Turbulent Diffusion Flame Burning Research Articles

Numerical Predictions of Soot Formation in Kerosene/Air Jet Diffusion Flame

Soot volume fraction in a turbulent diffusion flame burning kerosene/air is simulated using steady flamelet approach. The soot inception model based on the formation rate of three and two ring aromatics is used, which further accounts for inception, coagulation, surface growth, and oxidation processes. The Favre-averaged governing equations of mass, momentum, and energy are solved in conjunction with k-? turbulence model in ANSYS-FLUENT. The fairly detailed reaction mechanism for kerosene/air is coupled to the turbulent flow field by steady laminar flamelet approach, while the soot is calculated using Moss-Brookes model. The predictions are found to match well with the measurements where no significant effects are observed with the inclusion of higher order hydrocarbon as precursors and surface growth.

Radiation Emissions from Turbulent Diffusion Flames Burning Vaporized Jet and Jet-like Fuels

This study measured and compared the size and radiation emissions from turbulent diffusion flames burning large hydrocarbon fuels, including jet fuels. This effort is motivated by the need to understand how burning conventional or alternative jet-like fuels changes radiation emissions. Radiative heat transfer is significant because it is can alter pollutant formation and the lifetime of combustion devices. Eleven fuels were evaluated, consisting of traditional and alternative aviation fuels (e.g., Jet-A) and 1–3 component fuels. The flames were burned on a reduced-scale piloted Sydney turbulent diffusion burner with Reynolds numbers (Re) ranging from 7500 to 45 000. The vaporized fuel exited the burner near 300 °C. The radiative heat flux and the radiation intensity were measured using a radiometer (Medtherm, Model 64-0.2-20) and a mid-infrared camera (FLIR, Model SC6700), respectively. The radiant fraction (χR) is reported for the different fuels. χR typically varied by <15% at similar Re for the different flames, despite the aromatic content ranging from 0% to 31%. The χR had a tendency to be relatively insensitive to aromatic content, but was sensitive to the weight percent of hydrogen in the fuel. Differences in the molecular structure of the fuels, even for fuels with similar hydrogen contents, can significantly affect χR. χR values initially increased as the Re was increased, but then decreased monotonically for Re > 20 000. This trend is attributed to competing effects of flame stretch, entrainment, and surface-to-volume ratio on radiation emissions. The peak radiation intensity occurs near the location of the peak heat flux, which corresponds to the visible flame height. Radiation emitted from soot and unburned hydrocarbons is predominantly observed from within the flame sheet; significant variations in radiation emissions was observed between the different fuels. In contrast, emissions from CO2 for all the fuels varied by <10% and were more broadly distributed across the flame. Fluctuations in radiation emissions have a tendency to be greatest near the flame sheet for CO2 and within the flame sheet for soot and unburned hydrocarbons. Total radiation emissions from these flames have a tendency to be dominated by emissions from gaseous combustion products instead of emissions from soot or unburned hydrocarbons.

Effect of precursors and radiation on soot formation in turbulent diffusion flame

Soot formation in ‘Delft Flame III’, a pilot stabilized turbulent diffusion flame burning natural gas/air, is investigated using ANSYS FLUENT by considering two different approaches for soot inception. In the first approach soot inception is based on the formation rate of acetylene, while the second approach considers the formation rate of two and three-ringed aromatics to describe the soot inception (Hall et al., 1997). Transport equations are solved for soot mass fraction and radical nuclei concentration to describe inception, coagulation, surface growth, and oxidation processes. The turbulent–chemistry interactions and soot precursors are described by the steady laminar flamelet model (SLFM). Two chemical mechanisms GRI 3.0 (Gregory et al.) and POLIMI (Ranzi et al., 2012) are used to represent the effect of species concentration on soot formation. The radiative properties of the medium are included based on the non-gray modeling approach by considering four factious gases; the weighted sum of gray gas (WSGGM) approach is used to model the absorption coefficient. The effect of soot on radiative transfer is modeled in terms of effective absorption coefficient of the medium. A beta probability density function (β-PDF) in terms of normalized temperature is used to describe the effect of turbulence on soot formation. The results clearly elucidate the strong effect of radiation and species concentration on soot volume fraction predictions. Due to increase in radiative heat loss with soot, flame temperature decreases slightly. The inclusion of ethylene has less synergic effect than that of both benzene and ethylene. Both cases have less impact on the nucleation of soot. The increase in soot volume fraction with soot–turbulence interaction is in consistence with the DNS predictions.

Experimental comparison of soot formation in turbulent flames of Diesel and surrogate Diesel fuels

Soot formation is compared in turbulent diffusion flames burning a commercial Diesel and two Diesel surrogates containing n-decane and α-methylnaphthalene. A burner equipped with a high-efficiency atomisation system has been specially designed and allows the stabilisation of liquid fuels flames with similar hydrodynamics conditions. The initial surrogate composition (70% n-decane, 30% α-methylnaphthalene) was previously used in the literature to simulate combustion in Diesel engines. In this work, a direct comparison of Diesel and surrogates soot tendencies is undertaken and relies on soot and fluorescent species mappings obtained respectively by Laser-Induced Incandescence (LII) at 1064nm and Laser-Induced Fluorescence at 532nm. LIF was assigned to soot precursors and mainly to high-number ring Polycyclic Aromatic Hydrocarbons (PAH). The initial surrogate was found to form 40% more soot than the tested Diesel. Consequently, a second surrogate containing a lower α-methylnaphthalene concentration (20%) has been formulated. That composition which presents a Threshold Soot Index (TSI) very close to Diesel one is also consistent with our Diesel composition that indicates a relatively low PAH content. The spatially resolved measurements of soot and fluorescent soot precursors are quite identical (in shape and intensity) in the Diesel and in the second surrogate flames. Furthermore the concordance of the LII temporal decays suggests that a similar growth of the primary soot particles has occurred for Diesel and surrogates. In addition, the comparison of the LII fluence curves indicates that physical/optical properties of soot contained in the different flames might be similar. The chemical composition present at the surface of soot particles collected in Diesel and surrogate flames has been obtained by laser-desorption ionisation time-of-flight mass spectrometry. An important difference is found between Diesel and surrogate samples indicating the influence of the fuel composition on soot content.

Modeling soot formation in turbulent kerosene/air jet diffusion flames

Soot volume fraction and number density in a turbulent diffusion flame burning kerosene/air were predicted using two approaches. The first used a conventional soot inception model based on the acetylene concentration and is referred to as the acetylene model. The second used a soot inception model based on the formation rate of three and two ring aromatics [1] and is referred to as the PAH inception model. The soot models account for inception, coagulation, surface growth, and oxidation processes. The Favre-averaged governing equations of mass, momentum, and energy in the turbulent field were solved in conjunction with the k − ε turbulence model. A recently developed detailed reaction mechanism for kerosene/air [2] was coupled to the turbulent flow field by the stretched laminar flamelet approach. A radiation heat transfer model that considered the soot, water and CO 2 levels is included. Models are validated by comparing the numerical results to the experimental results of Young et al. [3] for a turbulent jet-flame burning pre-vaporized aviation kerosene. Significant improvements in the prediction of soot volume fraction are obtained using the PAH inception model for soot inception compared to the conventional acetylene approach.

Optical Properties in the Visible of Overfire Soot in Large Buoyant Turbulent Diffusion Flames

Nonintrusive measurements of the optical properties of soot at visible wavelengths (351.2–800.0 nm) were completed for soot in the overfire region of large (2–7 kW) buoyant turbulent diffusion flames burning in still air at standard temperature and pressure, where soot properties are independent of position and characteristic flame residence time for a particular fuel. Soot from flames fueled with gaseous (acetylene, ethylene, propylene, and butadiene) and liquid (benzene, cyclohexane, toluene, and n-heptane) hydrocarbon fuels were studied. Scattering and extinction measurements were interpreted to find soot optical properties using the Rayleigh-Debye-Gans/polydisperse-fractal-aggregate theory after establishing that this theory provided good predictions of scattering patterns over the present test range. Effects of fuel type on soot optical properties were comparable to experimental uncertainties. Dimensionless extinction coefficients were relatively independent of wavelength for wavelengths of 400–800 nm and yielded a mean value of 8.4 in good agreement with earlier measurements. Present measurements of the refractive index function for absorption, Em, were in good agreement with earlier independent measurements of Dalzell and Sarofim and Stagg and Charalampopoulos. Present values of the refractive index function for scattering, Fm, however, only agreed with these earlier measurements for wavelengths of 400–550 nm but otherwise increased with increasing wavelength more rapidly than the rest. The comparison between present and earlier measurements of the real and imaginary parts of the complex refractive index was similar to Em and Fm.[S0022-1481(00)02203-9]

Comprehensive Modeling of Turbulent Flames With the Coherent Flame-Sheet Model—Part I: Buoyant Diffusion Flames

A modified version of the computational fluid dynamics code KIVA-II was used to model the transient behavior of buoyant turbulent diffusion flames burning in still air. Besides extensions to the range of permitted boundary conditions and the addition of buoyancy terms to the turbulence model, KIVA-II was augmented by a version of the coherent flame-sheet model, Tesner’s soot generation model, Magnussen’s soot oxidation model, and an implementation of the discrete transfer radiation model that included both banded and continuum radiation. The model captured many of the features of buoyant turbulent flames. Its predictions supported experimental observations regarding the presence and frequency of large-scale pulsations, and regarding axial distributions of temperature, velocity, and chemical species concentrations. The radial structure of the flame was less well represented. The axial radiative heat flux distribution from the flame highlighted deficiencies in the soot generation model, suggesting that a model of soot particle growth was required.

Modeling of soot formation and oxidation in turbulent diffusion flames

Soot concentration in a turbulent diffusion flame burning propane was predicted using several formation and oxidation models. The Favre-averaged equations governing conservation of mass, momentum, and energy and transport equations for turbulent quantities, mixture fraction and its variance were solved using the k-t model and the laminar flamelet approach. Calculation of soot concentration was performed using the results of the flame field model as input data. The soot formation models of Khan and Greeves, and Moss and co-workers were used together with the soot oxidation models of Magnussen and Hjertager, Lee et al., and Nagle and Strickland-Constable. Comparison of the results with available measurements, and with the predictions obtained by Fairweather et al. using their own soot formation model, shows that reasonable predictions of soot concentration requires an adjustment of the constants of the model of Moss and co-workers. The soot formation model of Fairweather et al. appears to be less sensitive to the constants. The model of Khan and Greeves yields correct orders of magnitude, but fails to predict some important features of the data. Hence, there is a need to improve presently available soot formation and oxidation models to achieve a satisfactory predictive capability.

Structure of overfire soot in buoyant turbulent diffusion flames at long residence times

The structure of soot was investigated within the fuel-lean (overfire) region of overventilated buoyant turbulent diffusion flames burning in still air. The study was limited to the long residence time regime where characteristic flame residence times are roughly more than an order of magnitude longer than the laminar smoke point residence time and soot generation factors (the mass of soot emitted per unit mass of fuel carbon burned) are relatively independent of flame residence times. Both gaseous and liquid fuels were used to provide a range of H/C ratios (1–2.7) and fuel types (alkynes, alkenes, alkanes, aromatics, and alcohols) as follows: toluene, acetylene, benzene, propylene, ethylene, n-heptane, propane, and isopropanol. Measurements included transmission electron microscopy to find primary particle diameters, the number of primary particles per aggregate and aggregate geometrical and fractal dimensions. The results show that the structure of soot varies with fuel type but is relatively independent of both position in the overfire region and flame residence time for the long residence time regime. Mean primary particle diameters were 30–51 nm and the mean number of primary particles per aggregate were 255–552, with the larger values associated with the more heavily sooting fuels. Aggregate fractal dimensions, however, were less dependent on fuel type, only varying in the range 1.70–1.79. The structure measurements are used to estimate the optical properties of overfire soot, based on a recent approximate theory for polydisperse aggregates, finding significant differences between aggregate and Rayleigh scattering properties in the visible and near-infrared portions of the spectrum, even though the primary particles are well within the Rayleigh scattering regime.

Simultaneous Multiline Emission Absorption Measurements in Optically Thick Turbulent Flames

Abstract Simultaneous transient emission/absorption measurements at five wavelengths in strongly radiating, optically thick, turbulent diffusion flames burning acetylene in air were completed. The data were processed to obtain CO2, mole fractions, temperatures and soot volume fractions spatially resolved to the estimated local integral length scale of mixture fraction fluctuations. Temperatures and soot volume fractions based on emission intensities showed strong negative correlation due to radiative cooling effects. Probability density functions of soot volume fractions conditioned on CO2, mole fractions showed similarities with position. However, probable effects of negligible diffusivity of soot particles were observed. Probability density functions of soot volume fractions conditioned on both CO2, concentrations and temperature illustrate the important role of radiative heal transfer in determining the flame structure. A multivariate stochastic analysis resulted in good predictions of radiation intens...

Carbon monoxide and soot emissions from liquid-fueled buoyant turbulent diffusion flames

Carbon monoxide concentrations, soot concentrations, and mixture fractions were measured in the fuel-lean (overfire) region of liquid-fueled buoyant turbulent diffusion flames burning in still air. Pool-fire configurations were studied with the liquids burning from horizontal round wicks, considering both sooting (toluene, benzene, n-heptane, and isopropanol) and nonsooting (methanol and ethanol) fuels. Flame heights and characteristic residence times also were measured, both for the turbulent flames and at the normal smoke point (for the sooting fuels). Carbon monoxide and soot generation factors (mass of CO or soot emitted per unit mass of fuel carbon burned) were uniform throughout the overfire region and were relatively independent of flame residence times (which were generally an order of magnitude longer than the normal smoke point residence times of the sooting fuels). Processes of carbon monoxide and soot emission for the nonalcohols are closely related, based on the good correlation between their emission factors: 0.37 kg CO/per kg soot with a standard deviation of 0.09. However, nonsooting methanol and ethanol/air flames still emitted low levels of CO so that there is a component of CO emissions that is not associated with soot.

Scalar Measurements in a Co-Flowing Turbulent Diffusion Flame

Abstract —Measurements of a conserved scalar have been made in a horizontal, hydrogen turbulent diffusion flame burning in a co-flowing stream of air. Laser light scattered by refractory seeding particles was a measure of the fuel element mass fraction (the mixture fraction). Present values of the mean mixture fraction agree well with those obtained by probe-sampling. Results have been presented both as conventional statistics and as Favre or density-weighted statistics. Probability density functions of the mixture fraction are near Gaussian on the centre-line and are strongly skewed on the edge of the shear flow. Conversion to temperature probability density functions yielded results which were in qualitative agreement with those obtained by other techniques. These probability density functions revealed a widely-spread instantaneous reaction zone throughout the flame with combustion being indicated on the centre-line close to the nozzle. Spectra of the scalar fluctuations were also obtained.