Soot Number Density Research Articles

Effect of methane doping on nascent soot formation in ethylene-based laminar premixed flames

The impact of methane doping on sooting behavior of ethylene-base burner stabilized stagnation (BSS) flames was investigated by following the evolution of particle size distribution functions (PSDFs) of nascent soot. The M-series of flames with the equivalence ratio of 2.07 was doped by methane with the mixture ratio of 5%, 10% and 40%. All the methane-doping flames showed obvious bimodality in the PSDFs, and more particles in nucleation stage than the C3 reference flame. The synergistic effect appeared in small ratio (5% and 10%) methane doping cases, leading to the increase of the soot number density and soot mobility volume fractions. While thesynergisticeffect was not found under large methane doping condition (40%), in which the mobility volume fraction was less than that in C3 flame. By calculating the key species and analyzing the reaction pathways of pyrene formation with KM2-Mech, we found that small ratio of methane doping benefits C3H3 and pyrene (A4) formation, which strengthened the sooting tendency. However too much methane doping reduced the yield of C2H2 that is a critical intermediate of PAHs formation, leading to a lower sooting tendency compared with the pure ethylene flame.

Formation of Soot in Counterflow Diffusion Flames with Carbon Dioxide Dilution

ABSTRACTExperimental and numerical modeling studies have been performed to investigate the effect of CO2 dilution on soot formation in ethylene counterflow diffusion flames. Thermal and chemical effects of CO2 addition on soot growth was numerically identified by using a fictitious CO2 species, which was treated as inert in terms of chemical reactions. The results showed that CO2 addition reduces soot formation both thermodynamically and chemically. In terms of chemical effect, the addition of CO2 decreases soot formation through various pathways, including: (1) reduced soot precursor (PAH) formation leading to lower inception rates and soot number density, which in turn results in lower surface area for soot mass addition; (2) reduced H, CH3, and C3H3 concentrations causing lower H abstraction rate and therefore less active site per surface area for soot growth; and (3) reduced C2H2 mole fraction and thus a slower C2H2 mass addition rate. In addition, the sooting limits were also measured for ethylene counterflow flames in both N2 and CO2 atmosphere and the results showed that sooting region was significantly reduced in the CO2 case compared to the N2 case.

A Computational Study of Soot Formation in Methane Air Co-Flow Diffusion Flame Under Microgravity Conditions

An in-house developed code has been used to predict soot formation in a methane air co flow diffusion flame at normal gravity and at lower gravity levels of 0.5 G, and 0.0001 G (microgravity). There is an augmentation of soot formation at lower gravity levels because of lower buoyancy induced acceleration leading to an increased residence time. The peak temperature at microgravity is reduced by about 50 K than that at normal gravity level. The axial velocity under normal gravity and reduced gravity show negative values (relatively small in magnitude) near the wall at axial height beyond 15 cm; but axial velocity is never negative in microgravity condition. Peak value of soot volume fraction at 0.5 G and microgravity multiplies by a factor of ∼3 and ∼7, respectively of that at normal gravity. The zone of peak soot volume fraction shifts away from the axis towards the wings, as gravity level is lowered. In comparison to soot volume fraction, the factors of amplification of soot number density at reduced gravity and at microgravity are comparatively lower at 1.2 and 1.5 of that at normal gravity respectively. On the other hand, mean soot particle sizes at reduced gravity and microgravity increase to 1.5 and 2 times of that at normal gravity respectively.

Effects of non-unity Lewis number of gas-phase species in turbulent nonpremixed sooting flames

Turbulence statistics from two three-dimensional direct numerical simulations of planar n-heptane/air turbulent jets are compared to assess the effect of the gas-phase species diffusion model on flame dynamics and soot formation. The Reynolds number based on the initial jet width and velocity is around 15, 000, corresponding to a Taylor scale Reynolds number in the range 100 ≤ Reλ ≤ 150. In one simulation, multicomponent transport based on a mixture-averaged approach is employed, while in the other the gas-phase species Lewis numbers are set equal to unity. The statistics of temperature and major species obtained with the mixture-averaged formulation are very similar to those in the unity Lewis number case. In both cases, the statistics of temperature are captured with remarkable accuracy by a laminar flamelet model with unity Lewis numbers. On the contrary, a flamelet with a mixture-averaged diffusion model, which corresponds to the model used in the multi-component diffusion three-dimensional DNS, produces significant differences with respect to the DNS results. The total mass of soot precursors decreases by 20–30% with the unity Lewis number approximation, and their distribution is more homogeneous in space and time. Due to the non-linearity of the soot growth rate with respect to the precursors’ concentration, the soot mass yield decreases by a factor of two. Being strongly affected by coagulation, soot number density is not altered significantly if the unity Lewis number model is used rather than the mixture-averaged diffusion. The dominant role of turbulent transport over differential diffusion effects is expected to become more pronounced for higher Reynolds numbers.

Sectional soot model coupled to tabulated chemistry for Diesel RANS simulations

In future Euro norms, the soot volume fraction and the soot number density will be regulated. Car manufacturers need therefore accurate soot models for piston engine emissions prediction in order to develop future engine concepts. This paper addresses this question by coupling a sectional soot model with a tabulated combustion model for RANS simulations of Diesel engines. The sectional soot model, based on the work of Netzell et al. (2007), is implemented in the IFP-C3D RANS CFD code. At each time and location, transport equations are solved for several soot sections, including source terms for collisional and chemical processes. The soot model is coupled to a tabulated combustion model derived from the Engine Approximated Diffusion Flame one (EADF) (Michel and Colin, 2013). It allows to represent the minor species required by the soot model with a much lower computational cost than a kinetic solver. In order to evaluate the soot model coupled to the resulting combustion model called Variable Pressure Homogeneous Tabulated Chemistry (VPTHC), it is compared to the same soot model directly coupled to a complex chemistry solver. As this comparison can hardly be performed on a real Diesel engine case due to the very high CPU time involved by the chemical solver, it is performed on a variable volume and fuel/air ratio case which retains the essential features of a Diesel engine. Results show that the proposed coupling recovers with reasonable accuracy the evolution of the soot volume fraction and distribution. Finally, an experimental database of Diesel operating points is simulated. The database includes points with a commercial Diesel fuel and the computed surrogate (30% 1-Methylnaphthalene and 70% Decane in liquid volume) to validate the models against the experiments. Soot yields predictions from the model show an improvement against the current standards and reach industrial target of accuracy for most of the database while the model also provides a good estimation of the soot particles distributions in size.

Soot Particle Evolution and Transport in a Direct Injection Diesel Engine

Particle-based in-cylinder soot distribution study is becoming more important as the rules and regulations pertaining to particulate emission of diesel-powered vehicles have been increasingly more stringent. This paper focuses on the investigation of soot size evolution and its distribution and transport inside an engine cylinder. The overall process of soot formation includes soot nucleation, surface growth, oxidation, coagulation and agglomeration. The present study considers only soot surface growth, oxidation and coagulation to predict the in-cylinder soot particle size. The soot surface growth model was based on Hiroyasu’s soot formation model while soot oxidation was referred to Nagle & Strickland-Constable’s soot oxidation model. Coagulation rate was defined using Smoluchowski’s equation with constant proposed by Wersborg. From this study, it is demonstrated that soot particles with relatively larger size are gathered in the centre of the cylinder while smaller soot particles are found to be in the region near the wall. Soot number density is considerably high at the start of combustion and reduces sharply afterward while the soot particle size shows the opposite trend. Soot formation rate was found to be dominant at earlier crank angle and is overcome by soot oxidation and coagulation processes that caused lower soot number density but higher soot particle size.

Numerical Prediction of Radiation and Air Preheating Effects on the Soot Formation in a Confined Laminar Co-Flow Diffusion Flame

A numerical model has been developed for thorough investigation of the radiation and air-preheating effect on the sooting behavior of a diffusion flame. An explicit finite difference scheme has been adopted for the solution of different conservation equations for reacting flows along with the soot conservation equations. A variable size adaptive grid system has been considered using hyperbolic distribution to capture the sharp gradients of the field variables. Temperature distribution pattern does not change when radiation effect is considered, but peak temperature is lowered due to radiative heat loss. On the other hand, air-preheating not only increases the peak temperature, but also changes the distribution pattern significantly. Radiation decreases the soot volume fraction and soot number density to a large extent both for non-preheated and preheated case. Soot diameter is not much affected due to radiation, but it increases with preheating of air.

Computational Investigation of Oxygen Concentration Effects on a Soot Mechanism with a Phenomenological Soot Model of Acetone–Butanol–Ethanol (ABE)

A phenomenological soot model of acetone–butanol–ethanol (ABE) was proposed with modification of the fuel pyrolysis process, and the oxidation effect on soot number density was also included. Using KIVA-3 V Release 2 code coupled with this ABE soot model, multi-dimensional computational fluid dynamics (CFD) simulations of ABE spray combustion in a constant volume chamber were conducted to investigate the effects of ambient oxygen concentrations on combustion and soot emission characteristics. Validation experiments were also conducted in an optical constant volume combustion chamber under 1000 K initial temperature with varying oxygen concentrations (21, 16, and 11%). The results demonstrated that predicted time-related soot mass showed good agreement with experimental data qualitatively. The total yield of soot mass initially increased with decreasing oxygen concentrations from 21 to 16% and then decreased to the lowest level at 11% ambient oxygen. The non-monotonic ABE soot behavior with a decreasing am...

Interactive transient flamelet modeling for soot formation and oxidation processes in laminar non-premixed jet flames

The Interactive Transient Flamelet (ITF) model has been devised to realistically simulate slow processes such as soot formation and radiation in a laminar non-premixed flame with a relatively long residence time. In context with the ITF model, soot formation in the laminar non-premixed jet flame is modeled by the two-equation soot model to simulate physical processes such as nucleation, surface growth, oxidation and agglomeration. In the present ITF approach, gaseous chemistry is effectively coupled with soot chemistry. The present ITF model accounts for radiative cooling induced by gaseous species and soot particles in the same mixture fraction space simultaneously while it treats the unphysical diffusion of soot to be practically zero. Moreover, the present ITF procedure can maintain consistency to extend the present transient flamelet model to the simulation of turbulent non-premixed sooting flames. To validate the present ITF approach together with the two-equation soot model, in terms of soot volume fraction, number density, temperature, OH and C2H2 mass fractions, as well as reaction rates for nucleation, surface growth, and oxidation, numerical results are compared with those obtained by the full transport equation approach. Furthermore, to assess the applicability of the present ITF approach and to evaluate the effects of gaseous differential diffusion on the soot formation processes, numerical results obtained by the present ITF model with equal and differential diffusion are also compared with experimental data in terms of temperature; CH4, OH, H2O, CO2 and C2H2 mole fractions; and soot volume fraction. Based on these numerical results, the detailed discussion has been made for the capability and limitations of the present ITF approach to predict the precise flame structure and soot formation characteristics in the laminar non-premixed methane/air jet flames.

Soot modeling of counterflow diffusion flames of ethylene-based binary mixture fuels

A soot model was developed based on the recently proposed PAH growth mechanism for C1–C4 gaseous fuels (KAUST PAH Mechanism 2, KM2) that included molecular growth up to coronene (A7) to simulate soot formation in counterflow diffusion flames of ethylene and its binary mixtures with methane, ethane and propane based on the method of moments. The soot model has 36 soot nucleation reactions from 8 PAH molecules including pyrene and larger PAHs. Soot surface growth reactions were based on a modified hydrogen-abstraction-acetylene-addition (HACA) mechanism in which CH3, C3H3 and C2H radicals were included in the hydrogen abstraction reactions in addition to H atoms. PAH condensation on soot particles was also considered. The experimentally measured profiles of soot volume fraction, number density, and particle size were well captured by the model for the baseline case of ethylene along with the cases involving mixtures of fuels. The simulation results, which were in qualitative agreement with the experimental data in the effects of binary fuel mixing on the sooting structures of the measured flames, showed in particular that 5% addition of propane (ethane) led to an increase in the soot volume fraction of the ethylene flame by 32% (6%), despite the fact that propane and ethane are less sooting fuels than is ethylene, which is in reasonable agreement with experiments of 37% (14%). The model revealed that with 5% addition of methane, there was an increase of 6% in the soot volume fraction. The average soot particle sizes were only minimally influenced while the soot number densities were increased by the fuel mixing. Further analysis of the numerical data indicated that the chemical cross-linking effect between ethylene and the dopant fuels resulted in an increase in PAH formation, which led to higher soot nucleation rates and therefore higher soot number densities. On the other hand, the rates of soot surface growth per unit surface area through the HACA mechanism were similar among the different fuel mixing cases, although slightly lower compared with ethylene base case. The increased soot nucleation rates by fuel mixing and similar soot surface growth rates (per unit surface area) help to explain the increased soot volume fraction/number density and similar average sizes among the tested cases.

Application of a semi-detailed soot modeling approach for conventional and low temperature diesel combustion – Part I: Model performance

In the present study a previously developed soot model has been tested extensively for conventional and low temperature diesel combustion engine simulations. The soot model framework, which was implemented in the Computational Fluid Dynamics (CFD) code KIVA-CHEMKIN, is based on four fundamental steps, viz., soot inception through a four ring polycyclic aromatic hydrocarbon species, soot surface growth through acetylene and benzene, soot coagulation, and oxygen and OH-induced soot oxidation. The proposed model can be used not only for soot mass predictions, but also for soot number density and soot particle diameter predictions. Diesel combustion was simulated using reduced n-heptane/primary reference fuel (PRF) chemistry mechanisms and a reduced polycyclic aromatic hydrocarbon (PAH) chemistry mechanism, while the n-heptane chemistry mechanism was used for modeling constant volume n-heptane combustion. Soot model performance was evaluated by comparing the model predictions with available constant volume combustion chamber optical diagnostic experiments, optical engine in-cylinder soot data, and a light-duty single cylinder metal engine-out smoke data. In particular, a variety of combustion regimes were tested, including conventional diesel, premixed charge compression ignition (PCCI), and high exhaust gas recirculation (EGR) low temperature combustion (LTC). In general, the model was able to predict well the trends in soot mass over the range of operating conditions. With increasing degree of premixing, the model was able to predict relatively lower soot concentrations compared to diffusion combustion. In terms of soot particle diameter, the model computed particle size was seen to increase with increasing simulated EGR when the ambient oxygen concentration was varied from 21% to 8% by volume and with increasing in-cylinder density (14.8kg/m3 vs. 30kg/m3 at the same in-cylinder O2 level) under steady-state constant volume combustion conditions. It is also observed from the optical and metal engine studies that the soot particle size is larger for high temperature combustion conditions that are marked with high soot formation rates during the combustion process and may or may not lead to smaller particles at exhaust valve opening (EVO) depending on the oxidation rates. The soot size for LTC is generally lower through-out the combustion process including EVO conditions. All but one data-point show uni-modal type soot particle size distribution with respect to both soot mass and particle number.

Experimental and numerical study on soot formation and oxidation by using diesel fuel in constant volume chamber with various ambient oxygen concentrations

The study on effects of exhaust gas recirculation (EGR) on soot behavior is very important to reduce soot emissions and control the low temperature combustion process in diesel engines. In this work, high time-resolved quantitative soot measurements were experimented on a constant volume chamber by using European low-sulfur diesel fuel at three ambient oxygen concentrations (21%, 18%, 15%). Meanwhile, an improved semi-empirical soot model was coupled into computational fluid dynamics (KIVA-3V Release 2) code for in-depth understanding the soot formation and oxidation processes. Results demonstrated that numerical results of the improved semi-empirical soot model showed good agreement with experimental data in the whole processes of soot formation/oxidation and soot distribution under different oxygen concentrations. The mass concentration of acetylene, soot precursor species and soot mass initially increased with decreasing ambient oxygen concentrations from 21% to 15% and then began to decrease at 12% oxygen, while OH radicals reduced monotonically from 21% oxygen to 12%. At 12% oxygen, the concentrations of local rich sooty zone, acetylene and soot precursor species were as high as those under higher oxygen concentrations, but the total area of soot zone shrank remarkably at 12% oxygen. Compared to 21% ambient oxygen concentration, both soot formation and oxidation rates were increased under 18% oxygen, while the higher soot mass under 18% oxygen was the result of stronger soot formation mechanism. At 15% oxygen, both soot formation and oxidation rates were reduced and the maximum peak of soot mass concentration was caused by the suppressed oxidation mechanism. At 12% oxygen, reaction rates of soot formation and oxidation dropped to the very low level and the reduction in soot mass concentration and number density were achieved. Finally, it could be concluded that the retarded and reduced soot mass concentration at low ambient oxygen was the result of suppressed soot inception reaction from precursor species.

Formation, growth, and transport of soot in a three-dimensional turbulent non-premixed jet flame

The formation, growth, and transport of soot is investigated via large scale numerical simulation in a three-dimensional turbulent non-premixed n-heptane/air jet flame at a jet Reynolds number of 15,000. For the first time, a detailed chemical mechanism, which includes the soot precursor naphthalene and a high-order method of moments are employed in a three-dimensional simulation of a turbulent sooting flame. The results are used to discuss the interaction of turbulence, chemistry, and the formation of soot. Compared to temperature and other species controlled by oxidation chemistry, naphthalene is found to be affected more significantly by the scalar dissipation rate. While the mixture fraction and temperature fields show fairly smooth spatial and temporal variations, the sensitivity of naphthalene to turbulent mixing causes large inhomogeneities in the precursor fields, which in turn generate even stronger intermittency in the soot fields. A strong correlation is apparent between soot number density and the concentration of naphthalene. On the contrary, while soot mass fraction is usually large where naphthalene is present, pockets of fluid with large soot mass are also frequent in regions with very low naphthalene mass fraction values. From the analysis of Lagrangian statistics, it is shown that soot nucleates and grows mainly in a layer close to the flame and spreads on the rich side of the flame due to the fluctuating mixing field, resulting in more than half of the total soot mass being located at mixture fractions larger than 0.6. Only a small fraction of soot is transported towards the flame and is completely oxidized in the vicinity of the stoichiometric surface. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames. Finally, given the difficulties in obtaining quantitative data in experiments of turbulent sooting flames, this simulation provides valuable data to guide the development of models for Large Eddy Simulation and Reynolds Average Navier Stokes approaches.

Experimental and computational investigation of temperature effects on soot mechanisms

Effects of initial ambient temperatures on combustion and soot emission characteristics of diesel fuel were investigated through experiment conducted in optical constant volume chamber and simulation using phenomenological soot model. There are four difference initial ambient temperatures adopted in our research: 1000 K, 900 K, 800 K and 700 K. In order to obtain a better prediction of soot behavior, phenomenological soot model was revised to take into account the soot oxidation feedback on soot number density and good agreement was observed in the comparison of soot measurement and prediction. Results indicated that ignition delay prolonged with the decrease of initial ambient temperature. The heat release rate demonstrated the transition from mixing controlled combustion at high ambient temperature to premixed combustion mode at low ambient temperature. At lower ambient temperature, soot formation and oxidation mechanism were both suppressed. But finally soot mass concentration reduced with decreasing initial ambient temperature. Although the drop in ambient temperature did not cool the mean in-cylinder temperature during the combustion, it did shrink the total area of local high equivalence ratio, in which soot usually generated fast. At 700 K initial ambient temperature, soot emissions were almost negligible, which indicates that sootless combustion might be achieved at super low initial temperature operation conditions.

Development of a Phenomenological Soot Model Coupled with a Skeletal PAH Mechanism for Practical Engine Simulation

A new chemical mechanism with 12 species and 26 reactions for formation of polycyclic aromatic hydrocarbons (PAHs) was developed and integrated into a skeletal mechanism for oxidation of primary reference fuel (PRF). Coupled with the new skeletal PRF-PAH mechanism, an improved phenomenological soot model was further constructed based on our previous work. By validating against the experimental data on the related PAHs in four premixed laminar flames of n-heptane/iso-octane and three counterflow diffusion flames of n-heptane, it is indicated that the major species concentrations were well reproduced by the model. Moreover, validations of the new soot model show that the soot yield, particle diameter, and number density were predicted with reasonable agreement with the experimental data in a rich n-heptane shock tube over wide temperature and pressure ranges. Compared with the soot model with acetylene as precursor species, the new model agrees better with the measurement, which proves the necessity of incl...

LES/PDF based modeling of soot–turbulence interactions in turbulent flames

A large eddy simulation (LES)/probability density function (PDF) approach is used to describe the small-scale soot–turbulence–chemistry interactions. The PDF approach directly evolves the joint statistics of the gas-phase scalars and a set of moments of the soot number density function. This LES/PDF approach is then used to simulate a turbulent natural gas jet diffusion flame. Since the PDF equation is high dimensional, a Lagrangian method formulated in cylindrical coordinates is coupled to the Eulerian solution technique for the LES flow equations. The LES/PDF simulations show that soot formation is highly intermittent and is always restricted to the fuel-rich region of the flow. The PDF of soot moments has a wide spread leading to a large subfilter variance. Further, the conditional statistics of soot moments conditioned on mixture fraction and reaction progress variable show strong correlation between the gas phase composition and soot moments.

Absolute light calibration using S-type thermocouples

Absolute light calibration in optical measurements requires the use of a calibrated light source. The most common sources are blackbody furnaces or tungsten lamps, which are both problematic due to their expense and the requirement for frequent recalibration. In combustion research, the availability of an absolute light calibration makes possible the quantitative measurement of several quantities of interest (e.g., soot volume fraction and number density of species measured with fluorescence or chemiluminescence). In this study, a low-cost, simple, yet high-accuracy method is described that uses widely available S-type thermocouples to do absolute light calibration. The spectral emissivity of Pt and Pt–10% Rh wires was measured in the visible range. The measured spectral emissivity, temperature, and diameter of the heated thermocouple wires allow them to serve as a light source with spectral radiance that can be calculated by Planck’s law. The proposed absolute light calibration method with S-type thermocouples was used to determine the number density of excited-state CH radicals in a lifted, coflow laminar diffusion flame of methane and the soot volume fraction in coflow laminar diffusion flames of ethylene with varying amounts of nitrogen dilution. The results were compared with previous measurements and excellent agreement was achieved.

Monte Carlo simulation for soot dynamics

A new Monte Carlo method termed Comb-like frame Monte Carlo is developed to simulate the soot dynamics. Detailed stochastic error analysis is provided. Comb-like frame Monte Carlo is coupled with the gas phase solver Chemkin II to simulate soot formation in a 1-D premixed burner stabilized flame. The simulated soot number density, volume fraction, and particle size distribution all agree well with the measurement available in literature. The origin of the bimodal distribution of particle size distribution is revealed with quantitative proof.

Large eddy simulation subfilter modeling of soot-turbulence interactions

The small-scale interactions between turbulence, chemistry, and soot have a profound effect on the soot formation, growth, and destruction processes in turbulent reacting flows. In this work, the small-scale subfilter interactions between turbulence and soot are modeled using a presumed subfilter PDF for the statistical moments of the soot number density function. Due to a very large (infinite) Schmidt number, soot is confined to very thin structures. In addition, soot is formed initially from Polycyclic Aromatic Hydrocarbons, which exhibit a strong sensitivity to the local scalar dissipation rate in the flow field. These interactions of soot with the gas-phase chemistry, molecular transport, and turbulence result in very high spatial and temporal intermittency. Therefore, the soot subfilter PDF is presumed to be a pair of delta distributions with a sooting mode and a non-sooting mode. In addition to the mean values of the scalars, one additional parameter is needed to specify this distribution. The presumed soot subfilter PDF approach is evaluated a priori using a recent two-dimensional direct numerical simulation (DNS) database of soot evolution in a nonpremixed flame. The analysis shows that predictions of the soot intermittency as well as the soot source terms are improved with this presumed soot subfilter PDF approach compared to simply using the mean values of the soot scalars. Several choices for specifying the additional parameter of the PDF are also evaluated with the database. The results show that the parameter is best specified using the second moment of the soot number density. In addition, the transport equation for the second moment of the number density is briefly discussed. A model for the lone unclosed term is proposed and evaluated with the DNS data.

The effect of CO 2/H 2O on the formation of soot particles in the homogeneous environment of a rapid compression facility

Previous investigations show that soot particle volume fraction and number density were significantly reduced by exhaust gas recirculation (EGR) diluents CO 2 and H 2O. However, these investigations were often convoluted by their experimental flame configurations and primarily focused on soot volume fraction rather than soot inception. To isolate the effects on soot inception and the corresponding chemistry, the current study measured the reactivity of CO 2 (up to 9.5% volume fraction) for both C 2H 2 (1.00% volume fraction) and CH 4 (1.85% volume fraction) fuels in homogeneous mixtures. Computed effect of H 2O on these and other fuels are also presented. Experiments were performed at high temperature (1640 K and 1770 K) and high equivalence ratios ( Φ = 55 and 75) to understand the effect of CO 2 on polycyclic aromatic hydrocarbons (PAH) and formation of nascent soot particles with negligible oxygen influence. Experimental results show that CO 2 enhanced the soot inception rate when added to C 2H 2 but had an undetectable affect on CH 4. Gas chromatography confirmed that CO 2 increases CO mole fraction and reduces C 2H 2 fuel concentration. Chemical kinetic simulations showed that the C 2H 2 was being converted to soot precursors. CO 2 enhanced the soot inception rate for C 2H 2 by producing OH radicals. Images of nascent soot particles produced in the presence of CO 2 were used to determine the size of PAH molecules in the particles and particle morphology. Both attributes were similar to particles formed without CO 2. CO 2 had little impact on the long reaction pathway from CH 4 to PAH molecules because H and CH 3 radicals propagated these reactions more readily than OH radicals.