A new theory of premixed flames in near-stoichiometric mixtures
Department of Mathematical Sciences
Doctor of Philosophy
Bechtold, John Kenneth
Tilley, Burt S.
Lott, Dawn A.
Michalopoulou, Eliza Zoi-Heleni
In this dissertation, a new model of premixed flames in near-stoichiometric mixtures is derived. Unlike most previous theories, which are valid only for very lean or very rich, i.e. off-stoichiometric conditions, our model remains valid over the entire spectrum of mixture compositions, from lean to rich, including the near-stoichiometric regime. Since fuel-mixture composition is known to have a significant effect on flame behavior, such a model is expected to contribute new insights into classical problems in premixed combustion.
In the first part of this dissertation, we describe the derivation of a model for premixed flames in two-reactant mixtures in a formal asymptotic way. Using the method of matched asymptotics we are able to simplify the complicated governing equations of combustion and effectively decouple the hydrodynamic equations from those of heat and mass transport. Our model considers a two reactant mixture in which one reactant is slightly in excess and the other deficient. We show that, if the initially excess reactant is less mobile, then it doesn't diffuse as rapidly across a strained flow field and can be locally deficient, and hence consumed, at the reaction zone. This can have a significant effect on burning characteristics of the flame. There are two major differences between our model and previous models. First, we have an additional react ion-diffusion equation governing the transport of the second species. Second, the derived conditions relating the gradients across the reaction sheet are shown to take one of two different forms, depending on which of the two species is consumed in the reaction.
In the second part of the thesis, we use our model to study the behavior of planar and strained flames. For the planar flame in uniform flow, we find that many of the results of single-reactant theory apply under near-stoichiometric conditions, provided an effective Lewis number is introduced. On the other hand, for a flame in a nonuniform flow, the dynamics depend significantly on the mass diffusivities as well as mixture strength. In particular, we have analyzed the structure of flame in stagnation point flow and given a complete description of the combustion process including extinction conditions. Results are shown to compare favorably with experiments.
njit-etd2002-012 (80 pages ~ 3,290 KB pdf)
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Created February 28, 2003