Lean Premixed Methane-air Flames Research Articles

Direct numerical simulation of hydrogen-enriched lean premixed methane–air flames

The effect of hydrogen blending on lean premixed methane–air flames is studied with the direct numerical simulation (DNS) approach coupled with a reduced chemical mechanism. Two flames are compared with respect to stability and pollutant formation characteristics—one a pure methane flame close to the lean limit, and one enriched with hydrogen. The stability of the flame is quantified in terms of the turbulent flame speed. A higher speed is observed for the hydrogen-enriched flame consistent with extended blow-off stability limits found in measurements. The greater flame speed is the result of a combination of higher laminar flame speed, enhanced area generation, and greater burning rate per unit area. Preferential diffusion of hydrogen coupled with shorter flame time scales accounts for the enhanced flame surface area. In particular, the enriched flame is less diffusive-thermally stable and more resistant to quenching than the pure methane flame, resulting in a greater flame area generation. The burning rate per unit area correlates strongly with curvature as a result of preferential diffusion effects focusing fuel at positive cusps. Lower CO emissions per unit fuel consumption are observed for the enriched flame, consistent with experimental data. CO production is greatest in regions which undergo significant downstream interaction. In these regions, the enriched flame exhibits faster oxidation rates as a result of higher levels of OH concentration. NO emissions are increased for the enriched flame as a result of locally higher temperature and radical concentrations found in cusp regions.

Contribution of curvature to flame-stretch effects on premixed flames

This experimental investigation considers steady two-dimensional rich and lean premixed methane–air flames established in two configurations, one a two-dimensional slot burner and the other an axisymmetric coannular burner. The flames contain a curved premixed reaction zone that has a tip. Flame curvature is a parameter that significantly influences the structure and propagation of premixed flames through its contribution to flame stretch. In a previous investigation we focused primarily on stretch effects along the planar portion of the premixed reaction zones of various slot-burner methane–air rich (partially) premixed flames. In this study we concentrate on stretch effects along the curved portion of the reaction zones of those flames, employing the same methodology, and also extend our analysis to lean flames. Instantaneous temperature measurements were made by using holographic interferometry. Velocity vectors were determined by using particle image velocimetry, and the flame reaction-zone topography was characterized by imaging C 2∗-chemiluminescence. It is possible to identify a constant ≈1100 K temperature contour along the inner premixed reaction zone by comparing the C 2∗-chemiluminescence image and the holograms. The flame speed at the flame tip is enhanced above the corresponding unstretched value. We show that the Markstein relation S u/ S u o = 1 − Lκ/ S u o must be suitably modified along the curved portion of the reaction zones. The response to stretch differs in the planar and curved regions of the premixed reaction zone. However, this classical relation is well suited along the planar portions of the reaction zones. Although the propagation speed varies along the planar region of the reaction zone, the corresponding differences are larger along the curved region. We consider a variation of the Markstein expression S u = CS u o − Lκ. The value of C in the literature is generally considered to equal unity. We have developed an empirical relation for the curved region and have empirically determined from our data that the best fit for the constant C is related to the unstretched flame speed and the value of the reaction-zone speed at the inner premixed flame tip, i.e., C ≈ − S u,tip /10 S u o . The value of the constant C does not have a universal value due to geometry effects. A limitation of this expression is that it does not provide the expected solution S u = S u o at zero stretch. This discrepancy is most likely due to a poor resolution of the velocity data in the region where the flame transitions from a planar to a curved topography. The response of the flame speed at the larger stretch values differs from that at weaker stretch. The value of Ka can be increased by increasing the stretch rate κ or the mixture diffusivity D M,u , or by decreasing the laminar flame speed.

The reduced kinetic description of lean premixed combustion

Lean premixed methane–air flames are investigated in an effort to facilitate the numerical description of CO and NO emissions in LP (lean premixed) and LPP (lean premixed prevaporized) combustion systems. As an initial step, the detailed mechanism describing the fuel oxidation process is reduced to a four-step description that employs CO, H 2, and OH as intermediates not following a steady-state approximation. It is seen that, under conditions typical of gas–turbine combustion, this mechanism can be further simplified to give a two-step reduced description, in which fuel is consumed and CO is produced according to the fast overall step CH 4 + 3 2 O 2 → CO + 2H 2O, while CO is slowly oxidized according to the overall step CO + 1 2 O 2 → CO 2. Because of its associated fast rate, fuel consumption takes place in a thin layer where CO, H 2, and OH are all out of steady state, while CO oxidation occurs downstream in a distributed manner in a region where CO is the only intermediate not in steady state. In the proposed description, the rate of fuel consumption is assigned a heuristic Arrhenius dependence that adequately reproduces laminar burning velocities, whereas the rate of CO oxidation is extracted from the reduced chemistry analysis. Comparisons with results obtained with detailed chemistry indicate that the proposed kinetic description not only reproduces well the structure of one-dimensional unstrained and strained flames, including profiles of CO, temperature, and radicals, but can also be used to calculate NO emissions by appending an appropriate one-step reduced chemistry description that includes both the thermal and the N 2O production paths. Although methane is employed in the present study as a model fuel, the universal structure of the resulting CO oxidation region, independent of the fuel considered, enables the proposed formulation to be readily extended to other hydrocarbons.

The mechanism of two-dimensional pocket formation in lean premixed methane-air flames with implications to turbulent combustion

The mechanism of unburnt pocket formation in an unsteady two-dimensional premixed lean methane-air flame is investigated using direct numerical simulations. Theoretical results for nonlinear diffusion equations combined with analytical examples are used to interpret some of the results. Flame structure and propagation show three distinct stages of pocket formation: (1) flame channel closing involving head-on quenching of flames, (2) cusp recovery, and (3) pocket burnout. The flame channel closing and subsequent pocket burnout are mutual annihilation events that feature curvature, diffusion normal to the flame front, unsteady strain rate effects, and singularities in flame propagation and stretch rate. The results show that during channel closing and pocket burnout thermo-diffusive and chemical interactions result in the acceleration of the flames prior to annihilation; the time scales associated with the final stage of mutual annihilation and the initial stage of cusp recovery are significantly smaller than diffusive and convective time scales. As in earlier one-dimensional studies, the acceleration is attributed to enhanced diffusion and reaction rates, modifications to species profiles leading to shifts in balance between diffusion and reaction, and vanishing species and thermal gradients at the location in the channel where the pocket pinches off. Flame propagation and stretch rate are singular at this location. Enhanced radical production is initiated by a reversal of diffusion of H 2 towards the reaction zone during the early stages of thermo-diffusive interactions. Peak radical concentrations resulting from flame channel closing and pocket burnout exceed peak laminar values by as much as 25%. After the merging of the fuel consumption layers, radical production and flame structure shifts more towards an H 2/CO/O 2 system at the expense of hydrocarbon reactions. Species thermodiffusive interaction times are shorter than the unstrained one-dimensional counterpart due to unsteady strain and convection. Curvature effects on the flame propagation are prominent during pocket burnout and cusp recovery. The recovery stage shows strong dependence on diffusion of radicals left from the channel closing stage. This diffusion is amplified by the strong curvature of the flame cusp.

Simultaneous rayleigh, raman, and LIF measurements in turbulent premixed methane-air flames

Instantaneous measurements of temperature, major species, OH, CO, and NO are performed in turbulent premixed flames using simultaneous Rayleigh scattering, Raman scattering, and laser-induced fluorescence (LIF). Temperature is determined from Rayleigh scattering, and concentrations of CH 4 , O 2 , N 2 , H 2 O, CO 2 , and H 2 are obtained from Raman scattering. Linear LIF is used to measure OH and NO, while two-photon LIF is used to determine CO concentrations. The two-photon CO-LIF system provides significant improvements over CO-Raman measurements. This combination of diagnostic techniques is used to investigate the detailed compositional structure of turbulent lean premixed methane-air flames. Three turbulent flames having different equivalence ratios are considered. The ratio of rms velocity to laminar flame speed ranges from approximately 6 to 17. The correlation of major species, OH, and CO concentrations with temperature are similar to those predicted by one-dimensional laminar flame calculations. However, NO concentrations in the leaner flames are higher than the predicted values.