Flamelet Progress Variable Approach Research Articles

Evaluation of coal ammonia flames using a non-adiabatic three mixture fraction flamelet progress variable approach

A highly resolved large eddy simulation (LES) modeling of the co-firing of ammonia with pulverized coal in a single-burner test furnace of the Central Research Institute of Electric Power Industry (CRIEPI) has been performed using a non-adiabatic three mixture fraction flamelet/progress variable (3Z-FPV) approach. To parametrize the thermochemical space, a six-dimensional tabulation method based on non-premixed flamelets is introduced, which uses three mixture fractions, for ammonia, coal volatiles, and char-off gases, respectively, as well as variance of mixture fraction, reaction progress variable, and total enthalpy. The accuracy of non-adiabatic model has been tested by comparing it with experimental data and previously computed adiabatic FPV results for two different ammonia injection cases. The results show that the non-adiabatic FPV model accurately predicted the peak temperature and its location, while the peak NO, overpredicted by approximately 150 % in the adiabatic FPV model, is slightly underpredicted in the non-adiabatic FPV model. Ammonia injection in the primary reaction zone lowered NO concentrations in both cases, primarily due to dominant NO reduction reactions with ammonia under the reducing environment. Reactive conditions and higher temperature caused NH3 dissociation, increasing H2 and H2O concentrations near burner exit in burner ammonia injection case.

Pressure effects on soot formation and evolution in turbulent jet flames

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

Flamelet modeling of thermo-diffusively unstable hydrogen-air flames

In order to reduce CO2 emissions, hydrogen combustion has become increasingly relevant for technical applications. In this context, lean H2-air flames show promising features but, among other characteristics, they tend to exhibit thermo-diffusive instabilities. The formation of cellular structures associated with these instabilities leads to an increased flame surface area which further promotes the flame propagation speed, an important reference quantity for design, control, and safe operation of technical combustors. While many studies have addressed the physical phenomena of intrinsic flame instabilities in the past, there is also a demand to predict such flame characteristics with reduced-order models to allow computationally efficient simulations. In this work, a H2-air spherical expanding flame, which exhibits thermo-diffusive instabilities, is studied with flamelet-based modeling approaches both in a-priori and a-posteriori manner. A recently proposed Flamelet/Progress Variable (FPV) model, with a manifold based on unstretched planar flames, and a novel FPV approach, which takes into account a large curvature variation in the tabulated manifold, are compared to detailed chemistry (DC) calculations. Both flamelet approaches account for differential diffusion utilizing a coupling strategy which is based on the transport of major species instead of transporting the control variables of the manifold. First, both FPV approaches are assessed in terms of an a-priori test with the DC reference dataset. Thereafter, the a-posteriori assessment contains two parts: a linear stability analysis of perturbed planar flames and the simulation of the spherical expanding flame. Both FPV models are systematically analyzed considering global and local flame properties in comparison to the DC reference data. It is shown that the new FPV model, incorporating large curvature variations in the manifold, leads to improved predictions for the microstructure of the corrugated flame front and the formation of cellular structures, while global flame properties are reasonably well reproduced by both models.

Ignition under strained conditions: Unsteady flamelet progress variable modeling for diesel engine conditions in the transient counterflow configuration

The ignition of non-premixed n-dodecane-air flames is studied in a highly transient counterflow configuration under diesel engine operating conditions. The one-dimensional transient counterflow is configured such that ignition is initially physically inhibited through high strain rates. In a short relaxation period the inflow velocities (thereby the strain rate) are lowered and ignition can occur, a behaviour similarly observed in spray flames. The non-premixed flame, which is igniting in this unsteady flow environment, is computed with direct chemistry and tabulated chemistry based on the unsteady flamelet progress variable approach (UFPV). The flamelet look-up table is constructed from igniting unsteady flamelets considering also the parameter range between stable and unstable branches of the characteristic S-shaped curve which is obtained via a continuation method. The mixture fraction, the progress variable, and the local stoichiometric scalar dissipation rate serve as control variables for the three-dimensional table. Particular attention is put on the inclusion of thermal expansion effects and the treatment of the progress variable source term to capture the onset of ignition. Besides varying strain effects, the igniting flame exhibits low temperature chemistry and multi-stage ignition behaviour which is particular challenging for the UFPV modeling. By direct comparison to the reference solution the predictive capabilities of the UFPV approach are analyzed and assessed in detail. Furthermore, the influence of table resolution on computational efficiency is discussed. The systematic analysis and validation of the UFPV approach in the transient counterflow configuration fills a specific gap in the literature and underlines the fidelity of the approach for flames subject to complex transient processes. Furthermore, the 1D configuration may serve as an environment for adjusting and optimizing UFPV tabulation strategies prior to their application to more complex 3D CFD simulations such as spray LES.

Evaluation of the unsteady flamelet progress variable approach in Large Eddy Simulations of the ECN Spray A

Within the Unsteady Flamelet Progress Variable – Large Eddy Simulation (UFPV-LES) approach the local scalar dissipation rate represents one key parameter, significantly affecting the ignition behaviour. In this study, the UFPV-LES approach is evaluated for ECN Spray A baseline conditions, relevant for diesel engines. After confirming its general applicability, using experimental data under non-reacting and reacting conditions, special attention is paid to the distribution of the local scalar dissipation rate. Based on the findings of this analysis, a reduced modeling approach, considering only igniting flamelets starting from the adiabatic mixing line between the fuel and oxidizer, is investigated. The performance of this reduced approach is assessed systematically, using the UFPV-LES results as a reference. Based on an a-priori analysis, regions affected by the model reduction are identified and evaluated. A subsequent evaluation in an a-posteriori analysis, i.e. a coupled LES, reveals similar results in terms of local flame structure as well as global ignition characteristics and confirms the applicability of the reduced model under the ECN Spray A baseline conditions.

CFD Modeling of Reacting Diesel Sprays with Primary Reference Fuel

<div class="section abstract"><div class="htmlview paragraph">Computational fluid dynamics (CFD) modeling has many potentials for the design and calibration of modern and future engine concepts, including facilitating the exploration of operation conditions and casting light on the involved physical and chemical phenomena. As more attention is paid to the matching of different fuel types and combustion strategies, the use of detailed chemistry in characterizing auto-ignition, flame stabilization processes and the formation of pollutant emissions is becoming critical, yet computationally intensive. Therefore, there is much interest in using tabulated approaches to account for detailed chemistry with an affordable computational cost. In the present work, the tabulated flamelet progress variable approach (TFPV), based on flamelet assumptions, was investigated and validated by simulating constant-volume Diesel combustion with primary reference fuels - binary mixtures of n-heptane and iso-octane. Simulations were initially carried out to evaluate and compare the performance of two kinetic models in homogeneous reactors and laminar diffusion flames, followed by turbulent reacting spray simulations considering different fuels, ambient temperatures, and oxygen concentrations. The sensitivity study of the turbulent Schmidt number was then conducted, and results in terms of ignition delay and lift-off length were compared with experimental data to determine a more appropriate global constant. Finally, parametric variations of ambient temperature and oxygen concentration were performed for six fuel blends ranging from PRF0 (n-heptane) to PRF100 (iso-octane), confirming the validity of the TFPV model.</div></div>

Application of flamelet/progress-variable approach to the large eddy simulation of a turbulent jet flame of pulverized coals

In this study, the flamelet/progress-variable (FPV) approach was applied to a large eddy simulation of a pulverized coal jet flame. The FPV approach considers the characteristics of the pulverized coal flame, e.g., non-adiabatic system and several types of fuel streams, via additional representative variables. First, the applicability of the FPV approach to a turbulent flame with pulverized coals was confirmed through a comparison of the numerical solutions and experimental data. In this study, the pure pilot case was also investigated to clarify the effects of pulverized coals on the flame. The flame structure changes significantly upon the injection of pulverized coals, and the flame index suggests the coexistence of premixed and diffusion combustion modes even in the downstream region. In particular, the combustion mode fluctuates with time in the middle region of the flame. The fuel gas released from the pulverized coals should increase in this region; therefore, the release and combustion behavior of the volatile matter must be involved in the combustion mode variation. The evaluation of the combustion modes of fuel gas in the coal flame is useful for the design and optimization of pulverized coal combustors with next-generation technologies.

Hybrid Flamelet/Progress Variable Approach for NO Prediction in Pulverized Coal Flames

The flamelet/progress variable (FPV) approach is further extended for a better prediction of NO emission in pulverized coal flames. The predicted results are compared with detailed chemistry simulation solutions for model evaluation. The results show that the original FPV approach with a conventional progress variable consisting of major species can predict the gas temperature and major products well but can roughly predict the NO variation. Therefore, the extended FPV approach with a modified progress variable including both NO and major species is further evaluated. The results indicate that the prediction accuracy of NO is sensitive to the proportion of NO introduced in the modified progress variable. The best prediction of NO can be obtained with the modified progress variable where the concentration of NO is comparable to that of all major species. However, the predictions of other quantities of interest are simultaneously deteriorated to some extent. Based on these analyses, a hybrid FPV approach is developed in which two flamelet libraries are constructed with conventional and modified progress variables, respectively. The results indicate that the hybrid FPV approach can predict both NO and other quantities of interest quite well at the same time under various operating conditions of strain rate, coal particle diameter, coal mass flow rate, and initial temperature.

Large eddy simulations and analysis of NO emission characteristics in a laboratory pulverized coal flame

Large eddy simulation of a lab-scale turbulent pulverized coal jet flame is conducted using the extended flamelet/progress variable (FPV) approach, where normalized total enthalpy, char conversion parameter, progress variable, coal mixture fraction and coal mixture fraction variance are selected as the control parameters for a five-dimensional flamelet library. Two ways of fuel-N partitioning between volatiles and char are applied for NO formation in two cases, respectively. The model performance is evaluated by comparing with the experimental data. The results indicate that the present model can well reproduce the pulverized coal flame. Reasonably good agreements on particle dispersion and velocity, temperature as well as major species distributions are obtained. In addition, the distributions of NO obtained with different treatments are analyzed qualitatively as well as quantitatively. The results indicate that the extended FPV approach shows an advantage over the conventional post-processing approach. When reasonable fuel-N partitioning is considered, a more accurate NO prediction can be achieved. Finally, combustion and NO emission characteristics are further discussed.

Large-eddy simulation of hydrothermal flames using extended flamelet/progress variable approach

The primary purpose of this paper is to develop high-fidelity combustion models for hydrothermal flames. Based on the flamelet/progress variable (FPV) model, comprehensive real-fluid models are incorporated to treat the thermodynamic and transport properties. To account for the heat loss effects, the FPV model is extended to be non-adiabatic with the heat release damping (HRD) approach. Combined with a presumed probability density function closure, the model is then evaluated in laboratory-scale hydrothermal flames in the context of large-eddy simulation. Results show that the prediction from the FPV model is better than those from the EDM and EDC-MTS models, since it correctly predicts the flame lift-off height as well as the maximum temperature position. The wall heat loss has significant influence on the temperature and species distributions. The use of the non-adiabatic FPV approach improve considerably the predictions of the flame temperature by comparing with the experimental data.

Flame Structure Analysis and Flamelet/Progress Variable Modelling of DME/Air Flames with Different Degrees of Premixing

Laminar dimethyl ether (DME) coflow jet flames, defined by different levels of partial premixing in the fuel stream, provide canonical test cases for studying conditions in between perfectly premixed and non-premixed characteristics. This work investigates the structure of DME/air flames and the evaluation of suitable flamelet modeling approaches. Three laminar, partially premixed DME/air flames are studied using fully resolved numerical simulations and the flamelet/progress variable (FPV) approach. The flame structure of the partially premixed DME/air flames is discussed and selected slices are compared to each other as well as to Raman/Rayleigh data. Overall, the experimental data is well predicted and the local flame structure is represented by the fully resolved simulations. In the context of the FPV approach, an a priori analysis of the underlying tabulated manifold is carried out. Lookup tables based on strained counterflow flames were identified as the most suitable candidate for the fully coupled FPV simulations of all three partially premixed DME/air flames. The FPV results are further compared a posteriori to the fully resolved simulation data. The flame structure of the partially premixed DME/air flames was reproduced and good results were obtained for the temperature and the species mass fractions. This numerical investigation contributes to the understanding of local flame structures in partially premixed DME/air flames and has the potential to support model selection in complex combustion processes.

Evaluation of a flamelet/progress variable approach for pulverized coal combustion in a turbulent mixing layer

A steady flamelet/progress variable (FPV) approach for pulverized coal flames is employed to simulate coal particle burning in a turbulent shear and mixing layer. The configuration consists of a carrier-gas stream of air laden with coal particles that mixes with an oxidizer stream of hot products from lean combustion. Carrier-phase DNS (CP-DNS) are performed, where the turbulent flow field is fully resolved, whereas the coal is represented by Lagrangian point particles. CP-DNS with direct chemistry integration is performed first and provides state-of-the-art validation data for FPV modeling. In a second step the control variables for FPV are extracted from the CP-DNS and used to test if the tabulated manifold can correctly describe the reacting flow (a priorianalysis). Finally a fully coupled a posteriori FPV simulation is performed, where only the FPV control variables are transported, and the chemical state is retrieved from the table and fed back to the flow solver. The a priori results show that the FPV approach is suitable for modeling the complex reacting multiphase flow considered here. The a posteriori data is similarly in good agreement with the reference CP-DNS, although stronger deviations than a priori can be observed. These discrepancies mainly appear in the upper flame (of the present DNS), where premixing and highly unsteady extinction and re-ignition effects play a role, which are difficult to capture by steady non-premixed FPV modeling. However, the present FPV model accurately captures the lower, more stable flame that burns in non-premixed mode.

Evaluation of real-fluid flamelet/progress variable model for laminar hydrothermal flames

In this article, hydrothermal flames were studied and an extended flamelet model was developed accounting for the real-fluid effects. The newly formulated real-fluid flamelet/progress variable (FPV) model was evaluated in a laminar counterflow configuration through both a priori and a posteriori analyses. At supercritical conditions, the ideal-gas model results showed a higher flame temperature and intermediate species mass fractions compared to the real-fluid model results in both the detailed chemistry (DC) and FPV approaches. To thoroughly evaluate the real-fluid flamelet library, the a priori tests were carried out under atmospheric and supercritical conditions with different strain rates. The comparisons of tabulated quantities and reference values from DC simulations showed that the real-fluid FPV model can correctly predict the thermal properties and species mass fractions in hydrothermal flames. Finally, the a posteriori analysis showed overall good agreement between the FPV and DC results, verifying the newly proposed real-fluid FPV approach.

The role of tangential diffusion in evaluating the performance of flamelet models

For non-premixed combustion, the steady laminar flamelet model (SLFM) and flamelet/progress variable approach (FPVA) are two popular methods for tabulating flamelet manifolds. Even if the two methods are used to tabulate and parameterize the same flamelet database, their results sometimes differ in the subsequent simulation. In this work, a novel perspective is provided to assess the performance of the SLFM and FPVA. Both approaches are compared with respect to their capabilities to capture tangential diffusion (TD) of the thermochemical state variables along iso-surfaces of mixture fraction. The relevance of TD effects is identified using generalized flamelet equations and regimes by comparing flamelet solutions with and without TD terms to a FTC (full transport and chemistry) solution of a well-known non-premixed coflow flame. It is found that TD effects can play an important role in entire mixture fraction space, even in the classical flamelet regime. This suggests that the ability to characterize TD effects is an important performance indicator for tabulation strategies. Thereafter, an a priori analysis is conducted comparing the results from the FPVA and SLFM (using the same flamelet database) with the FTC results. The results show that the FPVA is able to more accurately describe the thermochemical state and the flame structure than the SLFM. For a more detailed assessment of the two tabulation strategies, the TD terms reconstructed from the FPVA and SLFM are compared to those from the FTC results. It is found that the FPVA can capture a significant portion of TD effects, while the SLFM can hardly characterize TD effects. This particular capability allows the FPVA to describe chemistry-transport interaction and flame structure more accurately than the SLFM.

Evaluation of flamelet/progress variable model for laminar pulverized coal combustion

In the present work, the flamelet/progress variable (FPV) approach based on two mixture fractions is formulated for pulverized coal combustion and then evaluated in laminar counterflow coal flames under different operating conditions through both a priori and a posteriori analyses. Two mixture fractions, Zvol and Zchar, are defined to characterize the mixing between the oxidizer and the volatile matter/char reaction products. A coordinate transformation is conducted to map the flamelet solutions from a unit triangle space (Zvol, Zchar) to a unit square space (Z, X) so that a more stable solution can be achieved. To consider the heat transfers between the coal particle phase and the gas phase, the total enthalpy is introduced as an additional manifold. As a result, the thermo-chemical quantities are parameterized as a function of the mixture fraction Z, the mixing parameter X, the normalized total enthalpy Hnorm, and the reaction progress variable YPV. The validity of the flamelet chemtable and the selected trajectory variables is first evaluated in a priori tests by comparing the tabulated quantities with the results obtained from numerical simulations with detailed chemistry. The comparisons show that the major species mass fractions can be predicted by the FPV approach in all combustion regions for all operating conditions, while the CO and H2 mass fractions are over-predicted in the premixed flame reaction zone. The a posteriori study shows that overall good agreement between the FPV results and those obtained from detailed chemistry simulations can be achieved, although the coal particle ignition is predicted to be slightly earlier. Overall, the validity of the FPV approach for laminar pulverized coal combustion is confirmed and its performance in turbulent pulverized coal combustion will be tested in future work.

A flamelet/progress variable approach for modeling coal particle ignition

In this work an extended flamelet/progress variable (FPV) approach is presented with the aim of simulating the homogeneous auto-ignition of single coal particles during devolatilization. The FPV approach allows chemistry to be decoupled from the solution of the flow field, using a pre-computed strained flamelet look-up table. In this way, the complex chemistry governing coal particle ignition can be accurately accounted for. In particular, one-dimensional unsteady laminar diffusion flamelet (ULDF) simulations are used to generate the table, considering different values of the mixture fraction and the scalar dissipation rate on the particle surface. A new analytical formulation for the scalar dissipation rate is used to solve the diffusion flamelet equations in mixture fraction space, characterized by maximum values on the particle side, rapidly decreasing to zero towards the oxidizer. The time of the ULDF simulations is mapped to progress variable to characterize the ignition process. The FPV approach is then used in laminar resolved CFD simulations of the reactive boundary layer around a single coal particle. The flamelet table look-up is performed using the cell values of the mixture fraction and progress variable along with the particle surface values of the mixture fraction and scalar dissipation rate. Results from the new approach are finally compared with results from a simulation fully resolving transport processes and the detailed chemistry by means of a priori and a posteriori analyses. The results show that the FPV method is able to correctly reproduce the flame structure before and during the ignition process.

Challenging modeling strategies for LES of non-adiabatic turbulent stratified combustion

Five different low-Mach large eddy simulations are compared to the turbulent stratified flame experiments conducted at the Technical University of Darmstadt (TUD). The simulations were contributed by TUD, the Institute for Combustion Technology (ITV) at Aachen, Lund University (LUND), the EM2C laboratory at Ecole Centrale Paris, and the University of Duisburg-Essen (UDE). Combustion is modeled by a premixed flamelet tabulation with local flame thickening (TUD), a premixed flamelet progress variable approach coupled to a level set method (ITV), a 4-steps mechanism combined with implicit LES (LUND), the F-TACLES model that is based on filtered premixed flamelet tabulation (EM2C), and a flame surface density approach (UDE). An extensive comparison of simulation and experimental data is presented for the first two moments of velocity, temperature, mixture fraction, and major species mass fractions. The importance of heat-losses was assessed by comparing simulations for adiabatic and isothermal boundary conditions at the burner walls. The adiabatic computations predict a flame anchored on the burner lip, while the non-adiabatic simulations show a flame lift-off of one half pilot diameter and a better agreement with experimental evidence for temperature and species concentrations. Most simulations agree on the mean flame brush position, but it is evident that subgrid turbulence must be considered to achieve the correct turbulent flame speed. Qualitative comparisons of instantaneous snapshots of the flame show differences in the size of the resolved flame wrinkling patterns. These differences are (a) caused by the influence of the LES combustion model on the flame dynamics and (b) by the different simulation strategies in terms of grid, inlet condition and numerics. The simulations were conducted with approaches optimized for different objectives, for example low computational cost, or in another case, short turn around.

Analysis of turbulent diffusion flames with a hybrid fuel of methane and hydrogen in high pressure and temperature conditions using LES approach

In the present study, large eddy simulations (LES) of a turbulent diffusion flame with a hybrid fuel of methane and hydrogen are carried out to investigate the effects of operating conditions on the flame shape and the pollutant formation. A parametric study is performed by changing the values of hydrogen concentration and the background pressure and temperature. A flamelet-progress variable approach (FPVA) is used for a tabulated chemistry and a dynamic subgrid scale (SGS) model is used as the LES model. The result shows a good agreement with a previous experiment in the statistics of flow fields and composition. Also, the results affected by each operating parameter show consistent tendency with previous studies. Then, we investigate how the effect of added hydrogen changes for different background pressure and temperature conditions including those of a direct injection internal combustion engine. It is found that both the high compression ratio and hydrogen addition can lead to more compact and focused reaction zone by changes in the reaction and mixing rates. This also affects the pollutant formation, and the reduction of CO and NO is observed with hydrogen addition.

Flamelet/progress variable modeling of partial oxidation systems: From laboratory flames to pilot-scale reactors

Partial oxidation (POX) reactors are characterized by different reaction regimes: the near burner zone is mainly governed by the mixing of the initially non-premixed reactants and by the fast oxidation zone, while the post-flame zone is mainly governed by the slow reforming reactions under almost fully premixed conditions. Advanced turbulence–chemistry interaction models are required to describe different regimes in POX reactors. This work investigates the use of the flamelet/progress variable (FPV) approach in laminar and turbulent POX reactors. The FPV approach allows different times and length scales to be captured, using a progress variable to describe the advancement of the reactions in reacting mixtures. The solution of the chemical reactions is generally separated from the solution of the flow and mixing by pre-calculating solutions of one-dimensional canonical flames. The solution of the chemistry is stored in look-up tables and linked to the solution of the flow using the mixture fraction and the progress variable scalars. In this work, different methods for generating the flamelet look-up tables are analyzed for a laboratory reference laminar POX flame. The results are compared to the direct numerical simulation with the goal of identifying the most suitable flame structure for tabulation in POX systems. These results are then used to build a FPV table. Finally, a pilot-scale high-pressure POX reactor was investigated and the numerical results are validated using the measured composition of the syngas at the exit of the reactor.

LES flamelet-progress variable modeling and measurements of a turbulent partially-premixed dimethyl ether jet flame

In the present study the flame structure of a piloted partially-premixed dimethyl ether flame (DME-D), which is based on the Sydney/Sandia piloted jet burner flame series, is investigated using an LES-flamelet-progress variable approach (LES-FPV). Simulation results are used together with a comprehensive experimental data set including multi-scalar measurements of temperature and major species from Raman/Rayleigh scattering, intermediate species CH2O and OH from laser induced fluorescence (LIF) and velocity data from particle image velocimetry (PIV). The comparison between numerical and experimental data includes the mean and the root mean square (RMS) radial profiles for velocity, mixture fraction, temperature and mole fractions of major species at different downstream locations. Furthermore, species distributions conditioned on the experimentally accessible mixture fraction are compared and differences between DME and methane flames are discussed. In addition to this comparison, the computation of CH2O-LIF and OH-LIF signals as well as the effective Rayleigh cross-section was incorporated into the flamelet-progress variable approach. The filtered and time-averaged numerical results are then directly compared with the corresponding experimental signals at different axial positions. The characteristic separation of instantaneous CH2O and OH fields, which was previously observed in simultaneous LIF measurements, is discussed and analyzed based on the underlying flamelet structures. Finally, modeling assumptions from the experimental post-processing for the effective Rayleigh cross-section, which were introduced to account for the experimentally inaccessible intermediate hydrocarbons, are evaluated using the detailed species composition from the numerical simulations.