Two-dimensional Soot Volume Fraction Research Articles

Experimental and numerical investigation of effects of premixing on soot processes in iso-octane co-flow flames

Abstract The goal of the current work is to understand the effects of premixing on soot processes in an iso-octane, axisymmetric, co-flow, laminar flame at atmospheric pressure. The flames investigated are non-premixed and partially-premixed (jet equivalence ratios of 24, 15, 12, 9 and 6). The total carbon flow rate is kept constant to facilitate comparison among the six flames. Laser-induced incandescence and laser extinction are applied to obtain two-dimensional soot volume fraction. The experimental results show that the spatial distribution of soot changes with premixing; the peak soot volume fraction location is in the annular region in the non-premixed flame and transitions to the centerline as the jet equivalence ratio is reduced. Numerical simulations are performed using a detailed iso-octane fuel chemistry and bi-variate soot model. The numerical model captures the changes in the spatial distribution of soot due to premixing, as in the experiment. Similar to the change in the soot distribution, the soot production processes, including nucleation, surface growth, and PAH condensation, show the transition behavior with premixing. The simulation shows that the location of peak PAH dimer concentration shifts from the annular region towards the centerline with premixing. As a result, the location where soot nucleation and PAH condensation rates peak show similar transition as observed in the PAH dimer concentration. Furthermore, PAH dimer concentration decreases due to premixing, leading to a decrease in the soot nucleation and soot growth due to PAH condensation. Additionally, soot growth due to surface reactions decrease with premixing due to the reduction in number of active sites on the soot surface.

Effect of aromatic fuels and premixing on aromatic species and soot distributions in laminar, co-flow flames at atmospheric pressure

The aim of the present study is to investigate the effects of aromatic fuel structure on the aromatic species and the soot volume fraction in laminar, co-flow flames at atmospheric pressure. Both non-premixed and partially-premixed flames (jet equivalence ratios of 24 and 6) are studied. The four fuels consist of binary mixtures of n-dodecane with one of four aromatic species: n-propylbenzene, toluene, m-xylene, and 1,3,5-trimethylbenzene. For all flame conditions, the total carbon flow rate of the fuel mixture is kept constant. Furthermore, in each binary mixture, the carbon mole fraction from the n-dodecane and the aromatic component is kept constant. Laser-induced incandescence (LII) and laser extinction are used to obtain two-dimensional soot volume fraction in the flames. Laser-induced fluorescence (LIF) is used to obtain the two-dimensional aromatic species distribution in the flames. The experimental results indicate that the 1,3,5-trimethylbenzene/n-dodecane flame has the highest peak soot volume fraction amongst the four fuels; the other three fuel mixtures, within measurement uncertainty, have similar peak soot volume fraction. In addition, premixing changes the spatial distribution of polycyclic aromatic hydrocarbons and soot in the flames. In the non-premixed cases, the peak soot volume fraction is located in an annular region near the flame sheet, whereas the soot field is more uniform across the cross-section of the flame at a jet equivalence ratio of 6. A published aromatic fuel chemical mechanism is used to understand the differences in the pathways to soot precursors among the aromatic fuels. The combined 2D LIF and LII result provide a unique dataset to validate soot and chemical numerical models for the four aromatic fuels: n-propylbenzene, toluene, m-xylene, and 1,3,5-trimethylbenzene.

Effect of fuel composition on soot and aromatic species distributions in laminar, co-flow flames. Part 2. Partially-premixed fuel

The goal of this work is to understand the effect of fuel molecular structure on soot precursors and soot in an axisymmetric, co-flow, laminar flame configuration at atmospheric pressure with partially-premixed fuel jets. Five fuels with varying molecular structure are investigated: methylcyclohexane/n-dodecane mixture, n-heptane/n-dodecane mixture, iso-octane/n-dodecane mixture, m-xylene/n-dodecane mixture and pure n-dodecane. The flames investigated have jet equivalence ratios of 24 and 6. The total carbon flow rate and carbon fraction of the two components are kept constant to facilitate comparison among fuels. A laser-induced fluorescence technique is used to obtain spatially-resolved polycyclic aromatic hydrocarbons, soot precursors, in the jet flames. The polycyclic aromatic hydrocarbons are identified into two classes: single/two ring aromatics (small) and aromatics having three to five rings (large). A laser-induced incandescence and laser extinction technique are applied to obtain two-dimensional soot volume fraction for all the flames. The experimental results indicate that the level of soot is highest for the m-xylene/n-dodecane fuel at approximately three times the peak soot levels in the paraffinic fuels. The data show the effects of premixing on the spatial distribution of aromatic species and soot, including the shift from a soot field that peaks near the flame front to one that has maximum soot volume fractions near the centerline in m-xylene/n-dodecane flame. The iso-octane/n-dodecane and methylcyclohexane/n-dodecane fuels show a similar transition of soot field that has an annular peak in the non-premixed flame to a more uniform soot field under premixed conditions. Normalizing the maximum soot volume fraction by the maximum for the n-dodecane base fuel, the data shows that, within measurement uncertainty, the effect of fuel structure on maximum soot volume fraction is independent of the equivalence ratio of the fuel jet.

Effect of fuel composition on soot and aromatic species distributions in laminar, co-flow flames. Part 1. Non-premixed fuel

The work described in this paper is part of Department of Defense-Industry-University collaboration to develop fundamental understanding of the effects of alternative fuels on emissions from military gas turbine engines. The study was conducted in an axisymmetric, co-flow, laminar flame configuration at atmospheric pressure to determine the effects of fuel structure on aromatic species and soot. Five fuels with varying molecular structure were investigated: pure n-dodecane and four binary mixtures: m-xylene/n-dodecane, n-heptane/n-dodecane, iso-octane/n-dodecane, and methylcyclohexane/n-dodecane. Flames with non-premixed and partially-mixed fuel jets were studied. This paper describes the results of flames with non-premixed fuel jets; a companion paper describes the results for the partially pre-mixed fuel jets. In all of the experiments, the total carbon flow rate and the fraction of carbon from the two components in the binary mixtures were kept constant to facilitate comparison among the fuels. A laser-induced fluorescence technique was used to obtain spatially-resolved information on the aromatic species. Signals from aromatic species were collected in two wavelength ranges corresponding to species with one or two rings and three–five rings. Laser-induced incandescence and laser extinction were applied to obtain two-dimensional soot volume fraction for all flames. The flames of the paraffin fuels are non-smoking and have similar spatial distributions of aromatic species and soot as well as maximum soot volume fractions. The flame from the m-xylene fuel is smoking and has spatial distributions of aromatic species and soot that are distinctly different than for the paraffin fuels. Maximum soot volume fraction produced by the m-xylene fuel is approximately three times that of the paraffin fuels.

Sooting tendencies of diesel fuels, jet fuels, and their surrogates in diffusion flames

Surrogates of transportation fuels are useful design tools for developing engines with cleaner and more efficient combustion. It is important that information about the sooting tendency of the target fuel be incorporated in the formulation of these surrogates. Toward that end, in this work we report experimentally measured sooting tendencies of several diesel and jet fuels, surrogates of these fuels, and pure compounds which are components of these surrogates. These fuels, surrogates, and compounds were separately doped in trace quantities (0.5% by mass) into the fuel of a methane-air laminar coflow flame. The sooting tendencies, expressed as Yield Sooting Indices (YSIs), were quantified from spatially resolved two-dimensional soot volume fraction distributions of these flames as measured with color ratio pyrometry. Previous means of matching the sooting behavior of the surrogate to the fuel, such as by matching the fuel’s hydrogen content or its double bond equivalent, were found to be inadequate because these approaches do not fully account for the effects on the fuel’s sooting tendency of branching in aliphatics and alkyl substitutions in aromatics. A strong correlation was observed between the YSI and the relative abundance of different types of carbon atoms (as quantified by 1H and 13C NMR spectroscopy) in these fuels and compounds, indicating that such NMR characterization can capture essential structure-specific sooting information of relevance to surrogate formulation. Based on these YSI measurements, we demonstrate two approaches to predict the sooting tendency of a fuel surrogate. These approaches, based on (1) the choice of pure compounds that are blended to create a surrogate, or (2) the distribution of carbon atom types in a surrogate, can be used to tune the surrogate’s formulation to match the sooting tendency of its target fuel.

Two-dimensional soot volume fraction measurements in flames doped with large hydrocarbons

Developing diesel surrogates that mimic the essential characteristics of target fuels such as their sooting tendency and volatility is a necessary but challenging endeavor, owing in part to the scarcity of available experimental sooting data for compounds in the appropriate high molecular weight range. To address this, in this work we demonstrate an experimental approach which provides quantitative sooting data for such compounds. Specifically, we have measured spatially resolved two-dimensional soot volume fraction distributions for three large hydrocarbons: 1,3,5-triisopropylbenzene (tipb), 1,3,5-triisopropylcyclohexane (tipcx), and perhydrophenanthrene (php), as well as benzene and n-hexane. Measurements were performed in methane/air coflowing flames whose fuel was doped separately with these compounds (dopant mass fraction 0.5%). Color-ratio pyrometry was used to measure soot temperature and soot volume fraction distributions. Sooting tendencies were tipb > benzene > php > tipcx > hexane. These data are expected to be useful in formulation of surrogates with greater fidelity to the volatility and sooting characteristics of real diesel, and in validation of numerical soot models involving such surrogates.

Soot particle size measurements in ethylene diffusion flames at elevated pressures

Soot particle size is investigated in laminar nitrogen-diluted ethylene coflow diffusion flames at 4, 8, 12 and 16 atm. Line of sight attenuation and scattering are used to measure two-dimensional soot volume fraction and particle size fields for the first time at elevated pressures. Soot volume fraction dependence on pressure is consistent with the observations of similar studies, scaling approximately with the square of pressure. Scattering intensity is analyzed through Rayleigh and Rayleigh–Debye–Gans polydisperse fractal aggregate theories to provide two estimates of particle size. An increase in overall particle sizes with pressure is found, consistent with similar one-dimensional studies. Particle diameters in the annulus of the flame increase faster with pressure than those on centerline. Contrary to previous studies, the dependence of particle size on pressure was found to taper off between 8 and 12 atm, with little observed growth beyond 12 atm. The measurements provide additional data for one of the International Sooting Flame (ISF) workshop’s target pressurized flames.

Evaluation of the sooting properties of real fuels and their commonly used surrogates in a laminar co-flow diffusion flame

In an effort to help determine the fidelity of simple surrogate fuels to represent real fuel chemistry in computational fluid dynamic simulations of engines, quantitative two-dimensional soot volume fraction measurements were made in a laminar coflow methane–air diffusion flame seeded with approximately 2200ppm of real and surrogate fuels. A combined laser extinction and laser-induced incandescence (LII) method was used to measure soot volume fraction. Additionally, soot particles were thermophoretically sampled from the flame and soot morphology data were collected with a transmission electron microscope (TEM). Vaporized liquid fuels were seeded at low concentrations to maintain constant thermodynamic conditions for each experiment. In all, 14 different fuels were investigated, including: three real fuels (gasoline, diesel, and jet fuel), two alkanes, and a variety of simple surrogate fuels. A toluene reference fuel (TRF) (30% aromatics) and gasoline (28% aromatics) were found to have similar soot volume fractions and soot morphology. The addition of toluene to the long-straight chain of n-tetradecane in similar concentrations (30vol.%) as the aromatic concentration of diesel (31.1vol.%) resulted in soot volume fractions that were very similar, although the primary particle size and mass-weighted radius of gyration were both smaller for the surrogate than for the conventional diesel fuel. Finally, the jet-fuel surrogate tested was found to have a lower sooting tendency than the jet-A fuel despite the jet-A fuel having a lower concentration of aromatics than the surrogate. Soot morphology between jet-A and the jet-fuel surrogate were the same within experimental uncertainty. The current work provides an experimental dataset for validation of fuel-surrogate chemistry and soot models.

Soot volume fraction fields in unsteady axis-symmetric flames by continuous laser extinction technique

A Laser Extinction Method has been set up to provide two-dimensional soot volume fraction field time history at a tunable frequency up to 70 Hz inside an axis-symmetric diffusion flame experiencing slow unsteady phenomena preserving the symmetry. The use of a continuous wave laser as the light source enables this repetition rate, which is an incremental advance in the laser extinction technique. The technique is shown to allow a fine description of the soot volume fraction field in a flickering flame exhibiting a 12.6 Hz flickering phenomenon. Within this range of repetition rate, the technique and its subsequent post-processing require neither any method for time-domain reconstruction nor any correction for energy intrusion. Possibly complemented by such a reconstruction method, the technique should support further soot volume fraction database in oscillating flames that exhibit characteristic times relevant to the current efforts in the validation of soot processes modeling.

Study on Soot Formation in Spray Flames (Effect of Droplet Size Distribution)

Effect of the droplet size distribution of fuel spray on the soot formation process in spray flames is investigated. The vibratory orifice atomizer is used to control the droplet size distribution of fuel spray. Two-dimensional soot volume fraction and primary soot particle size are measured by the Time-Resolved Laser Induced Incandescence (Time-Resolved LII) method and direct numerical simulation. The relationship between spatial distribution of fuel droplets and soot formation area are observed by the LII method and Interferometric Laser Imaging Droplet Sizing (ILIDS) method. The results show that flame structures of spray flame and soot formation process are affected critically by the mean droplet size of fuel spray. The mean soot formation area is increased by increase of the mean droplet size of fuel spray. The primary soot particles become large at around the edge of soot formation area. This trend mainly occurs from surface growth of soot particles. The group of some droplets and its vapor forms the soot formation area, and the large droplet of liquid fuel deforms shape of the soot formation area.

1839 対向流噴霧火炎中の燃料噴霧特性とすす生成に関する研究(J15-2 噴霧による混合気形成および燃焼(2),J15 噴霧による混合気形成および燃焼)

Effect of the droplet size distribution of fuel spray on the soot formation process in spray flames is investigated. The vibratory orifice atomizer is used to control the droplet size distribution of fuel spray. Two-dimensional soot volume fraction, primary soot particle size and relationship between spatial distribution of fuel droplets and soot formation area are observed by optical measurements. The results show that flame structures of spray flame and soot formation process are affected critically by the mean droplet size of fuel spray. The mean soot formation area is increased by increase of the mean droplet size of fuel spray. The primary soot particles become large at around the edge of soot formation area. This trend occurs from coagulation and surface growth of soot particles. The group of some droplets and its vapor forms the soot formation area, and the large droplet of liquid fuel deforms shape of the soot formation area.

The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O 2+(CO 2, N 2, and [Ar+CO 2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.

98/02301 A study on final volatile yield of pulverized coal pyrolysis with thermobalance

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O2+(CO2, N2, and [Ar+CO2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.

98/02295 Soot microstructure in steady and flickering laminar methane/air diffusion flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O2+(CO2, N2, and [Ar+CO2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.