Introduction to Combustion and Fire - Assignment Example

FV1001 Introduction to Combustion and Fire Name: Instructor: Course: Date: FV1001 Introduction to Combustion and Fire 1. Matter exists only in three states: solid, liquid, and gas. In solid state, the molecules are closely arranged with strong intermolecular forces that give rise to a fixed shape and volume. In liquids, intermolecular attractions keep the molecules in close proximity but not in fixed spatial relationship, i.e. liquids have definite volume but lack distinct shape, taking shape of their containers. On the other hand, in gas, molecules are freely packed and in constant motion due to existence of extremely weak intermolecular attraction forces, hence gases lack definite shape and volume. In addition, solids melt into liquids when they get sufficient energy to overcome the attraction forces, while liquids change into gases. Moreover, gases loose energy to change to liquids, while liquids change into solids (Rousseau & Fourie 2000). 2. Radicals are molecules, atoms, or ions with unpaired electrons on the outer valence shell. Almost all radicals are commonly referred to as free because they exist independently, free from any other species. They are usually chemically unstable, hence the name reactive intermediates. Their high reactive characteristic is associated with the unpaired electrons that need to pair in order for the radicals to stabilize. In relation to fire, different types of free radicals are produced and spread into the atmosphere during burning of fuels. These radicals depend on the type of matter burned and readily react with other contaminants in the atmosphere. In addition, some radicals such as Arseniodimethly or Kakodyl react spontaneously to fire (Glassman & Yetter 2008. 3. Heat of combustion refers to the heat generated or liberated when 1 mole of a substance undergoes complete combustion in presence of oxygen at constant pressure (Speight 2008, p.40). 4. Convert the following temperatures into Kelvin, i. 46.5°C Solution ii. 174°F Solution iii. 705°C Solution iv. 212°F Solution 5. Balance the following equations, Solution a. H3PO4 +3 KOH => K3PO4 + 3H2O b.2 H3PO4 +3 Mg(OH)2 => Mg3(PO4)2 + 6H2O c. 2C2H6 + 7O2 => 4CO2 + 6H2O d. Ca3(PO4)2 + 3SiO2 +5C => 3CaSiO3 +5CO +2P 6. Upward movement of gases and flames is mainly affected by temperature and density. This movement may also be affected the prevailing wind speed and the humidity of the atmosphere (Speight 2008) 7. Solution , where, Q = amount of heat required in joules c = specific heat capacity for gypsum (1090j/kg). m = mass = change in temperature Q = 1 ×1090(180-30) =163500j or 163.5kj 8. Gases react faster when the volume they occupy is reduced because the gas molecules are brought closer to each other leading to increased collision. Furthermore, reduction in volume also leads to increase in pressure, which speeds up the reaction (Stoessel 2008). 9. Temperature is the measurement of the degree of hotness or coldness of a body or object in degree Celsius, while heat is a form of energy measured in joules. In addition, heat is required for an increase in temperature and therefore, both have a direct relationship. 10. Solution Empirical formula mass for CLCH2 =1(atomic mass of chlorine, 35.5) + 1(atomic mass of carbon, 12) + 2 (atomic mass of hydrogen, 1) (Dumas et al. 2006) Therefore, empirical formula mass of CLCH2 = 35.5 + 12 + 2 = 49.5 n = 98.96/49.5 = 1.9, which is approximately 2 Therefore, molecular formula of CLCH2 = (CLCH2)2 =CL2C2H4. 11. Draw and explain a Semenov diagram for thermal explosions. From the diagram, points of intersections, S and I, between the heat release rate, and the heat removal, indicate equilibrated heat balance. S represents a stable operating point, I an instable operating point, and C a critical heat balance. 12. Solution Rate of heat transfer, Q/t = KA (T2-T1) d, where Q/t = rate of heat transfer K = thermal conductivity of gypsum (0.48W/m2k) A = surface area of the plaster wall T2 = temperature for one side of the wall T1 = temperature of the other side of the wall d = wall thickness Therefore, Q/t = 0.32  0.48 (675-32)  4.2 = 23.52 watts 13. There are various kinds of fire extinguishers, classified according to kinds of fires. Dry chemical extinguishers use compressed non-flammable gas as a propellant, and are usually applied for all kind of fires. Halon extinguisher contain a gas that interrupts chemical reactions that occur when fuels burn. They are normally applied to extinguish electrical fires with a limited range of about 4 to 6 feet. On the other hand, water extinguishers contain water and compressed gas and are used on ordinary combustibles or class A fires. Another important type is the carbon dioxide extinguisher used in extinguishing electrical and liquid fires. It is only effective in the range of about 3 to 8 feet and extinguishes fire due to the cooling effect of gas on the surroundings (Andrew 2001, p.119) 14. A premixed flame refers to any flame in which the oxidizer and the fuel are initially mixed, while diffusion flame imply any flame in which the fuel and the oxidizers are not initially mixed but exist as single elements. Another difference between the two flames is that in diffusion flame, the fuel-oxidizer ratios differ throughout the flame, while in premixed flames, the ratios are constant all over the flame. The temperature profile for diffusion flame comprises of a cool zone at low height that diminish with increase in height of the flame and a constant point of heat flux distributed in the same flame. On the other hand, premixed flames are characterized with an upstream preheat zone, maximum downstream temperature comprising of a blue flame, and a region of reduced temperature (Glassman & Yetter 2008). 15. Materials that undergo smoldering include synthetic foams, tobacco, cellulose, wood, coal, peat, humus, etc. 16. Heat transfer is the basic mechanism for spread of fire. Heat is normally transferred from materials with high temperature to those with low temperature through three principal mechanisms: radiation, convection, and conduction. Conduction forms the predominant method of heat transfer during the initial stages of fire development. Radiation is specifically important for fire development in compartments in which it serves as the primary mechanism of fire spread. Therefore, the extent of fire spread greatly relies on the three forms of heat transfer and the presence of the right media for heat transfer (Rousseau & Fourie 2000). 17. Turbulence remains essential in combustion because it increases mass combustion rate of reactants. Great combustion rates increase the chemical energy release rate from a combustor or internal engines. In fact, very few engines can function without increase in rate of reactant combustion brought about by turbulence. The importance of turbulence can be recognized in respect to spark timing in automotive engines. As the RPM of an engine increases, turbulence increases, which explains why spark timing does not require to be changed in such cases (Glassman & Yetter 2008) 18. Burning velocity is the speed of the flame front in respect to unburned gas. Laminar or gas mixture burning velocity depends on the initial condition of the mixture. Therefore, it varies with fuel equivalence ratio and reaches its maximum at rich mixtures for hydrocarbons fuels in which maximum flame temperature is present. There are about three reasons why a flame can move faster than its burning velocity. First, it can be possible if the flame increases the temperature of the gas, i.e. from 300k to about 2100k. This is because it causes expansion of the gas’ hot portion based on the perfect gas law, hence causing movement of the gas that carry the flame towards the unburned gas. Secondly, the concept of burning velocity assumes that the velocity vector of a gas moving into the surface of a flame is perpendicular to the flame surface. In such cases, the burning velocity is a normal component of the velocity vector and therefore, equals to V sin Ø (Rousseau & Fourie 2000). Thirdly, the burning velocity depends on laminar flame propagation, and hence, a turbulent premixed flame become propagated a number of times faster than a laminar one. 19. One of the reasons why thermal radiation remains important in fire safety is that the resulting fireballs and flash fires present potential injuries or damages. Another reason is that for large fires, thermal radiations cause secondary fires and burn hazards to people in the surrounding areas. In addition, establishment of the total thermal radiation is important because it helps fire fighters to put up safety measures to avoid spread and occurrence of simultaneous fire incidents in an area. 20. Detonation is a substance’s propagating chemical reaction whereby the front of reaction moves into the material, which has not reacted, at higher speed. On the other hand, deflagration is a propagating chemical reaction of a substance whereby the reaction front moves into the non-reacted material rapidly but at less sonic speed (Glassman & Yetter 2008). 21. Fire can be categorized into three: class A, class B, and class C. Class A fires, also known as carbonaceous fires, involve flammable solids such as paper, coal wood, etc. Class B or hydrocarbon fires entail flammable liquids where the combustibles include grease oils, etc. Class C fires involve gases such as propane, natural gas, butane etc. Extinguishing such fires entail shutting up the gas sources and are characterized with rapid spread hence can cause enormous damages (Andrew 2001, p.118). 22. Some of the toxins produced in Class A fires include hydrogen cyanide, nitrogen dioxide and other nitrogen oxides, sulfur dioxide, isocyanates, and acrolein. Toxic species from Class B fires include carbon monoxide, sulfur dioxide, lead from leaded oils, etc, while Class C fires’ toxins include sulfur dioxide, carbon monoxide, methane gas, and other toxic gases (Andrew 2001). 23. One factor that affects oxygen depletion rate during a fire in a compartment includes the degree of enclosure, which determines circulation of oxygen during the combustion process. Another factor is the size of fire, with large fires reducing the ambient oxygen concentration faster than small fire because the consumption rate is elevated. In addition, temperature also determines the concentration of oxygen in that extremely high temperatures lead to rise of air, which can easily be lost from the compartment (Andrew 2001). 24. The sequence of occurrence of fire hazards within a fire compartment proceeds in about three stages to affect occupants within the compartment. The first stage, incipient stage, marks the beginning of the fire hazard. It is where the fire does most of the developing in size, usually doubling every 30 seconds. During this stage, there is no oxygen depletion, but a cloud of hot gasses rising to the top of fire, transferring heat via convection. In the free burning phase, the second stage in the sequence, fire flames stretch upwards and outwards from the fire origin via all forms of heat transfer. Oxygen depletion occurs as oxygen-rich air is sucked into the flames and replaced with dense, hot, and thick smoke layers at the ceiling of the fire compartment, and gasses radiate intense heat to the floor of the compartment (Rousseau & Fourie, 2000, p.59). As the heat increases, rollover of gasses and smoke at the top of the compartment occurs, affecting the respiratory system of occupants of the compartment. At about 1,1000C, flashover occurs, with all combustible materials in the compartment igniting to an open flame. During the last stage in the sequence, smoldering phase, levels of oxygen decrease below 16% and the room becomes filled with hot gasses and smoke. Survival of occupants during this phase becomes impossible. 25. Figure 2: Standard fire curve (source: Rockwool International A/S n.d.) References Andrew 2001, Hotel housekeeping training manual, Tata McGraw-Hill, New Delhi. Dumas, PE, Reel, KR, Fikar, RM & Templin, JM 2006, AP chemistry exam, 9th edn., Research & Education Association, Piscataway, NJ. Glassman, I & Yetter, RA 2008, Combustion, 4th edn, Academic Press, New York, NY. Rockwool International A/S n.d., Fire curve, Rockwool Firesafe Insulation, viewed 26 Jan. 2011, Rousseau, PG & Fourie, NG 2000, Engineering science N2, issue 2, Pearson South Africa, Cape Town. Speight, J 2008, Synthetic fuels handbook: properties, process, and performance, McGraw- Hill Professional, New York, NY. Stoessel, F 2008, Thermal safety of chemical processes: risk assessment and process design, Wiley-VCH, Weinheim. Read More

Dry chemical extinguishers use compressed non-flammable gas as a propellant, and are usually applied for all kind of fires. Halon extinguisher contain a gas that interrupts chemical reactions that occur when fuels burn. They are normally applied to extinguish electrical fires with a limited range of about 4 to 6 feet. On the other hand, water extinguishers contain water and compressed gas and are used on ordinary combustibles or class A fires. Another important type is the carbon dioxide extinguisher used in extinguishing electrical and liquid fires.

It is only effective in the range of about 3 to 8 feet and extinguishes fire due to the cooling effect of gas on the surroundings (Andrew 2001, p.119) 14. A premixed flame refers to any flame in which the oxidizer and the fuel are initially mixed, while diffusion flame imply any flame in which the fuel and the oxidizers are not initially mixed but exist as single elements. Another difference between the two flames is that in diffusion flame, the fuel-oxidizer ratios differ throughout the flame, while in premixed flames, the ratios are constant all over the flame.

The temperature profile for diffusion flame comprises of a cool zone at low height that diminish with increase in height of the flame and a constant point of heat flux distributed in the same flame. On the other hand, premixed flames are characterized with an upstream preheat zone, maximum downstream temperature comprising of a blue flame, and a region of reduced temperature (Glassman & Yetter 2008). 15. Materials that undergo smoldering include synthetic foams, tobacco, cellulose, wood, coal, peat, humus, etc. 16. Heat transfer is the basic mechanism for spread of fire.

Heat is normally transferred from materials with high temperature to those with low temperature through three principal mechanisms: radiation, convection, and conduction. Conduction forms the predominant method of heat transfer during the initial stages of fire development. Radiation is specifically important for fire development in compartments in which it serves as the primary mechanism of fire spread. Therefore, the extent of fire spread greatly relies on the three forms of heat transfer and the presence of the right media for heat transfer (Rousseau & Fourie 2000). 17. Turbulence remains essential in combustion because it increases mass combustion rate of reactants.

Great combustion rates increase the chemical energy release rate from a combustor or internal engines. In fact, very few engines can function without increase in rate of reactant combustion brought about by turbulence. The importance of turbulence can be recognized in respect to spark timing in automotive engines. As the RPM of an engine increases, turbulence increases, which explains why spark timing does not require to be changed in such cases (Glassman & Yetter 2008) 18. Burning velocity is the speed of the flame front in respect to unburned gas.

Laminar or gas mixture burning velocity depends on the initial condition of the mixture. Therefore, it varies with fuel equivalence ratio and reaches its maximum at rich mixtures for hydrocarbons fuels in which maximum flame temperature is present. There are about three reasons why a flame can move faster than its burning velocity. First, it can be possible if the flame increases the temperature of the gas, i.e. from 300k to about 2100k. This is because it causes expansion of the gas’ hot portion based on the perfect gas law, hence causing movement of the gas that carry the flame towards the unburned gas.

Secondly, the concept of burning velocity assumes that the velocity vector of a gas moving into the surface of a flame is perpendicular to the flame surface. In such cases, the burning velocity is a normal component of the velocity vector and therefore, equals to V sin Ø (Rousseau & Fourie 2000). Thirdly, the burning velocity depends on laminar flame propagation, and hence, a turbulent premixed flame become propagated a number of times faster than a laminar one. 19.

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