Eulerian Particle Flamelet Model Research Articles

On the use of flamelet model approach in analyzing the mixing-controlled spray flame dynamics

This research examines the application of unsteady flamelet modeling to consider the impact of turbulent-chemistry interaction (TCI) on a mixing-controlled spray flame. The model incorporates the Representative Interactive Flamelets (RIF) approach to represent the sub-grid scaled heterogeneous mixture along with chemical reactions. This is combined with an Eulerian Particle Flamelet Model (EPFM), which uses multiple flamelets to account for the history of unsteady flames. The results are compared with those of a first-order moment method known as the Well-Stirred Reactor (WSR) model. The numerical simulations were carried out using a Reynolds Averaged Navier–Stokes (RANS) solver incorporated in two different CFD platforms, a commercially available CFD code, CONVERGE, and C++ based open-source CFD code, OpenFOAM. The test conditions were employed based on the Engine Combustion Network (ECN) Spray-A setup. The simulation results obtained interchangeably using both CFD software are investigated to address essential aspects of the RIF model. In order to ensure accurate predictions in the mixing field, adjustments were made to the original OpenFOAM code to enhance the treatment of time-stepping for spray source terms. This refinement allows for an adequate resolution of spray-induced mixing. While there are slight variations in the implementation of the RIF model between OpenFOAM and CONVERGE, both CFD codes effectively reproduce the physics of mixing-controlled combustion. This includes accurately representing important phenomena like mixing-controlled turbulent spray flame and combustion recession.

Entropy generation in turbulent syngas counter-flow diffusion flames

Efficiency is one of the major objectives when designing energy systems. Irreversibilities of combustion processes can be characterized by analyzing entropy generation which is proportional to exergy destruction. In this paper, entropy generation is investigated in turbulent non-premixed counter-flow syngas flames at a high strain rate over a wide range of hydrogen percentage (H2/CO molar fraction from 0.4 to 2.0). The aim is to define the most efficient syngas composition to reduce irreversibilities. Irreversibilities involved in NO formation process are also examined. RANS (Reynolds Averaged Navier Stokes) technique including k-ε turbulence model is used for the flow field estimation. Flame structure is calculated using SLFM (Steady Laminar Flamelet Model) and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Total entropy generation rate accounts for chemical, heat conduction, mixing and viscous effects. Computational results show that the total volumetric entropy generation decreases with H2 enrichment as well as its different contributing effects. Chemical effect is dominant, followed by heat conduction and mixing effects. Viscous effect is negligible. The maximum of both thermal and prompt NO formation routes are influenced by the three main entropy generation modes, with the predominance of the chemical effect. At high strain rates, H2-rich syngas flames are efficient in regards to irreversibilities and NO emissions reduction.

Numerical modelling of pollutant formation in a lifted methane–air vertical diffusion flame

AbstractA comparison of turbulence and combustion models have been performed for predicting CO2 and NOx formation from a methane diffusion flame firing vertically upwards. The flow field has been modeled using the Reynolds-Averaged Navier–Stokes equation incorporating the k-e realizable turbulence closure model, the k-ω shear-stress transport (SST) turbulence model and the transitional SST turbulence model and the three models have been compared. Combustion was modeled using the unsteady Stationary Laminar Flamelet Model (SLFM), the Eulerian Particle Flamelet Model (EPFM), and the Pollutant Model (PM) and the three models have also been compared. Numerical predictions show good agreement with experimental data. Furthermore, the experimental data showed that the k-e realizable turbulence model and the k-ω SST turbulence model performed better than transitional SST model in predicting the pollutant species from the flame. The result also shows that the PM performed better than flamelet models in predicting ...

A numerical investigation of structure and NO emissions of turbulent syngas diffusion flame in counter-flow configuration

This paper reports a numerical simulation of turbulent non-premixed counter-flow syngas flames structure and NO emissions at a high strain rate with a special focus on mixing. The analysis is conducted over a wide range of hydrogen percentage (H2/CO ratio between 0.4 and 2.0) and operating pressure (from 1 to 10 atm). Numerical model is based on RANS (Reynolds Averaged Navier–Stokes) technique including k-ε turbulence model. SLFM (Steady laminar flamelet model) is used for flame structure calculations and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Mixing is described by mixture fraction and its variance. Radiation effects are also considered.Computational results show an improvement of mixing with hydrogen enrichment and ambient pressure rise. Maximum flame temperature decreases with H2 addition and increases with pressure. NO levels decrease towards hydrogen-rich syngas flames and increase with pressure. Zeldovich route is found to be the main NO formation path in the operating conditions considered.

Numerical Study of Emission Characteristics of a Jet Flame in Cross-Flow

The authors numerically investigated the effect of changes in the fuel exit velocity and the cross-flow on the length, radiant fraction, and emission indices of pollutant species (NOx and CO), as well as the ratio NO2/NOx, of a high momentum jet flame in cross-flow. This flow configuration is of generic interest, and also practically relevant to flaring operations. The flow field is computed based on the Reynolds-Averaged Navier–Stokes equations incorporating the realizable k−ϵ turbulence closure. The combustion process is modeled using the unsteady Eulerian particle flamelet model based on the mixture fraction approach, and the heat loss by radiation is accounted for using the discrete ordinates method. Comparison of the predicted flame length and the trend of the emission indices with experimental measurements reveal good agreement for the range of jet-to-cross-flow momentum flux ratios investigated, namely 100–800.

Assessment of the Eulerian particle flamelet model for nonpremixed turbulent jet flames

Although the Eulerian particle flamelet model (EPFM) recently proposed by Barths et al. [Proc. Combust. Inst. 27 (1998) 1841–1847] has shown the potential capabilities to realistically predict detailed pollutant (NO x , soot) formation in a turbulent reacting flow occurring within practical combustion devices, there still exists room to improve the predicative capability in terms of local flame structure and turbulence–chemistry interaction. In this study, the EPFM approach was applied to simulate two turbulent nonpremixed jet flames of CO/H 2/N 2 fuel having the same jet Reynolds number but different nozzle diameters, and the capability of predicting the NO x formation as well as both similarity of major species and sensitivity of minor species to fluid-dynamic scaling for the two flames has been assessed deeply in terms of both conditional and unconditional mean structures. The present results indicate that the original EPFM substantially overpredicts the conditional scalar dissipation rate at the downstream region and consequently underpredicts the streamwise decay of superequilibrium radical concentrations to the equilibrium state. In this study, in order to correctly estimate the averaged conditional scalar dissipation rate, a new modeling of the conditional scalar dissipation rate based on a least-squares fit through a mass weighted spatial distribution has been devised. In terms of both conditional and unconditional means, the EPFM utilizing this new procedure yields nearly the same results as the Lagrangian flamelet model, and provides closer agreement with experimental data than the original EPFM approach.

Eulerian particle flamelet modeling of a bluff-body CH 4/H 2 flame

In this paper an axisymmetric RANS simulation of a bluff-body stabilized flame has been attempted using steady and unsteady flamelet models. The unsteady effects are considered in a postprocessing manner through the Eulerian particle flamelet model (EPFM). In this model the transient history of scalar dissipation rate, conditioned by stoichiometric mixture fraction, is required to generate unsteady flamelets and is obtained by tracing Eulerian particles. In this approach unsteady convective–diffusive transport equations are solved to consider the transport of Eulerian particles in the domain. Comparisons of the results of steady and unsteady calculations show that transient effects do not have much influence on major species, including OH, and the structure of the flame therefore can be successfully predicted by steady or unsteady approaches. However, it appears that slow processes such as NO formation can only be captured accurately if unsteady effects are taken into account, while steady simulations tend to overpredict NO. In this work turbulence has been modeled using the Reynolds stress model. Predictions of velocity, velocity rms, mean mixture fraction, and its rms show very good agreement with experiments. Performance of three detailed chemical mechanisms, the GRI Mech 2.11, the San Diego mechanism, and the GRI Mech 3.0, has also been evaluated in this study. All three mechanisms performed well with both steady and unsteady approaches and produced almost identical results for major species and OH. However, the difference between mechanisms and flamelet models becomes clearly apparent in the NO predictions. The unsteady model incorporating the GRI Mech 2.11 provided better predictions of NO than steady calculations and showed close agreement with experiments. The other two mechanisms showed overpredictions of NO with both unsteady and steady models. The level of overprediction is severe with the steady approach. GRI Mech 3.0 appears to overpredict NO by a factor of 2 compared to GRI Mech 2.11. The NO predictions by the San Diego mechanism fall between those of the two GRI mechanisms. The present study demonstrates the success of the EPFM model and when used with the GRI 2.11 mechanism predicts all flame properties and major and minor species very well, and most importantly the correct NO levels.

Conditional moment closure and transient flamelet modelling for detailed structure and NO x formation characteristics of turbulent nonpremixed jet and recirculating flames

This study has been mainly motivated to assess computationally and theoretically the conditional moment closure (CMC) model and the transient flamelet model for the simulation of turbulent nonpremixed flames. These two turbulent combustion models are implemented into the unstructured grid finite volume method that efficiently handles physically and geometrically complex turbulent reacting flows. Moreover, the parallel algorithm has been implemented to improve computational efficiency as well as to reduce the memory load of the CMC procedure. Example cases include two turbulent CO/H2/N2 jet flames having different flow timescales and the turbulent nonpremixed H2/CO flame stabilized on an axisymmetric bluff-body burner. The Lagrangian flamelet model and the simplified CMC formulation are applied to the strongly parabolic jet flame calculation. On the other hand, the Eulerian particle flamelet model and full conservative CMC formulation are employed for the bluff-body flame with flow recirculation. Based on the numerical results, a detailed discussion is given for the comparative performances of the two combustion models in terms of the flame structure and NO x formation characteristics.

Eulerian particle flamelet modeling for combustion processes of bluff-body stabilized methanol-air turbulent nonpremixed flames

The present study is focused on the development of the RIF (Representative Interactive Flamelet) model which can overcome the shortcomings of conventional approach based on the steady flamelet library. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the RIF model can effectively account for the detailed mechanisms of NOx formation including thermal NO path, prompt and nitrous NOx formation, and reburning process by hydrocarbon radical without any ad-hoc procedure. The flamelet time of RIFs within a stationary turbulent flame may be thought to be Lagrangian flight time. In context with the RIF approach, this study adopts the Eulerian Particle Flamelet Model (EPFM) with mutiple flamelets which can realistically account for the spatial inhomogeneity of scalar dissipation rate. In order to systematically evaluate the capability of Eulerian particle flamelet model to predict the precise flame structure and NO formation in the multi-dimensional elliptic flames, two methanol bluffbody flames with two different injection velocities are chosen as the validation cases. Numerical results suggest that the present EPFM model has the predicative capability to realistically capture the essential features of flame structure and NOx formation in the bluff-body stabilized flames.

Flamelet Modeling of Pollutant Formation in a Gas Turbine Combustion Chamber Using Detailed Chemistry for a Kerosene Model Fuel

Combustion and pollutant formation in a gas turbine combustion chamber is investigated numerically using the Eulerian particle flamelet model. The code solving the unsteady flamelet equations is coupled to an unstructured computational fluid dynamics (CFD) code providing solutions for the flow and mixture field from which the flamelet parameters can be extracted. Flamelets are initialized in the fuel-rich region close to the fuel injectors of the combustor. They are represented by marker particles that are convected through the flow field. Each flamelet takes a different pathway through the combustor, leading to different histories for the flamelet parameters. Equations for the probability of finding a flamelet at a certain position and time are additionally solved in the CFD code. To model the chemical properties of kerosene, a detailed reaction mechanism for a mixture of n-decane and 1,2,4-trimethylbenzene is used. It includes a detailed NOx submechanism and the buildup of polycyclic aromatic hydrocarbons up to four aromatic rings. The kinetically based soot model describes the formation of soot particles by inception, further growth by coagulation, and condensation as well as surface growth and oxidation. Simulation results are compared to experimental data obtained on a high-pressure rig. The influence of the model on pollutant formation is shown, and the effect of the number of flamelets on the model is investigated.

Numerical investigation of the autoignition of turbulent gaseous jets in a high-pressure environment using the multiple-RIF model

The autoignition and subsequent flame propagation of initially nonpremixed turbulent system have been numerically investigated. The unsteady flamelet modeling based on the RIF (Representative Interactive Flamelet) concept has been applied to account for the influences of turbulence on these essentially transient combustion processes. In this RIF approach, the partially premixed burning, diffusive combustion and formation of pollutants (NO x , soot) can be consistently modeled by utilizing the comprehensive chemical mechanism. To treat the spatially distributed inhomogeneity of scalar dissipation rate, the multiple RIFs are employed in the framework of Eulerian Particle Flamelet Model approach. Computations are made for the various initial conditions of pressure, temperature and fuel composition. The present turbulent combustion model reasonably well predicts the essential features of autoignition process in the transient gaseous fuel jets injected into high-pressure and high-temperature environments.

Turbulent Combustion

The book Turbulent Combustion by Norbert Peters is a concise monograph on single-phase gaseous low Mach number turbulent combustion. It is compiled from the author's review papers on this topic plus some additional material. Norbert Peters characterizes turbulent combustion both by the way fuel and air are mixed and by the ratio of turbulent and chemical time scales. This approach leads naturally to detailed models, which are based on results of turbulence modelling and asymptotic flame theory. In both areas Norbert Peters has contributed significantly over the last two decades. The book has four sections. In chapter 1 he discusses briefly the state of the art of combustion models as they are used by different authors. Important turbulent and chemical scales are introduced, which are then used to introduce and explain the different combustion models. He distinguishes between premixed and non-premixed combustion and also between infinitely fast and finite rate chemistry. The current turbulent combustion models are described in order of their complexity and physical accuracy. He explains Eddy BreakUp and Eddy Dissipation Models, the fundamentals of the PDF transport equation, and the laminar flamelet concept applied to non-premixed and premixed turbulent combustion. Then, the Conditional Moment Closure, the Linear Eddy Model, and combustion models used in Large Eddy Simulations are described very briefly. Chapter 2 is devoted to premixed turbulent combustion. After introducing some characteristic dimensionless numbers Peters uses the level set approach and the flamelet concept to formulate a combustion model valid in the thin zone and corrugated reaction zone regimes. He shows parallels between this more fundamental model and standard models like the Bray-Moss-Libby model. He also presents models for the turbulent burning velocity, the Flame Surface Area Ratio, and discusses the effects of gas expansion. Very helpful for the reader's understanding is the presentation of three worked examples of a slot burner, a propagating spherical flame and an oscillating counter flow. Peters' model for premixed turbulent combustion is based on the equations for the mean and the variance of the $G$-equation, some closure relations as well as the flamelet equation for premixed combustion. A numerical example is used to discuss its accuracy. Non-premixed turbulent combustion is the subject matter of chapter 3. Peters uses the mixture fraction variable and asymptotic flame theory to explain the regimes of non-premixed turbulent combustion. Two worked examples of a counterflow diffusion flame and the one-dimensional unsteady laminar mixing layer help the reader to understand the theory. After discussing turbulent jet diffusion flames and introducing the flamelet equation he develops steady and unsteady flamelet models for non-premixed turbulent combustion. In particular, the Eulerian Particle Flamelet Model and the RIF (Representative Interactive Flamelet) Model are discussed. These models have been used to predict pollutant formation in a gas turbine and a direct injection Diesel engine, respectively. Finally, partially premixed combustion is discussed in chapter 4. Lifted turbulent diffusion flames are reviewed and the prediction of the lift-off height is identified as a key problem. This leads directly to the introduction of the concept of a triple flame. Different models for partially premixed combustion are then presented and the numerical simulation and the scaling of lift-off heights in turbulent jet flames are studied. There is no doubt that this book is well written and is an important contribution to combustion literature. Peters uses asymptotic theory and scale separation to develop combustion models from first principles. Also, the book contains a comprehensive review of the current literature on turbulent combustion. It is clearly a `must have' for experienced combustion modellers and experimentalists. The book has not been written for non-experts and beginners; these readers would probably have liked more worked examples and exercises, a list of symbols, some material on the mathematical techniques of asymptotic analysis, and a more detailed discussion of the standard combustion models that are often used in the literature. Also, the numerical aspects of turbulent combustion modelling are not discussed in the book. Nevertheless, the book will be a useful source for advanced courses on combustion. Markus Kraft

Unsteady modelling of a piloted methane/air jet flame based on the Eulerian particle flamelet model

A piloted methane/air jet flame is numerically simulated by using both steady and unsteady flamelets. Unsteady calculations are performed in a postprocessing stage using Eulerian transport equations for passive scalars to describe the temporal evolution of the scalar dissipation rate, conditional on the stoichiometric mixture fraction. The influence of the number of particles, and the initial conditions of the fluid particle transport equations and unsteady flamelet equations on the predicted Favre-average species mass fractions are investigated. It is shown that the predictions obtained using unsteady flamelets are in closer agreement with the experimental data than the steady flamelet predictions, both for the minor and for the major species. Initialization of the probability of finding fluid particles over the fuel rich region and initialization of the unsteady flamelet profiles based on a piecewise-linear interpolation of the boundary conditions at the fuel, pilot flame and air inlets yield good agreement between the predicted and the experimental data. However, the OH and the NO mass fractions are overestimated.

Numerical simulation of a mild combustion burner

A combustor with high air preheating and strong internal exhaust gas recirculation is numerically simulated. The combustor operates in the mild combustion mode, characterized by relatively low flame temperatures, low NO x emissions, no visible flame and no sound. The Eulerian Particle Flamelet model by Barths et al. [1] is employed to calculate NO in a postprocessing stage. Predictions of the mean and fluctuating velocity components, as well as local residence times, are compared with experimental data. It is found that the predicted profiles reproduce the data reasonably well, but some discrepancies were found. The NO emission calculated using this unsteady flamelet model is relatively close to measurements available from a previous study, while steady flamelets overpredict the NO emission by an order of magnitude.

Computational fluid dynamics modelling of non-premixed combustion in direct injection diesel engines

An overview over flamelet modelling for turbulent non-premixed combustion is given. A short review of previous contributions to simulations of direct injection (DI) diesel engine combustion using the representative interactive flamelet concept is presented. A surrogate fuel consisting of 70 per cent (liquid volume) n-decane and 30 per cent α-methyl-naphthalene is experimentally compared to real diesel fuel. The resemblance of their physical and chemical properties is shown to result in very similar combustion and pollutant formation for both fuels. In order to account for variations of the scalar dissipation rate within the computational domain, a method using multiple flamelets, called the Eulerian particle flamelet model, is used. A strategy is described for subdividing the computational domain and assigning the resulting subdomains to different flamelet histories represented by Eulerian marker particles. Experiments conducted with an Audi DI diesel engine and diesel fuel are compared to simulations using the surrogate fuel. The use of multiple flamelets, each having a different history, significantly improves the description of the ignition phase, leading to a better prediction of pressure, heat release and exhaust emissions of soot and NOx. The effect of the number of flamelet particles on the predictions is discussed.

Simulation of combustion in direct injection diesel engines using a eulerian particle flamelet model

An overview of flamelet modeling for turbulent non-premixed combustion is given. A short review of previous contributions to simulations of direct injection (DI) diesel engine combustion using the representative interactive flamelet concept is presented. A surrogate fuel consisting of 70% (liquid volume) n-decane and 30% α-methylnaphthalene is experimentally compared to real diesel fuel. The similarity of their physical and chemical properties is shown to result in a very similar combustion process for both fuels. The mathematical derivation for the Eulerian particle flamelet model is outlined. A strategy based on physical arguments is described for subdividing the computational domain and assigning these domains to different flamelet histories associated with Eulerian marker particles. For each of these marker particles, a transport equation has to be solved. Experiments conducted with an Audi DI diesel engine equipped with a piezo injector and running with diesel fuel are compared to simulations using the surrogate fuel. The use of multiple flamelets, each having a different history, significantly improves the description of the ignition phase, leading to a better prediction of pressure, heat release, and exhaust emissions such as soot and NOx. The effect of the number of flamelet particles on the predictions is discussed.