Free Atoms and Radicals - Assignment Example

Exercises 1. Reduce the following dimension to its simplest form: (Power/Pressure)1/3 2. Convert in the SI units the following values: a) 5cm/microsecond b) 0.36 x 10 - 10 tons km/min2 1. What are free atoms and radicals? What is the difference between an ion, free atom and radical? Atom is the smallest indivisible particle that has an independent existence. Atoms are composed of protons, electrons and neutrons. (http://education.jlab.org/qa/atom.html) Radicals or free radicals are either atoms or molecules that have some unpaired electrons. Due to the presence of this electron, the free radicals play an important part in combustion. (Oakley, R.T. Prog. Inorg. Chem. 1998, 36, 299). An ion is an atom or group of atoms that contains a net electric charge due to either losing a cation or gaining an electron. (http://en.wikipedia.org/wiki/Category:Ions) Explain chemical bonds in the molecule of methane. The formula for methane is CH4. One molecule of methane consists of one atom of carbon, combined with four atoms of hydrogen. Exercise 1. Calculate the vapour densities (kg/m3) of pure C5H12 at 25oC and 1 atm = 1.013 105 Pa. (Assume ideal gas behaviour.) What are the stoichiometric, fuel lean and fuel rich mixtures? Give an example. Stoichiometric mixture is a mixture of substances that react to give products with no excess reactant. It is typically a fuel/air mixture in which both the fuel and the oxygen in the air are completely consumed. is called the "stoichiometric" mixture. For gasoline fuel, the stoichiometric air/fuel mixture is approximately 14.7 times the mass of air to fuel. Any mixture less than 14.7 to 1 is considered to be a rich mixture, whereas the mixtures in which the air/fuel ratio is more than 14.7 to 1 is a lean mixture. (http://en.wikipedia.org/wiki/Air-fuel_ratio) What is the concentration and its units of measurements? What is a mole? The mole is a unit of amount of substance. Its is the unit used to measure the physical quantity. The mole is also defined as the amount of substance of a system which contains atoms, molecules, ions, and electrons. Explain temperature and concentration dependence of the chemical reaction rate and Arrhenius equation. The rate of reaction typically increases with concentration. As reactant concentration increases, the frequency of collision increases. Similarly, the increase in temperature also increases the rate of reaction by enhancing the activation energy of the colliding particles. (http://en.wikipedia.org/wiki/Reaction_rate) The Arrhenius equation is a simple formula for determining the dependence of rate constant on the temperature. It therefore, also determines the rate of a chemical reaction. At higher temperatures, the collision rate of two molecules is also higher. This higher collision rate results in a higher kinetic energy, which has an effect on the activation energy of the reaction (http://www.shodor.org/UNChem/advanced/kin/arrhenius.html). Exercise 1. Compare 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). Analyze thermal explosion in adiabatic conditions and the mechanism of selfaccelerating reaction. Define induction period. In adiabatic condition, no heat is transferred. However, if the adiabatic temperature rise is sufficient to trigger undesired reactions, it can lead to a thermal explosion. Self accelerating reactions may occur when the combustible substances involved cross their self-accelerating decomposition temperature (SADT). Moreover, during storage there can be a loss of activity if the temperature is above the recommended storage temperature The induction period is defined as the initial slow phase of the chemical reaction after which the reaction accelerates. Induction period is commonly found in radical reactions, but may occur in other reactions as well. (http://www.iupac.org/goldbook/I03019.pdf) 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. 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. Exercise 1. Calculate 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. Describe the process of ignition of a solid combustible material by a hot plate. Explain evolution of the temperature field in the solid material. The hot plate technique contains significant temperature gradients for heating solid combustibles. The punches experience the highest temperatures, while the minimum is on the outer die surface, which is where the temperature is monitored for control purposes. The temperature difference between the specimen and die surface of the hot plate increases with temperature. Why do flames propagate through a combustible mixture? A flame is the light-emitting visible part of a fire. A flame is commonly caused by a highly exothermic reaction like combustion or a self-sustaining oxidation reaction. The color and temperature of a flame are dependent on the type of fuel involved in the combustion. What are the flame front and flame propagation velocitiy? Why a gas particle entering flame front is accelerated? A flame front develops when a flammable mixture of vapour or gas comes in contact with an ignition source. If a flame front propagates at a speed less than the speed of sound in the vapour, it is known as a deflagration. If a flame front that propagates at a shock wave at the speed of sound in the vapour is known as detonation. What is adiabatic flame temperature? In combustion, there are two types of adiabatic flame temperature depending on how the combustion process is completed, the constant volume and constant pressure. The constant volume adiabatic flame temperature is the temperature that results from a complete combustion process that occurs without any heat transfer or changes in kinetic or potential energy of the combustible substances. The comparatively lower temperature that results from a complete combustion process that occurs without any heat transfer or changes in kinetic or potential energy is known as the constant pressure adiabatic flame temperature is. 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? Compare the main features of premixed flame and detonation. In a premixed flame the oxidizer is mixed with the fuel before it reaches the flame front, thereby creating a thin flame front as all of the reactants are readily available. If the flow of the fuel–oxidizer mixture is laminar, the flame speed of premixed flames is dominated by the chemistry. The flame will move upstream until the fuel is consumed or until it encounters a flame holder if the flow rate is below the flame speed. As a result, the flame front of most premixed flames is roughly conical. Detonation is the combustion process wherein a supersonic shock wave is propagated through a fluid due to an energy release in a reaction zone. Detonation based combustion is a more powerful combustion. In a detonation, the shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. Explosives, reactive gaseous mixtures, certain dusts and aerosols can cause detonation. Describe internal structure of detonation wave. The internal structure of the detonation wave may vary with the combustible substance being used. Numerical exercise 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. Compare a jet fire and buoyancy dominated fire, using the flame height-jet velocity diagram to explain flame shape. Explain the low value of the Froude number for natural fires. Describe fluid-dynamic structure (air entrainment, buoyant flow, eddies) of a fire plume. Describe the flash point, fire point and auto-ignition temperature for combustible liquids. How these characteristic are measured in laboratories? Flash point is the minimum temperature at which a liquid fuel gives off sufficient vapours to form an ignitable mixture. The Firepoint is the temperature at which liquid produces enough vapours to support combustion once ignited. The auto-ignition temperature, also known as the kindling point of a substance is the lowest temperature at which the substance will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. What is BLEVE? Explain possible effects of accidental liquid fuel releases on the surrounding. BLEVE stand for boiling liquid expanding vapor explosion. It is a type of explosion that can occur when a vessel containing a pressurized liquid gets ruptured. Such explosions can be extremely hazardous. The liquid fuel release due to BELVE can result in environmental pollution, extensive fire, and release of hazardous gases. Discuss the main factors influencing flame spread over solid materials. Many factors influence fire spread within buildings, and one of the most important is the interior finish material, (Björn Karlsson, James G. Quintiere, Enclosure fire dynamics). The primary factors that influence the spread of flames include: Size and location of ignition source Type, amount, position, spacing of fuel packages Geometry of enclosure Material properties (http://www.acousticalsurfaces.com/soundproofing_tips/html/flame_spread.htm) Numerical exercise 1. Calculate the average flame height for a pool gasoline fire. Diameter of pool is 4m. Define heat of combustion, heat release rate and combustion efficiency. The heat of combustion denoted as (ΔHc0) is the amount of energy released when one mol of a compound undergoes complete combustion with oxygen. The heat release rate or the HRR is the engine that drives combustion. The heat release rate of HRR is the rate at which heat is generated by fire. It is measured in Joules per second, also called Watts. Combustion efficiency is a measure of the heating efficiency of a boiler. It is equivalent to the 100 percent minus the percentage of heat lost up the vent called "flue loss" or "stack loss". The combustion efficiency also measures the total heat energy that is spread through the heating system or escapes from the boiler jacket itself, rather then going up the stack or flue to warm up the environment. Describe three zones in turbulent diffusion flame (fire plume). Why is thermal radiation of importance in fire? Thermal radiation is the electromagnetic radiation in the wavelength range of 0.1 to 100 microns. This radiation arises due to an existing temperature difference between two bodies. This radiation enhances fire by transferring heat between bodies that are not in contact. (http://www.efunda.com/formulae/heat_transfer/radiation/overview_rad.cfm). Exercises 1. Estimate the gas velocity in in the fire plume. 2. Estimate the Froude number in fire plume. 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. What is a positive thermal feedback for fires in enclosures? What is flashover and backdraft? A flashover is the nearest simultaneous ignition of a conbustible material in an enclosed area when surfaces surrounding it are heated to the temperature at which the flammable gases that are being produced from the combustible materials in the space are hot enough to ignite. Flashover normally occurs at 500 °C (930 °F) or 1,100°F. A backdraft occurs when a fire has less oxygen and consequently combustion ceases but the fuel gases and smoke remain at high temperature. If oxygen is re-introduced to the fire by opening a door to a closed room, combustion can restart often resulting in an explosive effect as the gases heat and expand. 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. Explain fuel-controlled and oxygen-controlled regimes of fire in an enclosure. Describe the main flow patterns associated with fire development in enclosures. What are advantages and limitations of zone models? Zone models are simple computer models that divide the considered fire compartments into separate zones, where the condition in each zone is assumed to be uniform. The simplest model is a one-zone model for post-flashover fires, in which the conditions within the compartment are assumed to be uniform and represented by a single temperature. In addition, the cohesive zone model, which was initially applied to concrete and cementitious composites, can be used with success for other materials. What is field modelling of fires? What are objectives of CFD fire modelling? Fire field modelling is used for analyzing the fire situation and suggesting appropriate fire detector locations. The primary objective of the Computational Fluid Dynamics (CFD) based fire field modelling is to assist the fire safety approvals authority like fire brigade, local government authority in assessing the appropriateness of using a particular model for a particular fire application. (http://fseg.gre.ac.uk/fire/running_SMARTFIRE_in_parallel.html) Describe verification and validation for field modelling? Why validation is necessary? References 1. http://en.wikipedia.org/wiki/Category:Ions 2. Oakley, R.T. Prog. Inorg. Chem. 1998, 36, 299 3. http://wiki.answers.com/Q/What_is_the_chemical_formula_for_methane 4. http://www.visionlearning.com/library/module_viewer.php?mid=60 5. http://en.wikipedia.org/wiki/Air-fuel_ratio 6. http://www.ae.gatech.edu/people/bzinn/AE4451f08hw3.pdf 7. http://en.wikipedia.org/wiki/Reaction_rate 8. http://www.shodor.org/UNChem/advanced/kin/arrhenius.html 9. http://www.acousticalsurfaces.com/soundproofing_tips/html/flame_spread.htm 10. http://fseg.gre.ac.uk/fire/running_SMARTFIRE_in_parallel.html) Read More