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Formazione e Controllo di Inquinanti nella Combustione Impianti di trattamento effluenti Combustion Theory 2: premixed and diffusion flames Prof. L.Tognotti.

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Presentation on theme: "Formazione e Controllo di Inquinanti nella Combustione Impianti di trattamento effluenti Combustion Theory 2: premixed and diffusion flames Prof. L.Tognotti."— Presentation transcript:

1 Formazione e Controllo di Inquinanti nella Combustione Impianti di trattamento effluenti Combustion Theory 2: premixed and diffusion flames Prof. L.Tognotti Dipartimento di Ingegneria Civile e Industriale Corso di Laurea Magistrale in Ingegneria Chimica/ Ingegneria Energetica Anno Accademico

2 Combustion Theory 2 Introduction Basic Flame Types: Premixed flames: fuel and oxidizer are homogeneously mixed before reaction occurs. Laminar and turbulent premixed flamesPremixed flames: fuel and oxidizer are homogeneously mixed before reaction occurs. Laminar and turbulent premixed flames Non premixed flames: fuel and oxidizer come into contact during combustion process. Laminar and turbulent diffusion flamesNon premixed flames: fuel and oxidizer come into contact during combustion process. Laminar and turbulent diffusion flames

3 1.Laminar premixed flames

4 Combustion Theory 4 1.Laminar premixed flames A premixed flame is a self-sustaining propagation of a localized combustion zone at subsonic velocities (deflagration regime) The classical device to generate a laminar premixed flame is the Bunsen burner: Typical Bunsen burner flame

5 Combustion Theory 5 Inner cone (dark zone): fuel rich flame Preheating region containing fuel and air Outer cone (luminous zone): reaction and heat transfer Outer diffusion flame Typical Bunsen-burner flame is a dual flame: a fuel-rich premixed inner flame a diffusion outer flame: CO and H 2 from inner flame encounter ambient air Example: Typical Bunsen-burner CH 4 /Air flame

6 Combustion Theory 6 Experimental evidence for the presence of a cool inner preheating regionExperimental evidence for the presence of a cool inner preheating region A wire to reveal the presence of a cool preheating region containing unburned CH 4 and O 2 A match in preheating region does not ignite until it is moved to the inner cone

7 Combustion Theory 7 Fuel/Air Ratio Flame colour, i.e. colour of the outer cone Fuel leanStochiometric Fuel richVery fuel rich Deep Violet due to large concentrations of excited CH radicals BlueGreen due to large concentrations of C 2 species Yellow due to carbon particles High-T burned gases usually show a reddish glow due to radiation from CO 2 and H 2 O Flame characteristics for hydrocarbon-air stochiometric mixtures The flame is ~1 mm thick and moves at ~0.5 m/s Pressure drop through the flame is very small: ~1 Pa Temperature in reaction zone is ~ K Density ratio of reactants to products is ~7 2 sub regions exist: a fast chemistry zone, dominated by bimolecular reactions, and a slow chemistry zone (CO+OH=CO 2 +H) Basic features of laminar premixed flamesBasic features of laminar premixed flames

8 Combustion Theory 8 Thermal expansion through the flame front Normal component of velocity vector Tangential component of velocity vector At steady-state the burning velocity equals the flow velocity of the unburnt mixture normal to the flame front Kinematic balance for a steady oblique flameKinematic balance for a steady oblique flame

9 Combustion Theory 9 Laminar burning velocity, S L : It is the velocity with which the flame front propagates normal to itself into the unburned mixture Laminar flame theory: What do we particularly feel interested in?Laminar flame theory: What do we particularly feel interested in? Various flame theories attempt to predict the laminar flame propagation from physical and chemical properties: Thermal theory (Mallard and Le Chatelier; Zeldovich, Frank-Kamenetsky and Semenov): the mixture is heated by conduction to the point where the reaction is sufficiently rapid to become self-propagatingThermal theory (Mallard and Le Chatelier; Zeldovich, Frank-Kamenetsky and Semenov): the mixture is heated by conduction to the point where the reaction is sufficiently rapid to become self-propagating Diffusion theory (Tanford and Pease): diffusion of active species, such as atoms and radicals, from the reaction zone into the unreacted mixture causes reaction to occurDiffusion theory (Tanford and Pease): diffusion of active species, such as atoms and radicals, from the reaction zone into the unreacted mixture causes reaction to occur Steady combustion wave propagation REALITY: Diffusion Of Heat And Active Radicals

10 Combustion Theory 10 Thermal theory of Mallard and Le Chatelier:Thermal theory of Mallard and Le Chatelier: Energy balance on preheat zone: The flame consists of two zones: Pre-heat zone: the unburned gases are heated by conduction and reach ignitionPre-heat zone: the unburned gases are heated by conduction and reach ignition Reaction zone: chemical enthalpy is converted into sensible enthalpyReaction zone: chemical enthalpy is converted into sensible enthalpy Laminar flame-front thickness: Laminar flame speed: is a mean reaction rate, evaluated at T i

11 Combustion Theory 11 Example : Evaluate S L for a n-th order homogeneous chemical reaction: For 2-nd order reactions, laminar flame speed is not affected by pressureFor 2-nd order reactions, laminar flame speed is not affected by pressure Most of elementary reactions are second order reactionsMost of elementary reactions are second order reactions The reaction rate for species i (reactant) is ginen by: or, in terms of molar fractions: being The laminar flame speed dependence on pressure is then:

12 Combustion Theory 12 Comprehensive theory of Zeldovich, Frank-Kamenetsky and SemenovComprehensive theory of Zeldovich, Frank-Kamenetsky and Semenov Basic assumptions of the theory This theory is based on Mallard and Le Chatelier’s idea but takes into account: Species conservationSpecies conservation Energy equationEnergy equation 1-D and steady flame1-D and steady flame The pressure is constantThe pressure is constant Specific heat and thermal conductivity are constantSpecific heat and thermal conductivity are constant Unity lewis numberUnity lewis number Lean-fuel conditions and first order reaction rateLean-fuel conditions and first order reaction rate T i is replaced with T b in the estimation of reaction ratesT i is replaced with T b in the estimation of reaction rates SpeciesLe i H2H2 0.3 OH0.7 H2OH2O0.9 CH 4 1 O2O2 1.1 CO 2 1.4

13 Combustion Theory 13 What do we get?: Laminar flame speed Species and energy conservation H 2 is characterized by the maximum flame speed: H 2 diffusivity is much greater than that of hydrocarbon fuels. Le H2 <

14 Combustion Theory 14 Factors influencing flame velocity and thicknessFactors influencing flame velocity and thickness Effect of Equivalence Ratio Flame speed peaks occur at stochiometric or slightly fuel-rich mixtures (highest burned gases temperatures) Effect of Fuel Molecular Structure Flame speed is proportional to and, thus, it depends on M w,:

15 Combustion Theory 15 Factors influencing flame velocity and thickness Effect of temperature and pressure Laminar flame speed for stochiometric methane-air mixtures as a function of pressure and reactants’ temperature Laminar flame speed correlation for stochiometric methane-air mixtures

16 Combustion Theory 16 Turbulent flames Most of combustion devices operate in turbulent flow regime, i.e. internal combustion or aircraft engines, industrial burners and furnaces. Laminar combustion applications are almost limited to candles, lighters and some domestic furnaces. Turbulence increases the mixing processes thus enhancing combustion.Most of combustion devices operate in turbulent flow regime, i.e. internal combustion or aircraft engines, industrial burners and furnaces. Laminar combustion applications are almost limited to candles, lighters and some domestic furnaces. Turbulence increases the mixing processes thus enhancing combustion. Also combustion influences turbulence. Heat release due to combustion causes very strong flow accelerations through the flame front (flame- generated turbulence). Moreover, huge changes in kinematic viscosity associated with temperature changes may damp turbulence leading to flow re- laminarizationAlso combustion influences turbulence. Heat release due to combustion causes very strong flow accelerations through the flame front (flame- generated turbulence). Moreover, huge changes in kinematic viscosity associated with temperature changes may damp turbulence leading to flow re- laminarization

17 Turbulence

18 Turbulent shear flows

19 Combustion Theory 19 Fundamental aspects of turbulenceFundamental aspects of turbulence Turbulence is the most complex phenomena in non reacting fluid mechanics. Various time and length scales are involved and the structure and description of turbulence remain an open question Streamlines at a cross-section through pipe flow for three different Reynolds numbers The flow behaves chaotic in an ”eddy-like” pattern. With increasing Reynolds number the range of scales present in the flow also increases. Since the macro-structures are confined by the geometry of the flow, the broadening of the scales can only be achieved by introducing smaller scales. Re

20 Combustion Theory 20 Phenomenological description of turbulencePhenomenological description of turbulence Energy cascade theory (Rischardson, 1922) 1.Turbulence is organised as an hierarchy of eddies of various scales. 2.The biggest eddies, which are anisotropic and have the size of the flow geometry, break up in smaller eddies which on their turn break up in even smaller eddies. Kinetic energy is transferred from larger to smaller eddies, without any dissipation 3.This process continues until viscous forces can no longer be neglected. At the smallest scales the incoming energy is dissipated into heat. Kolmogorov (1941) hypothesis: 1.During the cascade process the anisotropy is gradually lost and from a certain eddy size the scales are homogenous and isotropic. So, a range of scales exist where the flow is locally homogeneous and isotropic. This regime is universal, independent of the type of flow, and determined by only two parameters, i.e. the energy dissipation rate and the coefficient of viscosity.

21 Combustion Theory 21 Phenomenological description of turbulencePhenomenological description of turbulence Energy dissipation may be defined as the ration of turbulent kinetic energy (u’ 2 ) and the characteristic energy transfer time in the larger eddies (l/u’):Energy dissipation may be defined as the ration of turbulent kinetic energy (u’ 2 ) and the characteristic energy transfer time in the larger eddies (l/u’): The macroscopic Reynolds number is:The macroscopic Reynolds number is:. The relation between large and small scales:The relation between large and small scales: With increasing Reynolds number, turbulence develops an even finer structure, which is very difficult to model.

22 Combustion Theory 22 Turbulence effect on transport processesTurbulence effect on transport processes 1.Turbulent diffusion (Taylor diffusion): as a consequence of the energy transfer from larger to smaller scales, scalar quantities gradients (temperature, species concentration) are reduced from a macro to a micro scale 2.Molecular diffusion: fluid volumes deformation due to turbulent whirls leads to a strong increase of contact surface area between regions of fluid with different properties, thus enhancing mixing. Deformation of a fluid element subject to turbulent scales (a) larger and (b) smaller than characteristic element size

23 Combustion Theory 23 Turbulence/Combustion interactionsTurbulence/Combustion interactions  Combustion enhancement Turbulence enhances mixing processes and combustion rate, leading to an increase of flame front area per unit cross sectional area. Typical turbulent flame speed are 5-10 m/s (laminar flame speed ranges between m/s)  Re-laminarization Kinematic viscosity of air increases roughly as T 1.7. For a flame with temperature ratio T b /T u =8 the Reynolds number is about 40 times smaller in burnt than in fresh gases. Flow may become laminar after ignition  Flame-generated turbulence: The flow acceleration due to heat release through the flame front may be significant. For typical hydrocarbon flames, the speed difference through the flame front is of order of 4 m/s (considering T b /T u =8 and S L =0.5).

24 Combustion Theory 24 Turbulent premixed flames combustion regimes: Borghi’s diagramTurbulent premixed flames combustion regimes: Borghi’s diagram What do we need? Some definitions: Damköhler number, Da: This number compares the turbulent time scale, τ T, and the chemical time scale, τ T :Damköhler number, Da: This number compares the turbulent time scale, τ T, and the chemical time scale, τ T : In the limit of high Damköhler numbers (Da>>1), the chemical time is short compared to the turbulent time, corresponding to a thin reaction zone distorted and convected by the flow field. The internal structure of the flame is not affected by turbulence and may be described as a laminar flame element, called flamelet, wrinkled and strained by the turbulence motions. On the other hand, a low Damköhler numbers (Da=1) corresponds to a slow chemical reaction. Reactants and products are mixed by turbulent motion before reaction and reaction rate is dominated by chemistry. This represents the limit of a perfectly stirred reactor, in which flame speed is independent of Reynolds number. Most practical situations correspond to high to medium values of the Damköhler numbers

25 Combustion Theory 25 Turbulent premixed flames combustion regimes: Borghi’s diagramTurbulent premixed flames combustion regimes: Borghi’s diagram What do we need? Some definitions: 2.Karlovitz number, Ka: This number refers to the smallest scales of the flow and compares the chemical time scale to the Kolmogorov time, τ k :

26

27 Combustion Theory 27 Borghi diagram J.F.Griffiths, J.A.Barnard ‘Flame and Combustion’ Blackie Academic London 1994 Damkohler = Il n.o di Karlovitz è una misura della curvatura e non stazionarietà della superficie della fiamma Intesità della turbolenza Velocità di fiamma laminare Macroscala della turbolenza spessore del fronte di fiamma l K =microscala della turbolenza

28 Combustion Theory 28 Turbulent premixed flames combustion regimesTurbulent premixed flames combustion regimes Wrinkled laminar-flame regime Flame regimes:Flame regimes: (a) Wrinkled flames(a) Wrinkled flames (b) Thin reaction sheets(b) Thin reaction sheets (c) Flamelets in eddies(c) Flamelets in eddies (d) Distributed reactions.(d) Distributed reactions.

29 Combustion Theory 29 1.Ka<1. The chemical time scale is shorter than any turbulent time scale and the flame thickness is smaller than the smallest turbulent length scale, the Kolmogorov scale. The flame front is thin, has an inner structure close to a laminar flame and is wrinkled by turbulent motions. This regime is called “thin flame regime” or “flamelet regime” and is divided in two sub-regions, depending on the ratio u’/S L Wrinkled laminar-flame regime Chemical reactions occur in thin sheets (thinner than Kolmogorov scale)Chemical reactions occur in thin sheets (thinner than Kolmogorov scale) Damkohler number always greater than 1. Fast chemistryDamkohler number always greater than 1. Fast chemistry Flame becomes wrinkled increasing flame surfaceFlame becomes wrinkled increasing flame surface Flame speed 3~5 times laminar burning velocityFlame speed 3~5 times laminar burning velocity

30 Combustion Theory 30 2.Da>1 Ka>1. The turbulent integral time scale is still larger than chemical time scale but Kolmogorov scales are smaller than the flame thickness and are able to modify the inner structure of the flame. The structure of the flame is no longer laminar but it is still wrinkled. 3.Da<1. Turbulent motions have shorter characteristic times than chemical reaction: mixing is fast and the overall reaction rate is limited by chemistry. This regime is known as perfectly stirred reactor regime

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32 Thin reaction zone regime

33 Broken reaction zone regime

34

35 Combustion Theory 35 Turbulent flame speedTurbulent flame speed Damkohler analysis (1940): S T as a function of turbulent Reynolds number Re < 2300: flame speed is independent of Reynolds numberRe < 2300: flame speed is independent of Reynolds number 2300 < Re < 6000: turbulent flame speed is proportional to the square root of Reynolds number. Turbulent eddies are much smaller than the flame thickness. Thus, the increase of flame speed is mainly due to the enhancement of transport processes within the combustion wave2300 < Re < 6000: turbulent flame speed is proportional to the square root of Reynolds number. Turbulent eddies are much smaller than the flame thickness. Thus, the increase of flame speed is mainly due to the enhancement of transport processes within the combustion wave Re > 6000: flame speed is proportional to the Reynolds number. Turbulent eddies are much larger than the flame thickness and they distort the otherwise smooth laminar flame front. The flame front surface per unit cross section increases and the flame speed raisesRe > 6000: flame speed is proportional to the Reynolds number. Turbulent eddies are much larger than the flame thickness and they distort the otherwise smooth laminar flame front. The flame front surface per unit cross section increases and the flame speed raises Flame front images obtained by LIF at various turbulence intensities. Numbers under the images indicate the magnitude of the normalized turbulence intensity, i.e. u’/S L

36 Combustion Theory 36 Turbulent flame speed calculationTurbulent flame speed calculation < Re < The molecular transport processes are enhanced. S L evaluation formula may be recalled: if both the turbulent Prandtl number (Pr=ν T /α T ) and the molecular Prandtl number (Pr=ν/α) are approximately equal to unity, then we obtain: For pipe flow ν T /ν=0.01Re: 2.Re > The inner structure of the flame is not altered (molecular transport properties remain the same) but flame surface is wrinkled. The laminar flame speed remain contsant and, thus, flame surface depends on flow velocity, u’:

37 Combustion Theory 37 Turbulent flame speed calculationTurbulent flame speed calculation 3.Some correlations for turbulent flame speed in the limit of high Reynolds number (Re>6000): Schelkin Analysis: surface are distorted into cones whose base area is proportional to the square of the average eddy diameter, l. Clavin e Williams Analysis Turbulent flame speed: state of the artTurbulent flame speed: state of the art 1.Definition of turbulent burning velocity is not uniform and universal 2.Experimental data scatter is significant between different experimental rigs 3.Numerical simulation, i.e. flamelets models, turbulent burning velocity closure, direct numerical simulation

38 Combustion Theory 38 Premixed flames ignition and stabilization Two ways exist to cause ignition: Self-ignition. Reactants’ temperature and pressure are such that combustion is self- sustainedSelf-ignition. Reactants’ temperature and pressure are such that combustion is self- sustained Induced ignition. The reactants’ mixture is ignited locally by means of sparks, piloted flames, hot wiresInduced ignition. The reactants’ mixture is ignited locally by means of sparks, piloted flames, hot wires Ignition criteria: Chemical-diffusive theory (Tanford and Pease). Ignition is caused by radicals recirculation in the preheating regionChemical-diffusive theory (Tanford and Pease). Ignition is caused by radicals recirculation in the preheating region Thermal theory. The heat provided to the fluid in the preheating region is enough to initiate oxidation processesThermal theory. The heat provided to the fluid in the preheating region is enough to initiate oxidation processes  Ignition occurs when the heat generated in the reaction zone equals heat losses to surrounding (Jost)  Ignition occurs when the cooling time of reactant’s mixture, heated up to adiabatic flame temperature, is greater than chemical reaction time (Zeldovich)  Ignition occurs when a portion of fluid as thick as a laminar propagating flame is heated up to adiabatic flame temperature (Lewis e Von Elbe)

39 Combustion Theory 39 Ignition process schematization with thermal ignition criterion

40 Combustion Theory 40 Important design criteriaImportant design criteria Avoid flash back and lift-off Flashback: flame enters and propagates through the burner upstream without quenching (Safety Hazard)Flashback: flame enters and propagates through the burner upstream without quenching (Safety Hazard) Lift-off: flame is not attached to burner tube but is stabilized at some distance from the tube. Further increase in incoming flow velocity causes blowoff, i.e. flame extinguishing.Lift-off: flame is not attached to burner tube but is stabilized at some distance from the tube. Further increase in incoming flow velocity causes blowoff, i.e. flame extinguishing. Flames stabilization: Laminar Flames: The fluid flow is adjusted in order to generate a region upon the burner rim where the burning velocity equals the gas flow velocityLaminar Flames: The fluid flow is adjusted in order to generate a region upon the burner rim where the burning velocity equals the gas flow velocity Turbulent Flames: Creation of a strong recirculation zone of hot products close to the burner exit. This provides a constant ignition source for the incoming mixture and generates a zone where local turbulent flame speed match local flow velocity (bluff- body flame holders, swirl or jet-induced recirculating flows, rapid increase in flow area creating recirculating separated flow)Turbulent Flames: Creation of a strong recirculation zone of hot products close to the burner exit. This provides a constant ignition source for the incoming mixture and generates a zone where local turbulent flame speed match local flow velocity (bluff- body flame holders, swirl or jet-induced recirculating flows, rapid increase in flow area creating recirculating separated flow)

41 Combustion Theory 41 Bunsen flame stabilization at burner rim Hypothesis: flow velocity equals flame velocity at location 2 and maximum S L is reached at location 3 If the flame is moved to location 3, S L >v and flame returns to location 2If the flame is moved to location 3, S L >v and flame returns to location 2 If the flame is moved to location 1, v>S L and flame returns to location 2If the flame is moved to location 1, v>S L and flame returns to location 2 Location 2 is then a stable operation point

42 Combustion Theory 42 Stability diagram for methane-air mixtures

43 Premixed vs diffusion flames

44 Laminar diffusion flames

45 Combustion Theory 45 The solid fuel is first heated by heat transfer induced by combustion. The liquid fuel reaches the flame by capillarity along the wick and is vaporized.The solid fuel is first heated by heat transfer induced by combustion. The liquid fuel reaches the flame by capillarity along the wick and is vaporized. Fuel oxidation occurs in thin blue layers (the color corresponds to the spontaneous emission of the CH radical)Fuel oxidation occurs in thin blue layers (the color corresponds to the spontaneous emission of the CH radical) Unburnt carbon particles are formed because the fuel is in excess in the reaction zone. The this soot is the source of the yellow light emission.Unburnt carbon particles are formed because the fuel is in excess in the reaction zone. The this soot is the source of the yellow light emission. Flow (entrainment of heavy cold fresh air and evacuation of hot light burnt gases) is induced by natural convectionFlow (entrainment of heavy cold fresh air and evacuation of hot light burnt gases) is induced by natural convection A very difficult flame: the candle flame

46 Example: gas lighter

47 Combustion Theory 47 Diffusion flames In a diffusion flame combustion occurs at the interface between the fuel and the oxidizer. Fuel burning rate depends more on reactants’ diffusion than on chemical reaction rates. It is more difficult to give a general treatment of diffusion flames, largely because no simple, measurable parameter, analogous to laminar burning velocity (S L ) in premixed flames, can be defined. Typical concentration profiles for diffusion flames: Diffusion flames equivalence ratio varies locally (Φ>1 and Φ 1 and Φ<1 regions) Real flame front is no zero-thickness layerReal flame front is no zero-thickness layer

48 Combustion Theory 48 Diffusion flame regimes Laminar diffusion flames: flame height is proportional to fuel flow rateLaminar diffusion flames: flame height is proportional to fuel flow rate Turbulent diffusion flames: flame height is independent of fuel flow rate. The fuel oxidized per unit time increases (combustion enhancement due tu turbulent motion)Turbulent diffusion flames: flame height is independent of fuel flow rate. The fuel oxidized per unit time increases (combustion enhancement due tu turbulent motion) Transition region: ….Transition region: …. Very high fuel flow rates cause flame lift-offVery high fuel flow rates cause flame lift-off Flame height evaluation Diffusion process is rate-determining: the reaction rate is directly related to the amounts of fuel and oxidizer diffusing into the reaction zone.Diffusion process is rate-determining: the reaction rate is directly related to the amounts of fuel and oxidizer diffusing into the reaction zone.

49 Turbulent jet diffusion flame

50 Combustion Theory 50

51 Combustion Theory 51 Methane air diffusion flames at high pressure

52 Combustion Theory 52

53 Combustion Theory 53

54 Single droplet combustion

55 Burnout time

56 Spray combustion


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