Structure of laminar sooting inverse diffusion flames

The flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene-air and methane-air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and airflow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH, and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAHs appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAHs and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAHs are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAHs.     

Effects of fuel droplet size on soot formation in spray flames formed in a laminar counterflow

The effects of fuel droplet size on soot formation in spray flames formed in a laminar counterflow are investigated experimentally and numerically. Sauter mean diameter (SMD) of quasi-monodispersed fuel spray (n-decane) is carefully controlled independently from the other spray characteristics using a frequency-tunable vibratory orifice atomizer, and the two-dimensional spatial distributions of soot volume fraction and soot particle size are measured by laser induced incandescence (LII) and time resolved LII (TIRE-LII), respectively. In addition, the soot formation processes are examined in detail by a two-dimensional direct numerical simulation (DNS) employing a kinetically based soot model with flamelet model. The results show that the soot formation area and location are strongly affected by the SMD of the fuel spray. As the SMD of the fuel spray increases, the average soot formation area expands, whereas local suppression of soot formation is instantaneously observed in the spray flames because of the appearance of groups of unburned droplets. The size of soot particles tend to be larger in the outer part of the soot formation area compared to soot in the inner part. This is because the surface growth of soot particles markedly proceeds compared to the coagulation and oxidation.     

Experimental and kinetic studies of soot formation in methanol-gasoline coflow diffusion flames

Methanol has been considered to be a potential alternative fuel to reduce soot emissions from GDI engine. In order to fully understand the effect of methanol addition on soot formation, the 2-D distribution of soot volume fraction in methanol/gasoline laminar diffusion flames was measured quantitatively with two-color laser induced incandescence (TC-LII) technique. In addition, the Methanol-TRF-PAH mechanism is constructed and used to analyze the formation pathways of soot precursors based on the CHEMKIN PRO 0-D constant pressure reactor. In this experiment, the blending ratio of methanol/gasoline was set as M0/20/40/60/80. Considering the carbon content decreasing due to methanol addition, carbon mass flow rate was remained constant. The experimental results showed that methanol is able to decrease the soot significantly, while the effect of methanol on soot reduction is weakened with the increasing methanol ratio. Compared with pure gasoline, the average soot volume fraction in the M20, M40, M60, and M80 flames decrease by 48.2%, 70.4%, 83.8%, and 97.7%, and the peak soot volume fraction decrease by 41.5%, 64.1%, 75.8% and 91.8% respectively. There is little soot formation in the M80 flame, inferring the pure methanol hardly forms soot. The kinetic analysis showed that mole fraction of A1-A4 decrease with the increasing methanol ratio. For the toluene-containing fuel M0-M80, A1 is mainly formed by C₆H₅CH₃ + H = A1 + CH₃ and oxidized by A1 + OH = A1- + H₂O. A4 is mainly produced by C₆H₅CH₂ + C₉H₇ = A4 + 2H₂ and oxidized by H-abstraction reaction with H or OH radical. The major reaction pathways of A1 and A4 formation are consistent under different methanol blending ratios. The soot reduction as methanol added mainly attributes to aromatics dilution effect. In addition, the formation process of soot precursors is largely affected by chemical processes of OH, CH₃, HO₂ radicals.     

An experimental and numerical study of the effects of dimethyl ether addition to fuel on polycyclic aromatic hydrocarbon and soot formation in laminar coflow ethylene/air diffusion flames

The effects of dimethyl ether addition to fuel on the formation of polycyclic aromatic hydrocarbons and soot were investigated experimentally and numerically in a laminar coflow ethylene diffusion flame at atmospheric pressure. The relative concentrations of polycyclic aromatic hydrocarbon species and the relative soot volume fractions were measured using planar laser-induced fluorescence and two-dimensional laser-induced incandescence techniques, respectively. Experiments were conducted over the entire range of dimethyl ether addition from pure ethylene to pure dimethyl ether in the fuel stream. The total carbon mass flow rate was maintained constant when the fraction of DME in the fuel stream was varied. Numerical calculations of nine diffusion flames of different dimethyl ether fractions in the fuel stream were performed using a detailed reaction mechanism consisting of 151 species and 785 reactions and a sectional soot model including soot radiation, inception of nascent soot particle due to collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition mechanism, soot particle coagulation, and PAH surface condensation. The addition of a relatively small amount of dimethyl ether to ethylene was found experimentally to increase the concentrations of both polycyclic aromatic hydrocarbons and soot. The synergistic effect on polycyclic aromatic hydrocarbons persists over a wider range of dimethyl ether addition. The numerical results reproduce the synergistic effects of dimethyl ether addition to ethylene on both polycyclic aromatic hydrocarbons and soot, though the magnitude of soot volume fraction overshoot and the range of dimethyl ether addition associated with the synergistic effect of soot are less than those observed in the experiment. The synergistic effects of dimethyl ether addition to ethylene on many hydrocarbon species, including polycyclic aromatic ones, and soot can be fundamentally traced to the enhanced methyl concentration with the addition of dimethyl ether to ethylene. Contrary to previous findings, the pathways responsible for the synergistic effects of benzene, polycyclic aromatic hydrocarbons, and soot in the ethylene/dimethyl ether system are found to be primarily due to the cyclization of l-C₆H₆ and n-C₆H₇ and to a much lesser degree due to the interaction between C2 and C4 species for benzene formation, rather than the propargyl self-combination reaction route, though it is indeed the most important reaction for the formation of benzene.     

Measurements of sooting tendency in laminar diffusion flames of n-heptane at elevated pressure

This research focuses on the effects of an increasing pressure on the soot formation during combustion of vaporized liquid fuel. Therefore soot formation is measured in a laminar diffusion flame, with n-heptane as fuel, over a range of pressures from 1.0 to 3.0 bar. The soot volume fraction in the diffusion flames has been measured using Laser-Induced Incandescence (LII) calibrated by means of the Line Of Sight Attenuation (LOSA) technique. The values of the calibration factors between LII intensities and soot volume fraction from LOSA are slightly varied for different pressure. The integral soot volume fractions show power law dependence on pressures, being proportional to pⁿ, with n being 3.4 ± 0.3 in the pressure range of 1.0–3.0 bar.     

A numerical study of soot aggregate formation in a laminar coflow diffusion flame

Soot aggregate formation in a two-dimensional laminar coflow ethylene/air diffusion flame is studied with a pyrene-based soot model, a detailed sectional aerosol dynamics model, and a detailed radiation model. The chemical kinetic mechanism describes polycyclic aromatic hydrocarbon formation up to pyrene, the dimerization of which is assumed to lead to soot nucleation. The growth and oxidation of soot particles are characterized by the HACA surface mechanism and pyrene-soot surface condensation. The mass range of the solid soot phase is divided into thirty-five discrete sections and two equations are solved in each section to model the formation of the fractal-like soot aggregates. The coagulation model is improved by implementing the aggregate coagulation efficiency. Several physical processes that may cause sub-unitary aggregate coagulation efficiency are discussed. Their effects on aggregate structure are numerically investigated. The average number of primary soot particles per soot aggregate np is found to be a strong function of the aggregate coagulation efficiency. Compared to the available experimental data, np is well reproduced with a constant 20% aggregate coagulation efficiency. The predicted axial velocity, OH mole fraction, and C₂H₂ mole fraction are validated against experimental data in the literature. Reasonable agreements are obtained. Finally, a sensitivity study of the effects of particle coalescence on soot volume fraction and soot aggregate nanostructure is conducted using a coalescence cutoff diameter method.     

Different chemical effect of hydrogen addition on soot formation in laminar coflow methane and ethylene diffusion flames

Previous studies showed that adding hydrogen (H₂) can have an opposite chemical effect on soot formation: its chemical effect enhances and suppresses soot formation in methane (CH₄) and ethylene (C₂H₄) diffusion flames, respectively. Such opposite chemical effect of H₂ (CE-H₂) remains unresolved. The different CE-H₂ is studied numerically in the two laminar coflow diffusion flames. A detailed chemical mechanism with the addition of a chemically inert virtual species FH₂ is used to model the gas-phase combustion chemistry in this study. Particularly, a reaction pathway analysis was performed based on the numerical results to gain insights into how H₂ addition to fuel affects the pathways leading to the formation of benzene (A₁) in CH₄ and C₂H₄ flames. The numerical results show that the CE-H₂ in CH₄ diffusion flame to prompt soot formation is ascribed that the higher mole fraction of H atom promotes the formation of A₁ and Acetylene (C₂H₂) and leads to higher nucleation rate and eventually higher soot surface growth rate. In contrast, adding H₂ to C₂H₄ diffusion flames decreases soot nucleation and surface growth rate. The lower soot nucleation rate is due to the lower mole fractions of pyrene (A₄), while the lower soot surface growth rate is due to the lower mole fractions of H atom and C₂H₂, higher mole fraction of H₂ and lower soot nucleation rate. Furthermore, the CE-H₂ in C₂H₄ diffusion flames promotes the formation of A₁, but suppresses the formation of A₄.     

Soot formation and temperature field structure in laminar propane-air diffusion flames at elevated pressures

The effect of pressure on soot formation and the structure of the temperature field was studied in coflow propane-air laminar diffusion flames over the pressure range of 0.1 to 0.73 MPa in a high-pressure combustion chamber. The fuel flow rate was selected so that the soot was completely oxidized within the visible flame and the flame was stable at all pressures. Spectral soot emission was used to measure radially resolved soot volume fraction and soot temperature as a function of pressure. Additional soot volume fraction measurements were made at selected heights using line-of-sight light attenuation. Soot concentration values from these two techniques agreed to within 30% and both methods exhibited similar trends in the spatial distribution of soot concentration. Maximum line-of-sight soot concentration along the flame centerline scaled with pressure; the pressure exponent was about 1.4 for pressures between 0.2 and 0.73 MPa. Peak carbon conversion to soot, defined as the percentage of fuel carbon content converted to soot, also followed a power-law dependence on pressure, where the pressure exponent was near to unity for pressures between 0.2 and 0.73 MPa. Soot temperature measurements indicated that the overall temperatures decreased with increasing pressure; however, the temperature gradients increased with increasing pressure.     

On evolution of particle size distribution functions of incipient soot in premixed ethylene-oxygen-argon flames

The evolution of the soot particle size distribution function (PSDF) and particle morphology are studied for premixed ethylene-oxygen-argon flat flames at equivalence ratio ?=2.07 over the maximum flame temperature range of 1600-1900 K. Experiments were carried out using an in-situ probe sampling method in tandem with a scanning mobility particle sizer (SMPS), yielding the PSDF for various distances from the burner surface. The morphology of the particles was examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Within the particle size range that can be detected, the PSDF transitions from an apparent unimodal PSDF for high temperature flames (Tf>∼1800 K) to a bimodal PSDF at lower temperatures (Tf<∼1800 K). The bimodal PSDF has a noticeable trough that separates the nucleation tail and log-normal mode. This mode-transition trough had been previously thought to occur at a fixed particle size, but these results show a continuous shift of the trough location towards smaller sizes with increasing flame temperature. TEM images show that the particles are spherical, even when the PSDF is bimodal, suggesting that the bimodality occurs as the primary particles undergoes mass and size growth, and is not a result of particle aggregation. Atomic force microscopy of substrate deposited particles shows that particles spread and form hill like structures upon impact with the substrate surface, indicating that the particles are liquid-like at the time of impact.     

Investigation of the effect of molecular structure on sooting tendency in laminar diffusion flames at elevated pressure

A specially designed High Pressure Vessel and Burner and fueling system (called “doped flame”) are presented in this paper. This setup allows for soot measurements in laminar diffusion flames of liquid fuel blends at elevated pressure. Fuels with two typical molecular structures, namely linear and saturated cyclic hydrocarbons, are examined in both non-oxygenated (n-hexane (C₆H₁₄) and cyclohexane (C₆H₁₂)) and oxygenated form (1-hexanol (C₆H₁₄O) and cyclohexanol (C₆H₁₂O)). All compounds are blended into n-heptane. Focus of the research is on soot volume fraction at elevated pressure in the range of 1.5–2.0 bar. Sooting tendency is evaluated by means of Laser Induced Incandescence (LII) with Line of Sight Attenuation calibration (LOSA), and the data suggests that soot is more prevalent for cyclic structures relative to their linear counterparts.     

Numerical and experimental study of an axisymmetric coflow laminar methane-air diffusion flame at pressures between 5 and 40 atmospheres

A numerical and experimental study of an axisymmetric coflow laminar methane-air diffusion flame at pressures between 5 and 40 atm was conducted to investigate the effect of pressure on the flame structure and soot formation characteristics. Experimental work was carried out in a new high-pressure combustion chamber described in a recent study [K.A. Thomson, Ö.L. Gülder, E.J. Weckman, R.A. Fraser, G.J. Smallwood, D.R. Snelling, Combust. Flame 140 (2005) 222-232]. Radially resolved soot volume fraction was experimentally measured using both spectral soot emission and line-of-sight attenuation techniques. Numerically, the elliptic governing equations were solved in axisymmetric cylindrical coordinates using the finite volume method. Detailed gas-phase chemistry and complex thermal and transport properties were employed in the numerical calculations. The soot model employed in this study accounts for soot nucleation and surface growth using a semiempirical acetylene-based global soot model with oxidation of soot by O₂, OH, and O taken into account. Radiative heat transfer was calculated using the discrete-ordinates method and a nine-band nongray radiative property model. Two soot surface growth submodels were investigated and the predicted pressure dependence of soot yield was compared with available experimental data. The experiment, the numerical model, and a simplified theoretical analysis found that the visible flame diameter decreases with pressure as . The flame-diameter-integrated soot volume fraction increases with pressure as between 5 and 20 atm. The assumption of a square root dependence of the soot surface growth rate on the soot particle surface area predicts the pressure dependence of soot yield in good agreement with the experimental observation. On the other hand, the assumption of linear dependence of the soot surface growth rate on the soot surface area predicts a much faster increase in the soot yield with pressure than that observed experimentally. Although pressure affects the gas-phase chemistry, the increased soot production with increasing pressure seems primarily due to enhanced mixture density and species concentrations in the pressure range investigated. The increased pressure causes enhanced air entrainment into the fuel stream around the burner rim, leading to accelerated fuel pyrolysis. In the pressure range of 20 to 40 atm both the model and experiment show a diminishing sensitivity of sooting propensity to pressure with a greater decrease in the predicted sensitivity of soot propensity to pressure than the experimental results.     

Experimental investigation and detailed modeling of soot aggregate formation and size distribution in laminar coflow diffusion flames of Jet A-1, a synthetic kerosene,and n-decane

A fully-coupled soot formation model is developed to predict the concentration, size, and aggregate structure of soot particles in the atmospheric pressure laminar coflow diffusion flames of a three-component surrogate for Jet A-1, a three-component surrogate for a Fischer–Tropsch Synthetic Paraffinic Kerosene (SPK), and n-decane. To model the chemical structure of the flames and soot precursor formation, a detailed chemical kinetic mechanism for fuel oxidation, with 2185 species and 8217 reactions, is reduced and combined with a Polycyclic Aromatic Hydrocarbon (PAH) formation and growth scheme. The mechanism is coupled to a highly detailed sectional particle dynamics model that predicts the volume fraction, structure, and size of soot particles by considering PAH-based nucleation, surface growth, PAH surface condensation, aggregation, surface oxidation, fragmentation, thermophoresis, and radiation. The simulation results are validated by comparing against experimental data measured for the flames of pre-vaporized fuels. The objectives of the present effort are to more accurately simulate the physical soot formation processes and to improve the predictions of our previously published jet fuel soot formation models, particularly for the size and aggregate structure of soot particles. To this end, the following improvements are considered: (1) addition of particle coalescence submodels to account for the loss of surface area, reduction of the number of primary particles, and increase of primary particle diameters upon collision, (2) consideration of a larger PAH molecule (benzopyrene instead of pyrene) for nucleation and surface growth to enhance the agreement between the soot model and the measured chemical composition of soot particles, and (3) implementation of a dimerization efficiency in the soot inception submodel to account for the collisions between PAH molecules that do not lead to dimerization. The results of two different particle coalescence submodels show that this process is too slow to account for the growth of primary particles, mainly because of the limited rate of particle collisions. Soot volume fraction predictions on the wings and at lower flame heights are considerably improved by using benzopyrene, due to the different distribution of the soot forming PAH molecule in the flame. The computed number of primary particles per aggregate and the diameters of primary particles agree very well with the experimentally measured values after implementing the dimerization efficiency for PAH collisions, because of the reduced rate of soot inception compared to growth by PAH condensation. Concentrations of major gaseous species and flame temperatures are also well predicted by the model. The underprediction of soot concentration on the flame centerline, observed in previous studies, still exists despite minor improvements.     

Effects of NH3/H2/N2 addition on soot morphology and nanostructure in laminar co-flow ethylene diffusion flame

The present study investigated the effects of NH₃/H₂/N₂ addition on soot morphology and nanostructure in laminar co-flow diffusion flame of ethylene, by adopting the method of thermophoretic sampling combined with transmission electron microscope (TEM). The volume fraction of NH₃ varies from 10% to 30%, and the volume fraction of H₂ and N₂ are all 30%. The soot morphology evolution images were analyzed with the parameters representing size and fractal characteristics determined. Among the three diluents, the addition of NH₃ is found to have the best suppressing effect with the smallest primary particle size and least accumulated aggregates, especially at the lower flame heights. Meanwhile, H₂ addition is observed to advance the soot formation process due to the higher flame temperature. The soot particle images with a magnification time of 600,000 obtained from high-resolution transmission electron microscope (HRTEM) were processed by the processing code. The parameters describing the soot nanostructure were measured and analyzed with the fitted curves. It is found that the fringes of particles sampled from NH₃ enriched flames tend to be shorter and more curved, and the inter fringe spacing tends to be larger, indicating the nanostructure is more disordered and the particles are easier to be oxidized. Compared with the flames with NH₃ addition, particles from H₂ enriched flames show a more ordered and compact nanostructure. Finally, simulations were performed to further interpret the effects. Calculated results show that the NH₃ and H₂ addition obviously suppress the production of larger PAHs, and NH₃ is more effective than H₂, but this difference gradually narrows for greater PAHs. Simulations also indicate that the peak value of precursors in NH₃ enriched flame is delayed.     

Soot formation in laminar diffusion flames

Laminar, sooting, coflow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated with laser diagnostics. Laser extinction has been used to calibrate the experimental soot volume fractions and an improved gating method has been implemented in the laser-induced incandescence (LII) measurements resulting in differences to the soot distributions reported previously. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry, and the dynamical equations for soot spheroid growth. The model also includes the effects of radiation reabsorption through an iterative procedure. An investigation of the computed rates of particle inception, surface growth, and oxidation, along with a residence time analysis, helps to explain the shift in the peak soot volume fraction from the centerline to the wings of the flame as the fuel fraction increases. The shift arises from changes in the relative importance of inception and surface growth combined with a significant increase in the residence time within the annular soot formation field leading to higher soot volume fractions, as the fuel fraction increases.     

A numerical study on the effects of pressure and gravity in laminar ethylene diffusion flames

The effects of pressure and gravity on sooting characteristics and flame structure were studied numerically in coflow ethylene–air laminar diffusion flames between 0.5 and 5 atm. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible, gaseous mixtures which include complex chemistry, detailed radiation heat transfer, and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which has been verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed for both normal- and zero-gravity conditions to help separate the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the flames through their effects on buoyancy and reaction rates. The zero-gravity flames have higher soot concentrations, lower temperatures and broader soot-containing zones than normal-gravity flames at the same pressure. The zero-gravity flames were also found to be longer and wider. Differences were observed between the two levels of gravity when pressure was increased. The zero-gravity flames displayed a stronger dependence of the maximum soot yield on pressure from 0.5 to 2 atm and a weaker dependence from 2 to 5 atm as compared to the normal-gravity flames. In addition, flame diameter decreased with increasing pressure under normal-gravity while it increased with pressure in the zero-gravity cases. Changing the prescribed wall boundary condition from fixed-temperature to adiabatic significantly altered the numerical predictions at 5 atm. When the walls were assumed to be adiabatic, peak soot volume fractions and temperatures increased in both the zero- and normal-gravity flames, emphasizing the importance of heat conduction to the burner rim on flame structure.     

Effect of oxygen-rich combustion on soot formation in laminar co-flow propane diffusion flames

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. In this work, the effects of oxygen-rich combustion on soot formation in the propane/(O₂+N₂) laminar flow coaxial jets diffusion flame were numerically investigated by using the detailed gas-phase chemical reaction model with the mechanism of tetracyclic aromatic hydrocarbons and the complex thermodynamic properties and transport characteristics parameters. Soot surface growth follows the hydrogen-abstraction-carbon-addition (HACA) model. A hybrid gas-phase mechanism was adopted, which contains a DLR-based polycyclic aromatic hydrocarbons (PAHs) formation, growth model and a gas-phase model. Results show that the oxygen-rich combustion has a great influence on the flame temperature, especially the high temperature region. With the increase of oxygen concentration, the soot formation region of flame broadens and the maximum of soot volume fraction increase from 3.95 ppm to 10.87 ppm. The extra oxygen makes PAHs increased around the nozzle, leading to larger rate in early soot nucleation and surface growth, eventually more soot yield.     

Morphological evolution of soot emissions from a laminar co-flow methane diffusion flame with varying oxygen concentrations

The effects of oxygen-enriched atmosphere on the morphological evolution of soot emitted from a methane co-flow laminar diffusion flame were studied using a 12-μm SiC fiber deposition sampling method combined with field emission scanning electron microscopy (FESEM) analysis. The temperature distribution and the soot morphological evolution along the flame radial and axial directions at different oxygen concentrations were systematically investigated. Results showed that the soot morphology was strongly dependent on its position and oxygen concentration in flames. At the same flame height, the soot morphology changed significantly when the oxygen concentration increased from 21% to 31%. The soot generation rate increased rapidly under the elevated oxidative condition, and the position of soot inception was closer to the burner nozzle. As the flame height and oxygen concentration increased, soot particles in the flame centerline were enlarged and evolved to fiber-like depositions. Furthermore, the large clusters of soot particles gradually evolved to even denser spongy and fibrous particles around the flame edges. In addition, unique sponge-like soot deposits were observed for the first time in methane flames.     

Effect of hydrogen and helium addition to fuel on soot formation in an axisymmetric coflow laminar methane/air diffusion flame

A detailed numerical study was conducted to investigate the effects of hydrogen and helium addition to fuel on soot formation in atmospheric axisymmetric coflow laminar methane/air diffusion flame. Detailed gas-phase chemistry and thermal and transport properties were employed in the numerical calculations. Soot was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Numerical results were compared with available experimental data. Both experimental and numerical results show that helium addition is more effective than hydrogen addition in reducing soot loading in the methane/air diffusion flame. These results are different from the previous investigations in ethylene/air diffusion flames. Hydrogen chemically enhances soot formation when added to methane. The different chemical effects of hydrogen addition to ethylene and methane on soot formation are explained in terms of the different effects of hydrogen addition on propargyl, benzene, and pyrene formation low in the flames.     

Soot formation in high pressure laminar diffusion flames

The details of the chemical and physical mechanisms of the soot formation process in combustion remain uncertain due to the highly complex nature of hydrocarbon flames, and only a few principles are firmly established mostly for atmospheric conditions. In spite of the fact that most combustion devices used for transportation operate at very high pressures (e.g., aircraft gas turbines up to 40 atm, diesel engines exceeding 100 atm), our understanding of soot formation at these pressures is not at a desirable level, and there is a fundamental lack of experimental data and complementary predictive models. The focus of this review is to assess the experimental results available from laminar co-flow diffusion flames burning at elevated pressures. First, a brief review of soot formation mechanisms in diffusion flames is presented. This is followed by an assessment of soot diagnostics techniques, both intrusive and non-intrusive, most commonly used in soot experiments including the laser induced incandescence. Then the experimental results of soot measurements done at elevated pressures in diffusion flames are reviewed and critically assessed. Soot studies in shock tubes and in premixed flames are not covered. Smoke point fuel mass flow rate is revisited, and shortcomings in recent measurements are pointed. The basic requirements for tractable and comparable measurements as a function of pressure are summarized. Most recent studies at high pressures with aliphatic gaseous fuels show that the soot yield displays a unified behaviour with reduced pressure. The maximum soot yield seems to reach a plateau asymptotically as the pressure exceeds the critical pressure of the fuel. Lack of experimental data on the sensitivity of soot morphology to pressure is emphasized. A short summary of efforts in the literature on the numerical simulation of soot formation in diffusion flames at high pressures is the last section of the paper.     

Numerical and experimental study of soot formation in laminar diffusion flames burning simulated biogas fuels at elevated pressures

The effects of pressure and composition on the sooting characteristics and flame structure of laminar diffusion flames were investigated. Flames with pure methane and two different methane-based, biogas-like fuels were examined using both experimental and numerical techniques over pressures ranging from 1 to 20 atm. The two simulated biogases were mixtures of methane and carbon dioxide with either 20% or 40% carbon dioxide by volume. In all cases, the methane flow rate was held constant at 0.55 mg/s to enable a fair comparison of sooting characteristics. Measurements for the soot volume fraction and temperature within the flame envelope were obtained using the spectral soot emission technique. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows, which included complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Overall, the numerical simulations correctly predicted many of the observed trends with pressure and fuel composition. For all of the fuels, increasing pressure caused the flames to narrow and soot concentrations to increase while flame height remained unaltered. All fuels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. Adding carbon dioxide to the methane fuel stream did not significantly effect the shape of the flame at any pressure; although, dilution decreased the diameter slightly at 1 atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO₂${\text{CO}}_2$ concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO₂${\text{CO}}_2$ dilution was largely attributed to the effects of dilution on chemical reaction rates, since the predicted maximum magnitudes of soot production and oxidation also varied linearly with dilution.