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AME513_Introduction_2008

AME513_Introduction_2008 - AME 513 Spring 2008 F.N...

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AME 513, Spring 2008, F.N. Egolfopoulos 1 Introduction 1. General Remarks Before any formal definitions are given, a preview of the level of complexity and scientific interest of the subject of Combustion or more general of Reacting Flows will be presented. This is very important, as it will enhance the motivation/interest of the student/reader to gain the highest possible insight into a number of physico-chemical processes that control a combustion process. Lack of such insight can result in casual and superficial treatment of the subject. It is assumed that the student/reader understands at this point that chemical reactions with the attendant heat release as well as the processes of mass and heat transport are present in a flowing reacting system. This typically happens in, say, the Bunsen burner (basic chemistry experiment) or in engines such as those used in automobiles, power generation, aircrafts, or spacecrafts to name a few. While the Bunsen-burner experiment typically results in laminar flows and a conical-shape flame, the flow configurations in engines are highly turbulent with flames shapes too complicated to be described experimentally and/or theoretically. Considering first the subject of turbulence, reference will be made to a private statement by Anatol Roshko: “I spent all my life trying to understand a very narrow area of turbulent flows and I think that my contributions have been substantial. However, the general subject of turbulence is so complex that if one of you derives a rigorous theory of turbulence in the future it will be too complex for me to understand !!!”. This reference was made to “cold” turbulent flows. Considering turbulent reacting flows can significantly augment this statement. In such flows there are additional, highly complex interactions between fluid mechanics, molecular transport, and chemical kinetics as well
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AME 513, Spring 2008, F.N. Egolfopoulos 2 substantial effects of dilatation resulting from the heat release, the underlying physics become far more complex compared to the case of non-reacting turbulent flows. While the above reference to the highly complex problem of reacting turbulent flows contains quite obvious remarks, the student/reader may better appreciate the complex nature of combustion by considering laminar reacting flows, characterized by well understood and frequently trivial fluid mechanics processes. Examples could be flows in tubes, stagnation-type configurations, and/or the Bunsen burner experiment. While the fluid mechanics in such configurations are readily being the simplest ones, the presence of chemical reactions results in phenomena that are still not well understood and/or characterized. For example, up to date we have not been able to: Directly measure flame propagation speeds Derive rigorous and universal criteria of the critical phenomena of ignition and extinction Understand the concept of flammability (term widely and casually used) Explain how flames are in general stabilized (e.g. through heat loss, fluid mechanics,
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