Particles In Flame Research Articles

Tracking the evolution of soot particles and precursors in turbulent flames using laser-induced emission

A point-wise, time-resolved optical technique is employed to track simultaneously the evolution of soot particles and soot precursors in a turbulent flame of ethylene/nitrogen (1/1 by vol.). A picosecond laser, 80ps@266nm is used to measure scattering and laser-induced fluorescence (LIF) in selected UV and visible bands while Laser-induced incandescence (LII) is excited using a pulsed laser with 8ns@1064nm. Fast photomultiplier tubes are used to ensure time-resolved measurements and LIF decay times of the order of 3.5ns are detected. The LIF signals are attributed to nanoparticles or condensed phase species made up of PAHs, whereas LII was correlated with soot. Data conditioning based on the noise level was made to delineate the potentially large influence of intermittency on the statistics of the detected signals. It was found that LIF signals with long decay times were detected well before the onset of soot particles, as seen by LII, and these continue well into the maximum soot formation region. The correlation between the LII and LIF signals confirmed the co-presence of small nanoparticles together with high concentrations of mature soot. Analysis of conditioned mean soot concentration suggests that the intermittency occurring at the edge and tip of the flame does not locally decrease the soot concentration since large LII signals were still detected. These simultaneous, time-resolved measurements are extremely useful to provide insights on the evolution of combustion-generated particles in turbulent sooting flames.

On the extinction of igniting kernels in near-isotropic turbulence

Extinction dynamics of ignition kernels of rich H2-air mixture (ϕ=4,Le>1) in near-isotropic turbulence, are studied using three sets of direct numerical simulations and the recently introduced flame particle tracking. Turbulence is found to extinguish a freshly ignited, initially spherical premixed flame kernel, which otherwise sustains in a quiescent flow field by propagating beyond the minimum radius. The mechanism of kernel extinction is investigated by tracking lifetime trajectories of flame particles on an O2 mass fraction isosurface in the flame speed-curvature (Sd, κ) space using the well-known concept of minimum radius from laminar flames. The classical S-curve in the temperature-Damköhler number (T, Da) space is also analyzed. Ensemble averaged Sd−κ and S-curves display corresponding turning points which help to elucidate the intricate mechanisms involved in turbulent premixed flame extinction dynamics. Turbulence locally wrinkles the isosurface into positively curved structures which lead to turning points in (Sd, κ) space, such that the minimum radius is never reached, locally and as an ensemble. A budget analysis of the principal curvature evolution equation highlights the role of turbulence in bending the surface to form positively curved, pointed structures where heat loss is enhanced, further lowering Da towards extinction.

Modeling soot oxidation with the Extended Quadrature Method of Moments

Modeling the oxidation of soot particles in flames is a challenging topic both from a chemical point of view and regarding the statistical treatment of the evolution of the soot number density function (NDF). The method of moments is widely-used for the statistical modeling of aerosol dynamics in various applications, and a number of different moment methods have been established and successfully applied to the modeling of soot formation and growth. However, a shortcoming of existing moment methods is the lack of an accurate, numerically robust, and computationally efficient way to treat soot oxidation, especially regarding the prediction of the particle number density. In this work, the recently developed Extended Quadrature Method of Moments (EQMOM) is integrated with a physico-chemical soot model and combined with a treatment for particle removal by oxidation. This leads to a modeling framework for the simulation of coupled inception, growth, coagulation, and oxidation of soot in flames. In EQMOM, the moment equations are closed by reconstructing the soot NDF with a superposition of continuous kernel functions. Various standard distribution functions can be used as kernel functions, and the algorithm has been implemented here using gamma and lognormal distributions. It is shown that and discussed why gamma distributions are more suitable as kernel functions than lognormal distributions in order to accurately predict soot oxidation. The integrated model is validated by comparisons with analytical solutions for the NDF, results from Monte Carlo simulations of soot formation and oxidation in flames, and experimental data.

Modelling of soot formation in laminar diffusion flames using a comprehensive CFD-PBE model with detailed gas-phase chemistry

A discretised population balance equation (PBE) is coupled with an in-house computational fluid dynamics (CFD) code in order to model soot formation in laminar diffusion flames. The unsteady Navier–Stokes, species and enthalpy transport equations and the spatially-distributed discretised PBE for the soot particles are solved in a coupled manner, together with comprehensive gas-phase chemistry and an optically thin radiation model, thus yielding the complete particle size distribution of the soot particles. Nucleation, surface growth and oxidation are incorporated into the PBE using an acetylene-based soot model. The potential of the proposed methodology is investigated by comparing with experimental results from the Santoro jet burner [Santoro, Semerjian and Dobbins, Soot particle measurements in diffusion flames, Combustion and Flame, Vol. 51 (1983), pp. 203–218; Santoro, Yeh, Horvath and Semerjian, The transport and growth of soot particles in laminar diffusion flames, Combustion Science and Technology, Vol. 53 (1987), pp. 89–115] for three laminar axisymmetric non-premixed ethylene flames: a non-smoking, an incipient smoking and a smoking flame. Overall, good agreement is observed between the numerical and the experimental results.

Microstructure and electrical properties of plasma sprayed Gd0.15Ce0.85O2-δ coatings from solution combustion synthesized flowable powders

Powders of Gadolinia doped Ceria (Gd0.15Ce0.85O2-δ (GDC)), a promising electrolyte material for intermediate temperature solid oxide fuel cell has been prepared by solution combustion (SC) synthesis. Variation of fuel to oxidizer ratio in the synthesis results in the GDC powder with varied powder characteristics. Fuel deficient combustion reaction results in GDC powder with smaller crystallites size but with larger agglomerates; whereas reaction with near stochiometric amount of fuel leads to the GDC powder with larger crystallite size and smaller agglomerates. Powders with large agglomerates possess required particle size, shape and flowability characteristics appropriate for plasma spraying. Electrolyte coatings fabricated from the synthesized plasma spray grade powders exhibit superior inter splat adhesion and conductivity (∼0.02S/cm at 600°C). Dense nature of the coating is ascribed to the complete melting of the porous GDC particles in the plasma flame. Powders prepared with stochiometric fuel are suitable for tape casting process.

Unsteady dynamics of PAH and soot particles in laminar counterflow diffusion flames

Due to their low chemical time scales, the production of soot particles in turbulent diffusion flames is highly impacted by large range of local strain rate fluctuations.In order to understand the response of soot production to strain rate fluctuations, unsteady laminar counterflow diffusion flames with an imposed oscillating strain rate are investigated both analytically and numerically. First an analytical linearized model is developed to predict the unsteady response of a flame quantity of interest from information on laminar steady flames. Three critical parameters governing flame response are identified: the Stokes number which compares the characteristic time associated to the mean imposed strain rate to the oscillation frequency, the Damköhler number associated to the quantity of interest, and a third one characterizing the response of this quantity to an imposed steady strain rate. This model is then applied to soot predictions. Parallely, the response of soot production in propane-air counterflow diffusion flames to unsteady strain harmonic oscillations is studied numerically using a detailed sectional soot model. A wide range of frequencies and amplitudes are considered. A specific trend is highlighted for soot precursors and particles production according to their respective chemical time scales: the bigger the PAH or soot particle, the higher its chemical time scale, resulting in a more damped and phase-lagged response. The particle size distribution evolves accordingly during the considered oscillations, so that the quasi-steady state behavior is not verified for high frequencies. The numerical results are compared to those obtained by the analytical approach and a very good agreement is obtained at low amplitudes. Non-linear response of soot precursors and soot particles production to strain oscillations are finally discussed in case of high oscillation amplitudes and the limits of the proposed analytical model are identified.

광 산란 기술을 이용한 연료 첨가제의 그을음 억제 효과 분석

본 논문에서는 이소 옥탄을 연료로 사용하는 층류 확산 화염에서 불완전 연소의 결과로 발생하는 그을음 입자의 성장 및 산화과정을 조사하였다. 또한, 철 연료첨가제의 그을음 발생 억제 효과를 평가하였다. 광 계측 기술을 이용하여 화염 안 그을음 입자의 탄성 산란 광을 화염의 높이에 따라 계측하고, 미광 보정 과정을 통하여 그을음 입자의 미분 산란 계수를 구하였다. 이 미분 산란 계수를 비교하여, 화염에서 그을음 입자의 성장 및 산화과정을 조사하였는데, 산화과정을 통하여 화염 내의 그을음 양이 감소하는 것을 관찰하였다. 철 연료 첨가제를 첨가한 화염에 대해서 동일한 연구 방법으로 구한 미분 산란 계수가 연료 첨가제가 첨가되지 않은 화염보다 더 작은 값을 보였다. 또한, 화염의 높이에 따라 반경방향으로 투과율을 측정하였는데, 연료 첨가제를 첨가한 연료의 화염은 투과율이 산화영역에서 약 5% 증가하는 것을 관찰하였고, 이는 미분 산란 계수와 동일한 경향을 보였다. 이것은 연료첨가제의 성분이 촉매 역할을 하여 그을음이 산화를 촉진해 화염 안의 그을음 양이 감소한 것으로 사료된다. This paper presents an experimental analysis of the growth and oxidation processes of soot particles generated in an isooctane diffusive laminar flame due to incomplete combustion. The effects of iron-based diagnostics were employed to measure the elastic scattering light from soot particles in a flame at different flame heights, and the differential scattering coefficients were calculated through a calibration process. The growth and oxidation of soot particles in flame was investigated by comparing differential scattering coefficients, and the soot volume fraction was seen to decrease in the soot oxidation process. In the same manner, the differential scattering coefficients were calculated for iron-based fuel-additive seeded flame, and these coefficients were revealed to be smaller than those obtained in the fuel-additive unseeded flame. In addition, transmission through the radial direction of the flame was measured, and transmission in the soot oxidation regime was approximately 5% higher for the seeded flame. The propensity of the data coincided well with the differential scattering coefficients, and it can be concluded that the iron component of the fuel additive plays a crucial role as a catalyst, which eventually enhanced soot particle oxidation.

Mid-infrared laser-induced thermal grating spectroscopy in flames

For the first time, laser-induced thermal grating spectroscopy (LITGS) in the spectral range around 3µm is demonstrated as a versatile diagnostic tool. This spectral region is of particular interest in combustion diagnostics as many relevant species such as hydrocarbons and water exhibit fundamental vibrational modes and hence can be probed with high sensitivity. Another benefit of the IR-LITGS is that it allows performing spectroscopy in the infrared combined with signal detection in the visible. Hence, the strong thermal radiation inherent in flames does not represent an interference. As the first step, we present the application of IR-LITGS to cold gas flows, where traces of ethylene and water vapor are detected. The time-resolved LITGS signals, which can be acquired in a single laser shot, are rich in information and allow deriving temperature and to some extend chemical composition. In the second step, the IR-LITGS technique is applied to ethylene/air flames stabilized on a flat flame burner. A proof-of-concept study is carried out, in which the temperature is determined in the burned region of flames with systematically varied equivalence ratio (0.72 < Φ <2.57). Moreover, in a highly sooty flame, LITGS signals were recorded as a function of height above the burner and allowed the determination of the temperature profile. The proposed IR-LITGS method has the potential for enabling single-shot measurements of several parameters at a time. Its applicability to sooty flame environments opens up new opportunities to study the complex formation of carbonaceous particles in flames.

Pressure dependence of primary soot particle size determined using thermophoretic sampling in laminar methane-air diffusion flames

Optical techniques, such as the light extinction and scattering as well as the laser-induced incandescence and the spectral soot emission, have been used routinely to measure soot concentrations in atmospheric flames. Laser-induced incandescence and light scattering have been proven to measure the primary soot particle size as well in atmospheric flames; however, using these two techniques for the purpose of primary soot particle sizing in diffusion flames at elevated pressures has been found to be problematic. One of the popular methods of studying soot morphology and primary size in atmospheric flames is thermophoretic sampling followed by transmission electron microscopy analysis. A high pressure thermophoretic sampling system was built and used successfully to measure the size of primary soot particles in laminar diffusion flames of methane at pressures above atmospheric. The multi-probe sampling system was fitted inside the high-pressure combustion chamber that had been used previously for high-pressure soot formation studies. Soot samples taken at various pressures were analyzed subsequently by transmission electron microscopy to estimate the primary soot particle sizes. The soot volume fractions and soot temperatures were measured by spectral soot emission technique at the same height above the burner rim as the thermophoretic sampling. The mean primary soot particle size, measured at a constant height from the burner exit at all pressures, decreased about 35% from 2 to 10 atm whereas the soot volume fraction increased by a factor of more than 50. Experimental results of mean primary soot sizes and the corresponding soot volume fractions imply that the number of soot nuclei in soot inception region of the laminar diffusion flames must have a strong sensitivity to pressure. The higher amounts of soot are mainly determined by the increasing nucleation leading to higher primary soot particle number densities as the pressure is increased.

Simulations and experiments on the ignition probability in turbulent premixed bluff-body flames

The ignition characteristics of a premixed bluff-body burner under lean conditions were investigated experimentally and numerically with a physical model focusing on ignition probability. Visualisation of the flame with a 5 kHz OH* chemiluminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work on non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability Pign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. Flows corresponding to flames close to the blow-off limit could not be ignited, although such flames were stable if reached from a richer already ignited condition. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high Pign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. A modelling effort that takes convection and localised flame quenching into account by tracking stochastic virtual flame particles, previously validated for non-premixed and spray ignition, was used to estimate the ignition probability. The applicability of this approach to premixed flows was first evaluated by investigating the model's flame propagation mechanism in a uniform turbulence field, which showed that the model reproduces the bending behaviour of the ST-versus-u′ curve. Then ignition simulations of the bluff-body burner were carried out. The ignition probability map was computed and it was found that the model reproduces all main trends found in the experimental study.

Probe effects in soot sampling from a burner-stabilized stagnation flame

Probe sampling of soot particles in laminar premixed flames is a common method for characterizing nascent soot formation. Probe intrusiveness into the flame can introduce significant uncertainty in interpretation of experimental data and comparison with numerical results. The aim of the present work is to study the probe-induced effects on soot sampling in a burner-stabilized stagnation (BSS) flame by numerical simulations. The thermophoretic effect is investigated first under non-reactive conditions. The relevant model formulation was tested against experimental data from the literature. Soot size distributions and global properties in the burner stabilized-stagnation flame configuration are studied using both a one-dimensional stagnation flow model and two-dimensional axisymmetric simulations using detailed kinetics and transport. A benchmark burner-stabilized stagnation flame fed with ethylene (Camacho et al., 2015 ) was employed as the target for detailed investigation, focusing on the quantification of the orifice flow effect on the soot size distribution. The results show that the orifice flow can introduce a notable impact on the local flow field, temperature, and particle residence time. Soot measurements have to be shifted some millimeters upstream from the stagnation surface because of the impact of the orifice on the local flow and temperature field. The extent of the spatial shift was quantified by comparing one-dimensional stagnation flow and two-dimensional axisymmetric simulations. The results showed that the spatial shift is weakly dependent on fuel chemistry, but it exhibits stronger dependencies on burner to stagnation separation, pressure drop across the orifice, unburned gas velocity, and the orifice diameter. The extent of spatial shift is parameterized with respect to these experimental parameters.

Transient ignition model of single coal particle in diffusion counterflow flame

Transient ignition model of single coal particle in diffusion counterflow flame

Effects of Combustion Gas Species on Y2O3 Film Produced from EDTA·Y·H Complex by Flame-Spraying Technique

ABSTRACTYttria (Y2O3) films were deposited on stainless steel substrates from ethylenediaminetetraacetic·yttrium·hydrogen (EDTA·Y·H) complexes by flame spraying. The gas mixtures of acetylene-oxygen (C2H2-O2) or hydrogen-oxygen (H2-O2) were used for the combustion flame and the effects of the combustion gas species on Y2O3 films were investigated. Experiments revealed that the particles propelled in the H2-O2 flame had lower temperature and higher velocity compared with the particles in the C2H2-O2 flame. The existence of Y2O3 crystalline phases and complete decomposition of the EDTA·Y·H were confirmed. The porosity of the film was 25% when the H2-O2 flame was used and 32% when the C2H2-O2 flame was used. In addition, the Y2O3 films showed excellent adherability in tape tests. The H2-O2 flame is thus considered suitable for fabricating dense Y2O3 films.

Modeling of Fine-Particle Formation in Turbulent Flames

The generation of nanostructured particles in high-temperature flames is important both for the control of emissions from combustion devices and for the synthesis of high-value chemicals for a variety of applications. The physiochemical processes that lead to the production of fine particles in turbulent flames are highly sensitive to the flow physics and, in particular, the history of thermochemical compositions and turbulent features they encounter. Consequently, it is possible to change the characteristic size, structure, composition, and yield of the fine particles by altering the flow configuration. This review describes the complex multiscale interactions among turbulent fluid flow, gas-phase chemical reactions, and solid-phase particle evolution. The focus is on modeling the generation of soot particles, an unwanted pollutant from automobile and aircraft engines, as well as metal oxides, a class of high-value chemicals sought for specialized applications, including emissions control. Issues arising due to the numerical methods used to approximate the particle number density function, the modeling of turbulence-chemistry interactions, and model validation are also discussed.

Effects of direct-current electric fields on flame shape and combustion characteristics of ethanol in small scale

The aim of this work is to investigate the effects of direct-current electric fields on the behavior of the small-scale diffusion ethanol flame. The flow rate of liquid ethanol, the flame temperatures, and the flame shapes were measured. The results showed that the stable working ranges of a small-scale combustor became narrower under the direct-current electric field. The main reason was that the evaporation velocity of liquid ethanol limited by great heat loss effect cannot keep up with the increasing of combustion velocity by the ionic wind effect. The movements of those charged particles in flame enhanced the combustion process, resulting in higher flame temperatures under positive or negative direct-current electric field. The flame heights decreased with increasing applied voltages, due to the ionic wind effect increasing the flame temperature and the diffusivity. The flame voltage–current characteristic was also examined. Three regions can be divided: the subsaturation region, the saturation region, and the supersaturation region. Finally, the ratios of electric active power to actual burning thermal power of ethanol flame were calculated. It can be inferred that using the external direct-current electric field with little power consumption to control combustion and flame is a feasible method.

A flame particle tracking analysis of turbulence–chemistry interaction in hydrogen–air premixed flames

Interactions of turbulence, molecular transport, and energy transport, coupled with chemistry play a crucial role in the evolution of flame surface geometry, propagation, annihilation, and local extinction/re-ignition characteristics of intensely turbulent premixed flames. This study seeks to understand how these interactions affect flame surface annihilation of lean hydrogen–air premixed turbulent flames. Direct numerical simulations (DNSs) are conducted at different parametric conditions with a detailed reaction mechanism and transport properties for hydrogen–air flames. Flame particle tracking (FPT) technique is used to follow specific flame surface segments. An analytical expression for the local displacement flame speed (Sd) of a temperature isosurface is considered, and the contributions of transport, chemistry, and kinematics on the displacement flame speed at different turbulence-flame interaction conditions are identified. In general, the displacement flame speed for the flame particles is found to increase with time for all conditions considered. This is because, eventually all flame surfaces and their resident flame particles approach annihilation by reactant island formation at the end of stretching and folding processes induced by turbulence. Statistics of principal curvature evolving in time, obtained using FPT, suggest that these islands are ellipsoidal on average enclosing fresh reactants. Further examinations show that the increase in Sd is caused by the increased negative curvature of the flame surface and eventual homogenization of temperature gradients as these reactant islands shrink due to flame propagation and turbulent mixing. Finally, the evolution of the normalized, averaged, displacement flame speed vs. stretch Karlovitz number are found to collapse on a narrow band, suggesting that a unified description of flame speed dependence on stretch rate may be possible in the Lagrangian description.

A high-speed camera based approach for the on-line analysis of particles in multi-fuel burner flames

A high-speed camera based approach for the analysis of the trajectories, the velocities and the burning behavior of alternative fuel macroscopic particles during co-firing in a coal dust flame is presented. The new approach can be used as on-line measurement for the burner control. It consists of an image processing and a particle tracking step. The image processing algorithm allows for the detection of particles within as well as outside of the coal dust flame. The tracking algorithm connects particle detections in single images to particle trajectories in image sequences. It is tolerant to temporarily missing particle detections. Based on the particle trajectories various characteristic parameters describing the distribution of light duration and of the particle velocity can be calculated on-line. The new approach is demonstrated on the basis of measurements from an experimental combustion chamber with a 1MW multi-fuel burner.

Physicochemical evolution of nascent soot particles in a laminar premixed flame: from nucleation to early growth

In this work, particle inception and early growth stages were investigated in an ethylene/air premixed flame by the evolution of the particle size and structure with flame residence time. Particle size distribution was measured by a scanning mobility particle seizer and chemical–physical investigation was carried out by Raman microspectroscopy, UV–visible light absorption and cyclic voltammetry.From early inception of particles, just downstream the flame front, to the formation of primary soot particles, in the post-oxidation flame zone, particles participate in a series of chemical and physical reactions that strongly modify their nanostructure and physicochemical properties, resulting in different optical and electronic characteristics. The results presented in this study show that the evolution from a mono-modal to a bi-modal size distribution is associated to a particle graphitization process consisting of a slight increase of the in plane average size of the polyaromatic units within the particles, La, and on the formation of stacks of polyaromatic planes. These outcomes suggest that in our flame conditions particle coagulation/coalescence has a major role in the initial soot formation, affecting both physical and chemical particle properties.

Firmation of condensed combustion products in dust flames of metals: Coagulation stage

The coagulation dynamics of condensed products of vapor-phase or gas-phase combustion of gas mixtures of microdispersed metal particles in dust laminar flame considered taking into account the ionization of the combustion zone due to additives of electronegative and electropositive atoms and due to thermionic emission. The influence of the degree of ionization of a monodisperse coagulating aerosol and the charge of the particles on the coagulation rate constant is studied. It is shown that the rate of coagulation of the aerosol is most significantly affected by the Coulomb interaction of like-charged condensed-phase particles, which, under certain conditions, leads to an early stop of this stage in the condensation of combustion products. Ionization of the coagulating particles gives rise to a dependence of the size of primary particles of combustion products on the environmental parameters affecting their electric charge, and can be used for targeted control of the degree of dispersion of the combustion products.

Effects of DME mixing on number density and size properties of soot particles in counterflow non-premixed ethylene flames

In order to investigate the effect of DME mixing on the number density and size of soot particles, DME was mixed in a counter flow non-premixed ethylene flame with mixture ratios of 5%, 14% and 30%. A laser extinction/scattering technique has been adopted to measure the volume fraction, number density, and mean size of soot particles. The experimental results showed that the highest soot concentrations were observed for flames with mixture ratios of 5% and 14%; however, for a mixture ratio of 30% the soot concentration decreased. Numerical results showed that the concentrations of propargyl radicals (C3H3) at the 5% and 14% ratios were higher than those measured in the ethylene-based flame, and the production of benzene (C6H6) in the 5% and 14% DME mixture flames was also increased. This indicates the crucial role of propargyl in benzene ring formation. These reactions generally become stronger with increased DME mixing, except for A1- + H2 → A1 + H (-R554) and n-C4H5 + C2H2 → A1 + H (R542). Therefore, it is indicated that adding DME to ethylene flames promotes benzene ring formation. Note that although the maximum C6H6 concentration is largest in the 30% DME mixing flame, the soot volume fraction is smaller than those for the 5% and 14% mixture ratios. This is because the local C6H6 concentration decreases in the relatively low temperature region in the fuel side where soot growth occurs.