Dss Lecture Overview of Combustion

7/28/2019 Dss Lecture Overview of Combustion

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Overview of

Combustion


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Ignition

Three things must be present at the same time

in order to produce fire:

Enough oxygen to provide combustion,

Enough heat to raise the material temperature to itsignition temperature,

Fuel or combustible material which produces highexothermic reaction to propagate heat to not-yet-

burnt material nearby


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Activation energy


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Different flame types of Bunsen Burner depending on air

flow through the throat holes (holes on the side of the bunsen

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

Premixed flame
http://en.wikipedia.org/wiki/Image:Bunsen_burner_flame_types_.jpg


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A burning candle. Within

the bluer, hotter region

near the base of the

wick, hydrogen

separates from the wax

vapor, burns and forms

water vapor. Withinthe brighter, yellower

part of the flame,

carbon soot oxidizes,

and forms carbondioxide.

Diffusion flame


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Spectrum of flame colour


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Flame

stabilisation


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Stabilisation using swirling


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

Combustion is encountered in many practical systems

such as boilers, heaters, domestic and industrialfurnaces, thermal power plants, waste incinerators,automotive and aeronautic engines, rocket enginesand even in refrigeration plants


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In most applications, combustion occurs in

gaseous flows and is characterized by:

A strong and irreversible heat release. Heat is

released in very thin fronts (typical flame thicknessesare usually < 0.5 mm) inducing strong temperature

gradients (temperature ratios between burnt and fresh

gases are of the order of 5 to 7).


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Highly nonlinear reaction rates . These rates follow

Arrhenius laws:

where the Ykare the

mass fractions of the N species involved in thereaction and Ta is an activation temperature. Ta is

generally large so that reaction rates are extremely

sensitive to temperature.

Nk 1 k aY exp T / T


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Combustion strongly modifies the flow field. In

simple one-dimensional flames,burnt gases are

accelerated because of thermal expansion but more

complex phenomena occur in turbulent flows:depending on the situation, turbulence may be either

reduced or enhanced by flames .

Fuel oxidation is generally faster compared to flow

time scales but pollutant formation (nitric oxides,soot) may be quite slower.


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Various coupling mechanisms occur in combustingflow fields:

Chemical reaction schemes deal with the fuelconsumption rate, the formation of combustion

products and pollutant species and should handleignition, flame stabilization and quenching (fullchemical schemes for usual hydrocarbon fuelsinvolve hundreds of species and thousands ofreactions).

Mass transfers of chemical species by moleculardiffusion, convection and turbulent transport alsooccur.


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For two (liquid fuel) and three (solid fuel) phasereacting systems, some other aspects must also beinvolved: spray formation, vaporization, dropletcombustion.

Even for gaseous combustion, multiphase treatmentsmay be needed: for example, soot particles (whichcan be formed in all flames) are carbon elements oflarge size transported by the flow motions.

Some of these phenomena are illustrated in Fig. 1 inthe simple configuration, but very complex case, of acandle.


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Figure 1. A very delicate flame: the candle.

Straight arrows correspond to mass transfer

Broken arrows denote heat transfer.


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The solid stearin fuel is first heated by heat transferinduced by combustion. The liquid fuel reaches theflame by capillarity along the wick and is vaporized.

Fuel oxidation occurs in thin blue layers (the colorcorresponds to the spontaneous emission of the CHradical).

Unburnt carbon particles are formed because the fuelis in excess in the reaction zone. Soot, which is

produced by imperfect combustion, is welcomed inthe case of the candle because it is the source of the

yellow light emission. Flow (entrainment of heavy cold fresh air and

evacuation of hot light burnt gases) is induced bynatural convection (a candle cannot burn in zero-gravity environment).


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To describe the various possible states observed in

reacting flows it is useful to introduce a classification

based on combustion regimes. Flames can be (see

Table 1):

a. premixed, non-premixed or partially premixedin

terms of how fuel and oxidiser are contacted

b. laminar or turbulentin terms of the shape of fluidflow

c. stable or unstablein terms of maintaining the

combustion phenomena


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Table 1. Some examples of practical applications in terms of

premixed/non-premixed flame and laminar/turbulent flow

field.


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

Fuel and oxidizer may be mixed before the reactiontakes place (premixed flames, Fig. 2a) or enter thereaction zone separately (non-premixed or diffusionflames, Fig. 2b).


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Figure 2. Classification of the combustion regime as a function

of the reactant introduction scheme.


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Criterion (b) corresponds to the usual definition ofturbulent states in which large Re numbers lead tounsteady flows. Most practical flames correspond toturbulent flows: turbulence enhances combustion

intensity and allows the design of smaller burners.

Criterion (c) is more specific of reacting flows: insome situations, a flame may exhibit strong unsteady

periodic motions (combustion instabilities) due to acoupling between acoustics, hydrodynamics and heatrelease.


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Premixed flames

In premixed combustion, the reactants, fuel and

oxidizer, are assumed to be perfectly mixed before

entering the reaction zone (Fig. 2a).

Premixed flames propagate towards the fresh gasesby diffusion/reaction mechanisms: the heat released

by the reaction preheats the reactants by diffusion

until reaction starts (reaction rates increase

exponentially with temperature).


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A one dimensional laminar premixed flame

propagates relatively to the fresh gases at the so-

called laminar flame speed sl depending on the

reactants, the fresh gases temperature and thepressure (Fig. 3). For usual fuels, the laminar flame

speed is about 0.1 to 1 m/s.

When fresh gases are turbulent, the premixed flame

propagates faster. Its speed sT is called the turbulentflame speed and is larger than the laminar flame

speed (sT >> sl).


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Figure 3. Structure of a one-dimensional premixed laminar

flame.


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For typical flames, the flame thickness, including

preheat zone, is about 0.1 to 1 mm whereas the

reaction zone itself is ten times thinner. In this figure,

the oxidizer is assumed to be in excess. The correlation between sT, sl and the turbulence

intensity of the incoming flow u:

(1)


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where and n are two model parameters of the order

of unity. Unfortunately, sT is not a well defined

quantity (Gouldin,1996) and depends on various

parameters (chemistry characteristics, flowgeometry).

Eq. (1) is consistent with the experimental

observation that the turbulent flame speed increases

with the turbulence intensity.


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Premixed flames offerhigh burning efficiency as the

reactants are already mixed before combustion.

Theburnt gases temperature, which plays an

important role in pollutant formation, can be easilycontrolled by the amount of fuel injected in the fresh

gases.


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But these flames may be difficult to design because

reactants should be mixed in well defined proportions

(fuel/oxidizer mixtures burn only for a limited range

of fuel mass fraction). A premixed flame may also develop as soon as the

reactants are mixed, leading to possible safety

problems.


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Non-premixed flames

In non-premixed flames (also called diffusion

flames), reactants are introduced separately in the

reaction zone.

The prototype of this situation is the fuel jetdischarging in atmospheric air (Fig. 5). This

configuration is very simple to design and to build:

no pre-mixing is needed and it is safer: the flame

cannot propagate towards the fuel stream because itcontains no oxidizer and vice versa.


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Nevertheless, diffusion flames are less efficientbecause fuel and oxidizer must mix by moleculardiffusion beforeburning.

The maximum burnt gases temperature is given by thetemperature of fuel and oxidizer burning instoichiometric proportions and cannot be controlledeasily.

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


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Figure 4. Structure of a one-dimensional non-premixedlaminar flame. Here fuel and oxidizer streams are assumed tohave the same temperature.


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Turbulence is also found to enhance combustion

processes in non-premixed flames as evidenced by

Hottel and Hawthorne (1949) who measured the

length of a diffusion flame burning a fuel jetdischarging in ambient air as a function of the fuel

flow rate (Fig. 5).

The flame length increases linearly with the fuel flow

rate as long as the flow remains laminar.


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


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When the jet becomes turbulent, the flame length

remainsconstant even when the flow rate increases,

showing an increase of the combustion intensity.

Very large flow rates will lead to lifted flames (theflame is no more anchored to the jet exit) and then to

blow-off or flame quenching.


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Partially premixed flames

The previously describedpremixed and non-

premixed flame regimes correspond to idealized

situations.

In practical applications, fuel and oxidizer cannot beperfectly premixed.

In some situations, an imperfect premixing is

produced on purpose to reduce fuel consumption

(toward premixed) and to reduce pollutant

emissions (toward diffusion).


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For example, in spark-ignited stratified chargeinternal combustion engines, the fuel injection istuned to produce a quasi-stoichiometric mixture in thevicinity of the sparkto promote ignition but a lean

mixture in the rest of the cylinder.

In non-premixed flames, fuel and oxidizer must meetto burn and ensure flame stabilization, leading to

partially premixed zones.

A small premixed flame develops and stabilizes adiffusion flame as shown in Fig. 6. As a consequence,

partially premixed flames have now become topics ofgrowing interest


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Figure 6. Structure of a triple flame. The flame is stabilized by

a premixed flame burning imperfectly premixed reactants (richand lean wings). A diffusion flame develops downstream.


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Stable and unstable flames:

Thermodiffusive instabilities

Laminar premixed flames exhibitintrinsic

instabilities depending on the relative importance of

reactant molecular diffusion and heat diffusion. An

example of such phenomena, studied in details innumerous papers (see, for example Williams, 1985) is

illustrated in Fig. 7.


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Assume that the molecular diffusivity of reactants ishigher than the thermal diffusivity (i.e. the Lewisnumber Le = k/( Cp D), comparing thermal andspecies diffusivities, < 1).

When the flame front is convex towards the freshgases, reactants diffuse towards burnt gases fasterthan heat diffuse towards cold fresh gases.

These reactants are heated and then burn fasterinreduced convex region, increasing the local flamespeed sl (sl > sl

o) with time


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On the other hand, for fronts convex towards the

burnt gases, reactants diffuse in a large zone thus

increasing convex region and the flame velocity is

decreased compared to slo

(sl 1), a similar analysis

shows that the flame is stable: the flame frontwrinkling decreases.


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Figure 7. Sketch of thermo-diffusive instabilities (in laminarpremixed flames). ForLe < 1, molecular diffusion (redarrows) > heat diffusion (blue arrows) and the wrinkling of the

flame front is enhanced by differential flame speeds (leftfigure).

ForLe > 1 (right figure), a stable planar flame is obtained inwhich molecular diffusion (blue arrows) < heat diffusion (redarrows)


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Stable and unstable flames:

Flame/acoustic interactions

Thermodiffusive instabilities (laminar premixedflames) are rarely observed in industrial devices.

However, another type of instability may develop inconfined flames. These instabilities come from acoupling between hydrodynamics, heat release andacoustics.

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


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In some cases, such instabilities may be generated onpurpose to increase efficiency like in pulsecombustors, but generally undesired.

A simple example of such combustion instability is

provided in Fig. 8 fora premixed turbulent laboratoryburner (Poinsot et al., 1987). Without combustioninstabilities, a turbulent reacting jet stabilized byrecirculation zones is observed (Fig. 9 left).

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


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The mechanism of such an instability may be

summarized as follows (Poinsot et al., 1987): a vortex

is generated at the jet inlet and convected

downstream. It induces an unsteady reaction rate,producing an acoustic wave moving upstream to

generate a new vortex at the burner inlet.


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Figure 8. Experimental turbulentpremixedburner of Poinsot et

al. (1987).


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Figure 9. Combustion instabilities in a turbulent premixed

flame. Schlieren views of the central jet through the quartz

window of Fig. 8: stable (left) and unstable (right) regimes.

The flow is going from the right to the left (Poinsot et al.,

1987).


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Dss Lecture Overview of Combustion

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