Soot Formation Dynamics Research Articles

Investigating the formation of soot in CH4 pyrolysis reactor: A numerical, experimental, and characterization study

Methane pyrolysis is a promising method for eco-friendly hydrogen production, but soot formation and carbon interaction pose challenges for scaling up. Therefore, understanding the dynamics of soot formation and carbon deposition is crucial. This study delves into the intricacies of soot formation in methane pyrolysis under industrially relevant conditions, namely operations at atmospheric pressure, employing a H2:CH4 ratio of 2 and exploring a range of hot zone temperatures (1473 K, 1573 K, 1673 K, and 1773 K) with a 5 s residence time. Utilizing a detailed gas-phase kinetic model with direct carbon deposition reactions, the research adopts the method of moments coupled with a one-dimensional plug flow reactor model to simulate soot formation. The model is validated by characterizing soot particles that were produced in a pyrolysis reactor by means of transmission electron microscopy, Dynamic light scattering (DLS), and Raman spectroscopy. Results show that lower temperatures lead to nucleation-dominated growth, whereas higher temperatures significantly restrain particle growth due to carbon deposition. DLS data indicate a complex balance between particle growth and deposition processes. These findings provide insights into operational parameters that can enhance reactor performance and sustainability in hydrogen production processes by mitigating soot and carbon deposition.

Influence of operating conditions on flow field dynamics and soot formation in an aero-engine model combustor

Large-eddy simulations of a sooting aero-engine model combustor are performed for three operating points. They are analyzed to investigate the influence of secondary air injection and primary equivalence ratio on soot formation dynamics. It will be shown how the causes of a strongly intermittent sooting behavior can be investigated quantitatively by mode analysis and correlated statistics. Simulations rely on Arrhenius reaction rate-based finite-rate chemistry, where 79 species transport equations are solved simultaneously to accurately predict the combustion of ethylene: 43 species represent the gas phase, 36 the soot and soot precursors. Soot evolution is described by a well validated sectional soot model including a sectional model for polycyclic aromatic hydrocarbons and their radicals. This approach captures the influence of changing operating conditions on soot formation well. Hence, soot prediction is found to be accurate enough to investigate the formation processes. The presence of coherent and incoherent flow field dynamics in the combustor necessitates the use of multiresolution proper orthogonal decomposition and correlated statistics for the analysis. While the precessing vortex core is found to support soot production continuously, low frequency dynamics are identified to cause the intermittence at all operating conditions. Secondary air injection significantly increases the intensity of the low frequency dynamics. Yet, the soot volume in the combustor varies by the same order of magnitude without secondary air injection. The soot formation intermittence is closely linked to an axially symmetric mixture fraction mode near the primary injector at all operating conditions. Low mode energy required an additional mode analysis of joint statistics which confirmed this conclusion. Increasing the equivalence ratio at the primary injector decreases the influence of the low frequency dynamics on soot formation.

Dynamics of soot surface growth and agglomeration by enclosed spray combustion of jet fuel

Understanding the dynamics of soot formation and growth during combustion of jet fuel is essential for mitigation of aircraft engine emissions. Here, soot formation during enclosed spray combustion of jet fuel is investigated for its capacity to form soot with comparable characteristics to that from aircraft engines. For this, microscopy, scanning mobility particle, X-ray diffraction & Raman spectroscopy measurements and discrete element modeling (DEM) are employed along the flame centerline at various Effective eQuivalence Ratios (EQR). The DEM-derived mobility and primary particle size distribution dynamics are in excellent agreement with those measured at 5–63 cm height above the burner (HAB) for the measured temperature and soot volume fraction. At low EQR (1.46 and 1.59), soot surface growth stops at residence time, t = 4–7 ms, resulting in median soot primary particle diameters, d¯p, of ∼ 14 nm. At longer t (high HAB), agglomeration takes over increasing the median mobility diameter from 16 to 88 or 145 nm at EQR of 1.46 or 1.59, respectively, without altering d¯p and having the disorder over graphitic Raman band ratio, D/G = 0.9 ± 0.01 and a crystallite length, Lc = 1.24 ± 0.02 nm. In contrast, increasing EQR from 1.59 to 1.88, enhances soot surface growth, increases d¯p up to 23 nm and results in more graphitic soot having D/G = 0.8 ± 0.01 and Lc = 1.47 ± 0.01 nm. Furthermore, the D/G of soot is inversely proportional to its d¯p that is determined largely by surface growth.

Impact of the maturation process on soot particle aggregation kinetics and morphology

Mature soot aggregates exhibit a morphology which is currently mimicked by diffusion-limited aggregation (DLCA) codes, i.e. with a sticking probability = 1 when two aggregates collide. Nevertheless, nascent soot particles may grow in the reaction-limited aggregation regime (RLCA), i.e. with a sticking probability ≪ 1. Yet, it remains to be seen how fast the transition from RLCA to DLCA occurs for soot particles and what is the impact on aggregation kinetics, particle size distribution and morphology. This work intends to fill this gap by exploring the aggregation of soot particles formed in a laminar premixed flame through numerical simulations. Results show that the transition from RLCA to DLCA is very fast and produces a moderate impact on soot formation dynamics and morphology. However, soot particles mass bulk density is found to play an important role and should be considered in future simulations of soot formation in flames.

Dynamics of soot formation and burnout in a gas diesel cylinder

Combustion of the methane-air mixture can be accompanied by intensive formation and burning of soot, which will lead to an increase in heat losses and heat stress of the chilindro-piston group. The complexity and difficulty of registering the concentration of instantaneous soot particles make it difficult to study the laws of soot formation kinetics in gas diesel. The mechanism of soot formation during combustion of a methane-air mixture in a gas-diesel fuel is even more complex and less studied, since there is charge heterogeneity and other supercritical parameters. The paper proposes a mathematical model of soot formation and burnout in a gas diesel cylinder, which attempts to take into account the features of combustion of atomized fuel and methane-air mixture.

A phenomenological modelling framework for particle emission simulation in a direct-injection gasoline engine

A phenomenological modelling framework is presented that allows the simulation of the engine-out particle emissions of direct-injection gasoline engines based on physical principles. It is applicable both at steady-state operating conditions and in transient driving cycles. Within the modelling framework, a multi-zone model with gas-phase reaction kinetics is coupled with a stochastic reactor model considering the soot formation dynamics. Both model parts are fed with inputs from an accompanying engine process simulation. Particle emissions from injector deposits and inhomgeneous gaseous mixture preparation are taken into account. Pyrolysis reactions are considered in a zone with remaining fuel film at the injector, whereas in gas-phase zones that arise from inhomogeneous mixture preparation, the reaction of the air–fuel mixture is calculated under sub-stoichiometric conditions. The setup of those mixture-induced zones is generated by an existing homogenisation sub-model. The modelling framework is evaluated by test bench measurements of the engine operating map, a variation of engine actuator settings and transient driving profiles. Hereby, an accuracy of less than 20% deviation for particle number and mass is achieved.

Effect of Dimethyl Ether Addition on Soot Formation Dynamics of Ethylene Opposed-Flow Diffusion Flames

Soot formation dynamics in C2H4/dimethyl ether (DME) opposed-flow diffusion flames was numerically studied in this paper, employing detailed gas-phase and dispersed-phase chemistries and transport models. A wide range of DME additions from pure C2H4 to pure DME was involved to systematically examine its impact on the soot formation process. It was found that the flow field had an important impact on the soot structure inside the flame. The soot volume fraction reached a maximum at the stagnation plane due to the infinite residence time, and then vanished abruptly on the fuel side of the stagnation plane. In the opposed-flow diffusion flames, enhancements of nucleation, H-abstraction–C2H2-addition (HACA) reaction, and polycyclic aromatic hydrocarbon (PAH) condensation rates in the near-stagnation region disappeared, such that the local particle size distribution function (PSDF) curve became unimodal, which was rather different from the burner-stabilized stagnation premixed flame. A synergistic effect of DM...

Numerical investigation of soot formation from microgravity droplet combustion using heterogeneous chemistry

The use of isolated droplets as idealized systems is an established practice to get an insight on the physics of combustion, and an optimal test field to verify physical submodels. In this context, this work examines the dynamics of soot formation from the combustion of hydrocarbon liquid fuels in such conditions. A detailed, heterogeneous kinetic mechanism, describing aerosol and particle behavior through a discrete sectional approach is incorporated. The developed 1-dimensional model accounts for (i) non-luminous and luminous radiative heat losses, and (ii) incomplete thermal accommodation in the calculation of the thermophoretic flux. The combustion of droplets of n-heptane, i.e., the simplest representative species of real fuels, was investigated as test case; an upstream skeletal reduction of the kinetic mechanism was carried out to limit calculation times. After checking the performance of the reduced mechanism against gas-phase experimental data, the transient evolution of the system was analyzed through a comprehensive study, including fiber-suspended (D0 < 1 mm) as well as free (D0 > 1 mm) droplets.The different steps of soot evolution were quantified, and localized in the region between the flame front and the soot shell, where particle velocity is directed inwards because of thermophoresis, and residence times are much higher than what usually found in diffusion flames. As a result, growth, coalescence, and aggregation steps are significantly enhanced, and soot accumulates in the inner shell, with an evident modification of the particle size distribution, if compared to what observed in conventional combustion conditions. The model exhibits a satisfactory agreement with experimental data on flame temperature and position around the droplet, while for larger droplets an increasing sensitivity to the radiation model was observed. It is found that the latter has a significant impact on the production of soot, while scarcely affecting the location of the soot shell. On the other side, the inclusion of incomplete thermal accommodation in the thermophoretic law brought about more accurate predictions of both volume fractions and shell location, and highlighted the primary role of thermophoresis in these conditions, as already found in literature through more simplified approaches.

Thermocouple temperature measurements in diesel spray flame for validation of in-flame soot formation dynamics

It is well known that the soot formation is governed by equivalence ratio and temperature. However, there are only few examples of temperature measurements in the soot formation region of diesel spray flame, which has been impeding better understanding and model validation. In this study, the time histories of temperature at different axial locations in a single-shot diesel spray flame were measured in a constant volume vessel using a 50-µm-thin wire type R thermocouple and used to demonstrate their usefulness for the model validation of in-flame soot formation dynamics. The measured temperature was (1) compared and cross-checked with two-color temperatures of diesel flame periphery and core, (2) compared with predicted temperature from large eddy simulation of the diesel flame for validation and (3) contrasted with previous laser spectroscopic measurements of soot precursors (polycyclic aromatic hydrocarbons) in diesel spray flame and predicted soot processes from the large eddy simulation of diesel spray flame employing detailed chemical kinetics. The measured steady-state temperature increased from upstream to downstream in diesel spray flame corresponding to the progress of mixing and combustion. The measured temporal histories of temperature exhibited notable increase after the end of injection duration considered due to entrainment of ambient gases and resulting heat release in the wake of the injection pulse. The thermocouple-measured “core” temperatures and two-color “peripheral” temperatures of diesel flame were significantly different as up to 700 K in the upstream and gradually converged to around 2000 K in the downstream. The thermocouple-measured and large eddy simulation–predicted temperature histories showed similar general trends; however, 500 K as significant discrepancy was observed between the measured and predicted temperatures of the polycyclic aromatic hydrocarbon onset locations in diesel spray flame, suspected partially due to deficiency in the current polycyclic aromatic hydrocarbon formation model employed in the simulation.

Validation and optimization of the thermal cycle for a diesel engine by computational fluid dynamics modeling

In this study, we developed a computational fluid dynamics (CFD) model of a turbocharged diesel engine (1CT107) powered by diesel fuel. The engine was part of the power generating set in the portable version. In the simulation tests, we analyzed the impact of the ignition timing on the thermodynamic parameters and emissions of toxic components. We verified the model of the test engine and it was then used to optimize the thermal cycle for the test engine. We found that the engine model had acceptable accuracy and it was suitable for emissions modeling. Under the full load, the NO emissions were 2.2g/kWh, which satisfied the EURO IV criteria. As the load increased, the soot emissions also increased. This model also confirmed that the dynamics of soot formation were the opposite of NO formation. In summary, CFD modeling provides a powerful tool for optimizing the internal combustion engine in terms of both the thermodynamic parameters and emissions. The model of the test engine was produced using AVL FIRE.

Effects of repetitive pulsing on multi-kHz planar laser-induced incandescence imaging in laminar and turbulent flames.

Planar laser-induced incandescence (LII) imaging is reported at repetition rates up to 100kHz using a burst-mode laser system to enable studies of soot formation dynamics in highly turbulent flames. To quantify the accuracy and uncertainty of relative soot volume fraction measurements, the temporal evolution of the LII field in laminar and turbulent flames is examined at various laser operating conditions. Under high-speed repetitive probing, it is found that LII signals are sensitive to changes in soot physical characteristics when operating at high laser fluences within the soot vaporization regime. For these laser conditions, strong planar LII signals are observed at measurement rates up to 100kHz but are primarily useful for qualitative tracking of soot structure dynamics. However, LII signals collected at lower fluences allow sequential planar measurements of the relative soot volume fraction with a sufficient signal-to-noise ratio at repetition rates of 10-50kHz. Guidelines for identifying and avoiding the onset of repetitive probe effects in the LII signals are discussed, along with other potential sources of measurement error and uncertainty.

Transient soot dynamics in turbulent nonpremixed ethylene–air counterflow flames

The dynamics of soot formation in turbulent ethylene–air nonpremixed counterflow flames is studied using direct numerical simulation (DNS) with a semi-empirical soot model and the discrete ordinate method (DOM) as a radiation solver. Transient characteristics of soot behavior are studies by a model problem of flame interaction with turbulence inflow at various intensities. The interaction between soot and turbulence reveals that the soot volume fraction depends on the combined effects of the local conditions of flow, temperature, and fuel concentration, while the soot number density depends predominantly on the high temperature regions. Depending on the relative strength between mixing and reaction, the effects of turbulence on the soot formation lead to three distinct paths in deviating the data points away from the laminar flame conditions. It is found that turbulence has twofold effects of increasing the overall soot yield by generating additional flame volume and of reducing soot by dissipating soot pockets out of high-temperature regions. The relative importance between the two effects depends on the relative length scales of turbulence and flame, suggesting that a nonmonotonic response of soot yield to turbulence level may be expected in turbulent combustion.

Dynamics of soot formation by turbulent combustion and thermal decomposition of natural gas

This study investigates the formation process of soot particles in turbulent flows within fuel-rich natural gas flames. Research on carbon formation from relatively simple hydrocarbons under real process conditions and harsh environments allows new insights into the general understanding of soot formation. Experimental work was undertaken on a small-scale gas furnace based on the design of a modern oil-fired carbon black furnace. The combustion system investigated is fired by a nonpremixed, swirl stabilized, confined methane-air flame. The study comprises establishing conditions under which the production of soot particles takes place, progressing toward quantified analysis. A large number of process parameters have been investigated, and the relative role of incomplete combustion and thermal decomposition in the process of carbon particulate formation has been illustrated. Maximum solid carbon formation was realized at maximum air temperature and maximum furnace temperature. Experimental investigations, using carbon particle concentration measurements coupled with on-line gas analysis showed that slightly fuel rich conditions are governed by incomplete combustion only, whereas richer combustion systems are governed by thermal decomposition processes. Both processes are found to be strongly temperature dependent, whereby an increase in temperature reduces particle production in the former process, but enhances it in the latter. A 3-D simulation of the simultaneous processes of incomplete fuel combustion and fuel decomposition in turbulent combustion systems, using a novel integration of fluid mechanics, chemical kinetics, and a standard soot model, facilitates discrimination between the two main sources of carbonaceous particulate matter, and indicates reasonable agreement with experimental trends.

Detailed kinetic modeling of soot formation in ethylene/air mixtures reacting in a perfectly stirred reactor

A series of perfectly stirred reactor (PSR) calculations has been performed that allow the comparison of numerical predictions for sooting trends in PSRs with those observed in experiments. A detailed chemical kinetics model for soot production is employed that has been extensively tested for laminar flames. The chemical kinetics model is used without any adjustments. The PSR code Aurora that is a CHEMKIN III application has been modified considerably to incorporate the method of moments. Gas-phase and surface reactions are rigorously coupled in the model. Predicted sooting characteristics are consistent with behavior observed experimentally for premixed laminar flames and well-stirred reactors. Specifically, soot volume fraction exhibited the same qualitative behavior with equivalence ratio, temperature, and pressure as observed for flames. Concentration profile shapes were consistent with profile shapes measured for flames as well. The model also predicts soot threshold values and the relationship of soot volume fraction with temperature and equivalence ratio that has been observed experimentally for well-stirred reactors. Under the conditions of the present investigation, soot formation dynamics was dominated by nucleation. The temperature dependence of surface reaction rates revealed the importance of the HACA mechanism (H-abstraction-C2H2 addition reaction sequence) in surface growth. Under PSR conditions, coupling the gasphase and surface chemistry is significant with the degree of coupling increasing with pressure.