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The kinetic modeling of high-temperature reforming of oxygen-free mixtures of methane with H2, H2O, CO, and CO2 additives into synthesis gas with strong dilution with argon under conditions of variable temperature and the formation of microheterogeneous soot particles was carried out. Such mixtures are typical for biomass gasification products, in which H2O, CO, and CO2 additives act as oxidizing agents. A direct comparison of kinetic calculations with the results of published experiments in a flow reactor at temperatures of 1100–1800 K, atmospheric pressure, and a reaction time of 0.68 s was carried out. The yields of soot were calculated for all test mixtures and conditions. A comparison of the results of kinetic calculations and experiments made it possible to evaluate the effect of soot formation on the reforming of methane with the additions of H2, H2O, CO, and CO2. The work analyzes two ways for carbon atoms to leave a reacting gas-phase system. The first way is the heterogeneous deposition of acetylene molecules from the gas phase onto the surface of the reactor with the subsequent formation of solid carbon, and the second way is the formation of microheterogeneous soot particles from nuclei in the gas phase. The paper compares the results of experiments in reflected shock waves and our kinetic calculations of the absolute concentration of CO for the process of methane oxidation in oxygen-free mixtures of methane and CO2. Mixtures with various CH4/CO2 ratios, 90/10, 75/25, and 50/50, were studied at temperatures above 2200 K and atmospheric pressure. It has been shown that the agreement between the calculated and measured CO concentrations improved with increasing temperature and CO2 fraction in the mixture. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Oxymethylene dimethyl ethers (OMEx), having a chemical formula of CH3O-(CH2O)x-CH3 where x varies from 1 to 5, have been widely considered as a promising fuel to partially replace diesel in CI engines in terms of reducing soot emissions. This work is focused on developing a reduced primary reference fuel (PRF)-OMEx chemical mechanism to better describe the combustion and emission characteristics of gasoline/diesel blends with OMEx. The novelty of this work lies in the fact that the OMEx part of the mechanism is represented not only by OME3 as done in most studies found in literature, but also with other OME chain lengths that is, OME2-4 which are considered to be optimum and better represent the commercial OMEx blends. For this purpose, a detailed OMEx mechanism is reduced by applying different reduction techniques considering a wide range of operating conditions including pressure, temperatures, equivalence ratios and fuel compositions. The result is merged with an already validated PRF mechanism to form a reduced PRF-OMEx mechanism consisting of 213 species and 840 reactions. The newly formed mechanism is validated against a wide set of experimental data including ignition delay times, laminar flame speeds and species concentration profiles. Furthermore, a rigorous set of numerical simulations for various diesel-OMEx blends in a compression ignition engine are carried out at two different operating points to validate the developed mechanism. Simulation results highlight that the developed mechanism not only replicates the experimental behavior in terms of in-cylinder pressure and heat release rate but exhibits a better combustion phasing closer to experimental data when compared with other mechanisms where only OME3 is utilized to represent OMEx. Overall, the developed PRF-OMEx mechanism proves to be realistic and suitable for application in engine combustion simulations involving gasoline/diesel and OMEx blends. [ABSTRACT FROM AUTHOR]

Creation of complex chemical mechanisms for hydrocarbon pyrolysis and combustion is challenging due to the large number of species and reactions involved. Reactive molecular dynamics (RMD) enables the simulation of thousands of reactions and the discovery of previously unknown components of the reaction network. However, due to the inherent imprecision of reactive force fields, it is necessary to verify RMD‐obtained reaction paths using more accurate methods such as Density Functional Theory (DFT). We demonstrate a method for identification and confirmation of reaction pathways from RMD that supplement an established mechanism, using the example of benzene formation from n‐heptane and iso‐octane pyrolysis. We establish a validation workflow to extract reaction geometries from RMD and optimize transition states using the Nudged‐Elastic‐Band method on semi‐empirical and quantum mechanical levels of theory. Our findings demonstrate that the widely recognized ReaxFF parameterization, CHO2016, can identify known pathways from a established soot formation mechanism while also indicating new ones. We also show that CHO2016 underestimates hydrogen migration barriers by up to 40kcalmol−1$40\,{\rm {kcal\,mol}}^{-1}$ as compared to DFT and can lower activation barriers significantly for spin‐forbidden reactions. This highlights the necessity for validation or potentially even reparametrization of CHO2016. [ABSTRACT FROM AUTHOR]

Abstract The application of biomass energy is one of the important ways to achieve carbon neutrality and deal with global warming. The study on the combustion mechanism of anisole, an oxygen-containing fuel, is helpful for biofuel large-scale application. In this study, the soot formation and laminar combustion characteristics of anisole were analyzed by reactive force field molecular dynamics (ReaxFF MD) and kinetic simulation, respectively. ReaxFF MD simulation studies had shown that soot formation of anisole combustion occurred in three stages, stage 1 (0–1 ns), stage 2 (1–2.5 ns), stage 3 (2.5–6 ns). The three stages represented the pyrolysis of the fuel, the developmental stage of the soot, and the graphitization stage of the soot, respectively. During the combustion of anisole, primary mechanisms for the soot formation were as follows: H-abstraction-C2H2-addition, carbon-addition-hydrogen-addition, internal ring formation and long carbon chain link. The formation of soot graphitization exhibited different morphologically behaviors: from flakes to onions to spheres with fewer branched chains. From the study of the laminar combustion characteristics of anisole, it can be found that the laminar burning velocities increased along with the increase of temperature, while the opposite trend was shown along with the increase of pressure. The sensitivity coefficient of naphthalene, the main soot precursor, revealed that the main promotional reactions for soot formation were R5 (O2 + H O + OH), R36 (CO + OH CO2 + H).

The addition of hydrogen in a dual-fuel mode based on natural gas and diesel offers great potential for improving engine efficiency and reducing emissions. Therefore, detailed numerical simulations have been used to explore the effects of soot formation characteristics of natural gas-diesel dual-fuel engines doped by hydrogen at elevated pressure. Numerical simulations are compared with available experimental data, the effects of H 2 addition on soot formation in laminar CH 4 + n-heptane, H 2 + CH 4 + n-heptane, FH 2 + CH 4 + n-heptane diffusion flames at 2, 4, 6 and 8 atm are simulated, FH 2 is added to separate the H 2 chemical effects. The results show that the dilution and thermal effects of H 2 inhibit soot formation, while the chemical effects of H 2 promote soot formation under different pressures. The addition of H 2 increases the concentration of H and OH radicals, which promotes the formation of C 2 H 2 and benzene (A1). By simplifying the reaction path and analyzing the production rate of A1, the mechanism of H 2 addition to promote A1 formation is illustrated. The concentrations of polycyclic aromatic hydrocarbon (PAH) pyrene (A4), benzo(ghi)fluoranthene (BGHIF) and benzo(a)pyrene (BAPYR) are inhibited at higher pressures that result in lower inception and condensation rates. Therefore, the increase of hydrogen abstraction acetylene addition (HACA) rate is the main reason for the increase of soot formation under chemical effects. While the O 2 oxidation rate decreases and the OH oxidation rate increases, that is due to the decrease of O 2 concentration and the increase of OH concentration in the soot formation area under chemical effects. • The dilution, thermal and chemical effects of H 2 addition on soot formation in methane/n-heptane flame were investigated at high pressures. • Key specie A1 formation pathways and related reactions were confirmed when H 2 addition. • The changes of soot formation and oxidation mechanisms with H 2 addition in different pressure were explained. [ABSTRACT FROM AUTHOR]

The 70 GB, which comprises 70% gasoline and 30% biodiesel, shows excellent potential for application in gasoline compression ignition due to its superior lubrication capability, renewability, environmental friendliness, high ignitability contributed by biodiesel namely as hydrogenated catalytic biodiesel (HCB), and high volatility conferred by gasoline. However, the spray combustion and emission characteristics of 70 GB fuel have not yet been quantitatively evaluated. In this work, we performed a comprehensive simulation focusing on the ignition delay, heat release rate, flame lift-off length, flame structure, and soot formation of 70 GB in a constant volume chamber under various fuel injection pressure. Numerical results showed that, different injection pressure strongly impact the heat release rate without affecting the maximum temperature. Increasing the injection pressure from 80-120 MPa, increased the heat release rates by 23%. The ignition delay was marginally affected by increasing injection pressure, while a 5.7 mm increase in flame lift-off length observed with higher injection pressure. Additionally, 65% lower soot formation was typically predicted for higher injection pressure 120 MPa. In particular, the soot mass is primarily controlled by enhancing the atomization and evaporation processes, as well as improving fuel-air mixing rate, which was achieved by increasing the injection pressure. Furthermore, the role of soot oxidation was insignificant in reducing soot with increasing injection pressure, while the soot initiation step and soot surface growth step play an important role in soot suppression with increasing injection pressure for 70 GB fuel. [ABSTRACT FROM AUTHOR]

The solution to the dilemma of carbon footprint of internal combustion engines and pollutant emissions is necessary for the survival of this technology. In this context, alternative fuels have shown great potential in terms of achieving cleaner combustion and compliance with ever increasing pollutant emissions regulations. This work is focused on the study of two promising alternative fuels as Hydrotreated vegetable oil (HVO), which is a biofuel and Dimethoxymethane also termed as OME1, which is an e-fuel. A comprehensive numerical approach has been followed to study these fuels. Primarily a compact reaction mechanism having 121 species and 678 reactions has been developed which can be utilized to perform 3D CFD simulations of blends of these fuels. Secondly, a detailed numerical investigation including combustion and emissions analysis has been carried out. Results show that the developed mechanism is able to offer predictions, which match the experimental behavior observed in various combustion parameters and thus can be utilized for compression ignition engine applications involving these promising fuels. In addition, the numerical analysis also highlights that a reduction of 50% and 37% in terms soot and NOx emissions respectively can be achieved by addition of 30% OME1 in the blend containing HVO, suggesting that these blends can be utilized in unmodified CI engines to break the soot-NOx tradeoff without significantly penalizing the energy loss. [ABSTRACT FROM AUTHOR]

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. At present, there are few detailed analyses on the mechanism changes of soot formation under oxygen enrichment of propane. In this paper, the effects of oxygen-rich (O2/N2, O2/CO2) combustion on soot formation in the propane laminar flow coaxial jet diffusion flame were investigated by using the experiment and numerical simulation. The flame image was taken by a color (CCD) camera in the visible band, and the two-dimensional distribution of temperature and soot volume fraction in the flame was reconstructed. In numerical simulation, the soot production model considers a detailed description of nucleation via collisions among heavy polycyclic aromatic hydrocarbons, particle aggregation, polycyclic aromatic hydrocarbons condensation, surface growth and oxidation through the hydrogen abstraction acetylene addition mechanism. The results show that the experimental results were compared with the numerical simulation results, and the numerical simulation results predict the temperature and soot change of oxygen-rich flame well. With the increase in oxygen concentration, the peaks of soot and temperature in the two oxygen-rich atmospheres gradually increased. With the increase in oxygen concentration, the peaks of soot and temperature in the two oxygen-rich atmospheres gradually increased. Comparing the combustion mechanism changes of the two oxygen-rich combustion systems, it was found that the hydrogen abstraction acetylene addition mechanism was the main cause of soot generation. The OH oxidation is dominated near the fuel nozzle side, and O2 oxidation is dominant downstream. With the substitution of CO2 for N2, the chemical effect of CO2 reduces the flame temperature and inhibits the formation mechanism and oxidation mechanism of soot to some extent. This in turn leaded to a reduction in the amount of soot produced.

AbstractIn coal gasification, soot forms from volatile matter (VM) and it influences the gasification efficiency. Recently, biomass or plastic wastes with higher VM than coal are used for gasification processes and the importance of soot is increasing. For predicting soot formation, elementary step models are often used. However, defining main reaction paths is difficult because the models include huge species and reactions. In this study, the polyaromatic hydrocarbon formation path was analyzed via an elementary step-like model with 257 species and 1107 reactions to understand soot formation. The main reaction paths were extracted. Initially, 257 species were classified into non-aromatic species, one-ring, two-ring, and three- or more-ring aromatics. Ten species in each group with high maximum concentrations were considered as the major species. The reactions were extracted as follows: extracting reactions according to their contribution to the major species mass balance; adding fast reactions; and identifying the main reaction path. Consequently, the main reaction path extracted (MRE) model for the combustor condition was reduced to under 100 reactions for two kinds of conditions. Simulation of coal gasification reaction with the MRE model successfully described soot formation behavior in experiments with a drop tube furnace.

AbstractThe objective of this study was to investigate the mechanism of soot formation in biodiesel by analyzing the combustion of individual components. The paper presents a numerical analysis of the effect of preheat temperatures on nucleation rates, coagulation rates, and soot volume fraction in methyl linolenate (MLe) co-flow flame. In this work, Moss-Brooke’s soot model and a reduced kinetic mechanism containing 177 chemical species and 2904 chemical reactions were used to simulate the pyrolysis and combustion of MLe. A laminar jet flame with inlet velocities of 0.4 m/s was studied. The preheat temperature of the fuel was varied between 300 and 450 K. The burner walls were stationary and no-slip conditions were applied. The pressure outlet had Neumann boundary conditions and the tangential velocity was set to zero at the wall. It was established that an increase in fuel preheat temperatures causes an increase in nucleation rates and the amount of soot due to accelerated fuel pyrolysis, improved diffusion, acceleration from buoyancy, and earlier formation of PAHs. It was discovered that increasing the fuel preheat temperature had a greater impact on soot formation along the centerline than on the wing.

This experimental study compares the flame structure, temperature, and soot formation characteristics of normal (NDFs) and inverse diffusion flames (IDFs), evaluates the effect of CO2 addition, and isolates the thermal and dilution effects of CO2 and N2 addition on soot formation in IDFs. A hyperspectral imager using the TR-GSVD algorithm measures IDFs with different CO2 addition (XD) in the oxidant, with N2 addition as the comparison. In contrast to NDFs, IDFs have an opened tip, and the visible flame height and flame width decrease as XD grows. IDFs have a higher peak temperature than NDFs, but their peak soot volume fraction is substantially lower. Compared with N2 addition at the same XD, CO2 addition narrows IDFs and makes the reaction zone visible, and the peak temperature of CO2 addition is 350 K lower than that of N2 addition. The suppression effect of CO2 on soot formation is more potent than N2, with a soot formation rate of 30% that of N2 addition and a soot loading of 20–60%. The activation energy of soot formation reaction with CO2 addition is higher than N2 addition. The isolating results reveal that the thermal effect contributes to the main suppression effect of CO2 addition on soot formation, while the dilution effect of N2 addition is stronger than its thermal effect. [ABSTRACT FROM AUTHOR]

The effect of the H/C ratio of feedstock on soot formation was investigated in this study. Soot was continuously generated by pyrolyzing C2 hydrocarbons in an atmosphere without oxygenated species at a maximum temperature of 1683 K. H 2 was added to the feedstock to vary the H/C ratio, and the particle size distribution of soot was determined using a scanning mobility particle sizer. Most of our results indicate that the ease of thermal dissociation of the feedstock is more important than the degree of unsaturation in the feedstock. With increasing H/C ratio of the feedstock, more radicals are generated, and polycyclic aromatic hydrocarbons (PAHs) are easily formed, leading to the formation of soot. Interestingly, many previous studies on combustion have reported the opposite behavior, indicating that the ease of radical formation is important in pure pyrolysis without oxygenated species, which generate radicals easily. In the case of C 2 H 2 as a feedstock in this study, the addition of a small amount of H 2 increased the amount of radicals and promoted soot formation. However, with increasing amounts of H 2 , soot formation was inhibited because the inhibitory effect of H 2 on PAH formation increased. When H/C = 2, the H/C ratio of the feedstock was dominant for soot formation, while the degree of unsaturation of the chemical species in the feedstock was dominant when the H/C ratios were 3 and 4. • Impact of chemical structure and H/C ratio in C2 hydrocarbon pyrolysis. • Experiment about impact of impurities in acetylene. • Simulation reveals importance of chemical structure in radical formation. • Ethane pyrolysis showed highest soot formation propensity. [ABSTRACT FROM AUTHOR]

Environmental pollution from transportation means and natural resource degradation are the top concern globally. According to statistics, NOx and PM emissions from vehicles account for 70% of total emissions in urban areas. Therefore, finding solutions to reduce NOx and PM emissions is necessary. Changing the engine's internal combustion method is considered promising and influential among the known solutions. One of the research directions is a combustion engine using the Premixed Charge Compression Ignition (PCCI) method combined with biofuels to improve the mixture formation and combustion process, reducing NOx and PM emissions. Therefore, this study presents the mechanism of the formation of PM and NOx emissions in the traditional combustion and the low-temperature combustion process of internal combustion engines. Besides, the theoretical basis of flame spread during combustion is also introduced. The key feature of this research is that it has modeled the combustion process in diesel engines under the PCCI modes. This was accomplished using blends of waste cooking oil (WCO)-based biodiesel and diesel fuel, as well as the ANSYS Fluent software. The results showed that PCCI combustion using B20 fuel can significantly reduce NOx and PM emissions, although HC and CO emissions tend to increase, and thermal efficiency tends to decrease. In further studies, different modes of the PCCI combustion process should be thoroughly examined so that this process can be implemented in practice to reduce pollutant emissions. [ABSTRACT FROM AUTHOR]

The molecular species composition of major toxic species, such as soot, CO and unburnt hydrocarbons from a boundary layer diffusion flame over heptane or dodecane surface at microgravity is numerically investigated. A two-step global reaction model for gas-phase chemistry and a simplified soot model consisting of laminar smoke point type for soot inception are used. Thermal radiation is calculated using the discrete-ordinates method coupled with a non-grey model for the radiative properties of CO, CO2, H2O and soot. The numerical results provide further insights into the intimate coupling between burning rate, flame length, thermal radiation, and toxic products at reduced gravity level. The importance of oxidizer flow speed to the flame structure, soot formation and thermal radiation at microgravity is demonstrated. The amount of soot from the microgravity heptane/dodecane flames augments with an increase of oxidizer flow velocity from 0.1 to 0.3 m/s due to an enhancement of burning rate. This finding is contrasted to the case with porous gas burners relative to toxic species production from microgravity flames. For the burning of various liquid fuels, the radiative loss fraction in general increase in a range of 0.5 to 0.7 due to enhanced soot formation with augmentation of the oxidizer flow velocity.

The soot formation model based on inverse ethylene diffusion flames was performed to study the sensitivity of the soot formation process to the prediction results. The effects of efficiency parameters such as soot inception, surface growth and coagulation on the simulation results were studied by using the adjustable efficiency model. In addition, the reversible soot model and conjugate heat transfer (CHT) model were also introduced to explore their advantages. Results indicated that, among adjustable efficiency parameters, the nucleation efficiency had the greatest influence on the predicted soot and PAHs distributions, while the H-abstraction-C2H2-addition (HACA) process and PAH adsorption surface growth efficiencies impacted little. The adjustable efficiency parameters had a significant effect on the concentration of soot gaseous precursors and soot particles, but their effects on temperature, gas phase molecules, and intermediate species were not obvious. When the nucleation efficiency increased from 2×10−6 to 1×10−4, the predicted value of the integrated soot was increased by nearly 50%, and the maximum primary particle number density and the number of aggregates were increased by an order of magnitude. The maximum concentration of BAPYR was doubled. However, the peak temperature along the axial direction increased by only 3.5 K. Using the reversible soot model, the approximation results of the adjustable efficiency parameters could be modified, which showed the feasibility of the model. The use of the CHT model promoted pyrolysis of the fuel below the outlet of the fuel tube, with high-temperature zones, soot zones, and PAHs zones moving towards higher flame heights. Besides, when using the reversible model and the CHT model, the maximum soot volume fraction decreased by 39% compared with the basic efficiency parameters, while the concentration of BAPYR increased by 162%, and the concentrations of gas phase species were decreased. [ABSTRACT FROM AUTHOR]

This paper presents a review of modern methods for producing hydrogen, including extracting hydrogen from exhaust and purge gases of chemical industries. The technology for producing hydrogen by separation from hydrogen-containing gas obtained by partial oxidation of hydrocarbons in a high-temperature reactor (HTR) is considered. In the HTR combustion chamber, the following zones are distinguished: a combustion zone, a coking zone, and a conversion zone, i.e., a zone of hydrocarbon conversion, soot formation and soot conversion. The paper presents the results of an experimental and theoretical study of soot formation as well as the results of studying the influence of various process parameters in the HTR chamber on soot formation and burnout. A physical and chemical model for the combustion of methane with air oxygen in an HTR chamber has been developed, taking into account soot formation. Recommendations have been formulated for optimizing the process in the HTR and improving its design. • Review and comparison of modern methods for producing hydrogen. • Hydrogen production by partial oxidation of organic raw materials. • Results of an experimental study of soot formation in a high-temperature reactor (HTR). • Physico-chemical model of combustion and soot formation in HTR. • Analysis of the kinetic scheme of HTR in order to minimize soot formation during combustion. [ABSTRACT FROM AUTHOR]

The objective of this study was to investigate the mechanism of soot formation in biodiesel by analyzing the combustion of individual components. The paper presents a numerical analysis of the effect of preheat temperatures on nucleation rates, coagulation rates, and soot volume fraction in methyl linolenate (MLe) co-flow flame. In this work, Moss-Brooke's soot model and a reduced kinetic mechanism containing 177 chemical species and 2904 chemical reactions were used to simulate the pyrolysis and combustion of MLe. A laminar jet flame with inlet velocities of 0.4 m/s was studied. The preheat temperature of the fuel was varied between 300 and 450 K. The burner walls were stationary and no-slip conditions were applied. The pressure outlet had Neumann boundary conditions and the tangential velocity was set to zero at the wall. It was established that an increase in fuel preheat temperatures causes an increase in nucleation rates and the amount of soot due to accelerated fuel pyrolysis, improved diffusion, acceleration from buoyancy, and earlier formation of PAHs. It was discovered that increasing the fuel preheat temperature had a greater impact on soot formation along the centerline than on the wing. [ABSTRACT FROM AUTHOR]

Soot formation in biodiesel combustion was studied by focusing on the major components in isolation. The aim of the study was to establish the influence of major components on soot production in combustion of biodiesel in co-flow jet flames. The components investigated in this study were methyl linolenate (MLe), methyl stearate (MS), methyl oleate (MO), methyl linoleate (MLi) and methyl palmitate (MP). The study was based on Moss-Brooke's soot model. A reduced kinetic mechanism for the pyrolysis and combustion of biodiesel surrogates with 177 chemical species and 2904 chemical reactions was implemented. Nucleation rate, coagulation rate, oxidation rate and soot volume fraction were investigated under laminar flow conditions. It was established that unsaturated methyl esters had higher soot formation rates than their saturated counterparts. MLe had the greatest influence in soot formation due to its relatively higher nucleation, coagulation and oxidation rates while MS had the least at the same boundary conditions. Analysis of the flame structure reveals that the higher nucleation rates are correlated to higher content of aromatic species in the immediate chemical reactions. [ABSTRACT FROM AUTHOR]

In this paper, the spray and combustion characteristics of diesel/butanol-blended fuels were studied within a high-temperature and high-pressure constant volume chamber equipped with a single-hole injector. Two blends with 80% diesel/20% butanol and 60% diesel/40% butanol mixed by volume were tested in this study. The pure diesel B0 was also tested here as a reference. The spray penetration, flame lift-off length, and soot optical thickness were obtained through high-speed schlieren imaging, OH* chemiluminescence, and diffused back-illumination extinction imaging technique, respectively. The thermogravimetric curves of different fuels were obtained through a thermogravimetric analyzer. The results showed that butanol/diesel blends presented a longer ignition delay (ID) and flame lift-off length compared with pure diesel, and such finding was mainly caused by the lower cetane number and higher latent heat of vaporization of n-butanol. With the increase in the n-butanol ratio, soot production in the combustion process decreased significantly. Given the shorter ID period, the soot distribution of pure diesel reached a steady state earlier than the blends. [ABSTRACT FROM AUTHOR]

This work investigates the chemical effects of ethane, propane, and butane substitution for methane on flame properties and sooting behaviors in inverse diffusion flames of natural gas in different ratios by a decoupling method. Results show that the influences of varying natural gas ratios on the concentration distributions of soot particles and soot gaseous precursors are greater than on temperature. After a certain amount of methane is replaced by C2–C4 alkanes, the soot volume fraction (SVF) becomes more centerline-dominated. Meanwhile, the average primary particle diameter increases while the number density of primary particles and the average number of primary particles per aggregate decrease. Due to their chemical effects, C2–C4 alkanes not only promote the formation of H2, C2H2, and C4H4 but also contribute to an increase in A2, A3, and A4 formation. As for the soot formation reaction rates, the condensation rates are promoted. However, under varying ratios, C2–C4 alkanes demonstrate diverse chemical effects on the coagulation rates. Among three alkanes, ethane has the most pronounced chemical suppression effect on soot inception, HACA surface growth, and oxidation process rates. The combined effects of increased condensation and delayed oxidation rates result in elevated SVF in natural gas flames.

The objective of this work is to assess the accuracy and limitations of two different semi-empirical soot models: the Laminar Smoke Point (LSP) and soot-yield approach. A global soot formation model based on the LSP concept is embedded within FDS6.7. Quantitative comparisons were made from turbulent buoyant pool fires between several computational results and well-instrumented experimental databases on the soot volume fraction, mass loss rate, heat release rate and gas temperature. The LSP model in combination with soot oxidation and surface growth is validated for most of the methane, ethylene and heptane turbulent buoyant pool fires, covering a wide range of fuel likely to form soot. This paper aims to broaden the scope of the validation of the available semi-empirical soot modelling. For the porous methane and ethylene burner, the LSP model was found to provide a better description of the soot volume fraction. The overall visual soot distribution is also numerically reproduced with the soot-yield approach, but as expected, there are some differences between the prediction and the measurement regarding the magnitude of soot volume fraction. The computed radiant heat flux was compared with experimental data for heptane flame, showing that predictions using both the LSP and soot-yield models were found to be twice the value of experimental data, although the measured HRR (Heat Release Rate) is reliably reproduced in the numerical simulation. For the heptane buoyant pool fires, a sufficient accuracy of the numerical model is confirmed only in some of the locations as compared to the experimental results. It is demonstrated that neither the temperature nor the soot volume fraction can be reliably calculated in the necking flame flapping region when the pyrolysis rate of condensed fuel (heptane) is coupled with radiation/convection heat feedback. This implies that an accuracy of prediction on the turbulent buoyant pool fires depends on the studied fire scenario regardless of the semi-empirical soot models.

Nowadays, the increasing consumption of conventional fuels has led to rising pollutant emissions, among which soot has received much attention due to its harmful effects. In this paper, previous typical soot models are reviewed. Based on empirical, semi-empirical and detailed models, phenomenological models, the most popular models of recent years, are also summarized. This work also summarizes the application of existing soot models in combination with other models and methods in turbulent flame and diesel engine development based on existing models. Besides, as the development of clean alternative fuels continues, new fuels have received significant attention as a method to reduce soot formation. For different fuels, the reasons for reducing soot formation have also attracted the interest of researchers, and thus this paper reviews the current research about relevant carbon soot modeling for biodiesel, natural gas, and other new fuels. A detailed understanding of soot generation and soot modelling can help further research into methods to reduce soot emissions. [Display omitted] [ABSTRACT FROM AUTHOR]

Thunderstorms are the main natural source of forest fires. The ignition mechanism of trees begins with the impact of cloud-to-ground lightning discharge. A common drawback of all predicting systems is that they ignore the physical mechanism of forest fire as a result of thunderstorm activity. The purpose of this article is to develop a physically based mathematical model for the ignition of a coniferous tree via cloud-to-ground lightning discharge, taking into account thermophysical, electrophysical, and physicochemical processes. The novelty of the article is explained by the development of an improved mathematical model for the ignition of coniferous trees via cloud-to-ground lightning discharge, taking into account the processes of soot formation caused by the thermal decomposition phase of dry organic matter. Mathematically, the process of tree ignition is described by a system of non-stationary nonlinear differential equations of heat conduction and diffusion. In this research, a locally one-dimensional method is used to solve three-dimensional partial differential equations. The finite difference method is used to solve one-dimensional heat conduction and diffusion equations. Difference analogues of the equations are solved using the marching method. To resolve nonlinearity, a simple iteration method is used. Temperature distributions in a structurally inhomogeneous trunk of a coniferous tree, as well as distributions of volume fractions of phases and concentrations of gas mixture components, are obtained. The conditions for tree trunk ignition under conditions of thunderstorm activity are determined. As a result, a complex three-dimensional mathematical model is developed, which makes it possible to identify the conditions for the ignition of a coniferous tree trunk via cloud-to-ground lightning discharge. [ABSTRACT FROM AUTHOR]

In this work, the kinetics of pyrolysis and subsequent soot yield in undiluted mixtures of acetylene with chemically active additives, namely acetone, hydrogen, oxygen, methane and propane–butane mixture, were investigated numerically over a wide range of pressures and temperatures using the modern kinetics mechanisms. It was shown that with different amounts of additives and under different temperature/pressure conditions, both acceleration and deceleration of acetylene pyrolysis was possible. The determining factors were either additional thermal effects or new reaction channels arising in the presence of certain additives. The greatest inhibitory effect was observed with the addition of alkanes, while the oxygen additives caused significant acceleration of pyrolysis due to exothermic oxidative reactions. The results of the calculations qualitatively agree with available experimental data in the literature obtained from different reactors. [ABSTRACT FROM AUTHOR]

The objective of this work is to assess the accuracy and limitations of two different semi-empirical soot models: the Laminar Smoke Point (LSP) and soot-yield approach. A global soot formation model based on the LSP concept is embedded within FDS6.7. Quantitative comparisons were made from turbulent buoyant pool fires between several computational results and well-instrumented experimental databases on the soot volume fraction, mass loss rate, heat release rate and gas temperature. The LSP model in combination with soot oxidation and surface growth is validated for most of the methane, ethylene and heptane turbulent buoyant pool fires, covering a wide range of fuel likely to form soot. This paper aims to broaden the scope of the validation of the available semi-empirical soot modelling. For the porous methane and ethylene burner, the LSP model was found to provide a better description of the soot volume fraction. The overall visual soot distribution is also numerically reproduced with the soot-yield approach, but as expected, there are some differences between the prediction and the measurement regarding the magnitude of soot volume fraction. The computed radiant heat flux was compared with experimental data for heptane flame, showing that predictions using both the LSP and soot-yield models were found to be twice the value of experimental data, although the measured HRR (Heat Release Rate) is reliably reproduced in the numerical simulation. For the heptane buoyant pool fires, a sufficient accuracy of the numerical model is confirmed only in some of the locations as compared to the experimental results. It is demonstrated that neither the temperature nor the soot volume fraction can be reliably calculated in the necking flame flapping region when the pyrolysis rate of condensed fuel (heptane) is coupled with radiation/convection heat feedback. This implies that an accuracy of prediction on the turbulent buoyant pool fires depends on the studied fire scenario regardless of the semi-empirical soot models. [ABSTRACT FROM AUTHOR]

The present work reports an experimental study of particle group combustion of pulverized bituminous coal in laminar flow conditions using advanced multi-parameter optical diagnostics. Simultaneously conducted high-speed scanning OH-LIF, diffuse backlight-illumination (DBI), and Mie scattering measurements enable analyses of three-dimensional volatile flame structures and soot formation in conventional (i.e., N 2 /O 2) and oxy-fuel (i.e., CO 2 /O 2) atmospheres with increasing O 2 enrichment. Particle-flame interaction is assessed by calculating instantaneous particle number density (PND), whose uncertainties are estimated by generating synthetic particles in DBI image simulations. Time-resolved particle sequences allow the evaluation of the particle velocity, which indicates a PND dependency and interactions between particles and volatile flames. 3D flame structure reconstruction and soot formation detection are first demonstrated in single-shot visualizations and then extended to analyze effects of O 2 concentration, PND, and inert gas composition statistically. The increasing O 2 concentration significantly reduces local flame extinction and suppresses soot formation in N 2 and CO 2 atmospheres. Volatile flames reveal higher intensities and lower lift-off heights as O 2 concentration increases. In contrast to that, an increased PND leads to earlier flame extinction and stronger soot formation due to the local gas temperature reduction and oxygen depletion. The lift-off height reduces with increasing PND, which is explained by the complex interaction between particle dynamics, heat transfer, and volatile reactions. Slightly stronger soot formation and delayed ignition are observed in CO 2 atmospheres, whereas CO 2 replacement reveals insignificant influences on the flame extinction behavior. Finally, non-flammability is quantified for particle group combustion at varying PNDs in different atmospheres. [ABSTRACT FROM AUTHOR]

This work reports an experimental and numerical analysis on the sooting characteristics of butanol isomers. Light extinction and gas chromatography were used to measure soot and gas-phase species, respectively. Kinetic analysis of the tested flames was performed with detailed gas-phase mechanism and a sectional soot model. The present work aims to provide an understanding on the effects of butanol isomeric structures on sooting tendencies. For a comprehensive analysis, flames of both neat butanol fuels and butanol/hydrocarbon mixtures were studied. The results showed that the relative ranking of sooting tendencies among the butanol isomers were similar in neat butanol flames and in butanol-doped ethylene flames. In addition, we show that a small amount of butanol addition in ethylene flame enhanced soot formation. It was found that different isomeric structures mainly affected the formation of C3H3, which led to the different concentrations of important aromatic soot precursors. In the butanol-doped ethylene flames, the blending of the four butanol isomers increased the number of C3H3 formation pathways, which in turn significantly increased the production of benzene. Structural effects explaining for the differences in the sooting tendencies of the four butanol isomers in neat butanol flames and ethylene/butanol flames were found to be the same. [ABSTRACT FROM AUTHOR]

Advanced combustion strategies are necessary for the use of more environmentally sustainable fuels than traditional diesel. Alcohol fuels and alcohol/gasoline blends are of particular interest as they are readily available in the marketplace. Heavy-duty engines typically use compression ignited, conventional diesel mixing controlled combustion. Mixing controlled combustion features a non-premixed diffusion flame with a wide range of local equivalence ratios, leading to potentially high rates of soot formation. This work studies the sooting behavior of iso -octane and ethanol as a function of equivalence ratio. Measurements are carried out in a rapid compression machine (RCM) and are reported for pre-ignition conditions of 10–30 bar and temperatures of 650–800 K. Theoretical equilibrium and bulk gas temperatures are calculated for both fuels. These data are used to identify the critical equivalence ratio, the lowest equivalence ratio where soot is detected with a single-pass laser extinction diagnostic. The critical equivalence ratio for iso- octane varies between 1.82 and 1.77 for compressed pressures of 10 and 20 bar, respectively. Ethanol, sometimes considered sootless, had a critical equivalence ratio between 2.37 and 2.12 for compressed pressures of 20 and 30 bar, respectively. When characterizing soot formation by oxygenated equivalence ratio, the critical equivalence ratios for ethanol approach those of iso -octane. This suggests the oxygenated nature of alcohol fuels reduces sooting tendency, but other factors such as fuel molecular structure and morphology may play a role. It was observed that ethanol will form soot at equivalence ratios only slightly higher than iso -octane, which could have implications in mixing controlled combustion. It was seen for both fuels that soot formation is pressure sensitive, with the critical equivalence ratio being inversely proportional to compressed pressure and the rate of soot formation. Future work will investigate the sooting behavior of gasoline/ethanol blends. [ABSTRACT FROM AUTHOR]

The second-order factor effect of burner optical ports and edge inter-matrices (EIM) and the first-order factor of pressure on the soot formation process and behavior of premixed sooting flames in a high-pressure burner are numerically investigated here. Three-dimensional computational fluid dynamics (CFD) simulations of a premixed flame C2H4/air at p = 1.01 and 10 bar using a one-step chemistry approach are first performed to justify the satisfied predictability of the prospective axisymmetric two-dimensional (2D) and one-dimensional (1D) simulations. The justified 2D simulation approach shows the generation of an axial vorticity around the EIM and axial multi-vorticities due to the high expansion rate of burnt gases at the high pressure of 10 bar. This leads to the development of axial multi-sooting zones, which are manifested experimentally by visible luminous soot streaks, and to the boosting of soot formation conditions of a relatively low-temperature field, fV) profile, fV < 3%, is manifested only at high heights above the burner of the atmospheric sooting flame C2H4/air ϕ = 2.1, and early at the high pressure of 10 bar of this flame, fV < 10%. Enhancing the combustion process reactivity by decreasing the rich equivalence ratio of the fuel/air mixture and/or rising the pressure results in the prior formation of soot precursors, which shifts the sooting zone upstream.

The simultaneous effect of the swirl number (SN) and equivalence ratio on NO and soot emissions in turbulent methane–air combustion was numerically investigated. The realizable k-ε model has been applied for modeling turbulence and eddy dissipation (ED) and probability density function (PDF) models were adopted for chemical reaction. The general gradient diffusion hypothesis (GGDH), high order algebraic model (HOGGDH), and simple eddy diffusivity model have been applied for modeling the turbulent heat flux (THF) vector. The Brooke–Moss model has been employed to predict the soot particle quantities. Comparing the results of the different numerical models (PDF and EDM with algebraic THF) with available experimental data indicated that employing the second-order models significantly leads to the modification of predicting temperature distribution in EDM. However, due to the effect of the PDF model on soot modeling, the PDF model provides more accurate results. The numerical simulations have been performed for various SNs (SN = 0–1.33) and equivalence ratios (0.125–0.75). In all of equivalence ratios the flame temperature and pollutants emission (NO and soot) were strongly affected by the SN. Also, the SN has different effects on NO and soot emission. Finally, for each of NO and soot emission, a correlation relative to effective combustion factors such as SN and equivalence ratio was presented. It is found that by increasing the equivalence ratio, the NO emission increases with a power of 2.3, and soot emission decreases with a power of 8.7. [ABSTRACT FROM AUTHOR]

A kinetic analysis of the noncatalytic carbon dioxide reforming of CH4 has been carried out in the temperature range of 1500–1800 K under conditions of variable temperature behind the reflected shock wave. The stages of methane conversion into syngas, the characteristic time intervals corresponding to these stages, and the most important elementary reactions have been established. At the first stage, as a result of thermal pyrolysis, methane molecules are sequentially converted into ethane, ethylene, and then acetylene, the most stable hydrocarbon in this temperature range. At the second stage, acetylene is normally converted into CO and H2 and also into soot particles in the case of rich mixtures. The conversion of CO2 proceeds at the second and third stages, when CH4 conversion is almost complete. It occurs as a result of the interaction of CO2 molecules with atoms arising in the reacting system and leads to the formation of CO molecules and radicals. Acetylene is predominantly consumed in the reaction with radicals. The high concentration of acetylene during methane reforming promotes active formation of soot nuclei, for which acetylene makes the highest contribution to the rate of their surface growth. At the same time, acetylene itself is not a precursor of soot particle nuclei, which mainly form from radicals. [ABSTRACT FROM AUTHOR]

The flamelet approach is widely used to model non-premixed turbulent combustion. In this model, chemistry is solved in mixture fraction space and the mixture fraction field is solved in physical space. Transport of reactive scalars in mixture fraction space is governed by two flow parameters: the scalar dissipation rate and the curvature of the mixture fraction field. While these two flow parameters can be easily evaluated if all scales are resolved, models are required if turbulence modeling is applied. However, conventional flow parameter models, which presume the shape of the flow parameters in mixture fraction space, do not consider that mixing in physical and mixture fraction space need to be modeled consistently, which can cause leading order errors. In this study, a consistent formulation of the governing equations and the flow parameters is derived, which is then applied within the Representative Interactive Flamelet (RIF) model to an auto-igniting, sooting, temporal n -dodecane jet under engine-relevant conditions. A Direct Numerical Simulation (DNS) is carried out and serves as reference solution for model validation. It is shown that the revised flow parameters yield superior model predictions of all investigated quantities in comparison to conventional models. Although the auto-ignition process is captured reasonably well by the conventional model, the modeling error in soot quantities is not tolerable. The revised model shows much better agreement with the DNS data and demonstrates that the RIF model is generally able to predict pollutants under the considered conditions. An analysis of the temporal soot mass evolution shows that an accurate description of both species and particle transport in mixture fraction space is essential for predicting soot formation. [ABSTRACT FROM AUTHOR]

An experimental and numerical study of combustion of a gasoline certification fuel ('indolene'), and four (S4) and five (S5) component surrogates for it, is reported for the configurations of an isolated droplet burning with near spherical symmetry in the standard atmosphere, and a single cylinder engine designed for advanced compression ignition of pre-vaporized fuel. The intent was to compare performance of the surrogate for these different combustion configurations and to assess the broader applicability of the kinetic mechanism and property database for the simulations. A kinetic mechanism comprised of 297 species and 16,797 reactions was used in the simulations that included soot formation and evolution, and accounted for unsteady transport, liquid diffusion inside the droplet, radiative heat transfer, and variable properties. The droplet data showed a clear preference for the S5 surrogate in terms of burning rate. The simulations showed generally very good agreement with measured droplet, flame, and soot shell diameters. Measurements of combustion timing, in-cylinder pressure, and mass-averaged gas temperature were also well predicted with a slight preference for the S5 surrogate. Preferential vaporization was not evidenced from the evolution of droplet diameter but was clearly revealed in simulations of the evolution of mixture fractions inside the droplets. The influence of initial droplet diameter (D o) on droplet burning was strong, with S5 burning rates decreasing with increasing D o due to increasing radiation losses from the flame. Flame extinction was predicted for D o =3.0 mm as a radiative loss mechanism but not predicted for smaller D o for the conditions of the simulations. [ABSTRACT FROM AUTHOR]

Air pollution in many major cities is endangering public health and is causing deterioration of the environment. Particulate emissions (PM) contribute to air pollution as they carry toxic polyaromatic hydrocarbons (PAHs) on their surface. Abatement of PM requires continuous strict emission regulation and, in parallel, the development of fuels with reduced formation of PM. Key processes in the formation of PM are the decomposition of hydrocarbon fuels and the synthesis of potential precursors that lead to the formation of benzene rings and thereafter growth to PAHs and eventually PM. Methane, ethane and butane are important components of natural gas and liquefied petroleum gas, and are also widely used in transportation, industrial processes and power generation. This paper reports on a quantitative investigation of the intermediate gaseous species present during pyrolysis of methane, ethane and butane in a laminar flow reactor. The investigation aimed to further the understanding of the decomposition process of these fuels and the subsequent formation of aromatic rings. The pyrolysis of methane, ethane and butane were carried out in a tube reactor under laminar flow conditions and within a temperature range of 869–1213 °C. The fuels were premixed in nitrogen carrier gas at a fixed carbon atom concentration of 10,000 ppm, and were pyrolysed under oxygen-free conditions. Intermediate gaseous species were collected from within the tube reactor at different residence times using a specially designed high-temperature ceramic sampling probe with arrangements to quench and freeze the reactions at entry to the probe. Identification and quantification of intermediate species were carried out using a gas chromatography-flame ionization detector (GC-FID). During methane pyrolysis, it was observed that as the concentration of acetylene increased, the concentration of benzene also increased, suggesting that the benzene ring is formed via the cyclo trimerisation of acetylene. With all three fuels, all intermediate species disappeared at higher temperatures and residence times, suggesting that those species converted into species higher than benzene, for example naphthalene. It was observed that increasing carbon chain length lowered the temperature at which fuel breakdown occurred and also affected the relative abundance of intermediate species. [ABSTRACT FROM AUTHOR]

The effects of H 2 addition on soot formation are investigated in counterflow diffusion n -heptane flames. Three effects including chemical, thermal, and dilution are fully isolated and characterized by additions of H 2 , He, and Ar. Soot volume fractions are measured using LE-calibrated LII technique, and flame temperatures are measured using OH-TLAF method along with a thermocouple. Numerical simulations are conducted with a detailed mechanism with soot model. The simulated soot volume fractions and flame temperatures are in good agreement with experimental data. The experimental results show that H 2 addition can greatly reduce the soot formation. It is also found that the chemical and dilution effects suppress soot formation, while the thermal effect with increasing flame temperature promotes soot formation. Kinetic analysis suggests that HACA growth rate could be the dominant factor that controls the final soot formation through the three effects due to H 2 addition. • Hydrogen effects on soot formation of n -heptane counterflow flame. • Speration of chemical, thermal, and dilution effects of hydrogen. • Soot promotion due to thermal effect with increasing flame temperature. • HACA growth domination in hydrogen effects. [ABSTRACT FROM AUTHOR]

As one of the important by-products of incomplete combustion due to a high local equivalence ratio, soot particles can affect the flame structure and species distributions through their radiative, diffusion, and chemical effects based on the conservation equation of energy. In this work, a fully step-by-step decoupling method was employed to numerically isolate the various effects of soot formation on flame properties and soot evolution in a laminar nonsmoking ethylene diffusion flame. The combustion process was solved by a detailed kinetic model coupled with a sectional soot model. Results show that, the peak temperature of the flame increases when the radiative effect of the particles is neglected; however, it decreases when the chemical effect is excluded due to the absence of particle oxidation. The flame height, which depends on OH concentration, increases under both the radiative and chemical effects of soot particles. When soot radiation is not considered, the peak concentration of CO increases because of a lower conversion of CO to CO2 under higher local temperature, and a further increase is observed when soot reactions are removed, since the formation of CO is the only pathway of carbon conversion under this condition. When the radiation of soot particles is not considered, soot volume fraction (SVF) is reduced owing to a lower hydrogen-abstraction-carbon-addition (HACA) rate caused by a shorter residence time. In addition, soot particles move inside the flame where both SVF and temperature are lower due to their thermophoresis diffusion. The inception and coagulation rates increase without the radiative effect of soot, therefore, the average diameters of primary particles are smaller and the aggregate characteristics are higher. [ABSTRACT FROM AUTHOR]

A single-cylinder full-metal engine with a real combustion chamber geometry was used to investigate particulate number emissions resulting from transient engine operation. The formation of particulate number emissions depends on mixture formation influenced by the in-cylinder flow and injection, and the formation of fuel films on the in-cylinder walls. For the investigation of this multi-parameter process, simultaneous endoscopic PIV and combustion visualization were applied. Hence, the measurement techniques allowed the investigation of in-cylinder flow, flame propagation, and soot formation. The test rig was modified to apply a generic load step and a realistic tip-in with Miller cycle. The reproducibility of the engine parameters during the transient allowed statistical analysis and the comparison between steady-state operating points. Cause-and-effect chains concerning the formation of soot are concluded by correlation analysis of parameters extracted from the flow field, the flame propagation and the soot luminosity. [ABSTRACT FROM AUTHOR]

The second-order factor effect of burner optical ports and edge inter-matrices (EIM) and the first-order factor of pressure on the soot formation process and behavior of premixed sooting flames in a high-pressure burner are numerically investigated here. Three-dimensional computational fluid dynamics (CFD) simulations of a premixed flame C2H4/air at p = 1.01 and 10 bar using a one-step chemistry approach are first performed to justify the satisfied predictability of the prospective axisymmetric two-dimensional (2D) and one-dimensional (1D) simulations. The justified 2D simulation approach shows the generation of an axial vorticity around the EIM and axial multi-vorticities due to the high expansion rate of burnt gases at the high pressure of 10 bar. This leads to the development of axial multi-sooting zones, which are manifested experimentally by visible luminous soot streaks, and to the boosting of soot formation conditions of a relatively low-temperature field, <1800 K, and a high mixing rate of gases in combustion around and above the EIM location. Nevertheless, a tolerable effect on the centerline soot volume fraction (fV) profile, fV < 3%, is manifested only at high heights above the burner of the atmospheric sooting flame C2H4/air ϕ = 2.1, and early at the high pressure of 10 bar of this flame, fV < 10%. Enhancing the combustion process reactivity by decreasing the rich equivalence ratio of the fuel/air mixture and/or rising the pressure results in the prior formation of soot precursors, which shifts the sooting zone upstream. [ABSTRACT FROM AUTHOR]

In the period from 2019 to 2022, the joint American–Russian space experiment (SE) Flame Design (Adamant) was carried out on the International Space Station (ISS). The purpose of the joint SE was to study the mechanisms of control of soot formation in a spherical diffusion flame (SDF) formed around a porous sphere (PS), and the radiative extinction of the SDF under microgravity conditions. The objects of this study were "normal" and "inverse" SDFs of gaseous ethylene in an oxygen atmosphere with nitrogen addition at room temperature and pressures ranging from 0.5 to 2 atm. A normal flame is a flame formed in an oxidizing atmosphere when fuel is supplied through the PS. An inverse flame is a flame formed in a fuel atmosphere when an oxidizer is introduced through the PS. This article presents the results of calculations of soot formation in normal and inverse SDFs. The calculations are based on a one-dimensional non-stationary model of diffusion combustion of gases with detailed kinetics of ethylene oxidation, supplemented by a macrokinetic mechanism of soot formation. The results indicate that soot formation in normal and inverse SDFs is concentrated in the region where the local C/O atomic ratio and local temperature satisfy the conditions 0.32 < C/O < 0.44 and T > 1300–1500 K. [ABSTRACT FROM AUTHOR]

Forest fires arise from anthropogenic load and lightning activity. The formation of a thunderstorm front is due to the influence of a number of factors, including the emission of aerosol particles from forest fires. The purpose of this study is mathematical modeling of heat and mass transfer in vegetation firebrand carried out from a forest fire front, taking into account the formation of soot particles to predict forest fire danger from thunderstorm activity. Research objectives: (1) development of a deterministic mathematical model of heat and mass transfer in a pyrolyzed firebrand of vegetation, taking into account soot formation; (2) development of a probabilistic criterion for assessing forest fire danger from thunderstorms, taking into account aerosol emissions; (3) scenario modeling of heat and mass transfer and the formation of a thunderstorm front; (4) and the formulation of conclusions and proposals for the practical application of the developed deterministic–probabilistic approach to the prediction of forest fires from thunderstorms, taking into account aerosol emissions. The novelty of this study lies in the development of a new model of heat and mass transfer in a pyrolyzed vegetation firebrand and a new probabilistic criterion for forest fire danger due to thunderstorm activity, taking into account aerosol emission. The distributions of temperature and volume fractions of phases in a firebrand are obtained for various scenarios. Scenarios of surface fires, crown forest fires, and a fire storm are considered for typical types of coniferous vegetation. Cubic firebrands are considered in the approximation of a two-dimensional mathematical model. To describe the heat and mass transfer in the firebrand structure, a differential heat conduction equation is used with the corresponding initial and boundary conditions, taking into account the kinetic scheme of pyrolysis and soot formation. Variants of using the developed mathematical model and probabilistic criterion in the practice of protecting forests from fires are proposed. Key findings: (1) linear deterministic–probabilistic mathematical model to assess forest fire occurrence probability taking into account aerosol emission and lightning activity; (2) results of mathematical modeling of heat and mass transfer in firebrand taking into account soot formation; (3) and results of scenario modeling of forest fire occurrence probability for different conditions of lightning activity and aerosol emission. [ABSTRACT FROM AUTHOR]

Numerical simulations of the formation and growth of soot particles in combustion processes are important for estimating the emission rate of particulate air pollutants and predicting the accurate size and morphology of industrial commodity particles. A new bi-modal soot aerosol model called the Soot Aerosol Moment Model (SAMM) was developed. The SAMM can predict the changes in the soot particle size distribution and morphology simultaneously by monitoring only five variables evolving due to nucleation, surface reaction, PAH condensation, coagulation, sintering, and condensational obliteration. The performance of the SAMM was evaluated by comparing its predictions with those of a sectional soot model and available measurements. The SAMM provided comparable information on the soot particle size distribution and morphology with a more than 100-fold shorter computation time than the sectional model, suggesting that it can be used effectively to develop and test sophisticated chemical mechanisms for soot formation. [ABSTRACT FROM AUTHOR]

An experimental study was performed to measure the effects of flame oscillations on soot concentration and flame temperature in co-flow laminar diffusion flames of ethylene. One novel aspect of this study is that flame oscillations were mechanically induced by moving the burner assembly in a reciprocating motion with an industrial computer-controlled linear stage. Motor speed was used to control the oscillation frequency. The peak-to-peak oscillation amplitude was defined as stroke, which was then used to calculate the Strouhal number. Oscillation frequencies of 5‒20 Hz and peak-to-peak amplitudes of 0.5‒3 mm were studied. At 1 mm peak-to-peak oscillation amplitude, flame oscillations switched from tip flickering type to tip-clipping type oscillations above 10 Hz. Time-averaged soot concentration increased from its value in the steady-state flame when the flame was perturbed by small oscillation frequencies and amplitudes but then decreased as the oscillation intensity was increased. The peak time-averaged soot volume fraction was 9.5 ppm in the steady-state flame and 4.5 ppm at a peak-to-peak oscillation amplitude and an oscillation frequency of 1 mm and 20 Hz, respectively. Similarly, the peak soot volume fraction was 5.8 ppm at a peak-to-peak oscillation amplitude and an oscillation frequency of 3 mm and 10 Hz, respectively. The time-averaged flame temperatures were in the range of 1500‒2000 K. The low-temperature zone was observed at the flame core in the steady-state flame and for low-intensity oscillations, and at the flame tip for high-intensity oscillations. The literature has discrepancies on how flame oscillations affect the total soot loading in ethylene flames. Our results show that the total soot loading increased by about 10% from steady-state to low-intensity oscillations, then slightly decreased as either oscillation frequency or amplitude was increased. The effects of increasing the oscillation frequency or amplitude were similar and well captured by the revised Strouhal number. [ABSTRACT FROM AUTHOR]

The influence of various oxygenated co-reactants on soot formation was addressed by studying the pyrolysis of gas mixtures using ethylene as a base fuel in the absence of molecular oxygen. Alcohols (methanol, ethanol, and n-butanol) and ethers (diethyl ether, dimethoxymethane, furan, and tetrahydrofuran) were selected as oxygenated co-reactants. The pyrolysis process was studied behind reflected shock waves at around 3 bar. Laser extinction at 633 nm was used to determine soot-inception times and the related optical densities as a function of reaction time. The temporal variation in temperature was measured via time-resolved two-color CO absorption using two quantum-cascade lasers at 4.73 and 4.56 µm. Soot particle sizes were determined by time-resolved laser-induced incandescence using a Nd:YAG laser at a wavelength of 1064 nm. The major finding is that based on C 2 H 4 as soot precursor, only CH 3 OH does not have a soot-promoting effect, whereas the addition of all other oxygenated hydrocarbons results in increased soot yields. The measured temperatures were compared with simulations based on a detailed chemical kinetics mechanism from the CRECK Modeling Group (Pejpichestakul et al. Proc. Combust. Inst. (2019)). In addition, kinetics modeling of hydrocarbon pyrolysis, PAH formation and soot growth was performed in OpenSMOKE++ software to give qualitative insight in the experimental results using the CRECK mechanism and an assembled mechanism composed of the CRECK and the recent PAH sub-mechanism (Sun et al. Proc. Combust. Inst. (2021)). The simulation reveals that the observed promotion of soot formation in presence of oxygenated co-reactants is associated with the release of CH 3 and C 3 H 3 during the thermal decomposition resulting in acceleration of C 6 H 6 ring formation. [ABSTRACT FROM AUTHOR]