Showing papers on "Diffusion flame published in 1981"

Turbulence Production in Premixed Turbulent Flames

Abstract: —A second order closure theory developed earlier is used to study the processes influencing the turbulent velocity field in a premixed turbulent flame with degrees of heat release of practical interest. The flow field is chosen so that the time-averaged flame structure is one-dimensional and statistically stationary. Earlier work suggests that in the absence of turbulence production due to Reynolds stresses as is the case in a flame orthogonal to the oncoming reactants, the case we consider, dilatation resulting from heat release reduces the level of turbulence. In contrast it is shown here that with sufficient heat release turbulence increases on passage through the flame because of a buoyancy production mechanism arising from the self-induced, mean pressure gradient. This mechanism overwhelms the effects of dilatation at temperature ratios characteristic of combustion. The same buoyancy mechanism also causes counter-gradient diffusion as predicted in an earlier paper and as observed in recent e...

Soot oxidation in flames

Abstract: The kinetics and mechanisms of soot oxidation were studied at temperatures of 1575 to 1865 K and O2 mole fractions of 10−5 to 0.05 in a two-stage atmospheric-pressure combustion system in which soot-laden gas from a primary flame was partially cooled, mixed with oxygen-containing gas, and burned in a downstream premixed flame. The rate of oxidation of soot was measured using light scattering and absorption techniques to determine the particle size and concentration of soot as a function of distance, and hence of time, in the secondary flame. Complementary measurements of soot particle size were performed using probe sampling, electron microscopy, and electrical mobility analysis, and the specific surface area of the soot at different stages of burnout was measured by the BET technique. Temperature and gas composition profiles in the secondary flame were determined using coated thermocouples with electrical compensation for radiation loss, chromatographic analysis of sampled stable gases, and a spectroscopic technique combined with partial equilibrium calculations for radical species. It was found that OH radical is the principal oxidant of soot under these conditions, with O2 being of secondary importance. The average value of the collision efficiency of OH with soot is found to be 0.28 if the optical (equivalent sphere) diameter of the soot aggregates is used in the calculations, or about 0.13 if the diameter of the individual spherical units within the aggregates is used. The actual value of the collision efficiency is expected to be bounded by these two values. The results indicate that soot burnout rates predicted from the Nagle and Strickland-Constable formula, which assumes O2 to be the oxidant, are drastically underestimated under fuel-rich flame conditions owing to the neglect of the OH contribution. Breakup of soot aggregates during burnout was observed under fuel lean conditions.

Laser Rayleigh thermometry in turbulent flames

Abstract: Nonintrusive, spatially and temporally resolved temperature measurements in turbulent flames, premixed and non-premixed, using laser Rayleigh scattering are reported. Through judicious design of experiments, laser Rayleigh scattering can be used to measure temperature (or concentration) at a data rate, and hence frequency response, DC-15 kHz, that is undemonstrated by any other present day laser based technique. Two demonstrations of laser Rayleigh thermometry are described: a turbulent premixed flame, and a turbulent jet diffusion flame. The turbulent jet diffusion flame uses a novel mixture of hydrogen and methane as fuel. This fuel mixture permitted the first laser Rayleigh thermometry in turbulent non-premixed flames. In these demonstrations, the Rayleigh scattered light intensity from a CW laser is used to produce a time series of temperature. From that time series, the temperature probability distribution function is generated as well as the power spectrum and autocorrelation. From the probability distribution function, the mean, variance and higher moments are easily generated.

Flamelet models of turbulent non-premixed combustion

Abstract: A flamelet model for thermodynamic state in turbulent non-premixed flames is described in which the microscopic element in the turbulent ensemble has the structure of an undisturbed laminar diffusion flame. The profiles of scalar properties through this flame uniquely define instantaneous mixture state and hence provide an alternative thermochemical relationship to that of chemical equilibrium, commonly employed in flowfield calculations. The need for a flowfield model is avoided by using published measurements of mean temperature and RMS temperature fluctuation in a turbulent flame to determine the necessary mixture fraction statistics, so that the instantaneous laminar flamelet composition profiles can be appropriately averaged.Comparison is made between species prediction using the alternative thermochemical assumptions, and experiments as reported in the literature. Substantial improvements in prediction are achieved in the fuel-rich regions of an open turbulent flame, notably in the sensitiv...

Laser rayleigh thermometry in turbulent flames

Abstract: Non-intrusive, spatially and temporally resolved temperature measurements in turbulent flames, premixed and non-premixed, using laser Rayleigh scattering are reported. We demonstrate that through judicious design of experiments, laser Rayleigh scattering can be used to measure temperature (or concentration) at a data rate, and hence frequency response, DC-15 kHz, that is undemonstrated by any other present day laser based technique. Two demonstrations of laser Rayleigh thermometry are described: a turbulent premixed flame, and a turbulent jet diffusion flame. The turbulent jet diffusion flame uses a novel mixture of hydrogen and methane as fuel. This fuel mixture permitted the first laser Rayleigh thermometry in turbulent non-premixed flames. In these demonstrations, the Rayleigh scattered light intensity from a CW laser is used to produce a time series of temperature. From that time series, the temperature probability distribution function is generated as well as the power spectrum and autocorrelation. From the probability distribution function, the mean, variance and higher moments are easily generated.

An experimental study of effect of inert gases on extinction of laminar diffusion flames

Abstract: The limiting fuel concentration and the limiting oxygen concentration required to maintain the diffusion flame and the limit flame temperatures were measured using a counterflow diffusion flame established in the forward stagnation region of a porous cylinder. The fuels used were methane and hydrogen, and three kinds of inert gas (nitrogen, argon, and helium) were used as the diluent. The flame temperature at the limiting fuel concentration coincides with that at the limiting oxygen concentration, and therefore, the controlling factor with diffusion flames under limiting conditions is the limit flame temperature. The limit flame temperatures for methane and for hydrogen, diluted with nitrogen, are 1,200°C and 740°C, respectively. The limit flame temperature is considerably higher with helium than with nitrogen or argon. The limit flame temperature of a diffusion flame diluted with an inert gas is closely related to the flame temperature at the lean flammability limit of the premixed combustible gas of the fuel and “air”, including the same inert gas. The lean flammability limit of a premixed flame and the limiting concentrations of the reactants for a diffusion flame are primarily controlled by the same factor. In the special case of the hydrogen diffusion flame where the flame lies on the fuel side of the stagnation point, the nonuniform extinction of the flame caused by the preferential diffusion of hydrogen occurs before the flame temperature is reduced to the limit temperature.

Flame propagation in heterogeneous mixtures of fuel droplets, fuel vapor and air

Abstract: A model of flame deflagration is proposed for application to quiescent combustible mixtures in which the fuel is present in the form of a multidroplet mist, or vapor, or both. In order to test the validity of the model, measurements of flame speed are carried out for three different fuels over a range of values of drop size, vapor concentration, equivalence ratio and pressure. Inaccuracies due to buoyancy effects and the settling of fuel, drops are minimized by conducting the flame speed measurements in a vertical tube while it falls freely under the influence of gravity. The accuracy of the data is further improved by incorporating, various acoustic devices into the tube to produce a ‘flat’ flame, instead of the normal semi-ellipsoidal shape. The experimental results of burning velocity show satisfactory agreement with predicted values, thus confirming the basic premise of the model.

Molecular Transport Effects in Turbulent Diffusion Flames at Moderate Reynolds Number

Abstract: : In flames molecular diffusivities are enhanced by the high temperatures and can be of the same order as turbulent diffusivities in flames of moderate Reynolds number. A perturbation analysis is used to quantify effects in a hydrogen/air diffusion flame which arise from differential molecular diffusivities. The analysis uses perturbations about the equal diffusivity, adiabatic, equilibrium theory commonly used and yields solutions for the average and higher moments for the departures in normalized element mass fractions and enthalpy. The results are compared with the laser-Raman measurements of Drake et al. in a relatively low Reynolds number flame. Generally the agreement is excellent. (Author)

Blow-Out Stability of Gaseous Jet Diffusion Flames Part II: Effect of Cross Wind

Abstract: A non-dimensional stability curve that describes the blow-out stability of diffusion flames in a cross-wind, for different gases, has been established experimentally. For a given burner and a given gas, if the cross-wind speed is greater than a limiting value, a stable flame is not possible. For cross-wind speeds less than this limit, there are usually two blow-out limits which are on either side of the blow-out limit in still air.

A model for solid propellant combustion

Abstract: A model has been developed to describe the combustion characteristics of composite solid propellants. The model is based on several new concepts. First, the oxidizer and binder have different surface temperatures rather than a single averaged temperature that has previously been assumed. Second, the overall burn rate is calculated from a time averaged approach rather than the conventional space averaging used in most propellant combustion models. A key contribution in the time averaging approach is use of an ignition delay time for the oxidizer. Third, a generalized flame standoff distance has been developed based on a modified Burke-Schumann diffusion flame analysis. The analysis accounts for variable oxidizer/fuel (O/F) ratio that differing oxidizer size fractions can experience. The analysis shows that the primary diffusion flame can extend over either the oxidizer or the fuel depending on the local O/F ratio. Calculated results for a series of HMX composite propellants show several key trends and excellent agreement when compared to experimental data. The propellants vary in oxidizer particle size and concentration over a range of pressures and temperatures. The model indicates that the fuel binder has a more significant influence than previously thought.

Peak gas velocities and flame heights of buoyancy-controlled turbulent diffusion flames

Abstract: It is found that a parameter ξ, which is closely related to the local Froude number in a turbulent plume over an axisymmetric fire source, can be considered to be a universal constant throughout the nonreacting plume and, often, even into the intermittent flame region. This constancy implies that maximum gas velocities associated with buoyancy-controlled turbulent diffusion flames can be readily predicted; the consequences are examined in a simple model for correlating flame-height observations. The model leads to a correlating parameter for non-dimensional flame height which, in practice, differs from Steward's “combustion number” primarily by the appearance of the fraction of total heat release carried as convective heat into the plume.

Pulsations in a burner-stabilized premixed plane flame*

Abstract: We employ a model for an edge-cooled flat flame burner to obtain expressions for the flame speed, flame temperature, standoff distance as well as the quenching distance for a plane flame front.For a given standoff distance there is a low-temperature as well as a high-temperature solution. We show by a linear stability analysis of the plane front that the high-temperature solution is unstable when the Lewis number is sufficiently large and the inflow velocity sufficiently less than the adiabatic flame speed. We also show that this instability is the type that will lead to a bifurcating time-periodic solution describing a pulsating flame.

Soot formation in a laminar diffusion flame

Abstract: Light scattering and extinction measurements have been made to measure profiles of soot volume fraction, particle number density and average size in a laminar ethylene diffusion flame. Velocity profiles by laser Doppler anemometry and temperature profiles were also obtained in order to solve the species conservation equations and deduce the soot formation and particle generation rates. Soot volume fraction and particle diameter profiles peak some 5 mm on the fuel side of the peak temperature and increase with height. Number densities drop steeply from the reaction zone to the sooting zone and are relatively invariant with height. Soot particle trajectories show strong entrainment of particles from the reaction zone into the fuel side of the flame. The soot formation rates and particle generation rates peak in different regions. The picture that emerges is that particles are generated in the reaction zone and move into the fuel side of the flame where the main increase in soot volume takes place.

Laser Light Scattering and Fluorescence Diagnostics of Rich Flames Produced by Gaseous and Liquid Fuels

Abstract: Rich combustion systems present a much more complex chemistry than stoichiometric or lean ones: compounds from unsaturated C2 hydrocarbons to polynuclear aromatic hydrocarbons (PAH) are formed and destroyed, soot particles nucleate, grow and agglomerate and eventually burn out at different stages of the combustion. Furthermore, the fuel is introduced in the form of liquid droplets in many practical systems.

Laser spark ignition and extinction of a methane‐air diffusion flame

Abstract: Laser sparks have been used to ignite and extinguish a methane/air diffusion flame. Simple shock models of strong explosions and convective transport appear capable of accounting for the probabilities of ignition and extinction as a function of the position of the spark.

Gas‐phase combustion in fluidized beds

Abstract: Kerosine or propane was injected near the base of a small, air-fluidized bed of sand at 940°C. The fraction of fuel burnt within the bed was determined from a heat balance, for various particle sizes, fluidizing velocities, and bed depths. Assuming the initial formation of a train of fuel vapor bubbles, theoretical analysis indicated two stages of combustion: (1) rapid consumption of the oxygen initially between fuel vapor bubbles; (2) slow consumption of oxygen initially outside the fuel vapor region by radial diffusion of oxygen and fuel vapor, analogous to a diffusion flame. Comparison of experiment with theory gave effective radial diffusion coefficients of the same order of magnitude as the molecular diffusion coefficient. It was inferred that combustion occurs largely by a diffusion flame within the bed, with diffusion through the particulate phase being the rate-controlling step. This explains why fuel distribution is so important in attaining efficient combustion.

A study of plasma jet ignition mechanisms

Abstract: Having demonstrated the effectiveness of high-velocity jets of chemically active plasmas for igniting and promoting fast flame propagation in lean fuel-air mixtures, the contributory mechanisms are studied with the object of designing improved ignition plugs. Incendivity and flame spread are measured in apparatus provided with time-resolved Schlieren, C2 emission and OH absorption measuring facilities. Theoretical and experimental studies indicate major contributions from chemical and fluid-mechanical effects. The former are prominent when the plasma medium is rich in H, and appear absent with Ar plasmas. When dominant, they enable sub-limit mixtures to be ignited and burnt, lead to rapid transition from an expanding plasma plume to a propagating flame and give rise to considerably increased burning velocities, persisting for appreciable times, during which the flame structure appears to be modified. The fluid mechanics determines the structure and turbulence of the jet and hence the initial location and surface area of the flame kernel. The increased flame area leads to increased flame speeds and conversion rates, even in the absence of chemically modified burning velocities. When propagating into quiescent reactants, the flame convolutions tend gradually to laminarise at a rate which increases with laminar burning velocity. The magnitude and persistence of fluid-mechanical effects thus depends on the chemical effectiveness of the plasma medium through both the formation and the rate of propagation of the flame. The two effects are complementary in that flame convolutions are most effective in chemically inactive plasmas. Either effect can be fostered by plug design. Liquid hydrocarbon fuelled plugs combine a plentiful supply of radicals with incompressibility of feedstock and suitability for practical applications.

Soot Formation — An Overview

Abstract: Formation of soot can take place in all kinds of practical combustion systems, especially in systems which operate on a diffusion flame concept. The influence of macroscopic system parameters on the amount of soot formed is rather well investigated for different systems. Even though these results often look unrelated to each other, the primary soot particles formed in various combustion processes are rather similar. Their mean diameter is usually a few hundred Angstroms. These particles often stick together and form long branched or straight chain aggregates and their number densities are very similar even under widely varied conditions, a consequence of their formation mechanism.

Measurement of scalar-velocity correlations in a turbulent diffusion flame

Abstract: Simultaneous measurements of scattered light intensity and axial velocity have been made in a turbulent hydrogen diffusion flame in a co-flowing stream with accelerating and decelerating pressure gradients. The light-scatter signal obtained from the seeded nozzle fluid is used as a measure of hydrogen element concentration, and is processed to derive records of density and mixture fraction which are correlated in digitized form with the simultaneously recorded streamwise velocity component. Results reported here include the mean and r.m.s. fluctuation of the streamwise velocity component and the density and correlations of the streamwise velocity component with the mixture fraction and the density as well as other correlations and Favre or density weighted averages. It is shown that the density velocity correlation is only large enough to make a few percent difference between the Favre and conventional mean velocities but that such correlations are important in the determination of the excess momentum flux. Correlations between velocity and mixture fraction are comparable with those found in isothermal flow but change sign where the excess velocity changes sign. The correlation between density and velocity shows similar behaviour and is shown to have a significant effect on the turbulent kinetic energy balance.

NOx formation in flat, laminar, opposed jet methane diffusion flames

Abstract: It is shown that the detailed structure of a flat laminar opposed jet diffusion flame can be modeled, by properly coupling the momentum and energy conservation equatins and by using detailed finite rate combustion kinetics Such a flame is one dimensional (for a given stretching rate) provided that the proper velocity boundary conditions, which are a result of the model are employed A laboratory CH 4 /N 2 /O 2 opposed jet diffusion flame was realized and gave good agreement with predictions with respect to both, one dimensinality and the temperature and species profiles within the reaction zone However, the actual location of the reaction zone was displaced by a small distance The theoretical/experimental tool developed was utilized to test kinetic mechanisms of NO formation from fuel nitrogen In this case, anhydrous ammonia was introduced first with the fuel and then with the oxidizer The formation of NO was well predicted, using a detailed kinetic mechanism developed by others, when ammonia was introduced with the fuel, but agreement was poor when ammonia was injected in the air side This indicates that there are some deficiencies in the ammonia pyrolysis kinetic mechanism when it is utilized in the absence of hydrocarbon fragments The qualitative dependence of the NO profile on flame stretching was correctly predicted by the model This also allowed the prediction of reaction zone thickness and the rate of formation of NO as a function of stretching rate and these results can be related to NO formation in strained laminar diffusion flamelets as they occur in turbulent diffusion flames

Effects of Flame Temperature and Air-Fuel Mixing on Emission of Particulate Carbon from a Divided-Chamber Diesel Engine

Abstract: The effect of flame temperature on particulate carbon emission from divided-chamber diesel engines was examined by adding various quantities of 02 and N2 to the intake air with the engines operating at several different loads and speeds. At a given operating condition for a fixed combustion chamber geometry, intake gas addition was expected to influence chemical kinetics without affecting the air-fuel mixing. Particulate carbon and CO were found to increase with N2 addition and decrease with 02 addition, whereas NOx emissions exhibited opposite trends.

Analysis of the Flat Laminar Opposed Jet Diffusion Flame with Finite Rate Detailed Chemical Kinetics

Abstract: Use of a similarity transform, followed by numerical integration, allowed the conservation equations describing a flat laminar opposed-jet, moist CO, diffusion flame to be solved. Predictions utilizing finite-rate, elementary combustion kinetics give excellent agreement with experiment. The flame is one-dimensional in concentrations and temperature and comprises a useful and unique tool for investigation and verification of kinetic mechanisms of pollutant formation.

On the Stability of Nonadiabatic Flames

Abstract: A linear stability analysis is carried out for the cellular instability of a nonadiabatic downward-propagating premixed flame. It is shown that if the molecular weight of the deficient reactant is sufficiently small, then an increase in heat loss may lead to destabilization of an adiabatically stable flame (i.e., a flame which is stable in the absence of heat losses).

Stability limits of natural gas diffusion flames with swirl

Abstract: An experimental study was made of flame stabilization in unconfined turbulent swirling natural gas flames using various degrees of swirl. Results of the experiments are reported in two parts: o (a) The evaluation of blow-off data shows that the stability limits of swirling flames can be successfully correlated by means of dimensionless Peclet numbers. It is also possible to indicate a relationship between maximum fuel gas throughput and swirl level, and to present the blow-off curves in a general form by means of a similarity plot. The equations developed are sufficient to calculate the stability limits for different degrees of swirl for all nozzle geometries investigated. (b) Measurements of temperature, concentration, velocity and flow direction under flame and no-flame conditions near the stability limits may explain the mechanism of swirling flame stabilization. The main conclusion is that flame stability depends on the location of the reaction zone within the flow field near to the burner exit. Reasons for flame extinction are: u - radial shift of the flame front in regions of excessive local fluid velocities. - lifting of the flames by exceeding the maximum possible fuel concentration within the stabilization region.

The effect of dc to 10 MHz electric field on flame luminosity and carbon formation

Abstract: The effect of electric fields, dc and ac at frequencies up to 10 MHz and the maximum strength of about 3 kV/cm, on the luminosity and soot emission of flames was investigated by using a counter-flow diffusion burner and a nozzle burner. The luminosity, with both diffusion and premixed flames of acetylene and air, was found to be a complicated function of the frequency, electric field strength and gas flow rate. Under certain conditions, the luminosity could be either increased or decreased by up to a factor of about two. There was no effect on the soot emission from the counter-flow diffusion flame at frequencies above 500 kHz. The observations can be successfully explained by adopting a simple model for the growth of carbon particles.

Modeling of Reaction Processes in Turbulent Flames with Special Emphasis on Soot Formation and Combustion

Abstract: The present paper reviews features of the eddy-dissipation concept developed by the author for treating chemical reactions in turbulent flow.

Smoke Filling in an Enclosure

Abstract: The filling of an enclosure by smoke generated by a diffusion flame was studied by measuring the smoke extinction coeffcient and temperature versus height during the filling process. Semi-quantitative agreement is found between the Bains-Turner theory and experiments with the burner located in the center of the room for heat fluxes over the range 11 to 32 KW. The filling process was found to be about twice as fast when the burner was moved off center. A novel technique for generating a thin smoke layer is described. Approximately 80% of the total heat generated by combustion was lost to the ceiling and walls. It was found that the conservation ratio of fuel to particulate is a non-linear function of fuel flow rate and that the shape of the curve is sigmoidal, which appears to be a common characteristics of sooting system.