F.A. Williams

Bio: F.A. Williams is an academic researcher. The author has contributed to research in topics: Premixed flame & Laminar flame speed. The author has an hindex of 1, co-authored 1 publications receiving 83 citations.

Dynamics of Stretched Flames

Abstract: Recent advances in the understanding of the structure, propagation, and extinction of laminar flames under the influence of stretch, as manifested by the existence of flame curvature, flow nonuniformity, and flame motion, are reviewed. The emphasis is on premixed flames because of the richness and subtlety of the phenomena involved. The review distinguishes the influences of the tangential and normal velocity gradients on the flame response, both at the hydrodynamic scale and within the flame structure, and emphasizes the importance of the preferential diffusion nature of heat and mass transport, as well as the extent to which the flame can freely adjust its location in response to stretch in order to achieve complete reaction. It is then demonstrated that stretch has only minimal effect on an adiabatic, unrestrained, diffusionally-neutral flame with complete reaction in that the temperature, propagation rate, and thickness of the flame are invariant to stretch, and that stretch alone cannot extinguish such a flame. In the presence of preferential diffusion and/or when the flame movement is restrained, the response of the flame to stretch becomes more sensitive and extinction is also possible. The concept of flame stretch is applied to interpret such practical flame phenomena as flame stabilization and flame-front instability, determination of laminar flame speeds and flammability limits, concentration and temperature modifications in flame chemistry, and modeling of turbulent flames. The properties of stretched diffusion flames are then briefly discussed. The review closes with suggestions for further research.

Dynamics of stretched flames

Abstract: Recent advances in the understanding of the structure, propagation, and extinction of laminar flames under the influence of stretch, as manifested by the existence of flame curvature, flow nonuniformity, and flame motion, are reviewed. The emphasis is on premixed flames because of the richness and subtlety of the phenomena involved. The review distinguishes the influences of the tangential and normal velocity gradients on the flame response, both at the hydrodynamic scale and within the flame structure, and emphasizes the importance of the preferential diffusion nature of heat and mass transport, as well as the extent to which the flame can freely adjust its location in response to stretch in order to achieve complete reaction. It is then demonstrated that stretch has only minimal effect on an adiabatic, unrestrained, diffusionally-neutral flame with complete reaction in that the temperature, propagation rate, and thickness of the flame are invariant to stretch, and that stretch alone cannot extinguish such a flame. In the presence of preferential diffusion and/or when the flame movement is restrained, the response of the flame to stretch becomes more sensitive and extinction is also possible. The concept of flame stretch is applied to interpret such practical flame phenomena as flame stabilization and flame-front instability, determination of laminar flame speeds and flammability limits, concentration and temperature modifications in flame chemistry, and modeling of turbulent flames. The properties of stretched diffusion flames are then briefly discussed. The review closes with suggestions for further research.

Flame stretch rate as a determinant of turbulent burning velocity

Abstract: A rational basis for correlating turbulent burning velocities is shown to involve the product of the Karlovitz stretch factor and the Lewis number. A generalized expression is derived to show how flame stretch is related to the velocity field. A new dimensionless correlation of experimental values of turbulent burning velocities is presented. Dimensionless groups also are used in correlations of laminar and turbulent flame extinction stretch rates. A distribution function of stretch rates in turbulent flames, based on an earlier one of Yeung et al ., is proposed and the experimental data are well predicted by a theory based on flamelet extinction by flame stretch with this distribution. Uncertainties arise concerning the role of negative stretch rate. Laminar flamelet modelling of complex combustion appears to have a broader validity than might be expected and some explanation for this is offered.

Turbulent flows with premixed reactants

Abstract: This chapter considers turbulent flows in which the reactants have been fully mixed prior to reaction. Application to combustion is emphasized both because many practical combustion systems require the fuel and oxidizer to be premixed, and also because the premixed flame provides a convenient test for contemporary ideas about turbulent reacting flows. Only gaseous species are considered. The rates of the chemical kinetic processes leading to combustion are strongly dependent on temperature. Consequently, the propagation of a premixed laminar flame requires thermal conduction and diffusion from the hot products to preheat the reactive mixture. In a turbulent flame, these molecular processes are augmented both indirectly, by distortion of flame surfaces, leading to an increase in their area, and directly, by turbulent mixing. The result is that the mean rate of heat release is generally more strongly influenced by the turbulence than by chemical kinetic factors so that premixed turbulent combustion is primarily a complex fluid mechanical problem. However, ignition and flame quenching provide examples of situations in which both chemical kinetics and fluid mechanics are likely to be important. Premixing leads to a significant simplification in the analysis as the composition of the flow is essentially uniform, in terms of the elements involved, i.e., the Z[s of (1.19) are constants. It also causes complications; the scalar thermodynamic variables of temperature, density, and composition often fluctuate strongly within this type of flame, between values characteristic of the unburned and fully burned mixtures. These intense scalar fluctuations pose theoretical and experimental difficulties, some of which have not yet been solved. Following a review of knowledge concerning premixed turbulent combustion, this chapter concentrates on theoretical models which are based on prior specification of a probability density function for the fluctuating thermodynamic state of the mixture. It is argued that such models provide the best available compromise between complexity and generality of application.