Flamelet Based NO x -Radiation Integrated Modelling of Turbulent Non-premixed Flame using Reynolds-stress Closure

A methodology of extending laminar flamelet model in its adiabatic form to a non-adiabatic form which can account for radiative heat loss as well as its effect on NO _x pollutant has been developed. Coupling of radiation submodel with flamelet model is based on the enthalpy defect concept. Pollutant NO _x has been calculated from solution of its transport equation containing source term which is derived from flamelet calculations. Flamelet calculations adopted GRI 2.11 reaction mechanism which accounts for detailed carbon and NO _x chemistry. Depending on consideration of variation in scalar dissipation within flamelet calculations, the non-adiabatic form has been further divided into non-adiabatic model with single (NADS) and multiple scalar dissipation rates (NADM). Bluff-body stabilized CH₄/H₂ flame has been chosen as the test case to assess the capability of non-adiabatic models. Turbulence closure has been achieved with a Reynolds stress transport model. Calculations have also been carried out with a modified k-ε model for evaluation of relative performance of the two turbulence closures. Performance of non-adiabatic flamelet models in regard to the overall structure of the flame is reasonably good and the agreement is similar to that of the adiabatic flamelet model thereby indicating weakly radiating nature of the flame. However, the NADM model results in minor but encouraging improvement in NO mass fraction predictions by reducing the extent of overprediction observed with the adiabatic model. In contrast, the NADS model results in overprediction over and above the adiabatic predictions thereby showing that, it is imperative to consider variation in scalar dissipation rate in flamelet calculations to capture the effect of radiation on NO. The results also show that employing the modified k-ε model instead of the Reynolds stress transport model for turbulence closure in NADM calculations results in considerable overprediction in centerline NO mass fractions.     

Development of an Analytical <Emphasis Type="Italic">β</Emphasis>-Function PDF Integration Algorithm for Simulation of Non-premixed Turbulent Combustion

Among many presumed-shape pdf approaches for modeling non-premixed turbulent combustion, the presumed β-function pdf is widely used in the literature. However, numerical integration of the β-function pdf may encounter singularity difficulties at mixture fraction values of Z = 0 or 1. To date, this issue has been addressed by few publications. The present study proposes the Piecewise Integration Method (PIM), an efficient, robust and accurate algorithm to overcome these numerical difficulties with the added benefit of improving computational efficiency. Comparison of this method to the existing numerical integration methods shows that the PIM exhibits better accuracy and greatly increases computational efficiency. The PIM treatment of the β-function pdf integration is first applied to the Burke–Schumann solution in conjunction with the k − ε turbulence model to simulate a CH₄/H₂ bluff-body turbulent flame. The proposed new method is then applied to the same flow using a more complex combustion model, the laminar flamelet model. Numerical predictions obtained by using the proposed β-function pdf integration method are compared to experimental values of the velocity field, temperature and species mass fractions to illustrate the efficiency and accuracy of the present method.     

Modelling of a Bluff-Body Nonpremixed Flame using a Coupled Radiation/Flamelet Combustion Model

A coupled radiation/flamelet combustion modelling technique is applied to the simulation of a bluff-body flame. Radiation heat transfer is incorporated into the laminar flamelet model for turbulent combustion through the enthalpy defect. A new method is developed for generating flamelet library with enthalpy defect. The radiation within the flame is modelled using a raytracing approach based on the discrete transfer method. The predicted results are compared with the reported experimental data. Comparison shows that the effects of radiative heat transferr on the temperature and major species are small for the flame considered. However, a significant improvement in the prediction of OH is achieved when radiation heat transfer is included. This revised version was published online in July 2006 with corrections to the Cover Date.     

Large-Eddy Simulation of Lean Premixed Turbulent Flames of Three Different Combustion Configurations using a Novel Reaction Closure

This large eddy simulation (LES) study is applied to three different premixed turbulent flames under lean conditions at atmospheric pressure. The hierarchy of complexity of these flames in ascending order are a simple Bunsen-like burner, a sudden-expansion dump combustor, and a typical swirl-stabilized gas turbine burner–combustor. The purpose of this paper is to examine numerically whether the chosen combination of the Smagorinsky turbulence model for sgs fluxes and a novel turbulent premixed reaction closure is applicable over all the three combustion configurations with varied degree of flow and turbulence. A quality assessment method for the LES calculations is applied. The cold flow data obtained with the Smagorinsky closure on the dump combustor are in close proximity with the experiments. It moderately predicts the vortex breakdown and bubble shape, which control the flame position on the double-cone burner. Here, the jet break-up at the root of the burner is premature and differs with the experiments by as much as half the burner exit diameter, attributing the discrepancy to poor grid resolution. With the first two combustion configurations, the applied subgrid reaction model is in good correspondence with the experiments. For the third case, a complex swirl-stabilized burner–combustor configuration, although the flow field inside the burner is only modestly numerically explored, the level of flame stabilization at the junction of the burner–combustor has been rather well captured. Furthermore, the critical flame drift from the combustor into the burner was possible to capture in the LES context (which was not possible with the RANS plus k–ɛ model), however, requiring tuning of a prefactor in the reaction closure.     

Heat and mass transfer study of impinging turbulent premixed flames

 Impinging jet combusting flows on granite plates are studied. A mathematical model for calculating heat release in turbulent impinging premixed flames is developed. The combustion including radiative heat transfer and local extinction effects, and flow characteristics are modeled using a finite volume computational approach. Two different eddy viscosity turbulence models, namely the standard k–ɛ and the RNG k–ɛ model with and without radiation (discrete transfer model) are assessed. The heat released predictions are compared with experimental data and the agreement is satisfactory only when both radiative heat transfer and local extinction modeling are taken into account. The results indicate that the main effect of radiation is the decrease of temperature values near the jet stagnation point and along the plate surface. Radiation increases temperature gradients and affects predicted turbulence levels independently of the closure model used. Also, the RNG k–ɛ predicts higher temperatures close the solid plate, with and without radiative heat transfer. Received on 13 November 2000 / Published online: 29 November 2001     

Effect of swirl intensity on the flow and combustion of a turbulent non-premixed flat flame

An experiment in a turbulent non-premixed flat flame was carried out in order to investigate the effect of swirl intensity on the flow and combustion characteristics. First, stream lines and velocity distribution in the flow field were obtained using PIV (Particle Image Velocimetry) method in a model burner. In contrast with the axial flow without swirl, highly swirled air induced streamlines going along the burner tile, and its backward flow was generated by recirculation in the center zone of the flow field. In the combustion, the flame shape with swirled air also became flat and stable along the burner tile with increment of the swirl number. Flame structure was examined by measuring OH and CH radicals intensity and by calculating Damkohler number (Da) and turbulence Reynolds number (Re _T ). It appeared that luminescence intensity decreased at higher swirl number due to the recirculated flue gas, and the flat flames were comprised in the wrinkled laminar-flame regime. Backward flow by recirculation of the flue gas widely contacted on the flame front, and decreased the flame temperature and emissions concentration as thermal NO. The homogeneous temperature field due to the widely flat flame was obtained, and the RMS in the high temperature region was rather lower at higher swirl number. Consequently, the stable flat flame with low NO concentration was achieved.     

AN ANALYTICAL DESIGN METHODOLOGY FOR AN ANNULAR COMBUSTOR

SUMMARY

This paper describes a computational procedure for the optimization of the performance parameters of a simulated annular combustor. This method has been applied to analyze the influence of the performance parameters and geometries on the annular combustor characteristics and provide a good understanding of combustor internal flow fields, and therefore it can be used for guiding the combustor design process. The approach is based on the solution of governing nonlinear, elliptic partial differential equations for 3-D axisymmetric recirculating turbulent reacting swirling flows and the modelling of turbulence, combustion, thermal radiation and pollutant formation. The turbulence effects are introduced via the modified two-equation κ-ε model. Turbulent combustion is modelled using the κ-ε-g model and a two-step turbulent combustion model is employed for the excess emission of carbon monoxide CO. For the evaluation of the NO pollutant formation rate, the NO pollutant formation model, which takes into account the influence of turbulence, presented here. The radiative heat transfer is handled by the heat flux model. The predictions of the combustor character-istics and performance parameters are made using the present approach.

Predictions of velocity, length of the recirculation zone, combustion efficiency and wall temperature are compared with measurements. Agreement between the predictions and experimental data is very satisfactory.     

Scalar turbulence model investigation with variable turbulent Prandtl number in heated jets and diffusion flames

Traditional turbulence models using constant turbulent Prandtl number fail to predict the experimentally observed anisotropies in the thermal eddy diffusivity and thermal turbulent intensity fields. Accurate predictions depend strongly on the turbulence model employed. Consequently, the objective of this paper is to assess the performance of turbulence model with variable turbulent Prandtl number in predicting of thermal and scalar fields quantities. The model is applied to axisymmetric turbulent round jet with variable density and in turbulent hydrogen diffusion flames using the flamelet concept. The k − ɛ turbulence model is used in conjunction with thermal field; the model involves solving supplemental scalar equations for the temperature variance and its dissipation rate. The model predictions are compared with available experimental data for the purpose of validating model. In reacting cases, velocity and scalar (including temperature and mass fractions) predictions agree relatively well in the near field of the investigated diluted hydrogen flames.     

Numerical Simulation of Delft-Jet-in-Hot-Coflow (DJHC) Flames Using the Eddy Dissipation Concept Model for Turbulence–Chemistry Interaction

In this paper, we report results of a numerical investigation of turbulent natural gas combustion for a jet in a coflow of lean combustion products in the Delft-Jet-in-Hot-Coflow (DJHC) burner which emulates MILD (Moderate and Intense Low Oxygen Dilution) combustion behavior. The focus is on assessing the performance of the Eddy Dissipation Concept (EDC) model in combination with two-equation turbulence models and chemical kinetic schemes for about 20 species (Correa mechanism and DRM19 mechanism) by comparing predictions with experimental measurements. We study two different flame conditions corresponding to two different oxygen levels (7.6% and 10.9% by mass) in the hot coflow, and for two jet Reynolds number (Re = 4,100 and Re = 8,800). The mean velocity and turbulent kinetic energy predicted by different turbulence models are in good agreement with data without exhibiting large differences among the model predictions. The realizable k-ε model exhibits better performance in the prediction of entrainment. The EDC combustion model predicts too early ignition leading to a peak in the radial mean temperature profile at too low axial distance. However the model correctly predicts the experimentally observed decreasing trend of lift-off height with jet Reynolds number. A detailed analysis of the mean reaction rate of the EDC model is made and as possible cause for the deviations between model predictions and experiments a low turbulent Reynolds number effect is identified. Using modified EDC model constants prediction of too early ignition can be avoided. The results are weakly sensitive to the sub-model for laminar viscosity and laminar diffusion fluxes.     

A Similarity Subgrid Model for Premixed Turbulent Combustion

The accuracy of large-eddy simulation (LES) of a turbulent premixed Bunsen flame is investigated in this paper. To distinguish between discretization and modeling errors, multiple LES, using different grid sizes h but the same filterwidth Δ, are compared with the direct numerical simulation (DNS). In addition, LES using various values of Δ but the same ratio Δ/h are compared. The chemistry in the LES and DNS is parametrized with the standard steady premixed flamelet for stochiometric methane-air combustion. The subgrid terms are closed with an eddy-viscosity or eddy-diffusivity approach, with an exception of the dominant subgrid term, which is the subgrid part of the chemical source term. The latter subgrid contribution is modeled by a similarity model based upon 2Δ, which is found to be superior to such a model based upon Δ. Using the 2Δ similarity model for the subgrid chemistry the LES produces good results, certainly in view of the fact that the LES is completely wrong if the subgrid chemistry model is omitted. The grid refinements of the LES show that the results for Δ = h do depend on the numerical scheme, much more than for h = Δ/2 and h = Δ/4. Nevertheless, modeling errors and discretization error may partially cancel each other; occasionally the Δ = h results were more accurate than the h ≤ Δ results. Finally, for this flame LES results obtained with the present similarity model are shown to be slightly better than those obtained with standard β-pdf closure for the subgrid chemistry.     

A pore scale study on turbulent combustion in porous media

This paper presents pore scale simulation of turbulent combustion of air/methane mixture in porous media to investigate the effects of multidimensionality and turbulence on the flame within the pores of porous media. In order to investigate combustion in the pores of porous medium, a simple but often used porous medium consisting of a staggered arrangement of square cylinders is considered in the present study. Results of turbulent kinetic energy, turbulent viscosity ratio, temperature, flame speed, convective heat transfer and thermal conductivity are presented and compared for laminar and turbulent simulations. It is shown that the turbulent kinetic energy increases from the inlet of burner, because of turbulence created by the solid matrix with a sudden jump or reduction at the flame front due to increase in temperature and velocity. Also, the pore scale simulation revealed that the laminarization of flow occurs after flame front in the combustion zone and turbulence effects are important mainly in the preheat zone. It is shown that turbulence enhances the diffusion processes in the preheat zone, but it is not enough to affect the maximum flame speed, temperature distribution and convective heat transfer in the porous burner. The dimensionless parameters associated with the Borghi–Peters diagram of turbulent combustion have been analyzed for the case of combustion in porous media and it is found that the combustion in the porous burner considered in the present study concerns the range of well stirred reactor very close to the laminar flame region.     

Radiation effect on temperature distribution and NO formation in a diffusion flame under reduced gravity conditions

Combustion of hydrocarbon fuel is accompanied with the formation of nitric oxide (NO) amongst other harmful emissions. In this work, a numerical investigation has been made for understanding the effect of radiative heat transfer on temperature distribution and formation of thermal NO in a methane–air diffusion flame under different reduced gravity environments. Conservation equations of overall mass, species concentration, momentum and energy for the reactive flows have been numerically solved with the use of finite difference scheme. In addition to that a semi-empirical soot model and an optically thin radiation model have been incorporated in the simulation. Gravity level is varied by the changed values of acceleration due to gravity. A thermal NO model incorporated accounts for the NO formation process which is decoupled from the hydrocarbon combustion. The relevant conservation equations have been solved as a post combustion reaction process. The flame height drops marginally with the reduction of gravity. Temperature becomes more uniformly distributed at lower gravity. NO formation boosts up with the fall of gravity below normal level when no radiation effect is considered. However, when radiation is considered, NO formation declines marginally with the reduction of gravity levels. Also in this case, concentration values of NO compare substantially lower with those without radiation. The upsurge of NO formation due to decline in gravity; and on the other hand, a shrinkage in concentration values of NO due to radiation effect can be attributed mainly to the rise and fall of temperature respectively in the computational zone.     

Large eddy simulation of turbulent premixed and stratified combustion using flame surface density model coupled with tabulation method

Large eddy simulations (LESs) are performed to investigate the Cambridge premixed and stratified flames, SwB1 and SwB5, respectively. The flame surface density (FSD) model incorporated with two different wrinkling factor models, i.e., the Muppala and Charlette2 wrinkling factor models, is used to describe combustion/turbulence interaction, and the flamelet generated manifolds (FGM) method is employed to determine major scalars. This coupled sub-grid scale (SGS) combustion model is named as the FSD-FGM model. The FGM method can provide the detailed species in the flame which cannot be obtained from the origin FSD model. The LES results show that the FSD-FGM model has the ability of describing flame propagation, especially for stratified flames. The Charlette2 wrinkling factor model performs better than the Muppala wrinkling factor model in predicting the flame surface area change by the turbulence. The combustion characteristics are analyzed in detail by the flame index and probability distributions of the equivalence ratio and the orientation angle, which confirms that for the investigated stratified flame, the dominant combustion modes in the upstream and downstream regions are the premixed mode and the back-supported mode, respectively.     

A 5-D Implementation of FGM for the Large Eddy Simulation of a Stratified Swirled Flame with Heat Loss in a Gas Turbine Combustor

Numerical simulations are foreseen to provide a tremendous increase in gas-turbine burners efficiency in the near future. Modern developments in numerical schemes, turbulence models and the consistent increase of computing power allow Large Eddy Simulation (LES) to be applied to real cold flow industrial applications. However, the detailed simulation of the gas-turbine combustion process remains still prohibited because of its enormous computational cost. Several numerical models have been developed in order to reduce the costs of flame simulations for engineering applications. In this paper, the Flamelet-Generated Manifold (FGM) chemistry reduction technique is implemented and progressively extended for the inclusion of all the combustion features that are typically observed in stationary gas-turbine combustion. These consist of stratification effects, heat loss and turbulence. Three control variables are included for the chemistry representation: the reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the stratification effect is expressed by the mixture fraction. The interaction between chemistry and turbulence is considered through a presumed beta-shaped probability density function (PDF) approach, which is considered for progress variable and mixture fraction, finally attaining a 5-D manifold. The application of FGM in combination with heat loss, fuel stratification and turbulence has never been studied in literature. To this aim, a highly turbulent and swirling flame in a gas turbine combustor is computed by means of the present 5-D FGM implementation coupled to an LES turbulence model, and the results are compared with experimental data. In general, the model gives a rather good agreement with experimental data. It is shown that the inclusion of heat loss strongly enhances the temperature predictions in the whole burner and leads to greatly improved NO predictions. The use of FGM as a combustion model shows that combustion features at gas turbine conditions can be satisfactorily reproduced with a reasonable computational effort. The implemented combustion model retains most of the physical accuracy of a detailed simulation while drastically reducing its computational time, paving the way for new developments of alternative fuel usage in a cleaner and more efficient combustion.     

Experimental and Computational Investigations of Flow and Mixing in a Single-Annular Combustor Configuration

A complementary experimental and computational study of the flow and mixing in a single annular gas turbine combustor has been carried out. The object of the investigation is a generic mixing chamber model, representing an unfolded segment of a simplified Rich-Quick-Lean (RQL) combustion chamber operating under isothermal, non-reacting conditions at ambient pressure. Two configurations without and with secondary air injection were considered. To provide an appropriate reference database several planar optical measurement techniques (time-resolved flow visualisation, PIV, QLS) were used. The PIV measurements have been performed providing profiles of all velocity and Reynolds-stress components at selected locations within the combustor. Application of a two-layer hybrid LES/RANS (HLR) method coupling a near-wall k − ε RANS model with conventional LES in the core flow was the focus of the computational work. In addition to the direct comparison with the experimental results, the HLR performance is comparatively assessed with the results obtained by using conventional LES using the same (coarser) grid as HLR and two eddy-viscosity-based RANS models. The HLR model reproduced all important flow features, in particular with regard to the penetrating behaviour of the secondary air jets, their interaction with the swirled main flow, swirl-induced free recirculation zone evolution and associated precessing-vortex core phenomenon in good agreement with experimental findings.     

Large-Eddy Simulation of Interactions Between a Reacting Jet and Evaporating Droplets

Large-eddy simulation of a turbulent reactive jet with and without evaporating droplets is performed to investigate the interactions among turbulence, combustion, heat transfer and evaporation. A hybrid Eulerian–Lagrangian approach is used for the gas–liquid flow system. Arrhenius-type finite-rate chemistry is employed for the chemical reaction. To capture the highly local interactions, dynamic procedures are used for all the subgrid-scale models, except that the filtered reaction rate is modelled by a scale similarity model. Various representative cases with different initial droplet sizes (St ₀) and mass loading ratios (MLR) have been simulated, along with a case without droplets. It is found that compared with the bigger, slow responding droplets (St ₀ = 16), smaller droplets (St ₀ = 1) are more efficient in suppressing combustion due to their preferential concentration in the reaction zones. The peak temperature and intensity of temperature fluctuations are found to be reduced in all the droplet cases, to a varying extent depending on the droplet properties. Detailed analysis on the contributions of respective terms in a transport equation for grid-scale kinetic energy (GSKE) shows that the droplet evaporation effect on GSKE is small, while the droplet momentum effect depends on St ₀. When the MLR is sufficiently high, the bigger (St ₀ = 16) droplets can have profound influence on GSKE, and consequently on the formation and evolution of large-scale flow structures. On the other hand, the turbulence level is found to be lower in the droplet cases than in the pure flame case, due to the dissipative droplet dynamic effect.     

Comparison of turbulence models for attached boundary layers relevant to aeronautics

With a single numerical method the performance of three classes of turbulence models is compared for different types of attached boundary layers, for which direct numerical simulations or experiments are available in the literature. The boundary-layer equations are solved with the following turbulence models: an algebraic model, two-equation models (k-ε andk-ω), and a differential Reynolds-stress model. The test cases are the channel flow, and boundary layers with zero, favourable and adverse streamwise pressure gradient. The differential Reynolds-stress model gives the best overall performance, whereas the performance of the algebraic model and thek-ω model is reasonably good. The performance of thek-ε model is less good for boundary layers with a non-zero streamwise pressure gradient, but it can easily be improved by an additional source term in the ε equation, which is also applied in the considered differential Reynolds-stress model.     

LES of a Multi-burner Annular Gas Turbine Combustor

In this study, Large Eddy Simulation (LES) has been used to predict the flow, mixing and combustion in both a single burner laboratory gas turbine combustor and in an 18 burner annular combustor, having identical cross sections. The LES results for the single burner laboratory combustor are compared with experimental data for a laboratory model of this combustor, and with other LES predictions, with good agreement. An explicit finite volume based LES model, using the mixed subgrid model together with a partially stirred reactor model for the turbulence chemistry interactions, is used. For the annular combustor, with the swirlers parameterized by jet inflow boundary conditions, we have investigated the influence of the a-priori unknown combustor exit impedance, the influence of the swirler characteristics and the fuel type. The combustion chemistry of methane–air and n-decane–air combustion is modeled by a two-step reaction mechanism, whereas NOx is separately modeled with a one-step mechanism. No experimental data exists for the annular combustor, but these results are compared with the single burner LES and experimental results available. The combustor exit impedance, the swirler- and fuel characteristics all seem to influence the combusting flow through the acoustics of the annular combustor. To examine this in greater detail time-series and eigenmodes of the combustor flow fields are analyzed and comparisons are made also with results from conventional thermoacoustic eigenmode analysis, with reasonable agreement. The flow and pressure distributions in the annular combustor are described in some detail and the mechanisms by which the burners interact are outlined.     

Study of a Filtered Flamelet Formulation for Large Eddy Simulation of Premixed Turbulent Flames

Despite significant advances in the understanding and modelling of turbulent combustion, no general model has been proposed for simulating flames in industrial combustion devices. Recently, the increase in computational possibilities has raised the hope of directly solving the large turbulent scales using large eddy simulation (LES) and capturing the important time-dependant phenomena. However, the chemical reactions involved in combustion occur at very small scales and the modelling of turbulent combustion processes is still required within the LES framework. In the present paper, a recently presented model for the LES of turbulent premixed flames is presented, analysed and discussed. The flamelet hypothesis is used to derive a filtered source term for the filtered progress variable equation. The model ensures proper flame propagation. The effect of subgrid scale (SGS) turbulence on the flame is modelled through the flame-wrinkling factor. The present modelling of the source term is successfully tested against filtered direct numerical simulation (DNS) data of a V-shape flame. Further, a premixed turbulent flame, stabilised behind an expansion, is simulated. The predictions agree well with the available experimental data, showing the capabilities of the model for performing accurate simulations of unsteady premixed flames.     

Experimental and Theoretical Sensitivity Analysis of Turbulent Flow Past a Square Cylinder

The Siemens SGT-800 3^rd generation DLE burner fitted to an atmospheric combustion rig has been numerically investigated. Pure methane and methane enriched by 80 vol% hydrogen flames have been considered. A URANS (Unsteady Reynolds Averaged Navier-Stokes) approach was used in this study along with the k ? ω SST and the k ? ω SST-SAS models for the turbulence transport. The chemistry is coupled to the turbulent flow simulations by the use of a laminar flamelet library combined with a presumed PDF. The effect of the mesh density in the mixing and the flame region and the effect of the turbulence model and reaction rate model constant are first investigated for the methane/air flame case. The results from the k ? ω SST-SAS along with flamelet libraries are shown to be in excellent agreement with experimental data, whereas the k ? ω SST model is too dissipative and cannot capture the unsteady motion of the flame. The k ? ω SST-SAS model is used for simulation of the 80 vol% hydrogen enriched flame case without further adjusting the model constants. The global features of the hydrogen enrichment are very well captured in the simulations using the SST-SAS model. With the hydrogen enrichment the time averaged flame front location moves upstream towards the burner exit nozzle. The results are consistent with the experimental observations. The model captures the three dominant low frequency unsteady motion observed in the experiments, indicating that the URANS/LES hybrid model indeed is capable of capturing complex, time dependent, features such as an interaction between a PVC and the flame front.