Structure And Stability Of Bouyant Diffusion Flames

Author : Sally Ann Cheatham
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (373 download)

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Download or read book On the Structure and Stability of Diffusion Flames written by Sally Ann Cheatham and published by . This book was released on 1997 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Diffusion flames are flames in which fuel and oxidant, initially separate, come together to react chemically in a relatively thin combustion zone. Asymptotic methods, supplemented by numerical computation, are used to analyze the structure and dynamical properties of such flames. Attention is first focused on diffusion flames in arbitrary fluid flows for general fuel and oxidant Lewis numbers, in which the combustion is complete or nearly complete. Thus the limit of large Damkohler number and large activation energy is considered. The resulting mathematical formulation consists of a free boundary problem, with the flame as the free boundary. Such a treatment greatly simplifies the governing system of equations, and provides a framework for the analysis of the flame structure as well as its dynamical properties. The stability of a planar flame is next considered, with attention focused on diffusional-thermal effects. A dispersion relation is obtained, which relates the growth parameter and wavenumber of disturbances to the fuel and oxidant Lewis numbers, supply temperatures, and supply concentrations. Analysis of the dispersion relations indicates that instability corresponding to cellularity of the flame may occur for lean flames under near extinction conditions when the Lewis numbers of fuel and oxidant are sufficiently low. The asymptotic model is also used to identify conditions under which flame extinction occurs. In particular, the burning characteristics of a dilute spray diffusion flame have been examined. The presence of fuel droplets in the fuel supply is shown to alter the extinction characteristics of the flame. The onset of oscillations in the spherical diffusion flame surrounding a fuel droplet is also considered. Motivated by recent microgravity candle flame experiments, the constraint of a weak oxidant environment is imposed. A linear stability analysis indicates that oscillations may indeed result when radiative heat losses and the Lewis numbers of fuel and oxidant are sufficiently large. Estimates for the critical conditions are given, along with a prediction of the resulting frequency of oscillation. A weakly non-linear analysis is then used to more fully investigate the transition to an oscillatory burning state.