5310 Combustion Introduction Lecture 10

5310 Combustion Introduction Lecture 10 - MAE 5310:...

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Unformatted text preview: MAE 5310: COMBUSTION FUNDAMENTALS Chemical Kinetics: Steady-State Approximation September 22, 2011 Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk STEADY-STATE APPROXIMATION What are we talking about: In many chemical combustion systems, many highly reactive intermediate species (radicals) are formed Physical explanation: Rapid initial build-up of radical concentration Then radicals are destroyed as quickly as they are being created Implies that rate of radical formation = rate of radical destruction Situation typically occurs when the reaction forming the radicals is slow and the reaction destroying the radicals is fast This implies that the concentration of radicals is small in comparison with those of the reactants and products The radical species can thus be assumed to be at steady-state MECHANISM FOR UNIMOLECULAR REACTIONS [ ] [ ][ ] [ ] [ ] [ ] [ ] [ ][ ] [ ] [ ] [ ] [ ][ ] [ ] [ ][ ] [ ] [ ] [ ] 1 1 * * * * * * * * + = + = + = = + =-- = + + + + M k k M k k products A M k k M A k k M k M A k k A k dt prod d k M k M A k A A k M A k M A k dt A d products A M A M A M A M A uni de e app k uni de e uni de e uni uni uni de e uni de e k k k app uni de e CHAIN AND CHAIN-BRANCHING REACTIONS Chain reactions involve production of a radical species that subsequently reacts to produce another radical. This radical in turn, reacts to produce yet another radical Sequence of events, called chain reaction, continues until a reaction involving the formation of a stable species from two radicals breaks the chain Consider a hypothetical reaction, represented by a global mechanism: M AB...
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5310 Combustion Introduction Lecture 10 - MAE 5310:...

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