Journal ArticleDOI
Effects of molecular diffusion and of thermal expansion on the structure and dynamics of premixed flames in turbulent flows of large scale and low intensity
P. Clavin,Forman A. Williams +1 more
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In this paper, the effects of flow inhomogeneities on the dynamics of laminar flamelets in turbulent flames, with account taken of influences of the gas expansion produced by heat release, were investigated.Abstract:
To study effects of flow inhomogeneities on the dynamics of laminar flamelets in turbulent flames, with account taken of influences of the gas expansion produced by heat release, a previously developed theory of premixed flames in turbulent flows, that was based on a diffusive-thermal model in which thermal expansion was neglected, and that applied to turbulence having scales large compared with the laminar flame-thickness, is extended by eliminating the hypothesis of negligible expansion and by adding the postulate of weak-intensity turbulence. The consideration of thermal expansion motivates the formal introduction of multiple-scale methods, which should be useful in subsequent investigations. Although the hydrodynamic-instability mechanism of Landau is not considered, no restriction is imposed on the density change across the flame front, and the additional transverse convection correspondingly induced by the tilted front is described. By allowing the heat-to-reactant diffusivity ratio to differ slightly from unity, clarification is achieved of effects of phenomena such as flame stretch and the flame-relaxation mechanism traceable to transverse diffusive processes associated with flame-front curvature. By carrying the analysis to second order in the ratio of the laminar flame thickness to the turbulence scale, an equation for evolution of the flame front is derived, containing influences of transverse convection, flame relaxation and stretch. This equation explains anomalies recently observed at low frequencies in experimental data on power spectra of velocity fluctuations in turbulent flames. It also shows that, concerning the diffusive-stability properties of the laminar flame, the density change across the flame thickness produces a shift of the stability limits from those obtained in the purely diffusive-thermal model. At this second order, the turbulent correction to the flame speed involves only the mean area increase produced by wrinkling. The analysis is carried to the fourth order to demonstrate the mean-stretch and mean-curvature effects on the flame speed that occur if the diffusivity ratio differs from unity.read more
Citations
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Journal ArticleDOI
Laminar Flamelet Concepts in Turbulent Combustion
TL;DR: In this article, it is shown that the inner structure of the flamelets is one-dimensional and time dependent, and a new coordinate transformation using the mixture fraction Z as independent variable leads to a universal description.
Journal ArticleDOI
Dynamic behavior of premixed flame fronts in laminar and turbulent flows
Paul Clavin,Paul Clavin +1 more
TL;DR: In this article, a review of recent developments in flame theory is provided, in sufficient detail to give the reader a comprehensive introduction to the field, including the stability and flammability limits of planar fronts, cellular flames, flame stretch, turbulent and self-turbulizing flames, hydrodynamic interactions between weakly turbulent gas flows and wrinkled flame fronts, molecular diffusion effects of intermediate species involved in chain reactions.
Journal ArticleDOI
Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels
Eliseo Ranzi,Alessio Frassoldati,Roberto Grana,Alberto Cuoci,Tiziano Faravelli,Andrew P. Kelley,Chung K. Law,Chung K. Law +7 more
TL;DR: In this paper, a detailed kinetic mechanism for the pyrolysis and combustion of a large variety of fuels at high temperature conditions is presented, and the authors identify aspects of the mechanism that require further revision.
Journal ArticleDOI
Flames as gasdynamic discontinuities
TL;DR: In this article, an equation for the propagation of the discontinuity surface for arbitrary flame shapes in general fluid flows is derived, where the structure of the flame is considered to consist of a boundary layer in which the chemical reactions occur, located inside another boundary layer, in which transport processes dominate.
Journal ArticleDOI
Flame acceleration and transition to detonation in ducts
Gaby Ciccarelli,S. Dorofeev +1 more
TL;DR: In this article, the state of knowledge on flame acceleration and deflagration-to-detonation transition (DDT) in smooth ducts and ducts equipped with turbulence-producing obstacles is reviewed.
References
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Journal ArticleDOI
Nonlinear analysis of hydrodynamic instability in laminar flames—I. Derivation of basic equations
TL;DR: In this paper, an asymptotic nonlinear integrodifferential equation for spontaneous instability of the plane front of a laminar flame is derived, and it is shown that in all cases spontaneous instability implies an increase in its propagation velocity.
Journal ArticleDOI
The asymptotic structure of counterflow diffusion flames for large activation energies
TL;DR: In this paper, the structure of steady state diffusion flames is investigated by analyzing the mixing and chemical reaction of two opposed jets of fuel and oxidizer as a particular example, and an Arrhenius one-step irreversible reaction in the realistic limit of large activation energies.
Journal ArticleDOI
Diffusional-Thermal Theory of Cellular Flames
TL;DR: In this article, the formation of cellular structure in a flame is conditioned by diffusion and heat conduction effects and is independent of the hydrodynamics of the perturbed flame, and it is proved that cellular flames are formed only when a sufficiently light reactant of the combustible mixture is present in a low concentration.
Journal ArticleDOI
Linear Stability Analysis of Nonadiabatic Flames: Diffusional-Thermal Model
Guy Joulin,P Clavin +1 more
TL;DR: In this paper, the authors applied matched asymptotic expansions to investigate the effects of heat losses on linear stability of a planar flame, which is governed by a one-step irreversible Arrhenius reaction.
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