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Examples of combustion applications: . Gas turbines and jet engines . Rocket propulsion . Piston engines . Guns and explosives . Furnaces and boilers . Flame synthesis of materials (fullerenes, nano- materials) . Chemical processing (e.g. carbon black produc- tion) . Forming of materials . Fire hazards and safety Combustion is a complex interaction of: . physical processes - fluid dynamics, heat and mass transfer . chemical processes - thermodynamics, and chemical kinetics Practical applications of the combustion phenomena also involve applied sciences such as aerodynamics, fuel technology, and mechanical engineering. ¨ . The transport of energy, mass, and momentum are the physical processes involved in combustion. . The conduction of thermal energy, the diffusion of chemical species, and the flow of gases all follow from the release of chemical energy in the exother- mic reaction. . The subject areas most relevant to combustion in the fields of thermodynamics, transport phenom- ena, and chemical kinetics can be summarized as follows: Thermodynamics: . Stoichiometry . Properties of gases and gas mixtures . Heat of formation . Heat of reaction . Equilibrium . Adiabatic flame temperature Heat and Mass Transfer: . Heat transfer by conduction . Heat transfer by convection . Heat transfer by radiation . Mass transfer Fluid Dynamics: . Laminar flows . Turbulence . Effects of inertia and viscosity . Combustion aerodynamics Chemical Kinetics: . Application of thermodynamics to a reacting system gives us - equilibrium composition of the combustion products, and - maximum temperature corresponding to this composition, i.e. the adiabatic flame tempera- ture. . However, thermodynamics alone is not capable of telling us whether a reactive system will reach equi- librium. Chemical Kinetics (cont’d): . If the time scales of chemical reactions involved in a combustion process are comparable to the time scales of physical processes (e.g. diffusion, fluid flow) taking place simultaneously, the system may never reach equilibrium. . Then, we need the rate of chemical reactions in- volved in combustion. ¨ Primary sources of combustion research literature: 1 Combustion and Flame (journal) 2 Combustion Science and Technology (journal) 3 Combustion Theory and Modelling (journal) 4 Progress in Energy and Combustion Science (re- view journal) 5 Proceedings of the Combustion Institute (Biennial Combustion Symposia (International) proceedings). 6 Combustion, Explosions and Shock Waves (journal translated from Russian) ¨ Fundamental Definitions Chemical Reaction: . exchange and/or rearrangement of atoms between colliding molecules Reactants ’ Products . The atoms are conserved (C, H, O) . On the other hand, molecules are not conserved. H+0.5(O+3.76N) ’ HO+1.88N 22222 Reactants ’ Products Amount of substance or mole numbers (mol): 23 . 1 mol of a compound corresponds to 6.023 · 10 particles (atoms, molecules, or any chemical species). 23 . Avogadro’s constant = 6.023 · 10 . Mole fraction Çof species i with mole number of i Nis i N i Ç= i S N j j=1 ¨ . Mass fraction Yof species i with mass of mis ii m i Y= i S m j j=1 . MolarorMolecularMass, M(molecular weight i is misleading and should not be used) - M=16 g/mol CH 4 - M=2 g/mol H 2 - M=32 g/mol O 2 ¨ 0.Introduction12AER 1304-OLG . Mean molar mass, M , of a mixture of species de- notes an average molar mass: 3 M =ÇM ii . S = number of species in the system MNMÇ iiii Y== i SS MNMÇ jjjj j=1j=1 YY/M iii Ç== i S MM iYj/Mj j=1 ¨ For a system of volume, V : 3 . Mass density (density), Á = m/V (kg/m) 3 . Molar density (concentration), c = N/V (kmol/m) . Mean molar mass is given by: Ám == M cN Chemical kinetics convention: concentrations c of chemical species are usually shown by species symbol in square brackets. c=[CO] CO2 2 ¨ For most conditions involved in combustion, it is satisfactory to use the perfect gas equation of state for the gas phase. o PV = NRT 3 (Pa)(m) = (mol)(J/molK)(K) o R= 8.314 J / mol K, universal gas constant P = pressure, Pa T = temperature, K ¨ When the gas phase temperatures are near or less than the critical temperatures, or when pressures are near or above the critical pressures, the density or con- centration is not correctly predicted by the perfect gas relationship. Real gas equations should be used. - van der Waals - Peng-Robinson Basic Flame Types: . Premixed Flames - Laminar - Turbulent . Non-Premixed (Diffusion) Flames - Laminar - Turbulent . Partially Premixed Flames - Laminar - Turbulent ` triple flames, edge flames,... ¨ Laminar (Turbulent) Premixed Flames: . Fuel (in gaseous form) and oxidizer are homoge- neously mixed before the combustion event . Flow is laminar (turbulent) . Turbulent premixed flames: - combustion in gasoline engines - lean-premixed gas turbine combustion Burned Unburned - Cross-section of a gasoline engine combustion chamber. Stoichiometry: . A premixed flame is stoichiometric if the premixed reactants contain right amount of oxidizer to con- sume (burn) the fuel completely. • If there is an excess of fuel: fuel-rich system • If there is an excess of oxygen: fuel-lean system • Standard air composition commonly used for com- bustion calculations: O+3.762N 22 ¨ Stoichiometry (cont’d): CH+5(O+3.762N) ’ 3822 4HO+3CO+18.81N 222 . (A/F )=air-to-fuel ratio (mass)= (mass of stoich air)/(mass of fuel) . (A/F )=[5(32+3.762*28)]/(44) = 15.6 stoich . ¦ =(A/F )/(A/F )= Fuel Equivalence stoichactual Ratio ¨ Stoichiometry (cont’d):: . ¦ =1: stoichiometric combustion . ¦ < 1: lean mixture, lean combustion . ¦ > 1: rich mixture, rich combustion . European convention (and to a certain extent Japanese) is to use Air equivalence ratio, »: . =1/¦ . In certain industries, excess air ratio, excess oxygen, and similar terminologies are also used. ¨ Laminar (Turbulent) Non- Premixed Flames: • Fuel (in gaseous form) and oxidizer are mixed/come in to contact during the combustion process • A candle flame is a typical laminar non-premixed (diffusion) flame • Turbulent non-premixed flames: - hydrogen rocket engine - current aero gas turbines - diesel engines A candle flame. ¾ Air Inlet ¾ Inlet Port Design ¾ Chamber Design ¾ Turbocharge AIR MOTION / TURBULENCE IN THE COMBUSTION CHAMBER FUEL-AIRPARTIALLYMOSTLY EXHAUST MIXINGIGNITION"PREMIXED"NON-PREMIXED EMISSIONS PROCESSCOMBUSTIONCOMBUSTION INJECTION AND SPRAY Fuel PropertiesEGR CHARACTERISTICS y HEAT RELEASE ¾ Injection Timing y RADIATION EXCHANGE BETWEEN ¾ Injection System Design HOT AND COLD POCKETS ¾ Injection Duration y NO & SOOT FORMATION ¾ Injection RateX y SOOT OXIDATION Processes in the diesel engine combustion. ¨ Spark-ignited gasoline engine TURBULENT Low-NO stationary gas turbine x PREMIXED Flat flame LAMINAR Bunsen flame Aircraft turbine Hydrogen-oxygen rocket motor TURBULENT Diesel engine NON-PREMIXEDPulverized coal combustion (DIFFUSION) Candle flame LAMINARRadiant burners for heating Wood fire FUEL/OXIDIZERFLUID EXAMPLES MIXINGMOTION Examples of combustion systems. ¨ |