Editing Combustion (section)

==Reaction mechanism==
{{More citations needed section|date=January 2017}}
Combustion in oxygen is a [[chain reaction]] in which many distinct [[Radical (chemistry)|radical]] intermediates participate. The high energy required for initiation is explained by the unusual structure of the [[dioxygen]] molecule. The lowest-energy configuration of the dioxygen molecule is a stable, relatively unreactive diradical in a [[triplet oxygen|triplet spin state]]. Bonding can be described with three bonding electron pairs and two antibonding electrons, with [[Spin (physics)|spins]] aligned, such that the molecule has nonzero total angular momentum. Most fuels, on the other hand, are in a singlet state, with paired spins and zero total angular momentum. Interaction between the two is quantum mechanically a "[[forbidden transition]]", i.e. possible with a very low probability. To initiate combustion, energy is required to force dioxygen into a spin-paired state, or [[singlet oxygen]]. This intermediate is extremely reactive. The energy is supplied as [[heat]], and the reaction then produces additional heat, which allows it to continue.

Combustion of hydrocarbons is thought to be initiated by hydrogen atom abstraction (not proton abstraction) from the fuel to oxygen, to give a hydroperoxide radical (HOO). This reacts further to give hydroperoxides, which break up to give [[hydroxyl radical]]s. There are a great variety of these processes that produce fuel radicals and oxidizing radicals. Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, and [[hydroperoxyl]]. Such intermediates are short-lived and cannot be isolated. However, non-radical intermediates are stable and are produced in incomplete combustion. An example is [[acetaldehyde]] produced in the combustion of [[ethanol]]. An intermediate in the combustion of carbon and hydrocarbons, [[carbon monoxide]], is of special importance because it is a [[Poison|poisonous gas]], but also economically useful for the production of [[syngas]].

Solid and heavy liquid fuels also undergo a great number of [[pyrolysis]] reactions that give more easily oxidized, gaseous fuels. These reactions are endothermic and require constant energy input from the ongoing combustion reactions. A lack of oxygen or other improperly designed conditions result in these noxious and carcinogenic pyrolysis products being emitted as thick, black smoke.

The rate of combustion is the amount of a material that undergoes combustion over a period of time. It can be expressed in grams per second (g/s) or kilograms per second (kg/s).

Detailed descriptions of combustion processes, from the chemical kinetics perspective, require the formulation of large and intricate webs of elementary reactions.<ref>{{cite book|last1=Law|first1=C.K.|title=Combustion Physics|date=2006|publisher=Cambridge University Press|location=Cambridge, UK|isbn=9780521154215}}</ref> For instance, combustion of hydrocarbon fuels typically involve hundreds of chemical species reacting according to thousands of reactions.

The inclusion of such mechanisms within computational flow solvers still represents a pretty challenging task mainly in two aspects. First, the number of degrees of freedom (proportional to the number of chemical species) can be dramatically large; second, the source term due to reactions introduces a disparate number of time scales which makes the whole [[dynamical system]] stiff. As a result, the direct numerical simulation of turbulent reactive flows with heavy fuels soon becomes intractable even for modern supercomputers.<ref>{{cite book|last1=Goussis|first1=D.|last2=Maas|first2=U.|title=Turbulent Combustion Modeling|date=2011|publisher=Springer Science|pages=193–220}}</ref>

Therefore, a plethora of methodologies have been devised for reducing the complexity of combustion mechanisms without resorting to high detail levels. Examples are provided by:
* The Relaxation Redistribution Method (RRM)<ref>{{cite journal|last1=Chiavazzo|first1=Eliodoro|last2=Karlin|first2=Ilya|title=Adaptive simplification of complex multiscale systems|journal=Phys. Rev. E|date=2011|volume=83|issue=3|pages=036706|doi=10.1103/PhysRevE.83.036706|pmid=21517624|arxiv = 1011.1618 |bibcode = 2011PhRvE..83c6706C |s2cid=7458232}}</ref><ref>{{cite journal|last1=Chiavazzo|first1=Eliodoro|last2=Asinari|first2=Pietro|last3=Visconti|first3=Filippo|title=Fast computation of multi-scale combustion systems|journal=Phil. Trans. Roy. Soc. A|date=2011|volume=369|issue=1945|pages=2396–2404|doi=10.1098/rsta.2011.0026|pmid=21576153|arxiv = 1011.3828 |bibcode = 2011RSPTA.369.2396C |s2cid=14998597}}</ref><ref>{{cite journal|last1=Chiavazzo|first1=Eliodoro|title=Approximation of slow and fast dynamics in multiscale dynamical systems by the linearized Relaxation Redistribution Method|journal=Journal of Computational Physics|date=2012|volume=231|issue=4|doi=10.1016/j.jcp.2011.11.007|arxiv = 1102.0730 |bibcode = 2012JCoPh.231.1751C|pages=1751–1765|s2cid=16979409}}</ref><ref>{{cite journal|last1=Kooshkbaghi|first1=Mahdi|last2=Frouzakis|first2=E. Christos|last3=Chiavazzo|first3=Eliodoro|last4=Boulouchos|first4=Konstantinos|last5=Karlin|first5=Ilya|title=The global relaxation redistribution method for reduction of combustion kinetics|journal=The Journal of Chemical Physics|date=2014|volume=141|issue=4|doi=10.1063/1.4890368|pmid=25084876|page=044102|bibcode = 2014JChPh.141d4102K |s2cid=1784716 |url=https://iris.polito.it/bitstream/11583/2553537/1/JChemPhys_2014a.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://iris.polito.it/bitstream/11583/2553537/1/JChemPhys_2014a.pdf |archive-date=2022-10-09 |url-status=live}}</ref>
* The Intrinsic Low-Dimensional Manifold (ILDM) approach and further developments<ref>{{cite journal|last1=Maas|first1=U.|last2=Pope|first2=S.B.|title=Simplifying chemical kinetics: intrinsic low-dimensional manifolds in composition space|journal=Combust. Flame|date=1992|volume=88|issue=3–4|pages=239–264|doi=10.1016/0010-2180(92)90034-m|bibcode=1992CoFl...88..239M }}</ref><ref>{{cite journal|last1=Bykov|first1=V.|last2=Maas|first2=U|title=The extension of the ILDM concept to reaction–diffusion manifolds|journal=Combust. Theory Model.|date=2007|volume=11|issue=6|pages=839–862|doi=10.1080/13647830701242531|bibcode=2007CTM....11..839B|s2cid=120624915}}</ref><ref>{{cite journal|last1=Nafe|first1=J.|last2=Maas|first2=U.|title=A general algorithm for improving ILDMs|journal=Combust. Theory Model.|date=2002|volume=6|issue=4|pages=697–709|doi=10.1088/1364-7830/6/4/308|bibcode = 2002CTM.....6..697N |s2cid=120269918}}</ref>
* The invariant-constrained equilibrium edge preimage curve method.<ref>{{cite journal|last1=Ren|first1=Z.|last2=Pope|first2=S.B.|last3=Vladimirsky|first3=A.|last4=Guckenheimer|first4=J.M.|title=The invariant constrained equilibrium edge preimage curve method for the dimension reduction of chemical kinetics|journal=J. Chem. Phys.|volume=124|issue=11|doi=10.1063/1.2177243|pmid=16555878|bibcode = 2006JChPh.124k4111R|page=114111|year=2006}}</ref>
* A few variational approaches<ref>{{cite journal|last1=Lebiedz|first1=D|title=Entropy-related extremum principles for model reduction of dissipative dynamical systems|journal=Entropy|date=2010|volume=12|issue=4|pages=706–719|bibcode = 2010Entrp..12..706L |doi = 10.3390/e12040706 |doi-access=free}}</ref><ref>{{cite journal|last1=Reinhardt|first1=V.|last2=Winckler|first2=M.|last3=Lebiedz|first3=D.|title=Approximation of slow attracting manifolds in chemical kinetics by tra trjectory-based optimization approaches|journal=J. Phys. Chem. A|date=112|pages=1712–1718|doi=10.1021/jp0739925|pmid=18247506|volume=112|issue=8|bibcode=2008JPCA..112.1712R|url=http://archiv.ub.uni-heidelberg.de/volltextserver/7352/1/modelred_optimization.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://archiv.ub.uni-heidelberg.de/volltextserver/7352/1/modelred_optimization.pdf |archive-date=2022-10-09 |url-status=live}}</ref>
* The Computational Singular perturbation (CSP) method and further developments.<ref>{{cite book|last1=Lam|first1=S.H.|last2=Goussis|first2=D.|title=Conventional Asymptotic and Computational Singular Perturbation for Symplified Kinetics Modelling|date=1991|publisher=Springer|location=Berlin}}</ref><ref>{{cite journal|last1=Valorani|first1=M.|last2=Goussis|first2=D.|last3=Najm|first3=H.N.|title=Higher order corrections in the approximation of low-dimensional manifolds and the construction of simplified problems with the csp method|journal=J. Comput. Phys.|date=2005|volume=209|issue=2|pages=754–786|doi=10.1016/j.jcp.2005.03.033|bibcode = 2005JCoPh.209..754V }}</ref>
* The Rate Controlled Constrained Equilibrium (RCCE) and Quasi Equilibrium Manifold (QEM) approach.<ref>{{cite journal|last1=Keck|first1=J.C.|last2=Gillespie|first2=D.|title=Rate-controlled partial-equilibrium method for treating reacting gas mixtures|journal=Combust. Flame|date=1971|volume=17|issue=2|pages=237–241|doi=10.1016/S0010-2180(71)80166-9|bibcode=1971CoFl...17..237K }}</ref><ref>{{cite journal|last1=Chiavazzo|first1=Eliodoro|last2=Karlin|first2=Ilya|title=Quasi-equilibrium grid algorithm: geometric construction for model reduction|journal=J. Comput. Phys.|date=2008|volume=227|issue=11|pages=5535–5560|doi=10.1016/j.jcp.2008.02.006|arxiv = 0704.2317 |bibcode = 2008JCoPh.227.5535C |s2cid=973322}}</ref>
* The G-Scheme.<ref>{{cite journal|last1=Valorani|first1=M.|last2=Paolucci|first2=S.|title=The G-Scheme: a framework for multi-scale adaptive model reduction|journal=J. Comput. Phys.|date=2009|volume=228|issue=13|pages=4665–4701|doi=10.1016/j.jcp.2009.03.011|bibcode = 2009JCoPh.228.4665V }}</ref>
* The Method of Invariant Grids (MIG).<ref>{{cite journal|last1=Chiavazzo|first1=Eliodoro|last2=Karlin|first2=Ilya|last3=Gorban|first3=Alexander|title=The role of thermodynamics in model reduction when using invariant grids|journal=Commun. Comput. Phys.|date=2010|volume=8|issue=4|pages=701–734|doi=10.4208/cicp.030709.210110a|bibcode=2010CCoPh...8..701C|url=http://www.math.le.ac.uk/people/ag153/homepage/ChiaKarGor2010.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.math.le.ac.uk/people/ag153/homepage/ChiaKarGor2010.pdf |archive-date=2022-10-09 |url-status=live|citeseerx=10.1.1.302.9316}}</ref><ref>{{cite journal|last1=Chiavazzo|first1=Eliodoro|last2=Karlin|first2=Ilya|last3=Frouzakis|first3=Christos E.|last4=Boulouchos|first4=Konstantinos|title=Method of invariant grid for model reduction of hydrogen combustion|journal=Proceedings of the Combustion Institute|date=2009|volume=32|issue=1 |doi=10.1016/j.proci.2008.05.014|pages=519–526|arxiv=0712.2386|bibcode=2009PComI..32..519C |s2cid=118484479}}</ref><ref>{{cite journal|last1=Chiavazzo|first1=Eliodoro|last2=Karlin|first2=Ilya|last3=Gorban|first3=Alexander|last4=Boulouchos|first4=Konstantinos|title=Coupling of the model reduction technique with the lattice Boltzmann method for combustion simulations|journal=Combust. Flame|date=2010|volume=157|issue=10|pages=1833–1849|doi=10.1016/j.combustflame.2010.06.009|bibcode=2010CoFl..157.1833C }}</ref>

===Kinetic modelling===

The kinetic modelling may be explored for insight into the reaction mechanisms of thermal decomposition in the combustion of different materials by using for instance [[Thermogravimetric analysis]].<ref>{{cite journal|last1=Reyes|first1=J.A.|last2=Conesa|first2=J.A.|last3=Marcilla|first3=A.|title=Pyrolysis and combustion of polycoated cartons recycling. kinetic model and ms analysis|journal=Journal of Analytical and Applied Pyrolysis|date=2001|volume=58-59|pages=747–763|doi=10.1016/S0165-2370(00)00123-6}}</ref>