Chemical kinetic modeling of hydrocarbon combustion

▪ Abstract Because calibrated light curves of type Ia supernovae have become a major tool to determine the local expansion rate of the universe and also its geometrical structure, considerable atte...

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Type IA Supernova Explosion Models

Dynamic behavior of premixed flame fronts in laminar and turbulent flows

In recent decades particular interest in applications of nonequilibrium plasma for the problems of plasma-assisted ignition and plasma-assisted combustion has been observed. A great amount of experimental data has been accumulated during this period which provided the grounds for using low temperature plasma of nonequilibrium gas discharges for a number of applications at conditions of high speed flows and also at conditions similar to automotive engines. The paper is aimed at reviewing the data obtained and discusses their treatment. Basic possibilities of low temperature plasma to ignite gas mixtures are evaluated and historical references highlighting pioneering works in the area are presented. The first part of the review discusses plasmas applied to plasma-assisted ignition and combustion. The paper pays special attention to experimental and theoretical analysis of some plasma parameters, such as reduced electric field, electron density and energy branching for different gas discharges. Streamers, pulsed nanosecond discharges, dielectric barrier discharges, radio frequency discharges and atmospheric pressure glow discharges are considered. The second part depicts applications of discharges to reduce the ignition delay time of combustible mixtures, to ignite transonic and supersonic flows, to intensify ignition and to sustain combustion of lean mixtures. The results obtained by different authors are cited, and ways of numerical modelling are discussed. Finally, the paper draws some conclusions on the main achievements and prospects of future investigations in the field.

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Plasma assisted ignition and combustion

The hydrogen-fueled internal combustion engine : a technical review.

For some time copies of the second edition of this book have been in short supply. In order to meet, the demand for additional copies the authors have decided in favor of a reprint of the book rather than an extensive revision at this time. The justification for this lies in the character of the book. It was conceived as an exposition of basic concepts of gaseous combustion phenomena. Since the publication of the second edition in 1961 the literature on the subject has grown in volume very greatly. However, virtually no new concepts in gaseous combustion have evolved. For this reason we feel that the book, in its present form, continues to serve well the purpose for which it was intended, namely, to provide a systematic presentation of the fundamentals of combustion for scientists and engineers. In addition, we feel that the present edition has not yet outlived its usefulness as a source of data. Furthermore, the utility of certain concepts is only now beginning to be appreciated. This applies in particular to the concept of flame stretch which permits a unified approach to seemingly widely different sets of phenomena, including ignition, flame extinction, stabilization, quenching, flammability limits, and maximum heat release rates.

Combustion, Flames and Explosions of Gases

It has been shown that the condition of equality of gas and burning velocities, required for stabilization of a flame above a burner, is established near the rim of the orifice or an obstruction within the stream by the effects of friction and inhibition of the explosive reaction. This condition is maintained between two critical gradients of the gas velocity at the solid surface, the lower gradient bordering on the flash‐back and the upper gradient on the blow‐off range. Values of the gradients in the range of laminar flow were determined by hydrodynamic equations from gas flow and tube dimensions; and their independence of tube diameter, except for extreme sizes, has been demonstrated both for upright and inverted flames. In the latter the critical velocity gradient for blow‐off was also found to be independent of the diameter of the centrally mounted wire within a considerable range. The effect of the surrounding atmosphere on the critical blow‐off gradient has been shown. The gas‐flow pattern was studied experimentally by photographing stroboscopically illuminated magnesium oxide dust particles. No appreciable redistribution of velocities over the cross section of the stream was observed below the combustion zone. The burning velocity was found constant over the surface of the inner cone, except at the tip, where it increases to the axial gas velocity, and at the base, where it decreases to zero. The experimental flame cone outline and flow pattern agree with the theoretical within the limitations imposed by simplifying assumptions. The temperature distribution in the flame was determined by the sodium line‐reversal method, only the center of the flame being colored, and observations of the width and emission spectrum of the luminous combustion zone were made. In natural gas‐air flames C–C and C–H bands are observed in the zone, and the temperature rises gradually behind the zone to a maximum that corresponds to the theoretical flame temperature. In natural gas‐oxygen flames no C–C or C–H bands are observed; and the temperature attains a maximum, exceeding the theoretical, immediately behind the combustion zone. Temperature distribution and flow pattern have been correlated. The flow pattern of inverted flames was also studied with particular attention to the region of flame attachment just above the central wire where the formation of an annular vortex is demonstrated. An interpretation of the formation of polyhedral flame cones, described by Smith and Pickering, has been given.

Stability and Structure of Burner Flames

A method has been developed for determining the velocity of flame relative to the mass movement of the gases, in a closed spherical bomb from an analysis of the time‐pressure record of the explosion. The speed of the flame can be evaluated at any moment during its progress from the center to the periphery of the bomb, as well as the temperature existing in the unburned phase, the temperature immediately behind the flame front, the temperature gradient from the latter point to the center of the bomb, and the pressure in the bomb at the same moment. Given a certain fraction burned of the total amount of gas, the volume occupied by the products can be determined for three conditions: (1) Before it has expanded against the rest of the unburned gas; (2) after it has expanded; and (3) when combustion is complete and it has been compressed by subsequent burning of gas nearer the periphery. Calculations of flame speeds, temperatures, etc., have been made and tabulated for explosions of mixtures of ozone and oxygen. The speed of flame increases from the center of the bomb to the wall. At the same time the pressure and temperature of the gas about to be burned increases. The temperature gradient in the gas from the center to the wall has been calculated when the combustion is complete. A complete diagrammatic description is given for one explosion. It is shown that the temperature gradient actually existing in the bomb does not affect the specific heat results obtained by the usual method of calculating the final temperature from the maximum pressure by means of the gas law. It is pointed out that the practical coincidence of the pressure curve with the zero line in the first part of the time‐pressure record is not due to a time lag between passage of the spark and ignition, but to the small fraction of gas which has burned during this time.

Determination of the Speed of Flames and the Temperature Distribution in a Spherical Bomb from Time-Pressure Explosion Records

The phenomenon of nonisotropic propagation consists in the spontaneous development of blisters or cells on the surface of a combustion wave. The present experiments on spherical flames and the experiments of Markstein on flames in wide tubes show that the phenomenon is characteristic of nonstoichiometric explosive mixtures in which the deficient reactant constituent is also the constituent of largest diffusivity. This suggests that the phenomenon is primarily caused by the effect of diffusion processes on the burning velocity. It is proposed that in curved areas of the wave that are convex with respect to the burned gas, the burning velocity is reduced because the lines of diffusion diverge, and hence the concentration of the faster diffusing constituent decreases; whereas in concave areas the lines of diffusion converge, and hence the concentration of the faster diffusing constituent increases. In rich mixtures of hydrocarbon and oxygen, additional evidence for the effect is furnished by the emergence of carbon streamers and by characteristic changes of light emission from convex wave areas, showing that the oxygen concentration, and probably therefore the burning velocity, is decreased in these areas below the average for the mixture.

Nonisotropic Propagation of Combustion Waves in Explosive Gas Mixtures and the Development of Cellular Flames

A new theory of flame propagation in slow inflammation is proposed which considers that the highly energized atoms or radicals formed in the flame front play a more important role than heat conductivity in bringing the unburned gas to the reacting state. A small number of atoms or radicals diffuse over into the unburned phase, there initiating the chemical reaction. This and other considerations allow one to propose that the sum of thermal and chemical energy per unit mass of gas in all layers from the burned phase to the unburned phase is constant. The fundamental equation for the velocity V of the flame is derived simply from the fact that the number of molecules of combustible gas entering the reaction zone (flame front) in unit time equals the number reacting in the zone in the same time. If the flame front extends from xu to xb and the unburned gas enters it at a rate just sufficient to maintain the flame front stationary, then for 1cm¯2 cross section the equation reads: VNc(u)= ∫ TuTb−∂Nc∂tdxdTdT wh...

Bernard Lewis

Papers

Combustion, Flames and Explosions of Gases

Stability and Structure of Burner Flames

Determination of the Speed of Flames and the Temperature Distribution in a Spherical Bomb from Time-Pressure Explosion Records

Nonisotropic Propagation of Combustion Waves in Explosive Gas Mixtures and the Development of Cellular Flames

On the Theory of Flame Propagation