Turbulent Sooting Flames Research Articles

A computationally efficient approach for soot modeling with discrete sectional method and FGM chemistry

A novel approach for the prediction of soot formation in combustion simulations within the framework of discrete sectional method (DSM) based univariate soot model and Flamelet Generated Manifold (FGM) chemistry, referred to as FGM-CDSM, is proposed in this study. The FGM-CDSM considers the clustering of soot sections derived from the original soot particle size distribution function (PSDF) to minimize the computational cost. Unlike conventional DSM, in FGM-CDSM, governing equations for soot mass fractions are solved for the clusters, by using a pre-computed lookup table with tabulated soot source terms from the flamelet manifold, while the original soot PSDF is re-constructed in a post-processing stage. The flamelets employed for the manifold are computed with detailed chemistry and the complete sectional soot model. A comparative assessment of FGM-CDSM is conducted in laminar diffusion flames for its accuracy and computational performance against the detailed kinetics-based classical sectional model. Numerical results reveal that the FGM-CDSM can favorably reproduce the global soot quantities and capture their dynamic response predicted by detailed kinetics with a good qualitative agreement. Furthermore, compared to detailed kinetics, FGM-CDSM is shown to substantially reduce the computational cost of the complete reacting flow simulation with soot particle transport. Primarily, the use of FGM reduces the overall calculation by about two orders of magnitude compared to detailed kinetics, which is advanced further with the clustering of sections at a low memory footprint. Therefore, the present work demonstrates the promising capabilities of FGM-CDSM in the context of computationally efficient soot calculations and provides an excellent framework for extending its application to the simulations of turbulent sooting flames.

Pressure effects on soot formation and evolution in turbulent jet flames

In this study, two series of pressurized turbulent jet sooting flames at 1, 3, and 5 bar with either fixed jet velocity or fixed Reynolds number are simulated to study the pressure effects on soot formation and evolution. Through a radiation flamelet progress variable approach with a conditional soot subfilter probability density function (PDF) model to consider the turbulence–chemistry–soot interactions, quantitatively good agreements are achieved for soot volume fraction (SVF) predictions compared with the experimental data, regardless different turbulent intensities and residence times. SVF source terms are then discussed to show the pressure effects on nucleation, condensation, surface growth, and oxidation at different axial positions in these flames. It is found that surface growth and oxidation increase by about three orders of magnitude from 1 to 5 bar, while nucleation and condensation only increase within one order of magnitude. The stronger SVF scaling on pressure than measured data is found to be attributed to the inaccurate surface growth and oxidation scaling on pressure. Further analysis indicates that (i) the uncertainty of C2H2 prediction at elevated pressures is likely a major reason for the too strong surface growth scaling; and (ii) taking account of pressure effects in the conditional subfilter PDF modeling for turbulence–soot–chemistry interactions is likely a key to improve oxidation prediction. The results in this study open up the possibilities for improving future turbulent sooting flame modeling by improving C2H2 chemistry and turbulence–chemistry–soot modeling at elevated pressures.

A turbulent combustion model for soot formation at the LES subgrid-scale using virtual chemistry approach

Designing efficient and clean combustion devices to reduce soot formation in practical applications is a major concern nowadays. This could be achieved by performing predictive simulations in turbulent sooting combustors. Large Eddy Simulation (LES) is an attractive approach to predict flame/turbulence interactions at small scales including soot phenomena. However, because of the peculiarly complex nature of soot production phenomena, turbulent sooting flames remain very difficult to simulate and are computationally expensive. Thus, reduced gas-phase kinetics and soot models should be included in numerical simulations. The reduced virtual chemistry approach is a great alternative to address this problem since it introduces an optimized and flexible strategy to tackle multi-mode turbulent combustion and pollutants prediction. Also, the virtual chemistry approach has recently demonstrated its capability in reproducing soot phenomena in laminar flames including radiative heat transfer. In the present work, the virtual chemistry methodology is extended to resolve turbulent sooting flames using LES. Based on analysis of flame and soot chemical time scales along with turbulent time scale, an original hybrid turbulent combustion model formulation is proposed, which degenerates to the flamelet regime in the flame front region and to a PSR formalism in the post-flame zone, characterized by slow soot chemistry. The subgrid-scale model for turbulence-chemistry-soot interactions is then implemented in an LES framework. The model is challenged in the turbulent non-premixed sooting flame from Sandia Laboratory. Results indicate that the LES subgrid-scale model has a significant impact on soot formation and confirms previous observations from the literature in LES of non-premixed turbulent sooting flames. Additionally, results show that the virtual chemistry can reproduce flame-soot characteristics of a turbulent sooting jet flame and are comparable to other state-of-the-art works.

Soot particle size distribution reconstruction in a turbulent sooting flame with the split-based extended quadrature method of moments

The Method of Moments (MOM) has largely been applied to investigate sooting laminar and turbulent flames. However, the classical MOM is not able to characterize a continuous particle size distribution (PSD). Without access to information on the PSD, it is difficult to accurately take into account particle oxidation, which is crucial for shrinking and eliminating soot particles. Recently, the Split-based Extended Quadrature Method of Moments (S-EQMOM) has been proposed as a numerically robust alternative to overcome this issue [Salenbauch et al., “A numerically robust method of moments with number density function reconstruction and its application to soot formation, growth, and oxidation,” J. Aerosol Sci. 128, 34–49 (2019)]. The main advantage is that a continuous particle number density function can be reconstructed by superimposing kernel density functions (KDFs). Moreover, the S-EQMOM primary nodes are determined individually for each KDF, improving the moment realizability. In this work, the S-EQMOM is combined with a large eddy simulation/presumed-probability density function flamelet/progress variable approach for predicting soot formation in the Delft Adelaide Flame III. The target flame features low/high sooting propensity/intermittency and comprehensive flow/scalar/soot data are available for model validation. Simulation results are compared with the experimental data for both the gas phase and the particulate phase. Good quantitative agreement has been obtained especially in terms of the soot volume fraction. The reconstructed PSD reveals predominantly unimodal/bimodal distributions in the first/downstream portion of this flame with particle diameters smaller than 100 nm. By investigating the instantaneous and statistical sooting behavior at the flame tip, it has been found that the experimentally observed soot intermittency is linked to mixture fraction fluctuations around its stoichiometric value that exhibits a bimodal probability density function.

Modelling of soot formation and aggregation in turbulent flows with the LES-PBE-PDF approach and a conservative sectional method

The present paper presents a comprehensive approach for modelling soot formation in turbulent flows that incorporates a recently developed conservative finite volume sectional method for the solution of the population balance equation (PBE) into the Large Eddy Simulation - Probability Density Function (LES-PDF) framework for modelling turbulent reacting flows. The approach thus predicts the particle size distribution of soot and accounts for the complete set of aerosol dynamic processes (nucleation, surface growth, oxidation, condensation and aggregation) as well as for the interaction between turbulence, chemistry and soot. We employ a combination of gas-phase chemistry, soot kinetics and aerosol dynamics recently used by the authors in the context of a laminar co-flow diffusion flame and maintain the same kinetics without adjustments in order to focus on the modelling issues in turbulent sooting flame simulations. The results show that several features such as the location of the soot forming zones and the qualitative profiles of soot volume fraction and species concentrations can be predicted reasonably well, but there is a considerable underprediction in the magnitude of soot volume fraction. The possible reasons for this underprediction are discussed in the context of the aforementioned laminar flame simulation and the previous validation of the components of our methodology. The CPU time analysis shows that the solution of the PBE by the conservative sectional method and the associated transport of the discretised number densities account for less than 25% of the total CPU time, indicating that its incorporation in LES-PDF simulations of turbulent combustion is computationally feasible.

A virtual chemistry model for soot prediction in flames including radiative heat transfer

Modeling soot formation is a very difficult task which has been assessed by the numerical combustion community. Numerical simulations of turbulent sooting flames remain very challenging due to the complexity of soot formation phenomena, which implies development of elaborate soot models often too computationally demanding, or simplified soot models that are limited to a small range of operating conditions of interest. Here an innovative optimized global approach called virtual chemistry is proposed. It consists of a mathematical formalism including virtual species and virtual reactions, whose thermochemical parameters are optimized in order to describe a combustion system. Thermochemical properties are trained through a genetic algorithm, employing a learning database made of reference flame elements. This makes it possible to reproduce multiple combustion regimes and operating conditions. The virtual chemistry approach reproduces then the structure of hydrocarbon-air flames, as well as predicting specific user-defined pollutant species. The objective of this article is to extend the virtual chemistry methodology for soot formation prediction. As the radiative heat losses are important in sooting flames, a new radiative virtual model is also developed to account for them. The final reduced chemistry consists of 12 virtual species and 6 virtual reactions considerably decreasing the computational time compared to detailed chemistry models. Simulations of 1-D and 2-D laminar ethylene-air sooting flames are performed to evaluate the virtual soot and radiative models. Temperature, soot volume fraction and radiative heat losses are well predicted and are in good agreement with the reference data.

Quantitative 2D thermometry in turbulent sooting non-premixed flames using filtered Rayleigh scattering.

This paper demonstrates the application of a thermometry method in turbulent sooting non-premixed flames using filtered Rayleigh scattering (FRS). Fuel tailoring is used to establish a specific C2H2-based fuel mixture such that temperature can be determined accurately by a single FRS measurement over the entirety of mixture fraction space, or equivalently, for all relevant thermo-chemical states. Evaluation is performed in a hierarchy of flows to establish measurement precision and accuracy. Initial assessments in a series of heated fuel mixtures; non-sooting, near-adiabatic flat flames; and laminar non-premixed sooting flames show accuracy of the approach over a full range of expected temperatures and high single-shot measurement precision (e.g., 65<SNR<80) for temperatures between 1900 and 2200 K. Subsequently, 2D temperature measurements are evaluated in a turbulent non-premixed sooting flame. For the current fuel mixture, the local mixture-averaged Rayleigh scattering cross section is nearly constant over all composition space, and thus traditional laser Rayleigh scattering (LRS) is used as a measurement standard in the absence of soot. The results show excellent agreement between the proposed FRS thermometry approach and LRS in non-sooting regions. In the presence of soot, the proposed FRS-based approach shows no signs of interference. In addition, 2D temperature imaging shows high SNR (60-74) over all temperature conditions. Thus, we believe the proposed methodology successfully provides a high-resolution 2D temperature method under turbulent sooting flame conditions.

Generating planar distributions of soot particles from luminosity images in turbulent flames using deep learning

We report a computational method based on deep learning (DL) to generate planar distributions of soot particles in turbulent flames from line-of-sight luminosity images. A conditional generative adversarial network (C-GAN) was trained using flame luminosity and planar laser-induced incandescence (LII) images simultaneously recorded in a turbulent sooting flame with an exit Reynolds number of 15,000. Such a training built up the underlying relationship between the two types of images i.e., a predictive model which was then used to predict LII images from luminosity images and the accuracy was assessed using four different methods. Results show that the model is effective and capable of generating LII images with acceptable prediction accuracies of around 0.75. The model was also found to be applicable over a range of heights in the flames, as well as for the flames with a range of exit Reynolds numbers spanning from 8000 to 20,000. Besides, the probability density function (PDF) of LII signals in different flames can also be predicated using the model. This work, for the first time, demonstrates the feasibility of predicting planar signals from corresponding line-of-sight signals from turbulent flames, which potentially offers a much simpler optical arrangement for a modest trade-off in accuracy.

Statistical relationship between soot volume fraction, temperature, primary particle diameter and OH radicals along transects normal to the local reaction zone in a turbulent flame

We present a new method to extract experimental data from simultaneous planar images in turbulent sooting flames, to derive transects of key species based on local coordinates that are aligned normal to the instantaneous, fluctuating reaction zone. Such statistics represent a milestone in the assessment of turbulent sooting flames, because they are one-dimensional and based on local coordinates conditioned on the distribution of soot sheets, in contrast to previous statistics, which are typically Eulerian and single point in nature. The local coordinate system was determined from instantaneous images of soot volume fraction (fv) based on a varying threshold and the local reaction zone was identified by further referring OH images. Quantitative experimental data of fv, temperature (T) and primary particle diameter (Dp), together with qualitative OH intensities, were extracted along the direction normal to the local reaction zone from simultaneous images of the four parameters. These results were measured in a pre-vaporized toluene diffusion flame (Reynolds number = 10,000) using advanced laser-based techniques. Statistical data were combined together and used to generate the profiles for these four parameters with high accuracy and level of resolution that is not possible using previous statistics methods based on Eulerian coordinates. These new kind results showed that the spatial relationship between fv, T and OH remains broadly similar anywhere in the flame, while the profiles of Dp significantly shift away from those of the other three parameters with the increase in the height of the flame. The mean values of fv, Dp and T also show a clear relationship as a function of the height, even though the trends of mean fv and Dp along the height are different from that of T, also suggesting a strong relationship exists between soot and temperature in the local reaction zone.

Soot Formation Model Performance in Turbulent Non-Premix Ethylene Flame: A Comparison Study

This paper presents results obtained from the application of a computational fluid dynamics (CFD) approach to modelling of non-premixed turbulent ethylene sooting flame. The study focuses on comparing the two soot models available in the Fluent in predicting the soot level in the turbulent non-premixed ethylene flame. A standard k-ε model and Eddy Dissipation model are utilized for the representation of flow field and combustion of the flame being investigated. For performance comparison study, a single step soot model of Khan and Greeves and two-step soot model proposed by Tesner are tested. The results of calculations are compared with experimental data for a turbulent sooting flame taken from literature. The results of the study show that a combination of the standard k-ε turbulence model and eddy dissipation model is capable of producing reasonable predictions of temperature both in axial and radial profiles; although further downstream of the flame over-predicted temperatures are evidence. With regard to soot model performance study, it shows that the two-step model clearly performed far better than the single-step model in predicting the soot level in ethylene flame at both axial and radial profiles.

Systematic assessment of the Method of Moments with Interpolative Closure and guidelines for its application to soot particle dynamics in laminar and turbulent flames

The Method of Moments with Interpolative Closure (MOMIC) is a widely used approach for numerical modeling of aerosol dynamics, where soot formation is an important area of application. Two basic variants of the algorithm were proposed in the past: In the original method, transport equations are solved for several non-negative order integer moments of the soot Number Density Function and one for the moment of order minus infinity. Based on these moments, two interpolation functions are constructed and used for the evaluation of positive and negative fractional order moments required for closure of the moment equation source terms. In the other variant, which is a simplification of the first, equations are solved only for non-negative order integer moments. Negative order moments then need to be evaluated by extrapolation. In addition to the choice between these two algorithms, it is not clear a-priori what is the optimal choice regarding the number of transported moments and hence the order of the interpolation polynomial. In this work, the effects of different choices in the setup of the MOMIC algorithm on the evaluation of soot source terms and the prediction of the soot number density and volume fraction are assessed systematically. To this end, MOMIC is combined with a state-of-the-art physico-chemical soot model and applied to the simulation of soot formation in a laminar premixed flame and along Lagrangian trajectories extracted from a Direct Numerical Simulation of a turbulent sooting flame. Monte Carlo simulations serve as reference solutions for a combined a-priori and a-posteriori study. Including the moment of order minus infinity is shown to be beneficial for an accurate calculation of the coagulation source term and hence the soot number density, because moments of negative order can be evaluated more accurately. Furthermore, a high interpolation order yields a more accurate prediction of the surface growth source term. This source term is systematically overpredicted, but errors are substantially reduced to a few percent with increasing interpolation order.

Simultaneously calibrated two-line atomic fluorescence for high-precision temperature imaging in sooting flames

Simultaneously calibrated, non-linear two-line atomic fluorescence (SC-nTLAF) thermometry for application in turbulent sooting flames has been developed to increase the precision of single-shot, planar measurements of gas temperature. The technique has been demonstrated in both steady and turbulent sooting flames, showing good agreements with previous optical measurements. The SC-nTLAF involves imaging simultaneously laser-induced fluorescence (LIF) of atomic indium in both the target flame and a non-sooting calibration flame for which the temperature distribution is known. The LIF intensities from the reference flame enable correction for fluctuations, not only in the laser power, but also in the laser mode. The resulting precision was found to be ±67 K and ±75 K (based on one standard deviation) in the rich and oxidizing regions of a steady sooting flame for which the measured temperature was 1610 K and 1854 K, respectively, with a spatial resolution of 550 × 550 µm2. This corresponds to a relative precision of ∼ 4.1%. The resulting precision in the single-shot temperature images for a well-characterized, lifted ethylene jet diffusion flame (fuel jet Reynolds number = 10,000) compares favorably with previously reported data obtained with shifted-vibrational coherent anti-Stokes Raman spectroscopy (CARS), together with increased spatial resolution. The planar imaging also provides more details of the temperature distribution, particularly in the flame brush region, which offers potential for measurement of more parameters, such as gradients and spatial corrections. The new calibration method has also achieved a significant time-saving in both data collection and processing, which is an estimated total of ∼ 60%–70% compared with conventional nTLAF.

Effect of soot model, moment method, and chemical kinetics on soot formation in a model aircraft combustor

The simulation of turbulent sooting flames requires a host of models, of which the two critical components are the chemical kinetics that describe soot precursor evolution and the description of the soot population. The purpose of this study is to understand the sensitivity of soot predictions in a realistic aircraft combustor to model choices for these components. Two different chemistry mechanisms, three different statistical approaches, and two different soot inception models are considered. The simulations show that acetylene-based soot inception produces very high soot volume fraction, with the soot particles present predominantly in the inner recirculation zone of the swirl-stabilized combustor. The PAH-based nucleation models lead to soot generation in the shear layers emanating from fuel injection. The two advanced statistical approaches (Hybrid and Conditional Quadrature Method of Moments) also show significant differences. While the Hybrid method produces lower soot number density, it also generates larger soot particles due to a faster predicted rate of coagulation. The Conditional Quadrature approach produces much higher soot number density, but its particle sizes are smaller compared to the Hybrid method for all kinetic mechanisms considered. This experimental combustor is strongly dominated by surface growth based soot mass addition. As a result, even if nucleation/condensation rates are different, the final soot mass yield is comparable for PAH-based soot models. These results indicate the importance of not only the chemical mechanism, which may be less important in this surface growth dominated combustor, but also the soot statistical model, to which coagulation and the soot surface area are relatively sensitive.

Simulation and analysis of the soot particle size distribution in a turbulent nonpremixed flame

A modeling framework based on Direct Simulation Monte Carlo (DSMC) is employed to simulate the evolution of the soot particle size distribution in turbulent sooting flames. The stochastic reactor describes the evolution of soot in fluid parcels following Lagrangian trajectories in a turbulent flow field. The trajectories are sampled from a Direct Numerical Simulation (DNS) of a n-heptane turbulent nonpremixed flame. The DSMC method is validated against experimentally measured size distributions in laminar premixed flames and found to reproduce quantitatively the experimental results, including the appearance of the second mode at large aggregate sizes and the presence of a trough at mobility diameters in the range 3–8 nm. The model is then applied to the simulation of soot formation and growth in simplified configurations featuring a constant concentration of soot precursors and the evolution of the size distribution in time is found to depend on the intensity of the nucleation rate. Higher nucleation rates lead to a higher peak in number density and to the size distribution attaining its second mode sooner. The ensemble-averaged PSDF in the turbulent flame is computed from individual samples of the PSDF from large sets of Lagrangian trajectories. This statistical measure is equivalent to time-averaged, scanning mobility particle size (SMPS) measurements in turbulent flames. Although individual trajectories display strong bimodality as in laminar flames, the ensemble-average PSDF possesses only one mode and a long, broad tail, which implies significant polydispersity induced by turbulence. Our results agree very well with SMPS measurements available in the literature. Conditioning on key features of the trajectory, such as mixture fraction or radial locations does not reduce the scatter in the size distributions and the ensemble-averaged PSDF remains broad. The results highlight and explain the important role of turbulence in broadening the size distribution of particles in turbulent sooting flames.

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.

Planar laser-induced incandescence of turbulent sooting flames: the influence of beam steering and signal trapping

The influence of beam steering and signal trapping on the accuracy of soot volume fractions measured using planar laser-induced incandescence (LII) has been investigated in turbulent non-premixed sooting flames at atmospheric pressure. In turbulent non-premixed C2H4/air flames, the influence of local de-focusing/focusing of the laser sheet from beam steering can result in the underestimate of the averaged LII signal by 30 %, even when operating within the so-called plateau regime of laser fluence. Beam steering was also found to be significant in both the upstream region of C2H4/air flames and non-reacting C2H4 flows, because the fuel has a relatively high refractive index compared with ambient air. The extent of beam steering at different heights of reacting and isothermal flows as well as its dependence on exit Reynolds number (Re) has been measured. The measurements reveal that even at low turbulence levels (2000 < Re < 3000), beam steering effects can be significant. Also found is that the LII signal at a 450 nm wavelength can be attenuated by a few per cent at high soot loading regions in turbulent flames due to signal trapping. Finally, the feasibility of directly evaluating the signal attenuation via planar LII results was assessed by comparing the virtual soot attenuation calculated based on the planar LII result with that measured using light-of-sight extinction.

Formation, growth, and transport of soot in a three-dimensional turbulent non-premixed jet flame

The formation, growth, and transport of soot is investigated via large scale numerical simulation in a three-dimensional turbulent non-premixed n-heptane/air jet flame at a jet Reynolds number of 15,000. For the first time, a detailed chemical mechanism, which includes the soot precursor naphthalene and a high-order method of moments are employed in a three-dimensional simulation of a turbulent sooting flame. The results are used to discuss the interaction of turbulence, chemistry, and the formation of soot. Compared to temperature and other species controlled by oxidation chemistry, naphthalene is found to be affected more significantly by the scalar dissipation rate. While the mixture fraction and temperature fields show fairly smooth spatial and temporal variations, the sensitivity of naphthalene to turbulent mixing causes large inhomogeneities in the precursor fields, which in turn generate even stronger intermittency in the soot fields. A strong correlation is apparent between soot number density and the concentration of naphthalene. On the contrary, while soot mass fraction is usually large where naphthalene is present, pockets of fluid with large soot mass are also frequent in regions with very low naphthalene mass fraction values. From the analysis of Lagrangian statistics, it is shown that soot nucleates and grows mainly in a layer close to the flame and spreads on the rich side of the flame due to the fluctuating mixing field, resulting in more than half of the total soot mass being located at mixture fractions larger than 0.6. Only a small fraction of soot is transported towards the flame and is completely oxidized in the vicinity of the stoichiometric surface. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames. Finally, given the difficulties in obtaining quantitative data in experiments of turbulent sooting flames, this simulation provides valuable data to guide the development of models for Large Eddy Simulation and Reynolds Average Navier Stokes approaches.

Investigation of detailed kinetic scheme performance on modelling of turbulent non-premixed sooting flames

This paper presents the results of an application of a first-order conditional moment closure (CMC) approach coupled with a semi-empirical soot model to investigate the effect of various detailed combustion chemistry schemes on soot formation and destruction in turbulent non-premixed flames. A two-equation soot model representing soot particle nucleation, growth, coagulation and oxidation, was incorporated into the CMC model. The turbulent flow-field of both flames is described using the Favre-averaged fluid-flow equations, applying a standard k-ɛ turbulence model. A number of five reaction kinetic mechanisms having 50–100 species and 200–1000 elementary reactions called ABF, Miller-Bowman, GRI-Mech3.0, Warnatz, and Qin were employed to study the effect of combustion chemistry schemes on soot predictions. The results showed that of various kinetic schemes being studied, each yields similar accuracy in temperature prediction when compared with experimental data. With respect to soot prediction, the kinetic scheme containing benzene elementary reactions tends to result in a better prediction on soot concentrations in comparison to those contain no benzene elementary reactions. Among five kinetic mechanisms being studied, the Qin combustion scheme mechanism turned to yield the best prediction on both flame temperature and soot levels.

On the formation and early evolution of soot in turbulent nonpremixed flames

A Direct Numerical Simulation (DNS) of soot formation in an n-heptane/air turbulent nonpremixed flame has been performed to investigate unsteady strain effects on soot growth and transport. For the first time in a DNS of turbulent combustion, Polycyclic Aromatic Hydrocarbons (PAH) are included via a validated, reduced chemical mechanism. A novel statistical representation of soot aggregates based on the Hybrid Method of Moments is used [M.E. Mueller, G. Blanquart, H. Pitsch, Combust. Flame 156 (2009) 1143–1155], which allows for an accurate state-of-the-art description of soot number density, volume fraction, and morphology of the aggregates. In agreement with previous experimental studies in laminar flames, Damköhler number effects are found to be significant for PAH. Soot nucleation and growth from PAH are locally inhibited by high scalar dissipation rate, thus providing a possible explanation for the experimentally observed reduction of soot yields at increasing levels of mixing in turbulent sooting flames. Furthermore, our data indicate that soot growth models that rely on smaller hydrocarbon species such as acetylene as a proxy for large PAH molecules ignore or misrepresent the effects of turbulent mixing and hydrodynamic strain on soot formation due to differences in the species Damköhler number. Upon formation on the rich side of the flame, soot is displaced relative to curved mixture fraction iso-surfaces due to differential diffusion effects between soot and the gas-phase. Soot traveling towards the flame is oxidized, and aggregates displaced away from the flame grow primarily by condensation of PAH on the particle surface. In contrast to previous DNS studies based on simplified soot and chemistry models, surface reactions are found to contribute barely to the growth of soot, for nucleation and condensation processes occurring in the fuel stream are responsible for the most of soot mass generation. Furthermore, the morphology of the soot aggregates is found to depend on the location of soot in mixture fraction space. Aggregates having the largest primary particles populate the region closest to the location of peak soot growth. On the contrary, the aggregates with the largest number of primary particles are located much further into the fuel stream.

Sooting turbulent jet flame: characterization and quantitative soot measurements

Computational fluid dynamics (CFD) modelers require high-quality experimental data sets for validation of their numerical tools. Preferred features for numerical simulations of a sooting, turbulent test case flame are simplicity (no pilot flame), well-defined boundary conditions, and sufficient soot production. This paper proposes a non-premixed C2H4/air turbulent jet flame to fill this role and presents an extensive database for soot model validation. The sooting turbulent jet flame has a total visible flame length of approximately 400 mm and a fuel-jet Reynolds number of 10,000. The flame has a measured lift-off height of 26 mm which acts as a sensitive marker for CFD model validation, while this novel compiled experimental database of soot properties, temperature and velocity maps are useful for the validation of kinetic soot models and numerical flame simulations. Due to the relatively simple burner design which produces a flame with sufficient soot concentration while meeting modelers’ needs with respect to boundary conditions and flame specifications as well as the present lack of a sooting “standard flame”, this flame is suggested as a new reference turbulent sooting flame. The flame characterization presented here involved a variety of optical diagnostics including quantitative 2D laser-induced incandescence (2D-LII), shifted-vibrational coherent anti-Stokes Raman spectroscopy (SV-CARS), and particle image velocimetry (PIV). Producing an accurate and comprehensive characterization of a transient sooting flame was challenging and required optimization of these diagnostics. In this respect, we present the first simultaneous, instantaneous PIV, and LII measurements in a heavily sooting flame environment. Simultaneous soot and flow field measurements can provide new insights into the interaction between a turbulent vortex and flame chemistry, especially since soot structures in turbulent flames are known to be small and often treated in a statistical manner.