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Combustion Mechanism Reduction & Ptimization Based on Sensitivity Analysis & Genetic Algorithm

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Tutor: LiuHong
School: Shanghai Jiaotong University
Course: Aircraft design
Keywords: chemical kinetic mechanism,sensitivity analysis,product rate analysis,quasi-stea
Type: Master's thesis
Year:  2012
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The ignition and combustion characteristics of fuel affect the efficiency of scramjet combustor seriously, numerical simulations of inner flow coupling with chemical reaction are very necessary during the design process of the combustor. While the reactions of a detailed combustion chemical kinetic mechanism involve great differences of the eigen time, this results in great numerical stiffness problem which makes strong against to the coupling numerical simulation. Thus the reduction of the chemical kinetic mechanisms is very important. In this paper, with the scramjet combustor design as the engineering background and the hydrocarbon fuel chemical kinetic mechanisms as studying object, reduction and optimization strategies based on sensitivity analysis and genetic algorithm are studied and proposed to do the best reduction with lest accuracy loss.For the first step, the chemical kinetic theory is studied and analyzed. With the most attention on the ignition delay, the Perfect Stirred Reactor (PSR) and Premixed Laminar Flame model(PLF) were chosen to study the detailed chemical kinetic mechanisms. Then the detailed chemical kinetic mechanisms of methane and ethylene (Gri-mech 3.0 for methane, containing 53 species and 325 elementary reactions, and the Konnov mechanism for ethylene, containing 127 species and 1207 elementary reactions) were studied through sensitivity analysis. The method of coupling sensitivity analysis with product rate analysis was proposed to reduce the reactions, and then by adopting the assumption of the quasi-steady-state, the mechanisms were further reduced. Not only the reactions but also the species can be greatly reduced through this reduction strategy. As the result of the work described above, a reduced mechanism of methane combustion which contains 14 species and 18 elementary reactions, and another reduced mechanism of ethylene combustion which contains 17 species and 23 elementary reactions, were abstracted from their detail mechanism respectively. As is known to all, every reduction work results in accuracy loss, and to bring back the accuracy, genetic algorithm was adopted in this paper to optimize and modify the reaction parameter system of the reduced mechanisms. The genetic algorithm calculates the fitness of each feasible solution with the ignition delay, equivalent temperature, and mole fractions of main products, which are obtained all by calling the PSR solver.Through the comparison of the reduced mechanisms of this paper with their detailed mechanisms and other reduced mechanisms, it indicated that by coupling sensitivity analysis with product rate analysis and quasi-steady-state assumption, detailed mechanisms can be greatly reduced and can still describe the combustion phenomena well; and it is feasible to improve the accuracy of the reduced mechanisms by optimizing the reaction parameter system with genetic algorithm.
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