5310 Combustion Introduction Lecture 17

5310 Combustion Introduction Lecture 17 - MAE 5310:...

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Unformatted text preview: MAE 5310: COMBUSTION FUNDAMENTALS Laminar Premixed Flames Example, Applications and Comments Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk LAMINAR PRE-MIXED FLAME EXAMPLE Laminar flame structure. Temperature and heat-release rate profiles based on experiments of Friedman and Burke Reference: Turns An Introduction to Combustion Estimate the laminar flame speed, S L , of a stoichiometric propane (C 3 H 8 )-air mixture using the simplified theory of Spalding developed in class Make use of a global, one-step reaction mechanism to estimate the mean reaction rate PRINCIPAL CHARACTERISTICS OF LAMINAR PREMIXED FLAMES Definition of flame speed, S L Temperature profile through flame Product density is less than the reactant density so that by continuity the velocity of the burned gases is greater than the velocity of the unburned gases For a typical hydrocarbon-air flame at atmospheric pressure, the density ratio is about 7 Convenient to divide the flame into two zones 1. Preheat zone : little heat is released 2. Reaction zone : most of the chemical energy is released 2.a Thin region of fast chemistry Destruction of fuel molecules and creation of intermediate species Dominated by bimolecular reactions At atmospheric pressure, fast zone is usually less than 1 mm Temperature and species concentration gradients are very large The large gradients provide the driving forces for the flame to be self-sustaining, i.e. diffusion of heat and radical species from the reaction zone to the preheat zone 2.b Wider region of slow chemistry Chemistry is dominated by three-body radical recombination reactions, such as the final burn-out of CO via CO + OH CO 2 + H At atmospheric pressure, this zone may extend several mm LAMINAR PREMIXED FLAMES: SIMPLIFIED ANALYSIS Analysis couples principles of heat transfer, mass transfer, chemical kinetics, and thermodynamics to understand the factors governing: Flame speed, S L Flame thickness, (ANSWER, =2 /S L ) Simplified approach using conservation relations Assumptions: 1. 1-D, constant area, steady flow 2. Neglect: kinetic and potential energy, viscous shear work, thermal radiation 3. Constant pressure (neglect small pressure difference across flame) 4. Diffusion of heat governed by Fouriers law 5. Diffusion of mass governed by Ficks law (binary diffusion) 6. Lewis number (Le /D) unity 7. Individual specific heats are equal and constant 8. Fuel and oxidizer form products in a single-step exothermic reaction 9. Oxidizer is present in stoichiometric or excess proportions; thus, the fuel is completely consumed at the flame. VON KARMAN INTEGRAL ANALYSIS OF F.P.B.L....
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5310 Combustion Introduction Lecture 17 - MAE 5310:...

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