Combustion
From CFDWiki
Contents 
What is combustion  Physics versus modelling
Combustion phenomena consists of many physical and chemical processes with broad range of time scales. Mathematical description of combustion is not always trivial. Analytical solutions exists only for basic situations of laminar flame and because of its assumptions it is often restricted to few problems solved usually in zero or onedimensional space.
Problems solved today concern mainly turbulent flows, gas as well as liquid fuels, pollution issues (products of combustion as well as for example noise pollution). These problems require not only extensive experimental work, but also numerical modelling. All combustion models must be validated against the experiments as each one has its own drawbacks and limits. However here the modelling part will be mainly addressed.
Reaction mechanisms
The combustion is mainly chemical process and although we can, to some extend, describe flame without any chemistry informations, for modelling of flame propagation we need to know the speed of reactions, product concentrations, temperature and other parameters. Therefore more or less detailed information about reaction kinetics is essential for any combustion model. Mixture will generally combust, if the reaction of fuel and oxidiser is fast enough to maintain until all of the mixture is burned into products. If the reaction is too slow, the flame will extinguish, if too fast, explosion or even detonation will occur. The reaction rate of typical combustion reaction is influenced mainly by concentration of reactants, temperature and pressure.
A stoichiometric equation of an arbitrary equation can be written as:

where $\nu$ is the stoichiometric coefficient, is arbitrary species. One prime specifies the reactants and double prime products of the reaction.
Reaction rate, expressing the rate of disappearance of reactant i of such a reaction, is defined as:

in which k is the specific reaction rate constant. Arrhenius found that this constant is a function only of temperature and this function is defined as:

where A is preexponential factor, E is activation energy and is temperature exponent. These constants for given reactions can be found in literature. The reaction mechanism can be given from experiments for every reaction resolved, it could be also constructed numerically by automatic generation method (see [Griffiths (1994)] for review on reaction mechanisms). For simple hydrocarbon tens to hundreds of reactions are involved. By analysis and systematic reduction of reaction mechanisms global reaction (from one to five step reactions) can be found (see [Westbrook (1984)]).
Governing Equations
The mass fraction transport equation for kth species
(1) 
where Ficks law is assumed for scalar diffusion and is the species reaction rate.
Infinitely fast chemistry
All combustion models can be divided into two main groups according to the assumptions on the reaction kinetics. We can either assume the reactions to be infinitely fast  compared to e.g. mixing of the species, or of the comparable time scale of the mixing process. The simpler approach assuming chemistry fast enough, that the limiting process is mixing of the species is historically older approach and even today can be appropriate approach. It is simpler to solve then #Finite rate chemistry models, but introduces errors to the solution which may or may not be important.
Premixed Combustion
Premixed flame occurs in mixtures of fuel and oxidiser, homogeneously premixed prior to the flame. These flames are not limited only to gas fuels, but also to the prevaporised fuels. Typical example of premixed laminar flame is bunsen burner, where the air enters the fuel stream. The mixture burns in the wake of the riser tube walls forming nice stable flame. The premixed flames has many advantages in terms of control of temperature and products and pollution concentration, but introduce also some dangers like the autoignition (in the supply system).
Turbulent flame speed model
Eddy BreakUp model
The Eddy BreakUp model is the typical example of mixedisburnt combustion model. It is based on the work of Magnussen and Hjertager, and Spalding and can be found in all CFD packages. The model assumes the reactions to be completed in the moment of mixing, so that the reaction rate is completely controlled by turbulent mixing. The combustion is described by a single step global chemical reaction:

in which F stands for fuel, O for oxidiser and P for products of the reaction. Alternativelly we can have multistep scheme, where each reaction has its own mean reaction rate. The mean reaction rate is given by:

denotes mean concentrations for fuel, oxidiser and products
respectively, A and B are model constants with typical values of 0.5
and 4.0 respectively. The values of these constants are fitted according
to the experimental results and they are suitable for most of the general cases.
Still they are just constants based on experimental fitting and they need not
be suitable for all the situations.
Care must be taken especially in highly strained regions, where the ratio of
to is large (flameholder wakes, walls ...). In those regions a positive reaction rate occurs and an artificial flame can be observed.
CFD codes usually has some remedies to overcome this problem.
This model largely overpredicts temperatures and concentrations of species like CO and other species. Still this model is quite popular for its simplicity and relatively easy convergence and implementation.
BrayMossLibby Model
Non premixed combustion
Conserved scalar equilibrium models
Finite rate chemistry
Premixed Combustion
Coherent Flame Model
Flamelets based on G equation
Nonpremixed Combustion
Flamelets based on conserved scalar
Conditional Moment Closure (CMC)
Multiple Mapping Closure (MMC)
Linear Eddy Model
PDF transport models
Lagrangian
Eulerian
References
 Griffiths J.F. (1994), "Reduced Kinetic Models and Their Application to Practical Combustion Systems", Prog. in Energy and Combustion Science,Vol. 21, pp. 25107.
 Westbrook, Ch.K., Dryer,F.L., (1984), "Chemical Kinetic Modeling of Hydrocarbon Combustion", Prog. in Energy and Combustion Science,Vol. 10, pp. 157.