Showing papers on "Laminar flame speed published in 1982"

Turbulent flame structure and speed in spark-ignition engines

Abstract: Recent exprimental, observations of flame structure and speed in spark-ignition enginesare discussed. Schlieren photographs, shadowgraphs and laser scattering measurements strongly suggest a highly wrinkled structure for the turbulent flame in such engines. Simultaneous pressure measurements and high-speed motion picture records are used to derive two closely related sets of empirical equations for calculating burning rates. One set suggests an eddy entrainment and laminar burn-out model and the other, a laminar flame stretching and wrinkling model. Tentative correlations relating the parameters in the burning equations to engine geometry and operating variables are derived. Statistical variations in the parameters produce cycle-to-cycle dispersions in the pressure, but correlations for predicting the magnitude of the dispersions have not yet been obtained.

The Effect of Viscosity on Hydrodynamic Stability of a Plane Flame Front

Abstract: This paper presents a linear analysis of the hydrodynamic stablity of the plane flame front of a premixed laminar flame. The technique of outer and inner asymptotic expansions is used to calculate the next approximation to the classical long-wave Landau limit. The resulting correction turns out to be independent of the Prandtl number. This implies that, although diffusivity, conductivity and viscosity in gases are of the same order of magnitude, viscosity exerts a secondary influence in comparison with diffusivity and conductivity.

Effects of lewis number on extinction behavior of premixed flames in a stagnation flow

Abstract: The combined effects of the velocity gradient and the Lewis number of deficient reactantsin the premixed gas on the behavior and extinction of a premixed flame in a stagnation flow were studied experimentally. The flat twin flames established in the stagnation region of the two dimensional opposed flow of the same mixture were used in order to make an adiabatic and noncatalytic stagnation surface. The flame extinction behavior observed is purely attributed to the flow-transport properties-chemical reaction interaction. The mixture employed was hydrogen, methane, propane, or butane with air. The flame temperature and the distance between the two flames were measured for the wide variations of the fuel concentration and the velocity gradient. As the stagnation velocity gradient was increased by increasing the opposed flow velocity, the flat two flames approached each other, and finally the extinction occurred abruptly. By observing the flame behavior, the extinction process of the flame was classified into two patterns corresponding to the ranges of the Lewis number Le of the deficient reactant in the premixed gas (fuel in the lean mixture and oxygen in the rich mixture). Its ranges are Le 1. Under the condition of Le 1 (lean hydrogen or methane-air and rich propane or butane-air), as the velocity gradient is increased, the flame temperature increases and reaches a maximum point at a certain value of the velocity gradient, beyond which the flame temperature decreases. The flame is extinguished close to the stagnation surface or in contact with the stagnation surface. The results of this experiment are in good agreement with the theoretical predictions.

The mechanisms of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube

Abstract: The mechanisms by which lean limit upward and downward propagating flames extinguish in a standard flammability tube (51mm ID, 1.8m long) were deduced using schlieren and direct light photography and temperature measurements and by observing the behavior of the flame under transient loading. The lean methane-air system was studied. The upward propagating flame takes the shape of a stable hemispherical cap attached to a trailing skirt. This flame always extinguishes first at the holding region on the axis of the tube and a failure wave subsequently washes down the flame until extinguishment is complete. The extinguishment occurs because the flame is stretched at the tip and heat loss to the walls is not important to this extinguishment. On the other hand the extinguishment of a downward propagating flame is a multistep process. First heat loss to the walls causes the flame to extinguish near the walls and a smaller residual flame is left in the center of the tube. Once this happens, differential buoyancy of the hotter central gases and the cooler surrounding gases forces the cooler gases to move ahead of the flame. Finally, the downward propagating flame is observed to rise slightly. At this point the flame extinguishes completely.

Flame zone in gas combustion in an inert porous medium

Abstract: The paper describes an experimental method for determining the propagation characteristics of a flame in a porous medium. The apparatus comprises a vertical tube filled with a medium of porosity 0.4. Photodiodes are provided for determination of the combustion zone boundaries, and photomultipliers are used to measure the extent of the combustion zone. The whole is filled with a stoichiometric methane air mixture and ignited with a single spark at the top end of the tube. Experiments are performed for mixtures at various initial pressures, and deductions are made concerning the simultaneous procession of the chemical transformations and combustion product cooling within the combustion zone. The flame propagation in a porous medium is determined to be turbulent in nature on the basis of observations concerning the relationship between the extent of the combustion zone and the thickness of a laminar flame front.

Investigation of the dynamic behavior of a bluff body diffusion flame using flame emission

Abstract: The evaluation of a research combustor in a study conducted by Roquemore et al. (1980) revealed the presence of unsteady flame behavior in the recirculation zone. The present investigation is concerned with the characteristics of the flame turbules at different air and fuel flow rate conditions. High speed cine pictures and time resolved measurements of CH emissions were used to study the characteristics of the flame turbules. It was found that the velocity of flame turbules is determined by the annulus air jet injection velocity for almost all air and fuel flow conditions studied. The formation of flame turbules downstream of the recirculation zone is believed to occur because combustible fuel and air mixtures escape the recirculation zone in discrete packets separated by very lean regions of noncombustible gases.

NO2 Formation in Laminar Flames

Abstract: The equations of one-dimensional laminar flame propagation were numerically resolved to study the formation of NO2 in the premixed methane-air and hydrogen-air flames at atmospheric pressure. The results show the formation of NO2 peak in the flame region prior to the rapid formation of NO (say prompt NO) in the methane flame, whereas they do not show the formation of NO2 peak in the hydrogen flame in which NO is not formed rapidly. The formation of NO2 in the flame is controlled by the concentration and formation rate of prompt NO as well as the concentrations of HO2 and H, mainly through the reaction of NO + HO2→NO2 + H.NO2 is consumed mainly through the reaction of NO2 + H→NO +OH coincidently with the rapid appearance of NO.

An experimental study on the opening of laminar diffusion flame tips

Abstract: Using an enclosed cylindrical burner, the possible opening of laminar diffusion flame tips was experimentally investigated for various kinds of fuel/inert gas mixtures. The results show that for the ternary mixture of hydrogen, propane, and the inert gas(nitrogen, argon or carbon dioxide), in which the molecular weight of the inert gas is much greater than that of hydrogen, tip opening is indeed possible when the mixture is considerably diluted with the inert gas, whereas for the ternary mixture of hydrogen, propane, and helium, the flame temperature (the maximum temperature in the reaction zone) increases towards the tip of the flame, burning is intensified at the tip, and hence tip opening is not possible. In addition, it is found in hydrogen/carbon dioxide mixtures that tip opening occurs at a constant hydrogen concentration independent of the inner mixture velocity and the outer air velocity. This tip behavior of diffusion flames is in qualitative agreement with the diffusional stratificationmechanism and is also similar to that of premixed flames, suggesting the interesting possibility of the dominance of the diffusional stratification mechanisms in the opening of both premixed bunsen flame tips and laminar diffusion flame tips.

Evaluation of a Mixing-Controlled Model for Engine Combustion

Abstract: Comparisons were made of computed and measured flame fronts, flame tip propagation rates, and pressures for eight configurations of a divided-chamber, stratified-charge engine differing in speed, equivalence ratio, and throat area. In the model, conversion from reactants to products was assumed to be mixing limited so that the controlling processes were turbulence and wall effects. Turbulence was represented by a k — e model and wall effects by the “law of the wall” and Reynolds analogy. The results were also compared with those from earlier models that attempted to account for finite rate kinetics too, through one overall irreversible reaction. The flowfield is controlled mostly by the flame speed, except near the walls and in the throat area. All models, including the mixing-controlled one, reproduced the flame speed for the eight cases, but tended to overpredict the pressure, particularly for rich cases. However the mixing-controlled model was shown to be inherently inadequate to reproduce com...

Two-point rayleigh scattering measurements in a V-shaped turbulent flame

Abstract: A Rayleigh scattering technique for simultaneous measurement of density at two pointsin a flame was developed This technique was used to deduce the spatial and time-space correlation and the two-point joint probability density function of the density in a premixed turbulent flame in three orthogonal directions A grid generated turbulent V-shaped flame at a flow velocity of 70 m/s and a half angle of 12° was used The analysis shows that this flame consists of structures of equal length scale in all three directions which decay as they are convected downstream by the flow It is also shown that this flame does not consist of flamelets

Characteristic scale of wrinkles in turbulent premixed flames

Abstract: The wrinkles have been well known to appear in most turbulent premixed flames Todetermine the scale of these wrinkles, experimental study was carried out using an open turbulent flame burner The effect of upstream turbulence on the scale was also examined The characteristics of upstream turbulence were measured by a laser doppler velocimeter and a hot wire anemometer To obtain the different turbulence characterstics, several types of turbulence producing grid were used, and also the flow velocity was changed In a lean propane-air turbulent flame, time records of temperature signal were obtained by a fine-wire thermocouple and were analyzed to afford the time scales of unburned and burned gas lumps Based on these time scales, the time scale of wrinkles was defined and converted into the length scale by using the flow velocity It was found that the length scale of wrinkles is around 5 mm independently of the characteristics of the upstream turbulence and the flow velocity Approximate agreement was found with the value derived previously from the correlation of signals from two thermocouples placed apart from each other in a similar flame The wavelength of wrinkles was deduced from the length scale and compared with the results of study on the laminar flame instability It was found that the wavelength of wrinkles in the turbulent premixed flame is in good agreement with that of the spontaneous oscillation of the laminar flame front observed only for lean propane-air mixture These facts suggest that although the upstream turbulence triggers the corrugation on the laminar flame front, the resulting scale of wrinkles, unless additional hydrodynamic disturbances caused by the burned gas, is determined by the physicochemical properties of the unburned gas itself

Two-point Rayleigh scattering measurements in a v-shaped turbulent flame. Interim report

Abstract: A Rayleigh scattering technique for simultaneous measurements of density at two points in a flame was developed. This technique was used to deduce the spatial and time-space correlation and the two-point joint probability density functions of the density in a premixed turbulent flame in three orthogonal directions. A grid generated turbulent V-shaped flame at a flow velocity of 7.0 m/s and a half angle of 12 degree was used. The analysis show that this flame consists of structures of equal length scale in all three directions which decay as they are convected downstream by the flow. It is also shown that this flame does not consist of flamelets.

An experimental study on the dynamic behavior of premixed laminar flames

Abstract: A premixed laminar flame was excited by a speaker-driver unit, and the velocity fluctuations in a methane-air premixed flame and the flame front movement were measured with a laser Doppler velocimeter and a CH* emission photodetector, respectively, for various mean velocities, excitation frequencies and mixture strengths. It was found that the fluctuation which propagates along the reaction zone of the flame is an instability wave similar to that observed in an isothermal jet (Tollmien-Schlichting wave). A transfer function of the flame is derived based on a simplified model taking into account the growth of the fluctuation. It is shown that the frequency dependencies of the gain and the phase observed in the measurement of CH* emission and the pyro-acoustic amplification are explained satisfactorily by the above function.

Comparison of experiments and theory on heterogeneous flame suppressants

Abstract: Experimental results obtained for heterogeneous flame suppression were compared with theoretical results. Flame propagation velocities inhibited by Al2O3 and NaHCO3 powders were measured by the burner method, and the flame retardant phenomenon was investigated. The results show that: o (1) The experimental results for Al2O3 powders in H2−O2−N2 flames support the thermalinhibition theory. Flame speed inhibited by large particles varies along the upper branch solution of the triple-valued function, and the flame suddenly extinguishes at the nose point of the upper branch. (2) Although the flame retardant effect of NaHCO3 is several times as great as that of Al2O3, the effectiveness can be explained mostly by the thermal inhibition effect including the decomposition heat of NaHCO3. (3) Some behaviors in experimental results can not be clarified only by the thermal effect.Possibilities of the chemical inhibition effect are discussed, although it has not a dominant role in the present flame retardant experiment.

Effect of tube diameter on homogeneous gas flame propagation limits

Abstract: This study examines the effect of tube diameter on flame propagation limits under conditions where convective factors can be expected to manifest themselves. Gives special attention to boundary conditions and the direction of flame propagation. Discusses flame propagation in the direction of gravity and against gravity. Suggests that the observed convective peculiarities of the flame hearth in the early stages of combustion (deformation and floating up of the hearth) indicate the strong effect of free convection on the flame propagation limits. The results of the experiments also demonstrate the presence of ''tube'' effects, i.e., peculiarities related to the limitation of space by the tube walls. In contrast to free space, the flame velocity, flame from surface area, and heat liberation in tubes are dependent on diameter.

Toward the Formulation of a Global Local Equilibrium Kinetics Model for Laminar Hydrocarbon Flames

Abstract: A global chemistry model for hydrocarbon combustion is formulated and applied to the computation of methane and ethane-air laminar flames. The formulation of the model is based on a first order expansion of the reaction rates about local equilibrium. The motivation for this local equilibrium model is discussed by first analysing the performance of standard global models. It is concluded that H2 and CO need to be included in these models if the energy release is to be predicted accurately over the range of equivalence ratios of interest (0.5 ≤ φ ≤ 3). However, the inclusion of H2 and CO in standard irreversible reaction mechanisms is difficult. The reason is that the stoichiometric coefficients of a one-step mechanism (or the rates of the individual reactions in the equivalent multistep irreversible mechanism) have to be selected empirically or computed from the complete thermodynamic equilibrium solution, and this latter approach is computationally inefficient. A multi-step reversible reaction scheme could also be used, but for the reactions addressed in this study this requires that three Arrhenius-type forward rate expressions be specified. The proposed local equilibrium model does allow H2 and CO and their equilibria to be included and the method is computationally efficient.The method can be applied using only one Arrhenius term and it can also be extended to include multiple time scales for the modeling of multi-step reactions. The results indicate that laminar flame speed trends such as the dependence of flame speed on equivalence ratio can be modeled satisfactorily.

Effect of strain fields on burning rate

Abstract: Flame propagation can be highly influenced by the presence of strain fields. Initial stagesof flame growth in a turbulent medium, initiated by a point ignition, source such as a spark, may be dominated by straining effects if the flame thickness is less than the Kolmogorov scale. Straining may influence both the flame surface area and the local burning velocity. Area variation (usually referred to as flame stretching for an aear increase) is characterized by the so-called Karlovitz number. The effect of area variations has been investigated by a number of workers. However, little attention has been given to the effect of velocity gradients on the local burning velocity. An analytical study is presented for the influence of strain rate on the laminar burning velocity with heat loss from the reaction zone. The analytical results show that, in general, the laminar burning velocity decreases when the strain rate is increased, and that the decrease is more pronounced with increasing the heat loss and/or increasing the Lewis number. Experimental work is carried out for an axisymmetric stagnation point flow, in which a laminar flat flame is established. Different values of the strain rate and heat loss from the reaction zone are imposed on the flame, and their effect on the laminar burning velocity is found. The strain rate and the laminar burning velocity are determined by measuring the velocity field using laser doppler aneomometry. The work is of importance in operating spark ignition engines on lean mixtures and athigh compression ratios. Cycle-by-cycle variations in combustion increase as the mixture is made leaner and it is now established that this is primarily due to mixture motion around the spark plug during the initial stage of flame development. It is show that, in internal combustion engines, straining can cause variations in the initial stage of flame growth, and thus cause cycle-by-cycle varitions in the combustion process.

Lean Flame Propagation with Competing Chemical Reactions

Abstract: The steady propagation of a planar flame through a premixed combustible mixture is studied for the case in which the mixture contains two distinct fuels which compete for available oxidizer. It is assumed that sufficient oxidizer is present in the unburned mixture to allow for eventual complete burning of both fuels. Employing the method of matched asymptotic expansions to derive a solution for large activation energies, the propagation velocity U and the effective separation distance H between the points where the two chemical reactions go to completion are determined as functions of standard flame parameters. The analysis extends a recent study by Margolis and Matkowsky (1982a) to include the effects of stoichiometry on flame propagation with multiple fuels.

An Analytical Study of Diffusion Flames in Vortex Structures

Abstract: The interaction of a laminar diffusion flame with two- and three-dimensional vortex structures is considered, in which the flame becomes severely distorted and is strained in its own plane. Fast chemical kinetics and unity stoichiometry are assumed. The resulting curved flame sheets are treated by applying the boundary layer approximation locally until neighboring flame sheets come sufficiently close to interact and consume the intervening reactant, thus creating a core of combustion products with external isolated flame sheets. The simplest example is the deformation of a diffusion flame by a viscous vortex of circulation Γ. For large Γ the radius of the core of combustion products increases in proportion to Γ1/3D1/6t1/2, where D is the binary diffusivity, indicating the overall transport quantity to be Γ2/3D1/3. The augmentation of reactant consumption due to the presence of the vortex is time-independent and behaves as Γ2/3D1/3. The interaction of a laminar flame with a viscous vortex undergoing constant axial straining also is examined. The growth of the core radius has the similarity relation ϒ · ~ Γ1/3D1/6[(1-e-et)1/2]/e1/2 indicating that the core eventually reaches a steady state size. The core continues to store products and the outer flame arms continue to consume reactants independently of time, however, due to axial extension. Hence there exist two different time scales governing the development of the flame: one associated with the flame-vortex interaction and one associated with the external strain rate. The effect of the release of heat (and subsequent density change) by the reaction on flame structure is examined by considering the interaction of a diffusion flame with a vortex undergoing a density change at the core. The decreased core density shifts the entire flowfleld radially outward, causing the burned core to be increased in size, while the radius of the unburned core decreases as [ρ1/ρ2 + 1]-1/6, where ρ1 is the reactant density and ρ2 is the product density. The augmented consumption rate of the flame also is reduced, since the flame is being strained further from the viscous core and thus to a lesser extent.

Influence of chamber length and equivalence ratio on flame propagation in a constant volume duct

Abstract: An investigation of flames propagating in a constant volume duct of square cross-section (38 mm by 38 mm) is reported. Two borosilicate glass windows provide full optical access to the combustion process. A line ignition source creates an approximately two-dimensional flame front. The relations among pressure, flame shape, flame speed, and mass consumption rate are examined. The investigation of flame characteristics is made for 1) stoichiometric methane-air flame propagation in ducts of lenghts from 30 mm to 150 mm and 2) ethylene-air flames with equivalence ratios from 0.6 to 1.1 propagating in a 150 mm duct. Uncertainties in the experimental approach are investigated and traced to deviations in the flame from its assumed two-dimensional shape and to the sensitivity of the flame speed calculation during the flame initiation period to noise in the pressure data. Results point to distinct regions of flame development, labeled cylindrical, planar, and folded. A correlation between flame shape development and flame speed is noted. The time to peak pressure for the methane experiments is a linear function of duct length. This indicates that despite the distinct flame development regions, the mean mass consumption rate is not sensitive to flame shape variations. Ethylene flames are foundmore » to demonstrate the same general characteristics as the methane flames. The ethylene flame propagation rates are found to be about twice as fast as methane flame propagation rates. The folded ethylene flames exhibit a greater amount of turbulence than comparable methane flames. At low equivalence ratios buoyancy alters the ethylene flame shape. 20 figures.« less

Flammability characteristics and structure of a pulverized coal, laminar opposed jet diffusion flame

Abstract: A novel use of the opposed jet diffusion flame in an experimental study of laminar, pulverized coal flames is described. The opposed jet configuration has previously only been used to study gaseous diffusion flames stabilized either between two jets or off evaporating surfaces. In those cases it yielded information on fuel pyrolysis and so its extension to pulverized coal seems well motivated. It is shown, both theoretically and experimentally, that the p.f. laminar opposed jet flameis one dimensional in species concentration and temperature. Particle size segregation does occur but, provided Stokes drag is applicable, the one dimensionality holds for particle size and number density even when particles do not follow gas streamlines. In general, the opposed jet configuration allowed investigation of the effects of non-premixingof pulverized fuel and oxygen on flame structure and characteristics. Although fundamentally different, this flame possessed some surprising similarities to premixed pulverized coal-air flames. Because of its one dimensionality in temperature and species concentrations and because of apparent similarities in propagation mechanisms, the opposed jet flame can be viewed as the diffusion flame analog to more traditional premixed flame studies.