Difference between combustion and flame

an introduction to combustion concepts and applications pdf and turbulent non-premixed combustion
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Published Date:23-07-2017
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Turbulent Non-Premixed Combustion CEFRC Combustion Summer School 2014 Prof. Dr.-Ing. Heinz Pitsch Copyright ©2014 by Heinz Pitsch. This material is not to be sold, reproduced or distributed without prior written permission of the owner, Heinz Pitsch. Course Overview Part II: Turbulent Combustion • Turbulence • Turbulent Premixed Combustion • Laminar Jet Diffusion Flames • Turbulent Non-Premixed • Turbulent Jet Diffusion Flames Combustion • Modelling Turbulent Combustion • Applications 2 Laminar Jet Diffusion Flames flame length L fuel air 3 Round Laminar Diffusion Flame • Fuel enters into the combustion chamber as a round jet • Forming mixture is ignited • Example: Flame of a gas lighter  Only stable if dimensions are small  Dimensions too large: flickering due to influence of gravity  Increasing the jet momentum → Reduction of the relative importance of gravity (buoyancy) in favor of momentum forces  At high velocities, hydrodynamic instabilities gain increasing importance: laminar-turbulent transition 4 Laminar Diffusion Fame: Influence of Gravity 1g 0g 5 Round Laminar Diffusion Flame • Starting point: Conservation equations for stationary axisymmetric boundary layer flow without buoyancy • Continuity: flame length L • Momentum equation in z-direction • Mixture fraction fuel air 6 Round Laminar Diffusion Flame • Schmidt number Sc = μ/ρD • Farfield area  r → ∞: u = u = 0 z r  From z-momentum equation  dp/dz = 0 • Boundary layer flow: 0 flame length L • Incompressible round jet  Quiescent ambient  Constant density  No buoyancy → Similarity solution • Simularity coordinate η = r/z (Schlichting, „Boundary Layer Theory“) fuel air 7 Round Laminar Diffusion Flame • If density not constant → Transformation flame length L • a: Distance of the virtual origin of the jet from the nozzle exit • For ρ = const. und a → 0 • Implies linear spreading of the roung jet fuel air 8 Round Laminar Diffusion Flame • Introduction of a stream function → Continuity equation identically satisfied • Applying the transformation rules to the convective terms in the momentum and mixture fraction equations yields 9 Round Laminar Diffusion Flame • Such manipulations are always possible for two-dimensional stationary boundary layer flows, if a stream function and a similarity coordinate ζ ≠ f(r) can be introduced • The diffusive terms become • C: Chapman-Rubesin-Parameter • For constant density (with η = r/ζ and μ = μ ): C = 1 ∞ 10 Round Laminar Diffusion Flame • Formal transformation of the momentum and concentration equations and assumption that C = f(ζ,η) • Ansatz for stream function and for the velocities • u und u can be expressed as a function of the nondimensional stream function F and its z r derivatives 11 Round Laminar Diffusion Flame • From the momentum equation  • Similarity solution only exists, if F ≠ f(ζ) • Then, u is proportional to 1/ζ (see previous slide) z → velocity decreases linearly with 1/(z + a) • Prerequesites: Boundary conditions and C are independent of z (e. g. u = 0 and u = 0 for η → 0) z r 12 Round Laminar Diffusion Flame • Equation for the nondimensional stream function • Let ω = Z(z,r)/Z (z), ratio of the mixture fraction Z (z) to its value at r = 0 a a • Applying the same transformations to the ω-equation yields • In case of a similarity solution 13 Round Laminar Diffusion Flame • If C = const.: • The assumption C = const. Holds if and ρμ/ρ μ = const. m ∞ • C = const. Often not a good assumption, since 14 Round Laminar Diffusion Flame • Constant of integration γ can be determined from the condition that the jet momentum is independent of ζ • Substitution of the solution into the momentum balance yields • ρ : density of the fuel stream 0 • Reynolds number Re = u d/ν z,0 ∞ 15 Round Laminar Diffusion Flame • Analogously for the mixture fraction (with Z = 1) 0 → Mixture fraction on the centerline Z (z) = Z(z,r=0): a → Z decreases with 1/ζ (as the velocity) a 16 Round Laminar Diffusion Flame • Determination of the flame contour r as function of z from the condition • Flame contour intersects centerline, flame r = 0, if Z = Z length L a st • Corresponding value of z defines the flame length • Valid for laminar jet flames without buoyancy fuel air 17 Round Laminar Diffusion Flame • For a given nozzle diameter, L increases linearly with the Reynolds number Re laminar fully developed flame transition turbulent flame flame length Sc =0,72 t L Reynolds number Re fuel air 18 flame length L/d Course Overview Part II: Turbulent Combustion • Turbulence • Turbulent Premixed Combustion • Laminar Jet Diffusion Flames • Turbulent Non-Premixed • Turbulent Jet Diffusion Flames Combustion • Modelling Turbulent Combustion • Applications 19 Turbulent Jet Diffusion Flame • Shear flow at nozzle exit • Flow instabilities (Kelvin-Helmholtz-instabilities) → laminar-turbulent transition • Ring shaped turbulent shear layer propagates in radial direction • Merging after 10 to 15 nozzle diameters downstream • Streamlines are parallel in th potential core • Velocity profile reaches self similar state after 20-30 nozzle diameters 20