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Accurate prediction of soot formation and evolution remains a formidable challenge due to the complex interaction between gas-phase composition and solid-phase particles. Recent studies have shown that the choice of gas-phase mechanisms is of primary importance in affecting predictability. In this work, a systematic analysis of ethylene (CH) combustion mechanisms denoted as KAUST, Stanford, Aachen, Polimi, ABF, DLR/UT, Naples, Caltech, and SJTU, which have been widely used in the soot community, is performed to investigate their differences in fundamental chemistry, polycyclic aromatic hydrocarbon (PAH) chemistry, and soot prediction. It is found that the nine mechanisms exhibit large differences even in predicting canonical combustion properties (e.g., ignition delay time, laminar flame speed, and extinction strain rate), indicating significant variations in the fundamental chemistry. This is due to the fact that although most mechanisms share very similar dominant fuel-oxidation reactions, there are notable differences in the rate coefficients of sensitive reactions used in these mechanisms. Owing to the uncertainties in the fundamental chemistry, the predictions of CH from the nine mechanisms show significant differences, which contributes to the differences in soot precursor prediction, in conjunction with the difference in benzene (A1) formation pathways demonstrated by the element flux analysis of the C atom. Furthermore, it is found that PAHs containing two rings play a dominant role in soot formation for most mechanisms. Moreover, it is observed that while Caltech and SJTU significantly under estimate soot formation, they reasonably reproduce its sensitivity to strain rate. These results indicate that despite substantial advances in the development of CH oxidation and PAH formation chemistry, the various existing mechanisms lead to significant differences in predicting soot concentrations which can be traced back to considerable differences in both fundamental chemistry and PAH chemistry. This suggests that the fundamental chemistry should be calibrated or improved before further development of PAH chemistry and soot models.

A direct numerical simulation (DNS) with finite rate chemistry was performed to evaluate the main influences on carbon monoxide (CO) emissions in gas turbine combustion. A lean methane/air mixture is burned in fully turbulent jet flames in a domain enclosed by isothermal walls. The formation of CO is found to be affected by the mean strain rate of the turbulent flow, the flame-wall interaction (FWI), and the interactions of the flame with the recirculation zones of the flow. The CO production and consumption in the turbulent flame differ strongly from the reaction rates in a freely propagating flame. In the upstream part of the domain, the mean strain rate of the turbulent flow mainly affects the CO formation, while wall heat loss influences the CO oxidation process towards the end of the domain, where the strain rate decreases. In an optimal estimator analysis, the relevant parameters that dominate the formation and consumption of CO are identified as the local CO mass fraction , the wall heat loss, described by the enthalpy defect , and the mass fraction of the OH radical . The heat loss is particularly influential close to the wall while the effects far from the wall are negligible. Using the local CO mass fraction as parameter describes the late-stage oxidation of CO well in the entire domain. In particular, should not be neglected at the wall. is well suited to describe the processes involved in CO oxidation, as it both parameterizes the turbulent strain and is the main reaction partner for CO oxidation. The combination of and was able to improve the domain-averaged irreducible error by almost half compared to only a progress variable. Adding to the parameter set further reduced the error to 25% of the original error.

Interactions of thermodiffusive instabilities and turbulence have been investigated by large-scale Direct Numerical Simulations (DNS) in this work. Two DNS of turbulent premixed lean hydrogen/air flames have been performed in a slot burner configuration at the same jet Reynolds number of Re=11,000 and Karlovitz number of Ka≈15 using a detailed chemical mechanism. Realistic transport models are employed in one case, which features the characteristic patterns of thermodiffusively unstable flames, such as strong variations of the heat release and super-adiabatic temperatures. In the other case, the diffusivities of all species are set equal to the thermal diffusivity (unity Lewis numbers assumption) and thermodiffusive instabilities are therefore suppressed. The local burning of the turbulent flame without thermodiffusive instabilities is similar to an unstretched laminar flame and the turbulent flame speed increases only due to the increase of flame surface area in agreement with previous studies for flames at similar conditions. In contrast, the thermodiffusively unstable flame features a strong enhancement of the turbulent flame speed, which is not only caused by flame wrinkling, but is greatly increased due to significant variations of the local reaction rates. These are caused by variations of the local equivalence ratio due to the differential diffusion of hydrogen. A comparison with a thermodiffusively unstable laminar flame at the same conditions reveals that the variations of the local equivalence ratio and local reaction rates are significantly enhanced in the turbulent flame due to higher fluctuations of curvature and an enhanced average strain rate induced by turbulence. Thus, turbulence and thermodiffusive instabilities show synergistic effects, which are reflected in a significantly higher fuel consumption rate per flame surface area. The flame surface area generation, which is governed by the tangential strain and the flame propagation in curved flame segments, is also different in the two cases. Most noteworthy, the tangential strain rate is shown to be determined by the smallest turbulent structures in both turbulent flames and to be unaffected by the thermodiffusive instability mechanism. However, thermodiffusive instabilities lead to a production of flame surface area in convexly curved flame segments, featuring the formation of tongue-like structures that penetrate into the unburned gas, which do not exist in the turbulent flame with unity Lewis numbers. This is linked to an enhancement of the flame displacement speed with curvature in the thermodiffusively unstable flame, while in the absence of instabilities, a reduction of the flame displacement speed with increasing curvature is observed, leading to a destruction of flame surface area. These findings suggest that thermodiffusive instabilities are sustained in turbulent flows and even show synergistic interactions with turbulence, which needs to be accounted for in turbulent combustion models.

The direct moment closure (DMC) approach for modeling the filtered chemical source terms in large eddy simulation of turbulent combustion is comprehensively assessed by using direct numerical simulation data of a turbulent premixed flame with a skeletal chemistry. In this approach, closure is achieved by linearizing the chemical source terms using the Taylor series expansions. Different from traditional method, the exponential term is treated as a single variable in the expansion, and the filtered chemical reaction rate can be readily modeled with the first-order and second-order moments, plus higher-order correlations. The a-priori investigations demonstrate that the DMC model is able to reasonably represent the chemical reaction rates, and the second-order correlation of reactive scalars can be well modeled with the algebraic gradient-type model. Compared to the first-order moment closure, the second-order one makes a significant improvement on the predictions. These improvements are also observed in the a-posteriori study in which the premixed flame can be better resolved by the second-order moment closure model. Consequently, the second-order moment closure model provides an alternative for turbulent combustion modeling and is recommended to be used as an improvement to the laminar chemistry closure model. The sensitivity of the DMC approach to the grid size needs further investigation.

An analysis of the combustion process along gradient trajectories is presented for a set of Direct Numerical Simulations of turbulent premixed jet flames at different Reynolds numbers and approxima...

Determining laminar flame speeds from spherical flame experiments is difficult for low flame speeds, when the flame shapes quickly become non-spherical due to the influence of gravity. Direct Numerical Simulations (DNS) of spherically expanding laminar premixed flames whose motion is affected by buoyancy have been performed. A lean methane/air flame and a series of rich hydrogen/air flames diluted with molecular nitrogen are considered to investigate the effects of different fuels. All flames feature low flame speeds such that the gravitational force quickly leads to a strong deformation of the initially spherical shapes. To assess the effects of non-unity Lewis numbers in the hydrogen flames, an additional DNS of one hydrogen flame is conducted with unity Lewis numbers for all species. All DNS employ finite rate chemistry. The effect of buoyancy on the flame dynamics is found to be governed by the Richardson number, which describes the relative importance of buoyancy with respect to the flame expansion, unless significant variations of the flame propagation speed due to strain rate and curvature occur, as observed for the hydrogen/air flames with non-unity Lewis numbers. In contrast to spherical flames, the linear dependence of the flame’s displacement speed on stretch is not observed. Unless effects due to non-unity Lewis numbers prevail, the displacement speed is found to be proportional to strain rate and curvature, which vary independently along the flame front of a buoyant flame. Based on the DNS data, five techniques to determine the burning velocity of the unstretched premixed flame from data collected in buoyant flames are discussed. Among these techniques, a methodology based on flame volume, surface area, and surface-averaged curvature shows promising results with relative errors below 2% for flames where effects due to non-unity Lewis numbers are negligible.

Modeling suites for Large-Eddy Simulations of soot evolution in turbulent flames include a number of submodels describing the chemistry of soot precursors, particle dynamics, heterogeneous soot chemistry, turbulent mixing, and combustion. To understand the reasons for model failure and to enhance the overall model performance, it is necessary to identify and subsequently improve model components with a leading order effect on the overall error. In this work, errors in soot predictions associated with flamelet-based combustion models are isolated and quantified in a combined a-priori and partial a-posteriori analysis using large-scale Direct Numerical Simulation (DNS) data of a sooting turbulent jet diffusion flame. Gas-phase quantities entering the calculation of the soot source terms and hence coupling the combustion model to the soot model are analyzed in the DNS first. The performance of a Flamelet/Progress Variable model with respect to these quantities is then analyzed a-priori for two DNS cases employing a mixture-averaged transport model and unity Lewis numbers. Then, the soot evolution along Lagrangian trajectories extracted from the DNS is re-computed using rate coefficients directly taken from the DNS and from the flamelet table. In the context of this partial a-posteriori analysis, flamelet-induced errors are also compared to errors induced by the chemical soot model. The soot surface growth and oxidation rate coefficients are reasonably well predicted by the flamelet library. Considerably larger errors in the model-predicted soot mass originate from the tabulated quantities entering the calculation of PAH-based soot growth rates. However, these errors can be reduced to a few percent if the rate is appropriately scaled with the mass fraction of polycyclic aromatic hydrocarbons (PAH). However, this requires the solution of a transport equation for the PAH mass fraction, and modeling the source term in this equation is shown to be challenging. Overall, the largest uncertainties can be attributed to the chemical mechanism for PAH formation and the model for the PAH source term.

Modeling soot evolution in turbulent reacting flows using Large Eddy Simulation (LES) is challenging due to the complex subfilter soot-turbulence-chemistry interactions. Soot particles form at fuel-rich mixtures and are subsequently oxidized as they are transported toward fuel-lean mixtures. In previous work, this phenomenology was explicitly encoded into a presumed subfilter PDF model for soot by confining soot strictly to mixtures where growth rates exceed oxidation rates. However, this model implicitly assumed that oxidation is infinitely fast. In this work, a new presumed subfilter PDF model for soot is proposed to account for finite-rate soot oxidation. The distribution of soot with respect to the flame structure (mixture fraction) is modeled by comparing the local relative motion of diffusionless soot particles with respect to mixture fraction iso-contours with the local oxidation rate. When the oxidation rate is suppressed or the transport rate is very fast, soot is allowed to penetrate further into fuel-lean mixtures. This model can allow for soot leakage across the flame, a critical phenomenon in smoking flames. The new model is validated a priori against Direct Numerical Simulation (DNS) databases of turbulent nonpremixed jet flames and then a posteriori against experimental measurements in a laboratory-scale turbulent jet flame. A priori results show remarkably good agreement with filtered DNS data and are shown to correctly allow for soot leakage at low Damköhler number. Finally, LES results of the turbulent sooting jet flame show an improvement of soot prediction using the new soot subfilter model compared to previous works. In this flame, as in experiments, evidence of soot leakage is found near the beginning of the sooting region, and this soot leakage cannot be predicted with previous subfilter models that presume infinitely fast oxidation.

The homogeneous ignition and volatile combustion of pulverized solid fuel in single-particle and particle group configurations were studied numerically in a laminar flat flame burner. Simulations with increasing particle streams were performed to investigate the influence of the interactions in particle groups on homogeneous ignition and combustion. An extensive set of simulations are conducted considering models with different levels of detail for both the gas-phase and solid fuel chemistry. The reference simulations employ the chemical percolation devolatilization model coupled with a detailed chemistry model for gas-phase reactions. The particle-fluid interactions were modeled with a fully coupled Eulerian-Lagrangian framework. Increased ignition delay times for higher particle streams were successfully validated against available experimental measurements. Furthermore, the transition from single-particle ignition to a conically shaped volatile flame with suppressed reactions near the flame base in particle group combustion was observed in both experiments and simulations. The subsequent detailed investigations revealed that the increased heat transfer to particles and, therefore, lower gas temperature for higher particle number densities together with the local oxygen depletion are the primary reasons for this transition. Based on the reference simulation, different simplified model combinations were assessed. The systematic model reduction investigation started with assessing the fixed volatile composition as a required assumption for flamelet models. Finally, the effects of gas-phase chemistry and different simple devolatilization models on ignition and combustion chemistry were studied. Overall, all model combinations provide reasonable predictions of volatile combustion with minor local deficits in the studied conditions.

Soot formation is experimentally and numerically investigated in laminar counterflow diffusion flames burning ethylene and three typical gasoline surrogate components; n-heptane, iso-octane, and toluene. Laser-induced incandescence and a light extinction technique are employed to determine the soot volume fraction within the well-controlled region of the burner. The experiments are performed across a wide range of strain rates and stoichiometric mixture fractions. From the experimental data, sensitivities of soot formation on strain rate and stoichiometric mixture fraction are derived for each fuel. The fuels show significantly different sensitivities. For iso-octane and n-heptane, a higher sensitivity of soot production on the strain rate is observed as compared to ethylene and toluene. Moreover, the sensitivities of soot formation on the strain rate increase with increasing stoichiometric mixture fraction. One-dimensional simulations of the flames investigated experimentally were performed using two different detailed chemical kinetic mechanisms, detailed chemical soot models, and the hybrid method of moments as well as a discrete sectional method to describe soot dynamics. The models are capable of predicting the soot volume fraction of the ethylene flames with remarkable accuracy, whereas for the gasoline surrogate components, the overall soot volume fractions are overpredicted for all tested models. In iso-octane flames, soot nucleation and PAH condensation rates are particularly enhanced. A reaction pathway analysis shows that in ethylene flames, the formation of benzene mostly originates from acetylene, while for iso-octane, large amounts of iso-butenyl form propyne, propargyl, and then benzene.

Turbulence and mixing statistics are investigated in a set of flames at a jet Reynolds number of 15000 achieving a Taylor’s scale Reynolds number in the range 100 ≤Reλ ≤ 150. In particular, the impact on small scale turbulence statistics of different levels of flame extinction, induced imposing different Damkohler numbers in the three simulated cases, is investigated. It is found that the non-dimensional scalar dissipation depends on the Damkohler number slightly. This deviation from self-similarity manifests itself as a decrease of the non-dimensional scalar dissipation with increasing occurrence of localized extinction events. This is caused by the decrease of molecular diffusion due to the lower flame temperatures in the low Damkohler number cases. Probability density functions of the scalar dissipation χ show important deviations from the log-normal distribution. The left tail of the pdf scales as χ1/2 while the right tail scales as $e^{-c\chi ^{\alpha }}$ , as shown for incompressible turbulence. In all flames, the vorticity vector displays a pronounced tendency to align with the direction of the intermediate strain and the gradient of mixture fraction aligns with the most compressive strain. Conditioning on the local values of mixture fraction and heat release does not affect the statistics. The alignment statistics of vorticity are in agreement with those in homogeneous isotropic turbulence while they show some difference compared to previous results in non premixed flames. The alignment between strain and mixture fraction gradient differs slightly from the homogeneous isotropic turbulent case but agree remarkably well with previous results observed in homogeneous shear incompressible flows.

Dissipation element (DE) analysis is a method for analyzing scalar fields in turbulent flows. DEs are defined as a coherent region in which all gradient trajectories of a scalar field reach...

The ignition of a stream of coal particles under oxy-fuel atmosphere is studied numerically in a laminar flow. Devolatilization of coal particles is modeled with the chemical percolation devolatilization (CPD) method coupled with a detailed chemistry model for homogeneous reactions. The particle and gas phase interactions are modeled with a fully coupled Euler-Lagrange code. A parametric study is performed to investigate the influence of particle number density, gas temperature, and velocity on ignition. We found that increasing particle number density delays the onset of ignition. Delayed ignition in denser streams is due to the lower gas temperature, which is caused by the higher energy required for particle heating. In addition, increasing particle number density leads to a more continuous and narrower flame front. Particle heating and ignition induce velocity variations in the gas phase through the coupling with particles. In the denser stream, velocity variations become significant and compromise the validity of a constant velocity assumption that is usually made in computing ignition delay time from the observed ignition location. It is also found that high particle slip velocities lead to a locally low volatile concentration and low temperature, and consequently increase ignition delay time.

The formation, growth, and oxidation of soot in turbulent prevaporized toluene diffusion flames stabilized on a jet-in-hot-coflow (JHC) burner are investigated in this study. Flame structure, local gas temperature as well as local soot volume fraction and primary soot particle diameter, are simultaneously detected by means of OH planar laser-induced fluorescence (PLIF), non-linear excitation regime two-line atomic fluorescence (nTLAF) of indium, and time-resolved (TiRe) laser-induced incandescence (LII), respectively. The collected data sets were used to generate joint statistics of soot properties and flame characteristics and provided new insights into the interaction of the OH layer and soot in turbulent flames. The interaction of OH and soot as a driving mechanism for soot oxidation is of particular interest as it has been proven to be challenging to model. Statistics of soot volume fraction and primary particle size in the OH layer are employed to gain deeper insights into the soot oxidation process. Mean soot volume fraction and primary soot particle size conditioned on temperature and OH signal intensity indicate that, due to differential diffusion of soot with respect to the chemical species, high soot volume fraction and primary soot particle diameter of up to 50 nm are present at low temperatures and low OH concentration. In the soot oxidation region, statistical analysis of the soot parameters disclose that clusters of high soot volume fraction mostly consist of large primary particles. Observations from instantaneous images and the presence of large primary particles inside the OH layer suggest that the oxidation is not sufficiently fast to burn the soot completely.

The ignition and combustion of coal particles are investigated numerically under conventional and oxy-fuel atmospheres. Devolatilization is computed using the chemical percolation devolatilization (CPD) model. The CPD model is coupled with a Lagrangian particle tracking method in the framework of a multiphysics, multiscale Navier–Stokes solver. Combustion in the gas phase is described using finite rate chemistry. The numerical results for ignition are compared with available experimental data and a remarkably good agreement is observed. The effect on flame ignition of the different phases characterizing the release of volatile gases is assessed. These different phases manifest themselves in two distinct peaks in the devolatilization rate and it is observed that ignition can occur during the first volatile release or on the onset of the second, depending on the particle size and gas temperature. It is found that an increase of ignition delay time in oxy-atmosphere compared to the air case is related to the depletion of radicals that react with the abundant carbon dioxide of the oxy-atmosphere, while the increased heat capacity of the mixture does not play a role.

Direct Numerical Simulations (DNS) are conducted to study the statistics of flame surface stretch in turbulent jet premixed flames. Emphasis is placed on the rates of surface production and destruction and their scaling with the Reynolds number. Four lean methane/air turbulent slot jet flames are simulated at increasing Reynolds number and up to Re ≈ 22 × 103, based on the bulk velocity, slot width, and the reactants’ properties. The Karlovitz number is held approximately constant and the flames fall in the thin reaction zone regime. The simulations feature finite rate chemistry and mixture-average transport. Our data indicate that the area of the flame surface increases up to the streamwise position corresponding to 80% of the average flame length and decreases afterwards as surface destruction overtakes production. It is observed that the tangential rate of strain is responsible for the production of flame surface in the mean and surface destruction is due to the curvature term. In addition, it is found that these two terms are both significantly larger than their difference, i.e., the net surface stretch.The statistics of the tangential strain rate are in good agreement with those for infinitesimal material surfaces in homogeneous isotropic turbulence. Once scaled by the Kolmogorov time scale, the means of both contributions to stretch are largely independent of location and equal across flames with different values of the Reynolds number. Surface destruction is due mostly to propagation into the reactants where the surface is folded into a cylindrical shape with the center of curvature on the side of the reactants. The joint statistics of the displacement speed and curvature of the reactive surface are nuanced, with the most probable occurrence being that of a negative displacement speed of a flat surface, while the surface averaged displacement speed is positive as expected.

Large-scale two-dimensional numerical simulations of thermodiffusively unstable, lean, premixed hydrogen flames have been performed using detailed finite rate chemistry to analyze flame intrinsic scales. The simulations feature a long integration time and large domain sizes to rule out effects of confinement on the dynamics of the flame front. For sufficiently large domain sizes, the total consumption speed of the flame is found to become independent of the domain size. An assessment of the characteristic scales of the flame front corrugation reveals the existence of a smallest and a largest flame intrinsic length scale. The smallest length manifests itself by local cusps, which lead to the formation of characteristic cells along the flame front. Their size is remarkably close to the most unstable wavelength predicted by a linear stability analysis of the flame front evolution in the linear regime. Independently of the domain size, a specific largest flame intrinsic structure, here referred to as flame finger, emerges from the interaction of multiple small-scale cusps. Thermodiffusively unstable flames are found to periodically form and destroy these flame fingers, but the formation of a global cusp that is known to emerge for purely hydrodynamically unstable flames is suppressed. The finite size of the largest scale fingers is explained by an instability in their movement. As they proceed towards the unburnt mixture, they tend to tilt and move laterally, thereby eventually being incorporated again by the rest of the flame. This behavior arises from the interaction of the flame fingers and the diverging velocity field ahead of them. Finally, the effect of equivalence ratio and unburnt gas temperature is investigated showing that flame fingers are found to develop only in case of a thermodiffusively unstable flame.

Background: It is well established that SARS-CoV-2 is readily transmitted in indoor environments; however questions remain about the relative importance of different transmission mechanisms, the risks associated with non-clinical indoor environments and activities and the role of ventilation and plumbing systems in mitigating or amplifying transmission. This rapid evidence review identifies and integrates evidence from three distinct disciplines: epidemiology, microbiology and fluid dynamics. Objectives: This review addresses the following research questions: 1. What evidence is there for aerosolised transmission? 2. What evidence is there for faecal-oral transmission? 3. What evidence is there regarding the role of ventilation systems in indoor transmission? 4. What evidence is there regarding the role of plumbing systems in indoor transmission? 5. What evidence is there regarding transmission via different indoor surfaces (materials and specific objects)? 6. What evidence is there for the transmission in indoor residential settings? 7. What evidence is there for transmission in indoor workplace settings? 8. What evidence is there for transmission in other indoor settings (social, community, leisure, religious, public transport)? 9. Do particular activities convey greater risk (e.g. shouting, singing, eating together, sharing bedrooms)? 10. What evidence is there for the appropriate length of distancing between people? Data sources: Searches were conducted in PubMed, medRxiv, arXiv, Scopus, WHO COVID-19 database, Compendex & Inspec. Eligibility criteria: We included studies reporting data on any indoor setting except schools, any indoor activities and any potential means of transmission. Additional screening criteria were modified slightly for the three study disciplines. Title and abstract and full text screening was conducted by a single reviewer, with rejections assessed by a second reviewer. Data extraction, quality appraisal and data synthesis: We used Joanna Briggs Institute and Critical Appraisal Skills Programme tools for evaluating epidemiological studies and developed bespoke tools for the evaluation of study types not covered by these instruments. Data extraction was limited to a minimum set of items, given the rapid nature of this review. Data extraction and quality assessment were conducted by a single reviewer. We conducted a meta-analysis of secondary attack rates in household transmission. Otherwise, data were synthesised narratively. Results and conclusion: Literature searches yielded a total of 1573 unique articles. After screening and quality assessment, fifty-eight articles were retained for analysis. Experimental evidence from fluid mechanics and microbiological studies demonstrates that aerosolised transmission is theoretically possible; however, we found no conclusive epidemiological evidence of this occurring. The evidence suggests that ventilation systems have the potential to decrease virus transmission near the source through dilution but to increase transmission further away from the source through dispersal. There is compelling evidence that, if infectious virus is present in faeces, defective plumbing systems can amplify transmission within high-rise buildings. However, although viral RNA can be detected in stool samples, we found no evidence for the detection of live infectious virus in faeces, nor did we find any examples of faecal-oral transmission. Laboratory studies suggest that the virus survives for longer on smooth surfaces and at lower temperatures. Environmental sampling studies have recovered small amounts of viral RNA from a wide range of frequently touched objects and surfaces; however, we did not find any such studies which had successfully cultured live virus from collected samples and epidemiological studies are inconclusive on the extent of fomite transmission. We found many examples of transmission in settings characterised by close and prolonged indoor contact, such as households (pooled secondary attack rate 11%, 95 % confidence interval 9, 13), communal residential settings, workplaces and religious gatherings. There were insufficient data to evaluate the transmission risks associated with specific activities; however workplace challenges related to poverty, such as continuing to work whilst ill, especially when attendance is incentivised, warrant further attention. Finally, the physical properties of droplets generated by coughing, speaking and breathing are well characterised and reinforce the importance of maintaining 2 metres social distance to reduce droplet transmission.

The characteristics of the internal layers of intense shear are examined in a mixing layer and in a jet, in the range of Reynolds numbers 134

International audience; Simulations of unsteady char particle combustion rely on various models that are necessary in order to correctly predict the governing flow and combustion processes. These models, in turn, rely on model parameters, which are determined by experiments or small scale simulations and contain a certain level of uncertainty. It is therefore, essential to correctly determine the sensitivities of quantities of interest measured using such simulations, with respect to the existing parameters. In this study, a discrete adjoint algorithm is employed to extract sensitivities of various quantities of interest with respect to physical and model parameters. This adjoint framework bears a great advantage in cases where a large input space is analyzed, since a single forward and backward sweep provides sensitivity information with respect to all parameters of interest. Sensitivities are extracted for relevant quantities of interest, such as burning rate and particle temperature, and are then compared as free stream composition changes from air to oxy atmosphere. The evolution of sensitivities in time is shown to be dependent on the selected quantity of interest. Model sensitivities with respect to heterogeneous reaction parameters (oxidation of carbon, in particular) are shown to be the highest, whereas the sensitivities with respect to free stream composition are shown to be significantly lower.

This article is dedicated to Moshe Matalon on the occasion of his 70th birthday, for his numerous contributions to the field of combustion and, in particular, to the rich and varied topic of premixed flame stability. Here, we follow in his footsteps and propose a subfilter modelling framework for thermo-diffusively unstable premixed flames, such as lean hydrogen–air flames. Performing an optimal estimator analysis for the unfiltered and filtered heat release rate of the lean premixed hydrogen–air flames, the latter is found to require at least two scalars for an appropriate representation while for large filter sizes, the heat release appears to require only one scalar for parametrisation. As a result, we develop a modelling strategy based on the construction of thermochemical tables for each unclosed term as a function of two variables as well as the filter size. The framework is based on the filtered tabulated chemistry approach, where, in lieu of a one-dimensional unstretched flame, we adopt a data-driven paradigm and filter fully resolved two-dimensional simulations of variable size. Models originating from small- and medium-sized simulations are tested a-priori on a large-size simulation, thus highlighting the role of the lateral domain in the dataset used for tabulation. The concept of a minimum domain size is thus discussed, leading to a dataset exhibiting the minimal properties for sufficiently accurate thermochemical tables. The strategy is shown to be more accurate than a classical one-dimensional filtered tabulated chemistry approach and shows promise in future LES modelling of laboratory and industrial scale hydrogen flames.

The impact of intrinsic combustion instabilities is studied for lean premixed hydrogen flames by means of a series of simulations at different equivalence ratios [0.4-1.0], unburned temperatures [298K-700K], and pressures [1bar-20bar]. In addition to the Darrieus-Landau, or hydrodynamic, instability, lean premixed hydrogen flames are prone to thermodiffusive instabilities, which lead to significant flame front wrinkling and a chaotic process of formation and destruction of cellular structures along the flame front. Theoretical models are not yet capable of accurately describing the evolution of such flames, so the propensity of lean hydrogen flames to develop instabilities is studied numerically in a parametric variation in this work. A stability analysis is conducted, in which planar flames are initially exposed to weak harmonic perturbations and the response of the flame is studied. In the initial phase referred to as linear phase, a growth or decrease of the initially imposed perturbation amplitude is observed, while for long times, chaotic cellular structures are formed along the flame front, which are studied in part 2 of this work (L.Berger et al., Combust. Flame, 2022). The growth rates of the perturbation amplitude that are obtained from the initial phase are a measure of the strength of the intrinsic instability mechanisms and vary with respect to the wave number of the harmonic perturbation yielding characteristic dispersion relations. A decrease of equivalence ratio and unburned temperature and an increase of pressure are found to enhance the growth rates and hence intrinsic instabilities. The variation of dispersion relations is analyzed with respect to variations of the expansion ratio, the effective Lewis number of the mixture, and the Zeldovich number. For the lean hydrogen flames, with increasing pressure a decrease of the cut-off wave number, which represents the change of the sign of the dispersion relation at high wave numbers, is observed. This is the opposite trend compared to flames that are not affected by thermodiffusive instabilities. Further, numerical growth rates are compared to theoretical models. The results show that lean hydrogen flames are prone to develop instabilities at conditions that are relevant to several combustion devices such as gas turbines that operate at lean equivalence ratios, elevated pressures and temperatures or domestic and industrial heaters that operate at low temperatures and ambient pressure.

In the flamelet regime of turbulent premixed combustion the enhancement in the burning rates originates primarily from surface wrinkling. In this work we investigate the Reynolds number dependence of burning rates of spherical turbulent premixed methane/air flames in decaying isotropic turbulence with direct numerical simulations. Several simulations are performed by varying the Reynolds number, while keeping the Karlovitz number the same, and the temporal evolution of the flame surface is compared across cases by combining the probability density function of the radial distance of the flame surface from the origin with the surface density function formalism. Because the mean area of the wrinkled flame surface normalized by the area of a sphere with radius equal to the mean flame radius is proportional to the product of the turbulent flame brush thickness and peak surface density within the brush, the temporal evolution of the brush and peak surface density are investigated separately. The brush thickness is shown to scale with the integral scale of the flow, evolving due to decaying velocity fluctuations and stretch. When normalized by the integral scale, the wrinkling scale defined as the inverse of the peak surface density is shown to scale with Reynolds number across simulations and as turbulence decays. As a result, the area ratio and the burning rate are found to increase as , in agreement with recent experiments on spherical turbulent premixed flames. We observe that the area ratio does not vary with turbulent intensity when holding the Reynolds number constant.

The dissipation element analysis is applied to the mixture fraction fields of a series of datasets from direct numerical simulations of non-premixed temporally evolving jet flames with jet Reynolds numbers ranging from 4500 to 10 000 and varying stoichiometric mixture fractions. Dissipation elements are space-filling regions where a scalar field behaves monotonically and allow for the analysis of scalar fields in homogeneous isotopic turbulence as well as in complex, highly inhomogeneous and anisotropic flows such as turbulent flames. Statistics of the dissipation element parameters of non-premixed flames are compared to those obtained from non-reacting jets. It is found that the universality of the normalized length distribution of the dissipation elements observed in non-reacting cases also holds true for the reacting flows. The characteristic scaling with the Kolmogorov micro-scale is obtained as well. The effects of combustion on the scalar difference in the dissipation elements are shown and are found to diminish as the Reynolds number and the fuel dilution is increased. The dissipation elements provide the means for a local comparison of the turbulent and characteristic flame scales. A new regime diagram for non-premixed combustion is introduced using coherent structures in the scalar fields, the dissipation element parameters for a local classification of the turbulent flame surface into flamelet-like zones and fine-scale mixing zones in addition to the burning and non-burning zones. The soundness of the regime diagram and the potential consequences for combustion modelling in the individual regimes is demonstrated by the investigation of the correlation between the chemical field and the dissipation element parameters in the individual regimes.

Direct Numerical Simulations (DNS) of reacting flows provide high-fidelity data for combustion model reduction and validation, although their interpretation is not always straightforward because of the massive amount of information and the data high-dimensionality.

A series of Direct Numerical Simulations (DNS) of lean methane/air flames was conducted in order to investigate the enhancement of the turbulent flame speed and modifications to the reaction layer structure associated with the systematic increase of the integral scale of turbulence $l$ while the Karlovitz number and the Kolmogorov scale are kept constant. Four turbulent slot jet flames are simulated at increasing Reynolds number and up to $Re \approx 22000$, defined based on the bulk velocity, slot width, and the reactants' properties. The turbulent flame speed $S_T$ is evaluated locally at select streamwise locations and it is observed to increase both in the streamwise direction for each flame and across flames for increasing Reynolds number, in line with a corresponding increase of the turbulent integral scale. In particular, the turbulent flame speed $S_T$ increases exponentially with the integral scale for $l$ up to about 6 laminar flame thicknesses, while the scaling becomes a power-law for larger values of $l$. These trends cannot be ascribed completely to the increase in the flame surface, since the turbulent flame speed looses its proportionality to the flame area as the integral scale increases; in particular, it is found that the ratio of turbulent flame speed to area attains a power-law scaling $l^{0.2}$. This is caused by an overall broadening of the reaction layer for increasing integral scale, which is not associated with a corresponding decrease of the reaction rate, causing a net enhancement of the overall burning rate. This observation is significant since it suggests that a continuous increase in the size of the largest scales of turbulence might be responsible for progressively stronger modifications of the flame's inner layers even if the smallest scales, i.e., the Karlovitz number, are kept constant.

Proceedings of the Combustion Institute (2020). doi:10.1016/j.proci.2020.06.208, Published by Elsevier, Amsterdam [u.a.]

The interpretation of high-dimensional data, like those obtained from Direct Numerical Simulations (DNS) of turbulent reacting flows, constitutes one of the biggest challenges in science and engineering. Although these simulations are a source of key information to advance the knowledge of turbulent combustion, as well as to develop and validate modeling approaches, the dimensionality of the data often limits the full opportunity to leverage the detailed and comprehensive information stored in datasets. The Principal Component Analysis (PCA) and its local formulation (LPCA) are widely used in many fields, including combustion. During the last 20 years, they have been used in combustion for the identification of low-dimensional manifolds, data analysis, and development of reduced-order models. Lower dimensional structures, either global or local, can provide better insights on the underlying physical phenomena, and lead to the formulation of high-fidelity models. This chapter aims to offer to the reader a comprehensive introduction of the PCA potential for data analysis, firstly introducing the main theoretical concepts, and then going through all the required computational steps by means of a MATLAB® code. Finally, the methodology is applied to data obtained from a DNS of a turbulent reacting non-premixed n-heptane jet in air. The latter can be regarded as an optimal case for data analysis because of the complex physics characterized by turbulence–chemistry interaction and soot formation.

The effect of pressure on the hydrodynamic stability limits of lean methane–air premixed flames is investigated with Direct Numerical Simulation based on multi-step chemistry and using a simplified one-step chemistry formulation. The dependency on pressure p of the cut-off length scale λc, that separates stable from unstable wavelengths of the initial perturbation, is computed for a number of different conditions. An increase of pressure causes a significant decrease of the cut-off length, as observed already in previous simulations and experiments. However, this decrease cannot be ascribed only to the decreased flame thickness due to elevated pressures, but the cut-off is reduced significantly even if normalized by either the thermal flame thickness lT or the diffusive flame thickness lD. For the conditions analyzed, the cut-off can be well approximated by the power-law λ c ∝ p − 0.8 , while the thermal and diffusive flame thicknesses, in accordance with previous experiments, are proportional and scale as l T ∝ l D ∝ p − 0.3 . Therefore, the non-dimensional cut-off scales as λ c / l T ∝ λ c / l D ∝ p − 0.5 . This behavior is linked to the increase of the Zeldovich number with pressure, caused by higher inner layer temperatures at higher pressures, which is a result of increased importance of chain termination reactions. The same behavior is observed also in a one-step chemistry approach if the Zeldovich number, appearing explicitly in the one-step model equations, is varied with pressure according to the results from multi-step chemistry. The analysis is extended to the non-linear phase of the instability, when typical strong cusps are observed on the flame surface, simulating a two-dimensional slot burner for different pressures; it is confirmed that the same pressure effects are observed also in more complex settings and in the non-linear regime.

Dissipation elements provide a procedure for compartmentalizing scalar fields into physically meaningful sub-units which provides a direct measure for turbulent scales. Furthermore, dissipation elements enable a variety of additional ways of investigating non-local effects in reacting and non-reacting turbulent flows. After the underlying physical ideas of dissipation elements are explained and a parameterization of dissipation elements is defined, the method of detecting dissipation elements with gradient trajectories is explained and physical and numerical prerequisites are presented. Common characteristics of dissipation elements are interpreted and compared for a large range of selected reacting and non-reacting flow configurations. To provide the reader with a degree of familiarity, dissipation element statistics are then related to more commonly used methods of obtaining statistics. The additional benefit of using the dissipation element analysis in free shear flows is highlighted by using it as an alternative way of identifying turbulent core regions. Next, a dissipation element-based procedure for the local investigation of the turbulence–combustion interaction in the context of non-premixed flames is presented. The chapter is concluded with the application of a dissipation element statistics-based modeling procedure for computational fluid dynamics of a passenger car diesel engine, employing the previously gained insight into the structure of turbulent scalar fields.

Flow, turbulence and combustion (2020). doi:10.1007/s10494-020-00222-1, Published by Springer Science + Business Media B.V., Dordrecht [u.a.]

Simulations of pulverised coal combustion rely on various models, required in order to correctly approximate the flow, chemical reactions, and behavior of solid particles. These models, in turn, rely on multiple model parameters, which are determined through experiments or small-scale simulations and contain a certain level of uncertainty. The competing effects of transport, particle physics, and chemistry give rise to various scales and disparate dynamics, making it a very challenging problem to analyse. Therefore, the steady combustion process of a single solid particle is considered as a starting point for this study. As an added complication, the large number of parameters present in such simulations makes a purely forward approach to sensitivity analysis very expensive and almost infeasible. Therefore, the use of adjoint-based algorithms, to identify and quantify the underlying sensitivities and uncertainties, is proposed. This adjoint framework bears a great advantage in this case, where a large input space is analysed, since a single forward and backward sweep provides sensitivity information with respect to all parameters of interest. In order to investigate the applicability of such methods, both discrete and continuous adjoints are considered, and compared to the conventional approaches, such as finite differences, and forward sensitivity analysis. Various quantities of interest are considered, and sensitivities with respect to the relevant combustion parameters are reported for two different freestream compositions, describing air and oxy-atmospheres. This study serves as a benchmark for future research, where unsteady and finally turbulent cases will be considered., Comment: Submitted to Combustion Theory and Modelling

Fuel-rich combustion is a promising technique to produce hydrogen by reforming hydrocarbons. Predictive modeling of the fuel reforming process needs a reliable chemical mechanism which is usually validated using laminar flame speed (LFS) data. While LFSs have been extensively employed to optimize mechanisms under fuel-lean, stoichiometric, and slightly fuel-rich conditions, it remains a formidable challenge to measure the LFSs of very rich mixtures (e.g., equivalence ratio ϕ ≥ 2 ) due to significant soot formation in such conditions, which leads to the LFS data under fuel-rich conditions being rather scarce. To overcome the challenge, a clear understanding of soot formation and its impact on LFS measurements is required. In this work, a series of one-dimensional outwardly propagating spherical flames (OPFs) with and without considering soot formation are simulated for rich ethylene/air mixtures ( 2.0 ≤ ϕ ≤ 3.5 ), in order to understand soot dynamics and morphology in the OPFs and to quantify the effects of soot formation on the determination of LFS using the OPF method under fuel-rich conditions. To this end, a detailed chemistry accounting for major pathways of PAH formation up to A7 (coronene, C 24 H 12 ) is employed and coupled with a state-of-the-art soot model considering nucleation, condensation, coagulation, surface growth, oxidation, and fragmentation. It is found that soot dynamics and morphology are very sensitive to the change in equivalence ratio and flame radius. Specifically, surface growth and oxidation play a dominant role in soot formation/evolution and can balance each other at ϕ = 2.0 , resulting in limited soot formation. However, the PAH-based condensation can dominate over other processes at ϕ ≥ 2.5 and hence a large amount of soot is observed. Moreover, the maximum value of primary particle diameters would tend to a constant as the flame radius increases. The results also show that the ratio between total soot mass and the mass of burned gas, m soot / m burned , follows an exponential law given by m soot / m burned = 10 0.47 ϕ − 3.76 , implying that soot formation can be exponentially enhanced by increasing equivalence ratio. Furthermore, it is found that under very rich conditions, the presence of soot has a significant impact on the determination of LFS, which leads to the measured LFS at ϕ = 3.5 being 22% lower than 1D planar adiabatic flame speed. This is mainly attributed to the thermal and flow effects of soot radiation, i.e., the reduction of flame temperature and negative flow speed of the burned gas. The above findings suggest that the impact of soot radiation should be carefully treated when determining the LFSs of rich premixed mixtures at ϕ ≥ 2.0 .

Coflow diffusion flames are a canonical laboratory-scale flame configuration, which is routinely employed in fundamental combustion studies on flame stabilization, chemical kinetics, and pollutants’ emissions. In particular, pressurized coflow flames are used to study the effect of pressure on soot formation. In this work, we explore the opportunity to scale sooting coflow flames at constant Reynolds and Grashof numbers as pressure increases. This is achieved by decreasing the bulk velocity and the diameter of the fuel nozzle with increasing pressure. Despite some minor departures from the ideal scaling due to the effect of radiative heat losses from soot, the coflow flames are shown to be self-similar to a very good approximation. By keeping the Reynolds and Grashof numbers constant, one obtains a set of pressurized flames, which have self-similar nondimensional flow fields. Self-similarity is observed experimentally via direct photography and documented thoroughly by direct numerical simulation of steady axisymmetric coflow flames of methane and air at pressures from 1 to 12 atm. Although the study does not include data on soot yields, the implications for soot formation are explored with emphasis on the field of scalar dissipation rate and on the residence time, temperature, and mixture fraction experienced by a parcel of fluid moving along the centerline and along a streamline on the flame’s wing. We explain how the proposed approach to scaling pressurized flames facilitates the analysis of the effect of pressure on soot formation.

A new skeletal mechanism, consisting of 16 species and 72 reactions, has been developed for lean methane–air premixed combustion from the GRI-Mech 3.0. The skeletal mechanism is validated for eleva...

The formation, growth, and oxidation of soot are studied in a set of laminar coflow diffusion flames at pressures ranging from 1 to 8 atm. The modeling approach combines detailed finite rate chemical kinetics mechanisms that model the formation of Polycyclic Aromatic Hydrocarbon (PAH) species up to pyrene, and a bivariate method of moments that describes soot particles and aggregates by their volume and surface area. The spatial distribution of soot observed experimentally and that predicted numerically are in good qualitative agreement with the peak soot volume fraction located at the flame tip and soot appearing on the flame wings and closer to the nozzle as pressure increases. A detailed analysis of the effect of hydrodynamics and mixing on soot formation is presented. We show that the scalar dissipation rate is lower for the higher pressure flames, promoting the formation of PAH species and soot. Thus, the observed increase in soot volume fraction across flames with increasing pressure is not due solely to mixture density and kinetics effects, rather is affected by hydrodynamics and mixing processes also. Similarly, our results indicate that the decrease in the scalar dissipation rate contribute to changing the location where soot forms in the flame, with soot formation occurring closer to the nozzle and outward on the flame’s wings as pressure increases. Radiative heat losses are found to lower the flame temperature, inducing a reduction of the PAH species and associated rates of soot formation. However, heat losses are responsible for a slightly longer flame, which allows for more soot. The overall effect is a modest variation of soot volume fraction if radiation is included.

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.

The ignition and volatile combustion of single coal particles were investigated under laminar conditions. Relevant physico-chemical processes were analyzed under conventional and oxy-fuel atmospheres with varying O2 contents in experiments and simulations. An optically accessible laminar flow reactor with well-defined boundary conditions measured with PIV and quantitative OH-LIF was employed. Multi-parameter optical diagnostics were conducted, including OH-LIF, luminescence imaging, and backlight illumination. Simultaneously acquired experimental data allowed for the evaluation of particle size, ignition delay time, and volatile combustion duration for individual particles. A statistical analysis revealed the improved accuracy of OH-LIF compared to luminescence imaging regarding ignition detection. Simulations within an Eulerian–Lagrangian framework were introduced and validated against experiments. On this basis, particle temperatures, local gas temperatures, and fuel mass fraction were evaluated, providing insights into the devolatilization. Both experimental and numerical results indicated that increasing particle sizes significantly retarded homogeneous ignition and volatile consumption. When increasing the O2 content, a shorter ignition delay time and volatile combustion duration were observed experimentally, which was more significant for larger particles. High slip velocities accelerated convective transport resulting in an earlier ignition and faster volatile combustion. An atmosphere change from N2 to CO2 showed an earlier ignition and increased volatile combustion duration for larger particles, whereas the differences were insignificant for small particles. Simulation results suggested that the local heat transfer was improved by CO2, mainly due to the lower temperature sink close to particles and hence higher volatile release rates. As the initial ambient temperatures were similar, the introduction of CO2 favored homogeneous ignition and slowed down the volatile consumption.

Turbulence statistics from two three-dimensional direct numerical simulations of planar n -heptane/air turbulent jets are compared to assess the effect of the gas-phase species diffusion model on flame dynamics and soot formation. The Reynolds number based on the initial jet width and velocity is around 15, 000, corresponding to a Taylor scale Reynolds number in the range 100 ≤ Re λ ≤ 150. In one simulation, multicomponent transport based on a mixture-averaged approach is employed, while in the other the gas-phase species Lewis numbers are set equal to unity. The statistics of temperature and major species obtained with the mixture-averaged formulation are very similar to those in the unity Lewis number case. In both cases, the statistics of temperature are captured with remarkable accuracy by a laminar flamelet model with unity Lewis numbers. On the contrary, a flamelet with a mixture-averaged diffusion model, which corresponds to the model used in the multi-component diffusion three-dimensional DNS, produces significant differences with respect to the DNS results. The total mass of soot precursors decreases by 20–30% with the unity Lewis number approximation, and their distribution is more homogeneous in space and time. Due to the non-linearity of the soot growth rate with respect to the precursors’ concentration, the soot mass yield decreases by a factor of two. Being strongly affected by coagulation, soot number density is not altered significantly if the unity Lewis number model is used rather than the mixture-averaged diffusion. The dominant role of turbulent transport over differential diffusion effects is expected to become more pronounced for higher Reynolds numbers.

The interaction between scales is investigated in a turbulent mixing layer. The large-scale amplitude modulation of the small scales already observed in other works depends on the crosswise location. Large-scale positive fluctuations correlate with a stronger activity of the small scales on the low speed-side of the mixing layer, and a reduced activity on the high speed-side. However, from physical considerations we would expect the scales to interact in a qualitatively similar way within the flow and across different turbulent flows. Therefore, instead of the large-scale fluctuations, the large-scale gradients modulation of the small scales has been additionally investigated. INTRODUCTION AND MOTIVATION In the present work the interaction between turbulence scales in a mixing layer is investigated at relatively high Reynolds number (1). The interaction between the large and the small scales has been the topic of several studies. Recent investigations showed that in a boundary layer the large scales of turbulence modulate the small-scale motions (2) (3). Close to the wall, large-scale positive fluctuations are associated with a stronger activity of the small-scale motions, whereas they are related with reduced small-scale activity in the outer region. The top-down interaction between large and small scales presents an important phase delay, and therefore it is not concurrent (4). These findings were based on time series from hot-wire anemometry. The interaction between scales was studied experimentally in a jet by Fiscaletti et al. (5), using hot-wire anemometry, and PIV. It was found that the small-scale signal is stronger in amplitude if it is conditioned on positive large-scale fluctuations. Surprisingly, the strength of the small-scale amplitude modulation in the spatial signal obtained with PIV was only 25% of the value obtained from the time signal from hot-wire anemometry. The elevated level of the amplitude modulation from hot-wire anemometry was attributed to the fixed spectral band filter used to obtain the large- and the small-scale signals, which does not consider the local convection velocity. Moreover, the inhomogeneous distribution of the structures of vorticity, and their preferential location in high velocity regions of the flow could explain the spatial amplitude modulation. Buxton & Ganapathisubramani (2014) (6) investigated experimentally the fully developed region of a mixing layer. They found that negative large-scale fluctuations coincide with regions characterized by a high amplitude of the small-scale signal. Large and small scales appear therefore to interact similarly to the outer region of the turbulent boundary layer. The work did not consider different crosswise locations within the mixing layer. Therefore, the first goal of the present study is the investigation of the interaction between the large and the small scales of turbulence at different crosswise locations in the mixing layer. A strong flow dependency with respect to the nature of the scale interaction, i.e. increased vs decreased small activity with positive large scale fluctuations, has been observed (Bandyopadhyay & Hussein 1984, (2)). In addition, the location within the same flow seems to affect large-scale amplitude modulation. On the other hand, this appears at conflict with the classical theories of turbulence, according to which the transfer of turbulent kinetic energy across the scales is a universal process. Then, we would expect the scale interactions to be qualitatively similar across flows. Recent findings have related the activity of the viscous scales to the large-scale shear layers within the flow ((7), (8)). Large-scale shear layers, characterized by strong velocity gradients are expected to play an important role in the cascade of turbulent kinetic energy. As a second goal of this work, we intend to examine possible interactions between the large-scale gradients (rather than the large-scale velocity used before) and the small scales of turbulence, by determining the correlation between the large-scale gradients and the local activity of the small scales (local enstrophy). METHODS

The effect of Damkohler number on turbulent nonpremixed sooting flames is investigated via large scale direct numerical simulation in three-dimensional n -heptane/air jet flames at a jet Reynolds number of 15,000 and at three different Damkohler numbers. A reduced chemical mechanism, which includes the soot precursor naphthalene, and a high-order method of moments are employed. At the highest Damkohler number, local extinction is negligible, while flames holes are observed in the two lowest Damkohler number cases. Compared to temperature and other species controlled by fuel oxidation chemistry, naphthalene is found to be affected more significantly by the Damkohler number. Consequently, the overall soot mass fraction decreases by more than one order of magnitude for a fourfold decrease of the Damkohler number. On the contrary, the overall number density of soot particles is approximately the same, but its distribution in mixture fraction space is different in the three cases. The total soot mass growth rate is found to be proportional to the Damkohler number. In the two lowest Da number cases, soot leakage across the flame is observed. Leveraging Lagrangian statistics, it is concluded that soot leakage is due to patches of soot that cross the stoichiometric surface through flame holes. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames.

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.

The thin interface separating the inner turbulent region from the outer irrotational fluid is analysed in a direct numerical simulation of a spatially developing turbulent mixing layer. A vorticity threshold is defined to detect the interface separating the turbulent from the non-turbulent regions of the flow, and to calculate statistics conditioned on the distance from this interface. The conditional statistics for velocity are in remarkable agreement with the results for other free shear flows available in the literature, such as turbulent jets and wakes. In addition, an analysis of the passive scalar field in the vicinity of the interface is presented. It is shown that the scalar has a jump at the interface, even stronger than that observed for velocity. The strong jump for the scalar has been observed before in the case of high Schmidt number (Sc). In the present study, such a strong jump is observed for a scalar with Sc ≈ 1. Conditional statistics of kinetic energy and scalar dissipation are presente...

A Lagrangian particle scheme is applied to the solution of soot dynamics in turbulent nonpremixed flames. Soot particulate is described using a method of moments and the resulting set of continuum advection-reaction equations is solved using the Lagrangian particle scheme. The key property of the approach is the independence between advection, described by the movement of Lagrangian notional particles along pathlines, and internal aerosol processes, evolving on each notional particle via source terms. Consequently, the method overcomes the issues in Eulerian grid-based schemes for the advection of moments: errors in the advective fluxes pollute the moments compromising their realizability and the stiffness of source terms weakens the stability of the method. The proposed scheme exhibits superior properties with respect to conventional Eulerian schemes in terms of stability, accuracy, and grid convergence. Taking into account the quality of the solution, the Lagrangian approach can be computationally more economical than commonly used Eulerian schemes as it allows the resolution requirements dictated by the different physical phenomena to be independently optimized. Finally, the scheme posseses excellent scalability on massively parallel computers.