Showing papers on "Diffusion flame published in 1985"

On the determination of laminar flame speeds from stretched flames

Abstract: The effects of stretch on the determination of the laminar flame speed are experimentally studied by using the positively-stretched stagnation flame and negatively-stretched bunsen flame, and by using lean and rich mixtures of methane, propane, butane, and hydrogen with air whose effective Lewis numbers are either greater or less than unity. Results demonstrate that flame speed determination can be influenced by stretch through two factors: (1) Preferential diffusion which tends to increase or decrease the flame temperature and burning rate depending on the effective Lewis number, and (2) Flow divergence which causes the flame speed to assume higher values when evaluated at the upstream boundary of the preheat zone instead of the reaction zone. Recent data on flame speed including the present ones are then examined from the unified viewpoint of flame stretch, leading to satisfactory resolution of the discrepancies between them. The present study also proposes a methodology of determining the laminar flame speeds by using the stagnation flame and linearly extrapolating the data to zero stretch rate.

Visible structure of buoyant diffusion flames

Abstract: Natural gas diffusion flames stabilized on 0.10, 0.19 and 0.50 m. diameter porous bed burners have been studied for heat release rates ranging from 10 to 200 kW. Flame heights were measured from video tape recordings and by eye averaged techniques. The dependence of flame height on a dimensionless heat addition parameter shows a transition for values of the parameter around unity. For flames taller than three burner diameters, the initial diameter of the fire does not affect the length of the flame whereas for short flames, initial geometry becomes important. Another prominent feature of these flames is the presence of large scale axisymmetric structures which are formed close to the burner surface with more or less regular frequency and which rise through the flame region. These structures are responsible for the fluctuations of the flame top and strongly influence the geometry of the flame.

Growth of a Diffusion Flame in the Field of a Vortex

Abstract: A simple diffusion flame with fast chemical kinetics is initiated along the horizontal axis between a fuel occupying the upper half-plane and an oxidizer below. Simultaneously a vortex of circulation Γ is established at the origin. As time progresses the flame is extended and “wound up” by the vortex flow field and the viscous core of the vortex spreads, converting the motion in the core to a solid-body rotation.

Calculation of the structure and extinction limit of a methane-air counterflow diffusion flame in the forward stagnation region of a porous cylinder

Abstract: The paper addresses itself to the numerical representation of the structure of counterflow methane-air diffusion flames, with the use of complex chemistry and detailed formulation of the transport fluxes. A similarity solution is briefly outlined which allows the problem to be treated as one-dimensional in space. Following this, discretization and solution of the resulting algebraic equations has been achieved by five different methods. Although there are some differences between the results of the separate computations, these results agree within the probable experimental error with the observations of Tsuji and Yamaoka as regards the temperature and species mole fraction profiles in a particular flame. The more detailed chemical models are able to predict the profiles of C 2 hydrocarbons with reasonable precision; but even somewhat less detailed models are able satisfactorily to predict the major structural features. Despite this agreement, the system overall does not behave as a straightforward boundary layer flow, and in order fully to match the solution to the experimental results it was necessary to modify the measured velocity gradient of 100 s −1 for the cold flow to near 130 s −1 in the flame region. The discrepancy is probably caused by modification of the pressure field due to apparatus effects or to the flame itself, and a full solution requires a two-dimensional treatment to include this effect. As the velocity gradient which characterises these flames is increased, the increased strain in the flame reaction zone (or flame stretch) may lead to extinction. The effect of increasing strain on the flame structure and position near the porous cylinder is examined. There are differences between the two available predictions of the quenching velocity gradient, but both predictions are consistent with the measured, nominal gradient. It is shown that thermal quenching effects at the cylinder do not contribute appreciably to the observed limits. The significance of the extinction limits in the context of non-premixed turbulent combustion is briefly discussed.

Experimental investigation on the stabilization mechanism of jet diffusion flames

Abstract: Two contrary concepts have been suggested in order to explain the mechanism of flame stabilization based on premixed and diffusion flamelet combustion respectively. To contribute to the understanding of which model represents the true stabilization mechanism, two different natural gas jet diffusion flames, at exit velocities between flame detachment and blow-off, were investigated. The measured profiles of gas composition and velocity around the stabilization zone were used to derive the rate of mixing and fuel burnout. The results show that, for the flames investigated, about forty to fifty percent of the total fuel flow is already mixed at a molecular level upstream of the flame stabilization zone. This mixture then reacts over a very short distance, supporting the concept of premixed combustion in lifted jet diffusion flames.

Soot formation in diffusion flames: Flow rate, fuel species and temperature effects

Abstract: A detailed study of soot particle formation in diffusion flames has been made for a series of laminar diffusion flames using a laser scattering/extinction technique. The effects of flow rate, fuel species and temperature on the evolution of the soot particle field have been investigated. Temperature effects have been examined through nitrogen dilution of the fuel. Fluorescence measurements from gas phase species in the initial particle formation region have been used to examine the relationship between these potential soot precursors and the observed particle formation. Flow rate, fuel species and temperature variations have been observed to significantly effect the annular region of the flame, where particles first appear. Increasing the fuel flow rate results in a larger soot volume fraction while the maximum particle size observed in the flame remains nearly constant. Thus, particle number concentration appears to be the quantity most sensitive to the flow rate variation. Temperature measurements made in the flames indicate that fuel flow rate increases result in higher temperatures in the particle formation region, leading to increased soot formation. At later stages, the flames with larger soot concentrations exhibit lower temperatures which facilitates soot particle survival in the oxidation region of the flame. Measurements made in flames with different fuels but similar adiabatic flame temperatures, indicate that the temperature exerts the strongest influence on soot formation. However, distinct effects due to fuel structure still need to be considered.

Effects of preferential diffusion on the burning intensity of curved flames

Abstract: Using the Bunsen flame as a model curved flame, the coupled influence of preferential diffusion and flame stretch on the burning intensity of lean and rich mixtures of methane, propane, butane, ethylene, and hydrogen with air has been experimentally studied. The results substantiate theoretical predictions and quantify previous experimental observations that, for mixtures whose effective Lewis numbers (Le) are less than unity, the flame temperature is less than the adiabatic flame temperature. This temperature also decreases towards the flame tip, which has the largest curvature and therefore may locally extinguish. The dominance of diffusional transport in influencing the intensity of curved flames is demonstrated by showing that the tip opening of the highly diffusive hydrogen/air flames occurs at constant hydrogen equivalence ratios of about 1.15 to 1.20, being almost independent of the flow intensity and uniformity.

On the behavior of premixed flames in a rotating flow field: Establishment of tubular flames

Abstract: The behvior of premixed flames in a rotating flow field has been investigated using a swirl type burner, in which a fuel/air mixture is ejected tangentially into a cylindrical combustion chamber. The results show that for appropriate conditions for the ejection velocity and the fuel concentration of the mixture, a tubular flame of circular cross-section is established inside the burner. By this flame front, the combustion field is separated into two regions, an outer unburned gas region and an inner burned gas region, and the burned gas flows inside the flame with rotation. This rotating hot gas core is very stable according to the Rayleigh stability criterion. However, further experiments show that this type of tubular flame can be established for lean methane/air or hydrogen/air mixtures, but not for lean propane/air mixtures. In the former case, the diameter of the flame decreases with a decrease of the fuel concentration, and the flame is eventually extinguished after the tubular flame is merged into a luminous rod, whereas in the latter case, an unsteady, corrugated flame is established around the wall of the cylindrical burner, and the flame is extinguished without forming a uniform flame front. The fuel concentration at the extinction limits is close to the lean flemmability limit in the former case, whereas very far from that in the latter case. These two distinct behaviors of flames in the rotating flow field have been discussed on the basis of the tangential velocity distribution of the Rankine's combined vortex and the flame stretch theory considering the Lewis number effect.

An experimental investigation on flame interaction and the existence of negative flame speeds

Abstract: Downstream interaction between two counterflow premixed flames of different stoichiometries are investigated. Various flame configurations are observed and quantified; these include the binary system of two lean or rich flames, the triplet system of a lean and a rich flame separated by a diffusion flame, and single diffusion flames with some degree of premixedness. Extinction limits are determined for methane/air and butane/air mixtures over the entire range of mixture concentrations. Results show that these extinction limits can be significantly modified in the presence of interaction such that a mixture much beyond the flammability limit can still burn if it is supported by a stronger flame. The experiment also demonstrates the existence of negative flames whose propagation velocity is in the same general direction as that of the bulk convective flow. Implications of the present results on the flammability of stratified mixtures and on the modeling of turbulent flames are discussed.

Partially premixed diffusion flamelets in non-premixed turbulent combustion

Abstract: The structure and extinction of partially premixed diffusion flamelets in a turbulent flow field is analyzed. It is shown that diffusion flamelets are disconnected if the variation of the mixture fraction around stoichiometric is large enough for the corresponding adiabatic flame temperature to be less than a characteristic freezing temperature. At the freezing temperature, a premixed flame structure develops. If stretch (expressed in terms of the instantaneous scalar dissipation rate) is increased, the premixed flame merges into the non-equilibrium diffusion flame layer that exists around the maximum temperature. Quenching of the partially premixed diffusion flamelet occurs in the merged flame layer. The inner structure of the merged flame layer is identical to that for unpremixed diffusion flamelets with, however, modified boundary conditions. A numerical analysis of the merged flame structure is performed and extinction conditions are derived. It is shown that local premixing of an initially unpremixed flow leads to lower values of the scalar dissipation rate at quenching. The analysis of the flame structure is general and may also be applied to initially partially premixed systems.

Lifting mechanism of free jet diffusion flames

Abstract: The lifting phenomena of free jet diffusion flames of pure hydrogen and hydrogen-inert gas mixtures stabilized on a thin-walled burner tube were studied in unconfined, quiescent, oxygen-inert gas atmospheres. Nitrogen, argon, and helium were used as the inert gases. A laminar flame base, which exists even in a fully-developed turbulent jet diffusion flame, controls the flame stability. A simplified flame stability model is presented based on the concept that the flame base is the very end of the diffusion flame and it provides a continuous ignition source to propagate towards the combustible zone between the flame base and the burner tip. The local burning velocity parallel to the diffusion flame zone is considered at the flame base to be equilibrated with the entrained stream velocity component opposite to the direction of the burning velocity. Velocity measurements were made in the flame stabilizing region under near-limit conditions by means of the laser-Doppler technique. The measured velocity component parallel to the flame zone at the flame base was found to be directly proportional to the maximum laminar burning velocity obtainable by mixing the jet and external fluids. The result is independent of whether the mixture strength was obtained by varying the hydrogen content of the jet fluid or the oxygen content of the external fluid.

An excess enthalpy flame combustor for extended flow ranges

Abstract: An experimental study was conducted on the stability and combustion characteristics of the combustor system, based on the excess enthalpy flame, for extended flow rate ranges using a city gas (natural gas) and air mixture, and an entirely new combustor. The combustor was designed in light of previous studies, and the inner tube for combustion was surrounded by a heat exchanger to facilitate external heat recirculation, while a bundle of narrow ceramic tubes was used to produce internal heat recirculation. The stability limits in flow rate and equivalence ratio of the three flames stabilized ahead of, in, and behind the tubes were extended remarkably as compared to those of the previous studies. It became possible to burn stably mixtures as lean as equivalence ratio of 0.151 for large flow rate of 10.0 L/s. The measured temperature distributions showed that the combustion proceeded through the one-dimensional laminar flame, and that the combined external and internal heat recirculation produced very high flame temperature of more than twice the adiabatic flame temperature. The emission characteristics of the combustor were found excellent, while the pumping loss to force the flow through the narrow pores remained in the tolerable range.

Visualization of the Structure of a Pulsed Methane-Air Diffusion Flame

Abstract: Experiments have been carried out in a variable pressure flow facility with the objective of studying the structure of a co‐flowing jet diffusion flame. The flow is visualized using an optical scheme which superimposes the luminous image of the flame on its Schlieren image. This gives a useful picture of the relationship between the bright, yellow‐orange, soot‐laden core flow and the edge of the surrounding hot gas envelope. A loudspeaker is used to force the central fuel jet at several frequencies. In the unforced flow and over most of the driving frequency range in the forced flow, a double structure is observed with two distinct wavelengths: a long wavelength associated with the luminous, buoyancy‐driven core flow and a short wavelength associated with the shear‐driven outer flow. Excitation at the proper frequency causes strong coupling to occur. In this case the core flow pinches off and the flame breaks up into a series of flamelets moving with a single wavelength.

Turbulent diffusion flames of hydrocarbon fuels stabilized on a bluff body

Abstract: In order to be able to test and improve the theoretical models which can predict the chemical kinetic effects in turbulent nonpremixed flames, it is important to have a bank of data for flames which have been specially designed so that turbulent mixing is very intense and the chemical kinetic effects are substantial. The turbulent diffusion flame of hydrocarbon fuel, stabilized on a bluff body, is a candidate flame. The fules used are commercial LPG and natural gas. Nitrogen could be mixed with the fuel at various volumetric ratios. In this paper, the general characteristics of this flame have been studied. Blow-off limits and electrical connetedness limits are plotted. Shadowgraph pictures for “electrically connected” flames close to the blow-off limits indicate that turbulence is well developed and turbulent mixing is most intense in a region which extends for about two bluff body diameters downstream of the recirculation zone. The temperature profiles indicate that, at a fixed mean external air velocity, ū e , increasing the mean jet velocity, ū j towards the blow-off limit leads to a decrease in the peak mean temperature of the radial profile, at the region where mixing is most intense, of 300 to 400°C. This large decrease indicates the existence of important chemical kinetic effects in this region of the flame.

Temperature and fuel effects in sooting diffusion flames

Abstract: An investigation was carried out into the differing sooting tendencies of various fuels in free round laminar diffusion flames. Measurements included soot concentrations, soot temperatures and flame reaction zone and centreline temperatures. It is shown that flames emit smoke when the soot temperature in the oxidation zone falls below about 1300 K. This temperature is controlled by heat losses through radiation from the soot formed. Measured flame temperatures are well below adiabatic temperatures in the sooting flames and are determined by fuel flow rate and radiation losses. Thus acetylene and butene produced the highest soot yields at the lowest flame temperatures for the fuels tested. Fuel dilution with N2 reduced soot yield more than could be explained by variation of the flame temperature. Carbon to hydrogen ratio of the fuel is not itself a parameter determining soot yeild. Soot yield profiles were normalized according to the stoichiometric flame length. Under these conditions the profiles from differing fuels showed good axial similarity except for C2H2.

Imaging of atomic hydrogen in flames with two-step saturated fluorescence detection

Abstract: The hydrogen profile in a laminar hydrogen-air diffusion flame is mapped from a study of the Balmer-..cap alpha.. emission. (AIP)

A study of the controlling mechanisms of flow assisted flame spread

Abstract: An experimental study has been performed of the spread of flames over the surface of thick PMMA sheets in a forced gaseous flow of varied oxygen concentration moving in the direction of flame spread. It is shown that the rate of spread of the pyrolysis front is time independent, linearly dependent on the gaseous flow velocity, and approximately square power dependent on the oxygen concentration of the gas. The spread rate data can be correlated very well in terms of parameters deduced from heat transfer models of the flame spread process indicating that in the flow assisted mode of flame spread, heat transfer from the flame to the condensed fuel is the primary mechanism controlling the spread of the flame. Finite rate chemical kinetic effects have apparently a small influence on the flame spread process itself. Extinction processes are limited primarily to the upstream leading edge of the flame and to the flame tip.

Prediction and measurement of a non-equilibrium turbulent diffusion flame

Abstract: Superequilibrium radical concentrations in a turbulent CO/H2/N2 jet diffusion flame are computed using a two-scalar pdf model and directly measured using single pulse laser saturated OH fluorescence. The model is based on the averaged Navier-Stokes equations and the k∈l turbulence model. Non-equilibrium chemistry is accounted for by including CO in the partially equilibrated oxyhydrogen radical pool. Two scalars (mixture fraction and eaction progress suffice to describe the thermochemical system. Laser saturated fluorescence is used to directly measure the mean and fluctuating components of OH concentrations and thus the radical pool. Measurements and model both find mean OH concentrations which are four to six times larger than equilibrium with rms values of OH concentration also reasonably predicted. Superequilibrium effects are predicted to lower the mean temperature by as much as 250 K in agreement with experiments. Evidence of the breakdown of partial equilibrium was found in cool fuel-rich zones where predictions of temperature and OH concentration were too high. Extensions of the model to predict thermal NO formation and CO burnout are discussed.

Solution of Premixed and Counterflow Diffusion Flame Problems by Adaptive Boundary Value Methods

Abstract: Combustion models that simulate pollutant formation and study chemically controlled extinction limits in flames often combine detailed chemical kinetics with complicated transport phenomena. Two of the simplest models in which these processes are studied are the premixed laminar flame and the counterflow diffusion flame. In both cases the flow is essentially one-dimensional and the governing equations can be reduced to a set of coupled nonlinear two-point boundary value problems with separated boundary conditions.