F. Xu

Soot formation in laminar premixed ethylene/air flames at atmospheric pressure☆

Abstract Soot formation was studied within laminar premixed ethylene/air flames (C/O ratios of 0.78–0.98) stabilized on a flat-flame burner operating at atmospheric pressure. Measurements included soot volume fractions by both laser extinction and gravimetric methods, temperatures by multiline emission, soot structure by thermophoretic sampling and transmission electron microscopy, major gas species concentrations by sampling and gas chromatography, concentrations of condensable hydrocarbons by gravimetric sampling, and velocities by laser velocimetry. These data were used to find soot surface growth rates and primary soot particle nucleation rates along the axes of the flames. Present measurements of soot surface growth rates were correlated successfully by predictions based on typical hydrogen-abstraction/carbon-addition (HACA) mechanisms of Frenklach and co-workers and Colket and Hall. These results suggest that reduced soot surface growth rates with increasing residence time seen in the present and other similar flames were mainly caused by reduced rates of surface activation due to reduced H atom concentrations as temperatures decrease as a result of radiative heat losses. Primary soot particle nucleation rates exhibited variations with temperature and acetylene concentrations that were similar to recent observations for diffusion flames; however, nucleation rates in the premixed flames were significantly lower than in the diffusion flames for reasons that still must be explained. Finally, predictions of yields of major gas species based on mechanisms from both Frenklach and co-workers and Leung and Lindstedt were in good agreement with present measurements and suggest that H atom concentrations (relevant to HACA mechanisms) approximate estimates based on local thermodynamic equilibrium in the present flames.

Soot surface oxidation in hydrocarbon/air diffusion flames at atmospheric pressure

Abstract Soot surface oxidation was studied experimentally in laminar hydrocarbon/air diffusion flames at atmospheric pressure. Measurements were carried out along the axes of round fuel jets burning in co-flowing dry air considering acetylene-nitrogen, ethylene, propylene-nitrogen, propane and acetylene-benzene-nitrogen in the fuel stream. Measurements were limited to the initial stages of soot oxidation (carbon consumption less than 70%) where soot oxidation occurs at the surface of primary soot particles. The following properties were measured as a function of distance above the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of major stable gas species (N2, H2O, H2, O2, CO, CO2, CH4, C2H2, C2H4, C2H6, C3H6, C3H8, and C6H6) by sampling and gas chromatography, concentrations of some radical species (H, OH, O) by deconvoluted Li/LiOH atomic absorption and flow velocities by laser velocimetry. For present test conditions, it was found that soot surface oxidation rates were not affected by fuel type, that direct rates of soot surface oxidation by O2 estimated from Nagle and Strickland-Constable (1962) were small compared to observed soot surface oxidation rates because soot surface oxidation was completed near the flame sheet where O2 concentrations were less than 3% by volume, and that soot surface oxidation rates were described by the OH soot surface oxidation mechanism with a collision efficiency of 0.14 and an uncertainty (95% confidence) of ±0.04 when allowing for direct soot surface oxidation by O2, which is in reasonably good agreement with earlier observations of soot surface oxidation rates in both premixed and diffusion flames at atmospheric pressure.

Soot Formation in Laminar Premixed Methane/Oxygen Flames at Atmospheric Pressure

Abstract Flame structure and soot formation were studied within soot-containing laminar premixed methane/oxygen flames at atmospheric pressure. The following measurements were made: soot volume fractions by laser extinction, soot temperatures by multiline emission, gas temperatures (where soot was absent) by corrected fine-wire thermocouples, soot structure by thermophoretic sampling and transmission electron microscope (TEM), major gas species concentrations by sampling and gas chromatography, and gas velocities by laser velocimetry. Present measurements of gas species concentrations were in reasonably good agreement with earlier measurements due to Ramer et al. as well as predictions based on the detailed mechanisms of Frenklach and co-workers and Leung and Lindstedt; the predictions also suggest that H atom concentrations are in local thermodynamic equilibrium throughout the soot formation region. Using this information, it was found that measured soot surface growth rates could be correlated successfully by predictions based on the hydrogen-abstraction/carbon-addition (HACA) mechanisms of both Frenklach and co-workers and Colket and Hall, extending an earlier assessment of these mechanisms for premixed ethylene/air flames to conditions having larger H/C ratios and acetylene concentrations. Measured primary soot particle nucleation rates were somewhat lower than the earlier observations for laminar premixed ethylene/air flames and were significantly lower than corresponding rates in laminar diffusion flames, for reasons that still must be explained.

Soot formation in laminar acetylene/air diffusion flames at atmospheric pressure

The flame structure and soot-formation (soot nucleation and growth) properties of axisymmetric laminar coflowing jet diffusion flames were studied experimentally. Test conditions involved acetylene-nitrogen jets burning in coflowing air at atmospheric pressure. Measurements were limited to the axes of the flames and included soot concentrations, soot temperatures, soot structure, major gas species concentrations, radical species (H, OH, and O) concentrations, and gas velocities. The results show that as distance increases along the axes of the flames, detectable soot formation begins when significant H concentrations are present, and ends when acetylene concentrations become small. Species potentially associated with soot oxidation—O2, CO2, H2O, O, and OH—are present throughout the soot-formation region so that soot formation and oxidation proceed at the same time. Strong rates of soot growth compared to soot nucleation early in the soot-formation process, combined with increased rates of soot nucleation and oxidation as soot formation proceeds, causes primary soot particle diameters to reach a maximum relatively early in the soot-formation process. Aggregation of primary soot particles proceeds, however, until the final stages of soot oxidation. Present measurements of soot growth (corrected for soot oxidation) in laminar diffusion flames were consistent with earlier measurements of soot growth in laminar premixed flames and exhibited encouraging agreement with existing hydrogen-abstraction/carbon-addition (HACA) soot growth mechanisms in the literature that were developed based on measurements within laminar premixed flames. Measured primary soot particle nucleation rates in the present laminar diffusion flames also were consistent with corresponding rates measured in laminar premixed flames and yielded a crude correlation in terms of acetylene and H concentrations and the temperature.

Structure of the soot growth region of laminar premixed methane/oxygen flames

The structure of the soot growth region of laminar premixed methane/oxygen flames (fuel-equivalence ratios of 1.60-2.77) was studied both experimentally and computationally. Measurements were carried out in flames stabilized on a flat flame burner operated at standard temperature and pressure, and included velocities by laser velocimetry, soot volume fractions by laser extinction, soot temperatures by multiline emission, gas temperatures (where soot was absent) by corrected fine-wire thermocouples, major gas species concentrations by sampling and gas chromatography, and hydrogen atom concentrations by the Li/LiOH technique in conjunction with atomic absorption to find the proportion of free lithium in the flames. The measured concentrations of major gas species were in reasonably good agreement with predictions based on the detailed mechanisms of Leung and Lindstedt, and Frenklach and coworkers. The measurements also confirmed predictions of both these mechanisms that H-atom concentrations are in local thermodynamic equilibrium throughout the soot growth region even through the concentrations of major gas species are not. Thus, present findings support recent evaluations of the hydrogen-abstraction/carbon-addition (HACA) soot growth mechanism in similar flames, where the approximation that H-atom concentrations were in local thermodynamic equilibrium was adopted, based on predictions using the two mechanisms, due to the absence of direct H-atom concentration measurements.

Soot Surface Growth in Laminar Hydrocarbon/Air Diffusion Flames

Thestructureandsootsurfacegrowthpropertiesofround, laminar, jetdiffusione ameswerestudied experimentally. Measurements were made along the axes of ethylene-, propylene-, propane-, and acetylene ‐benzene-fueled e ames burning in coe owing air at atmospheric pressure with the reactants at normal temperature. The measurements included soot structure, soot concentrations, soot temperatures, major gas species concentrations, some radial species (H, OH, and O) concentrations, and gas velocities. The results suggested that soot surface growth involved decomposition of theoriginal fuel to form acetyleneand H, which werethe main reactantsforsoot surface growth, and that the main effect of the parent fuel on soot surface growth involved its yield of acetylene and H for present test conditions. Measurements of soot surface growth rates (corrected for soot surface oxidation ) in laminar jet diffusion e ames were consistent with earlier measurements of soot surface growth rates in laminar premixed e ames and exhibited good agreement with existing hydrogen-abstraction/carbon-addition soot surface growth mechanisms in the literature with steric factors in these mechanisms having values on the order of unity, as anticipated.

SOOT SURFACE GROWTH IN LAMINAR HYDROCARBON/AIR DIFFUSION FLAMES AT ATMOSPHERIC PRESSURE

The soot surface growth and flame structure properties of round laminar coflowing jet diffusion flames were studied experimentally. Test conditions involved ethylene-, propyleneand propane-fueled diffusion flames burning in coflowing air at atmospheric pressure. Measurements were limited to the axes of the flames and included soot concentrations, soot temperatures, soot structure, major gas species concentrations, radical (H,OH,O) species concentrations, and gas velocities. The results show that fuel decomposition yields significant acetylene concentrations at fuel-rich conditions, that significant soot formation begins when significant H-radical concentrations are present, and soot formation ends when acetylene concentrations become small. Hydrogen, OH and O radicals are present throughout the soot formation region so that soot formation and oxidation proceed at the same time. Strong rates of soot growth compared to rates of soot nucleation early in the soot formation process, combined with increased rates of soot nucleation and oxidation as soot formation proceeds, causes primary soot particle diameters to reach a maximum relatively early in the soot formation process. Present measurements in diffusion flames exhibit encouraging agreement with existing Hydrogen-Abstraction/Carbon-Addition (HACA) growth mechanisms, with earlier soot growth measurements in acetylene-fueled diffusion flames, and with earlier soot growth measurements in premixed flames fueled with a variety of hydrocarbons.

Soot Formation and Oxidation in Laminar Flames

The flame and soot structure, including soot primary particle nucleation, surface growth and oxidation properties, of 6 premixed and 20 diffusion flames were studied experimentally for various fuels, at temperatures of 1400-2350 K and at pressures of 13-811 kPa (0.1-8.0 atm). Measurements were made along the axes of flames with the reactants at normal temperature (300 K). The following properties were measured as a function of distance from the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using transmission electron microscopy (TEM), concentrations of major stable gas species by isokinetic sampling and gas chromatography, concentrations of radial species (H, OH, O) by deconvoluted Li/LiOH atomic absorption, and flow velocities by laser velocimetry. The measurements were analyzed to determine local soot surface growth, oxidation and nucleation rates, as well as local flame properties that are thought to affect these rates. The measurements of soot surface growth rates (corrected for soot surface oxidation) were consistent with the measurements in laminar premixed and diffusion flames involving a variety of hydrocarbons at variable pressure. In addition, the growth rates from all the available flames were in good agreement with each other and with existing hydrogen- abstraction/carbon-addition (HACA) soot surface growth mechanisms available in the literature yielding values of steric factors on the order of unity, as expected. Measurements of early soot surface oxidation rates (corrected for soot surface growth and prior to consumption of 70 % of the maximum mass of the primary soot particles) in laminar diffusion flames were consistent with all flames regardless of flame types, fuels, temperatures and pressures. The soot surface oxidation rates from all available flames could be explained by reaction with OH, having a collision efficiency of 0.12, and supplemented to only a minor degree by direct soot surface oxidation by O2.

Effects of Flow Properties on Soot Formation and Oxidation Rates in Flame Environments

Soot and flame structure were measured in a variety of round laminar premixed and nonpremixed (diffusion) jet flames fueled with hydrocarbons to study the properties of soot formation (primary soot particle nucleation and surface growth) and surface oxidation. The following properties were measured along the axes of the flames: soot concentrations by laser extinction, soot temperatures by multiline emission, soot structure by thermophoretic sampling and electron microscopy, concentrations of major gas species by sampling and gas chromatography, concentrations of some radical species (H, OH and O) by atomic absorption and flow velocities by laser velocimetry. These measurements were analyzed to yield local primary soot particle surface growth and surface oxidation rates, as well as primary soot particle nucleation rates. It was found that local soot surface growth rates could be correlated effectively using Hydrogen-Abstraction/CarbonAddition (HACA) soot surface growth rate mechanisms in the literature; that local soot surface oxidation rates could be correlated effectively using OH/O2 mechanisms in the literature; and that local soot particle nucleation rates (observable using electron microscopy techniques) could be correlated by assuming that the rate of development of large PAH molecules by the HACA mechanism controlled nucleation rates. These results were found considering a broad range of conditions in laminar flame environments: premixed and diffusion flames, various hydrocarbon fuel types (methane, acetylene, ethylene, propylene, propane and benzene), gas temperatures of 1500-2350 K and pressures of 10-100 kPa. NOMENCLATURE