Laminar Counterflow Diffusion Flames Research Articles

Structual Studies on Locally Strained Diffusion Flames. 2nd Report. Analysis of Flame Structure and Extinction Phenomena.

Numerical studies are made of H2/N2-air nonpremixed laminar counterflow diffusion flames strained by impinging micro fuel or air jet from fuel or air side. Numerical computations taking into account detailed chemical kinetics and multicomponent diffusion show that the temperature of the flame strained by micro fuel jet from fuel side easily decreases to reach extinction, and the flame strained from air side, on the contrary, hardly extinguishes and the flame temperature tends to increase. The present numerical results predict well the flame characteristics recognized by the experiments and this study reveals that these characteristics of those strained flames are mainly dominated by dilution or concentration of H2 in the fuel due to the preferential diffusion in relation to convex or concave flame curvature. In addition, it is also shown that maximum flame temperature cannot be rationalized by the local stretch rate and changes widely depending on the flame configuration such as the flame curvature relating with the preferential diffusion effects.

Prediction of NO x production rate in the turbulent diffusion flame

A new method to predict NO x emission index of turbulent diffusion flame is proposed. The procedure is as follows: 1. (1) To calculate emission index as a function of scalar dissipation rate χ q at an apparent flame surface position of steady laminar counterflow diffusion flame by adopting the detailed kinetics of large mechanisms. 2. (2) To calculate probability density function PDF of scalar dissipation rate at flame surface position in fuel jet turbulent diffusion flame by adopting the flame surface model of infinite reaction rate and by solving directly the time dependent Navier-Stokes equation and conserved scalar equation by using a finite difference method. 3. (3) To take average of local indices in terms of the above PDF to yield emission index of the whole flame. In this way, the combination of these two types calculation, (1) and (2), makes it possible to predict the emission index of the turbulent diffusion flame. The paper will describe some results of the prediction for methane-air turbulent diffusion flame and their comparison with existing experimental data.

Quantitative technique for imaging mixture fraction, temperature, and the hydroxyl radical in turbulent diffusion flames

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.

Structures of CO Diffusion Flames Near Extinction

Computational results are reported for structures of laminar counterflow diffusion flames between carbon monoxide and air, initially at room temperature and pressure from 1 to 100 atm, with total hydrogen-atom mole fractions in the system ranging from zero to about 0.02. All strain rates considered are within a factor of ten of the critical extinction strain rate. This critical strain rate is calculated as a function of pressure and of hydrogen content and is shown to lie below measured values under most conditions. For hydrogen-free flames, activation-energy asymptotics is employed and supports the computational results. It is reasoned that trace hydrogen amounts in air and preferential hydrogen diffusion through nonplanar diffusive-thermal instability contribute to enhanced flame robustness in the experiments, while increasing buoyant convective heat loss with increasing pressure promotes extinction at the higher pressures.

Prompt NO Scaling in Diffusion Flames: Effects of Strain and Pressure

Nitric oxide emission control is one of the major problems encountered in the development of combustion chambers for all types of applications. Scaling rules describing the dependence of NOx emission on operating parameters such as pressure, mixture ratio or turbulence intensity provide useful design guidelines. While such rules are well established for thermal NO, much less is available for prompt NO. This specific problem is considered in this article. The influence of pressure and strain rate on prompt NO production is investigated experimentally in a laminar counterflow diffusion flame. The results relative to the pressure dependence compare well with complex chemistry calculations in well-stirred reactors. The strain rate dependence is in good agreement with previous theoretical results found in the literature. The scaling established for the laminar flame is then applied to a turbulent nonpremixed combustor in order to describe the influence of strain on prompt NO production. The results are analyzed on the basis of a decomposition between global chemical effects and local effects due to strain. The final expression given for the prompt NO scaling in the turbulent diffusion flame provides a good correlation of experimental measurements.

Morphological evolution of nanoparticles in diffusion flames: Measurements and modeling

The morphological evolution of flame-generated primary spherules and inorganic aggregates was studied at low particle volume fractions [O(10 -1 ppm] in a well-defined/characterized laminar nonpremixed combustion environment which produces particle heating rates of 10 4 K/s. Pure Al 2 O 3 particles synthesized in an Al(CH 3 ) 3 (TMA-) seeded atmospheric pressure laminar counterflow diffusion flame fueled with CH 4 /O 2 /N 2 were used as the model material/combustion system. Experimental techniques included spatially resolved laser light scattering (LLS) and thermophoretic sampling/transmission electron microscopy. Local aggregate morphology was characterized in terms of spherule (grain') size, aggregate size, aggregate shape and fractal structure. Effects of flame temperature and TMA concentrations on particle inception location, sizes and morphology studied systematically were interpreted based on parallel theoretical studies. LLS signals and TEM images show particle/aggregate size and morphology evolution as a result of two competing rate processes. Mean spherule diameters prior to high-temperature coalescence are explained in terms of the strong size dependence of nanoparticle restructuring kinetics due to surface melting, even at 500 K Mean fractal aggregate sizes reached only 15-27 spherules near a local temperature of only 1,250 K Final particulate products were isolated spherical particles resulting from complete collapse of the aggregates in an interval of only 24 ms immediately upstream of the maximum gas temperature (2,280 K). Experimental results are compatible with the characteristic times governing each participating unit rate process. Some of these methods can be applied in controlling the larger-scale synthesis of valuable nanopowders and guide rational extensions into the domain of turbulent nonpremixed combustors generating ultrafine particles of tailored composition and morphology at high mass loadings.

Prediction of Radiative Heat Transfer in Laminar Flames∗

Abstract A detailed radiation model is proposed for radiative heat transfer calculations in laminar flames. The discrete transfer method is implemented in order to solve the equation of radiative transfer; the exponential wide band model is modified to avoid that thermal properties predicted by the original wide band model depend on the size of the numerical grid cells used in flame calculation. The overlapping correction among gases and mixture of soot and gases is considered. In addition, a phenomenological soot model for laminar premixed flat flame is formulated. The applications of the detailed radiation model in CO/H2/N2-Air and H2 — O2 laminar counterflow diffusion flames and C2H4-Ar premixed burner stabilized flame are presented.

Global characteristics and structure of hydrogen-air counterflow diffusion flames

A model based on similarity transformation, for the nitrogen-diluted, H 2-air opposed-jet laminar counterflow diffusion flame (CFDF), was developed independently of earlier models, and numerically solved to study flame location and flame structure and extinction limits. Numerical stiffness is handled by a special treatment of the species production term. Flame location with respect to the stagnation plane is identified as an important parameter that governs H2-air diffusion flames, and physical explanations are given to show how flame location is affected by fuel dilution, strain rate, and Lewis number. Results show very good agreement with experimental extinction conditions. The effect of thermal diffusion on the flame is found to be negligible. The simpler, constant Lewis number model produced extinction at half the strain rate compared to the species-dependent Lewis number model. The hydrogen-air CFDF exhibits several characteristics not observed for hydrocarbon flames. The underlying reasons are discussed in terms of the fluid dynamic and chemical kinetic aspects. Am = a = C = cfl = D = Di = Dj = Em = / = h = hi = K = k =

The influence of carbon dioxide on smoke formation and stability in methane-oxygen-carbon dioxide flames

The effect of replacing nitrogen in combustion air by carbon dioxide in a laminar, atmospheric methane diffusion flame was investigated experimentally and by numerical modelling. Measurements included flame temperature, carbon monoxide concentrations and direct observation and photographic investigation of the flame shape and behaviour. The experimental results indicate a substantial reduction of scattered light intensity and flame volume. When a sufficient amount of carbon dioxide is added to the oxidant, the sooting tendency and flame volume are reduced. In addition, the effect of replacing nitrogen by carbon dioxide in the flame gases on the rate of formation of key combustion intermediates, such as hydroxyl radicals and ethyne, was investigated theoretically by using the Sandia laminar counterflow diffusion flame computer model. This showed that, in addition to a reduction in flame temperature, there were significant reductions in hydroxyl radical and ethyne concentrations, the net result being a reduction in the soot-forming tendency of these flames.

Studies on strained non-premixed flames affected by flame curvature and preferential diffusion

Experimental and numerical studies are made of H 2 -N 2 -air laminar counterflow diffusion flames strained by steadily impinging micro fuel jet or air jet from the fuel side or the air side. Direct observation and two-dimensional temperature measurements by laser Rayleigh scattering method show that the flame strained by the micro fuel jet from the fuel side (flame 1) is easily quenched locally, but the flame strained from the air side (flame 2) hardly extinguishes and the flame temperature tends to increase. Numerical computations, taking into account detailed chemical kinetics and multicomponent diffusion, predict well the characteristics of the experimental findings noted above for the strained flames 1 and 2. They reveal the factors dominating the flame characteristics; that is, (1) H 2 of the fuel component is diluted by nitrogen compared to the original fuel near the central axis at stoichiometric condition in flame 1 with micro fuel jet but is concentrated in flame 2 with micro air jet due to the preferential diffusion among species in relation to the flame curvature; and (2) significant excess enthalpy is induced at the off-axis region in the lean and stoichiometric region due to the nonunity Lewis number effect. It is also noted that the maximum flame temperature cannot be rationalized by the local stretch rate but changes widely depending on the flame configuration.

Fullerenes versus soot in benzene flames

Many practical soot-emitting combustion systems such as diesel and jet engines rely on diffusion flames for efficient and reliable operation. Efforts to mitigate soot emissions from these systems are dependent on fundamental understanding of the physicochemical pathways leading from fuel to soot in laminar diffusion flames. Existing diffusion flame−based soot studies focused primarily on over-ventilated coflow flame where the fuel gas (or vapor) issues from a cylindrical tube into a co-flowing oxidizer, and counterflow flame, where a reacting zone is established between two opposing streams of fuel and oxidizer. As a canonical diffusion flame configuration, laminar counterflow diffusion flames have been widely used as a highly controllable environment for soot research, enabling significant progress in the understanding of soot formation for several decades. In view of the possibility of fuel/oxidizer premixing in practical systems, counterflow partially premixed flames have also been studied. In the present work we intend to provide a comprehensive review of the researches on various aspects of soot formation utilizing counterflow flames. Major processes of soot formation (formation of gas phase soot precursors, soot inception and surface reactions, as well as particle-particle interactions) are examined first, with focus on the most recent (post-2010) research progress. Experimental techniques and associated challenges for the measurement of soot-related properties (some knowledge of which is helpful for full appreciation of the experimental data to be reviewed) are then introduced. This is followed by a detailed description of soot evolution in counterflow flames, which is complemented by a discussion on the similarity and differences of the sooting structures between counterflow and coflow diffusion flames. Parametric studies of the effects of fuel molecular structure, fuel additive, partial-premixing, pressure, temperature, stoichiometric mixture fraction, and residence time on soot formation in counterflow flames will then be addressed in detail. This review concludes with a summary of the knowledge and challenges gathered and demonstrated through decades of research, and an outlook on opportunities for future counterflow flame−based soot research towards a more complete understanding of soot formation and the development of novel techniques for soot mitigation in practical combustion devices.

The Non-Equilibrium Structure of a Trichloro-ethene Methane Laminar Diffusion Flame

ABSTRACT Experimental measurements of scalar quantities in laminar counter-flow diffusion flames of mixtures of trichloroethylene (TCE) with methane have been compared with an equilibrium calculation. The data are correlated in terms of a mixture fraction. Data for stable major and minor species were obtained by using gas sampling with gas chromatography (GC) and a GC/mass spectrometer in flames with high (248 s−1) and low (175 s−1) strain rates. Calculations indicated that TCE mass fractions at equilibrium were very small, even at a relatively low temperature (600 K). However, the GC measurements showed that TCE persisted well into the flame. Strain rate was found to have a small but measurable effect on the correlation of species mass fractions and temperature with mixture fraction. The comparison of equilibrium predictions with measurements in mixture fraction space suggests that finite rate chemistry and chemical kinetics should be considered in the modeling of chlorinated hydrocarbon turbulent diffus...

An experimental investigation of strain-induced extinction of diluted hydrogen-air counterflow diffusion flames

Previous studies about turbulent combustion in hydrogen-air systems for application in supersonic-combustion devices such as the National Aerospace Plane have suggested that combustion in such devices takes place in thin reaction sheets that are convoluted, distorted, and in some cases locally extinguished by turbulence. Sharing this view, a number of researchers have focused their attention on characterizing the structure and extinction properties of laminar hydrogen-air diffusion flames. Most of the research has been of a numerical or theoretical nature and only a few experiments have been performed to test the detailed numerical diffusion-flame models. The objective of the present communication is to report results of some experiments designed to characterize the extinction properties of diluted laminar counterflow diffusion flames between air and hydrogen-nitrogen mixtures.

Charge-Induced Secondary Atomization in Diffusion Flames of Electrostatic Sprays

Abstract The combustion of electrostatic sprays of heptane in laminar counterflow diffusion flames was experimentally studied by measuring droplet size and velocity distributions, as well as the gas-phase temperature. A detailed examination of the evolution of droplet size distribution as droplets approach the flame shows that, if substantial evaporation occurs before droplets interact with the flame, an initially monodisperse size distribution becomes bimodal. A secondary sharp peak in the size histogram develops in correspondence of diameters about one order of magnitude smaller than the mean. No evaporation mechanism can account for the development of such bimodality, that can be explained only in terms of a disintegration of droplets into finer fragments of size much smaller than that of the parent. Other evidence in support of this interpretation is offered by the measurements of droplet size-velocity correlation and velocity component distributions, showing that, as a consequence of the ejection process, the droplets responsible for the secondary peak have velocities uncorrelated with the mean flow. The fission is induced by the electric charge. When a droplet evaporates, in fact, the electric charge density on the droplet surface increases while the droplet shrinks, until the so-called Rayleigh limit is reached at which point the repulsion of electric charges overcomes the surface tension cohesive force, ultimately leading to a disintegration into finer fragments. We here report on the first observation of such fissions in combustion environments. If, on the other hand, insufficient evaporation has occurred before droplets enter the high temperature region, there appears to be no significant evidence of bimodality in their size distribution. In this case, in fact, the concentration of flame chemi-ions or, in the case of positively charged droplets, electrons may be sufficient for them to neutralize the charge on the droplets and to prevent disruption.

Direct measurement of mixture fraction in reacting flow using Rayleigh scattering

The mixture fraction variable, ξ, is very useful in describing reaction zone structure in nonpremixed flames. Extinction limits and turbulent mixing are often described as a function of this variable. Experimental evaluation of ξ is critical for improving our understanding of the influence of turbulent mixing on the chemistry process. Heretofore, the evaluation of mixture fraction in combusting flow required multiple simultaneous concentration measurements. In this paper we present a fuel designed to permit measurements of mixture fraction by Rayleigh scattering technique only. A Rayleigh intensity/mixture fraction correspondence has been obtained experimentally in a laminar coflow flame. The influence of strain rate and differential diffusion effects have been investigated using laminar counterflow diffusion flame and shifting equilibrium chemistry models. The results obtained from comparisons between experiments and these models are very encouraging and suggest that the Rayleigh/mixture fraction correspondence established is valid under both the turbulent mixing and laminar strained flamelet combustion regimes.

Thermophoretic Effects on Particles in Counterflow Laminar Diffusion Flames

Abstract Thermophoresis, meaning particle drift down a local gas temperature gradient, is now known to be important to many combustion-related technologies. Until now, however, no direct experimental determinations of primary and aggregated particle thermophoretic diffusivities, αT D, in high temperature combustion environments have been reported. To perform such measurements, we selected a seeded laminar counterflow diffusion flame (CDF) operated at low strain-rate as a well-defined combustion system, offering at the same time a low velocity and high temperature gradient environment. We established a CH4/ O2Inert opposed jet diffusion flame in which the gaseous fuel/oxygen ratio, and the diluent flow rates were adjusted to obtain a flat, stable flame, approximately coincident with the gas stagnation plane (GSP). Particles fed to or formed on either or both sides of the GSP move toward this plane until the local axial velocity is exactly counterbalanced by the thermophoretic velocity. As a result of this ...

The effect of strain on laminar diffusion flames of chlorinated hydrocarbons

Laminar counterflow diffusion flames of mixtures of methyl chloride and trichloroethylene with methane have been studied. Measurements of the temperature profiles were obtained through the flames, the velocity profiles, and the profiles of major species and selected minor species such as polycyclic aromatics. The critical strain rate at which flame extinction occurred was measured for a range of chlorine loading in these flames. It was found that the critical strain rate varied in an approximately linear fashion with the mole fraction of the chlorinated species in the flame. Furthermore, the results suggested that the fuel structure did not affect flame extinction greatly; the chlorine loading appeared to be the only important parameter. Measurements of minor species showed that chlorinated compounds responded more sensitively to the strain rate of the flow than other, nonchlorinated hydrocarbons.

Chemical kinetic effects in nonpremixed flames of H 2/CO 2 fuel

Finite rate chemical kinetic effects are investigated in diffusion flames of H 2/CO 2 fuel. The CO 2 diluent is not totally inert since at high temperature some of it will react to give CO. Calculations of laminar flames and measurements in turbulent flames are presented. The calculations are performed, using a detailed chemical kinetic mechanism with 13 species and 34 chemical reactions. The structure of laminar counterflow diffusion flames with a range of stretch rates are calculated. Using the joint Raman-Rayleigh-LIF technique, simultaneous space- and time-resolved measurements of temperature and the Raman scattering from CO, CO 2, H 2, H 2O, O 2, and N 2, as well as laser-induced fluorescence (LIF) from OH have been made in turbulent nonpremixed flames of H 2/CO 2. As these flames are pilot-stabilized they blow off, at high enough jet velocities, in a region down stream of the pilot where turbulent mixing rates are high and the finite rate chemistry effects are significant. It is found that for laminar flames with low stretch rates, the peak calculated temperatures exceed the equilibrium temperature and the peak temperatures measured in turbulent flames. This is mainly due to differential diffusion effects that are substantial in the laminar flame calculations and cause hydrogen enrichment of lean mixtures. Such effects are not as significant in turbulent flames. The hydrogen radical plays a major role at extinction, where the competition is very intense between the formation-destruction processes within the reaction zone and diffusive processes towards nonreactive regions. In laminar flames, the radicals OH, H, and O are in superequilibrium concentrations regardless of the stretch rates. Measurements in turbulent flames show that the hydroxyl radical is in superequilibrium concentrations as well. The turbulent's flame approach to global blowoff is monomodal. Local extinction in turbulent flames increases very sharply only in the flame's final approach to blowoff when the jet velocities are greater than ∼90% of the blowoff velocity.

A Generalized Conditional Scalar Dissipation-Mixture Fraction Joint PDF for Flamelet Modeling of Non-Premixed Flames

Abstract One problem in flamelet modeling of turbulent non-premixed flames is that of formulating a scalar dissipation rate-mixture fraction joint probability density function (pdf). While the individual flamelet calculation provides a conditional scalar dissipation rate, the turbulent flame calculation provides a marginal scalar dissipation rate. Thus, linking the laminar and turbulent flame calculations through the joint pdf is not straightforward, and, for simplicity, it is often assumed that the marginal and conditional probability densities are equal. The present paper assesses the validity of this assumption and examines its consequences in a numerical example. A conditional, scalar dissipation rate-mixture fraction joint pdf is developed which is based upon numerical solutions of the laminar counterflow diffusion flame and functional relationships among the scalar dissipation rate, the conditional scalar dissipation rate, and the mixture fraction. Interestingly, the result, when applied to a turbulent non-premixed flame, results in predictions of NO concentration which are not significantly different. The analysis also suggests experiments which may provide insight into flamelet structure, thereby assessing the validity of the idealized, underlying laminar counterflow configuration used in the flamelet modeling of non-premixed flames.

PDF calculations of piloted turbulent nonpremixed flames of methane

A velocity-composition joint pdf transport equation has been solved by the Monte Carlo method to calculate the structure of pilot-stabilized turbulent nonpremixed flames of methane. Three components of velocity and a conserved scalar, namely, mixture fraction, ξ are jointly represented in the pdf. A new model is used for turbulent frequency. Turbulent dissipation and the fluctuating pressure gradient terms are conditionally modeled. Two simple models for thermochemistry are used. In one, density is a piecewise function of ξ, and in the other, density is obtained from calculations of a laminar counterflow diffusion flame of methane with a stretch rate, a = 100 s −1. Calculations are compared with the corresponding experimental measurements performed on a number of flames ranging from flames with low mixing rates to ones close to extinction. The velocity, turbulence, and mixing fields are predicted with reasonable accuracy down to x D j ∼ 30 . The agreement with experiments is less satisfactory at x D j = 50 . This will improve with the use of better pdf models which have recently been developed [27, 30, 31]. The reactive scalars are very well predicted only for flames which are far from extinction. This is expected considering the simplicity of the thermochemical model. This paper is a first step towards implementing realistic chemistry in the code, and hence predicting the observed finite rate kinetic effects in these flames.