Soot Particle Number Density Research Articles

Comparative study of NH3 and H2 impacts on combustion and emission characteristics of n-Hexadecane under different flame conditions

The addition of renewable fuel to hydrocarbon fuel mitigated the carbon monoxide, carbon dioxide and soot particle emissions of internal combustion engines. A comparative study of the ammonia and hydrogen effect on combustion and emission characteristics of n-hexadecane flames was executed at various flame conditions. The results showed that alternative fuels had a stronger impact on peak temperature at diffusion flame conditions rather than at premixed flame conditions. The premixed flame released carbon monoxide emission lower than the diffusion flames with/without additional fuels. The carbon monoxide emissions decreased highly with ammonia enrichment instead of hydrogen. At premixed and coflow diffusion flame conditions, the ammonia highly reduced carbon dioxide emission compared to hydrogen. Hydrogen addition reduced the nitrogen oxide emission, whereas it was increased by ammonia. The soot particle number density was reduced highly by hydrogen addition instead of ammonia at the premixed flow flame and diffusion coflow flame conditions. The impact of hydrogen on the reduction of soot mass concentration emissions was greater than that of ammonia at all combustion conditions. It is suggested that H2 ≥ 70% should be mixed in n-hexadecane fuel because a higher burning velocity can be achieved with a higher emission reduction. A reduction in the emission of carbon-related species can be realized with a small reduction in burning velocity at NH3 < 70%.

Effect of Pressure on Burning and Soot Characteristics of RP-3 Kerosene Droplets under Sub-Atmospheric Pressure.

The burning and soot characteristics of RP-3 kerosene droplets under sub-atmospheric pressure were experimentally investigated in a pressure chamber. The droplet size during combustion was continuously recorded using a high-speed camera, and the burning rate based on the d 2-law was determined. The flame temperature was calculated from ICCD camera spectral data using two-color pyrometry, and the carbon soot volume fraction was measured by the calibrated laser-induced incandescence (LII) technique. Soot particles were also sampled using a thermophoretic deposition probe and characterized using a transmission electron microscope (TEM) for particle size and morphology. The results showed that the droplet burning rate increased monotonically with increasing pressure under sub-atmospheric pressure, and the flame temperature slightly decreased with increasing pressure. Sub-atmospheric pressure environment significantly inhibits the formation of soot particle clusters during the ignition of droplets. The average soot volume fraction in the flame increases approximately with increasing pressure at 0.2-1.0 bar with a power of 2.044 ± 0.066. As the pressure decreases from 1.0 to 0.2 bar, the average soot volume fraction decreases significantly from 11.801 to 0.437 ppm. This is mainly due to the fact that the sub-atmospheric environment not only inhibits the collision growth of soot particles but also promotes the oxidation process of soot particles. The collected soot particles reveal a significant reduction in particle size under sub-atmospheric pressure with average primary soot particles of 48.23, 40.06, 27.096, and 18.718 nm at 1.0, 0.6, 0.4, and 0.3 bar pressures, respectively. The change in the number density of carbon soot particles is not significant, which reveals that the change in the diameter of soot particles under sub-atmospheric pressure plays a key role in the change in the volume fraction of soot.

H2 impact on combustion kinetics, soot formation, and NOx emission of hydrocarbon fuel flames

Soot and carbon dioxide released from internal combustion engines became the key issues when using fossil fuels. The use of zero-carbon fuel, hydrogen, with hydrocarbon fuels can play an important role in reducing the exhaust effect on the environment and mitigating the reliance on nonrenewable energy resources. A numerical approach was executed to elucidate the H2 impact on combustion kinetics, soot formation, and NOx emission of hydrocarbon fuel under premixed and diffusion flame conditions. The results showed that hydrogen deteriorated the formations of particle number density, the volume fraction of soot, carbon monoxide, carbon dioxide and polycyclic aromatic hydrocarbons thru the reduction of small chain hydrocarbons at both premixed and diffusion flame conditions. The soot volume fraction and soot particle number density formation were prevailing in the air region of the combustion chamber at diffusion flame conditions. NO3 produced from fuel-related species such as CHO and air-related species such as NO. H2 reduced the peak molar fraction of small chain HC including CHO related species, resulting in a reduction in NO3 emissions. C3H3 species dominantly participated to reduce the CO2 and NO3 emissions, while C2H2 and C5H4CH2 were the main species to control the soot precursors formation in both HC fuel flames. Besides, NH3 emission was limited by C2H2 in DS flames.

Effects of blending methane, propane, and propylene on soot evolution in ethylene diffusion flames based on optical diagnostics

Soot evolution of the ethylene-based laminar coflow diffusion flame doping with methane, propane, and propylene with the blending ratio ranging from 0 to 50% has been investigated experimentally. These fuels were selected for their different fuel structure. Spatially resolved distribution maps of the flame temperature, soot volume fraction, the fluorescence signals of polycyclic aromatic hydrocarbons (PAHs), and primary soot particle diameter and number density were determined utilizing laser-induced incandescence (LII), laser-induced fluorescence (LIF), and ratio pyrometry (RP). It was found that the flame height surged and the flame temperature dropped dramatically when the propylene blending ratio was 20% while they were almost constant with the increasing addition of methane or propane. The soot/PAHs loading decreased monotonically with the addition of methane and the opposite was true in the cases of blending propylene. The synergistic effect of mixing propane into ethylene was found on both the soot/PAHs loading. The same trends were observed in the percentage of carbon conversion to soot and the primary soot particle diameter. It can be seen from the soot evolution analysis that methane addition suppressed the soot nucleation and surface growth simultaneously. The enhanced soot surface growth for the flames doping with propane at a low addition level and for the whole flame sets doping with propylene were attributed to the earlier inception time and the longer mass growth time. The preferential diffusion and weakened oxidation of soot particles because of the decreased flame temperature perhaps accounted for the transition from a non-smoking flame to a smoking flame when the propylene blending ratio was greater than 40%.

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.

Soot volume fraction and size measurements over laminar pool flames and pre-vaporised non-premixed flames of biofuels, methyl esters and blends with diesel

Previous studies have demonstrated that methyl ester-based biodiesels produce significantly less soot compared to petroleum diesel in standard diffusion flames. However, the roles of the oxygen content and the degrees of unsaturation (DoU) of the biodiesels on their sooting propensity have not been independently analysed. In the present study, the relationship between soot yield of biodiesels and the DoU of the fuel is systematically analysed in laminar pool flames and pre-vaporised diffusion flames. The spatial distribution of soot volume fraction of the flames fuelled with four biodiesel fuels, two methyl esters and their blends with diesel are quantitatively measured using laser induced incandescence. All six biofuels have different DoU but similar oxygen mass fractions, so that the effect of the DoU on soot formation could be separately considered. The soot yield using biodiesels was measured to be lower than that of diesel fuel by a factor up to 7.5 in pool flames and by a factor of 4.0 in vapour flames. In both flame setups, the soot reduction effect of biodiesels ranks in order of DoU. The more saturated the fuel molecule is, the lower the soot yield. The soot reduction effect of the blending ratio with biodiesel was found to be non-linear, and different in pool and vapour flames. The morphology, size, and number density of soot particles were analysed using scanning electronic microscopy. The results suggest that the shape of primary soot particles is close to spherical and the structure of the point-contact aggregates is similar in all cases. Biofuels not only produce smaller particle sizes, but also fewer particles compared to diesel. Chemical kinetic simulations suggest that blending of biodiesels reduces the soot yield primarily by slowing the soot surface growth process, and secondarily by prohibiting the soot nucleation and inception. Both the nucleation (C3H3 and benzene) and growth species (C2H2) for soot formation take place more quickly and intensely for the combustion of more unsaturated fuels, which results in the correspondingly larger soot yield.

Flamelet LES of a turbulent pulverized solid fuel flame using a detailed phenomenological soot model

In this work, large-eddy simulation (LES) is conducted for a turbulent pulverized coal flame with a detailed phenomenological model to characterize the soot evolution and a flamelet model to describe the detailed gas phase kinetics. In particular, a quadrature-based moment method (QBMM) is adopted to describe the soot statistics with the physicochemical processes of soot particle nucleation, condensation, coagulation, chemical surface growth and oxidization being considered. The simulation results are compared to the experimental data, including the direct photograph images, particle velocities, laser induced fluorescence (LIF) for polycyclic aromatic hydrocarbons (PAHs), laser induced incandescence (LII) for soot, scanning electron microscope (SEM) with the thermophoretic sampling (TS), and the time-resolved LII (TiRe-LII) for spatial soot particle size distribution. The comparisons show that the ignition position and the mean velocity of the coal particles can be accurately predicted with the proposed flamelet/QBMM/LES method, and an overall good agreement is also obtained when comparing against the PAHs-LIF, soot-LII and SEM images. The importance of the contributions of the different physicochemical processes to the soot particle volume growth is quantified by analyzing the corresponding moment source terms. It is found that for the pulverized coal flame studied, the chemical surface growth is the most important contribution to the soot volume growth at different locations along the streamwise direction. Although the nucleation does not make a significant contribution to soot volume growth, it determines the number density of the primary soot particles formed in the main flow region.

Soot Size Determination In A Swirl Stratified Premixed Ethylene-Air Flame By A Sampling Probe And Elastic Light Scattering

An experimental study was performed to measure the size and number density distributions of soot particles produced in turbulent, stratified (high spatial equivalence ratio gradient), swirled (rotating flow) premixed ethylene-air flames at atmospheric pressure. Soot particle size and number density measurements are initially performed using a Scanning Mobility Particle Sizer (SMPS). For this purpose, two-stage sampling and dilution device designed for hot gas analysis was used. The dilution rate of the sampling apparatus was evaluated to ensure a most representative probing. In a second step, the ex-situ measurements were replaced by single-shot measurements recorded with a high cadency planar multi-angle light scattering (2D-MALS) laser diagnostic technique, based on the relationship between aggregate size and light scattering angle. In a third step, results obtained by the SMPS as well as by the optical diagnostic were compared. Because a direct confrontation of the obtained results obtained is not feasible due to difference in nature of both techniques, a two-step post-processing methodology as developed. For this purpose, the median electric mobility diameters measured by SMPS were converted into diameters of gyration by means of a semi-empirical conversion tool. Moreover, a time average data processing was applied to the soot size distributions recorded by 2D-MALS, considering that SMPS measurements performed over a relatively large acquisition time converge to a single value. Results obtained in stratified swirled premixed flames by SMPS exhibit an overall good agreement with the optical measurements. This good similarity between the results provides a remarkably high degree of confidence in the quality of the measurements for both measurement systems investigated. Furthermore, the results suggest that a comparison of soot aggregate size and number density measurements between a sampling probe and a light scattering technique is still possible in turbulent flames at atmospheric pressure. While the current work has focused on the performances of SMPS and the 2D-MALS approach on a one-to-one basis, future work will also address the application of these diagnostics to high-pressure two-phase flames for which measurements are made in confined environments.

A numerical approach to elucidate the combustion and emission characteristics of n-dodecane under hydrogen enrichment

Soot and CO2 emission are the major challenges when using fossil fuels in internal combustion engines. To mitigate the exhaust effect on the environment and reduce the dependency on fossil fuels, it is a better idea to utilize zero-carbon fuel, H2, with hydrocarbon fuels. A numerical approach was executed to elucidate the combustion and emission characteristics of n-dodecane under hydrogen enrichment. It was found that H and OH made a huge contribution to the formation of benzene to pyrene aromatic rings, while C2H2, C3H3 and C5H4CH2 linear hydrocarbons produced the first benzene aromatic rings of PAH (polycyclic aromatic hydrocarbons). The trend of peak molar fraction of benzene and pyrene was observed correlated to that of small and linear HC (hydrocarbons) at various H2 ratios. Hydrogen mitigated the production of acetylene, C3H3, C5H4CH2, C2H3, ethylene and C3H2, and later these species impeded the formation of benzene and pyrene. Besides, the diminution in the production rate of CH3O and CH2O species indicated a reduction in CO2 emission when adding H2. H2O and CO2 encouraged NH3 and NO2 production, respectively. H2 mitigated the production of peak soot mass concentration and soot particle number density, whereas gravity impact only reduced the peak soot particle number density in the flame.

Evolution of Soot Particle Number, Mass and Size Distribution along the Exhaust Line of a Heavy-Duty Engine Fueled with Compressed Natural Gas

An experimental study has been conducted to provide a characterization of the transformations that particle size distributions and the number density of soot particles can encounter along the exhaust line of a modern EURO VI compliant heavy-duty engine, fueled with compressed natural gas. Being aware of the particles history in the exhausts can be of utmost importance to understand soot formation and oxidation dynamics, so that, new strategies for further reducing these emissions can be formulated and present and future regulations met. To this purpose, particle samples were collected from several points along the exhaust pipe, namely upstream and downstream of each device the exhaust gases interact with. The engine was turbocharged and equipped with a two-stage after-treatment system. The measurements were carried out in steady conditions while the engine operated in stoichiometric conditions. Particle emissions were measured using a fast-response particle size spectrometer (DMS500) so that size information was analyzed in the range between 5 and 1000 nm. Particle mass information was derived from size distribution data using a correlation available in the literature. The reported results provide more insight on the particle emission process related to natural gas engines and, in particular, point out the effects that the turbine and the after-treatment devices produce on soot particles. Furthermore, the reported observations suggest that soot particles might not derive only from the fuel, namely, external sources, such as lubricant oil, might have a relevant role in soot formation.

A progress review of practical soot modelling development in diesel engine combustion

Abstract Great efforts have been made over the last few decades in understanding of the mechanisms and phenomenology of soot formation and burnout in combustion systems. In this paper, theoretical advances in modelling soot formation and oxidation under engine circumstance are surveyed based on the published works, particularly focus on the practical soot modelling coupled with engine computational fluid dynamics (CFD) numerical studies. The types of soot models can be divided up into three main groups: empirical, semi-empirical and detailed soot model. With the development of computing technology and increasingly comprehensive soot fundamental knowledge, the semi-empirical phenomenological soot model with major generic processes of soot formation was proposed, which is one of the most extensively investigated soot models in recent years. It is highlighted with solving mathematical equations to get soot particle number density, soot volume and mass fraction. When coupled with fuel chemistry, the detailed soot model seeks to predict soot characteristics based on molecule structure, bringing further insight of the soot evolution and transient behavior of soot-relevant species. Therefore, the sooting tendency from engine combustion fueled with alternative and oxygenated fuels are able to be further explored, which is conductive to propose new engine technologies for soot mitigation and future fuel strategy.

Numerical optimization of a diesel combustion system to reduce soot mass and particle number density

In view of the recent emission legislations imposing a limit on the soot particle number density, a numerical optimization has been presented for a constant speed heavy-duty diesel engine to reduce soot particle number density while ensuring no penalty on NOx emissions. Closed cycle combustion simulations have been performed at three loads with the sectional soot model coupled to gas phase kinetics. The combustion model has been tuned at each load to ensure a good agreement for in-cylinder pressure, heat release rate and exhaust emissions. The validated combustion model has been used to study the effect of various parameters like intake swirl, start of injection, number of nozzle holes, spray angle and piston bowl shapes. Combustion simulations have been performed for every case of a full factorial design of experiments with 486 cases. Change in plume dynamics due to each parameter has been discussed in detail with the help of in-cylinder soot contours. A reentrant bowl with a higher swirl, lower number of nozzle holes and a deeper spray angle has been found to be the most promising option as it reduces the exhaust soot mass, number density with no penalty on NOx and specific fuel consumption.

Soot formation and evolution characteristics in premixed methane/ethylene-oxygen-argon burner-stabilized stagnation flames

The soot formation and evolution characteristics in premixed methane/ethylene-oxygen-argon flames are studied experimentally and numerically. The soot particles sampled with an in-situ probe sampling method are measured for different light hydrocarbon fuels (i.e., methane and ethylene), heights above the exit surface of the burner (HAB), equivalence ratios, ϕ and Reynolds numbers of flame jet, Rej. A novel stochastically weighted operator splitting Monte Carlo (SWOSMC) method coupled with detailed soot model is developed to simulate the evolution of soot particle size distribution (PSD) in premixed methane/ethylene-oxygen-argon flames. The flame temperature decreases while geometric mean diameter of soot particles increases with increasing ϕ. With increasing ϕ, condensation and nucleation processes are enhanced in methane flame while coagulation and nucleation processes are enhanced in ethylene flame. Hence, these processes lead to an increase of total soot number concentration for methane flame but a decrease for ethylene flame. A distinctly different variation trend of the normalized total number density of soot particles for methane flame along with HAB can be found for low and high studied Rej ranges. Similarly, a distinctly different variation trend of geometric mean diameter of soot particles for ethylene flames along with HAB can also be found for low and high studied Rej ranges. The numerical simulation results obtained via the fully validated SWOSMC method show excellent agreement with experimental results in the present study. Consistent with the experimental results, the numerical simulation results show that both condensation and nucleation rates increase and become dominant processes in the methane flame while coagulation rate increases in ethylene flame with increasing ϕ. Meanwhile, the simulated nucleation and coagulation rates along the centerline of both methane and ethylene burner flames are reversed with increasing Rej. The present study not only verifies the competition phenomenon among different soot dynamic processes but also shows that this competition can be reversed for different Rej ranges. These results show that the soot formation and evolution characteristics in the studied premixed methane/ethylene-oxygen-argon flames are determined by competition among different soot dynamic processes.

Experimental determination of distributions of soot particle diameter and number density by emission and scattering techniques**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0601900), the National Natural Science Foundation of China (Grant Nos. 51827808, 51821004, and 51406095), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 2018ZD03 and 2017ZZD005), and Science and

A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504C cameras were utilized for the scattering measurement, with consideration of the attenuation effect in the flames according to corresponding absorption coefficients. Distributions of soot particle diameter and number density were simultaneously determined using the measured scattering coefficients and absorption coefficients under multiple wavelengths already measured with a SOC701V hyper-spectral imaging device, according to the Mie scattering theory. A flame was produced using an axisymmetric laminar diffusion flame burner with 194 mL/min ethylene and 284 L/min air, and distributions of particle diameter and number density for the flame were presented. Consequently, the distributions of soot volume fraction were calculated using these two parameters as well, which were in good agreement with the results calculated according to the Rayleigh approximation, demonstrating that the proposed diagnostic method is capable of simultaneous determination of the distributions of soot particle diameter and number density.

A numerically robust method of moments with number density function reconstruction and its application to soot formation, growth and oxidation

Several method of moments (MOM) models were developed recently to describe the evolution of soot particle populations. However, especially soot oxidation is still very challenging in MOM, as pointwise information of the underlying soot particle number density function (NDF) is usually unknown and thus, the prediction of smallest particles that oxidize completely is not easy. The recently proposed Extended Quadrature Method of Moments (EQMOM) (Yuan, Laurent, & Fox, 2012) has the potential to resolve this issue providing a continuous NDF reconstruction based on the set of transported moments. While its general suitability for soot was already demonstrated in the literature, the EQMOM moment inversion procedure reveals several numerical difficulties.In this work, we propose an EQMOM modification called split-based EQMOM that builds upon the general idea of EQMOM to represent the NDF by a weighted sum of superimposed, non-negative, continuous kernel density functions (KDFs), avoiding, however, the numerical problems of its original version. The model is based on a split of the entire NDF into a sum of overlaying density functions, whose evolution is governed by individual population balance equations (PBEs). These PBEs are derived and implemented in a novel Monte Carlo (MC) framework which is applied to simulate two fuel-rich burner-stabilized laminar premixed flames with different sooting behaviours. Results are compared to a classical MC model to demonstrate the suitability. Next, the MC data is used to assess the suitability of lognormal, gamma and inverse Gaussian distributions to approximate the NDF shape by employing the Wasserstein metric as criterion. This information is finally used to formulate an improved EQMOM soot model, which is then evaluated for both fuel-rich and oxidizing conditions. For the latter, the two-stage burner experiments of Echavarria, Jaramillo, Sarofim, & Lighty (2011) is considered, where particle oxidation is the dominant process. It is demonstrated that the proposed MOM model allows an accurate and numerically robust description of soot formation, growth and also oxidation.

Detailed investigation of soot formation from jet fuel in a diffusion flame with comprehensive and hybrid chemical mechanisms

A numerical and experimental study is performed to investigate soot formation from jet fuel in a laminar coflow diffusion flame. The combustion chemistry of the fuel is simulated using (1) the MURI jet fuel surrogate (Dooley et al. 2012) with a modestly reduced Ranzi mechanism (Ranzi et al. 2012), and (2) the recently proposed HyChem model (Xu et al. 2018) combined with the KAUST PAH mechanism 2 (Wang et al. 2013). The two reaction mechanisms are coupled with a sectional soot model to simulate a coflow diffusion flame of methane doped with the MURI jet fuel surrogate. The combined laser extinction and two-angle elastic light scattering method is used to perform non-intrusive in situ measurements of soot volume fraction, primary particle diameter and number density. The good agreement including soot particle size and number density between the experimental data and the simulation results computed with the reduced Ranzi mechanism demonstrate the robustness of the soot model to changes in fuel composition, as the model parameters are unchanged with a previous numerical study of soot formation of n-propylbenzene/n-dodecane mixtures (Zhang and Thomson, 2018). The computation with the combined HyChem/KAUST mechanism predicts similar results as the computation with the detailed chemistry of the reduced Ranzi mechanism for fuel breakdown, thus the basic premise of the HyChem model that the fuel decomposition process can be greatly simplified with the lumped reaction steps is supported. The results also show that by adding a PAH growth scheme to the HyChem model, the approach can be used to predict soot formation from jet fuel combustion in a laminar coflow diffusion flame. Finally, the dependency of the soot prediction on PAH chemistry is discussed and it is suggested that more experimental data is needed to validate the PAH mechanism and improve the predictive accuracy of the model.

Characteristics of evolution of in-cylinder soot particle size and number density in a diesel engine

Particulate matter (PM) emission from diesel engines has been a critical issue due to its environmental impact. The soot size and number density inside the cylinder can severely affect the size and mass of PM emission emitted from the diesel exhaust pipe. In the present study, the temporal and spatial variations of the mean and instantaneous in-cylinder soot size and number density in a single cylinder direct injection diesel engine have been simulated numerically at different operating conditions, injection system parameters, fuel types and piston bowl geometry using AVL-FIRE code. The results show that the engine load has great effects on the mean soot particle size and number density with crank angle while the engine speed and the fuel injection system parameters have no obvious effects on them. It is observed that there is an obvious shift to larger particles at higher load. The mean soot number densities firstly increase with the increase of loads, but decrease at later crank angle. The in-cylinder soot size distribution shows a unimodal shape at different crank angle under all operating conditions. The soot particle numbers are concentrated from the start of combustion to 40 °CA after top dead center (ATDC). The spatial soot particle size distribution in the combustion chamber is closely related to engine load, impinging angle and airflow motion. The piston bowl diameter has large effect on the mean soot size and number density during the latter phase of the expansion stroke. The results also indicate that both in-cylinder mean soot size and number from biodiesel-diesel blends are affected not only by the oxygen content, but also engine operating conditions.

Sooting limits of non-premixed counterflow ethylene/oxygen/inert flames using LII: Effects of flow strain rate and pressure (up to 30 atm)

An absolute irradiance-calibrated Laser Induced Incandescence (LII) technique and a standard particle image velocimetry (PIV) technique were utilized to collect quantitative data on soot volume fraction and corresponding flow strain rates of diluted ethylene-air non-premixed counterflow flames. Pressures up to 30 atm were explored with increasing dilution with nitrogen or helium to minimize flow strain limits at which incipient soot was detected and to maintain the flame in laminar mode. For weakly strained flames considered, the species and velocity boundary conditions were used to predict the gas-phase flame structure (e.g., temperature and major species). The predicted gas properties, together with soot particle temperature decay rate measured by two-color pyrometry were used in the LII heat transfer model to extract the effective soot particle size and particle number density. Estimates of global activation energy of incipient soot yield with pressure indicated a sudden change around a pressure of 20 atm, which may be attributed to a shift in soot nucleation and growth pathways.

Extending the Flamelet Generated Manifold for Soot and NOx Modeling in Diesel Spray Combustion

Diesel spray combustion is prone to lead to high soot and nitrogen oxides (NOx) emissions. Regulations impose lowering the emission limits drastically because of the severe impact on health and environment. Apart from health and environmental issues, soot production also results in an efficiency loss due to the heat loss through radiation and late oxidation of the soot particles. Thermal NOx and soot modeling for practical engine applications is challenging. In this study, a two-equation multi-step phenomenological (MSP) soot model is fully coupled with the gas phase chemistry to solve the 1D detailed flame (flamelet) equations for the so-called Spray-A conditions from the Engine Combustion Network (https://ecn.sandia.gov/). Two different soot kinetics models are used that either utilize C2H2 or PAH as the soot-precursor. For the gas phase, a chemical reaction mechanism of n-dodecane comprising of 253 species and 1437 reactions is utilized. The resulting flamelet database includes emission information, NOx and soot carbon concentrations, and the soot particle number density. Detailed flamelet results are inspected in the mixture fraction (Z) domain to characterize the soot formation and oxidation, and NOx formation. The other important controlling variable, the progress variable (PV), is used for the tabulation of the flamelet results. The definition of PV is optimized for key parameters such as soot and NOx. Furthermore, the detailed solutions are stored in a low-dimensional manifold, i.e. a Flamelet Generated Manifold (FGM), with respect to the controlling variables (Z and PV). Here, the FGM tabulation is extended with soot and NOx emissions for non-premixed combustion. Finally, FGM is coupled with CFD and validated by studying an igniting diesel spray and comparing the simulation results with the Spray-A experiments.

Structural effects of biodiesel on soot formation in a laminar coflow diffusion flame

Structural effects of biodiesel in terms of unsaturation, i.e. C=C double bonds, its position and the presence of the ester moiety on soot formation are studied in this paper. Laminar coflow diffusion flames of 5-decene, 1-decene, n-decane, and a biodiesel surrogate consisting of a 50%/50% molar blend of n-decane and methyl-octanoate are selected for the investigation. It is observed that the presence of unsaturation promotes soot formation in the flames by increasing soot chemical surface growth species and soot inception species (aromatics) as it is evident from the larger primary particle size and number density of soot particles in the 1-decene, and 5-decene flames compared to the n-decane flame, respectively. More centrally located C=C double bond further speeds up soot inception as it promotes the formation of aromatic species. This is also evident from earlier detection of soot on the centerline and higher soot primary particle number density of the 5-decene flame compared to 1-decene and n-decane flames measured by the laser extinction and thermophoretic sampling methods, respectively. Effects of the ester moiety on soot formation are more on the concentrations of soot chemical surface growth species because the biodiesel surrogate flame has similar number density of primary particles compared to the n-decane flame but soot primary particles have smaller diameters.