Detailed Soot Model Research Articles

Computational assessment of soot models in ethylene/air laminar diffusion flames

To improve the accuracy of soot formation and evolution predictions, several physical and chemical models have been developed over the last decades. These models include (i) detailed chemical kinetic mechanisms describing both gas-phase chemistry related to combustion processes and reaction pathways leading to large-sized aromatic molecules, which are needed for modelling soot formation, and (ii) soot models providing a comprehensive description of soot particle dynamics and interactions with gas-phase chemical species. Accordingly, in this work, two detailed soot models, the method of moments (MOM) and the discrete sectional method (DSM), are evaluated in ethylene/air laminar diffusion flames, and their corresponding results are compared with experimental measurements. Furthermore, the NBP and KM2 chemical kinetic mechanisms are assessed and compared with each other by examining key chemical species related to soot formation and evolution. To compute gas mixture’s radiative properties, the weighted sum of grey gases model considering a grey medium is also utilised. Finally, the contributions of the soot precursors known as PAH (polycyclic aromatic hydrocarbon) to soot formation are also analysed. The main results show that the discrepancies in PAH concentrations obtained with different chemical kinetic mechanisms can be significant. In addition, compared to MOM ones, DSM results obtained here show a better agreement with experimental data. Finally, the analysis of PAH shows that those with two (A2) to four (A4) aromatic rings impact the most on soot modelling. Specifically, contributions of A4 were found to be more significant at lower heights above the burner, whereas A2 was found to be more impactful downstream as the flame develops. Maximum contributions of A2 and A4 to the soot inception rate were 66% and 85%, respectively, whereas the maximum summed contribution of PAH with five (A4R5) to seven (A7) aromatic rings accounted for only 13% of the inception rate.

An easy but quantitative assessment of soot production rate and its dependence on temperature and pressure

The challenge of soot emission persists in combustion research due to the complexities of tracking the crucial stages of growth from fuel to soot nuclei and ultimately mature particles. Studying soot formation in flames often requires a sophisticated approach, involving detailed measurements of gaseous soot precursors and soot particles using multiple complementary diagnostics. On the other end of the spectrum of studies are simpler methods that capture the sooting tendency using a single index, akin to the cetane number in compression ignition engines and the octane number in spark ignition engines. This article seeks a middle ground, aiming to quantify the soot production rate while maintaining the simplicity of single-index characterizations. The approach involves establishing counterflow diffusion flames, measuring soot volume fraction through pyrometry, and accurately computing velocity and temperature profiles using a commercial code. These data allow for the quantification of the production rate from the soot governing equation. The methodology is applied to counterflow ethylene diffusion flames to examine the temperature dependence of the soot production rate across peak temperatures varying by several hundred degrees and pressures in the 1–32 atm range. The soot production rate per unit flame area falls within the range of 10−7–10−3 g/(cm2s) range and, when normalized with respect to the carbon flux, it ranges between 10−6 and nearly 10−2. On a logarithmic scale, it linearly correlates with the peak temperature at a fixed pressure. Although this study deals only with flames of ethylene, the approach can be generalized to any fuel. The resulting database should be valuable not only for industry practitioners but also to the scientific community for the global validation of detailed soot models.

Numerical and experimental investigations on the particle formation in oxymethylene ethers (OME[formula omitted], n = 2−4)/ethylene premixed flames

Alternative synthetic fuels can be produced by renewable energy sources and represent a potential route for solving long-term energy storage. Among them, oxygenated fuels have the advantage of significantly reducing pollutant emissions and can therefore be used as carbon-neutral substitute fuels for transportation. In this work, the sooting propensity of different oxymethylene ethers (OMEs) is investigated using a combined experimental and numerical study on a series of burner-stabilized premixed flames under mild to severe sooting conditions. Herein, mixtures of ethylene in combination with the three individual oxymethylene ether (OMEn) for n=2,3,4 are compared in terms of soot formation behavior with pure ethylene flames. The kinetic mechanism from Sun et al. (Proc. Combust. Inst. 36 (2017) 1269–1278) for OMEn combustion with n=1,2,3 is extended to include OME4 decomposition and combustion kinetics. It is combined with a detailed quadvariate soot model, which uses the Conditional Quadrature Method of Moments (CQMOM), and the soot simulation results are validated with Laser-Induced Fluorescence (LIF) and Laser-Induced Incandescence (LII) measurements. It is observed that the three investigated OMEn with n=2,3,4 show similar sooting behavior, mainly reducing larger aggregates while not significantly affecting the formation of smaller particles. Furthermore, the extent of soot reduction is comparable among the three OMEn. The trends and overall reduction are captured very well by the model. The modeling results are analyzed through reaction pathway analyses and sensitivity studies that show the importance of OMEn decomposition and the formation of formaldehyde (CH2O) under rich conditions for reducing species relevant for soot precursor formation. These findings are based on the negligible differences in soot formation between the different OMEn fuels observed in this study.

A numerical study on soot formation in methane-ethanol diffusion flames

• Soot volume fraction and particle number density increase with ethanol blending ratio. • The degree of soot enhancement decreases with further increase in ethanol blending ratio. • Effect of ethanol on soot concentration follows the order: chemical > dilution > thermal. • Thermal effect has a dominant influence on the flame temperature, but not on soot. • Ethanol addition promotes the HACA pathway and surface growth mechanism. Numerical simulation was carried out to investigate the effect of ethanol addition on the soot formation characteristics of methane diffusion flame. The ethanol blending ratio in methane ranged from 0 to 40 % in terms of the carbon contribution, ensuring a constant carbon flow rate for all cases. The sooting diffusion flames were modelled in an open-source environment, OpenFOAM, whereby-two different soot modelling approaches were attempted, including one detailed soot model and one semi-empirical soot model. Numerical results revealed that ethanol addition increased the soot volume fraction; however, further increase in the ethanol blending ratio (>20 %) reduced the relative enhancement of the soot concentration. Decoupling analysis was conducted to study the dilution, thermal, and chemical effects of ethanol separately. It was found that the dilution effect increased with the ethanol blending ratio. Also, the flame temperature was primarily affected by the thermal effect of ethanol. Despite these factors, the chemical effect of ethanol still played a dominant role in increasing soot formation. Analysis of the soot formation pathway revealed that ethanol addition promoted soot formation mainly through the acetylene addition reactions and polycyclic aromatic hydrocarbons (PAHs) addition reactions, which contributed to the hydrogen-abstraction-acetylene-addition (HACA) and surface growth mechanisms, respectively. Soot oxidation was enhanced with ethanol addition, which resulted in greater soot reduction; however, the overall effect of ethanol on soot formation remained positive.

A highly robust numerical approach for handling the bi-variate soot population balance models in internal combustion engines

ABSTRACT It is widely acknowledged that the soot particulate pollutants pose a serious threat to both the environment and human health. Reducing the emission of these soot particulate pollutants from engines is very urgent, to which a highly efficient and robust modeling tool for soot emission prediction could contribute significantly. In this work, a new and robust numerical approach to handle the detailed soot models in engine combustion simulations is presented. The new approach adopts an advanced bi-variate moment projection method to treat the complex soot particle dynamics. The bi-variate moment projection method is superior to other methods of moments in terms that it is able to solve the soot moment closure problem due to oxidation, which governs the evolution of soot particle populations inside the engine cylinder. The proposed approach has been validated by simulating the detailed evolution processes of the soot particle population inside a dual-fueled, four-cylinder compression ignition engine. It is shown to be highly efficient and robust. It is able to provide a comprehensive investigation into the soot particle dynamics inside the engine cylinder at low computational cost. The emissions of soot particles under different engine operating conditions could be predicted with high accuracy. These findings suggest that the new approach proposed in this work is a promising framework for prediction and analysis of the emission of soot particulate pollutants from internal combustion engines. It is a useful tool to explore clean combustion technology.

Research on the Soot Generation of Diesel Surrogate Mechanisms of Different Carbon Chain Length

This paper studies the generation process and emission characteristics of soot from Marine diesel engine. On the basis of Particulate size mimic (PSM) detailed soot model, the parameters related to soot generation obtained from the reaction mechanism calculation of 3 surrogate of different carbon chain length: n-heptane, n-tetradecane and n-tetradecane-toluene were compared and analyzed including precursor of soot, quality and density of soot, particle size distribution of empirical soot model. The results show that the soot nucleation stage of n-tetradecane-toluene mechanism was slightly more consistent than that of n-tetradecane mechanism with the experimental results, far more consistent than that of n-heptane mechanism. The intensity of surface growth and aggregation stage is greater, which is reflected in the soot precursor: acetylene and A4, and leads to that the soot produced by the mechanism of n-tetradecane and n-tetradecane-toluene is small in quality but large in quantity. The particle size distribution calculated by n-tetradecane and n-tetradecane-toluene mechanism is closer to the experimental data than that calculated by n-heptane mechanism. Then combustion process of n-tetradecane and n-tetradecane-toluene mechanism is more suitable for marine engine, which is reflected in the higher temperature and the smaller equivalent ratio in the cylinder.

Temperature dependence of ethanol and dimethyl ether chemical mixing effects on soot and gas products in ethylene pyrolysis

The present work aims to understand the temperature dependence of the fuel chemical mixing effects on soot and gas product formation in ethylene pyrolysis in a laminar flow reactor. The chemical and dilution effects of ethanol and dimethyl ether addition were studied experimentally by gas chromatography measurements and computationally using a detailed soot model coupled with PAHs chemistry. The chemical effects of ethanol and dimethyl ether mixing increased the production of CO, H2, and CH4, but the dilution effects decreased the production of H2, CH4, and C2H2. The chemical mixing effects inhibited the formation of C2H2 except for the case in which 50% ethanol was mixed with 50% ethylene at 1273 K. Ethanol and dimethyl ether mixing inhibited soot production, but the fuel mixing effects on soot reduction decreased with increasing temperature. The temperature dependence of the fuel mixing effects on soot production was dominated by the chemical effects rather than the dilution effects. Kinetic analysis of the soot formation process indicated that the methyl radical (CH3) from ethanol and dimethyl ether decomposition combined with C2 species (C2H2, C2H4) from ethylene pyrolysis to generate plenty of C3 species (C3H4–P, C3H6, C3H5-A, C3H4-A, C3H3), which further converted into benzene (C6H6) and polycyclic aromatic hydrocarbons (C10H8, C14H10, and C16H10). The reaction rate of the above C1+C2→C3→C6→PAHs pathway increased noticeably with increasing pyrolysis temperature, which strengthened the chemical effects of ethanol and dimethyl ether mixing on soot surface growth rate contributed by PAHs and therefore soot yield under high-temperature conditions.

Modeling soot particles as stable radicals: a chemical kinetic study on formation and oxidation. Part I. Soot formation in ethylene laminar premixed and counterflow diffusion flames

Based on recent theoretical and experimental evidences, this work proposes a detailed discrete sectional soot model considering particles and aggregates behaving as stable radicals, paving the way for the implementation of a game-changing assumption in soot kinetic models. The concentration of large closed-shell and open-shell polycyclic aromatic hydrocarbons (PAHs) levels out with the increase of their aromatic structure and, contrary to conventional assumptions in existing soot models, their reactivity approaches that of persistent radical species. These findings were implemented in the model here proposed to describe soot formation (Part I) and oxidation (Part II). While the number of lumped pseudo-species involved is significantly reduced compared to previous discrete sectional soot models since no distinction between the kinetics of closed-shell and open-shell particles is taken into account, the physical meaning and consistency with recent evidences improves together with model performances. Rate parameters of gas-phase reactions involving resonantly stabilized radical PAHs were used as a reference for soot particle kinetics. The model was validated against 65 target ethylene laminar premixed flames and counterflow diffusion flames, with a general good agreement between experimental data and numerical simulations at different pressures (P = 1–8 atm), maximum flame temperatures (Tmax∼1500–2400 K) and soot yields (fv∼0.01 to ∼170 ppm). Parity diagrams allow to identify systematic deviations of the present model from the experiments, while the comparison between all the flames included in the database highlights the relative contribution to soot formation and growth of the main reaction classes constituting the soot model, spotting analogies and peculiarities of the two flame configurations considered. Similar kinetic patterns characterize most of the flames within the database, independently of flame configuration and sooting tendency of the specific case. Large PAH condensation dominates the formation of the first soot nuclei, while the condensation of smaller PAHs up to four aromatic rings is primarily responsible for the growth of larger soot particles and aggregates. Finally, the contribution of the different fluid dynamics in premixed and counterflow flames to particle evolution, resulting in a higher hydrogenation level of soot particles in the latter, is discussed.

Modeling soot particles as stable radicals: a chemical kinetic study on formation and oxidation. Part II. Soot oxidation in flow reactors and laminar flames

Recent experimental and theoretical studies have shed light on the persistent radical behavior of large polycyclic aromatic hydrocarbons (PAHs) and soot particles. The first detailed soot model developed based on these findings is here proposed. While its good performances in predicting soot formation and growth when soot oxidation is negligible are presented in the companion paper of this work (Part I), the model is here validated against 45 cases of particle size distribution functions (PSDF) and soot volume fraction under oxidizing conditions. Notably, we show that by considering stable radical particles and aggregates it is possible to reproduce soot oxidation by O2 in plug flow reactors under very diluted conditions, whereas existing models could not predict particle consumption in the absence of particle surface activating species. Therefore, this work paves the way for a different approach to soot modeling that could help to better interpret soot reactivity. Oxidation of soot produced from different fuels (ethylene, n-heptane, and toluene, i.e. leading to different particle graphitization) at different temperatures (950–1073 K) and O2 concentrations (1000–10000 ppm) is successfully modeled adopting reference oxidation rates of gas-phase resonantly stabilized PAH radicals, featuring analogous reactivity compared to soot species, and by also accounting for different particle hydrogenation levels. Good model performances are also obtained in both rich and lean premixed ethylene flames (Tmax∼1480–1510 K) and in two sets of counterflow ethylene soot formation/oxidation flames (SFO, Tmax∼2560–2825 K), where OH radical becomes the dominant oxidizer. Finally, model limitations emerging in some of the cases considered are discussed through a sensitivity analysis and with references to other oxidation mechanisms proposed in the literature in order to highlight possible future improvement of the model predictive capability

Numerical Investigation of Ducted Fuel Injection Strategy for Soot Reduction in Compression Ignition Engine

The ducted fuel injection strategy is a method that significantly reduces soot emissions in direct injection compression ignition engines. Fuel is injected into the combustion chamber through a duct enhancing the air-fuel mixture. It guarantees more efficient combustion and less soot formation by reducing the equivalence ratio at the autoignition zone inside the combustion chamber. The effects of the duct fuel injection on the performance, combustion, and emission of the compression ignition engine were numerically investigated in this study. The duct geometries with varying diameters, lengths, and stand-off distances were examined to find the most appropriate size using an experimentally validated CFD model and detailed soot model. Results show that up to 66.7% reduction in soot emissions were observed with the usage of the ducted fuel injection strategy compared to conventional diesel combustion. In addition to reducing soot emissions, the ducted fuel injection strategy decreased CO and HC emissions by 20.4% and 7.8%, respectively. While the ducted fuel injection strategy reduces emissions, it does not decrease engine performance; on the contrary, it increases gross IMEP by 0.58%.

A Detailed PAH and Soot Model for Complex Fuels in CFD Applications

A model to predict soot evolution during the combustion of complex fuels is presented. On one hand, gas phase, hbox {polycyclic aromatic hydrocarbon (PAH)} and soot chemistry are kept large enough to cover all relevant processes in aero engines. On the other hand, the mechanisms are reduced as far as possible, to enable complex computational fluid dynamics (CFD) combustion simulations. This is important because all species transport equations are solved directly in the hbox {CFD}. Moreover, emphasis is placed on the applicability of the model for a variety of fuels and operating conditions without adjusting it. A kinetic scheme is derived to describe the chemical breakdown of short- and long-chain hydrocarbon fuels and even blends of them. hbox {PAHs} are the primary soot precursors which are modeled by a sectional approach. The reversibility of the interaction between different hbox {PAH} classes is achieved by the introduction of hbox {PAH} radicals. Soot particles are captured by a detailed sectional approach too, which takes a non-spherical growth of particles into account. In this way the modeling of surface processes is improved. The applicability and validity of the gas phase, hbox {PAH}, and soot model is demonstrated by a large number of shock tube experiments, as well as in atmospheric laminar sooting flames. The presented model achieves excellent results for a wide range of operating conditions and fuels. One set of model constants is used for all simulations and no case-dependent optimization is required.

Modelling of laminar diffusion flames with biodiesel blends and soot formation

Modelling of soot formation and oxidation of biodiesel fuels is a challenge, due to the complexity of the chemical reactions and soot formation pathways. In this paper, the discretised population balance approach and a PAH-HACA based detailed soot model have been coupled with an in-house CFD code to simulate a laminar diffusion flame with blends of different components of oxygenated biodiesel fuel, and employed to predict combustion performance and soot formation. The simulation aims to reproduce a target experiment which investigated the effects of dibutyl ether (DBE) addition to the biodiesel surrogate (methyl decanoate, MD) by increasing the mole fraction of DBE from 0 to 40%. A combined and reduced MD–DBE–PAH mechanism developed from three sub-mechanism branches has been employed in the simulation. The simulation results show that temperature rises as the DBE percentage increases to 40%. The swallow-tail shape of the soot occurrence zone and the quantity of soot production are correctly predicted. Regarding the effect of soot suppression, the model has basically captured the reducing tendency of soot formation in the measurements as the DBE addition increases from 0% to 40%. Concentrations of PAHs and C3 species contributing to the formation of aromatic rings are slightly reduced due to the addition of DBE, which is a leading cause of soot suppression. However, on the whole, the numerical solution featured much smaller differences than those observed in the experiment among laminar flames with different MD/DBE ratios, because the combined MD–DBE–PAH mechanism only present a slight difference of concentrations of soot precursor species.

In-Cylinder Validation of a Method of Moments-Based Soot Model for Diesel Engines

<div class="section abstract"><div class="htmlview paragraph">Detailed soot models based on method of moments have been reported for combustion engines for more than twenty years. However, these models have always been validated against soot measured at exhaust and none of the modeling works have validated the spatial and temporal evolution of modeled soot with in-cylinder data. In this article a soot model based on method of moments has been evaluated by comparing simulations with the published experimental data for varying operating conditions of the heavy-duty optical engine at Sandia National Laboratories. Closed cycle combustion simulations have been performed using CONVERGE, a computational fluid dynamics solver for reacting multiphase flows. Before modeling soot, confidence in non-reactive modeling has been ensured by validating spray and equivalence ratio distributions from simulations with experimental optical images at different crank angles for each operating condition. Reduced kinetic schemes available in the literature for diesel engines have been assessed to identify the most promising one based on its performance across all operating conditions. The soot mass modeled by the method of moments has shown less sensitivity to the involved soot model constants, and a good agreement for temporal evolution and spatial soot distribution for the considered cases.</div></div>

Lagrangian particle tracking with new weighted fraction Monte Carlo method for studying the soot particle size distributions in premixed flames

PurposeThe purpose of this paper is to study the soot formation and evolution by using this newly developed Lagrangian particle tracking with weighted fraction Monte Carlo (LPT-WFMC) method.Design/methodology/approachThe weighted soot particles are used in this MC framework and is tracked using Lagrangian approach. A detailed soot model based on the LPT-WFMC method is used to study the soot formation and evolution in ethylene laminar premixed flames.FindingsThe LPT-WFMC method is validated by both experimental and numerical results of the direct simulation Monte Carlo (DSMC) and Multi-Monte Carlo (MMC) methods. Compared with DSMC and MMC methods, the stochastic error analysis shows this new LPT-WFMC method could further extend the particle size distributions (PSDs) and improve the accuracy for predicting soot PSDs at larger particle size regime.Originality/valueCompared with conventional weighted particle schemes, the weight distributions in LPT-WFMC method are adjustable by adopting different fraction functions. As a result, the number of numerical soot particles in each size interval could be also adjustable. The stochastic error of PSDs in larger particle size regime can also be minimized by increasing the number of numerical soot particles at larger size interval.

Numerical Investigation on the Effect of the Oxymethylene Ether-3 (OME3) Blending Ratio in Premixed Sooting Ethylene Flames

Synthetic fuels, especially oxygenated fuels, which can be used as blending components, make it possible to modify the emission properties of conventional fossil fuels. Among oxygenated fuels, one promising candidate is oxymethylene ether-3 (OME3). In this work, the sooting propensity of ethylene (C2H4) blended with OME3 is numerically investigated on a series of laminar burner-stabilized premixed flames with increasing amounts of OME3, from pure ethylene to pure OME3. The numerical analysis is performed using the Conditional Quadrature Method of Moments combined with a detailed physico-chemical soot model. Two different equivalence ratios corresponding to a lightly and a highly sooting flame condition have been investigated. The study examines how different blending ratios of the two fuels affect soot particle formation and a correlation between OME3 blending ratio and corresponding soot reduction is established. The soot precursor species in the gas-phase are analyzed along with the soot volume fraction of small nanoparticles and large aggregates. Furthermore, the influence of the OME3 blending on the particle size distribution is studied applying the entropy maximization concept. The effect of increasing amounts of OME3 is found to be different for soot nanoparticles and larger aggregates. While OME3 blending significantly reduces the amount of larger aggregates, only large amounts of OME3, close to pure OME3, lead to a considerable suppression of nanoparticles formed throughout the flame. A linear correlation is identified between the OME3 content in the fuel and the reduction in the soot volume fraction of larger aggregates, while smaller blending ratios may lead to an increased number of nanoparticles for some positions in the flame for the richer flame condition.

Thermodynamics second-law analysis of hydrocarbon diffusion flames: Effects of soot and temperature

In this paper, a computation of the ethylene co-flow diffusion flame is carried out using the Co-flame code with a detailed gas-phase mechanism and soot model. A numerical analysis based on the thermodynamics second-law was conducted for developing a high-efficiency combustion condition of hydrocarbon diffusion flames. The entropy generations and exergy loss ratios due to thermal radiation, heat conduction and convection, and chemical reactions are numerically calculated, in which entropy generations caused by both gasses and soot particle are considered. The effects of soot (controlled by changing stoichiometric mixture fraction Zst from 0.1 to 0.62) and adiabatic flame temperature Tad (2300 K, 2500 K, 2700 K) on entropy generation and exergy loss ratio in flame are investigated. The results suggest that, entropy generation due to soot radiation and soot surface growth and oxidation accounts for about 15% and 5% in total radiation entropy generation and total chemical entropy generation, respectively. Entropy generation due to thermal radiation decreases when soot reduces, and entropy generation due to heat conduction and convection, mass diffusion and chemical reactions increases when Zst increases. When Tad is 2300 K, the change of exergy loss ratio is slight with a reducing of soot and increasing of Zst. When Tad is 2500 K and 2700 K, the exergy loss ratio has tendency of increasing with a reducing of soot and increasing of Zst. The exergy loss ratio increases with increasing of Tad.

Comparison of soot models for reacting sprays in diesel engine-like conditions

Stringent emission legislations and growing health concerns have contributed to the evolution of soot modeling in diesel engines from simple empirical relations to methods involving detailed kinetics and complex aerosol dynamics. In this paper, four different soot models have been evaluated for the high temperature, high pressure combusting dodecane spray cases of engine combustion network (ECN) spray A which mimics engine-relevant conditions. The soot models considered include an empirical, a multistep, a method of moments based, and a discrete sectional method soot model. Two experimental cases with ambient oxygen volume of 21% and 15% have been modeled. A good agreement between simulations and experiments for vapor penetration and heat release rate has been obtained. Quasi-steady soot volume fraction contours for the four soot models have been compared with experiments. Contours of the species and source terms involved in soot modeling have also been compared for a better understanding of soot processes. The empirical soot model results in higher magnitude and spread of soot due to a lack of modeling framework for oxidation through OH species. Among the four models studied, the multistep soot model has been observed to provide the most promising agreement with the experimental data in terms of distribution of soot and location of peak soot volume fraction. Due to a two-way coupling of soot models, the detailed models predict an upstream location for soot as compared to the multi-step soot model which is one way coupled. A significant difference (of an order of magnitude) in the concentration of PAH (polycyclic aromatic hydrocarbons) precursor between multistep and detailed soot models has been observed because of precursor consumption due to the coupling of detailed soot models with chemical kinetics. It is recommended that kinetic schemes, especially those concerning PAH, be validated with experimental data with a kinetics-coupled soot model.

Numerical modelling to study the effect of DC electric field on a laminar ethylene diffusion flame

An applied DC electric field was experimentally demonstrated to modify the flame structure and gas dynamic in an ethylene diffusion flame. The aim of this paper is to investigate the influence of the electric field on the flow field and its impacts on the flame behavior. A numerical study has been performed to elucidate the experimental observations and to monitor the effect of electric body force on the flame. The numerical model was validated by comparing the computed results to experimental measurements from the literature. The resulting computed flame shape was compared to a visible image taken during the experiment. The simulated OH mole fraction, the burning rate and the computed velocity and temperature are presented. The developed model proved the ability to reproduce qualitatively the experimental flame behavior when submitted to the electric field. The electric field is shown to modify the flame shape (flame tip, flame shortness and flame deformation), to promote the burning process and to improve the ion production. Results show that the modifications are due to an air entrainment acting specifically near the burner zone enhancing the mixture and changing the fluid dynamic in this region. The ionic wind is demonstrated to increase the maximum burning rate and promoting ions' formation mostly near the burner. A more detailed model (detailed ions' chemistry and soot model with charged particles, detailed electric diffusion) is necessary to gain a better understanding of the influence of electric field on diffusion combustion and soot formation.

Soot emission reduction in oxygenated co-flow jet flames

A computational study was performed for ethylene/air non-premixed laminar co-flow jet flames using an axisymmetric CFD code to explore the effect of oxygenation on PAH and soot emissions. Oxygenated flames were established using N2 diluted fuel stream along with O2 enriched air stream such that the stoichiometric mixture fraction (Ζst) is varied but the adiabatic flame temperature is not materially changed. Simulations were carried out using a spatially and temporally accurate algorithm with detailed chemistry and transport. A detailed kinetic model involving 111 species and 784 reactions and a fairly detailed soot model were incorporated into the code. Two different approaches, one with constant flame height and other with constant inlet velocity are comprehensively examined to bring out the effects of changes in flame structure and residence time on soot emissions with respect to Zst. With increase in Ζst, a drastic reduction in the formation of soot precursors (acetylene and benzene) and thus in soot emissions are observed. In the present study, oxygenated flames with Ζst ≥ 0.424 are considered as blue flames or completely soot free. For various oxygenated flames a C/O ratio between 0.45 and 0.6 is found to be most favorable for soot formation.

Soot Emission Simulations of a Single Sector Model Combustor Using Incompletely Stirred Reactor Network Modeling

AbstractThe simulation of soot evolution is a problem of relevance for the development of low-emission aero-engine combustors. Apart from detailed CFD approaches, it is important to also develop models with modest computational cost so that a large number of geometries can be explored, especially in view of the need to predict engine-out soot particle size distributions (PSDs) to meet future regulations. This paper presents an approach based on Incompletely Stirred Reactor Network (ISRN) modeling that simplifies calculations, allowing for the use of very complex chemistry and soot models. The method relies on a network of incompletely stirred reactors (ISRs), which are inhomogeneous in terms of mixture fraction but characterized by homogeneous conditional averages, with the conditioning performed on the mixture fraction. The ISRN approach is demonstrated here for a single sector lean-burn model combustor operating on Jet-A1 fuel in pilot-only mode, for which detailed CFD and experimental data are available. Results show that reasonable accuracy is obtained at a significantly reduced computational cost. Real fuel chemistry and a detailed physicochemical sectional soot model are consequently employed to investigate the sensitivity of ISRN predictions to the chosen chemical mechanism and provide an estimate of the soot PSD at the combustor exit.