Increase In Soot Volume Fraction Research Articles

Effects of oxygen addition to the fuel stream on the soot formation in an ethylene coflow partially premixed flame based on the orthogonal decoupling method

In this work, an orthogonal decoupling method based on the orthogonal design and decoupling method was used to simulate the effects of adding oxygen (O2) to the fuel stream on the soot formation characteristics in an ethylene (C2H4) coflow partially premixed flame. The dilution effect, thermal effect and chemical effect of the added O2, as well as the interaction between these effects were decoupled and analyzed. Results show that the peak soot volume fraction (SVF) in the flame decreases first and then increases as the O2 mole fraction gradually increases to 30%, and is lowest at an addition of 20%. The orthogonal decoupling calculations were performed for two O2 concentration ranges of 5%–10% and 25%–30%, and the differences on soot formation at these two ranges were found to be dominated by the chemical effects of O2 through the extremum difference analysis of the calculated peak SVF, number density of primary particle and particle aggregation degree. The chemical effect of O2 inhibits the formation of soot at an O2 addition of 5%–10%, while promotes the formation of soot at an O2 addition of 25%–30%, by affecting the inception, hydrogen-abstraction-carbon-addition (HACA) reaction, and polycyclic aromatic hydrocarbon (PAH) condensation rates of soot formation. When the O2 concentration is 25%–30%, the chemical effect of O2 promotes the soot surface growth rate and therefore increases the soot particle size, which in turn retards the complete oxidation process, leading to an increase in SVF.

On the soot formation characteristics of an ammonia-ethylene counterflow diffusion flame based on the orthogonal decoupling method

Ammonia is regarded to be one of the most competitive fuels for achieving carbon neutral due to its carbon-free properties. In this work, the effects of adding ammonia (NH3) to the fuel stream of an ethylene counterflow diffusion flame on the soot formation characteristics were numerically investigated. The orthogonal decoupling method was used to completely separate the dilution, chemical and thermal effects of the added NH3 by introducing two different virtual NH3. The influence of these effects and their interactions were explored by the range analysis, respectively. The results show that the soot volume fraction (SVF) of the flame decreases under the dilution, chemical and thermal effects of NH3, with the dominated effect being the chemical effect, followed by the dilution effect, and the least is the thermal effect. Besides, the chemical-thermal interaction also has an important impact on the SVF. The increase of SVF is dominated by the HACA surface growth, while the decrease of that is dominated by the OH oxidation. The HACA surface growth reaction is inhibited under the three effects of NH3 addition mainly due to a lower concentrations of C2H2 and H radical. The number density of soot increases under the chemical effect, while decreases under the dilution effect. The mole fractions of polycyclic aromatic hydrocarbons (PAHs) reduces under these three effects, leading to a lower inception rate, which in turn has a significant effect on the number density of soot.

Special features of soot formation in a standard premixed ethylene/air flame diluted with DME

This paper presents the experimental and calculated results of the investigation of soot formation in a standard premixed ethylene/air flame with the addition of 15–100% dimethyl ether. The soot volume fraction was measured using the laser light extinction method at a wavelength of 520 nm. The flame temperature versus height above the burner was measured using standard Pt-Rh thermocouples. It was experimentally shown that the addition of 15% dimethyl ether to ethylene/air flame resulted in a 20% increase of soot volume fraction. The further replacement of ethylene by 60% dimethyl ether decreased the soot volume fraction at 5 fold relatively to a pure ethylene/air flame. Kinetic modeling of soot volume fraction growth with height above the burner was carried out based on the kinetic mechanisms developed by CREСK group (http://creckmodeling.chem.polimi.it/). The calculations were carried out using open software package OpenSMOKE ++, which includes the modeling of gas-dynamic processes in the premixed laminar flame reactor. Good agreement between the experimental and calculated data was obtained. The analysis of kinetic mechanism used in this study allowed determining the peculiarity of kinetic pathways of benzene formation, which is proposed as an initial substance for soot precursor. It was found, that increase of soot concentration at 15% dimethyl ether addition to the ethylene/air flame is caused by intensification of channel of benzene formation via C3H3 recombination, which additionally produced from C3H4-p (propyne). In turn, the increase of C3H4-p concentration occurred due to reactions of acetylene with CH3 radical formed during DME decomposition. These results obtained in a standard ethylene/air flame could be used for development and validation of various kinetic mechanisms of soot formation.

Revealing the competitive relationship between soot formation and chemiluminescence

This paper reveals the competitive relationship between soot formation and chemiluminescence. Soot volume fraction (SVF) and chemiluminescence are measured in the experiment. A mechanism coupled with the soot model and OH*/CH*/C2*/CO2* reactions is constructed for kinetic analysis. By varying oxygen concentrations and CO2 additions, it is experimentally observed that chemiluminescence is inhibited in the soot-laden regions/flames rather than obscured by the soot radiation, implying that soot formation and chemiluminescence are in competition. Rate of production analysis shows that soot formation and chemiluminescence compete for fuel pyrolysis products (C2H3, C2H2, etc.) because their reaction paths are opposite. CH*, C2*, and OH* decrease with the increase of SVF because the pyrolysis products that can be used for chemiluminescence participate in soot formation. The rise in oxygen concentration also promotes the production of C4H4 from C2H. CO2 addition enhances the chemiluminescence by forming OH and CO to accelerate CH* and CO2* formation and consuming H to suppress the consumption of pyrolysis products in soot formation/inception.

Effect of Toluene Content on Soot Particle Morphology and Evolution in Coflow Diffusion Flames of Diesel Surrogate Fuels

ABSTRACT Toluene is a representative of aromatics in diesel surrogate fuels, and its influence mechanism on the formation and evolution of soot particles has attracted much attention. In this work, the evolution of the microscopic morphology of soot particles in n-heptane laminar diffusion flame doped with toluene of 0% to 30% was investigated experimentally, and the effects of toluene ratio on particle dynamics such as inception, surface growth and oxidation were analyzed numerically. The experimental results showed that as the toluene ratio increases, the primary particle size increases, and the aggregates first grows from short chains to long chains, then develops to multi-branched chains, which results in Df first decreasing and then increasing, reaching the minimum at 10% toluene. The numerical results showed that the peak inception rate increases linearly with the increase of toluene ratio. The peak rate of HACA surface growth increases slightly due to the increase of particles that could provide active sites and reaches the maximum at ratio of 10% toluene, then decreases slightly due to the limited production of H and C2H2. On the other hand, the PAHs condensation rate increases monotonously, which makes the aggregates more likely to form a loose multi-branched chain structure, and in the subsequent oxidation process, the weak points of the connections between the particles are more likely to be oxidized, resulting in the fracture and fragmentation of the aggregates. For particles oxidation, toluene blending has an opposite effect on soot oxidation by O2 and OH. As the toluene ratio increases, the O2 oxidation rate increases gradually due to the increase of soot volume fraction, while the OH oxidation rate decreases due to the inhibited production of OH. However, the total oxidation rate of particles increases with the increase of toluene ratio.

Effects of oxygen depletion on soot production, emission and radiative heat transfer in opposed-flow flame spreading over insulated wire in microgravity

The main objective of this article is to investigate experimentally and numerically the effects of reduced-oxygen contents on the soot production and emission from solid fuels in microgravity, which constitute an important issue in terms of fire safety for manned space missions. Due to its convenience for the implementation of soot-related optical diagnostics, the configuration of a flame spreading in an opposed flow over thin nickel chromium (NiCr) wires coated by low density PolyEthylene (LDPE) is considered. Experiments are conducted at a pressure of 101.3 kPa and an oxidizer velocity of 150 mm/s. The oxygen mole fraction in the oxidizer, XO2, is varied from 18% to 21% by nitrogen dilution of air. The modeling strategy lies on a surrogate fuel to mimic the combustion of LDPE by preserving its stoichiometry and the laminar smoke-point (LSP) flame height. The numerical model considers a detailed chemistry, a two-equation soot production model involving laminar smoke point (LSP)-based soot formation rates and oxidation by OH and O2, a radiation model coupling the Full-Spectrum correlated-k method with the finite volume method, and a simple degradation model for LDPE. Based on experimental evidence, the soot formation rate is scaled by the adiabatic flame temperature to account for thermal effects due to variation in XO2. The model reproduces quantitatively the increase in flame size, residence time, and soot volume fraction observed experimentally as XO2 is enhanced as well as the transition from a non-smoking to a smoking flame, which occurs for XO2 between 19% and 20%. The increase in soot volume fraction results of a combined enhancement in both residence time, owing to an increase in the fuel mass flow rate, and soot formation rate due to higher temperature in the soot formation region. The radiant fraction increases significantly with XO2 from about 17% for XO2=18% to about 36% for XO2= 21%. This increase in radiative losses is accompanied by a reduction of the temperature in the soot oxidation region. Therefore, for increasing XO2, the soot oxidation process is governed by a competition between oxygen-enhanced conditions that promote the formation of soot oxidizing species and the increase in radiative losses that dampens their formation. For the present flames, the first mechanism prevails as XO2 increases from 18% to 19% whereas the second dominates as XO2 is further increased, leading to smoking flames as actually observed for XO2=20 and 21%. The radiant fraction at the smoke-point transition and the soot oxidation freezing temperature are in line with those reported at normal gravity. Finally, model results show that, whatever XO2, the contribution of radiation to the heating process is negligible ahead of the pyrolysis front and is largely overcome by surface radiative losses along the pyrolysis region.

Evolution of the soot particle size distribution along the centreline of an n-heptane/toluene co-flow diffusion flame

A newly developed experimental set-up for studying liquid hydrocarbon combustion in the well-established Yale burner was used to investigate the correlation between fuel composition and its sooting propensity. Soot particle size distributions (PSDs) and flame temperatures along the centreline of an n-heptane/toluene co-flow diffusion flame are reported. The results are compared to soot temperature and volume fraction profiles obtained using colour ratio pyrometry. The addition of toluene (0, 5, 10, and 15 mol%) to heptane moved soot inception to lower heights above the burner (HAB). The earlier inception extended the soot growth zone in the toluene-laden flames, leading to larger soot primary and agglomerate particles. Toluene addition had little influence on the maximum soot number density, indicating that the observed increase in soot volume fraction can mainly be attributed to the increase in particle size. The reported PSDs inside a vapour-fed diffusion flame are the first of their kind and provide a comprehensive dataset for future studies of combustion chemistry and soot particle models.

The Role of Methyl Radical in Soot Formation

ABSTRACT It was noted a significant increase in soot formation with the addition of methane in the pyrolysis of acetylene in shock tubes. Qualitative kinetic analysis revealed an increase of propargyl formation due to methyl/methylene and acetylene recombination. Here, we study the role of methyl radicals in the soot formation process more carefully. Mixtures of acetylene with various sources of methyl radicals – methane, diacetyl and dimethyl ether (DME) – were studied under shock tube pyrolysis conditions over a temperature range of 1650–2250 K and a pressure range of 3.5–4.5 bar. Laser light extinction was used for soot volume fraction and induction time of the particle inception measurements. A more than threefold increase in soot volume fraction with the addition of 1% of methane and 2% of acetylene diluted in argon was observed. Additions of diacetyl and DME resulted in less significant increases in soot volume fraction. All additions of methane, diacetyl and DME reduced the induction time of particle inception. The numerical modeling of the initial gas phase part of the process confirmed our hypothesis that in all cases the increase in the soot yield was proportional to the appearance of an additional amount of CH3 radicals, which promoted the formation of propargyl, benzene and pyrene.

Influence of methane addition on soot formation in pyrolysis of acetylene

Time-resolved laser-induced incandescence for particle sizing and laser light extinction for soot volume fraction was applied simultaneously to study the influence of methane addition on soot formation in acetylene pyrolysis. Three series of the experiments with initial mixtures of 2% C2H2 + Ar, 1% CH4 + Ar and 2% C2H2 + 0.5/1/2% CH4 + Ar in the temperature range of 1600–2300 K and the pressure range of 4–5 bar behind reflected shock waves were carried out. The kinetic characteristic of the soot formation process—the induction time of soot particle inception as well as the temperature dependences of final values of soot volume fraction and particle sizes have been determined and analyzed. An essential increase of soot volume fraction, particle sizes and a decrease of induction time of soot inception at methane addition to acetylene were observed. The gas phase kinetic modeling of the investigated processes up to the soot nuclei precursors formation has been performed. Analysis of gas kinetic stages of acetylene decomposition with methane addition has demonstrated the significant increase of the rates of pyrene formation followed by PAH growth due to effective propargyl C3H3 formation.

Effect of soot self-absorption on color-ratio pyrometry in laminar coflow diffusion flames.

This Letter reports on the effect of self-absorption on measured temperature for color-ratio soot pyrometry with a color camera. A series of increasingly nitrogen diluted atmospheric pressure ethylene/air laminar coflow diffusion flames are studied, providing flames with different optical path lengths, soot loading, and soot optical properties. Numerical calculations are used to simulate the change in collected flame emission signal with and without light attenuation using experimentally measured maps of the soot absorption coefficient. This parameter implicitly contains information about soot volume fraction and soot optical properties. The ratio of these calculations is used to correct the raw color-channel signals, resulting in temperature maps with improved accuracy. The change in calculated temperature varies spatially within each flame, with the maximum correction quantified to be 22K for a flame with a maximum optical depth of 0.31. This correction is as much as 42 and 75K for simulated flames with the same optical properties, structure, and a factor of two and five increase in soot volume fraction, respectively.

Effects of hydrodynamics and mixing on soot formation and growth in laminar coflow diffusion flames at elevated pressures

The formation, growth, and oxidation of soot are studied in a set of laminar coflow diffusion flames at pressures ranging from 1 to 8 atm. The modeling approach combines detailed finite rate chemical kinetics mechanisms that model the formation of Polycyclic Aromatic Hydrocarbon (PAH) species up to pyrene, and a bivariate method of moments that describes soot particles and aggregates by their volume and surface area. The spatial distribution of soot observed experimentally and that predicted numerically are in good qualitative agreement with the peak soot volume fraction located at the flame tip and soot appearing on the flame wings and closer to the nozzle as pressure increases. A detailed analysis of the effect of hydrodynamics and mixing on soot formation is presented. We show that the scalar dissipation rate is lower for the higher pressure flames, promoting the formation of PAH species and soot. Thus, the observed increase in soot volume fraction across flames with increasing pressure is not due solely to mixture density and kinetics effects, rather is affected by hydrodynamics and mixing processes also. Similarly, our results indicate that the decrease in the scalar dissipation rate contribute to changing the location where soot forms in the flame, with soot formation occurring closer to the nozzle and outward on the flame’s wings as pressure increases. Radiative heat losses are found to lower the flame temperature, inducing a reduction of the PAH species and associated rates of soot formation. However, heat losses are responsible for a slightly longer flame, which allows for more soot. The overall effect is a modest variation of soot volume fraction if radiation is included.

Simultaneous measurements of acetylene and soot during the pyrolysis of ethylene and benzene in a shock tube

Acetylene is one of the most important precursors of soot and contributes to soot growth by the hydrogen-abstraction acetylene-addition (HACA) mechanism. In this work, we undertake time-resolved simultaneous measurements of acetylene and soot behind reflected shock waves at temperatures of 1600–2200K and pressures of 3–5bar. Acetylene mole fraction time-histories are measured from the absorption of a quantum-cascade laser operating around 13.6µm. The soot volume fraction, particle size and number densities are calculated from the extinction and scattering of a cw Nd:Yag laser at 532nm. Acetylene and soot are generated from the pyrolysis of 1% benzene in argon, 2.35% ethylene in argon, and binary mixtures of ethylene with propane/methane in argon. We note that acetylene time-histories exhibit a two-stage growth during the pyrolysis of benzene, which can be correlated to the initial rapid increase of soot volume fraction and a later plateauing. In comparison to ethylene pyrolysis, the pyrolysis of benzene results in larger values of the soot volume fraction, particle diameter and number density. We compare the measured data against the values simulated using the method-of-moments routine in Chemkin-Pro and a detailed PAH mechanism based on KM2 [1] and AramcoMech 1.3 [2]. Large discrepancies are observed between the measured and predicted values of the soot parameters. The data obtained from our experiments may assist future validation and development of soot mechanisms.

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.

Pressure effect on soot formation in turbulent diffusion flames

Soot formation in a methane–air turbulent jet diffusion flame is investigated numerically using a semi-empirical model. The temperature, density and species (the soot precursor C 2H 2) fields are calculated using detailed chemical kinetic mechanism based on the flamelet library approach. The influence of pressure on the soot formation and the behavior of the semi-empirical model in different flame situations are investigated. It is found that the flame shape and the flame temperature can be well predicted by the flamelet library approach. The calculated soot yield is mostly sensitive to the soot surface growth rate and the increase of pressure. The increase of pressure leads to the increase of soot surface growth rate and therefore to the increase of soot volume fraction. By adjusting a model constant in the soot surface growth rate, the soot emissions in both pressure p=1 atm and p=3 atm are properly simulated by the current semi-empirical soot model.

Soot Study in Laminar Diffusion Flames at Elevated Pressure Using Two-Color Pyrometry and Abel Inversion.

Soot temperatures were measured using a two-color pyrometry and an Abel inversion technique in co-flow diffusion flames at pressures up to 0.4 MPa. The measured soot temperatures are analyzed along with the soot volume fractions. The oxidation of soot particles ceased at the temperature close to 1400 K. The addition of small amount of air into ethylene does not change the temperature in sootformation regions. The added propane (up to χC3H8=0.27) increases the soot volume fraction without changing soot temperatures in ethylene-propane mixture flames. These results support that the enhanced C3H3 recombination reaction for incipient ring formation is the major contribution for the increase of soot volume fraction and the synergistic effect. The radial profiles of soot temperature are calculated from the line-of-sight soot emission data through an Abel inversion procedure. The effective soot particle brightness temperature is not simply weighted toward the regions of highest soot loading.

Sooting Counterflow Diffusion Flames with Varying Oxygen Index

Abstract Varying oxygen concentrations was found to have pronounced effects on the sooting characteristics of diffusion flames. Ethylene and propane flames were stabilized around a porous ceramic cylindrical burner mounted in a vertically oriented, low turbulence wind tunnel. Laser light scattering and extinction, laser Doppler velocimetry and thermocouple temperature measure- ments were obtained along the forward stagnation line. Measurements of soot particle size, number density and volume fraction showed that with an increasing oxygen index the soot volume fractions in both ethylene and propane flames increased at all locations along the stagnation stream- line. For ethylene flames (when the oxygen index increased from 0.18 to 0.28). the maximum soot volume fraction increased by almost an order of magnitude. Propane was found to produce about one half the volume of soot of an ethylene flame at the same oxygen index. The increase in soot volume fraction with increasing oxygen index was attributed to bot...