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.     

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.     

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.     

Laminar flame speeds of primary reference fuels and reformer gas mixtures

The laminar flame speeds of neat primary reference fuels (PRFs), n-heptane and iso-octane, PRF blends, reformer gas, and reformer gas/iso-octane/air mixtures are measured over a range of equivalence ratios at atmospheric pressure, using counterflow configuration and digital particle image velocimetry (DPIV). PRF blends with various octane numbers are studied. The synthetic reformer gas mixture employed herein has a composition that would be produced from the partial oxidation of rich iso-octane/air mixture into CO and H₂, namely, 28% H₂, 25% CO, and 47% N₂. Computationally, the experimentally determined laminar flame speeds are simulated using the detailed kinetic models available in the literature. Both experimental and computational results demonstrate that the flame speeds of hydrocarbon/air mixtures increase with addition of a small amount of reformer gas, and the flame speeds of reformer gas/air mixtures are dramatically reduced with addition of a small amount of hydrocarbon fuel. Furthermore, the number density effect of seeding particles on flame speed measurement is assessed, and the experimental uncertainties associated with the present DPIV setup as well as the linear extrapolation method employed herein are discussed.     

Soot growth mechanism in C2H2 combustion with H2 addition: A reactive molecular dynamics study

A numerical investigation of the effect of H₂ addition on soot formation was conducted in C₂H₂ combustion process from the molecular level. The results showed that the chemical effect of H₂ (CE-H₂) inhibited the soot formation of C₂H₂ combustion. With the gradual increase of H₂ doping ratio, the soot morphology became sparser and the soot particle size decreased. The CE-H₂ significantly inhibited the surface growth and coalescence of soot. Addition of H₂ led to an increase in the H₂ concentration in the combustion process, which inhibited the dehydrogenation reaction of the primary soot particle with C₂H₂, free radicals, and polycyclic aromatic hydrocarbons (PAHs). The CE-H₂ increased the consumption rate of C₂H₂ and the formation rate of H₂O, and it decreased the formation rate of CO. Moreover, the addition reaction of free radicals and PAHs played an important role in the soot growth process, which effectively promoted the soot surface growth process. And the coalescence process significantly increased the soot mass and affected the soot size and morphology although its frequency was relatively low.     

Soot concentration and temperature measurements in co-annular, nonpremixed CH4/air laminar flames at pressures up to 4 MPa

Laminar nonpremixed methane-air flames were studied over the pressure range of 0.5 to 4 MPa using a new high-pressure combustion chamber. Flame characterization showed very good flame stability over the range of pressures, with a flame tip rms flicker of less than 1% in flame height. At all pressures, soot was completely oxidized within the visible flame. Spectral soot emission (SSE) and line-of-sight attenuation (LOSA) measurements provided radially resolved measurements of soot volume fraction and soot temperature at pressures from 0.5 to 4.0 MPa. Such measurements provide an improved understanding of the influence of pressure on soot formation and have not been reported previously in laminar nonpremixed flames for pressures above 0.4 MPa. SSE and LOSA soot concentration values typically agree to within 30% and both methods exhibit similar trends in the spatial distribution of soot concentration. Maximum soot concentration depended on pressure according to a power law, where the exponent on pressure is about 2 for the range of pressures between 0.5 and 2.0 MPa, and about 1.2 for 2.0 to 4.0 MPa. Peak carbon conversion to soot also followed a power-law dependence on pressure, where the pressure exponent is unity for pressures between 0.5 and 2.0 MPa and 0.1 for 2.0 to 4.0 MPa. The pressure dependence of sooting propensity diminished at pressures above 2.0 MPa. Soot concentrations measured in this work, when transformed to line-integrated values, are consistent with the measurements of Flower and Bowman for pressures up to 1.0 MPa [Proc. Combust Inst. 21 (1986) 1115-1124] and Lee and Na for pressures up to 0.4 MPa [JSME Int. J. Ser. B 43 (2000) 550-555]. Soot temperature measurements indicate that the overall temperatures decrease with increasing pressure; however, the differences diminish with increasing height in the flame. Low down in the flame, temperatures are about 150 K lower at pressures of 4.0 MPa than those at 0.5 MPa. In the upper half of the flame the differences reduce to 50 K.     

Measurement of smoke point in velocity-matched coflow laminar diffusion flames with pure fuels at elevated pressures

Using a coflow burner, a quartz chimney, and a pressure vessel with good optical access, smoke points of pure fuels were measured in a laminar jet diffusion flame. The smoke point is a fundamental kinetic parameter, as this is the point where production of soot is exactly offset by its oxidation. Ethylene and methane, burning in a velocity-matched, overventilated coflow of air, were tested over a range of pressures from 1 to 16 atm. Fuel flow rate and air coflow rate were iteratively increased, keeping the exit velocity equal, until the smoke point was reached. The volumetric fuel flow and flame height were measured as a function of pressure to determine the functional relationship between these parameters and pressure. The volumetric fuel flow at the smoke point is observed to scale as a power law with pressure, while the smoke point height is best described by a log law with pressure. The residence time, based on flame height and exit velocity, was also calculated as a function of pressure and found to have a nonmonotonic behavior, with a peak at lower pressures.     

Effects of CO addition on laminar flame characteristics and chemical reactions of H2 and CH4 in oxy-fuel (O2/CO2) atmosphere

An experimental study is conducted to investigate the effect of CO addition on the laminar flame characteristics of H₂ and CH₄ flames in a constant-volume combustion system. In addition, one-dimensional laminar premixed flame propagation processes at the same conditions are simulated with the update mechanisms. Results show that all mechanisms could well predict the laminar flame speeds of CH₄/CO/O₂/CO₂ mixtures, when Z_CO is large. For mixtures with lower CO, the experimental laminar flame speeds are always smaller than the calculated ones with Han mechanism. For mixtures with larger or smaller Z_CO2, GRI 3.0, San diego and USC 2.0 mechanisms all overvalue or undervalue the laminar flame speeds. When CO ratio in the CH₄/CO blended fuels increases, laminar flame speed firstly increases and then decreases for the CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, San diego, Davis and Li mechanisms all undervalue the laminar flame speeds of H₂/CO/CO₂/CO₂ mixtures. Existing models could not well predict the nonlinear trend of the laminar flame speeds, due to complex chemical effects of CO on CH₄/CO or H₂/CO flames. Then, the detailed thermal, kinetic and diffusive effects of CO addition on the laminar flame speeds are discussed. Kinetic sensitivity coefficient is far larger than thermal and diffusive ones and this indicates CO addition influences laminar flame speeds mainly by the kinetic effect. Based on this, radical pool and sensitivity analysis are conducted for CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures. For CH₄/CO/O₂/CO₂ mixtures, elementary reaction R38H + O₂ ↔ O + OH and R99 OH + CO ↔ H + CO₂ are the most important branching reactions with positive sensitivity coefficients when CO ratio is relative low. As CO content increases in the CH₄/CO blended fuel, the oxidation of CO plays a more and more important role. When CO ratio is larger than 0.9, the importance of R99 OH + CO ↔ H + CO₂ is far larger than that of R38H + O₂ ↔ O + OH. The oxidation of CO dominates the combustion process of CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, the most important elementary reaction with positive and negative sensitivity coefficients are R29 CO + OH ↔ CO₂ + H and R13H + O₂(+M) ↔ HO₂(+M) respectively. The sensitivity coefficient of R29 CO + OH ↔ CO₂ + H is increasing and then decreasing with the addition of CO in the mixture. Chemical kinetic analysis shows that the chemical effect of CO on the laminar flame propagation of CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures could be divided into two stages and the critical CO mole fraction is 0.9.     

Application of an enhanced PAH growth model to soot formation in a laminar coflow ethylene/air diffusion flame

A recently developed chemical kinetic scheme for C₂ fuel combustion with PAH growth has been implemented in a parallelized coflow flame solver. The reaction mechanism has been developed to include almost all reasonably well-established reaction classes for aromatic ring formation and soot particle precursor molecular weight growth. The model has recently been validated for zero- and one-dimensional premixed flame systems [N.A. Slavinskaya, P. Frank, Combust. Flame 156 (2009) 1705–1722] and has now been updated and extended to a sooting ethylene/air diffusion flame in the coflow geometry. Updates to the mechanism reflect the latest advances in the literature and address numerical stiffness that was present in diffusion flame systems. The chemical kinetic mechanism has been coupled to a sectional aerosol dynamics model for soot growth, considering PAH-based inception and surface condensation, surface chemistry (growth and oxidation), coagulation, and fragmentation. The sectional model predicts the soot aggregate number density and the number of primary particles per aggregate in each section, so as to yield information on particle size distribution and structure. Flame simulation data for the present mechanism is compared to data computed using two other reaction schemes [J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136; N.M. Marinov, W.J. Pitz, C.K. Westbrook, A.M. Vincitore, M.J. Castaldi, S.M. Senkan, Combust. Flame 114 (1998) 192–213]. The computed data are also compared to numerous experimental data sets. Whereas the fuel oxidation chemistry in all three mechanisms are essentially the same, the PAH growth pathways vary considerably. It is shown that soot concentrations on the wings of the flame (where soot formation is dominated by surface chemistry) can be predicted with two of the three mechanisms. However, only the present mechanism with its enhanced PAH growth routes can also predict the correct order of magnitude of soot volume fraction in the low-sooting, inception-dominated, central region of the flame. In applying this chemical mechanism, the parameter α, which describes the portion of soot surface sites that are available for chemical reaction, has been reduced to a theoretically acceptable range, thus improving the quality of the model.     

Physical and chemical comparison of soot in hydrocarbon and biodiesel fuel diffusion flames: A study of model and commercial fuels

Data are presented to compare soot formation in both surrogate and practical fatty acid methyl ester biodiesel and petroleum fuel diffusion flames. The approach here uses differential mobility analysis to follow the size distributions and electrical charge of soot particles as they evolve in the flame, and laser ablation particle mass spectrometry to elucidate their composition. Qualitatively, these soot properties exhibit a remarkably similar development along the flames. The size distributions begin as a single mode of precursor nanoparticles, evolve through a bimodal phase marking the onset of aggregate formation, and end in a self preserving mode of fractal-like particles. Both biodiesel and hydrocarbon fuels yield a common soot composition dominated by C_x ions, stabilomer PAHs, and fullerenes in the positive ion mass spectrum, and and C_2xH⁻ in the negative ion spectrum. These ion intensities initially grow with height in the diffusion flames, but then decline during later stages, consistent with soot carbonization. There are important quantitative differences between fuels. The surrogate biodiesel fuel methyl butanoate substantially reduces soot levels, but soot formation and evolution in this flame are delayed relative to both soy and petroleum fuels. In contrast, soots from soy and hexadecane flames exhibit nearly quantitative agreement in their size distribution and composition profiles with height, suggesting similar soot precursor chemistry.     

Soot surface growth and oxidation at pressures up to 8.0 atm in laminar nonpremixed and partially premixed flames

The flame structure and soot particle surface reaction properties, including growth and oxidation, of laminar jet nonpremixed flames were studied experimentally at pressures of 1.0-8.0 atm. Ethylene-helium mixtures were used in an oxygen/helium coflow at normal temperature (300 K) in order to minimize the effects of buoyancy. The following properties along the axis of flames were measured as a function of distance from the burner exit: soot concentrations by laser extinction, soot temperatures by 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 radical species (H, OH, O) by Li/LiOH atomic absorption, and flow velocities by laser velocimetry. The measurements were analyzed to determine local flame properties in order to find soot surface growth and oxidation rates. The measurements of soot surface growth rates (corrected for soot surface oxidation) were found to be consistent with earlier measurements at atmospheric and subatmospheric pressures involving laminar premixed and diffusion flames fueled with a variety of hydrocarbons. 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. Measurements of early soot surface oxidation rates at pressures of 1.0-8.0 atm (corrected for soot surface growth and prior to consumption of 70% of the maximum mass of the primary soot particles) were found to be consistent with earlier measurements at atmospheric and subatmospheric pressures. The oxidation rates of up to 8 atm in flame environment could be explained by reaction with OH, having a collision efficiency of 0.12.     

Soot suppression by nonthermal plasma in coflow jet diffusion flames using a dielectric barrier discharge

The effect of nonthermal plasma on diffusion flames in coflow jets has been studied experimentally by adopting a dielectric barrier discharge (DBD) technique. The plasma reactor had wire-cylinder-type electrodes with AC power supply operated at 400 Hz. The effect of flame on the behavior of electrical discharge was first investigated to identify the regime of plasma generation, discharge onset voltage, and delivered power to the plasma reactor. The generation of streamers was enhanced with a flame by the increase in the reduced electric field intensity due to high-temperature burnt gas and by the abundance of ions in the flame region. The effect of streamers on flame behavior reveals that the flame length was significantly decreased as the applied voltage increased. The yellow luminosity by the radiation of soot particles was also significantly diminished. The temperature of burnt gases, the concentrations of major species, and the spatial distribution of OH radical, PAH, and soot have been measured. The formation of PAH and soot was influenced appreciably by the nonthermal plasma, while the flame temperature and the concentration of major species were not influenced much with the plasma generation. The results demonstrated that the application of nonthermal plasmas can effectively suppress PAH and soot formation in the flames with low power consumption even in the order of 1 W.     

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.     

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 characteristics and PAH formation process in a micro flow reactor with a controlled temperature profile

A micro flow reactor with a controlled temperature profile was examined with regard to its capabilities to investigate soot formation characteristics of rich methane/air mixtures and the formation process of polycyclic aromatic hydrocarbons (PAHs) of rich acetylene/air mixtures. In the experiment for a methane/air mixture, four kinds of flame and soot responses to equivalence ratio (1.5–4.5) and inlet mean flow velocity (5–40 cm/s) were observed: soot formation without a flame; a flame with soot formation; a flame without soot formation; and neither a flame nor soot formation. Soot formation was observed at high equivalence ratio and low flow velocity. Sooting limits depending on equivalence ratio and flow velocity (residence time) were successfully identified by the present micro flow reactor. To investigate the PAH formation process, the micro flow reactor was employed for a rich acetylene/air mixture at equivalence ratios of 4, 5 and 6 at an inlet mean flow velocity of 2 cm/s and gas sampling experiments were conducted at temperatures from 600 to 1000 K. Temperature dependence of mole fractions of benzene, styrene, naphthalene, phenanthrene, indene, acenaphthylene and biphenyl was successfully obtained and larger PAHs such as pyrene and coronene were not observed in this study. One-dimensional computation with the ABF 2.99 mechanism predicted a benzene mole fraction three times higher than the experimental result. The modification of the ABF 2.99 mechanism using recent benzene reactions greatly improved the prediction of the benzene mole fraction. The rate of production analysis was carried out and PAH formation in the micro flow reactor was investigated in detail.     

A model of particulate and species formation applied to laminar, nonpremixed flames for three aliphatic-hydrocarbon fuels

A detailed kinetic mechanism is developed that includes aromatic growth and particulate formation. The model includes reaction pathways leading to the formation of nanosized particles and their coagulation and growth to larger soot particles using a sectional approach for the particle phase. It is tested against literature data of species concentrations and particulate measurements in nonpremixed laminar flames of methane, ethylene, and butene. Reasonably good predictions of gas and particle-phase concentrations and particle sizes are obtained without any change to the kinetic scheme for the different fuels. The model predicts the low concentration of particulates in the methane flame (about 0.5 ppm) and the higher concentration of soot in the ethylene and butene flames (near 10 ppm). Model predictions show that in the methane flame small precursor particles dominate the particulate loading, whereas soot is the major component in ethylene and butene flames, in accordance with the experimental data. The driving factors in the model responsible for the quite different soot predictions in the ethylene and butene flames compared with the methane flame are benzene and acetylene concentrations, which are higher in the ethylene and butene flames. Soot loadings in the ethylene flame are sensitive to the acetylene soot growth reaction, whereas particle inception rates are linked to benzene in the model. A coagulation model is used to obtain collision efficiencies for some of the particle reactions, and tests show that the modeled results are not particularly sensitive to coagulation at the rates used in our model. Soot oxidation rates are not high enough to correctly predict burnout, and this aspect of the model needs further attention.     

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.     

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 graphitic order in laminar diffusion flames and a large-scale JP-8 pool fire

High-resolution transmission electron microscopy (HRTEM) has been performed on soot samples collected from two smoking laminar ethylene diffusion flames (one steady and one unsteady) and from the active-flaming region of a 5-m diameter JP-8 pool fire. The motivation for this study is to improve the understanding of the influence of soot microstructure on its optical properties. The soot sampling positions in the steady ethylene flame correspond to locations of maximum soot mass growth, partial soot oxidation, and quenched oxidation along a common streamline. Visual examination of the HRTEM images suggests that the graphitic crystalline layers of soot undergo increased densification along the sampled streamline in the steady laminar flame. Quantitative image analysis reveals a small decrease in the mean graphitic interlayer spacing (d₀₀₂) with increasing residence time in the high-temperature region. However, the differences in the mean interlayer spacing are far smaller than the spread of interlayer spacings measured for any given soot sample. Post-flame samples from the unsteady ethylene flame show interlayer spacing distributions similar to the lower region of the steady flame. The soot samples from the pool fire show little evidence of oxidized soot and have interlayer spacings similar to the unsteady ethylene flame. Previous research in the carbon black field has demonstrated a direct relation between the graphitic interlayer spacing and the optical absorptivity of the carbon. Consequently, the current HRTEM results offer support to recent measurements of the dimensionless extinction coefficient of soot that suggest that the optical absorptivity of agglomerating soot shows only minor variations for different fuels and flame types.     

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.