Presentation on theme: "Lecture 20: Laminar Non-premixed Flames – Introduction, Non-reacting Jets, Simplified Description of Laminar Non- premixed Flames Yi versus f Experimental."— Presentation transcript:
1Lecture 20: Laminar Non-premixed Flames – Introduction, Non-reacting Jets, Simplified Description of Laminar Non- premixed Flames Yi versus f Experimental DataQualitative characteristics of laminar non-premixed or diffusion (of fuel and oxidizer) flames.Review of conserved scalar concept.Role of the momentum equation in deflagration regime: Non-reacting jet mixing solution.Simplified theoretical description of a laminar non-premixed (or diffusion) flame.
4Nonpremixed flamesFuel (F) and oxidizer (O) are stored apart.When combustion is desired, F and O must come together at the molecular level.How many molecules of each decides interim and final products and their temperature.Staged pre or post reaction mixing and rich and lean reactions all lead to different products.Specific strategies such as R-Quench-L, Lean Direct Injection, Direct Injection-Spark Ignition have emerged.Learning the non-premixed flame regime is important.Learning about equipment for specific strategies is critical
5Mixture Fraction, Mixedness, Progress Variables, Reaction Fraction, and ReactednessConcept of a conserved scalar is very useful for nonpremixed flames.A conserved scalar is a quantity defined such that there are no sink or source terms in the conservation equation for that quantity. (Sink and source terms result from reactions, heat transfer, and work transfer) Quantification of how non-(pre)mixed and whenTotal Energy is a conserved scalar in the absence of net heat loss to or work done on boundaries.Elemental Mass Fractions, Fraction of Mass that originated in the fuel stream(s) and Fraction of Mass that originated in the oxidizer stream(s) are all conserved scalars.
6Review of Conserved Scalar, Definition of Mixture Fraction In nonpremixed flames species mass fractions very continuously as mixing at the molecular level and chemical reaction occurs.Definition of mixture fraction f:Independent of the progress ofreaction. That meansCH4+O2 have the same mixturefraction as CO+H2O+H2Both are f=16/(16+32)=(12+2+2)/( )=16/48= 1/3Fuel Mass+ Mass in products that came fromfuelf stoich= 16/16+64= 16/80=0.2
7Review of Conserved Scalar, Definition of Mixture Fraction Consider the three-"species" reaction:For this system the mixture fraction will be:
8Conservation equation for the mixture fraction. Assume all species diffuse at the same rate:
9Review of Conserved Scalar, Definition of Mixture Fraction Divide the product species conservation equation by and add to the fuel species conservation equation:Substituting for the mixture fraction we obtain:
10Review of Conserved Scalar, Definition of Mixture Fraction When kinetic energy is neglected (along with potential energy, thermal radiation, viscous dissipation, differential diffusion) we can write a similar equation for the absolute enthalpy:These conserved scalar equations, in cylindrical coordinates, will be very useful for our study of laminar non-premixed flames.
11Solution for a Non-reacting, Constant Density Laminar Jet Consider first the case of a laminar jet of fuel issuing into air with no chemical reaction. Assuming the air and fuel have the same density, there is an analytical solution for the flow field away from the potential core region of the jet.
12Solution for a Non-reacting, Constant Density Laminar Jet
13Assumptions: Non-reacting, Constant Density Laminar Jet - MW(jet fluid) = MW(air), ideal gases.- Constant P, T, and r throughout the flow field.- Steady state.- Fick's law applies.- Equal species and momentum diffusivities, Sc = n /D = 1.- Neglect axial diffusion of momentum and species.- Solution applies downstream of the jet core region.14/36
15Boundary Conditions for a Non-reacting, 𝜌=C Laminar Jet Along the jet centerline:Far from the jet:At the jet exit plane , r ≤ R:At the jet exit plane, r > R:
16Solution for a Non-reacting Constant Density Laminar Jet The solution to this problem can be found in Schlichting Boundary Layer Theory for the region of the flow beyond the jet core where the flow is similar. The solution is given by:
17Axial velocity distribution: Fuel mass fraction distribution (assuming Sc = n/D = 1):
18Infinitely Fast Kinetics Infinitesimal Flame Sheet Approximation for Nonpremixed FlamesLaw, Combustion Physics, 2006Finite-rate KineticsInfinitely Fast Kinetics
19Simplified Theoretical Description of Laminar Jet Diffusion Flame • Assume:1. Laminar, steady, axisymmetric flow2. Three "species": fuel, product, oxidizer3. Flame (reaction) sheet assumption, infinitely fast chemical kinetics4. Equal species diffusivities5. Le = 16. No radiation transport7. Axial diffusion is neglected8. Vertical flame axis
20Conservation Equations: Cylindrical Coordinates, Thin Flame Conservation of MassConservation of Axial MomentumConservation of Fuel Mass Fraction (Inside the Flame Sheet)Conservation of O2 Mass Fraction (Outside the flame sheet)Conservation of Product Mass Fraction (Everywhere)
21Simplified Theoretical Description of Laminar Jet Diffusion Flame Conservation of Species Mass Fraction
22Conserved Scalar Equations for Laminar Jet Flame Boundary ConditionsAt the jet exit plane
23Non-dimensional Laminar Jet Diffusion Flame A dimensionless enthalpy is defined:The non-dimensional conservation equations and boundary conditions for h* and f are identical, and therefore h* = f.The non-dimensional conservation equations and boundary conditions for h* and f are identical, and therefore h* = f.
24Description of Global Fast Chemistry Assume that the reaction kinetics are described by a single-step, three-species reaction:Inside the flame sheetOutside the flame sheet
25State Relationship for Temperature: Fuel Side Assume constant heat capacities, and that for the jet fluid and oxidizer far from the jet, T = 298 K.Inside the flame sheet:Oxidizer = air Solve the h* = f equation for T :
26State Relationship for Temperature: Oxidizer Side
27Experimental Support for State Relationships for Major Species