Turbulent Premixed Jet Flames Research Articles

LES/PDF Modeling of Turbulent Premixed Flames with Locally Enhanced Mixing by Reaction

Large-eddy simulations (LES) combined with the transported probability density function (PDF) method are carried out for two turbulent piloted premixed methane-air jet flames (flame F1 and flame F3) to assess the capability of LES/PDF for turbulent premixed combustion. The conventionally used model for the sub-filter scale mixing time-scale (or the mixing frequency) fails to capture the premixed flames correctly. This failure is expected to be caused by the lack of the sub-filter scale premixed flame propagation property in the sub-filter scale mixing process when the local flame front is under-resolved. It leads to slower turbulent premixed flame propagation and wider flame front. A new model for specifying the sub-filter scale mixing frequency is developed to account for the effect of sub-filter scale chemical reaction on mixing, based on past development of models for the sub-filter scale scalar dissipation rate in premixed combustion. The new model is assessed in the two turbulent premixed jet flames F1 and F3. Parametric studies are performed to examine the new model and its sensitivity when combined with the different mixing models. Significantly improved performance of the new mixing frequency model is observed to capture the premixed flame propagation reasonably, when compared with the conventional model. The sensitivity of the flame predictions is found be relatively weak to the different mixing models in conjunction with the new mixing frequency model.

Comparative behavior of piloted turbulent premixed jet flames of C1[sbnd]C8 hydrocarbons

An exploratory study was performed on highly turbulent piloted premixed jet flames of straight chain (paraffin and olefin), branched, cyclo, and aromatic C1C8 hydrocarbons. The goal was to investigate fuel effects on global and local flame observables under flow conditions corresponding to the transition from the thin to the broken reaction zone regimes. The experiments were performed at atmospheric pressure, reactant temperature of 298K, jet Reynolds numbers of 12,500 and 25,000, and various equivalence ratios and fuels in order to assess the attendant effects on the flame characteristics. The effects of heat losses from the flame were considered also in order to sensitize kinetic effects. Measurements were performed using particle image velocimetry and spontaneous luminosity imaging. The averaged and fluctuating velocity components as well the average flame height were determined to scale in general with the laminar flame speed. The exception to this scaling was the height of methane flames that exhibit notable differences compared to large molecular weight fuels. Subtle differences were identified even among the large hydrocarbon flames and became evident from measurements of the instantaneous luminosity field. Finally, high-speed luminosity imaging revealed that there are apparent fuel effects on the local structure of the flame as evident by the distribution and size of luminous pockets. These results suggest that the fuel effects should be also the focus of further investigations by the community.

A comparison between direct numerical simulation and experiment of the turbulent burning velocity-related statistics in a turbulent methane-air premixed jet flame at high Karlovitz number

A three-dimensional (3D) direct numerical simulation (DNS) of an experimental turbulent premixed jet flame at high Karlovitz number was studied. The DNS resolution adequately resolves both the flame and turbulence structures. A reduced chemical mechanism for premixed CH4/air flames with NOx based on GRI-Mech3.0 was used, including 268 elementary reactions, and 28 transported species. Consistent post-processing methods were applied to both the DNS and experimental data to evaluate turbulent burning velocity-related statistics, namely the flame surface density (FSD), and the flame curvature. Good agreement was achieved for the 2D comparisons. The DNS data were further analysed and provide 3D statistics unattainable from the experiment. The ratio of the 3D and 2D flame surface densities was estimated. The results are comparable with other values reported for various experimental flames. The 3D and 2D flame curvatures were also compared and their distributions are shown to be quite different owing to the round on-average geometry. Instantaneous images of the heat release surrogate, [CH2O][OH], between the DNS and experiment agreed qualitatively. Various other experimentally obtainable surrogates for heat release rate including [CH2O][H], [CH2O][O], [HCO], and [CH] are also evaluated and compared using the DNS. The inner structure of the flame was compared between the DNS and experiment in terms of the joint PDFs of OH concentration and temperature. Generally good agreement was obtained; discrepancies may be due to the inconsistency of assumed equilibrium levels of OH concentration in the co-flow.

Experimental and numerical studies of fuel and hydrodynamic effects on piloted turbulent premixed jet flames

An experimental and numerical investigation of fuel and hydrodynamic effects is performed on piloted premixed jet flames. The investigation is carried out at a constant laminar flame speed, varying heat losses, jet Reynolds number, fuel molecular weight, and fuel chemical classification. Large Eddy Simulations are performed in an attempt to reproduce the behaviors observed experimentally. Simulations are compared against well-characterized boundary conditions, well-resolved two-dimensional velocity fields from particle image velocimetry, and line-of-sight CH⁎ profiles. Experimental results indicate that small amounts of heat losses may play a significant role on the jet reactivity as the flame heights scale with the heat loss from the jet. However, differences between flames with different fuels can still be seen in the absence of heat losses and these differences are magnified at higher Reynolds numbers. Particularly, methane flames are consistently taller and ethylene flames consistently shorter while other fuels present approximately the same flame height. LES reproduce the experimentally observed trends in global flame heights (effects of heat losses and Reynolds number) but some of the differences between fuels are not captured.

Visualization of multi-regime turbulent combustion in swirl-stabilized lean premixed flames

Simultaneous two-species imaging using single-shot planar laser-induced fluorescence have been performed to record high quality image pairs of CH/OH, CH/CH2O and OH/CH2O to visualize the flame front structures in swirl-stabilized lean premixed methane/air flames. The results show that the investigated flames exhibit various flame front structures distinctly in space, which covers: (1) the corrugated flamelet at the leading front; (2) the thin reaction-zone layer with distorted preheat zone in the shear-layer downstream; and (3) quenching, re-ignition and distributed reactions further downstream. The large variation of the flame characteristics in space stems from the entrainment of ambient cold air to the flame that results in flame quenching at the trailing edge of the flame. Thereafter, the unburned fuel/air mixture in the downstream region mixes with the entrained air and the hot combustion products from the upstream leading flame front, leading to reignition with distributed reactions. The current results provide a direct experimental evidence that distributed reactions can be a common combustion mode along with the results (Ref. [1], Zhou et al., 2015) recently reported in the highly turbulent premixed jet flames.

Fractal scaling of turbulent premixed flame fronts: Application to LES

The fractal scaling properties of turbulent premixed flame fronts have been investigated and considered for modeling sub-grid scales in the Large-Eddy-Simulation framework. Since the width of such thin reaction fronts cannot be resolved into the coarse mesh of LES, the extent of wrinkled flame surface contained in a volume is taken into account. The amount of unresolved flame front is estimated via the “wrinkling factor” that depends on the definition of a suitable fractal dimension and the scale at which the fractal scaling is lost, the inner cut-off length ∊i. In this context, the present study considers laboratory experiments and one-step reaction DNS of turbulent premixed jet flames in different regimes of turbulent premixed flames. Fractal dimension is found to be substantially constant and well below that typical of passive scalar fronts. The inner cut-off length shows a clear scaling with the dissipative scale of Kolmogorov for the regimes here considered. These features have been exploited performing Large Eddy Simulations. Good model performance has been found comparing the LES against a corresponding DNS at moderate Reynolds number and experimental data at higher Reynolds numbers.

Transport of inertial particles in a turbulent premixed jet flame

The heat release, occurring in reacting flows, induces a sudden fluid acceleration which particles follow with a certain lag, due to their finite inertia. Actually, the coupling between particle inertia and the flame front expansion strongly biases the spatial distribution of the particles, by inducing the formation of localized clouds with different dimensions downstream the thin flame front. A possible indicator of this preferential localization is the so-called Clustering Index, quantifying the departure of the actual particle distribution from the Poissonian, which would correspond to a purely random spatial arrangement. Most of the clustering is found in the flame brush region, which is spanned by the fluctuating instantaneous flame front. The effect is significant also for very light particles. In this case a simple model based on the Bray-Moss-Libby formalism is able to account for most of the deviation from the Poissonian. When the particle inertia increases, the effect is found to increases and persist well within the region of burned gases. The effect is maximum when the particle relaxation time is of the order of the flame front time scale. The evidence of this peculiar source of clustering is here provided by data from a direct numerical simulation of a turbulent premixed jet flame and confirmed by experimental data.

Finite Rate Chemistry Effects in Highly Sheared Turbulent Premixed Flames

Detailed scalar structure measurements of highly sheared turbulent premixed flames stabilized on the piloted premixed jet burner (PPJB) are reported together with corresponding numerical calculations using a particle based probability density function (PDF) method. The PPJB is capable of stabilizing highly turbulent premixed jet flames through the use of a small stoichiometric pilot that ensures initial ignition of the jet and a large shielding coflow of hot combustion products. Four lean premixed methane-air flames with a constant jet equivalence ratio are studied over a wide range of jet velocities. The scalar structure of the flames are examined through high resolution imaging of temperature and OH mole fraction, whilst the reaction rate structure is examined using simultaneous imaging of temperature and mole fractions of OH and CH2O. Measurements of temperature and mole fractions of CO and OH using the Raman–Rayleigh–LIF-crossed plane OH technique are used to examine the flame thickening and flame reaction rates. It is found that as the shear rates increase, finite-rate chemistry effects manifest through a gradual decrease in reactedness, rather than the abrupt localized extinction observed in non-premixed flames when approaching blow-off. This gradual decrease in reactedness is accompanied by a broadening in the reaction zone which is consistent with the view that turbulence structures become embedded within the instantaneous flame front. Numerical predictions using a particle-based PDF model are shown to be able to predict the measured flames with significant finite-rate chemistry effects, albeit with the use of a modified mixing frequency.

Terascale direct numerical simulations of turbulent combustion using S3D

Computational science is paramount to the understanding of underlying processes in internal combustion engines of the future that will utilize non-petroleum-based alternative fuels, including carbon-neutral biofuels, and burn in new combustion regimes that will attain high efficiency while minimizing emissions of particulates and nitrogen oxides. Next-generation engines will likely operate at higher pressures, with greater amounts of dilution and utilize alternative fuels that exhibit a wide range of chemical and physical properties. Therefore, there is a significant role for high-fidelity simulations, direct numerical simulations (DNS), specifically designed to capture key turbulence-chemistry interactions in these relatively uncharted combustion regimes, and in particular, that can discriminate the effects of differences in fuel properties. In DNS, all of the relevant turbulence and flame scales are resolved numerically using high-order accurate numerical algorithms. As a consequence terascale DNS are computationally intensive, require massive amounts of computing power and generate tens of terabytes of data. Recent results from terascale DNS of turbulent flames are presented here, illustrating its role in elucidating flame stabilization mechanisms in a lifted turbulent hydrogen/air jet flame in a hot air coflow, and the flame structure of a fuel-lean turbulent premixed jet flame. Computing at this scale requires close collaborations between computer and combustion scientists to provide optimized scaleable algorithms and software for terascale simulations, efficient collective parallel I/O, tools for volume visualization of multiscale, multivariate data and automating the combustion workflow. The enabling computer science, applied to combustion science, is also required in many other terascale physics and engineering simulations. In particular, performance monitoring is used to identify the performance of key kernels in the DNS code, S3D and especially memory intensive loops in the code. Through the careful application of loop transformations, data reuse in cache is exploited thereby reducing memory bandwidth needs, and hence, improving S3D's nodal performance. To enhance collective parallel I/O in S3D, an MPI-I/O caching design is used to construct a two-stage write-behind method for improving the performance of write-only operations. The simulations generate tens of terabytes of data requiring analysis. Interactive exploration of the simulation data is enabled by multivariate time-varying volume visualization. The visualization highlights spatial and temporal correlations between multiple reactive scalar fields using an intuitive user interface based on parallel coordinates and time histogram. Finally, an automated combustion workflow is designed using Kepler to manage large-scale data movement, data morphing, and archival and to provide a graphical display of run-time diagnostics.

The compositional structure of highly turbulent piloted premixed flames issuing into a hot coflow

Simultaneous line measurements of major species and temperature by the Raman–Rayleigh technique, combined with CO two-photon laser-induced fluorescence and crossed-plane OH planar laser-induced fluorescence have been applied to a series of flames in the Piloted Premixed Jet Burner (PPJB). The PPJB is capable of stabilizing highly turbulent premixed jet flames through the use of a stoichiometric pilot and a large coflow of hot combustion products. Four flames with increasing jet velocities and constant jet equivalence ratios are examined in this paper. The characteristics of these four flames range from stable flame brushes with reaction zones that can be described as thin and “flamelet-like” to flames that have thickened reaction zones and exhibit extinction re-ignition behaviour. Radial profiles of the mean temperature are reported, indicating the mean thermal extent of the pilot and spatial location of the mean flame brush. Measurements of carbon monoxide (CO) and the hydroxyl radical (OH) reveal a gradual decrease in the conditional mean as the jet velocity is increased and the flame approaches extinction. Experimental results for the conditional mean temperature gradient show a progressive trend of reaction zone thickening with increasing jet velocities, indicating the increased interaction of turbulence with the reaction zone at higher turbulence levels. For the compositions examined, the product of CO and OH mole fractions ([CO][OH]) is shown to be a good qualitative indicator for the net rate of production of carbon dioxide. The axial variation of [CO][OH] is shown to correlate well with the mean chemi-luminescence of the flames including the extinction re-ignition regions. The experimental findings reported in this paper further support the hypothesis of an initial ignition region followed by extinction and re-ignition regions for certain PPJB flames.

Hydroxyl time-series measurements and simulations for turbulent premixed jet flames in the thickened preheat regime

Quantitative measurements of OH concentration time series are presented for turbulent lean-premixed, methane-air jet flames theoretically in the thickened preheat regime. Picosecond time-resolved laser-induced fluorescence (PITLIF) reveals unique differences between these premixed flames and previous non-premixed jet flames. Time-averaged [OH] measurements are used to identify mean flame structures and to discern how these structures are affected by varying bulk flow velocities and heat release. More importantly, hydroxyl time series are inspected to distinguish among three main regions in these turbulent premixed flames. These regions include the reacting side of the flame brush, the mixing side of the flame brush (radially outside the location of heat release), and above the flame tip. Although the main reaction zone appears to be broadened by its associated high turbulent intensity, a combination of statistical analysis plus flamelet simulations suggests that the primary internal structure responsible for the OH distribution remains constant across the mean flame brush. Therefore, the absolute concentration of OH depends principally on the intermittency of this instantaneous internal structure. Outside the mean flame brush, mixing of OH with co-flow air shifts the distribution of absolute OH concentrations. Distinct autocorrelation functions are found within the three different regions identified for these premixed flames. Across the flame brush, integral time scales are dominated by turbulent convection, as verified by flamelet simulations. Above the flame tip, integral time scales are determined by a competition between turbulent convection and the reaction rate for OH destruction.

Topology of turbulent premixed flame fronts resolved by simultaneous planar imaging of LIPF of OH radical and rayleigh scattering

Planar images of Rayleigh scattering and laser-induced predissociative OH-fluorescence (OH-LIPF) have been obtained simultaneously in turbulent premixed jet flames on a single-shot basis. The geometric structure of temperature and OH isocontours were extracted for fractal analysis. A power-law fractal behavior can be identified in the ensemble-averaged flame length measure. It was found that the inner and outer cut-off scales of OH contours are larger than those of the iso-temperature contours; while the OH images show comparatively smaller fractal dimensions. The joint-pdf ’s between flame temperature and OH LIPF signals at different heights are also derived to evaluate the flame stretch effect on local flame structure. Comparison of image pairs near the extinction limit suggests that Rayleigh thermometry is more adequate to characterize the fine-scale flame front wrinkling in highly stretched turbulent premixed flames.

An evaluation of flame surface density models for turbulent premixed jet flames

We report a computational study of a turbulent premixed jet flame stabilized in a laminar coflow of a reactant mixture at the same stoichiometric ratio as the turbulent flame. The performance of four different flame surface density (FSD) models, earlier used by Duclos et al. [1] for studying flame propagation in constant density frozen turbulence, and that of the reaction progress variable approach of Bray and Moss (BM) is evaluated by comparison with existing experimental data. We also describe a sensitivity analysis of predictions to inlet turbulence quantities and inlet flame surface density. The predictions of the Mantel and Borghi (MB) model are observed to be highly sensitive to inlet turbulence quantities, while those of the coherent flame (CF) model and BM models are less sensitive. The mean velocity and temperature predictions are insensitive to the inlet FSD distribution because of the strong production and dissipation terms in the transport equation for FSD. The predictions of mean velocity and temperature profiles are compared with the experimental data of Chen et al. [2]. Among the FSD models, the CF and MB models gave good estimates of mean velocity and temperature. The Cant, Pope, and Bray (CPB) model overpredicted the mean temperatures slightly. The flame surface density model of Cheng and Diringer (CD) predicted too high temperatures. This is shown to be resulting from an overprediction of the generation of flame surface density. All the models predict the increase in flame brush thickness with distance from the injector exit, but the MB model provides the best quantitative estimates of the flame brush thickness.

Two-Dimensional Measurements of the Time Development of a Turbulent Premixed Flame

The fuel-air mixture concentration has been measured as a function of time in the central plane of a turbulent premixed jet flame. A stoichiometric hydrogen-air mixture was uniformly seeded with submicron-sized aerosols. The aerosols were consumed at the reaction front, and their presence was used to map the concentration of the fuel-air mixture. A sheet of laser light was formed with an Ar- laser and the elastically-scattered light from the flame was recorded in two dimensions at 48 kHz using a framing camera to record 20 frames. The interface between the unburned fuel-air mixture and reaction products was clearly visible in the scattered intensity distribution. From the sequence of images, the motion of the flame front and the convective motion of the flow structures are evident. Information on the local burning velocity was obtained by observing the change in the size of structures in a convective frame of reference.

NO x from fuel nitrogen in two-stage combustion

Gaseous fuels simulating those produced by coal gasification have been burned in single-stage and in two-stage processes. The first stage consisted of a fuel-rich turbulent premixed jet flame; the second was produced by injection of dilution air into the hot primary products. Product gases were sampled. Fuel-lean mixtures and also fuel-rich mixtures of methane-free fuels burned readily; when methane was a fuel component, auxiliary pilot air jets were required for rich flames. Small quantities of NH3 were incorporated into the fuel and fractions converted to NOx (yields) were measured. For single-stage burning of a fuel containing 1410 ppm of NH3 and no CH4 at an equivalence ratio (ER)=0.8 the yield was around 0.8; as the flame was made richer the yield grew progressively smaller until at the rich experimental limit it was around 0.1. When dilution air to produce a final ER=0.8 was injected into the hot rich products of CH4-free fuels there was only a slight increase in NOx; thus the two-stage process reduced the yields of NOx by roughly a factor of 8. When dilution air was added in the same way to the rich primary products of CH4-containing fuels, the result depended strongly on the primary ER. For primary mixtures not too close to the rich limit the final yield of NOx was decreased only slightly; but near the rich limit, the final overall yield for ER=0.80 increased sharply. this is attributed to passage of unburned NH3 through the primary flame; and it sets a lower limit on the attainable NOx levels.