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Turbulent combustion (Lecture 3) Non-premixed flames

Turbulent combustion (Lecture 3) Non-premixed flames

Turbulent combustion (Lecture 3) Non-premixed

Turbulent combustion (Lecture 3)! Motivation (Lecture 1)! Basics of turbulence (Lecture 1)! Premixed-gas flames! Turbulent burning velocity (Lecture 1)! Regimes of turbulent combustion (Lecture 1)! Flamelet models (Lecture 1)! Non-flamelet models (Lecture 1)! Flame quenching via turbulence (Lecture 1)! Case study I: “Liquid flames” (Lecture 2)! ! !(turbulence without thermal expansion)! Case study II: Flames in Hele-Shaw cells (Lecture 2) ! ! !(thermal expansion without turbulence)! Nonpremixed gas flames (Lecture 3)! Edge flames (Lecture 3)! AME 514 - Spring 2013 - Lecture 9 1 Non-premixed flames! Nonpremixed: no inherent propagation rate (unlike premixed flames where propagation rate = S L )! No inherent thickness δ (unlike premixed flames where thickness ~ α/S L ) - in nonpremixed flames, determined by equating convection time = δ/U = Σ -1 to diffusion time δ 2 /α ⇒ δ ~ (α/Σ) 1/2 ! Have to mix first then burn! Burning occurs near stoichiometric contour where reactant fluxes in stoichiometric proportions (otherwise surplus of one reactant)! Burning must occur near highest T since ω ~ exp(-E/RT) is very sensitive to temperature (like premixed flames)! Simplest approach: “mixed is burned” - chemical reaction rates faster than mixing rates! Recall stoichiometric mixture fraction Z st = mass fraction of fuel stream in a stoichiometric mixture of fuel and oxidant streams! Z st = 1 1+ S ;S ! ! M Y ox ox F,"# ! F M F Y ox,# ν = stoichiometric coefficient, M = molecular weight, Y = mass fraction ! AME 514 - Spring 2013 - Lecture 9 2 • 1

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