Showing papers on "Diffusion flame published in 1977"

The coherent flame model for turbulent chemical reactions. Final report 1 Mar 75--31 Jan 77

Abstract: A description of the turbulent diffusion flame is proposed in which the flame structure is composed of a distribution of laminar diffusion flame elements, whose thickness is small in comparison with the large eddies. These elements retain their identity during the flame development; they are strained in their own plane by the gas motion, a process that not only extends their surface area, but also establishes the rate at which a flame element consumes the reactants. Where this flame stretching process has produced a high flame surface density, the flame area per unit volume, adjacent flame elements may consume the intervening reactant, thereby annihilating both flame segments. This is the flame shortening mechanism which, in balance with the flame stretching process, establishes the local level of the flame density. The consumption rate of reactant is then given simply by the product of the local flame density and the reactang consumption rate per unit area of flame surface. The proposed description permits a rather complete separation of the turbulent flow structure, on one hand, and the flame structure, on the other, and in this manner permits the treatment of reactions with complex chemistry with a minimum of added labor. The structuremore » of the strained laminar diffusion flame may be determined by analysis, numerical computation, and by experiment without significant change to the model.« less

The Coherent Flame Model for Turbulent Chemical Reactions

Abstract: A description of the turbulent diffusion flame is proposed in which the flame structure is composed of a distribution of laminar diffusion flame elements, whose thickness is small in comparison with the large eddies. These elements retain their identity during the flame development; they are strained in their own plane by the gas motion, a process that not only extends their surface area, but also establishes the rate at which a flame element consumes the reactants. Where this flame stretching process has produced a high flame surface density, the flame area per unit volume, adjacent flame elements may consume the intervening reactant, thereby annihilating both flame segments. This is the flame shortening mechanism which, in balance with the flame stretching process, establishes the local level of the flame density. The consumption rate of reactant is then given simply by the product of the local flame density and the reactang consumption rate per unit area of flame surface. The proposed description permits a rather complete separation of the turbulent flow structure, on one hand, and the flame structure, on the other, and in this manner permits the treatment of reactions with complex chemistry with a minimum of added labor. The structure of the strained laminar diffusion flame may be determined by analysis, numerical computation, and by experiment without significant change to the model.

Development of the eddy-break-up model of turbulent combustion

Abstract: A new expression is presented for the time-average reaction rate in a turbulent flame, whether of uniform or non-uniform fuel-air ratio. It is based on the idea of coherent gas “parcels,” which are subjected to a stretching process while reaction and small-scale mixing take place. The expression has been used for the prediction of flame spread behind a baffle, and for the turbulent diffusion flame. An outline is given of a more complete theory, still under development.

Hybrid combustor with staged injection of pre-mixed fuel

Abstract: A combustor for a gas turbine engine which includes a fuel nozzle at the head end of the combustor, to provide a diffusion flame, and downstream inlet means at a plurality of axial dimensions of the combustor to inject pre-mixed lean fuel/air into the combustor for admission downstream from the diffusion flame resulting in a series of low temperature premixed flames to provide relatively high turbine inlet temperatures from the combustor with a minimum of thermally formed NOx compounds.

Propagation Velocity and Structure of Flames in Droplet-Vapor-Air Mixtures

Abstract: Abstract-Flame speeds of ethanol and n-octane mixtures of mono-sized droplets, vapor and air have been measured to study the fundamental aspects of spray combustion. The state of the mixtures, which were prepared in an apparatus similar to the Wilson cloud chamber, was experimentally verified in detail. A rugged, undulated and thickened flame front with the cellular structure is peculiar to the flame propagation in the mixtures containing large droplets and is in sharp contrast to the smooth and continuous flame front observed with premixed gas mixtures or mixtures containing droplets of much smaller size. This difference in the flame structure, caused by the heterogeneous nature of the unburned mixture, can explain the observation that the burning velocity in a droplet-vapor-air mixture is larger than in a homogeneous mixture of the same overall fuel-air ratio, even on the lean side of the stoichiometric point, provided that the droplet diameter is large enough. Another aspect of the flame propa...

Fine Wire Thermocouple Measurements of Fluctuating Temperature

Abstract: A technique for the measurement of the statistics of the temperature fluctuation, notably probability density function and power spectrum, in turbulent diffusion flames which employs fine wire thermocouples, compensated for the effects of thermal inertia, is assessed. Measurements made in an open turbulent methane diffusion flame are reported and compared, from the standpoint of identifying the interaction between measurement technique and turbulent flame structure, with simple model predictions. The flame-intermittent zone is seen to be characterised by large fluctuations in temperature and velocity which lead to significant local variations in heat transfer to the thermocouple and hence in its time constant. Attention is therefore focussed on an assessment of the impact on measured temperature statistics of both the level of compensation and the manner it is effected. Techniques involving electrical networks and computational procedures are discussed.

High pressure flame system for pollution studies with results for methane-air diffusion flames

Abstract: A high pressure flame system was designed and constructed for studying nitrogen oxide formation in fuel air combustion. Its advantages and limitations were demonstrated by tests with a confined laminar methane air diffusion flame over the pressure range from 1 to 50 atm. The methane issued from a 3.06 mm diameter port concentrically into a stream of air contained within a 20.5 mm diameter chimney. As the combustion pressure is increased, the flame changes in shape from wide and convex to slender and concave, and there is a marked increase in the amount of luminous carbon. The height of the flame changes only moderately with pressure.

Further experiments on the structure of a spray combustion flame

Abstract: In previous studies, the authors conducted experiments on spray combustion flames of axial jets of kerosene under comparatively low turbulence conditions. The results showed that droplets within the flames do not burn individually with envelope flames, but that a cloud of fuel vapor generated by them burns as a turbulent gas diffusion flame, and that the spray combustion flame is very similar to the turbulent gas diffusion flame in structure. The present study is intended to examine the extent to which the above conclusions are valid, that is: (1) whether the similarity between the spray combustion flame and the turbulent gas diffusion flame also exists in nitric oxide formation; (2) whether the above conclusions can be applied to the flame of a low volatile heavy oil. In the radial distribution within the kerosene flame, the profile of NO concentration showed two peaks symmetrical with respect to the flame axis. These peaks coincide approximately with the peaks of the temperature profile, and with the points where the local equivalence ratio is unity. The above tendency is the same as in the turbulent gas diffusion flame and confirms that the spray combustion flame is also similar to the turbulent gas diffusion flame in the NO formation process. Next, a heavy oil flame was experimentally compared with a kerosene flame under the same condition. The result showed that the shape and the measured profiles of various quantities were almost the same for both flames. This shows that the heavy oil flame does not differ significantly in structure from the kerosene flame under the conditions of this experiment.

Structure analysis of righ fuel-air flames in the forward stagnation region of a porous cylinder

Abstract: An experimental study was made of the structure of rich fuel-air flames established in the forward stagnation region of a porous cylinder, from the surface of which premixed methane and air were ejected into a uniform air stream (the secondary air). Aerodynamic, temperature, and stable species concentration profiles were measured for flames at atmospheric pressure. These distributions were analyzed to yield the reaction-rate profiles of stable species throughout the flame zone. The stabilized flame is really a double flame. For the equivalence ratioof the ejected mixture >2.7, the stagnation point lies between the inner flame and the cylinder surface, and net reaction-rate profiles of various species are similar to those of a pure diffusion flame. Therefore, for >2.7, the flame can be treated essentially as a diffusion flame. For mixtures inside the rich limit of flammability ( For Besides the rich limit of flammability usually defined, there is another limit of flame propagation. In the mixture beyond this limit a flame will never propagate. This limit can be regarded as fundamental in the sense that it depends only on the internal properties of the mixture. The equivalence ratio of this limit is 2.7 for methane-air mixture.

A Model of Erosive Burning of Composite Propellants

Abstract: : Development of solid rocket motor designs which result in high velocity flows of product gases across burning propellant surfaces(notably, nozzleless rocket motors) is leading to increased occurrence of erosive burning. In this paper, a physically realistic picture of the effect of such crossflow on composite propellant combustion, based on the bending of columnar diffusion flames by the crossflow, is presented. This bending results in shifting of the diffusion flame heat release zone toward the surface, with consequent increased heat feedback flux from this flame to the surface and thus increased burning rate. A relatively simple analytical model based on this picture is developed for prediction of propellant burnings rate as a function of pressure and crossflow velocity, given only zero-crossflow burning rate versus pressure data. Model predictions and experimental results are compared, with reasonably good agreement being found. (Author)

Characteristics of laminar gas jet diffusion flames under the influence of elevated gravity

Abstract: Laminar gas jet diffusion flames of hydrogen, methane, ethane, and propane, stabilized on small cylindrical burners, were studied at elevated gravity in a centrifuge to investigate the role of buoyancy in such flames. Photographic observation of the hydrocarbon flames indicated that length and carbon luminosity decreased with increasing buoyancy. These flames eventually separated from the burner rim and finally extinguished as gravity increased. Hydrogen flames, however, did not separate or extinguish up to about 31 times earth normal gravity. Length data are reported and interpreted with the aid of a simplified model which describes the flame as a heated vertical cylindrical sheet to which oxygen is supplied by a free convection boundary layer.

Unsteady burning of unconfined fuel vapor clouds

Abstract: The rapid accidental escape of a combustible vapor into open surroundings can produce a cloud of gas which can burn as an unsteady, turbulent diffusion flame if ignited at its edge. In such a flame, buoyancy forces induce a mixing of the fuel with air, promoting combustion which ultimately consumes the initial fuel charge. This paper considers three principal characteristics of such a combustion process: the flame height, the time to complete combustion, and the maximum diameter of the flame. A simple entrainment model is proposed which leads to the determination of these flame characteristics, to within two unknown parameters to be determined from experiments. The principal consequence of the model is the prediction that the rise height and the maximum diameter of the flame vary as the 1/3 power of the initial volume of vapor cloud, while the combustion time varies as the 1/6 power. Experimental observations of the burning of small (laboratory scale) vapor samples are described. The experimental observations were lognormally distributed about a mean line whose slope conformed with the scaling laws derived from the theoretical model. From the mean line, the unknown parameters were determined. Within the limits of the laboratory experiments, the principal flame characteristics are nearly independent of the initial conditions of the vapor cloud and also of the fuel type, and are adequately correlated by the equations given by the theoretical model.

Flame Propagation in Closed Vessels and Flammability Limits

Abstract: Abstract-Flame propagation in near-limit mixtures of ethylene and air has been investigated by using a spherical vessel. A model of the flame behavior in closed vessels has been formulated, which takes into account the complete motion equation and the heat losses by the flame to the surroundings. The model forecasts two possible mechanisms of flame failure: the "homogeneous" flame extinction due to the exchanges of the rising flame towards the unburned gases, and the "heterogeneous" extinction due to conductive heat losses to the walls after flame impingement on the vessel top. The theory and the experimental data fairly agree; in particular the dependence of the measured "heterogeneous" limit on the igniter position is explained.

A Theoretical and Experimental Study of Radiation–Convection Interaction in a Diffusion Flame

Abstract: An experimental and theoretical investigation of the interaction of gaseous thermal radiation with natural convection was made for a laminar methane-air diffusion flame in the lower stagnation region of a horizontal porous cylinder. The exponential wide-band gas radiation model was employed in this nonhomogeneous (nonuniform in temperature and composition) problem through the use of scaling techniques. Using a numerical scheme, the compressible energy, flow, and species-diffusion equations were solved simultaneously with and without the radiative component. In the experiment, methane was blown uniformly from the surface of the porous cylinder, setting up (upon ignition) a diffusion flame within the free-convection boundary layer. Using a Mach-Zehnder interferometer and a gas chromatograph, temperature and composition measurements were obtained along the stangation line. Excellent agreement was found between the results based the the nongray wide-band model and the experimental data. Furthermore, it was found that the wide-band model yielded results that were superior to those results that excluded radiation-interaction effects. Thus, this study demonstrates that the exponential wide-band model can be accurately applied to nonhomogeneous combustion situations in order to account for the radiation-convection interactions. 10 figures, 4 tables, 31 references.

Characteristic time emissions correlations: the T-63 helicopter gas turbine combustor

Abstract: Gaseous pollutant emissions from conventional diffusion flame T-63 combustors are correlated as functions of combustor geometry and inlet conditions in terms of a characteristic time model. New aspects of the present study show partial inclusion of effects of alternate fuel properties, suggest a generalization of the Lipfert NO/sub x/ correlation, compare advanced film injected and prevaporizing/premixing with conventional combustors, and demonstrate a quantitative relation between emissions of unburned hydrocarbons and CO. 9 figures, 4 tables.

Some Studies on Hydrogen-Oxygen Diffusion Flame

Abstract: The opposed-jet diffusion flame has been considered with four step reaction kinetics for hydrogenoxygen system. The studies have revealed that the flame broadening reduces and maximum temperature increases as pressure increases. The relative importance of different reaction steps have been brought out in different regions (unstable, near extinction and equilibrium). The present studies have also led to the deduction of the oveall reaction rate constants of an equivalent single step reaction using matching of a certain overall set of parameters for four step reaction scheme and equivalent single step reaction.

Effects of Diluents on the Flame Structure and Radiation of Propane Jet Flames in a Concentric Stream

Abstract: Abstract-Data are presented on the structure of propane diffusion flames burning in a concentric stream of air, with and without the addition of diluents (carbon dioxide and nitrogen) to the fuel jet and air stream. The flame length and the fraction of heat release radiated are documented as functions of diluent concentrations in fuel jet and air stream. The effects of diluent addition on the temperature profiles, composition profiles showing species CO2, H2O, CO, N2, O2, C3H8,NO and pyrolysis species of C3H8 and axial profiles of radiant power emitted are also presented. The results show that the diluents reduce flame length, stability, radiation and concentrations of nitric oxide in the flames. The effects of diluents on the flame structure can be attributed primarily to the changes in concentrations of various species rather than to the changes in temperature. The problems of using flue gas recirculation in large combustion systems are indicated.

Supersonic Combustion of Hydrogen in a Vitiated Airstream Using Transverse Injection

Abstract: Theme I N this paper, an investigation of the two-dimensional supersonic jet interaction flowfield with and without chemical heat release is presented, and the spontaneous ignition limits of the bulk flame produced far downstream are determined. Highly underexpanded hydrogen and nitrogen jets are injected from a converging slot nozzle perpendicular to the supersonic vitiated air stream, at various airstream and injectant conditions. The parameters varied during the experiments include the airstream stagnation temperature and oxygen concentration, and the injection pressure. The temperature profiles and the Schlieren photographs of the resulting flowfield with chemical heat release are compared with those of nonreacting case. From these results, the ignition mechanism of supersonic diffusion flame and the extent to which chemical heat release changes the jet interaction flowfield are discussed.

The hydrocarbon turbulent diffusion flame in subsonic cross-flow

Abstract: This paper describes a simple model of a hydrocarbon turbulent diffusion flame in a horizontal subsonic cross flow of air. The flame is modeled as a bent-over initially vertical circular jet with top-hat profiles of composition, temperature, and velocity. The hydrocarbon pyrolyzes in a zero-order reaction and the pyrolysis products are oxidized at a rate proportional to the rate of entrainment of air into the flame. Entrainment is modeled by the two-component law developed in connection with buoyant chimney plumes. Three parameters (the pyrolysis time T, the proportionality factor between the rate of entrainment and the rate of oxidation g, and the entrainment constant a) are evaluated by comparison with data obtained in wind-tunnel experiments. The closed-form solution of the limiting case of the governing equation provides useful information on the shape of the flame near the nozzle, on the physical significance of g, and on the effects of scaling on the relative influences of initial momentum and buoyancy. The numerical solutions of the full governing equations show that the model reproduces some of the principal trends in the variation of flame shape and length with aerodynamic parameters.