Steady Flamelet Approach Research Articles

Numerical Predictions of Soot Formation in Kerosene/Air Jet Diffusion Flame

Soot volume fraction in a turbulent diffusion flame burning kerosene/air is simulated using steady flamelet approach. The soot inception model based on the formation rate of three and two ring aromatics is used, which further accounts for inception, coagulation, surface growth, and oxidation processes. The Favre-averaged governing equations of mass, momentum, and energy are solved in conjunction with k-? turbulence model in ANSYS-FLUENT. The fairly detailed reaction mechanism for kerosene/air is coupled to the turbulent flow field by steady laminar flamelet approach, while the soot is calculated using Moss-Brookes model. The predictions are found to match well with the measurements where no significant effects are observed with the inclusion of higher order hydrocarbon as precursors and surface growth.

REDIM reduced chemistry for the simulation of counterflow diffusion flames with oscillating strain rates

The dynamic behaviour and structures of laminar counterflow diffusion methane flames under oscillatory strain rates are investigated based on both detailed and reduced chemistry. It is known both from experiment and numerical studies that flame can deviate from its quasi-steady manner for high frequencies, which makes the well-known steady laminar flamelet model questionable. In this work, the Reaction-diffusion manifolds (REDIM) concept for simplified chemistry is applied to study the laminar counterflow diffusion flames with oscillating strain rates. In the construction of REDIM, one needs little knowledge of the studied system, and the REDIM reduced chemistry can cover both stable and unstable (extinction) regimes of the studied system. It is shown that the REDIM can capture both the steady and unsteady behaviours of the flame structures even for high frequencies, where the standard laminar steady flamelet approach is not appropriate.

Large Eddy Simulation on the Effects of Pressure on Syngas/Air Turbulent Nonpremixed Jet Flames

ABSTRACT The influence of increasing pressure on nonpremixed syngas/air turbulent jet flames is numerically investigated using large eddy simulations in conjunction with a steady laminar flamelet approach. The applicability of the steady flamelet approach is assessed through an extensive parametric study of laminar counterflow flames and tangential stretching rate analysis on target flame structures at different pressures. Two sets of large eddy simulations, exploring pressure values up to atm, are carried out. The first one (series A) is characterized by a constant jet Reynolds number, while the second one (series B) is characterized by a constant jet inlet velocity. Both campaigns show narrower flame brushes and reduced radical concentrations with increasing pressure. While for series A the flame length is not sensitive to pressure, a longer flame brush is noticed for series B, being mainly caused by the increased mass flow rate. The sensitivity of the local flame behavior to pressure, such as the OH layer thickness and position, is compared to the available experimental results, showing similar trends with a satisfactory agreement.

Zonal Hybrid Reynolds-Averaged Navier–Stokes/Large-Eddy Simulation of a Hydrogen-Fueled Scramjet Combustor

A zonal hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation framework is implemented to investigate the diffusion combustion of hydrogen–air in a model scramjet combustor. The computational domain is segregated into Reynolds-averaged Navier–Stokes and large-eddy simulation regions. An upwind convective flux discretization method and shear-stress transport turbulence models are applied to the Reynolds-averaged Navier–Stokes region, and the low-dissipative convective flux discretization method and improved delayed detached-eddy simulation are applied to the large-eddy simulation region. To generate turbulence at the Reynolds-averaged Navier–Stokes/large-eddy simulation interface, a synthetic-eddy method is employed. The geometries and boundary conditions of the model scramjet combustor are based on an experiment conducted at German Aerospace Center. The turbulent hydrogen diffusion flame is modeled using the steady flamelet approach. A total of nine computation cases are examined to analyze the influence of the numerical methods. As a result, the zonal hybrid method using the low-dissipative convective flux discretization method and synthetic-eddy method shows the best agreement with the experimental data in terms of quantitative temperature and rms velocity fluctuation profiles.

Application of the steady flamelet model on a lab-scale and an industrial furnace for different oxygen concentrations

In the present study a numerical and experimental investigation was done on the impact of oxy-fuel combustion in a lab-scale furnace. For combustion and radiation modelling the steady flamelet approach with 17 species and 25 reactions associated with a WSGG (weighted sum of grey-gases) model was used. CFD (computational fluid dynamics) model was validated by measured temperatures and heat fluxes with different O2 concentrations. It was found that simulated temperatures and heat fluxes were in close agreement with the measurements in the full range of oxygen enrichment. Although 17 species were considered the calculation time was significantly reduced by the steady flamelet approach compared to commonly used eddy dissipation concept models. Predicted and measured data revealed gas savings of 8.2% by an O2 concentration of 25 vol% instead of 21 vol%. Maximum gas savings were determined for 100 vol% O2 with a value of 16.7%. The CFD model was also applied to a simulation of an 18.2 MW walking hearth furnace under air-fired conditions which should be adapted for oxy-fuel combustion in the future. Results from CFD showed a heat flux of 9.15 MW compared to the required 9.33 MW according to the material data and production rate.

CFD and experimental analysis of a 115 kW natural gas fired lab-scale furnace under oxy-fuel and air–fuel conditions

This paper investigates a natural gas fired lab-scale furnace with a thermal input between 28 and 115kW under different O2/N2 ratios in the oxidizer by computational fluid dynamics (CFD). Results of the simulation were confronted with temperature measurements inside the furnace and heat flux measurements on a water cooled plate inside the furnace. The main goal of this work was to use a detailed chemical mechanism and reduce the calculation time. This was achieved with the steady flamelet (SFM) approach. The advantage of the SFM approach is that the computational chemical calculation can be pre-processed and stored in look up tables. Only two additional equations have to be solved to determine the chemical reaction in the flow field. For the simulation the detailed mechanism skeletal25 was used with 17 species and 25 reactions. Additionally the furnace was simulated with the eddy dissipation concept model (EDC) which other authors mainly use to describe oxy-fuel combustion. For the EDC simulation a refined version of the 4-step mechanism proposed by Jones and Lindstedt was used. The EDC simulation was used as a benchmark to determine the time saving potential of the SFM approach. With the SFM approach the calculation time could be reduced from 4weeks to 4days on 8 CPU cores, although a detailed mechanism was being used. The predicted temperatures of the CFD simulations were in good accordance with the measurements and showed the applicability of the skeletal25 mechanism with the SFM approach under different combustion environments. In metal melting or reheating furnace the right prediction of the heat flux on the goods is crucial. Therefore the heat flux on a water cooled plate inside the furnace was determined by measurements and CFD calculations for different combustion environments. For the heat flux at a temperature level of 1070°C the CFD calculation showed a maximum relative error of 5% to the measurements for 21, 25, 30, 45 and 100Vol% O2 in the oxidizer. For the temperature level of 1200°C the maximum error increases, especially for O2 concentration in the oxidizer higher than 45% up to 12%. Both the experiments and the numerical model showed an increase in furnace efficiency with increasing oxygen in the oxidizer. A maximum efficiency of 76% was observed for 100Vol% in the oxidizer compared to 48% at 21Vol% O2. This shows the fuel saving potential of oxygen or oxygen enriched combustion.

The usability and limits of the steady flamelet approach in oxy-fuel combustions

This paper investigates two furnaces which work under oxy-fuel condition with natural gas. One is a 0.8 MW furnace where detailed inflame measurements are available. The other furnace is an 11.5 kW lab-scale furnace with temperature measurements. The furnaces were investigated by CFD (Computational fluid dynamics) analysis. The main focus was on using combustion models that are not computationally demanding. Therefore the SFM (steady flamelet) approach was used with two detailed mechanisms. The advantage of the SFM is that the calculation time can be reduced from 4 weeks to 4 days on 8 CPU-cores. The applicability of two detailed mechanisms under oxy-fuel condition is pointed out in this paper. The investigation showed that the skeletal25 mechanism and the SFM are in very good accordance with measurements. If the strain rate between CH4 and O2 stream is too low, the SFM fails to predict the flame shape correctly. The influence of three different turbulence models was also investigated. Furthermore simulations with the eddy dissipation model and numerically expensive eddy dissipation concept model were conducted. Different WSGGM (weighted sum of grey gases model) were applied. The comparison of the WSGGMs showed that the difference between them is insignificant for small furnaces.

Numerical investigation of the steady flamelet approach under different combustion environments

In the present work a CFD study of a natural gas fired lab-scale furnace under different oxygen enrichments was done. Results were compared to measured temperatures and heat fluxes on a thermal sink inside the furnace. The main emphasis was to apply the steady flamelet model (SFM) with a detailed chemical mechanism to predict the flow field, temperature and species concentrations in high temperature applications. A skeletal mechanism, which considered 25 reversible reactions and 17 species, showed close agreement with the measurement for all oxygen concentrations in the oxidizer. Turbulence modelling was done by the standard and realizable k-epsilon models as well as the Reynolds stress model (RSM). It was found, that standard k-epsilon model calculates too high temperatures in the vicinity of the burner. Results from realizable k-epsilon model and RSM were in close accordance with marginal differences in flame length and maximum temperatures. The CFD calculation with the SFM was also compared with a time expensive eddy dissipation concept (EDC) simulation with 46 reactions and 17 species. Both approaches calculated similar results for temperature and species concentrations. The calculation time was reduced significantly from 4weeks (EDC) to 6days with the SFM although dissociation effects and forming of radicals were taken into account. Considering melting and annealing furnaces, the heat flux to a thermal sink constitutes a major role for combustion processes. Heat flux to the thermal sink was determined by measurement and CFD predictions for different O2/N2 ratios in the oxidizer. CFD calculations revealed a maximum relative error to the measurement for the heat flux of 5% for 25, 30, 45 and 100Vol% O2 in the oxidizer. The numerical model and experiments highlighted a major increase on the furnace efficiency due to oxygen enrichment. A maximum efficiency of 67% was detected for natural-gas combustion with pure oxygen compared to 44% in case of 25Vol% O2. It was also found, that high oxygen concentrations lead to a homogeneous temperature distribution on the sink which is desired for melting and annealing applications.

Evaluation of a steady flamelet approach for use in oxy-fuel combustion

The present work investigates CFD analysis of an 11.5kW lab-scale furnace with oxy-natural-gas combustion for high temperature processes. In recent years many researches were done for oxy-fuel combustion with flue gas recycle in power plants. However, this paper considers combustion models which are applicable for oxy-fuel environment in melting and annealing furnaces. The main emphasis was to use the steady laminar flamelet model (SFM) associated with three detailed chemical mechanisms to predict the combustion process in the furnace. This approach was chosen to avoid time expensive chemical calculations in CFD while calculating the flow field, temperature and species concentrations. Just two additional transport equations for mixture fraction and mixture fraction variance have to be solved to determine the reactions in the flow field. It was found, that the skeletal mechanism with 17 species and 25 reversible reactions generated the best results with SFM. CFD calculations with SFM were compared with results from a 2-step eddy dissipation model (EDM) and a numerically expensive eddy dissipation concept (EDC) model with 17 species and 46 reversible reactions. The EDM was exposed to be unsuitable for modelling oxy-fuel combustion due to dissociation effects and forming of radicals in high temperature applications. In contrast to the EDM results, temperatures calculated by EDC and SFM showed close agreement with measured data in the furnace. Also for the species concentrations, obtained with both models, no significant differences were observed. A significant improvement was the reduction of computational time from 3weeks to 4days on 8 CPU-cores by using the SFM. The P1 and discrete ordinates (DO) model were also applied to solve the radiative transfer equation. A weighted sum of grey-gases model (WSGGM) was applied to model the radiative properties of the flue gas. Due to the special composition of the flue gas in oxy-fuel combustion, which consists mainly of H2O and CO2, the radiative properties should be treated with a non-grey gas approach. Nevertheless, the WSGGM was used in this study because of the small beam length of the furnace. Simulation with the P1 model overestimates the radiation, which leads to lower temperatures in the furnace. The DO model calculated temperatures in good agreement with the measurement with the SFM and EDC.

A numerical study on the effects of H2 addition in non-premixed turbulent combustion of C3H8–H2–N2 mixture using a steady flamelet approach

This study has been implemented in two sections. At first, the turbulent jet flame of DLR-B is simulated by combining the k–ε turbulence model and a steady flamelet approach. The DLR-B flame under consideration has been experimentally investigated by Meier et al. who obtained velocity and scalar statistics. The fuel jet composition is 33.2% H2, 22.1% CH4 and 44.7% N2 by volume. The jet exit velocity is 63.2 m/s resulting in a Reynolds number of 22,800. Our focus in the first part is to validate the developed numerical code. Comparison with experiments showed good agreement for temperature and species distribution. At the second part, we exchanged methane with propane in the fuel composition whilst maintaining all other operating conditions unchanged. We investigated the effect of hydrogen concentration on C3H8–H2–N2 mixtures so that propane mole fraction extent is fixed. The hydrogen volume concentration rose from 33.2% up to 73.2%. The achieved consequences revealed that hydrogen addition produces elongated flame with increased levels of radiative heat flux and CO pollutant emission. The latter behavior might be due to quenching of CO oxidation process in the light of excessive cold air downstream of reaction zone.

Modelling effects of subgrid-scale mixture fraction variance in LES of a piloted diffusion flame

A posteriori analysis of the statistics of two large-eddy simulation (LES) solutions describing a piloted methane–air (Sandia D) flame is performed on a series of grids with progressively increased resolution reaching about 10.5 million cells. Chemical compositions, density and temperature fields are modelled with a steady flamelet approach and parametrised by the mixture fraction. The difference between the LES solutions arises from a different numerical treatment of the subgrid scale (SGS) mixture fraction variance – an important quantity of interest in non-premixed combustion modelling. In the first case (model I), the variance transport equation is solved directly, while in the second (model II), an equation for the square of the mixture fraction is solved, and the variance is computed from its definition. The comparison of the LES solutions is based on the convergence properties of their statistics with respect to the turbulence resolution length scale. The dependence of the LES statistics is analysed for velocity and the mixture fraction fields, and tested for convergence. For the most part, the statistics converge for the finest grids, but the variance of the mixture fraction shows some residual grid dependence in the high-gradient regions of the jet near field. The SGS variance given by model I exhibits realisability everywhere, whereas in regions of the flame model II is non-realisable, predicting negative variances. Furthermore, the LES statistics of model I exhibit superior convergence behaviour.

Numerical analysis of the acoustic field of reacting flows via acoustic perturbation equations

The acoustic radiation of a turbulent non-premixed flame using a hybrid method is numerically simulated. The two-step method consists of an incompressible large-eddy simulation (LES) and acoustic perturbation equations (APE), which are reformulated to account for reacting flow effects (APE-RF). In reacting flows, hybrid methods to compute acoustic radiation have some advantages compared to the direct simulation of the acoustic field using a compressible LES. Considering the different characteristic length scales optimized schemes can be applied to each subproblem, i.e., to the hydrodynamic and the acoustic problem. This is of interest since the fluid mechanics is governed by the combustion process and to compute the highly intricate chemistry tailor-made schemes with reasonable computational costs combustion models can be implemented to resolve this phenomenon by, for instance, preprocessed databases for the chemical reactions, like in the steady flamelet approach. The APE-RF system possesses several source terms on the right-hand side (RHS), which are thoroughly discussed to their relation to various sound mechanisms. The acoustic sources describe the impact of unsteady heat release, non-isomolar combustion, species diffusion, heat diffusion, viscous effects, non-uniform mean flow and non-constant combustion pressure effects, and the influence of acceleration of density inhomogeneities. Moreover, an additional source term within the APE-RF pressure–density relation can be identified to describe the local acoustic wave amplification due to acoustic–flame interaction. It is evidenced that the well-known Rayleigh criterion can be directly given by this source. The unsteady heat release is shown to occur in the total time derivative of the density that is directly provided from an LES solution. By analyzing via the two-step method the acoustic field of an open turbulent non-premixed flame being generated just by the total temporal derivative of the density, and by comparing the numerical data with experimental findings the total substantial derivative is shown to describe for a wide frequency range the essential sound propagation caused by reacting flows. Nevertheless, it is also discussed that to simulate all the details over the complete frequency range additional source mechanisms occurring on the RHS of the APE-RF system are to be considered in the investigation.

Investigation of lengthscales, scalar dissipation, and flame orientation in a piloted diffusion flame by LES

This work investigates the structure of a diffusion flame in terms of lengthscales, scalar dissipation, and flame orientation by using large eddy simulation. This has been performed for a turbulent, non-premixed, piloted methane/air jet flame (Flame D) at a Reynolds-number of 22,400. A steady flamelet model, which was represented by artificial neural networks, yields species mass fractions, density, and viscosity as a function of the mixture fraction. This will be shown to suffice to simulate such flames. To allow to examine scalar dissipation, a grid of 1.97 × 10 6 nodes was applied that resolves more than 75% of the turbulent kinetic energy. The accuracy of the results is assessed by varying the grid-resolution and by comparison to experimental data by Barlow, Frank, Karpetis, Schneider (Sandia, Darmstadt), and others. The numerical procedure solves the filtered, incompressible transport equations for mass, momentum, and mixture fraction. For subgrid closure, an eddy viscosity/diffusivity approach is applied, relying on the dynamic Germano model. Artificial turbulent inflow velocities were generated to feature proper one- and two-point statistics. The results obtained for both the one- and two-point statistics were found in good agreement to the experimental data. The PDF of the flame orientation shows the tilting of the flame fronts towards the centerline. Finally, the steady flamelet approach was found to be sufficient for this type of flame unless slowly reacting species are of interest.

FLAMELET MODELING FOR COMBUSTION PROCESSES AND NOx FORMATION IN THE TURBULENT NONPREMIXED CO/H2/N2 JET FLAMES

Two turbulent nonpremixed jet flames having the same jet Reynolds number but different nozzle diameters have been numerically investigated by using the steady and unsteady flamelet models. These validation cases have been chosen to critically evaluate the predicative capability of the present turbulent combustion models whether the model is able to predict correctly both the similarity of temperature and major species by scaling with nozzle diameter, sensitivity of minor species to fluid-dynamic scaling and distribution of radicals and NO. In order to accurately resolve the physically and geometrically complex reacting flow fields, the present study adopts the unstructured grid finite-volume method with the cell-centered scheme and the edge-based storage. In terms of unconditional mean scalar structures, the steady flamelet model with the additional treatment for the slow processes such as radiation and NOx formation is able to yield the reasonable results. On the other hand, without additional ad-hoc procedure, the unsteady flamelet model based on a Lagrangian approach can correctly predict the experimentally observed streamwise evolutions of conditional mean scalar structure and unconditional means as well as the full NOx chemistry. Furthermore, in the context of unsteady flamelet model, the detailed discussions have been made for the potential error of the optically thin radiation model, the full NO chemistry and the effect of differential diffusion, which the steady flamelet approach hardly addresses.