Flame speed and tangential strain measurements in widely stratified partially premixed flames interacting with grid turbulence

Methane-air partially premixed flames subjected to grid-generated turbulence are stabilized in a two-slot burner with initial fuel concentration differences leading to stratification across the stoichiometric concentration. The fuel concentration gradient at the location corresponding to the flame base is measured using planar laser induced fluorescence (PLIF) of acetone in the non-reacting mixing field. Simultaneous PLIF of the OH radical and particle image velocimetry (PIV) measurements are performed to deduce the flow velocity and the flame front. These flames exhibit a convex premixed flame front and a trailing diffusion flame, with flow divergence upstream of the flame, as indicated by the instantaneous OH–PLIF, Mie scattering images, and PIV data. The mean streamwise velocity profile attains a global minimum just upstream of the flame front due to expansion of a gases caused by heat release. The flame speed measured just upstream of the flame leading edge is normalized with respect to the turbulent stoichiometric flame speed that takes into account variations in turbulent intensity and integral length scale. The turbulent edge flame speed exceeds the corresponding stoichiometric premixed flame speed and reaches a peak at a certain concentration gradient. The mean tangential strain at the flame leading edge locally peaks at the concentration gradient corresponding to the peak flame speed. The strain varies non-monotonically with the flame curvature unlike in a non-stratified curved premixed flame. The mechanism of peak flame speed is explained as the competition between availability of hot excess reactants from the premixed flame branches to the flame stretch induced due to flame curvature. The results suggest that the stabilization of lifted turbulent partially premixed flames occurs through an edge flame even at a relatively gentle concentration gradient. The strain is also evaluated along the flame front; it peaks at the flame leading edge and decreases gradually on either side of the leading edge. The present results also show qualitatively similar trends as those of laminar triple flames.     

Measurements of the heat release rate integral in turbulent premixed stagnation flames with particle image velocimetry

A new definition of turbulent consumption speed is proposed in this work that is based on the heat release rate integral, rather than the mass burning rate integral. Its detailed derivation and the assumptions involved are discussed in a general context that applies to all properly defined reaction progress variables. The major advantage of the proposed definition is that it does not require the thin-flame assumption, in contrast to previous definitions. Experimental determination of the local turbulent displacement speed, SD, and the local turbulent consumption speed, SC, is also demonstrated with the particle image velocimetry technique in three turbulent premixed stagnation flames. The turbulence intensity of these flames is of the same order of the laminar burning velocity. Based on the current data, a model equation for the local mean heat release rate is proposed. The relationship between SD and SC is discussed along with a possible modeling approach for the turbulent displacement speed.     

Relevance of the Bray number in the small-scale modeling of turbulent premixed flames

The present study is devoted to the analysis of the influence of expansion phenomena on turbulent small scales in premixed reactive flows. It is shown that, under certain conditions, the expansion that takes place across wrinkled laminar flamelets can be sufficient to control the fluctuating velocity gradients and associated dissipation rate functions. These conditions are fixed by the respective values of a set of nondimensional parameters, namely the turbulence Reynolds number ReT, the Bray number, and the ratio between integral length scale of turbulence and thermal flame front thickness. A new criterion is introduced that makes it possible to delineate the influence of expansion phenomena on small-scale turbulent premixed reactive flows. The relevance of this criterion is analyzed in the light of experimental results represented in the classical diagram of combustion regime. The present analysis confirms that special care is required to represent and include the influence of expansion phenomena when using either RANS or LES closures to model turbulent premixed combustion.     

Effects of turbulence and carrier fluid on simple, turbulent spray jet flames

This paper presents simultaneous LIF images of OH and the two-phase acetone fuel concentration as well as detailed single-point phase-Doppler measurements of velocity and droplet flux in three turbulent spray flames of acetone. This work forms part of a larger program to study spray jets and flames in a simple, well-defined geometry, aimed at providing a platform for developing and validating predictive tools for such flows. Spray flames that use nitrogen or air as droplet carrier are investigated and issues of flow field, droplet dispersion, size distribution, and evaporation are addressed. The joint OH/acetone concentration images reveal a substantial similarity to premixed flame behavior when the carrier stream is air. When the carrier is nitrogen, the reaction zone has a diffusion flame structure. There is no indication of individual droplet burning. The results show that evaporation occurs close to the jet centerline rather than in the outer shear layer. Turbulence does not have a significant impact on the evaporation rates. A small fraction of the droplets escapes the reaction zone unburned along the centerline and persists far downstream of the flame tip. The proportion of this droplet residue increases with shorter residence times as observed for the higher velocity flame.     

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.     

Suppression limits of low strain rate non-premixed methane flames

The suppression of low strain rate non-premixed flames was investigated experimentally in a counterflow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global strain rate of 20 s⁻¹. The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N₂, CO_2, and CF₃Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global strain rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global strain rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N₂ concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstrate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global strain rates was measured to be 0.841 ± 0.01 for N₂, 0.773 ± 0.009 for CO₂, and 0.437 ± 0.005 for CF₃Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global strain rates was measured as 0.299 ± 0.004 for N₂, 0.187 ± 0.002 for CO₂, and 0.043 ± 0.001 for CF₃Br.     

Effects of position and frequency of obstacles on turbulent premixed propagating flames

This paper studies the effects of the number and location of solid obstacles on the rate of propagation of turbulent premixed flames. A vented explosion chamber is constructed where controlled premixed flames are ignited from rest to propagate past grids or baffles plates as well as other solid obstacles strategically positioned in the chamber. Laser Induced Fluorescence (LIF) is used to image OH which is used as an indicator of the reaction zone while pressure transducers are used to obtain pressure-time traces. Single grids or baffle plates located at different distances from the ignition source are tested. Two as well as three baffle plates are also investigated in varying configurations. It is found that while the peak overpressure increases with increasing number of grids or baffle plates, a limit is reached where the pressure starts to decrease. The location of the obstacles is found to have a significant effect on the overpressure and the flame structure. Higher overpressures are obtained when the baffle plates and obstacles are stacked closer together hence not allowing turbulence to decay. LIF images for OH show that the reaction zones become more contorted with increasing number of baffle plates in the flame path.     

Effects of Lewis number on flame surface density transport in turbulent premixed combustion

The transport of flame surface density (FSD) in turbulent premixed flames has been studied using a database obtained from Direct Numerical Simulation (DNS). Three-dimensional freely propagating developing statistically planar turbulent premixed flames have been examined over a range of global Lewis numbers from 0.6 to 1.2. Simplified chemistry has been used and the emphasis is on the effects of Lewis number on FSD transport in the context of Reynolds-averaged closure modelling. Under the same initial conditions of turbulence, flames with low Lewis numbers are found to exhibit counter-gradient transport of FSD, whereas flames with higher Lewis numbers tend to exhibit gradient transport of FSD. Stronger heat release effects for lower Lewis number flames are found to lead to an increase in the positive (negative) value of the dilatation rate (normal strain rate) term in the FSD transport equation with decreasing Lewis number. The contribution of flame curvature to FSD transport is found to be influenced significantly by the effects of Lewis number on the curvature dependence of the magnitude of the reaction progress variable gradient, and on the combined reaction and normal diffusion components of displacement speed. The modelling of the various terms of the FSD transport equation has been analysed in detail and the performance of existing models is assessed with respect to the terms assembled from corresponding quantities extracted from DNS data. Based on this assessment, suitable models are identified which are able to address the effects of non-unity Lewis number on FSD transport, and new or modified models are suggested wherever necessary.     

Simultaneous stereo particle image velocimetry and double-pulsed planar laser-induced fluorescence of turbulent premixed flames

A new technique was developed and applied to the study of flame structure and flame-vortex interaction in turbulent premixed flames. Turbulent premixed flames were probed using simultaneous stereo particle image velocimetry (PIV) and a double-pulsed acetone planar laser-induced fluorescence system (PLIF). Two double-pulsed Nd:YAG lasers operating at 532 and 266 nm were used for the PIV and acetone PLIF measurements, respectively. The stereo PIV images were acquired using two double-frame CCD cameras, and two ICCD cameras were used to capture the PLIF signal. The diagnostic system was applied to study turbulent methane-air stoichiometric premixed flames at relatively high Reynolds numbers. Flame merging and the creation of pockets of both products and reactants were detected, and a very strong interaction between the flame front and the vortex structures was suggested in the simultaneous PIV/PLIF images. Double-pulsed PLIF data obtained for different time delays allowed statistical study of flame development. Three-dimensional turbulent fluxes of mean progress variable were obtained. It was shown that the fluxes obey the gradient diffusion hypothesis. The proposed diagnostic increases flexibility and range of measurements available for premixed flames.     

Effect of differential diffusion on turbulent lean premixed hydrogen enriched flames through structure analysis

This study presents the flame structure influenced by the differential diffusion effects and evaluates the structural modifications induced by the turbulence, thus to understand the coupling effects of the diffusively unstable flame fronts and the turbulence distortion. Lean premixed CH₄/H₂/air flames were conducted using a piloted Bunsen burner. Three hydrogen fractions of 0, 30% and 60% were adopted and the laminar flame speed was kept constant. The turbulence was generated with a single-layer perforated plate, which was combined with different bulk velocities to obtain varied turbulence intensities. Quasi-laminar flames without the plate were also performed. Explicit flame morphology was obtained using the OH-PLIF. The curvature, flame surface density and turbulent burning velocity were measured. Results show that the preferential transport of hydrogen produces negatively curved cusps flanked with positively curved bulges, which are featured by skewed curvature pdfs and consistent with the typical structure caused by the Darrieus-Landau instability. Prevalent bulge-cusp like wrinkles remain with relatively weak turbulence. However, stronger turbulence can break the bulges to be finer, and induce random positively curved cusps, therefore to destroy the bulge-cusp structures. Evident positive curvatures are generated in this process modifying the skewed curvature pdfs to be more symmetric, while the negative curvatures are not affected seriously. From low to high turbulence intensities, the hydrogen addition always strengthens the flame wrinkling. The augmentation of flame surface density and turbulent burning velocity with hydrogen is even more obvious at higher turbulence intensity. It is suggested that the differential diffusion can persist and even be strengthened with strong turbulence.     

Substantiating a fractal-based algebraic reaction closure of premixed turbulent combustion for high pressure and the Lewis number effects

A comprehensive set of nearly 100 atmospheric and high-pressure flame data of Kobayashi et al. are a good source for numerical analysis to address two main aspects in premixed turbulent combustion—high-pressure influence and effects of fuel type on the reaction rate. The present work deals with the lucid and realizable fractal-based reaction rate closure from Lindstedt and Váos (LV model) for premixed flames in the thin-flame limit. In this study, the reaction source term is customized on the eddy viscosity closure of turbulent transport, for practical reasons. Computed results from the LV model show the right qualitative trends with the experimental findings, as a function of turbulence. However, quantitative predictions of the original model are partly too low, and preclude the effects of pressure and fuel type on the reaction rate. With an extensive parametric study, based on numerical findings as well as on theoretical argumentation, the LV model is substantiated for these two effects. Results from the proposed tuned LV model are found to be in very good agreement with most of the measured data.     

Emission characteristics and heat release rate surrogates for ammonia premixed laminar flames

Ammonia is a carbon-free fuel that has the potential to meet increasing energy demand and to reduce CO₂ emissions. In the present work, the characteristics of pollutant emissions in ammonia premixed laminar flames are investigated using one-dimensional simulations, and heat release rate (HRR) surrogates for ammonia combustion are proposed. Both atmospheric and high-pressure conditions were considered, and four representative mechanisms for ammonia combustion were employed. It is shown that the total emission of NO and NH₃ achieves a minimum around an equivalence ratio (?) of 1.1 under atmospheric conditions, and there is no noticeable emission of NO and NH₃ for ? = 1.1 ~ 1.5 under high-pressure conditions. Three HRR surrogates, [NH₃][OH], [NH₂][O], and [NH₂][H], were proposed based on the analysis of HRR and elementary reaction profiles. The performance of HRR surrogates was found to vary with equivalence ratios. For example, with the Miller mechanism, [NH₃][OH], [NH₂][O], and [NH₂][H] have the best performance under atmospheric conditions at ? = 1.15, 0.95 and 1.05, respectively, and under high-pressure conditions at ? = 1.11, 0.87 and 0.96, respectively. Similar conclusions can also be drawn with other mechanisms. These findings provide valuable insights into emission control and flame identification of ammonia combustion.     

Strained flamelets for turbulent premixed flames, I: Formulation and planar flame results

A strained flamelet model is proposed for turbulent premixed flames using scalar dissipation rate as a parameter. The scalar dissipation rate of reaction progress variable is a suitable quantity to describe the flamelet structure since it is governed by convection-diffusion-reaction balance and it is defined at every location in the flamelets, which are represented by laminar flames in reactant-to-product opposed flow configuration. The mean reaction rate is obtained by using the flamelets reaction rate and the joint pdf of the progress variable and its dissipation rate. The marginal pdf of the progress variable is presumed to be β-pdf and the pdf of the conditional dissipation rate is taken to be log-normal. The conditional mean dissipation rate is obtained from modelled mean dissipation rate. This reaction rate closure is assessed using RANS calculations of statistically planar flames in the corrugated flamelets and thin reaction zones regimes. The flame speeds calculated using this closure are close to the experimental data of Abdel-Gayed et al. (1987) [27] for flames in both the regimes. Comparisons with other reaction rate closures showed the benefits of the strained flamelets approach.     

Effects of ammonia substitution on extinction limits and structure of counterflow nonpremixed hydrogen/air flames

The potential of partial ammonia substitution to improve the safety of hydrogen use was evaluated computationally, using counterflow nonpremixed ammonia/hydrogen/air flames at normal temperature and pressure. The ammonia-substituted hydrogen/air flames were considered using a recent kinetic mechanism and a statistical narrow-band radiation model for a wide range of flame strain rates and the extent of ammonia substitution. The effects of ammonia substitution on the extinction limits and structure, including nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions, of nonpremixed hydrogen/air flames were investigated. Results show reduction of the high-stretch extinction (i.e., blow-off) limits, the maximum flame temperature and the concentration of light radicals (e.g., H and OH) with ammonia substitution in hydrogen/air flames, supporting the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use in nonpremixed hydrogen/air flames. For high-stretched flames, however, NO_x and N₂O emissions substantially increase with ammonia substitution even though ammonia substitution reduces flame temperature, implying that chemical effects (rather than thermal effects) of ammonia substitution on flame structure are dominant. Radiation effects on the extinction limits and flame structure are not remarkable particularly for high-stretched flames.     

Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor

Effects of H₂-enrichment on structures of CH₄/air turbulent swirling premixed flames affected by high intensity turbulence in a gas turbine model combsutor are investigated by conducting direct numerical simulations. Two stoichiometric mixture conditions, of which volume ratio of CH₄:H₂ = 50:50 and 80:20, are simulated by considering a reduced chemistry (25 species and 111 reactions). Results showed qualitatively different flame shapes and reaction zone characteristics between the cases. For the higher H₂-ratio case, the flame is stabilized both in the inner and outer shear layers. For the lower H₂-ratio case, the flame is stabilized only in the inner shear layer and extinction occurs in the outer shear layer. Comparison of the reaction zone characteristics with unstrained and strained laminar flames in phase space showed that H₂ mass fraction for the lower H₂-ratio case and reaction rate profiles for both cases deviate from the corresponding laminar values. Analysis of fuel species conservation equation suggests that the turbulent transports are substantially influential to determine local and global flame structures. These findings would be useful for designing practical H₂-enriched gas turbine combustor in the aspect of flame structures under high intensity turbulence.     

The effects of heat loss on the burning velocity of cellular premixed flames generated by hydrodynamic and diffusive-thermal instabilities

The effects of heat loss on the burning velocity of cellular premixed flames are investigated by two-dimensional unsteady calculations of reactive flows based on the compressible Navier-Stokes equation and on the diffusive-thermal model equation. Hydrodynamic and diffusive-thermal instabilities are taken into account as contributing to the intrinsic instability of premixed flames. A sufficiently small disturbance is superimposed on a planar flame to obtain the relation between the growth rate and the wavenumber, i.e., the dispersion relation. As the heat loss becomes larger, the growth rate decreases and the unstable range narrows. This is because hydrodynamic instability caused by thermal expansion weakens for nonadiabatic flames. To investigate the characteristics of cellular flames, the disturbance with the linearly most unstable wavenumber, i.e., the critical wavenumber, is superimposed. As the superimposed disturbance evolves, the cellular-flame front forms due to the intrinsic instability. The lateral movement of cellular flames is observed at low Lewis numbers, and the behavior of cellular-flame fronts becomes more unstable for nonadiabatic flames. As the heat-loss parameter increases, the burning velocity of a cellular flame normalized by that of a planar flame increases at Lewis numbers lower than unity. By contrast, when the Lewis number is not less than unity, the burning-velocity increment decreases by increasing the heat loss. Diffusive-thermal instability thus has a pronounced influence on the unstable behavior and burning velocity of nonadiabatic cellular flames.     

Effects of premixed flames on turbulence and turbulent scalar transport

Experimental data and results of direct numerical simulations are reviewed in order to show that premixed combustion can change the basic characteristics of a fluctuating velocity field (the so-called flame-generated turbulence) and the direction of scalar fluxes (the so-called countergradient or pressure-driven transport) in a turbulent flow. Various approaches to modeling these phenomena are discussed and the lack of a well-elaborated and widely validated predictive approach is emphasized. Relevant basic issues (the transition from gradient to countergradient scalar transport, the role played by flame-generated turbulence in the combustion rate, the characterization of turbulence in premixed flames, etc.) are critically considered and certain widely accepted concepts are disputed. Despite the substantial progress made in understanding the discussed effects over the past decades, these basic issues strongly need further research.