Ethylene-air Flames Research Articles

Conditional moment closure prediction of soot formation in turbulent, nonpremixed ethylene flames

Results obtained from incorporating a semiempirical soot model into a first-order conditional moment closure (CMC) approach to modeling turbulent nonpremixed flames of ethylene and air are presented. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, with acetylene and benzene employed as the incipient species responsible for soot nucleation, and the concentrations of these species calculated using a detailed gas-phase kinetic scheme involving 463 reactions and 70 species. The study focuses on the influence of differential diffusion of soot particles on soot volume fraction predictions. The results of calculations are compared with experimental data for three sooting ethylene flames and, in general, predictions of mixing and temperature fields within the three flames show good agreement with data. Soot volume fraction predictions are found to be in significantly better accord with data when differential diffusion is accounted for in the CMC-based soot model, supporting the importance of such effects in sooting flames, as previously noted by Kronenburg et al. in relation to methane combustion. Overall, the study demonstrates that the CMC-based soot model, when used in conjunction with a model of differential diffusion effects, is capable of accurately predicting soot formation in turbulent nonpremixed ethylene–air flames.

Temperature measurement by holographic interferometry for non-premixed ethylene-air flame with a series of state relationships

Abstract This investigation developed a systematic method to measure the temperatures and the mass concentrations of local species in a non-premixed ethylene–air flame using the holographic interferometry (HI) technique. A nearly linear state relationship between the Gladstone–Dale constant and the mixture fraction was deduced. Normalized state relationships between the refractive index and the mixture fraction were established from detailed combustion simulations for the NPFs. The maximum error of temperature caused by the assumption of composition of air was reduced from 48.84% to under 1.6% for the NPFs using the systematic method. The method can overcome the difficulty that the concentrations of local species must be known when measuring the temperature by HI for NPFs.

Addition of NO 2 to a laminar premixed ethylene–air flame: Effect on soot formation

Experiments were conducted on a laminar premixed ethylene–air flame at equivalence ratios of 2.34 and 2.64. Comparisons were made between flames with 5% NO 2 added by volume. Soot volume fraction was measured using light extinction and light scattering and fluorescence measurements were also obtained to provide added insight into the soot formation process. The flame temperature profiles in these flames were measured using a spectral line reversal technique in the non-sooting region, while two-color pyrometry was used in the sooting region. Chemical kinetics modeling using the PREMIX 1-D laminar flame code was used to understand the chemical role of the NO 2 in the soot formation process. The modeling used kinetic mechanisms available in the literature. Experimental results indicated a reduction in the soot volume fraction in the flame with NO 2 added and a delay in the onset of soot as a function of height above the burner. In addition, fluorescence signals—often argued to be an indicator of PAH—were observed to be lower near the burner surface for the flames with NO 2 added as compared to the baseline flames. These trends were captured using a chemical kinetics model that was used to simulate the flame prior to soot inception. The reduction in soot is attributed to a decrease in the H-atom concentration induced by the reaction with NO 2 and a subsequent reduction in acetylene in the pre-soot inception region.

Characterization of nanoparticles of organic carbon (NOC) produced in rich premixed flames by differential mobility analysis

Differential mobility analysis (DMA) is used to measure on-line the size distributions of inception particles in atmospheric pressure premixed ethylene air flames ranging from C/O = 0.61 to 0.69, just at the onset of soot formation. DMA is also used, in combination with electrospray, to measure the size distributions of suspended flame products captured in water samples. The DMA systems used for this work employ detectors sensitive to the full range of molecular clusters/nanoparticles in gas-to-particle conversion processes (as small as about 1 nm) and they have much larger sheath gas flow rates than is typically used to reduce losses and peak broadening by diffusion. The measured size distributions show that the first particles observed in flames have a size of 2 nm, consistent with previous in situ measurements by light scattering and extinction (LSE) and the off-line measurements of material captured in water samples from the same flames. For richer flames, the quantity of the 2 nm particles measured increases, and the width of its size distribution shifts asymmetrically toward larger sizes. A numerical coagulation model assuming size-dependent coagulation efficiency predicts well the experimentally measured size distributions in the flames examined. Similarly, the slightly larger size distributions measured by atomic force microscopy of inception particles deposited on surfaces can also be attributed to the size-dependent coagulation/adhesion efficiency. The results imply that the smaller nanoparticles formed in combustion processes have a longer lifetime than those larger than 6–7 nm and may play an important role in the formation of fine organic carbon particulate in the atmosphere.

Reduction of PAH and soot in premixed ethylene–air flames by addition of ethanol

Results are presented from a combined experimental and modeling study undertaken to understand the pathways by which the addition of ethanol to fuel-rich ethylene flames causes reductions in PAH and soot. The experimental work was conducted in a flat-flame burner at equivalence ratios of 2.34 and 2.64. Ethanol was added to the ethylene at two levels corresponding to 5 and 10% oxygen by weight in the fuel. Soot was measured by laser-induced incandescence calibrated with light extinction, and aromatic species were measured using laser-induced fluorescence. Modeling was based on a 1-D premixed flame model and kinetic mechanisms available in the literature. The modeling work captures the trends in aromatic species with changes in equivalence ratio and oxygen concentration in the fuel. However, the soot predictions do not match the increases observed at the higher equivalence ratio. Analysis of the modeling results for the lower equivalence ratio shows that the addition of ethanol to the ethylene reduces the aromatic species mainly by reducing the amount of carbon that is available to form precursor species.

Implementation of the population balance equation in CFD codes for modelling soot formation in turbulent flames

The simulation of soot formation in turbulent diffusion flames is carried out within a CFD code, by coupling kinetics and fluid dynamics computations with the solution of the population balance equation via the Direct Quadrature Method of Moments, a novel and efficient approach based on a quadrature approximation of the size distribution of soot particles. A turbulent non-premixed ethylene–air flame is used as the test case for validation of the model. Simplified kinetic expressions are employed for modelling nucleation, molecular growth and oxidation of particles, along with a Brownian aggregation kernel. A recently proposed approach for modelling the evolution of fractal dimension is used with a monovariate population balance to predict the morphological properties of aggregates.

Surface deposition and coagulation efficiency of combustion generated nanoparticles in the size range from 1 to 10 nm

The size distribution of the nanoparticles formed in premixed ethylene–air flames and collected thermophoretically on mica cleaved substrates is obtained by atomic force microscopy (AFM). The distribution function extends from 1 to about 5 nm in non-sooting flames and in the soot pre-inception region of the richer flames, while it becomes bimodal and larger particles are formed in the soot inception region of the slightly sooting flames. The distribution is compared with the size distribution of nano-sized organic carbon (NOC) and soot particles, obtained by “in situ” multi-wavelength extinction and light scattering methods. The deposition efficiency is estimated from the differences between these two size distribution functions as a function of the equivalent diameter of the nanoparticles. Furthermore, the coagulation coefficient of particles in flame is obtained from the temporal evolution of the number concentration of the nanoparticles inside the flames. NOC particles, which are rapidly produced in locally rich combustion regions, have peculiar properties since their sticking coefficient both for coagulation and adhesion result to be orders of magnitudes lower than that expected by larger aerosols, like soot particles. The experimental results are interpreted by modelling the van der Waals interactions of the nanoparticles in terms of Lennard-Jones potentials and in the framework of the gas kinetic theory. The estimated adhesion and coagulation efficiencies are in good agreement with those calculated from AFM and optical data. The very low efficiency values observed for the smaller particles could be ascribed to the high energy of these particles due to their Brownian motion, which causes thermal rebound effects prevailing over adhesion mechanisms due to van der Waals forces.

Nano-organic carbon and soot particle measurements in a laminar ethylene diffusion flame

The formation of carbonaceous particulates in a co-flow laminar diffusion flame has been studied using UV–visible spectroscopy and laser scattering/extinction techniques for measurements of volume fractions and particle sizes. Measurements were performed in a non-smoking ethylene–air flame at atmospheric pressure. UV–visible spectroscopy allowed the identification of two classes of particles: soot particles, which absorb light in the whole spectral range and nano-organic carbon particles (NOC) which are transparent to the visible radiation. The mean size of nano-organic carbon particles was estimated to be about 2–3 nm. This agreed with previous results obtained in rich premixed flames with equivalence ratios across the soot formation threshold. The experimental results indicate that nano-organic carbon particles are formed in the fuel side of the flame front closer to the flame centre line than soot particles and with a concentration level comparable to that of soot. The intermediate spatial location of these particles between the fuel- and soot-containing regions and their high concentration in flame suggest that soot formation is just the consequence of coagulation of NOC particles without a major role of surface growth in the soot loading process.

Measurements of the soot volume field in laminar diffusion flames at elevated pressures

Soot volume fraction ( f v ) is measured quantitatively in a laminar diffusion flame, with either methane or ethylene as fuel, at elevated pressures up to 2.5 MPa in order to gain a better understanding of the effects of pressure on the soot formation process. Soot continues to be of interest because it is a sensitive indicator of the interactions between combustion chemistry and fluid mechanics and known to be detrimental to human health. To examine the effects of increased pressure on soot production, laser-induced incandescence (LII) is used to obtain the desired spatially resolved measurements of f v as the pressure is incrementally increased to 2.5 MPa. The effects of pressure on the physical characteristics of the flame are also observed. Using a laser light extinction technique, the path-integrated soot volume fraction scales with pressure as p 1.0 and p 1.2 for the methane–air and ethylene–air flames, respectively, at 65% of the flame height. From the LII images, it is observed that the soot layer radius decreases with increasing pressure, scaling as approximately p −0.5 at 65% of the flame height, for both methane and ethylene flames. The local peak f v is found to scale with pressure as p 1.2 for methane and p 1.7 for ethylene flames, which is different than the path-integrated soot pressure dependence. The location of peak soot is observed to move from the edges toward the tip of the flame as the pressure is increased for both fuels.

Interference effects of C2‐radicals in nitrogen vibrational CARS thermometry using a frequency‐doubled Nd:YAG laser

AbstractGenerally, in coherent anti‐Stokes Raman spectroscopy a frequency‐doubled Nd:YAG laser emitting at 532 nm along with a broadband dye laser is used for nitrogen vibrational CARS thermometry, resulting in a signal wavelength of 473 nm. Unfortunately, emission and absorption bands of C2 radicals occur in the same spectral region, hence the CARS signal can be disturbed by interferences. C2 radicals are an intermediate species especially in hydrocarbon flames and, moreover, they can be produced by laser‐induced vaporization of soot particles present in the flame. The described interferences can be a severe problem resulting in erroneous temperature data. The influence of the interferences on the determined temperatures was analyzed in this work. For this purpose, an experimental set‐up was chosen that simultaneously generates the vibrational nitrogen signal at 473 and 487 nm, i.e. in a undisturbed spectral region. This was realized by using the dual‐pump CARS technique with an annular phase‐matching geometry (USED CARS). Hence a direct comparison of temperature results from disturbed and undisturbed spectra can be performed. Measurements conducted in one‐dimensional ethylene–air flames established on a McKenna‐type burner are presented. Copyright © 2004 John Wiley & Sons, Ltd.

Flame synthesis of carbon nanofibres and nanofibre composites containing encapsulatedmetal particles

We have synthesized carbon nanofibres (CNFs) in an ethylene–air flame on astainless steel substrate. Ten-minute specimens consisted of both nodules and carbonmicro- and nanofibres of various diameters up to . There are two kinds of primary fibres. The first are of nm thickness and show numerous kinks along their length. The second variety can havethicknesses of the order of and can grow to a length of . Nanofibre branching is also evident. The inclusion of periodic metal deposits in thenanofibres reveals a strategy by which it might be possible to encapsulate metal particles inorder to form connected CNF networks.

The flame preheating effect on numerical modelling of soot formation in a two-dimensional laminar ethylene–air diffusion flame

Numerical modelling of soot formation is conducted for an axisymmetric, laminar, coflow diffusion ethylene–air flame by two different methods to investigate the effect of flame preheating. The first method cannot account for the preheating effect, while the second one can. A detailed gas-phase reaction mechanism and complex thermal and transport properties are used. The fully coupled elliptic equations are solved. A simple two-equation soot model is used to model the soot process coupled with detailed gas-phase chemistry. The numerical results are compared with experimental data and indicate that the flame preheating effect has a significant influence on the prediction of soot yields. Both methods give reasonable flame temperature and soot volume fraction distributions. However, quantitatively the second method results in improved flame temperatures and soot volume fractions, especially in the region near the fuel inlet, although the maximum flame temperatures from both methods are slightly lower than that from the experiment.

UV-visible spectroscopy of organic carbon particulate sampled from ethylene/air flames

A systematic comparison of spectra obtained with extra and in situ diagnostics in the soot preinception region of rich, premixed ethylene air flames suggests that combustion generated organic carbon (OC) particulate can be extracted from flames and isolated from other flame material for further chemical analysis. Both the trend with height above the burner and the form of UV fluorescence and absorption spectra from extra situ sampled material captured in water agree with those measured in situ. These results show that the OC particulate formed in flames is partially water soluble. However, the collection efficiency can be increased using less polar solvents, like acetonitrile and dichloromethane. The fluorescence spectra from the water samples are comprised both a naphthalene-like component and a broad band UV fluorescence component similar to that observed in situ which is attributed to flame generated OC particulate. The broad band UV fluorescence centered around 320 nm is also observed very early in flames and does not change considerably with increasing flame residence time. These results support previous hypotheses that the UV broad band fluorescence is from carbonaceous material comprised two-ring aromatics, formed earlier than soot in the flame, and is still present along with soot at higher heights or flame residence times.

Laser-induced incandescence for soot particle size measurements in premixed flat flames

Measurements of soot properties by means of laser-induced incandescence (LII) and combined scattering-extinction were performed in well-characterized premixed ethylene-air flames. In particular, the possibility of using LII as a tool for quantitative particle sizing was investigated. Particle sizes were evaluated from the temporal decay of the LII signal combined with heat balance modeling of laser-heated particles, and these sizes were compared with the particle sizes deduced from scattering-extinction measurements based on isotropic sphere theory. The correspondence was good early in the soot-formation process but less good at later stages, possibly because aggregation to clusters began to occur. A critical analysis has been made of how uncertainties in different parameters, both experimental and in the model, affect the evaluated particle sizes for LII. A sensitivity analysis of the LII model identified the ambient-flame temperature as a major source of uncertainty in the evaluated particle size, a conclusion that was supported by an analysis based on temporal LII profiles.

Dynamic Modeling of Soot Particle Coagulation and Aggregation: Implementation With the Method of Moments and Application to High-Pressure Laminar Premixed Flames

The method of moments was extended to include a more complete description of particle coagulation, namely, the transition and continuum regimes as well as the formation and growth of fractal aggregates. The formalism preserves the numerical efficiency and the physical rigor of the method of moments by inclusion of just two additional moment equations and without prescribing a mathematical form to the particle size distribution function. The extended model was used to simulate soot formation in several 10 bar laminar premixed ethylene-air flames. The results demonstrate that the more complete formulation improves significantly model prediction of experimental data, thereby explaining the appearance of a catastrophic decrease in coagulation rates, which could not be rationalized within the classical theory of Brownian coagulation.

Experimental and numerical study on soot formation in laminar high-pressure flames

Soot volume fractions and flame temperatures have been measured in laminar premixed ethylene-air flames at pressures between 5 and 20 bar for equivalence ratios ranging 1.7 to 2.8. The measurements have been carried out by applying laser-induced incandescence (LII) and light extinction for determining flame structure and soot volume fractions, and N2 coherent anti-Stokes Raman spectroscopy (CARS) for measuring temperatures in the pressooting zone. A one-dimensional flame model has been used. The equations have been solved with a computer code that includes both a detailed gas phase and transport model. The gas-phase reaction mechanism comprises recent results of kinetic data of aromatic compounds. Moreover, a standard soot formation and oxidation model is employed. Analysis of the experimental results obtained between 5 and 20 bar reveal major deficits in the submodels of polycyclic aromatic hydrocarbon (PAH) formation and of soot oxidation by OH radicals.

OH and soot concentration measurements in a high-temperature laminar diffusion flame

OH absorption and soot extinction coefficients have been determined in a sooting ethylene laminar diffusion flame produced above a Wolfhard-Parker burner. An oxidizer rich in oxygen provided a flame with a high-temperature combustion zone ( T ∼ 2500 K). Compared to an ethylene-air flame, the production of OH is more pronounced, and soot burns out; thus, it is easier to study the oxidation of soot. 2D fluorescence images were obtained with a laser sheet tuned on and off the OH resonance frequency. A strategy, based on the relative shape of the signal in the direction of the laser at a given height above the burner, was developed to derive the local OH absorption coefficient and, in the sooting zone, the extinction coefficient. On the flame's axis, the temperature was determined by two-colour pyrometry; the absolute concentration of OH was derived from the absorption coefficients. Along the flame's axis, profiles of OH and soot concentrations were obtained over the zone where soot oxidation takes place. The collision efficiency for OH was determined from these measurements. Just above the soot burn-out zone, [OH] was found to be lower than the value calculated assuming equilibrium.

Evolution of char chemistry, crystallinity, and ultrafine structure during pulverized-coal combustion

Detailed modeling of soot particle nucleation, growth and oxidation in laminar premixed ethylene-air flames at a pressure of 10 bar is presented. The employed kinetic model of soot formation, developed and tested earlier for subatmospheric and atmospheric flames, can quantitatively describe the experimental soot volume fraction profiles collected in a number of ehtylene flames at high pressures. The kinetic model consists of initial aromatic ring formation from nonaromatic species, planar polycyclic aromatic hydrocarbon (PAH) growth through the hydrogen-abstraction-acetylene-addition reaction mechanism, particle nucleation through coalescence of PAHs, particle growth by coagulation and surface reactions of the forming clusters and particles, and particle oxidation. Analysis of the computational results obtained at 10 bar and additional tests performed for a 1-bar flame suggest that the influence of pressure on soot production arises mainly from the larger concentration of acetylene, which affects both the soot surface growth and the nucleation of soot particles.

Detailed modeling of soot formation in laminar premixed ethylene flames at a pressure of 10 bar

Detailed modeling of soot particle nucleation, growth and oxidation in laminar premixed ethylene-air flames at a pressure of 10 bar is presented. The employed kinetic model of soot formation, developed and tested earlier for subatmospheric and atmospheric flames, can quantitatively describe the experimental soot volume fraction profiles collected in a number of ehtylene flames at high pressures. The kinetic model consists of initial aromatic ring formation from nonaromatic species, planar polycyclic aromatic hydrocarbon (PAH) growth through the hydrogen-abstraction-acetylene-addition reaction mechanism, particle nucleation through coalescence of PAHs, particle growth by coagulation and surface reactions of the forming clusters and particles, and particle oxidation. Analysis of the computational results obtained at 10 bar and additional tests performed for a 1-bar flame suggest that the influence of pressure on soot production arises mainly from the larger concentration of acetylene, which affects both the soot surface growth and the nucleation of soot particles.

Dynamics of a strongly radiating unsteady ethylene jet diffusion flame

Time-dependent numerical simulations of an axisymmetric ethylene-air diffusion flame are used to quantify the way in which radiation transport affects the development, structure, and dynamics of the flame. The numerical model solves the time-dependent Navier-Stokes equations coupled to submodels for chemical reaction and heat release (ethylene combustion), soot formation, and radiation transport. The soot formation model includes algorithms for soot nucleation, surface growth, coagulation, thermophoresis, and oxidation. The radiative heat flux is found by solving the radiative transfer equation using the Discrete Ordinates Method and includes radiative effects from soot, CO 2 and H 2O. The model is tested by comparing simulation results with previously published experimental data for a coflowing laminar ethylene-air flame. Simulations of a higher-speed jet at 5 m/s show that radiative heat losses reduce the flame temperature, which decreases the chemical heat release rate. The reduction in heat release rate decreases the volumetric expansion, causing the flame to shrink considerably, and hence changes the overall temperature, species concentration, and soot volume fraction distributions in the flame. The computations show that radiative intensity is attenuated significantly within the heavily sooting region. Radiative heat flux vectors are primariy directed in the radial direction; however, there is a significant axial component that follows the curvature of the sooting region. The computations for an undiluted fuel jet show that heat transfer by radiation dominates transfer by conduction and convection in the heavily sooting regions of the flame.