Structure and Stability of Buoyant Diffusion Flames PDF Download
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Author: Graham Christopher Fleming
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 300
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Book Description
Author: Graham Christopher Fleming
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 300
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Book Description
Author: D. A. Scholefield
Publisher:
ISBN:
Category :
Languages : en
Pages : 22
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Author: John Barr
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author: Derek Arthur Scholefield
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author: Graham Christopher Fleming
Publisher:
ISBN:
Category :
Languages : en
Pages : 150
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Author: Kezhong Jia
Publisher:
ISBN:
Category :
Languages : en
Pages : 268
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Book Description
Author: San-Mou Jeng
Publisher:
ISBN:
Category : Dwellings
Languages : en
Pages : 237
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Author: Sally Ann Cheatham
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Category :
Languages : en
Pages :
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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.
Author: F. Takahashi
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author: National Aeronautics and Space Adm Nasa
Publisher: Independently Published
ISBN: 9781723745058
Category : Science
Languages : en
Pages : 42
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Book Description
Numerical simulations of 2-D triple flames under gravity force have been implemented to identify the effects of gravity on triple flame structure and propagation properties and to understand the mechanisms of instabilities resulting from both heat release and buoyancy effects. A wide range of gravity conditions, heat release, and mixing widths for a scalar mixing layer are computed for downward-propagating (in the same direction with the gravity vector) and upward-propagating (in the opposite direction of the gravity vector) triple flames. Results of numerical simulations show that gravity strongly affects the triple flame speed through its contribution to the overall flow field. A simple analytical model for the triple flame speed, which accounts for both buoyancy and heat release, is developed. Comparisons of the proposed model with the numerical results for a wide range of gravity, heat release and mixing width conditions, yield very good agreement. The analysis shows that under neutral diffusion, downward propagation reduces the triple flame speed, while upward propagation enhances it. For the former condition, a critical Froude number may be evaluated, which corresponds to a vanishing triple flame speed. Downward-propagating triple flames at relatively strong gravity effects have exhibited instabilities. These instabilities are generated without any artificial forcing of the flow. Instead disturbances are initiated by minute round-off errors in the numerical simulations, and subsequently amplified by instabilities. A linear stability analysis on mean profiles of stable triple flame configurations have been performed to identify the most amplified frequency in spatially developed flows. The eigenfunction equations obtained from the linearized disturbance equations are solved using the shooting method. The linear stability analysis yields reasonably good agreements with the observed frequencies of the unstable triple flames. The frequencies and amplitudes of dist
