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Chemical_Kinetics_2008 - 1 AME 513 Spring 2008 F.N...

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AME 513 Spring 2008, F.N. Egolfopoulos 1 Chemical Kinetics 1. General Remarks Although equilibrium calculations reveal important information regarding the state of the burned gases, they can not provide any measure of the time that is required for the chemical energy to be converted to thermal. This time is very important aspect in combustion processes as its magnitude although small (typically of the order of nanoseconds to microseconds) it can still be of the same order with other time scales. For example, under certain conditions it can be comparable to the time scales that control the processes of molecular transport and fluid mechanics, and under extreme conditions to the residence time available in a practical combustor. Under such conditions combustion efficiency can be compromised and pollutants emissions could be increased. Accurate accounting of all chemical times can be done by considering the detailed description of all successive paths of the oxidation of the parent fuel molecule to the final products. That description includes the rates of conversion between successive paths characterized by “true” elementary reactions. Chemical kinetics is the discipline that provides the necessary tools to describe the reaction rates of such elementary paths. Thus, time becomes a controlling parameter. The importance of chemical kinetics in combustion is apparent, but it has been frequently overemphasized in the past. More specifically, the kinetics community has systematically neglected any non-chemical effects on the rate that the chemical reactions proceed. As it will be explained in detail in a later chapter, fluid mechanics can affect the rate of transport of the reactants as well as the topology of the flame, and as a result can have a first order effect on the kinetics. This is a topic of great complexity and currently attracts major
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AME 513 Spring 2008, F.N. Egolfopoulos 2 research efforts in view if the importance of the coupling of chemistry and fluid mechanics in turbulent reacting flows. A rigorous description of the chemical kinetics describing the oxidation of a practical fuel molecule includes a large number of species and elementary reactions. Implementing that level of rigor though can make any realistic simulations prohibitive as the CPU time requirement scales with N 2 , where N is the number of species. Representative number of species and elementary reactions are: H 2 /air 9 species 20 reactions CH 4 /air without NO x chemistry 35 species 200 reactions CH 4 /air with NO x chemistry 50 species 300 reactions iso-octane/air without NO x chemistry 170 species 1,200 reactions On the other hand, the chemical kinetics have been oversimplified by the fluid mechanics community, in attempts to simulate complex reacting flow configuration by having a manageable system of equations. Quite frequently a one-step kinetic model has been used such as, for example: CH 4 + 2O 2 CO 2 + 2H 2 O which also happens to be the correct description of the initial and final equilibrium states of the combustion process.
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Chemical_Kinetics_2008 - 1 AME 513 Spring 2008 F.N...

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