Overview of Overview of CombustionCombustion

IgnitionIgnition

Three thingsThree things must be present at the same time must be present at the same timein order to produce fire: in order to produce fire: Enough Enough oxygenoxygen to provide combustion, to provide combustion, Enough Enough heatheat to raise the material temperature to its to raise the material temperature to its

ignition temperature, ignition temperature, FuelFuel or combustible material which produces high or combustible material which produces high

exothermic reaction exothermic reaction to propagate heat to not-yet- to propagate heat to not-yet- burnt material nearbyburnt material nearby

Activation energyActivation energy

Flammability limitFlammability limit

Different flame types of Bunsen Burner depending on air Different flame types of Bunsen Burner depending on air flow through the throat holes (holes on the side of the bunsen flow through the throat holes (holes on the side of the bunsen

burner). 1. air baffle closed (Safety flame) 2. air baffle half burner). 1. air baffle closed (Safety flame) 2. air baffle half open 3. air baffle nearly fully open 4. air baffle fully openopen 3. air baffle nearly fully open 4. air baffle fully open

Premixed flame

A burning candle. Within A burning candle. Within the bluer, hotter region the bluer, hotter region near the base of the near the base of the wick, wick, hydrogenhydrogen separates from the wax separates from the wax vapor, burns and forms vapor, burns and forms water vapor. Within water vapor. Within the brighter, yellower the brighter, yellower part of the flame, part of the flame, carbon sootcarbon soot oxidizes, oxidizes, and forms carbon and forms carbon dioxide.dioxide.

Diffusion flame

Spectrum of flame colour

Flame stabilisation

Stabilisation using swirling

Burning fossil fuels produces > 2/3 of our energy Burning fossil fuels produces > 2/3 of our energy productionproduction today and probably still will in a century. today and probably still will in a century.

Combustion is encountered in many practical systems Combustion is encountered in many practical systems such as boilers, heaters, domestic and industrial such as boilers, heaters, domestic and industrial furnaces, thermal power plants, waste incinerators, furnaces, thermal power plants, waste incinerators, automotive and aeronautic engines, rocket engines automotive and aeronautic engines, rocket engines and even in refrigeration plantsand even in refrigeration plants

In most applications, combustion occurs in In most applications, combustion occurs in

gaseous flows and is characterized by:gaseous flows and is characterized by: A strong and irreversible heat releaseA strong and irreversible heat release. Heat is . Heat is

released in very thin fronts (typical flame thicknesses released in very thin fronts (typical flame thicknesses are usually < 0.5 mm) inducing strong temperature are usually < 0.5 mm) inducing strong temperature gradients (temperature ratios between burnt and fresh gradients (temperature ratios between burnt and fresh gases are of the order of 5 to 7).gases are of the order of 5 to 7).

Highly nonlinear reaction ratesHighly nonlinear reaction rates . These rates follow . These rates follow Arrhenius laws:Arrhenius laws:

where the Ywhere the Ykk are the are the

mass fractions of the N species involved in the mass fractions of the N species involved in the reaction and Treaction and Taa is an activation temperature. T is an activation temperature. Taa is is

generally large so that reaction rates are extremely generally large so that reaction rates are extremely sensitive to temperature.sensitive to temperature.

N

k 1 k aY exp T / T

Combustion strongly modifies the flow fieldCombustion strongly modifies the flow field. In . In simple one-dimensional flames, simple one-dimensional flames, burnt gases are burnt gases are accelerated because of thermal expansionaccelerated because of thermal expansion but more but more complex phenomena occur in turbulent flows: complex phenomena occur in turbulent flows: depending on the situation, turbulence may be either depending on the situation, turbulence may be either reduced or enhanced by flames . reduced or enhanced by flames .

Fuel oxidationFuel oxidation is generally faster compared to is generally faster compared to flow flow time scales but pollutant formationtime scales but pollutant formation (nitric oxides, (nitric oxides, soot) may be quite soot) may be quite slowerslower..

Various coupling mechanisms occur in combustingVarious coupling mechanisms occur in combustingflow fields:flow fields: Chemical reactionChemical reaction schemes deal with the fuel schemes deal with the fuel

consumption rate, the formation of combustion consumption rate, the formation of combustion products and pollutant species and should handle products and pollutant species and should handle ignition, flame stabilizationignition, flame stabilization and and quenching quenching (full (full chemical schemes for usual hydrocarbon fuels chemical schemes for usual hydrocarbon fuels involve hundreds of species and thousands of involve hundreds of species and thousands of reactions). reactions).

Mass transfersMass transfers of chemical species by of chemical species by molecular molecular diffusion, convection and turbulent transportdiffusion, convection and turbulent transport also also occur. occur.

The The heatheat released by chemical reactions induces released by chemical reactions induces strong strong conductive, convective or radiative heat conductive, convective or radiative heat transfertransfer inside the flow and with the surrounding inside the flow and with the surrounding walls. walls.

For two (liquid fuel) and three (solid fuel)For two (liquid fuel) and three (solid fuel) phase phase reacting systems, some other aspects must also be reacting systems, some other aspects must also be involved: spray formation, vaporization, droplet involved: spray formation, vaporization, droplet combustion. combustion.

Even Even for gaseous combustionfor gaseous combustion, multiphase treatments , multiphase treatments may be needed: for example, soot particles (which may be needed: for example, soot particles (which can be formed in all flames) are carbon elements of can be formed in all flames) are carbon elements of large size transported by the flow motions. large size transported by the flow motions.

Some of these phenomena are illustrated in Fig. 1 in Some of these phenomena are illustrated in Fig. 1 in the simple configuration, but very complex case, of a the simple configuration, but very complex case, of a candle.candle.

Figure 1. A very delicate flame: the candle. Straight arrows correspond to mass transfer Broken arrows denote heat transfer.

The The solid solid stearin fuel is first heated by heat transfer stearin fuel is first heated by heat transfer induced by combustion. The induced by combustion. The liquidliquid fuel reaches the flame fuel reaches the flame by capillarity along the wick and is by capillarity along the wick and is vaporizedvaporized. .

Fuel oxidationFuel oxidation occurs in occurs in thin blue layersthin blue layers (the color (the color corresponds to the spontaneous emission of the corresponds to the spontaneous emission of the CH CH radicalradical). ).

Unburnt carbon particles are formed because the fuel is in Unburnt carbon particles are formed because the fuel is in excess in the reaction zone. excess in the reaction zone. SootSoot, which is produced by , which is produced by imperfect combustion, is welcomed in the case of the imperfect combustion, is welcomed in the case of the candle because it is the source of the yellow light candle because it is the source of the yellow light emission. emission.

Flow (Flow (entrainmententrainment of heavy cold fresh air and of heavy cold fresh air and evacuationevacuation of hot light burnt gases) is induced by natural convection of hot light burnt gases) is induced by natural convection (a candle cannot burn in zero-gravity environment). (a candle cannot burn in zero-gravity environment).

To describe the various possible states observed inTo describe the various possible states observed in

reacting flows it is useful to introduce a classificationreacting flows it is useful to introduce a classification

based on combustion regimes. based on combustion regimes. Flames Flames can be (seecan be (see

Table 1):Table 1):

a.a. premixed, non-premixed or partially premixedpremixed, non-premixed or partially premixed in terms in terms of how fuel and oxidiser are contactedof how fuel and oxidiser are contacted

b.b. laminar or turbulentlaminar or turbulent in terms of the shape of fluid flowin terms of the shape of fluid flow

c.c. stable or unstablestable or unstable in terms of maintaining the in terms of maintaining the combustion phenomenacombustion phenomena

Table 1. Some examples of practical applications in terms of Table 1. Some examples of practical applications in terms of premixed/non-premixed flame and laminar/turbulent flow premixed/non-premixed flame and laminar/turbulent flow field.field.

Criterion (a) depends on the way how to introduce the Criterion (a) depends on the way how to introduce the reactants into the combustion zone and is one of the reactants into the combustion zone and is one of the main parameters controlling the flame regime. main parameters controlling the flame regime.

Fuel and oxidizer may be mixed before the reaction Fuel and oxidizer may be mixed before the reaction takes place (takes place (premixed flames, Fig. 2apremixed flames, Fig. 2a) or enter the ) or enter the reaction zone separately (reaction zone separately (non-premixed or diffusion non-premixed or diffusion flames, Fig. 2bflames, Fig. 2b).).

Figure 2. Classification of the combustion regime as a function Figure 2. Classification of the combustion regime as a function of the reactant introduction scheme.of the reactant introduction scheme.

Criterion (b) corresponds to the usual definition of Criterion (b) corresponds to the usual definition of turbulent states in which large Re numbers lead to turbulent states in which large Re numbers lead to unsteady flows. Most practical flames correspond to unsteady flows. Most practical flames correspond to turbulent flows: turbulent flows: turbulence enhances combustion turbulence enhances combustion intensity and allows the design of smaller burners. intensity and allows the design of smaller burners.

Criterion (c) is more specific of reacting flows: in Criterion (c) is more specific of reacting flows: in some situations, a flame may exhibit some situations, a flame may exhibit strong unsteadystrong unsteady periodic motions (combustion instabilities) due to a periodic motions (combustion instabilities) due to a coupling between acoustics, hydrodynamics and heat coupling between acoustics, hydrodynamics and heat release.release.

Premixed flamesPremixed flames

In premixed combustion, the reactants, fuel and In premixed combustion, the reactants, fuel and oxidizer, are assumed to be perfectly mixed before oxidizer, are assumed to be perfectly mixed before entering the reaction zone (Fig. 2a). entering the reaction zone (Fig. 2a).

Premixed flames propagate towards the fresh gases Premixed flames propagate towards the fresh gases by diffusion/reaction mechanismsby diffusion/reaction mechanisms: the heat released : the heat released by the reaction preheats the reactants by diffusion by the reaction preheats the reactants by diffusion until reaction starts (reaction rates increase until reaction starts (reaction rates increase exponentially with temperature). exponentially with temperature).

A one dimensional laminar premixed flame propagates A one dimensional laminar premixed flame propagates relatively to the fresh gases at the so-called relatively to the fresh gases at the so-called laminar flame laminar flame speed sspeed sll depending on the reactants, the fresh gases depending on the reactants, the fresh gases

temperature and the pressure (Fig. 3). For usual fuels, the temperature and the pressure (Fig. 3). For usual fuels, the laminar flame speed is about 0.1 to 1 m/s.laminar flame speed is about 0.1 to 1 m/s.

When When fresh gases are turbulent, the premixed flame fresh gases are turbulent, the premixed flame propagates faster. Its speed spropagates faster. Its speed sTT is called the turbulent flame is called the turbulent flame

speedspeed and is larger than the laminar flame speed ( and is larger than the laminar flame speed (ssTT >> s >> sll).).

Figure 3. Structure of a one-dimensional premixed laminar Figure 3. Structure of a one-dimensional premixed laminar flame. flame.

For typical flames, For typical flames, the flame thickness, including the flame thickness, including preheat zone, is about 0.1 to 1 mm whereas the preheat zone, is about 0.1 to 1 mm whereas the reaction zone itself is ten times thinnerreaction zone itself is ten times thinner.. In this figure, In this figure, the oxidizer is assumed to be in excess.the oxidizer is assumed to be in excess.

The correlation between sThe correlation between sTT, s, sll and the turbulence and the turbulence

intensity of the incoming flow u’:intensity of the incoming flow u’:

(1)(1)

where where and n are two model parameters of the order and n are two model parameters of the order of unity. Unfortunately, sof unity. Unfortunately, sTT is not a well defined is not a well defined

quantity (Gouldin,1996) and depends on various quantity (Gouldin,1996) and depends on various parameters (chemistry characteristics, flow parameters (chemistry characteristics, flow geometry).geometry).

Eq. (1) is consistent with the experimental Eq. (1) is consistent with the experimental observation that observation that the turbulent flame speed increases the turbulent flame speed increases with the turbulence intensitywith the turbulence intensity. .

Premixed flames offer Premixed flames offer high burning efficiencyhigh burning efficiency as the as the reactants are already mixed before combustion. reactants are already mixed before combustion.

The The burnt gases temperatureburnt gases temperature, which plays an , which plays an important role in pollutant formation, important role in pollutant formation, can be easily can be easily controlledcontrolled by the amount of fuel injected in the fresh by the amount of fuel injected in the fresh gases. gases.

But these But these flames may be difficult to designflames may be difficult to design because because reactants should be mixed in well defined proportions reactants should be mixed in well defined proportions (fuel/oxidizer mixtures burn only for a limited range (fuel/oxidizer mixtures burn only for a limited range of fuel mass fraction). of fuel mass fraction).

A premixed flame may also develop as soon as the A premixed flame may also develop as soon as the reactants are mixed, leading to possible reactants are mixed, leading to possible safety safety problemsproblems..

Non-premixed flamesNon-premixed flames

In In non-premixed flamesnon-premixed flames (also called diffusion (also called diffusion flames), flames), reactants are introduced separately in the reactants are introduced separately in the reaction zonereaction zone. .

The prototype of this situation is the fuel jet The prototype of this situation is the fuel jet discharging in atmospheric air (Fig. 5). This discharging in atmospheric air (Fig. 5). This configuration is very simple to design and to build: configuration is very simple to design and to build: no pre-mixing is needed and no pre-mixing is needed and it is saferit is safer: the flame : the flame cannot propagate towards the fuel stream because it cannot propagate towards the fuel stream because it contains no oxidizer and vice versa. contains no oxidizer and vice versa.

Nevertheless, Nevertheless, diffusion flames are diffusion flames are less efficientless efficient because fuel and oxidizer must mix by molecular because fuel and oxidizer must mix by molecular diffusiondiffusion before before burning. burning.

The The maximum burnt gases temperaturemaximum burnt gases temperature is given by the is given by the temperature of fuel and oxidizer burning in temperature of fuel and oxidizer burning in stoichiometric proportions and stoichiometric proportions and cannot be controlled cannot be controlled easilyeasily. .

The structure of a one-dimensional non-premixed The structure of a one-dimensional non-premixed laminar flame is sketched in Fig. 4.laminar flame is sketched in Fig. 4.

Figure 4. Structure of a one-dimensional non-premixed Figure 4. Structure of a one-dimensional non-premixed laminar flame. Here fuel and oxidizer streams are assumed to laminar flame. Here fuel and oxidizer streams are assumed to have the same temperature.have the same temperature.

Turbulence is also found to Turbulence is also found to enhance combustionenhance combustion processesprocesses in non-premixed flames as evidenced by in non-premixed flames as evidenced by Hottel and Hawthorne (1949) who measured the Hottel and Hawthorne (1949) who measured the length of a diffusion flame burning a fuel jet length of a diffusion flame burning a fuel jet discharging in ambient air as a function of the fuel discharging in ambient air as a function of the fuel flow rate (Fig. 5). flow rate (Fig. 5).

The The flame length increases linearly with the fuel flow flame length increases linearly with the fuel flow raterate as long as the flow remains as long as the flow remains laminar.laminar.

Figure 5. Non-premixed jet flame. Figure 5. Non-premixed jet flame. A fuel jet discharges in the A fuel jet discharges in the ambient air. Top: flow configuration; Bottom: flame length ambient air. Top: flow configuration; Bottom: flame length versus fuel jet velocity. (Hottel and Hawthorne, 1949.versus fuel jet velocity. (Hottel and Hawthorne, 1949.

When the jet becomes When the jet becomes turbulentturbulent, the , the flame length flame length remainsremains constantconstant even when the flow rate increases, even when the flow rate increases, showing an increase of the combustion intensity. showing an increase of the combustion intensity.

Very large flow rates will lead to lifted flamesVery large flow rates will lead to lifted flames (the (the flame is no more anchored to the jet exit) and then to flame is no more anchored to the jet exit) and then to blow-off or flame quenching.blow-off or flame quenching.

Partially premixed flamesPartially premixed flames

The previously described The previously described premixed and non-premixed and non-premixed flamepremixed flame regimes correspond to regimes correspond to idealized idealized situations. situations.

In practical applications, fuel and oxidizer cannot be In practical applications, fuel and oxidizer cannot be perfectly premixed. perfectly premixed.

In some situations, an imperfect premixing is In some situations, an imperfect premixing is produced on purpose to produced on purpose to reduce fuelreduce fuel consumption consumption (toward (toward premixedpremixed) and to ) and to reduce pollutantreduce pollutant emissions (emissions (toward diffusiontoward diffusion). ).

For example, in spark-ignited stratified charge For example, in spark-ignited stratified charge internal combustion engines, the fuel injection is internal combustion engines, the fuel injection is tuned to produce a tuned to produce a quasi-stoichiometricquasi-stoichiometric mixture in mixture in the vicinity of the spark the vicinity of the spark to promote ignitionto promote ignition but a lean but a lean mixture in the rest of the cylinder. mixture in the rest of the cylinder.

In non-premixed flames, fuel and oxidizer must meet In non-premixed flames, fuel and oxidizer must meet to burn and to burn and ensure flame stabilizationensure flame stabilization, leading to , leading to partially premixed zones. partially premixed zones.

A small premixed flame develops and stabilizes a A small premixed flame develops and stabilizes a diffusion flame as shown in Fig. 6. As a consequence, diffusion flame as shown in Fig. 6. As a consequence, partially premixed flames have now become topics of partially premixed flames have now become topics of growing interest growing interest

Figure 6. Structure of a triple flame. The flame is Figure 6. Structure of a triple flame. The flame is stabilized by stabilized by a premixed flame burninga premixed flame burning imperfectly premixed reactants (rich imperfectly premixed reactants (rich and lean wings). A diffusion flame develops downstream.and lean wings). A diffusion flame develops downstream.

Stable and unstable flames:Stable and unstable flames:Thermodiffusive instabilitiesThermodiffusive instabilities

Laminar premixed flames exhibitLaminar premixed flames exhibit intrinsic intrinsic instabilities depending on the relative importance of instabilities depending on the relative importance of reactant molecular diffusion and heat diffusionreactant molecular diffusion and heat diffusion. An . An example of such phenomena, studied in details in example of such phenomena, studied in details in numerous papers (see, for example Williams, 1985) is numerous papers (see, for example Williams, 1985) is illustrated in Fig. 7.illustrated in Fig. 7.

Assume that the molecular diffusivity of reactants is Assume that the molecular diffusivity of reactants is higher than the thermal diffusivity (i.e. the higher than the thermal diffusivity (i.e. the Lewis Lewis number Le = k/(number Le = k/( C Cp p D), comparing thermal and D), comparing thermal and species diffusivities, < 1species diffusivities, < 1). ).

When the flame front is convex towards the fresh When the flame front is convex towards the fresh gases, gases, reactants diffuse towards burnt gases faster than reactants diffuse towards burnt gases faster than heat diffuse towards cold fresh gasesheat diffuse towards cold fresh gases..

These reactants are heated and then burn faster These reactants are heated and then burn faster in in reduced convex regionreduced convex region, increasing the local flame , increasing the local flame speed sspeed sll ( (ssl l > s> sll

oo) with time) with time

On the other hand, for fronts convex towards the On the other hand, for fronts convex towards the burnt gases, burnt gases, reactants diffuse in a large zonereactants diffuse in a large zone thus thus increasing convex regionincreasing convex region and the flame velocity is and the flame velocity is decreased compared to sdecreased compared to sll

oo ( (ssl l << sslloo). This situation is ). This situation is

unstable: the flame front wrinkling increases. unstable: the flame front wrinkling increases. When the When the species molecular diffusivity < the heat species molecular diffusivity < the heat

diffusivity (Lewis number > 1diffusivity (Lewis number > 1), a similar analysis ), a similar analysis shows that the flame is stable: the flame front shows that the flame is stable: the flame front wrinkling decreases. wrinkling decreases.

Figure 7. Sketch of thermo-diffusive instabilities (Figure 7. Sketch of thermo-diffusive instabilities (in laminar in laminar premixed flamespremixed flames). For ). For Le < 1, molecular diffusion (red Le < 1, molecular diffusion (red arrows) > heat diffusion (blue arrows)arrows) > heat diffusion (blue arrows) and the wrinkling of the and the wrinkling of the flame front is enhanced by differential flame speeds (left flame front is enhanced by differential flame speeds (left figure). figure). For For Le > 1Le > 1 (right figure), a stable planar flame is obtained in (right figure), a stable planar flame is obtained in which which molecular diffusion (blue arrows) < heat diffusion (red molecular diffusion (blue arrows) < heat diffusion (red arrows)arrows)

Stable and unstable flames:Stable and unstable flames:Flame/acoustic interactionsFlame/acoustic interactions

Thermodiffusive instabilitiesThermodiffusive instabilities (laminar premixed (laminar premixed flames) are rarely observed in industrial devices. flames) are rarely observed in industrial devices.

However, another type of instability may develop in However, another type of instability may develop in confined flames. These instabilities come from a confined flames. These instabilities come from a coupling between hydrodynamics, heat release and coupling between hydrodynamics, heat release and acoustics. acoustics.

Strong unsteady motions develop producing noise, Strong unsteady motions develop producing noise, enhancing combustion intensity and leading sometimes enhancing combustion intensity and leading sometimes to the system destruction.to the system destruction.

In some cases, such instabilities may be generated In some cases, such instabilities may be generated on on purposepurpose to increase efficiency like in pulse to increase efficiency like in pulse combustors, but generally undesired.combustors, but generally undesired.

A simple example of such combustion instability is A simple example of such combustion instability is provided in Fig. 8 for provided in Fig. 8 for a premixed a premixed turbulentturbulent laboratory laboratory burnerburner (Poinsot et al., 1987). (Poinsot et al., 1987). Without combustion Without combustion instabilities, a turbulent reacting jet instabilities, a turbulent reacting jet stabilized by stabilized by recirculation zonesrecirculation zones is observed (Fig. 9 left). is observed (Fig. 9 left).

Changing the equivalence ratio Changing the equivalence ratio (i.e. the amount of (i.e. the amount of fuel in the air stream) fuel in the air stream) leads to a strong instabilityleads to a strong instability (Fig. 9 right): large mushroom vortices are formed at (Fig. 9 right): large mushroom vortices are formed at a frequency of 530 Hz, increasing the combustion a frequency of 530 Hz, increasing the combustion intensity by about 50 %. intensity by about 50 %.

The mechanism of such an instability may be The mechanism of such an instability may be summarized as follows (Poinsot et al., 1987): a summarized as follows (Poinsot et al., 1987): a vortexvortex is generated at the jet inlet and is generated at the jet inlet and convected convected downstreamdownstream. It induces an unsteady reaction rate, . It induces an unsteady reaction rate, producing producing an acoustic wave moving upstream to an acoustic wave moving upstream to generate a new vortexgenerate a new vortex at the burner inlet. at the burner inlet.

Figure 8. Experimental Figure 8. Experimental turbulentturbulent premixed premixed burner of Poinsot burner of Poinsot et al. (1987).et al. (1987).

Figure 9. Combustion instabilities in a Figure 9. Combustion instabilities in a turbulent premixed turbulent premixed flameflame. Schlieren views of the central jet through the quartz . Schlieren views of the central jet through the quartz window of Fig. 8: stable (left) and unstable (right) regimes. window of Fig. 8: stable (left) and unstable (right) regimes. The flow is going from the right to the left (Poinsot et al., The flow is going from the right to the left (Poinsot et al., 1987).1987).