Combustion and Fire: Assessment - Assignment Example

Assignment Solutions Sol 1. The primary role of a fire engineer is safety of life which is achieved through implementation of the code of fire engineering practice, benchmarking a lucid engineering methodology. Also, a fire engineer is always required to enumerate the objectives of a fire safety design so as to enable a clear understanding of the same for all those involved. This is often done using a pre laid down format known as Qualitative Design review (QDR). Thus, it may be ascertained that the role of a fire engineer is not one which may help reducing fire precautions but, should aim at providing the right fire precautions at a specified place. Sol 2. The different states of matter are: Solid, Liquid and Gas. The molecular difference between them can be enumerated as: Solid Liquid Gas Molecules are closely packed with strongest mutual forces of attraction Molecules are less closely packed with mutual forces of attraction weaker than those in solids Molecules are sufficiently apart from one another and have almost negligible mutual forces of attraction Position of molecules in crystal lattice are fixed and hence solids do not have translator or rotator motion but only vibratory motion Molecules have greater freedom of movement. They have some translator and rotator motion in addition to the vibratory motion Molecules have large rotator, vibratory and translator motions Molecules possess least energy Molecules have higher energies than those solids Molecules are most energetic Sol 3. A free atom is an atom which is unaffected by its nearby atoms, ions or molecules since it has no electrical charge due to equal number of protons and electrons. Free radicals are an atoms or group of atoms which are highly reactive, with an uneven (unpaired) number of electrons (with neither positive nor negative charge) and are capable of propagating chain reactions. These can be formed when oxygen reacts with certain molecules. Free radicals form one of the four essential components of a fire tetrahedron. Free radicals the propagation of fire and its elimination is necessary to ensure that a fire is suppressed. This can be done by interrupting the free radical chain reaction which can be achieved by absorbing the heat/energy created when the uncharged fragments of free radicals collide with other molecules. Sol 4. The difference between ion, free atom and radical is: Ion Free atom Radical Ion is an atom or molecule where the total number of electrons are not equal to the total number of protons Free atom has equal number of protons and electrons Radical has unpaired electrons It has a positive or negative charge It has no electrical charge It has neither positive nor negative charge Sol 5. Now, t°C = (t + 273.15) K and b°F = (5/9) (b - 32) °C i. 36.5°C = (36.5 +273.15) K = 309.65 K ii. 104°F = (5/9) (104 - 32) °C = 40 °C = (40 +273.15) K = 313.15 K iii. 196°C = (196 + 273.15) K = 469.15 K iv. 212°F = (5/9) (212 - 32) °C = 100 °C = (100 +273.15) K = 373.15 K Sol 6. a. H3PO4 + 3 KOH => K3PO4 + 3 H2O b. 2 H3PO4 + 3 Mg(OH)2 => Mg3(PO4)2 + 6 H2O c. C2H6 + 7/2 O2 => 2 CO2 + 3 H2O d. Ca3(PO4)2 + 3 SiO2 + 5 C => 3 CaSiO3 + 5 CO + 2 P Sol 7. Conditions which affect a chemical reaction rate are: 1. Concentration of reacting substances 2. Temperature under which the reaction takes place 3. Nature of the reactants ( state and type of reactants ) 4. Surface area ( heterogeneous system involving a solid ) 5. Presence of Catalysts Sol 8. Heat Energy, Q = m * c * Δt where, m = mass of the substance; c = specific heat capacity of the substance (in J/Kg 0C); and Δt = Change in temperature Specific Heat capacity, ‘c’ for Gypsum = 1.09 KJ/Kg K Thus, heat energy required, Q = 1 * 1.09 * [(150-20) + 273.15] = 439.43 KJ Sol 9. Gases are affected by change in pressure. When the volume occupied by a gas is decreased (i.e. pressure increased or concentration increased), gas molecules collide more frequently. Thus, increased collision enhances the reaction rate and thus gases react faster. On the other hand, increasing the volume of a gas, decreases pressure which actually decreases the frequency of collision and hence reduces the rate of reaction. Sol 10. In coal mines, silos etc, high ratio of surface area to volume due to which the pressure on the largely available (high concentration) powdered combustible materials increase thus leading to increased rates of reaction which in uncontrolled situation leads to dust explosion. Also, a relatively minor explosion is propagated by the presence of fine coal dust. Sol 11. Molecular weight = Common factor * Empirical weight Thus, Common Factor =  =  = 1.999 ≈ 2 Therefore, the Molecular formula = Cl2C2H4 Sol 12. In case of thermal explosions of sensitised reaction with variable heat loss, for different values of heat loss parameter β, the Frank-Kamenetskii parameter δ and maximum temperature of the slab θ varies with activation energy Є. Under adiabatic conditions, i.e. β = 0, there is no exchange of heat between walls of the slab and combustible material. Thus in this case, δ and θ are monotonically increasing and decreasing respectively for increasing values of Є. Induction period is the primary slow stage of a chemical reaction which later accelerates. Induction periods are often observed with radical reactions, but they may also occur in other systems (for example before steady-state concentration of the reactants is reached) Sol 13. A qualitative interpretation of the Semenov diagram: the intersections S and I between the heat release rate of a reaction and the heat removal by a cooling system represent an equilibrated heat balance. Intersection S is a stable operating point, whereas I represent an instable operating point. Point C corresponds to the critical heat balance. The heat balance is in equilibrium when the heat production is equal to heat removal (qrs = qex). Sol 14. In Thermal explosions, an increase in temperature causes a very large increase in rate of reaction. Also, variations in the size of the vessel affects the explosion because the greater the surface area, the more readily the material can burn in air, because the burning reaction takes place at the air/solid interface. Sol 15. Given, Thickness, d = 3 cm = 0.03 m Area, A = 0.4 m2 Inside surface temperature T1 = 600 0C Outside surface temperature, T2 = 25 0C Thermal conductivity of plaster, k = 0.5 W/m x °C Therefore, Rate of heat transfer, Q =  =  = 3833.33 W = 3.83 KJ/s Sol 16. The two types of smoke alarms and their trigger mechanisms commonly found are: 1. Ionization technology: Ionization smoke alarms are triggered when electrically charged ("ionized") particles released in a fire interfere with the electrical current that flow through the alarm's detection chamber. The interruption in the electrical current causes the smoke alarm to ring. 2. Photoelectric technology: Photoelectric smoke alarms are triggered when particles of smoke interfere with and reflect the light beam of alarm which propagates through the alarm's chamber of detection. This triggers the alarm. Sol 17. The burning of fuel in which the supply of oxidiser can be regulated is a controlled reaction, which otherwise would be an uncontrolled burning reaction. There are various types of fire extinguishers and are so designed to determisne types of burning material based on the fact that certain extinguishing agents are better than others when used on wood, liquids or electrical equipments: Class A: These types of extinguishers can be used for combustible materials based on cellulose, like paper, wood or cardboard, or materials like cloth, styrofoam, trash or plastics. Class B: These types of extinguishers can be used for flammable liquids and greases such as gasoline, kerosene, oil, paint, solvent and rubber cement. Class C: These extinguishers are made especially for electrical fires created within a computer, an appliance or a fuse box. Class D: This extinguisher is mainly used for flammable metals, like sodium, magnesium or titanium. Class K: Although it’s also used for oils, this extinguisher was specially created for commercial kitchens. Sol 18. Premixed Flame Diffusion Flame A flame in which fuel and oxidiser are initially separated A flame in which fuel and oxidiser are initially mixed Fuel-Oxidiser ratio varies throughout the flame Fuel-Oxidiser ratio remains constant everywhere in the flame Example: Candle flame, Wood flame, Forest fire. Example: Uniform combustible mixture inside a tube ignited by a spark The temperature profiles of 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). The temperature profiles of diffusion flame are: Wax migrates up the wick and is pyrolysed at approx 600-800 0C Bright yellow flame at centre with insufficient oxygen leading to cyclisation , aromatisation and formation of soot particles Sol 19. Whenever a chemical reaction involving adding oxygen to the compound and losing electrons occurs, the same is oxidation while a complex chemical reaction involving oxygen so as to break the combustible organic compound into carbon-di-oxide and water accompanied by release of heat and light, the reaction is termed as combustion. Sol 20. Materials which are porous, permeable to flow and formed by aggregates which facilitate surface reaction with oxygen readily undergo smouldering combustion. For e.g. cellulose, biomass fuel, polyurethane, coal, tobacco etc. Sol 21. Fire spread is an instantaneous process for many crude oils in their natural state, loss of highly volatile compounds, due to weathering, and presence of water emulsions might lead to fire spread that needs to be assisted by external radiation. Here, a minimum size is necessary to provide radiative heat to self sustain fire spread. For fire spread, the minimum external heat flux that will sustain propagation together with the parameter Ф (function of fuel properties) will serve to describe the fire spread characteristics. Sol 22. Turbulence in both Pre-mixed or diffusion combustion flames enable heat release at faster rates of combustion energy release per unit volume and increased flame propagation as it helps in mixing the fuel and the oxidiser. Sol 23. Burning velocity of a gaseous mixture is the speed of a two-dimensional flame front perpendicular to its surface and with respect to the unburned gas-fuel oxidizer mixture, i.e. the speed with which flame approaches unburned fuel gaseous mixture. Inside of a closed tank, the increase in volume of the high-temperature combustion materials pushes the flame ahead even faster than burning velocity, and the flame appears to move even faster, 5 to 10 ft/s, up to 15 m/hr. Sol 24. Thermal radiation from a flame is important in fire safety because radiation which is transfer of heat through a fluid or vacuum by means of electromagnetic waves, the surfaces themselves begin to radiate heat energy which influence in the following three crucial ways: 1. Influences temperature rise in enclosures 2. Influences the burning rate 3. Influences fire development and growth Sol 25. Detonation is a process of combustion wherein a supersonic shock wave disseminates through a mass of detonable substance, such as an oxygen-methane mixture or a high explosive. Detonation is different from a deflagration, which is another class of combustion. It propagates at a subsonic rate through heat conduction. Detonations are much more destructive than deflagrations owing to the generation of high pressures by them. Read More