen each. The third bond is a co-ordinate bond formed by the lone pair of electrons on oxygen. Co-ordinate or Dative bonding in Carbon monoxide Exercise II: Example of single bonds  Methane (CH4) H Example of double bonds  Ethene (C2H4) Question 3 Theoretical I: Fluids are gases or liquids. Density is mass per unit volume of the substance. With pressure, density of gas varies, unlike liquid which has a constant density at normal pressures. At very high temperatures, fluids tend to expand. Gases tend to flow in all directions, whereas liquids flow in one direction at a comparatively slower speed than flow of gases. Thus, gases ave pretty high velocities. Theoretical II: Pressure id the net force per unit area of an object. Theoretical III: Equation of state for an ideal gas; PV = nRT where, P = Pressure V = Volume N = Number of moles R = Boltzmann Constant T = Temperature Exercise I: Boyle’s Law  V α  or, PV = constant at constant temperature. At constant pressure, V α T Exercise II: Molecular weight, M =  = (2.58 kg/m3 x 0.0826 L.atm/mol/K x 273 K ) x  = 29.1 kg/mol Exercise III: Density, d =  = (3 atm x 0.028 kg/mol)/ (0.0826 L.atm/mol/K x 900 K) = 0.00113 kg/m3 Exercise IV: T (in Kelvin) = 30 + 273 = 303 K T (in Farenheit) = 30 + 32 = 62 F Question 4 Theoretical I: Cl2  Cl- is a stochiometric equation with same atoms on both sides of the equation. The same can be balances by putting an equal number of atoms on both sides of the equation as follows: Cl2  2 Cl-. Theoretical II: Stochiometric mixture is a mixture of a ratio of some substances which can react to give products with no excess reactants. For example an air: fuel mixture in the ratio 10:1. Theoretical III: As the concentration of reactant increases, so increases the chances of collision of reactant molecules and therefore the chemical reaction rate also increases. According to rate law, aA + bB  cC + dD here, Rate = k [A]x [B]y is the relationship between rate of reaction and concentration of reactants raised to some powers. Theoretical IV: Arrhenius equation states the relationship or dependence of reaction rates on temperature: k = Ae –Ea/RT where, A Ea are Arrhenius parameters. Theoretical V: Energy of Activation Ea is the minimum threshold kinetic energy required to allow the reaction to occur. It is indicated in KJ/mol. Exercise II: ln (k’/k) = Ea/R (1/T – 1/T’) or, ln (k’/k) = 180000 J/mol / 8.314 J/mol K (1/300 – 1/600) K = 2494.2 Question 6 Theoretical I: Conduction is transfer of heat through a material from a high temperature zone to a lower temperature zone. Convection is transfer of heat through or by a fluid by means of induced convective currents within the fluid. Radiation is transfer of heat through or by a fluid or vacuum by means of electromagnetic waves where surfaces themselves radiate heat energy influencing temperature rise. Exercise I: Heat Transfer, Q = kA(ΔT/L)t = 0.5 W/m 0C * 0.4 m2 * (575 0C/0.03 m)* 1 = 3833.33 W every sec if t = 1 sec. Exercise II: Radiative heat flux emitted, q” = ε * б * T4 = 0.55 * (5.67 x 10-8Wm-2K-4) * (800+273) 4 K = 41337.54 W/m2 Question 8 Theoretical I: Flames propagate through a combustible mixture because the flame propagation is mainly supported by the combustion of smaller particles gasifying across the schlieren front. Theoretical II: Flame front and Flame propagation velocity Theoretical III: Premixed flame on Bunsen burner moves at a fairly low velocity and mechanically created laminar conditions Theoretical IV: Three regions in premixed flame are Pre heat zone (0.33 mm thick , premixed gas heated to ignition temperature), Reaction zones (1 mm thick hydrocarbons, combustion occurs in visible flames) and Post-flame zones (high temperature with a local equilibrium). Question 9 Theoretical I: Internal structure of detonation wave can be characterised by a speed of decomposition more than the speed of decomposition of sound. Question 10 Theoretical I: Candle burns through pure molecular diffusion. The wax migrates up the wick and is pyrolyzed at 600-800 0C. Soot formed due to the cyclization and aromatization owing to insufficient oxygen forming soot particles which glows with a bright yellow flame. Energy for combustion id provided reaction of pyrolysis gases with oxygen in presence of high energy radicals. Question 11 Theoretical I: Flash point is the lowest temperature at which there is sufficient vapour to cause a momentary burning when a flame is applied. Fire point is the lowest temperature at which vapour from a liquid may be ignited and continue to burn until all the liquid has been consumed by fire. Auto ignition temperature is the lowest temperature at which a substance will ignite in the absence of any source of ignition like a flame or spark. Exercise I: Mean flame height, L = 0.235 Q2/5 – 1.02 D, where D = 4m and Q = Afm”χΔHc Now, Af , Area of fire = π/4*(4)2; ΔHc, heat of combustion for gasoline = 43.7 MJ/Kg; χ = 1 as clean burning and m” = 0.029 Kg m-2 s-1 . Thus, L = 0.235 *(12.56 * 0.029 * 1 * 43.7) 2/5 – 1.02 * 4 = 0.712 – 4.08 = Question 12 Theoretical V: Radiation is important in fire because transfer or radiation of heat through fluid or vacuum through electromagnetic waves influence the temperature rise in fire enclosures, its burning rate and fire’s development and growth. Exercise I: Heat release rate, Q = Af m” χ ΔHc Now, ΔHc, heat of combustion for gasoline = 25.2 MJ/Kg; χ = 0.55 as clean burning and m” = 0.035 Kg m-2 s-1 . Q = 0.035 Kg m-2 s-1 * 0.55 * 25.2 MJ/Kg = 0.4851 MJ m-2 s-1 Question 13 Theoretical I: After flashover which is a sudden increase in the amount of burning liquid or solid substance, fire enclosures exhibit certain unique characteristics related to smoke aggregation and air ventilation. Thermal feedback or pre-heating has a positive effect on the flame burning rate. Theoretical II: Temperature-time curve for fire development in an enclosure is a bell shaped curve with the initial stages of fire growth (growth in its temperature) increases with time and then after a peak again reduces with time as seen in fire development stage of an enclosure. Theoretical III: The conditions necessary for a flashover include a radiant heat flux at the floor of 15-20 kW/m2 (radiant heat transfer enough to rapidly increase average combustibles to their ignition temperature) and average upper layer temperature of 500o-600o C. Theoretical IV: Fires in compartment are usually fuel controlled while in the nascent and initial development stage and then as the fire decays and the demand for oxygen is reduced. However, as the fire grows the demand for oxygen increases, and at a certain point, this demand exceeds the available oxygen. Here, the fire transitions to ventilation control. Theoretical V: Fire resistance is creation of a barrier such as walls or floor/ceiling assemblies. The major criterion for fire resistance is utilization of unexposed face temperature. Theoretical VI: Fire development can also be segregated into several stages with regards to flow through gaps, and depend on the amount of pressure in and around the opening. The following are the main stages illustrated: Stage i: Expansion of boiling gases, flow of cold gases out through gap, positive pressure difference in compartment Stage ii: Expansion and escape of gases, flow of hot and cold gases out through gap, positive pressure difference in compartment Stage iii: The layered case, smoke layer placed at some a level higher than floor level, hot gases flow out through upper part of gap, cold gases flow in through lower part of gap Stage iv: The properly mixed case, compartment filled with hot smoke, hot gases flow out through upper part of gap, cold gases flow in through lower part of gap Theoretical VII: Backdraft is the creation of an explosive situation due to the sudden induction of oxygen into a combustion process starved of oxygen but containing combustible smoke and gases at a very high temperature. Question 14 Theoretical I: The weather and fuel moisture assumptions of fire modelling can be describes as below: Example of fire modelling can be the burn treatments applied in case of various alternatives. Theoretical II: Advantages of fire models: It shows an improvement in case of various alternatives as compared to no action. Limitations of fire models: It is only a simulation of reality. It is only a support tool for decision making and not a tool to make decisions. Theoretical III: Zone modelling of fires is basically compartmentalization of fire divisions into zones in a computer based model so as to have uniform conditions in all the zones. The objective of zone modelling is to model the fire compartments in more detail taking into consideration the heat release rate due to combustible substances, flow of mass, temperatures etc. Theoretical IV: Necessity of verification and validation of fire modelling: It introduces risk awareness and performance oriented evaluation into the fire protection engineering practice. It can reliably predict the outcome of fires. Read More