Stratified Flames Research Articles

A Presumed Joint pdf Model for Turbulent Combustion with Varying Equivalence Ratio

Modeling of the joint probability density function of the mixture fraction and progress variable with a given covariance value is studied. This modeling is validated using experimental and direct numerical simulation (DNS) data. A very good agreement with experimental data of turbulent stratified flames and DNS data of a lifted hydrogen jet flame is obtained. The effect of using this joint pdf modeling to calculate the mean reaction rate with a flamelet closure in Reynolds averaged Navier–Stokes (RANS) calculation of stratified flames is studied. The covariance effect is observed to be large within the flame brush. The results obtained from RANS calculations using this modeling for stratified jet- and rod-stabilized V-flames are discussed and compared to the measurements as a posteriori validation for the joint probability density function model with the flamelet closure. The agreement between the computed and measured values of flame and turbulence quantities is found to be good.

A Priori DNS Modeling of the Turbulent Scalar Fluxes for Low Damköhler Number Stratified Flames

Statistically planar turbulent stratified flames for different initial intensities of decaying turbulence have been simulated for global equivalence ratios <φ> = 0.7 and <φ> = 1.0 using three-dimensional simplified chemistry–based direct numerical simulations (DNS). The simulation parameters are chosen in such a manner that all the cases considered here represent low Damköhler number thin reaction zones regime combustion. The DNS data have been used to analyze the statistical behaviors of the turbulent fluxes of fuel mass fraction Y F and mixture fraction ξ, and their variances and covariances. It has been found that turbulent scalar flux of fuel mass fraction predominantly exhibits gradient-type transport in all cases considered in the current study, but countergradient-type transport has also been observed in the globally stoichiometric flame for small values of turbulence intensity where the flame normal acceleration due to chemical heat release overcomes the effects of turbulent velocity fluctuation. Models have been identified for the turbulent fluxes of fuel mass fraction Y F and mixture fraction ξ, and their variances and covariances, which are shown to satisfactorily capture the qualitative and quantitative behaviors of the corresponding quantities extracted from DNS data. It has been shown that the accurate modeling of the scalar fluxes of fuel mass fraction and mixture fraction plays a key role in the modeling of scalar fluxes of the fuel mass fraction variance and the covariance of fuel mass fraction and mixture fraction.

Reaction-diffusion manifolds for unconfined, lean premixed, piloted, turbulent methane/air systems

The efficient simulation of technical combustion processes requires a reduced description of the thermochemical state. Especially in the case of turbulent combustion, appropriate reduction methods are required to save computational costs, while preserving a desired accuracy. The current work deals with the application of the so-called reaction-diffusion manifold (REDIM) method. It is used to identify a two-dimensional manifold in the composition space, describing the thermochemical states of an unconfined, lean premixed, piloted, turbulent methane/air flame. This stratified flame is an interesting case, because it coveres different flamelet regimes, namly premixed flamelets and premixed flamelets mixing with air, which complicates the application of flamelet methods. Although the REDIM method is not restricted to a specific flamelet regime, previous works dealt either with premixed or diffusion flames. Thus in the current work, the application of the REDIM method is extended to an intermediate flamelet regime. The application of the method is demonstrated step by step, which includes the detailed description of determining an initial manifold and of estimating gradients. Eventually a two-dimensional manifold for a physically motivated gradient estimation is presented and analyzed by a comparison to detailed flamelet trajectories. The discussion shows, that the manifold accounts for important premixed and diffusion scenarios of the turbulent flame. Therefore the REDIM can be used in subsequent CFD simulations, which is not part of the current work.

Large eddy simulations of the Darmstadt turbulent stratified flame series

Large-eddy simulations of the Darmstadt turbulent stratified flame (TSF) burner are presented. The TSF burner is operated under a wide variety of conditions. A case without stratification or shear, and one with stratification but no shear, are considered for validation purposes. A combined mixture fraction and reaction progress variable approach is used, with the reaction rate modelled by a fractal flame wrinkling flame surface density model, where the laminar flame speed is obtained as a function of the mixture fraction. The simulation results and the suitability of the modelling approach are verified and validated through comparison of mean and variance of axial and radial velocity, temperature and mixture fraction with experimental data. The effects of stratification on the flame flow field are then studied by comparing the two reactive simulations, and studying the impact of the cross-scalar dissipation rate term. Finally, the occurrence of potential back- or front-supported stratification is examined through statistical analysis of the flame normal directions. The modelling approach employed was found to produce very good predictions of the TSF burner, with both back- and front-supported stratification modes occurring, the former being considerably more likely.

The structure of turbulent stratified and premixed methane/air flames II: Swirling flows

Experimental results are presented from a series of turbulent methane/air stratified flames stabilized on a swirl burner. Nine operating conditions are considered, systematically varying the level of stratification and swirl while maintaining a lean global mean equivalence ratio of ϕ¯=0.75. Scalar data are obtained from Rayleigh/Raman/CO laser induced fluorescence (CO-LIF) line measurements at 103μm resolution, allowing the behavior of the major combustion species—CH4, CO2, CO, H2, H2O and O2—to be probed within the instantaneous flame front. The corresponding three-dimensional surface density function and thermal scalar dissipation rate are investigated, along with geometric characteristics of the flame such as curvature and flame thickness. Hydrogen and carbon monoxide levels within the flame brush are raised by stratification, indicating models with laminar premixed flame chemistry may not be suitable for stratified flames. However, flame surface density, scalar dissipation and curvature all appear insensitive to the degree of stratification in the flames surveyed.

The structure of turbulent stratified and premixed methane/air flames I: Non-swirling flows

A series of flames in a turbulent methane/air stratified swirl burner is presented. The degree of stratification and swirl are systematically varied to generate a matrix of experimental conditions, allowing their separate and combined effects to be investigated. Non-swirling flows are considered in the present paper, and the effects of swirl are considered in a companion paper (Part II). A mean equivalence ratio of ϕ¯=0.75 is used, with ϕ for the highest level of stratification spanning 0.375–1.125. The burner features a central bluff-body to aid flame stabilization, and the influence of the induced recirculation zone is also considered. The current work focuses on non-swirling flows where two-component particle image velocimetry (PIV) measurements are sufficient to characterize the main features of the flow field. Scalar data obtained from Rayleigh/Raman/CO laser induced fluorescence (CO-LIF) line measurements at 103μm resolution allow the behavior of key combustion species—CH4, CO2, CO, H2, H2O and O2—to be probed within the instantaneous flame front. Simultaneous cross-planar OH-PLIF is used to determine the orientation of the instantaneous flame normal in the scalar measurement window, allowing gradients in temperature and progress variable to be angle corrected to their three dimensional values. The relationship between curvature and flame thickness is investigated using the OH-PLIF images, as well as the effect of stratification on curvature.The main findings are that the behavior of the key combustion species in temperature space is well captured on the mean by laminar flame calculations regardless of the level of stratification. H2 and CO are significant exceptions, both appearing at elevated levels in the stratified flames. Values for surface density function and by extension thermal scalar dissipation rate are found to be substantially lower than laminar values, as the thickening of the flame due to turbulence dominates the effect of increased strain. These findings hold for both premixed and stratified flames. The current series of flames is proposed as an interesting if challenging set of test cases for existing and emerging turbulent flame models, and data are available on request.

A Filtered Tabulated Chemistry model for LES of stratified flames

The model called Filtered Tabulated Chemistry for LES (F-TACLES), recently developed for turbulent premixed combustion [1], is extended in the present article to stratified flames where the equivalence ratio is not spatially uniform. The principle is to generate a look-up table where thermo-chemical variables, computed from a set of filtered laminar unstrained premixed flamelets, are stored as a function of the filtered progress variable Y˜c, the filtered mixture fraction c˜, and the mixture fraction subgrid scale variance z″2∼ for a given value of the LES filter size Δ. From this database and using a classical subgrid scale flame surface wrinkling model, a closure model for the filtered progress variable balance equation is then proposed. As only three balance equations (respectively for Y˜c, z˜ and z″2∼) are added to the Navier–Stokes system, this turbulent combustion model is straightforward to implement and is not expensive in terms of CPU time. After preliminary tests on 1-D planar flames, the proposed strategy is applied to a 3-D simulation of a swirled turbulent stratified flame. The agreements observed between numerical and measured data validate the modeling strategy. Finally, it is observed that the flame position cannot be captured without accounting for the subgrid scale mixture fraction heterogeneities that impact on the flame propagation. This result highlights the importance of the mixture fraction filtered density function on the filtered chemical reaction rate.

Experimental and numerical analysis of a lean premixed stratified burner using 1D Raman/Rayleigh scattering and large eddy simulation

A joint experimental and numerical approach is conducted to investigate a turbulent lean premixed stratified flame (flame TSF-A of the Darmstadt stratified burner). First, the distribution of the temperature and main species is obtained experimentally by 1D Raman/Rayleigh scattering. These measurements are used to provide insight into the physics of stratified combustion and to serve as validation data for numerical models. As a second step Large Eddy Simulations (LES) are carried out using tabulated chemistry combined with a thickened flame approach. The chemistry table uses the progress variable and additionally the mixture fraction as a second controlling variable to account for the variation in equivalence ratio. To test the applicability of the model, the influence of artificial thickening on the simulation of stratified flames is investigated by means of a one-dimensional test case. Furthermore, two different grids are used in the three-dimensional simulations to assess the modeling impact. The data obtained from the measurements and simulations are presented and compared along radial profiles at several axial positions. Further information about the interaction of the reaction zone with the mixing layer has been extracted from the LES which is currently not accessible by experiments.

Thermodynamic and optical characterizations of a high performance GDI engine operating in homogeneous and stratified charge mixture conditions fueled with gasoline and bio-ethanol

UltraViolet–visible imaging measurements were carried out in a gasoline direct injection (GDI) engine in order to investigate the spray and combustion evolution of gasoline and pure bio-ethanol fuel. Two different starts of injection, early injection (homogeneous charge) and late injection (stratified charge), were tested in two different engine conditions, 1000rpm idle and 1500rpm medium load as representative point of urban new European driving cycle (NEDC).Measurements were performed in the optically accessible combustion chamber made by modifying a real 4-stroke, 4-cylinder, high performance GDI engine. The cylinder head was instrumented by using an endoscopic system coupled to high spatial and temporal resolution cameras in order to allow the visualization of the fuel injection and the combustion process.All the optical data were correlated to the in-cylinder pressure-based indicated analysis and to the gaseous and solid emissions. Wide statistics were performed for all measurements in order to take into account the cycle-to-cycle variability that characterized, in particular, the idle engine condition.Optical imaging showed that gasoline spray was more sensible to air motion and in-cylinder pressure than ethanol’s, for all the investigated conditions. The stratified flame front for both fuels was about 40% faster compared to homogeneous in the first phase, due to the A/F ratio local distribution. It leads to better performance in terms of stability and maximum pressure, even if the late injections produce more soot and UHC emissions due to fuel impingement.Ethanol combustion shows less diffusive flames than gasoline. A lower amount of soot was evaluated by two color pyrometry method in the combustion chamber and measured at the exhaust.

The Impact of Variable Inlet Mixture Stratification on Flame Topology and Emissions Performance of a Premixer/Swirl Burner Configuration

The work presents the assessment of a low emissions premixer/swirl burner configuration utilizing lean stratified fuel preparation. An axisymmetric, single- or double-cavity premixer, formed along one, two, or three concentric disks promotes propane-air premixing and supplies the combustion zone at the afterbody disk recirculation with a radial equivalence ratio gradient. The burner assemblies are operated with a swirl co-flow to study the interaction of the recirculating stratified flame with the surrounding swirl. A number of lean and ultra-lean flames operated either with a plane disk stabilizer or with one or two premixing cavity arrangements were evaluated over a range of inlet mixture conditions. The influence of the variation of the imposed swirl was studied for constant fuel injections. Measurements of turbulent velocities, temperatures, OH* chemiluminescence and gas analysis provided information on the performance of each burner set up. Comparisons with Large Eddy Simulations, performed with an 11-step global chemistry, illustrated the flame front interaction with the vortex formation region under the influence of the variable inlet mixture stratifications. The combined effort contributed to the identification of optimum configurations in terms of fuel consumption and pollutants emissions and to the delineation of important controlling parameters and limiting fuel-air mixing conditions.

Fluid dynamics modeling of a stratified disk burner in swirl co-flow

The work describes Large Eddy Simulations of an unconfined partially premixed propane-air burner with swirl. A premixing section consisting of two axisymmetric cavities, formed along three concentric disks, supplies the afterbody disk recirculation with a radial equivalence ratio gradient. Flame stabilization is established there supported by a swirling co-flow of air. Lean and ultra-lean conditions are examined by regulating the fuel-air stratification and the swirl. An implicit finite-volume LES method is applied, employing the dynamic Smagorinsky model and the Eddy Dissipation Concept. Propane combustion and NOx production are modelled with an eleven-step scheme. The LES reveals intrinsic features of the flame front anchoring close to the burner rim under the influence of inlet mixture stratification and reproduces large and small scale features of the primary vortex flame stabilization and its interaction with the downstream vortex breakdown zone. Comparisons with measurements indicate a satisfactory agreement in the mean and turbulent velocity fields, while the impact of combustion and heat release on the wake development is reproduced well by the model. Deviations in the mean and fluctuating fields increase in the vicinity of the interaction of the twin vortex system. Preliminary comparisons between simulations and global species measurements suggest that the turbulence-chemistry model represents satisfactorily the experimental trends. ▸ Large Eddy Simulations of stratified disk burner are performed. ▸ Combustion is stabilized on a cavity premixer/disk burner under the action of swirl. ▸ Turbulent reactions were treated with the EDC model with eleven-step chemistry. ▸ Simulations represented satisfactorily both large and small scale burner characteristics. ▸ Intrinsic features of stratified flame stabilization in the primary vortex are exposed.

The nonlinear heat release response of stratified lean-premixed flames to acoustic velocity oscillations

This paper analyzes the forced response of swirl-stabilized lean-premixed flames to high-amplitude acoustic forcing in a laboratory-scale stratified burner operated with CH 4 and air at atmospheric pressure. The double-swirler, double-channel annular burner was specially designed to generate high-amplitude acoustic velocity oscillations and a radial equivalence ratio gradient at the inlet of the combustion chamber. Temporal oscillations of equivalence ratio along the axial direction are dissipated over a long distance, and therefore the effects of time-varying fuel/air ratio on the response are not considered in the present investigation. Simultaneous measurements of inlet velocity and heat release rate oscillations were made using a constant temperature anemometer and photomultiplier tubes with narrow-band OH ∗/CH ∗ interference filters. Time-averaged and phase-synchronized CH ∗ chemiluminescence intensities were measured using an intensified CCD camera. The measurements show that flame stabilization mechanisms vary depending on equivalence ratio gradients for a constant global equivalence ratio ( ϕ g = 0.60). Under uniformly premixed conditions, an enveloped M-shaped flame is observed. In contrast, under stratified conditions, a dihedral V-flame and a toroidal detached flame develop in the outer stream and inner stream fuel enrichment cases, respectively. The modification of the stabilization mechanism has a significant impact on the nonlinear response of stratified flames to high-amplitude acoustic forcing ( u′/ U ∼ 0.45 and f = 60, 160 Hz). Outer stream enrichment tends to improve the flame’s stiffness with respect to incident acoustic/vortical disturbances, whereas inner stream stratification tends to enhance the nonlinear flame dynamics, as manifested by the complex interaction between the swirl flame and large-scale coherent vortices with different length scales and shedding points. It was found that the behavior of the measured flame describing functions (FDF), which depend on radial fuel stratification, are well correlated with previous measurements of the intensity of self-excited combustion instabilities in the stratified swirl burner. The results presented in this paper provide insight into the impact of nonuniform reactant stoichiometry on combustion instabilities, its effect on flame location and the interaction with unsteady flow structures.

Alignment statistics of active and passive scalar gradients in turbulent stratified flames

The scalar gradient alignment statistics for active and passive scalar gradients (e.g., fuel mass fraction gradient and mixture fraction gradient) are presented here for turbulent stratified flames. It has been shown that, under some conditions, the active scalar gradient may align with the most extensive principal strain rate in contrast to the passive scalar gradient that remains aligned with the most compressive principal strain rate even when the active scalar gradient aligns preferentially with the most extensive principal strain rate. Physical explanations are provided for the observed behaviors.

The structure of premixed and stratified low turbulence flames

Practical combustion systems typically operate in stratified regimes to leverage the advantages of a spatially varying mixture fraction field. Although these benefits are well known in industry, the fundamental physics underpinning these effects are currently an area of active research. In this paper simultaneous Rayleigh–Raman–LIF measurements of temperature and major species concentrations along a line are used to investigate the structure of a weakly turbulent stratified flame. Concurrent cross-planar OH-PLIF enables the extraction of flame orientation relative to the measurement line, as well as flame front curvature. The behavior of major species concentrations with respect to temperature is found to agree well with laminar flames at the local mixture fraction, even in stratified flows. However, measurements of the surface density function, ∣∇ c∣, and scalar dissipation, χ c , suggest that both premixed and stratified flames are spatially thicker at the microscale than corresponding laminar flames.

Petascale direct numerical simulation of turbulent combustion—fundamental insights towards predictive models

The advent of petascale computing applied to direct numerical simulation (DNS) of turbulent combustion has transformed our ability to interrogate fine-grained ‘turbulence-chemistry’ interactions in canonical and laboratory configurations. In particular, three-dimensional DNS, at moderate Reynolds numbers and with complex chemistry, is providing unprecedented levels of detail to isolate and reveal fundamental causal relationships between turbulence, mixing and reaction. This information is leading to new physical insight, providing benchmark data for assessing model assumptions, suggesting new closure hypotheses, and providing interpretation of statistics obtained from lower-dimensional measurements. In this paper the various roles of petascale DNS are illustrated through selected examples related to lifted flame stabilization, premixed and stratified flame propagation in intense turbulence, and extinction and reignition in turbulent non-premixed jet flames. Extending the DNS envelope to higher Reynolds numbers, higher pressures, and greater chemical complexity will require exascale computing in the next decade. The future outlook of DNS in terms of challenges and opportunities in this regard are addressed.

Statistical Analysis of Displacement Speed in Turbulent Stratified Flames: A Direct Numerical Simulation Study

Statistically planar turbulent stratified flames under different initial root mean square turbulent velocity fluctuation u′ levels are simulated for global equivalence ratios <φ> = 1.0 and <φ> = 0.7 using 3-dimensional compressible direct numerical simulations (DNS) with a modified single-step Arrhenius-type chemistry. For the simulations of stratified flames, a sinusoidal variation of equivalence ratio is introduced in the unburned reactants ahead of the flame. The simulation parameters are chosen in such a manner that the combustion situation belongs to the thin reaction zones regime in all cases. The DNS data has been used to analyze the statistical behavior of the displacement speed S d and its components (i.e., the reaction rate component S r , the normal molecular diffusion component S n , the tangential diffusion component S t , and the component arising due to reactant inhomogeneity S ξ) and their curvature and tangential strain rate dependence. The mean behavior of displacement speed S d and its local strain rate and curvature dependences have been analyzed in detail in terms of the statistical behaviors of the various components of displacement speed (i.e., S r , S n , S t , and S ξ). It has been observed that the contribution of S d due to reactant inhomogeneity S ξ is negligible in comparison to the other components and therefore S d is most significantly affected by the contribution of (S r + S n + S t ). The displacement speed S d statistics are presented in terms of its marginal probability density functions (PDFs) as well as its joint PDFs with curvature and tangential strain rate. It has been observed that S d shows a nonlinear negative correlation with curvature but the correlation strength decreases with increasing u′. The tangential strain rate response of S d has also been found to be dependent upon the turbulence velocity fluctuation level u′ with weak correlation in higher initial u′ cases and a modest positive correlation in the lower u′ cases. It has been found that the marginal PDFs of S r and S n in stratified flames show significant differences in comparison to the corresponding PDFs obtained for perfectly premixed flames with the same values of global equivalence ratio and u′. However, the local curvature and strain rate dependences of S d and its components are found to be qualitatively similar to the corresponding statistical behaviors obtained for perfectly premixed flames with the same values of global equivalence ratio and u′. It has been demonstrated that the curvature dependence of S d has important implications in the context of modeling the curvature stretch term in the Flame Surface Density transport equation.

Effects of equivalence ratio variation on lean, stratified methane–air laminar counterflow flames

The effects of equivalence ratio variations on flame structure and propagation have been studied computationally. Equivalence ratio stratification is a key technology for advanced low emission combustors. Laminar counterflow simulations of lean methane–air combustion have been presented which show the effect of strain variations on flames stabilized in an equivalence ratio gradient, and the response of flames propagating into a mixture with a time-varying equivalence ratio. ‘Back supported’ lean flames, whose products are closer to stoichiometry than their reactants, display increased propagation velocities and reduced thickness compared with flames where the reactants are richer than the products. The radical concentrations in the vicinity of the flame are modified by the effect of an equivalence ratio gradient on the temperature profile and thermal dissociation. Analysis of steady flames stabilized in an equivalence ratio gradient demonstrates that the radical flux through the flame, and the modified radical concentrations in the reaction zone, contribute to the modified propagation speed and thickness of stratified flames. The modified concentrations of radical species in stratified flames mean that, in general, the reaction rate is not accurately parametrized by progress variable and equivalence ratio alone. A definition of stratified flame propagation based upon the displacement speed of a mixture fraction dependent progress variable was seen to be suitable for stratified combustion. The response times of the reaction, diffusion, and cross-dissipation components which contribute to this displacement speed have been used to explain flame response to stratification and unsteady fluid dynamic strain.

Temperature and mixing field measurements in stratified lean premixed turbulent flames

Turbulent combustion under lean stratified conditions appears in many practical applications. However, relatively few experimental or theoretical studies have investigated stratified flames in detail. We present qualitative mixing and quantitative temperature field measurements of jet-like flames in a canonical burner geometry with the focus on comparisons of stratified and premixed flames. Planar laser-induced fluorescence of OH and acetone are used to mark the flame front and the mixing field, respectively. Laser Rayleigh scattering is used to probe the detailed structure of the temperature field. The burner consists of three staged concentric tubes, and flame stabilization is facilitated by a pilot flame burning inside the central tube. This arrangement minimizes the effects of the pilot flame on the shear and stratification layer in-between the two annular slots. Operating conditions are selected in accordance with previous flow field studies in this burner. Shear and stratification between the annular slots are varied independently from each other. Comparisons between stratified and premixed flames show that differences emerge primarily at large scales and are dominated by shear layers. The effect of stratification appears in instantaneous temperature profiles but is washed out in radial profiles of mean and rms temperatures. In combination with earlier flow field measurements, these data serve as well for validation of numerical simulations.

Experimental analysis of an oblique turbulent flame front propagating in a stratified flow

This paper details the experimental study of a turbulent V-shaped flame expanding in a nonhomogeneous premixed flow. Its aim is to characterize the effects of stratification on turbulent flame characteristics. The setup consists of a stationary V-shaped flame stabilized on a rod and expanding freely in a lean premixed methane-air flow. One of the two oblique fronts interacts with a stratified slice, which has an equivalence ratio close to one and a thickness greater than that of the flame front. Several techniques such as PIV and CH * chemiluminescence are used to investigate the instantaneous fields, while laser Doppler anemometry and thermocouples are combined with a concentration probe to provide information on the mean fields. First, in order to provide a reference, the homogeneous turbulent case is studied. Next, the stratified turbulent premixed flame is investigated. Results show significant modifications of the whole flame and of the velocity field upstream of the flame front. The analysis of the geometric properties of the stratified flame indicates an increase in flame brush thickness, closely related to the local equivalence ratio.

A comparative analysis of flame surface density metrics inpremixed and stratified flames

Flame surface density (FSD) is an important quantity in premixed turbulent combustion models, many of which use it in order to predict flame behaviour and characteristics. This quantity may be determined using a statistical method based on measurements of temperature and its gradient, or a geometric method based on 2D temperature or LIF imaging. Measurements of FSD obtained using both methods have been reported in the literature, but no direct comparisons have been made between these methods in premixed or stratified flames. This paper compares these methods in both premixed and stratified datasets (up to a stratification ratio of 3) from a slot burner and a swirl burner. Temperature, local equivalence ratio and three dimensional flame normal orientations were obtained from simultaneous cross plane OH-LIF and Rayleigh–Raman line measurements. Three dimensional progress variable gradients, which are used in the statistical FSD calculations, are derived by angle-correcting gradients along the measurement line. Differences between the statistical and geometrical methods of evaluating FSD are shown to be negligible for values of progress variable between 0.2 and 0.9, and differences at extreme values are detailed and explained. This good agreement between methods is demonstrated to hold over a broad range of flame operating condition.