Flamelet/progress Variable Research Articles

Computing supersonic non-premixed turbulent combustion by an SMLD flamelet progress variable model

This paper presents a statistically more likely distribution (SMLD) approach for the evaluation of the presumed probability density function (PDF) in flamelet progress variable (FPV) models for non-premixed supersonic combustion. The numerical simulation of the NASA Langley Research Center supersonic H2–Air combustion chamber is performed using two approaches: the first one is a standard FPV model, built presuming the functional shape of the PDFs of the mixture fraction, Z, and of the progress parameter, Λ; the second approach employs the SMLD technique to presume the joint PDF of Z and Λ. The standard and FPV-SMLD models have been developed using the low Mach number assumption. In both cases, the temperature is evaluated by solving the total-energy conservation equation, providing a more suitable approach for the simulation of supersonic combustion. By comparison with experimental data, the proposed SMLD model is shown to provide a clear improvement with respect to the standard FPV model, especially in the auto-ignition and stabilization regions of the flame.

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.

An SMLD Joint PDF Model for Turbulent Non-Premixed Combustion Using the Flamelet Progress-Variable Approach

This paper provides an improved flamelet/progress variable (FPV) model for the simulation of turbulent combustion, employing the statistically most likely distribution (SMLD) approach for the joint probability density function (PDF) of the mixture fraction, Z, and of the progress parameter, {\Lambda} . Steady-state FPV models are built presuming the func- tional shape of the joint PDF of Z and {\Lambda} in order to evaluate Favre-averages of thermody- namic quantities. The mixture fraction is widely assumed to behave as a passive scalar with a mono-modal behaviour modelled by a \b{eta} -distribution. Moreover, under the hypothesis that Z and {\Lambda} are statistically independent, the joint PDF coincides with the product of the two marginal PDFs. In this work we discuss these two constitutive hypotheses. The proposed model evaluates the most probable joint distribution of Z and {\Lambda} , relaxing some crucial as- sumption on their statistical behaviour. This provides a more general model in the context of FPV approach and an effective tool to verify the adequateness of widely used hypotheses. The model is validated versus experimental data of well-known test cases, namely, the San- dia flames. The results are also compared with those obtained by the standard FPV approach, analysing the role of the PDF functional form on turbulent combustion simulations.

IDDES simulation of hydrogen-fueled supersonic combustion using flamelet modeling

A turbulent combustion modeling method combining the improved delayed detached eddy simulation (IDDES) model and the flamelet/progress variable (FPV) model is developed and then applied in the hybrid RANS/LES simulation of two canonical configurations for hydrogen-fueled supersonic combustors, namely the DLR supersonic combustor and Gamba model combustor. Results of both cases show good agreement with experiment, validating the accuracy of this method in both two- and three-dimensional combustion simulations. Moreover, the results suggest that the IDDES model is capable of capturing large scale vortex structures, and verify that the FPV model is applicable under supersonic conditions. Meanwhile, the k-ω SST model combined with the FPV model is applied in both cases. It is found that the application of the k-ω SST model leads to larger deviations compared with that of the IDDES model. Further analysis indicates that large scale vortex structures can influence the distribution of control variables of the FPV model, which further affects the species mixing and reaction progress. Therefore, the simulation of turbulent transport process has a significant effect on the performance of the FPV model.

Large eddy simulation of a partially-premixed gas turbine model combustor

Large-eddy simulations of a dual-swirl gas turbine model combustor (GTMC) are performed. This burner was experimentally studied by Meier et al. (2006), and is operated in the partially-premixed combustion regime. Two different LES-combustion formulations are employed to separately investigate modeling strategies for representing the combustion regime and the turbulence/chemistry interaction. A prior model analysis is conducted to examine the accuracy of describing the turbulent combustion regime inside this burner in terms of premixed or non-premixed flamelet solutions. Modeling results from three different LES computations are compared with measurements for velocity, temperature, and species-mass fractions of CO2, CO, and H2. Overall, the simulation results, obtained with a flamelet/progress-variable (FPV) model and a premixed-based filtered tabulated chemistry LES (F-TACLES) formulation, are in good agreement with experimental data. It is shown that the flow-field structure exhibits sensitivity to the representation of the turbulence/chemistry interaction. This is a result of the flame wrinkling at the lifted flame base in the nozzle-near region.

Numerical study of methane/air jet flame in vitiated co-flow using tabulated detailed chemistry

Two different combustion models, the autoignition (AI) model and flamelet/progress variable (FPV) model, have been applied to study the auto-ignition process of methane/air jet flame in vitiated co-flow. A priori study was conducted to test the validity of the two models. Results show that the different range of predicted reaction rates is mainly responsible for their different performances in large eddy simulation (LES) studies. In this paper, beta PDF was used to model the mixture fraction distribution, while two different shapes of PDF, delta function and beta function, were applied for the reaction progress. Compared to the FPV model, the AI model combined with beta function for reaction progress could capture the auto-ignition process and predict the exact lifted height. Also the results indicate that the variance of reaction progress plays an important role in predicting the flame lifted height.

Steady Flamelet Progress-Variable (FPV) Modeling and Simulation of a High-Pressure Gasifier

A steady computational fluid dynamics (CFD)-flamelet/progress-variable (FPV) model is implemented for the simulation of a gasifier. In this model, the open source code OpenFOAM is used to solve the partial differential equations of mass, momentum, turbulence, fuel mixture fraction mean of fuel and its variance, mean mixture fraction of steam, and progress variable. On the other hand, the species mass fractions are retrieved from a precomputed flamelet look-up table. Finally, the simulation results obtained with the steady CFD–FPV are compared to the available pilot plant data. The flame zone shifts toward the inlet when using the FPV model compared to the standard flamelet model without the progress variable. In both models, the observed outlet values are in reasonably good agreement with the experimental values.

Large-Eddy Simulation of a Jet-in-Hot-Coflow Burner Operating in the Oxygen-Diluted Combustion Regime

Large-eddy simulations of moderate and intense low-oxygen dilution (MILD) combustion of a jet-in-hot-coflow (JHC) burner are performed. This burner configuration consists of three streams, providing fuel, oxygen-diluted coflow, and air to the burner. To account for the mixing between the three reactant streams, a three-stream flamelet/progress variable (FPV) formulation is utilized. This model was previously applied to a condition corresponding to the upper range of MILD-combustion, and the objective of this contribution is to further investigate this model in application to highly diluted operating conditions. Comparisons of mean and conditional results show that the model accurately captures effects of increasing oxygen-depletion on the flame-structure and heat-release, and predictions for temperature and species mass fractions are in overall good agreement with experiments.

A Comparative Study of Flamelet and Finite Rate Chemistry LES for a Swirl Stabilized Flame

Present-day demands on combustion equipment are increasing the need for improved understanding and prediction of turbulent combustion. Large eddy simulation (LES), in which the large-scale flow is resolved on the grid, leaving only the small-scale flow to be modeled, provides a natural framework for combustion simulations as the transient nature of the flow is resolved. In most situations; however, the flame is thinner than the LES grid, and subgrid modeling is required to handle the turbulence-chemistry interaction. Here we examine the predictive capabilities between LES flamelet models, such as the flamelet progress variable (LES-FPV) model, and LES finite rate chemistry models, such as the thickened flame model (LES-TFM), the eddy dissipation concept (LES-EDC) model, and the partially stirred reactor model (LES-PaSR). The different models are here used to examine a swirl-stabilized premixed flame in a laboratory gas turbine combustor, featuring the triple annular research swirler (TARS), for which high-quality experimental data is available. The comparisons include velocity and temperature profiles as well as combustor dynamics and NO formation.

LES flamelet modeling of a three-stream MILD combustor: Analysis of flame sensitivity to scalar inflow conditions

Large-eddy simulations (LES) of a jet issuing into a hot and diluted coflow are performed. To model this three-stream burner configuration, which is operated in the moderate and intense low-oxygen dilution (MILD) combustion regime, a flamelet/progress variable (FPV) formulation is extended by introducing an additional conserved scalar. This additional scalar is associated with the oxidizer split, and is used to identify flamelets of different mixture composition. Due to the extended spatial structure of this jet diffusion flame, the moderate Reynolds number, and the overall lean operating condition, different flamelets interact only weakly in mixture composition space, so that the thermochemical state-space is populated from the solution of the one-dimensional flamelet equations. To account for the turbulence/chemistry interaction on numerically unresolved scales, a presumed probability density function (PDF) is used in the LES combustion model. This three-stream FPV combustion model is applied in LES of the MILD combustor, which was experimentally investigated by Dally et al. (2002) [4]. The comparison with results obtained from the single-mixture fraction FPV formulation shows that the coflow mixture composition can only inadequately be represented by a single mixture fraction, resulting in a significant overprediction of the flame temperature and CO mass fraction. The second part of this work addresses the sensitivity of the flow field and flame structure to the specification of scalar inflow conditions under kinetics-controlled, low-Damköhler number combustion conditions. To this end, LES calculations are performed that employ an increasing level of fidelity in the specification of the scalar boundary conditions, including homogeneous and intermittent turbulent scalar inflow conditions that are derived from experimental data. From this analysis, it is shown that the consideration of turbulent fluctuations in the scalar composition leads to improved predictions for temperature and mass fractions of CO and OH. Furthermore, the results from this simulation also suggest that effects of scalar inflow conditions are not only confined to the nozzle-near region but extend throughout the entire flame. It is anticipated that these findings could also be of relevance to other simulations of kinetics-controlled and low-temperature combustion systems, including autoignition, lifted flames, and premixed systems in which flames are stabilized by vitiated and hot coflows.

Analysis of Different Sound Source Formulations to Simulate Combustion Generated Noise Using a Hybrid LES/APE-RF Method

Combustion noise and sound source mechanisms of the DLR-A flame are investigated. A hybrid large-eddy simulation/computational aeracoustics (LES/CAA) approach is employed. In the first step of the hybrid analysis the flamelet/progress variable (FPV) model is employed as combustion model followed by the acoustic simulation in the second step using the acoustic perturbation equations for reacting flows (APE-RF). The flamelet/progress variable database has been extended in terms of acoustic source terms. The analysis of the acoustic field of low MACH number reacting flows induced by the thermoacoustic sources such as the unsteady heat release leads to a very stiff problem formulation, since the related sources require highly resolved regions in the source area, which restricts the possible time step. To simulate combustion generated noise using such a hybrid approach, a suitable source description has to be used, which preferably satisfies two requirements, i.e, to efficiently and accurately predict the generated sound field, while the source term can be easily evaluated from the LES. Using the source term, which is expressed via the scaled partial time derivative of the density, the acoustic field can be reproduced best up to a maximum Strouhal number of StD = 2. However, this source formulation requires a rigorous constraint at the interface of the hybrid approach to avoid spurious noise due to artificial acceleration caused by interpolation. To be more precise, the convection speed of density inhomogeneities has to be preserved during interpolation. A compromise between efficiency and accuracy can be achieved using the source formulation expressed via the scaled material derivative of the density, since by definition this formulation does not describe the convection of density inhomogeneities.

Prediction of extinction and reignition in nonpremixed turbulent flames using a flamelet/progress variable model: 2. Application in LES of Sandia flames D and E

An extension of the flamelet/progress variable (FPV) model for the prediction of extinction and reignition is applied in large-eddy simulation (LES) of flames D and E of the Sandia piloted turbulent jet flame series. This model employs a presumed probability density function (PDF), in which the marginal PDF of a reactive scalar is modeled by a statistically most likely distribution. This provides two advantages. First of all, the shape of the distribution depends on chemical and mixing time-scale information, and second, an arbitrary number of moments can be enforced. This model was analyzed in an a priori study in the first part of this work. In the present LES application, the first two moments of mixture fraction and reaction progress variable are used to constrain the shape of the presumed PDF. Transport equations for these quantities are solved, and models for the residual scalar dissipation rates, which appear as unclosed terms in the equations for the scalar variances, are provided. Statistical flow field quantities for axial velocity, mixture fraction, and temperature, obtained from the extended FPV model, are in good agreement with experimental data. Mixture-fraction-conditioned data, conditional PDFs, and burning indices are computed and compared with the delta-function flamelet closure model, which employs a Dirac distribution as a model for the marginal PDF of the reaction progress parameter. The latter model considerably underpredicts the amount of local extinction, which shows that the consideration of second-moment information in the presumed PDF of the reaction progress parameter is important for the accurate prediction of extinction and reignition. Mixture-fraction-conditioned results obtained from the extended FPV model are in good agreement with experimental data; however, the overprediction of the consumption of fuel and oxidizer on the fuel-rich side results in an overprediction of minor species. The predictions for the conditional PDFs and burning indices are in good agreement with measurements.

MODELING DIESEL SPRAY IGNITION USING DETAILED CHEMISTRY WITH A PROGRESS VARIABLE APPROACH

In this work, a progress variable approach is used to model diesel spray ignition with detailed chemistry. The flow field and the detailed chemistry are coupled using the flamelet assumption. A flamelet progress variable is transported by the computational fluid dynamics (CFD) code. The progress variable source term is obtained from an unsteady flamelet library that is evaluated in each grid cell. The progress variable chosen is based on sensible enthalpy. By using an unsteady flamelet library for the progress variable, the impact of local effects, for example variations in the turbulence field, effects of wall heat transfer etc. on the autoignition chemistry can be considered on a cell level. The coupling between the unsteady flamelet library and the transport equation for total enthalpy follows the ideas of the representative interactive flamelet (RIF) approach. The method can be compared to having an interactive flamelet in each computational cell in the CFD grid. The results obtained using the proposed model are compared to results obtained using the RIF model. Differences are exhibited during the autoignition process. After ignition, the results obtained using the proposed model and RIF are virtually identical. The model was used to study lift-off lengths in sprays as function of nozzle diameter and injection pressure. A good agreement between model predictions and experimental trends was found.