Laminar Non-premixed Flames Research Articles

Feasibility on equivalence ratio measurement via OH*, CH*, and C2* chemiluminescence and study of soot emissions in co-flow non-premixed DME/C1–C2 hydrocarbon flames

The effects of dimethyl ether (DME) addition to methane and ethylene fuels on the combustion characteristics of heat release, soot emissions, and flame temperature were investigated experimentally and numerically in a non-premixed laminar flame configuration. The flame-heat release soot-volume fraction was measured experimentally using CH*, OH*, and C2* chemiluminescence and planar two-color soot pyrometry, respectively. The CH*, OH*, and C2* were used to locate flame-heat release regions as well as to investigate the soot signal’s effect on their measurements. The ratios of the chemiluminescence pairs (OH*/CH* and OH*/C2*) were studied for the feasibility of map local equivalence ratios. Numerical calculations across a full range of DME mixing ratios were performed through 1D laminar flame simulations implemented with a detailed mechanism to provide an indication of the flame structures and profiles of key species including OH*, OH, CH*, CH, CH3, C3H3, C2H2, heat release rate (HRR), and flame temperature. An existing developed soot model was used in a 2D computational study to investigate its validity for modeling soot for DME (oxygenated fuel)/C2H4/N2 flames. Parametric studies have been carried out on some key parameters in the soot model to find optimum values that can be used in future studies. Although soot radiation intensities increased at a small amount (25%vol) of DME addition in the DME/methane flames, the soot pyrometry results showed a reduced soot volume fraction with an increased DME mixture ratio in both DME/methane and DME/ethylene flames studied, agreeing with the key conclusion of 1D numerical results. The flame HRR decreases with the increasing addition of DME to methane and ethylene flames and correlates with the trend of OH* and CH* profiles. The 1D simulation showed a non-monotonic correlation between OH*/CH* ratios and equivalence ratios, implying a limited use of OH*/CH* for the equivalence ratio measurement in non-premixed flames with DME additions.

Assessing PAHs-based soot inception models in various laminar non-premixed flame configurations

Accurate soot modeling is challenging due to the complex underlying physical and chemical processes. To improve the fidelity of the soot prediction in inverse coflow flame configurations, an empirical reactive soot inception model with radical involvement was developed in our previous work [Guo et al. Combust Flame 254 (2023) 112,853]. In this study, the model is further assessed by comparing the predictions of polycyclic aromatic hydrocarbons (PAHs) and soot parameters, including volume fraction, number density, and particle size, with measurements in various laminar diffusion flame configurations. These configurations encompass counterflow diffusion flames (CDF), normal coflow diffusion flames (NDF), and inverse coflow diffusion flames (IDF). The empirical reactive inception model is also compared to the reported irreversible and reversible inception models. The results show that the empirical reactive inception model with radical involvement most accurately capture axial soot behavior in IDF configuration, while irreversible and reversible models predict a sustained increase in soot mass, inconsistent with measurements. In CDF configurations, the empirical reactive model and the reported reversible model have better performance than the irreversible inception model, regarding the predictions of spatial distributions of soot and PAHs. Moreover, in NDF configuration, the soot peaks predicted by the reactive and reversible models are in reasonable agreement with measurements within a factor of 2–3. For the soot spatial distribution in NDF, the difference in the predictions is mainly reflected in the flame wings, which is caused by the temperature dependences of the inception rate provided by the different models.

Direct Numerical Simulation of a Reacting Turbulent Hydrogen/Ammonia/Nitrogen Jet in an Air Crossflow at 5 Bar

The article aims to analyze the fluid dynamics and combustion characteristics of a non-premixed flame burning a fuel mixture derived from ammonia partial decomposition injected in an air crossflow. Nominal pressure (5 bar) and inlet air temperature (750 K) conditions are typical of micro-gas turbines. The effects of strain on the maximum flame temperature and NO generation in laminar non-premixed counter-flow flames are initially explored. Then, the whole three-dimensional fluid dynamic problem is investigated by setting up a numerical experiment: it consists of a Direct Numerical Simulation, based on accurate transport, chemical, and numerical models. The flow topology of the specific reacting jet in crossflow configuration is described in terms of its main turbulent structures, like shear layers, ring, and horse-shoe vortices, as well as of its leeward recirculation region anchoring the flame. The reacting region is characterized by providing instantaneous spatial distributions of temperature, heat release, and some transported chemical species, including NO, and calculating the Flame Index to identify non-premixed and premixed combustion local conditions. The latter is quantified by looking at the distribution of the volume fraction associated with a certain Flame Index versus the Flame Index and at the distribution of the average values of both the Heat Release Rate and NO versus the Flame Index and the mixture fraction.

Maturity characterization of soot in laminar coflow diffusion flames of methane/anisole under different oxygen indices

Anisole fuel has gained increased attention, since it is considered a biofuel, and its addition to gasoline increases the octane number, improving the engine performance. This study focuses on the comprehensive characterization of soot maturity and sooting propensity of a well-controlled laminar coflow diffusion flame fueled with anisole and using methane as the carrier gas. Four laminar non-premixed flames were established below the smoke point. The oxidizer composition ranged from a molar fraction of oxygen (oxygen index, OI) of 21% to 35% (doped with pure oxygen), while keeping constant the total volumetric flow rate of oxidizer stream. Multi-wavelength line-of-sight attenuation experiments were performed from visible to infrared wavelengths, and the measurements of the extinction coefficient and flame temperature were used to retrieve two dimensional fields of soot temperature, volume fraction, radiative intensity and degree of soot maturity. In general, the sooting propensity and radiative behavior of the anisole flames follows a similar trend than typical hydrocarbon fuels when increasing the OI. However, the spatial distribution of soot volume fraction is enhanced, particularly near the flame centerline. In contrast, the soot production is promoted by the OI, near the flame wings. The same effect can be observed for the soot temperature. Indeed, temperature increases, promoting formation and oxidation processes, specially at the locations near the maximum soot volume fraction. In general, the maturity of soot particles is affected as the OI is changed. Also, it is observed that the degree of maturity is enhanced by flame temperature. In order to support these local observations, a statistical analysis was carried out. Finally, the results obtained yield a curated database for validating detailed soot production models of oxygenated fuels, and to gain further insights on the evolution of soot maturity and propensity of these particular fuels under different oxidizer conditions.

Reduced-order modeling with reconstruction-informed projections

Reduced-order models (ROMs) are commonly employed to address the large computational cost of simulations for high-dimensional, dynamical systems such as those found in combustion. Two key components of building these ROMs include an effective dimension reduction or projection of the high-dimensional system onto a low-dimensional manifold, alongside an accurate method for reconstructing quantities of interest (QoIs) over the projection parameters. A QoI-based approach to projection has been recently introduced, combining these aspects of building ROMs into a single training that learns the projection parameters based on nonlinear reconstruction errors of QoIs. A key advantage to this approach is the ability to inform the projection with reconstruction errors of QoIs that are functions of the projection itself, such as projected source terms influencing evolution over the low-dimensional manifold. We expand on the QoI-based projection method and implement Partition of Unity Network (POUnet) models for reconstruction, which are able to reach high levels of accuracy with small memory footprints by combining classification with localized polynomial regression. Furthermore, we find that sign changes in the projected source terms can negatively impact evolution over the low-dimensional manifold. Therefore, we also include a penalty function favoring positivity in projected source terms of the QoI-based projection method. These modeling improvements are demonstrated in one- and two-dimensional direct numerical simulations (DNS) of a non-premixed laminar flame.

Soot properties characterization of aviation Jet A-1 laminar non-premixed coflow flame

A canonical laminar coflow non-premixed flame was produced using a Jet A-1 fuel burning in coflowing ambient air environment using a Gülder type burner configuration. The aviation fuel was vaporized and carried by methane to the reaction zone. Non-intrusive laser-based multi-wavelength line-of-sight attenuation and emission measurements were employed to quantify soot properties in a spatially-resolved fashion. The experimental methodology allowed to evaluate soot properties such as temperature, absorption function, soot maturity, percentage of organic and graphitic compound contents, soot volume fraction and the divergence of the radiative heat flux.The distribution of soot temperature was found to be similar to that of alkenes and alkanes fuels, increasing radially toward the flame edge and axially toward the tip, peaking at 1950 K. The level of maturity was found to be higher (increased proportion of graphitic contents) in high soot temperature regions, revealing a relationship between temperature and the maturation process. Also, graphitic soot particles were predominantly found at the top of the flame, near the centerline zone and in the vicinity of the flame wings. As expected, the maturity influenced the absorption function, which was found to vary from 0.2 to 0.37 as the percentage of graphitic compounds changes from approximately 60% to 80% (mean values). This spatial variation of the absorption function led to significant differences on the retrieved soot volume fraction, around 1 ppm for the maximum values, as compared to the use of a constant value assuming fully graphitic soot particles. Despite these observations, the radiative heat flux released by the flame was not significantly affected, even considering the importance of taking into account the maturity for soot experimental diagnostics.

Effects of n-decane substitution on structure and extinction limits of formic acid diffusion flames

Formic acid (FA) is a low-carbon fuel that can be produced from renewable hydrogen (H2) and carbon dioxide (CO2). However, transitioning completely to low carbon fuels might be lengthy and challenging, so it makes sense to gradually introduce low carbon fuels into the energy sector in conjunction with existing fuels. One practical approach could be blending FA with existing fossil fuels, such as kerosene in aviation. This study investigates the behavior of flames comprising FA and its blends with n-decane. The extinction limits of pure FA and FA/n-decane mixtures were measured in a counterflow non-premixed laminar flame setup at various blending ratios. Additionally, the flame structure was studied by measuring temperature and species distribution using probe-based sampling. The results indicate that replacing FA with n-decane in the FA-N2 flame greatly enhances the flame's reactivity. Blending the fuels also led to a decrease in CO2 production in the flame, while increasing the flame temperature and the concentration of H2 and CO species. Experimental results were modeled using an updated FA and n-decane model. The kinetic model agrees with the experimental trends, but slightly overpredicts H2, CH4 species, and flame temperature. The kinetic model was also used to investigate the kinetic coupling between FA and n-decane in non-premixed laminar flames. Kinetic analyses indicate that HOCO and OCHO intermediates play a crucial role in pure FA flames by reacting with active radicals like H and OH to produce CO2 and CO, while in n-decane blended flames, fuel decomposition proceeds through two other pathways leading to H2 and H2O2.

Global instability phenomenon as a physical mechanism controlling dynamics of a nitrogen-diluted hydrogen flame

We investigate the dynamics of counter-current reacting flow under varying strengths of the counterflow and report the occurrence of global instability phenomena. This clarifies the role of global instability in the transition between attached and lifted flames. The research is performed with the help of the Large Eddy Simulation method using a high-order numerical method. The combustion process is modelled by applying detailed chemical kinetics for hydrogen combustion with chemical reaction terms computed from the resolved scale. The flow configuration consists of a central jet of nitrogen-diluted hydrogen fuel surrounded by an annular nozzle with a suction slot enforcing counterflow in the vicinity of the fuel stream. Suction strength is controlled by the ratio between the velocity in the suction slot (Usuc) and the velocity of the fuel jet (Uj) (I=Usuc/Uj). The Reynolds number based on the fuel parameters and the central nozzle diameter is Re=1600. Two values of the suction strength are considered (I=0.1,0.2) as well as the case without suction (I=0.0). A hot oxidizer (air) provided in the region outside the co-axial nozzle causes fuel auto-ignition far from the injection system after which the flame propagates downstream. It is shown that increasing the suction strength postpones the ignition and shifts its axial location downstream. Depending on the I parameter the flame stabilizes as attached (I=0.0,0.1) or lifted (I=0.2). It is shown that the flames in the cases with I=0.0,0.1 are very similar to each other appearing as laminar non-premixed flames both in the nozzle vicinity and far downstream. The flame in case I=0.2 is turbulent and much more dynamic. Its lifting is the result of global instability appearing at increased suction in the region of counter-current flow. It is shown that the global instability induces strong toroidal vortices that create a premixed reaction zone at the flame base. This flow structuring prevents upstream flame propagation and intensifies the mixing process ensuring almost complete fuel burning in a short distance from the nozzle.

Entropy generation in an opposed-flow laminar non-premixed flame—Effects of using reduced and global chemical mechanisms for methane–air and syngas–air combustion

The distribution of entropy generation is calculated with the aim of comparing second-law results for reduced and global mechanisms against a detailed mechanism. For methane, the DRM19 mechanism and a global, one-step irreversible mechanism are used for comparison with GRI3.0. For an equimolar CO/H2 mixture, the Davis et al. mechanism and a global mechanism are compared with GRI3.0 for flames at 1, 10 and 20 atm. Conduction is the largest contributor to entropy generation, followed by mass diffusion and chemical reactions. For the conduction and mass-diffusion components, the reduced mechanisms give results close to the full mechanism. The global mechanisms have some deviations due to, among other things, inaccurate prediction of flame position and temperature. Overall, entropy generation by chemical reactions of the reduced mechanisms correspond to full mechanism reasonably well, in spite of larger deviations between the individual reactions included in both mechanisms. When reactions are left out in mechanism reduction, their effects are compensated by adjustments in the remaining reactions. These are clearly made with objectives other than entropy generation. Reaction-by-reaction comparison shows that the importance with respect to entropy generation can be very different to that of heat release. Some simplified models are also investigated.

Counterflow flame extinction of ammonia and its blends with hydrogen and C1-C3 hydrocarbons

Ammonia as a fuel offers the potential to avoid carbon emissions, but its combustion is hindered by low reactivity. Here, the extinction limits of NH3 and NH3 plus reactivity enhancers were measured in the counterflow laminar non-premixed flames. A stable NH3-N2 flame was established with an oxygen-enriched oxidizer stream, and when the fuel was blended with CH4, C2H6, C3H8, and H2. For blended mixtures, results showed that CH4 has the least potential to enhance the stability of NH3 flames compared to the other additives. The extinction limits of C2H6 and C3H8 blended NH3 flames are nearly identical. At low percentage addition, H2-blended flames extinguish earlier than those blended with C1-C3 hydrocarbons, but this trend is reversed at higher H2 blends. Experimental conditions were simulated using Okafor et al. 2018 model and an extended Zhang et al 2021 model developed here. The models captured the measured trends, including the crossover between NH3-H2 and NH3-C2/C3 hydrocarbon fuels. Quantitatively, both models under-predicted the extinction limits of NH3-N2/enriched oxidizer flame. Better quantitative agreement is observed for the blended fuels using the model developed here. Discrepancies have been observed in the reported rates for reactions involving HNO (+OH, H), and if addressed, could improve models' capability in predicting extinction behavior in non-premixed flames. Numerical analyses were carried out to understand the kinetic coupling between NH3 and H2/C2-C3 in counter-flow flames. Extinction limits of NH3-C2-C3/H2 flames are shown to be affected by H abstraction and NH3 related chain termination reactions, heat producing reactions, and chain branching reactions. It has also been observed that at high blending ratios, C2H6/C3H8 addition in NH3 flames reduced the peak H and OH concentration via recombination and termination reactions, which compete with branching pathways. H2-blended flames are mostly influenced by reactions producing active radicals.

Effects of radiation, curvature, and preferential diffusion on the extinction of laminar non-premixed flames

The combined effects of radiative heat loss, curvature, and preferential diffusion on laminar non-premixed flames (or flamelets) are investigated in this work by using asymptotic analysis. A general theoretical description of flame temperature and extinction is derived for curved flames with non-unity Lewis numbers and radiative heat loss. Special attention is paid to the effects of curvature and radiative heat loss on the flammability limits. The results show that (1) a curved flamelet always has two extinction limits: one is the kinetic extinction limit, and the other is the curvature-induced extinction limit for the adiabatic case or the radiative extinction limit for the radiative case; (2) the curvature exerts a different influence on the adiabatic and radiative flames. Specifically for the adiabatic flame, it is found that both flame temperature and flame position significantly decrease as the curvature increases and that a new extinction limit at a low stretch rate occurs due to the existence of curvature. Furthermore, a higher curvature coupled with the increase in the Lewis number results in a lower flammability limit and narrower flammable zone. Therefore, the presence of curvature has a negative impact on the adiabatic flame. On the contrary, for the radiative flame, the results show that the increase in curvature has a positive effect on the flammability limit and thereby increases the flammable zone. It is expected that curved flamelets hold smaller (larger) flammable zones than planar flamelets under the adiabatic (radiative) condition. All results show that the change in flame curvature has a stronger effect on the flame structure and extinction than the deviation of the Lewis number from unity.

Effects of curvature on soot formation in steady and unsteady counterflow diffusion flames

A numerical study has been conducted in order to understand the effects of flame curvature on soot formation in laminar non-premixed flames. For the fundamental understanding, the canonical configuration of a counterflow diffusion flame is employed as it exhibits the essential combustion physics associated with non-premixed flames. Numerical results for the steady counterflow ethylene flames confirmed that the response of soot in curved flames is governed by the intricate coupling between flow convection, soot kinetics, and differential transport of soot impacting the rates of soot formation sub-processes. The sensitivity of the soot formation in curved flames to the strain rate variation is also analyzed to understand their competing effects. Furthermore, the dynamic response of soot formation to the unsteady curvature is investigated by imposing harmonic oscillations to the curvature. Numerical results revealed that for an increase in the frequency of curvature oscillations, the soot formation gets attenuated and amplitudes of the induced oscillations exhibit large phase-shift with respect to imposed fluctuations. The higher characteristic time scales of larger-sized particles proved to control the overall dynamic response of soot under unsteady fluctuations of curvature. To examine the dynamic response of soot formation under a more complex case of flow-flame-soot interaction, the unsteady analysis of flames is extended by subjecting them to the simultaneous strain rate and curvature oscillations. The correlation between imposed frequencies and variability of soot formation response under fluctuating strain rate and curvature is elaborated.

Experimental and Numerical Study on the Combustion Characteristics of a Laminar Non-Premixed Methane Jet Flame in Oxygen/Carbon Dioxide Coflow

The combustion characteristics of laminar non-premixed CH4 jet flame in an O2/CO2 coflows with different oxygen mole fractions were studied experimentally. The flame heights at different oxygen concentrations and fuel jet velocity were obtained. The experimental observation shows that the luminosity of the CH4 jet flame in O2/CO2 coflow is different from that of the flame in air stream. A two-dimensional numerical study of a laminar non-premixed CH4 jet flame in the O2/CO2 coflow with the O2 mole fraction of 0.35 was conducted to analyze the effects of CO2 dilution on the flame. The distribution of OH radicals in the flame was measured experimentally using planar laser-induced fluorescence (PLIF) to validate the computational method adopted in this work, and the computational and experimental results of the OH distributions showed good consistency at various fuel flow velocities. Three artificial species were created in the numerical experiment to analyze the effects of the chemical reactions, third-body collisions, and transport properties of CO2 on the height, width, and temperature distribution of the flame. The results showed that CO2 participation in chemical reactions exerts significant effects on the flame. However, the influences of the third-body effects and transport properties of CO2 on the jet flame are unremarkable. The global reaction pathways and distributions of important species in the laminar non-premixed CH4 jet flame were analyzed in detail to investigate the influence mechanisms of CO2 on the flame height and temperature. The entire flame can be divided into two oxidation parts, which separated by the boundary of the HCCO. The H, O, and OH concentrations and distributions in different parts of the flame were influenced by CO2 dilution, resulting in different flame heights and temperature distributions.

Numerical study of laminar non-premixed biogas-air flames behind backward facing steps

Numerical study of laminar non-premixed biogas-air flames behind backward facing steps

Entropy Generation in an Opposed-Flow Laminar Non-Premixed Flame – Effects of Using Reduced and Global Chemical Mechanisms for Methane-Air and Syngas-Air Combustion

Entropy Generation in an Opposed-Flow Laminar Non-Premixed Flame – Effects of Using Reduced and Global Chemical Mechanisms for Methane-Air and Syngas-Air Combustion

Real-fluid thermophysicalModels: An OpenFOAM-based library for reacting flow simulations at high pressure

Although OpenFOAM is a widely-used open source computational fluid dynamics (CFD) tool, it is limited to numerical simulations of multi-dimensional reacting/nonreacting flows at relatively-low pressures. This is not only because real-fluid models that can evaluate thermophysical properties at high pressures are not available in the thermophysicalModels library of OpenFOAM, but also because the existing mixing model cannot handle various mixing rules of real-fluid models. In the present study, we develop a novel algorithm applicable for a mixture model incorporating various mixing rules in OpenFOAM. Based on the new algorithm, we update the thermophysicalModels library of OpenFOAM 6.0 by implementing a set of real-fluid models such as the Soave-Redlich-Kwong/Peng-Robinson equation of state, Chung's model for dynamic viscosity and thermal conductivity, mixture averaged model for mass diffusivity using Takahashi's correction for binary diffusion coefficients at high pressure. The new library is validated against experimental data and is further assessed for compressible reacting flows by performing two-dimensional numerical simulations of axisymmetric laminar non-premixed counterflow flames and one-dimensional numerical simulations of premixed CH4/air flames at high pressures. The developed library can be used for any reacting flow solvers in OpenFOAM 6.0 that adopt a set of implemented real-fluid models. Program summaryProgram title: Real-fluid thermophysicalModelsCPC Library link to program files:https://doi.org/10.17632/n8zb2wpjp6.1Developer's repository link:https://github.com/danhnam11/realFluidThermophysicalModels-6Licensing provisions: GPLv3Programming language: C++Nature of problem: The mixing rules required for evaluating the real-fluid based thermophysical properties of mixtures are relatively complicated and different from each other, while the existing thermophysicalModels library offers only a mixing rule based on the mass fraction weighted average of each species, for which overloading operator functions (“*” and “+=”) are utilized to update parameters for a mixture. However, this approach is not applicable for complicated mixing rules in the real-fluid models.Solution method: To build a mixture class containing different mixing rules in different models such as the equation of states and thermodynamics/transport properties in OpenFOAM, we adopt void-type functions (parameter updating functions) to update parameters of a mixture instead of the original overloading operator functions in OpenFOAM. The parameters in the mixing rules that depend on pressure and temperature are handled by decomposition such that they can be updated in the mixture class.

Measurement and simulation of sooting characteristics by an ATJ-SKA biojet fuel and blends with Jet A-1 fuel in laminar non-premixed flames

We investigate the sooting propensity of an Alcohol-to-Jet-Synthetic Kerosene with Aromatics (ATJ-SKA) biojet fuel. The soot volume fraction and primary particle size in the pre-vaporised diffusion flames using ATJ-SKA biojet and blends with Jet A-1 at atmospheric conditions were measured experimentally and compared to numerical simulations. The measurements were conducted using extinction calibrated laser induced incandescence (LII). The soot volume fractions measured using the ATJ-SKA fuel do not show significant differences relative to measurements with Jet A-1. A comparison of the chemical composition of the fuels suggests that the Degree of Unsaturation (DoU) may not determine the sooting propensity of biojet fuels. The SEM analysis shows that diffusion flames using neat Jet A-1 produce finer soot particles and larger number density compared to the biojet and biojet surrogate. The soot model employs a semi-detailed chemical kinetic mechanism and a physical model which integrates the population balance equation governing the soot particle size distribution with an in-house reactive flow solver for multicomponent ideal gases. The model predicts the maximum soot volume fraction (SVFm) in the neat biojet case and the blended cases with Jet A-1 fuels within an error margin of 13% of the measured values. However, the predicted soot volume fraction distribution patterns differ from the measured one and the possible causes are discussed.

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.

Non-Premixed Filtered Tabulated Chemistry: Filtered Flame Modeling of Diffusion Flames

The flame front filtering is a well-known strategy in turbulent premixed combustion. An extension of this approach for the non-premixed combustion context has been proposed by means of directly filtering counterflow diffusion flamelets. Promising results were obtained for the non-premixed filtered tabulated chemistry formalism on 1-D and 2-D unresolved counterflow flame configurations. The present paper demonstrates the soundness of this approach on a 3-D real laminar non-premixed coflow flame. The model results are compared against the direct filtering of the fully resolved laminar diffusion flame showing that the formalism adequately describes the underlying physics. The study reveals the importance of the one-dimensional counterflow flamelet hypothesis, so that the model activation under this condition is ensured by means of a flame sensor. The consistent coupling between the model and the flame sensor adequately retrieves the flame lift-off and satisfactorily predicts the profile extension due to the filtering process.

Modelling of soot coalescence and aggregation with a two-population balance equation model and a conservative finite volume method

The objective of the present paper is to develop a population balance approach for modelling soot formation that distinguishes between coalescence and aggregation and accounts for finite-rate fusing of primary particles within aggregates, while providing a numerically accurate description of primary particle surface growth and oxidation within aggregates. To this end, the recently developed conservative finite volume sectional method for the solution of the population balance equation (PBE) due to Liu and Rigopoulos (2019, Combust. Flame 205, 506–521) is extended to a two-PBE approach that allows for a more accurate modelling of primary particle surface growth and oxidation and furthermore involves a timescale for the fusing of primary particles. The accuracy of the numerical method is first tested by calculating the self-preserving distributions of aggregates with varying fractal dimension. Subsequently, the one-PBE and two-PBE approaches are coupled with CFD and applied to the simulation of the Santoro laminar non-premixed co-flow sooting flame. The results show that both approaches can provide good prediction of the soot volume fraction, but the two-PBE approach yields a significant improvement in the prediction of soot morphology. At present, the information available for modelling the gradual fusing of soot primary particles is based on experiments on silica and titania nanoparticles, and therefore a comprehensive study of the impact of the sintering model parameters is conducted. Finally, conclusions are drawn regarding the predictive potential of the one-PBE and two-PBE approaches.