Interaction-by-exchange-with-the-mean Research Articles

Reactive mixing in rotating packed beds: On the packing’s role and mixing modeling

Rotating packed beds (RPBs) are some of the most promising modular and intensified apparatuses for continuous mixing of large liquid streams. In this study, the influence of packing depth, liquid distribution, and volumetric ratio on mixing processes are for the first time investigated in detail. It is shown that the packing seems to be more important at low volumetric ratios and an overlapping liquid distribution, but less important – or even interfering with fast mixing – at higher volumetric ratios and an opposite liquid distribution. Based on these findings, a promising novel type of HiGee equipment is suggested, that is particularly suitable for fast mixing at high volumetric ratios. Additionally, the linear and the exponential approaches of the incorporation model, as well as of the Interaction-by-Exchange-with-the-Mean (IEM) model, are investigated for their suitability to accurately describe the mixing process. On the basis of revised kinetics for the Dushman reaction, micromixing time parameters in the order of 10−1 ms to 101 ms are obtained for the investigated operating conditions.

RANS modelling of a lifted hydrogen flame using eulerian/lagrangian approaches with transported PDF method

In this study, the eulerian probability density function approach (EPDF) has been applied to simulate turbulent diffusion flame in a Vitiated Coflow. EPDF is the eulerian method to solve the PDF transport equations with a direct-quadrature-method-of-moments (DQMOM) closure. The PDF transport equation represented by a set of governing equations for the probability of occurrence in any environmental condition and the probability weighted species mass fractions, which are solved in an eulerian solution [1]; it is considered a product of the delta function, in order to model the turbulence–chemistry interaction. Among these advantages are the prediction and the kinetic control of the species such as CO and NOX. Even though the EPDF approach has been improved in recent years, most improvements have been achieved with parametric study in order to investigate the impact of the model accuracy. The main objective of this investigation is the numerical evaluation of the PDF approach accuracy, using different mixing models and turbulence models, to predict the lift-off height, the extinction and ignition of the flame. Another study comparative of eulerian (EPDF) and lagrangian monte-carlo (LPDF) was applied by equivalent physical models and numerical parameters for evaluating the performances of each approach such as precision and computational specification. The chosen mixture model is the IEM (Interaction by Exchange with the Mean) for micro-mixing closure. The number of environment in the eulerian approach EPDF is (2.0). Through the use of a dynamic model for the mixing time-scale, by computing the individual time-scales for the reactive scalars dynamically in each cell during the course of the simulation using the ANSYS-Fluent/MM-INTAS CFD codes and the chemical reaction mechanism injected is GRI mech 2.1. The results such as mixture fraction, temperature, species mass fraction is validated with experimental data and discussed.

Modeling and numerical study of H2/N2 jet flame in vitiated co-flow using Eulerian PDF transport approach

The multi-environment Eulerian approach (MEPDF) is the eulerian method to solve the PDF transport equations; it is considered a product of the delta function. Its advantages are the prediction of extinction and ignition of the flame, also the kinetic control of the species as CO and NOX. Even though the MEPDF approach has been improved in recent years, most improvements have been achieved with parametric study in order to investigate the impact of the model accuracy. The main objective of this work is to improve further the model accuracy, the prediction of the lift off height by a parametric study of the mixing constant and Schmidt number and to understand its impact in flame stabilization. The numerical investigation of H2/N2 jet flame in vitiated co-flow is presented using MEPDF approach. The study was applied with K-epsilon modified model of turbulence. The chosen mixture model is the IEM (Interaction by Exchange with the Mean). The number of environment in the multi-environment Eulerian approach MEPDF is (2.0). The model was solved in this work by the commercial CFD code, ANSYS fluent and the chemical reaction mechanism injected is GRI mech 2.1. The results are validated with experimental data and discussed.

Multi-environment Probability Density Function Modeling for Turbulent CH4 Flames under Moderate or Intense Low-Oxygen Dilution Combustion Conditions with Recirculated Flue Gases

The present study has adopted the multi-environment probability density function (MEPDF) approach to simulate the turbulent CH4 flames under flameless combustion conditions with recirculated flue gases. The MEPDF approach is based on an Eulerian PDF formulation with computational efficiency. Micromixing can be represented via the IEM (interaction by exchange with the mean) model, and the chemistry is based on the GRI 2.11 mechanism including the NOx chemistry. Special emphasis is given to the effects of the micromixing model constants on the structure and characteristics of recirculated moderate or intense low-oxygen dilution (MILD) combustion processes. In terms of the temperature and species mole fraction, the multi-environment PDF model with a micromixing constant of 0.5 yields reasonably good agreement with experimental data. In terms of the integrated NO production rate for the MILD combustion condition, the N2O path yields the highest level, followed by the prompt, reburn, NO2, thermal, and NNH mech...

Analysis of Mixing Models for use in Simulations of Turbulent Spray Combustion

The present study concerns the investigation of different mixing models for use in the transported probability density function (PDF) modeling of turbulent (reacting) spray flows. The modeling of the turbulent mixing and other characteristic scalar variables such as gas enthalpy using transported (joint) PDFs has become an important method to describe turbulent (reacting) spray flows since the evaporation process causes the PDF of the mixture fraction to deviate from the widely used β function, which is typically used in models for turbulent gas flows. In the PDF transport equation, the molecular mixing does not appear in closed form so that modeling strategies are required. For gas combustion, the interaction-by-exchange-with-the-mean (IEM) model, the modified Curl (MC) model, and the Euclidean minimum spanning tree (EMST) models are used. More recently, a new mixing model, the PSP model, which is based on parameterized scalar profiles has been developed. The present study focuses on the use and analysis of the IEM, MC and PSP models for turbulent spray flames. For this purpose, the models are reconsidered with respect to the evaporation process that must be included and evaluated if spray combustion is considered. For model evaluation, turbulent ethanol/air spray flames are simulated, and the results are compared to experimental data by A. Masri, University of Sydney, Australia.

A spray flamelet/progress variable approach combined with a transported joint PDF model for turbulent spray flames

A spray flamelet/progress variable approach is developed for use in spray combustion with partly pre-vaporised liquid fuel, where a laminar spray flamelet library accounts for evaporation within the laminar flame structures. For this purpose, the standard spray flamelet formulation for pure evaporating liquid fuel and oxidiser is extended by a chemical reaction progress variable in both the turbulent spray flame model and the laminar spray flame structures, in order to account for the effect of pre-vaporised liquid fuel for instance through use of a pilot flame. This new approach is combined with a transported joint probability density function (PDF) method for the simulation of a turbulent piloted ethanol/air spray flame, and the extension requires the formulation of a joint three-variate PDF depending on the gas phase mixture fraction, the chemical reaction progress variable, and gas enthalpy. The molecular mixing is modelled with the extended interaction-by-exchange-with-the-mean (IEM) model, where source terms account for spray evaporation and heat exchange due to evaporation as well as the chemical reaction rate for the chemical reaction progress variable. This is the first formulation using a spray flamelet model considering both evaporation and partly pre-vaporised liquid fuel within the laminar spray flamelets. Results with this new formulation show good agreement with the experimental data provided by A.R. Masri, Sydney, Australia. The analysis of the Lagrangian statistics of the gas temperature and the OH mass fraction indicates that partially premixed combustion prevails near the nozzle exit of the spray, whereas further downstream, the non-premixed flame is promoted towards the inner rich-side of the spray jet since the pilot flame heats up the premixed inner spray zone. In summary, the simulation with the new formulation considering the reaction progress variable shows good performance, greatly improving the standard formulation, and it provides new insight into the local structure of this complex spray flame.

Effects of molecular transport in LES/PDF of piloted turbulent dimethyl ether/air jet flames

We report large-eddy simulation (LES)/probability density function (PDF) modeling of piloted turbulent dimethyl ether (DME)/air jet flames with a skeletal chemical mechanism. The modeled PDF transport equation with three different implementations of molecular transport in the interaction-by-exchange-with-the-mean (IEM) mixing model is solved using the NGA/HPDF code in order to assess the a posteriori performance of these LES/PDF methodologies applied to the DME flames D and F proposed in the TNF Workshop. The three implementations considered are the classical random-walk model (IEM-RW), the mean-drift model with a single molecular diffusivity (IEM-MD), and the mean-drift model with differential diffusion (IEM-DD). Better quantitative agreement with experimental data for the mean and root-mean-square profiles of mixture fraction, temperature and major species mole fractions and conditional means on mixture fraction are obtained with the IEM-MD model than with the IEM-RW model. In LES of the DME flame series, the molecular diffusivity dominates the turbulent diffusivity in the centerline region, so the accurate modeling of molecular transport plays a significant role in LES/PDF of these jet flames. Nevertheless, the IEM-DD model shows only minor effects on the model predictions compared to IEM-MD in these DME flames.

Micro-mixing measurement by chemical probe in homogeneous and isotropic turbulence

The chemical probe is commonly used to evaluate the performance of chemical reactors. By a localized injection of chemical reagents, it is possible to measure the local micro-mixing, which is readily related to the selectivity of chemical reactions, since mixing at the molecular scale is the limiting factor for a wide range of chemical systems. The raw result of the chemical-probe method is a segregation index that allows comparison of different situations (various locations in the reactor, various Reynolds numbers, various geometries, etc.). Beyond the qualitative assessment provided by this segregation index, it is possible to obtain the intrinsic micro-mixing time by means of a micro-mixing model, describing the temporal evolution of a chemical reaction whose rate is governed by the micro-mixing. Here a key step is the choice of micro-mixing model. Several micro-mixing models available in the literature have been used in some specific cases without evaluating their appropriateness for the problem in hand. The main difficulty in this evaluation is that the real flows often do not fully satisfy the basic model assumptions, in particular the condition of homogeneous and isotropic turbulence (HIT).The present work aims at assessing the validity of a micro-mixing model under “ideal” experimental conditions, i.e. HIT with no mean flow, to avoid the bias due to the flow gradients. The HIT is obtained here by a system of oscillating grids placed in a vessel. The chemical probe measurements carried out by the iodide/iodate reaction system are applied to the two most commonly used phenomenological models in the literature: the IEM (Interaction by Exchange with the Mean) and the EDD (Engulfment, Deformation and Diffusion) models. The benchmark for the micro-mixing models is based on comparison of the local turbulent kinetic energy (TKE) dissipation rate both drawn from the micro-mixing time by the theoretical model of Bałdyga and the reference direct experimental determination by laser Doppler velocimetry measurements. It is shown that the engulfment model EDD seems the more appropriate to analyze the chemical data and provide a quantitative characterization of micro-mixing.

Effects of combined dimension reduction and tabulation on the simulations of a turbulent premixed flame using a large-eddy simulation/probability density function method

A turbulent lean-premixed propane–air flame stabilised by a triangular cylinder as a flame-holder is simulated to assess the accuracy and computational efficiency of combined dimension reduction and tabulation of chemistry. The computational condition matches the Volvo rig experiments. For the reactive simulation, the Lagrangian Large-Eddy Simulation/Probability Density Function (LES/PDF) formulation is used. A novel two-way coupling approach between LES and PDF is applied to obtain resolved density to reduce its statistical fluctuations. Composition mixing is evaluated by the modified Interaction-by-Exchange with the Mean (IEM) model. A baseline case uses In Situ Adaptive Tabulation (ISAT) to calculate chemical reactions efficiently. Its results demonstrate good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. For dimension reduction, 11 and 16 represented species are chosen and a variant of Rate Controlled Constrained Equilibrium (RCCE) is applied in conjunction with ISAT to each case. All the quantities in the comparison are indistinguishable from the baseline results using ISAT only. The combined use of RCCE/ISAT reduces the computational time for chemical reaction by more than 50%. However, for the current turbulent premixed flame, chemical reaction takes only a minor portion of the overall computational cost, in contrast to non-premixed flame simulations using LES/PDF, presumably due to the restricted manifold of purely premixed flame in the composition space. Instead, composition mixing is the major contributor to cost reduction since the mean-drift term, which is computationally expensive, is computed for the reduced representation. Overall, a reduction of more than 15% in the computational cost is obtained.

Transported Joint Probability Density Function Modeling of Turbulent Dilute Spray Flows

A transported joint probability density function (PDF) model for turbulent spray flows is presented, where a one-point one-time statistical description of the gas-phase mixture fraction and the gas velocity is used. This approach requires the closure of the molecular mixing, which is achieved through use of the extended interaction-by-exchange-with-the-mean (IEM) model and a simplified Langevin model for the closure of the gas velocity both of which are extended through additional terms accounting for spray evaporation. These equations require the solution of the turbulent time scales and the mean pressure field through a Eulerian description. The numerical approach includes a Lagrangian Monte Carlo method for the solution of modeled joint PDF equation with a Eulerian finite-volume algorithm to determine the turbulent time scale and the mean pressure field. For the dispersed liquid phase, Lagrangian equations are used to describe the droplet heating, evaporation, and motion in the framework of a discrete droplet model. The convective droplet evaporation model is employed, and the infinite conductivity model with consideration of non-equilibrium effects based on the Langmuir-Knudsen law is used. The droplet turbulent dispersion is modeled with two different Lagrangian stochastic models. The resulting spray evolution equations are solved by a Lagrangian discrete droplet method using the point source approximation for a dilute spray. The numerical results are compared with experimental data of Gounder et al. [1], where the experimental set B of the acetone spray flows SP2 and SP6 are simulated. Comparison of numerical and experimental results includes droplet size, liquid volume flux as well as the mean and fluctuating velocities. Generally, good agreement is achieved, although the radial droplet dispersion is somewhat under-predicted by the computations. The droplet fluctuating velocities show sensitivity to the different dispersion models.

Joint Gas-Phase Velocity-Scalar PDF Modeling for Turbulent Evaporating Spray Flows

The article proposes a joint gas-phase velocity-scalar probability density function (PDF) model for turbulent spray flows. The joint gas-phase velocity-mixture fraction PDF transport equation for turbulent spray flows is derived and modeled. The simplified Langevin model and interaction-by-exchange-with-the-mean (IEM) model are extended to account for spray evaporation, and they close the PDF transport equation, which is solved by a Lagrangian Monte-Carlo particle method, and the droplet characteristics including heating, vaporization, and motion are solved by a Lagrangian droplet parcel method using the point-source approximation. A steady, two-dimensional, axi-symmetric, turbulent methanol/air dilute non reacting spray flow without swirl is simulated using the present method. The results of the gas velocity obtained by the joint PDF method are in good agreement with experimental data, and the PDF method improves the simulation results obtained using a moment closure model.

Large-eddy simulation/probability density function modeling of a non-premixed CO/H2 temporally evolving jet flame

We report a large-eddy simulation (LES)/probability density function (PDF) study of a non-premixed CO/H2 temporally-evolving turbulent planar jet flame, which has previously been studied using direct numerical simulations (DNS) with a skeletal chemical mechanism. The flame exhibits strong turbulence–chemistry interactions resulting in local extinction followed by re-ignition. In this study, the filtered velocity field in LES and the PDF transport equations with the interaction-by-exchange with the mean (IEM) mixing model (with molecular transport) are solved by the highly-scalable NGA/HPDF codes with second-order accuracy in space and time. The performance of the hybrid LES/PDF methodology is assessed through detailed a posteriori comparisons with DNS of the same flame. The comparison shows overall good agreement of the temporal evolution of the temperature and mass fractions of major chemical species, as well as the prediction of local extinction and re-ignition. The modeling of multi-scalar mixing is analyzed using the DNS and LES/PDF results. The DNS results exhibit an attracting manifold of the streamlines of the diffusion velocity in composition space, and the LES/PDF results show qualitative agreement on the manifold and joint PDFs of compositions.

Mixing performance assessment of a multi-channel mini heat exchanger reactor with arborescent distributor and collector

A mini reactor with 16 channels and with channel diameter of 1mm or 2mm is designed, fabricated and studied experimentally. Two fluids are divided into 16 channels through distributor, then contacted at T-mixers, and finally collected. Both distributor and collector are designed following the arborescent shape. Visualization with pH indicator bromothymol blue (BTB) is used to qualitatively observe the global mixing; while for the micromixing performance assessment, we used competitive iodide/iodate (Villermaux/Dushman) reactions. Segregation index and micromixing time, with respect to energy dissipation rate are used to characterize micromixing performance.Visualization with pH indicator shows that complete global mixing at the collector by liquid impingement is obtained under our tested conditions. Iodide/iodate reaction shows a rapid mixing at molecular level as the flowrate increases and impingement being enhanced. Micromixing time estimated by IEM (Interaction by Exchange with the Mean) model is compared with other micro-structured mixers in the literature. Comparable mixing performance is obtained using our millimetric reactor with that of micro-structured designs.The design of multi-channel reactor with arborescent distributor and collector makes possible the numbering-up of multiple channels. Process intensification is highlighted by high throughput and compact size, thus may be attractive for industrial application.

A 3D Lagrangian micromixing dispersion model LAGFLUM and its validation with a wind tunnel experiment

A stochastic numerical model, LAGFLUM (LAGrangian FLUctuation Model), is developed to determine the mean and the fluctuation statistics of scalar fields generated in 3D turbulent flows from continuous sources. The model couples a macromixing scheme with a micromixing scheme. The macromixing scheme, based on the so called “well-mixed” criterion, is utilized to evaluate the mean concentration, while the micromixing IECM (Interaction by Exchange with the Conditional Mean) scheme is integrated to calculate the higher statistical moments of the concentration. The model LAGFLUM is tested by comparison with the MUST (Mock Urban Setting Test) wind tunnel experiment, which is the laboratory counterpart of the original (real-atmosphere) MUST experiment. In both these experiments the dispersion of a passive tracer in a 3D flow field, in the presence of obstacles, is measured. Using a flow field reconstructed from the experimental flow measurements, the ensemble mean, standard deviation, intensity of fluctuations and skewness of the concentration predicted by LAGFLUM are compared with the available measurements. The results show a generally good agreement between the observations and the simulation, in particular for the mean and the standard deviation of the concentration. Predictions obtained with the flow field replaced by a flow field calculated using a numerical model and with the IECM model replaced by the IEM (Interaction by Exchange with the Mean) model are also compared with the measurements.

A PDF method for HCCI combustion modeling : CPU time optimization through a restricted initial distribution

Probability Density Function (PDF) is often selected to couple chemistry with turbulence for complex reactive flows since complex reactions can be treated without modeling assumptions. This paper describes an investigation into the use of the particles approximation of this transport equation approach applied to Homogeneous Charge Compression Ignition (HCCI) combustion. The model used here is an IEM (Interaction by Exchange with the Mean) model to describe the micromixing. Therefore, the fluid within the combustion chamber is represented by a number of computational particles. Each particle evolves function of the rate of change due to the chemical reaction term and the mixing term. The chemical reaction term is calculated using a reduced mechanism of n-heptane oxidation with 25 species and 25 reactions developed previously. The parametric study with a variation of the number of particles from 50 up to 104 has been investigated for three initial distributions. The numerical experiments have shown that the hat distribution is not appropriate and the normal and lognormal distributions give the same trends. As expected when the number of particles increases for homogenous mixture (i.e. high turbulence intensity), the in-cylinder pressure evolution tends towards the homogeneous curve. For both homogeneous and inhomogeneous (i.e. low turbulence intensity) cases, we have found that 200 particles are sufficient to model correctly the system, with a CPU time of a few minutes when a restriction of initial distribution is adopted.

A comparison between IECM and IEM Lagrangian models

A three-dimensional Lagrangian stochastic model to evaluate pollutant dispersion in terms of both mean and fluctuating concentration fields has been developed. The mean concentration field is predicted by using a macromixing model, while the micromixing scheme IECM (Interaction by Exchange with the Conditional Mean) is adopted to determine the moments of concentration. The results agree with experimental data and with other numerical studies found in literature. The performance of a second micromixing scheme, namely, the IEM (Interaction by Exchange with the Mean), is tested. Some inaccuracies of the IEM model are evaluated by comparison against measurements and IECM results.

Micromixing effects on series–parallel and autocatalytic reactions in a turbulent single-phase gas flow

A hybrid solution algorithm is implemented to simulate turbulent reactive single-phase gas flow in an isothermal tubular reactor. This algorithm is a combination of a Finite Volume (FV) and a Probability Density Function (PDF) method. The FV method is used to solve the mass and momentum conservation equations combined with a standardized k– ε model for single-phase gas flow. The PDF method is applied to solve the species continuity equations. The advantage of using the PDF method is the fact that there is no need of any closures for chemical reaction source terms in a turbulent flow . The mesomixing and the micromixing contributions in the PDF equation are closed using the gradient-diffusion model and the Interaction-by-Exchange-with-the-Mean (IEM) model, respectively. This hybrid solution algorithm is applied to simulate a series–parallel and an autocatalytic reactive single-phase gas flow. The mechanical-to-scalar time-scale ratio, i.e. the IEM model parameter, is found to have an influence on the simulation results that cannot be neglected. As expected, in series–parallel reactions, the desired product selectivity increases when the reaction rate coefficient, corresponding to its formation, increases. Moreover multiple solutions are observed in the autocatalytic reaction for a given feed ratio, Damköhler (Da I ) and Péclet (Pe) numbers. To validate the hybrid FV–PDF solution algorithm, the calculated results are compared with the results obtained when using the Reynolds-averaged species continuity equation model. A good agreement is observed when infinite-rate mixing is applied.

Simulation of a turbulent spray flame using coupled PDF gas phase and spray flamelet modeling

A joint mixture fraction–enthalpy probability density function (PDF) is proposed for the simulation of turbulent spray flames. The PDF transport equation is deduced and modeled. The interaction-by-exchange-with-the-mean (IEM) model that has been developed for gas-phase flows is extended to describe molecular mixing in nonreactive and reactive spray flows. The joint PDF transport equation is solved by a hybrid finite-volume and Lagrangian Monte Carlo method. Standard spray and turbulence models are used to describe the gas phase and the liquid phase. A turbulent methanol/air spray flame is simulated using the present method. Detailed chemistry is implemented through the spray flamelet model. The precalculated spray flamelet library for methanol/air combustion comprises 23 species and 168 elementary reactions. Thus, the model is capable of predicting the formation of radicals and of pollutants. Different values for the model constant C ϕ in the IEM model are tested. The numerical results for the gas velocity, the gas temperature, and the mass fraction of methanol vapor are compared with experimental data in the literature. Good agreement with experiment is obtained when C ϕ = 2.0 . Marginal PDFs of mixture fraction, enthalpy, and gas temperature are presented. The computed PDFs of mixture fraction are compared with the presumed standard β function and modified β function. The results show that the standard β function fails to reproduce bimodal shapes observed in transported PDF computation, while the modified β function, fits the computed PDFs very well. Moreover, joint PDFs of mixture fraction and enthalpy are presented and analyzed. The enthalpy and mixture fraction are strongly correlated. The samples that deviate from the linear correlation are due to the energy consumption of local spray evaporation.

Multi-environment probability density function method for modelling turbulent combustion using realistic chemical kinetics

A computational fluid dynamics (CFD) tool for performing turbulent combustion simulations that require finite-rate chemistry is developed and tested by modelling a series of bluff-body stabilized flames that exhibit different levels of finite-rate chemistry effects ranging from near equilibrium to near global extinction. The new modelling tool is based on the multi-environment probability density function (MEPDF) methodology and combines the following: the direct quadrature method of moments (DQMOM); the interaction-by-exchange-with-the-mean (IEM) mixing model; and realistic combustion chemistry. Using DQMOM, the MEPDF model can be derived from the transport PDF equation by depicting the joint composition PDF as a weighted summation of a finite number of multi-dimensional Dirac delta functions in the composition space. The MEPDF method with multiple reactive scalars retains the unique property of the joint PDF method of treating chemical reactions exactly. However, unlike the joint PDF methods that typically must resort to particle-based Monte-Carlo solution schemes, the MEPDF equations (i.e. the transport equations of the weighted delta-peaks) can be solved by traditional Eulerian grid-based techniques. In the current study, a pseudo time-splitting scheme is adopted to solve the MEPDF equations; the reaction source terms are computed with a highly efficient and accurate in-situ adaptive tabulation (ISAT) algorithm. A 19-species reduced mechanism based on quasi-steady state assumptions is used in the simulations of the bluff-body flames. The modelling results are compared with the experimental data, including mixing, temperature, major species and important minor species such as CO and NO. Compared with simulations using a Monte-Carlo joint PDF method, the new approach shows comparable accuracy.

A consistent LES/filtered-density function formulation for the simulation of turbulent flames with detailed chemistry

A hybrid large-Eddy simulation/filtered-density function (LES–FDF) methodology is formulated for simulating variable density turbulent reactive flows. An indirect feedback mechanism coupled with a consistency measure based on redundant density fields contained in the different solvers is used to construct a robust algorithm. Using this novel scheme, a partially premixed methane/air flame is simulated. To describe transport in composition space, a 16-species reduced chemistry mechanism is used along with the interaction-by-exchange with the mean (IEM) model. For the micro-mixing model, typically a constant ratio of scalar to mechanical time-scale is assumed. This parameter can have substantial variations and can strongly influence the combustion process. Here, a dynamic time-scale model is used to prescribe the mixing time-scale, which eliminates the time-scale ratio as a model constant. Two different flame configurations, namely, Sandia flames D and E are studied. Comparison of simulated radial profiles with experimental data show good agreement for both flames. The LES–FDF simulations accurately predict the increased extinction near the inlet and re-ignition further downstream. The conditional mean profiles show good agreement with experimental data for both flames.