Yung-g Chen

The detailed flame structure of highly stretched turbulent premixed methane-air flames

Abstract The premixed stoichiometric turbulent methane flames are investigated on a piloted Bunsen burner with a nozzle diameter of 12 mm and mean nozzle exit velocities of 65, 50, and 30 m/s. Advanced laser diagnostics of the flow field using two-component and two-point laser Doppler anenometer (LDA), as well as of the scalar fields with 2-D Rayleigh thermometry and line Raman/Rayleigh laser-induced predissociation fluorescence (LIPF)-OH techniques, are applied to obtain both the instantaneous and mean flame structure in terms of velocity, temperature, and major species concentrations, as well as turbulent kinetic energy and length scales. In terms of their location on the combustion diagram, the three flames cover the entire range of the distributed-reaction-zones regime from the borderline to the well-stirred reactor regime to the flamelet regime. Measurements were from X/D = 2.5 above the nozzle exit plane to X/D = 12.5 downstream. Thus, a complete database is established for comparison with the numerical predictions. Within the mixing layer between the unburnt gas and the pilot flame, the instantaneous temperatures are much lower than the adiabatic flame temperature due to the short residence time and heat loss to the burner. With increasing residence time the mean flame temperature increases in the axial direction. The radial mixing of the turbulence generated with the shear layers between the nozzle jet stream and surrounding pilot stream is surpressed, such that the turbulence kinetic energy remains nearly constant on the centerline. From the two-dimensional (2D) temperature fields instantaneous iso-temperature contours are plotted showing broad regions where burnt and unburnt gas are partially mixed. These regions are interpreted in terms of the quench scale lq = (eτc3)1/2. The measured values of the flame brush thickness are proportional to the quench scale for the two high-velocity flames, whereas the low-velocity flame exhibits essential flamelet behavior.

Experimental investigation of three-dimensional flame-front structure in premixed turbulent combustion—I: hydrocarbon/air bunsen flames

Comprehensive laser measurements of three-dimensional flame-front structures for turbulent lean hydrogen/air premixed Bunsen flames are reported in this continuation paper. The local scalar front appears lamella-like for both the reaction progress variable and the OH mole fraction. This lamella-like feature cannot be predicted by the commonly accepted combustion-regime diagrams. The flame residence time relevant to the turbulent flames investigated here may be much smaller than the unstretched laminar value used for constructing these regime diagrams. Superadiabaticy and flame-front bulges convex toward the reactants are clearly observed, representing effects of the less-than-unity Lewis number of the mixture. The average size of flame bulges is found to increase with the turbulence integral length scale. Moderate positive correlations exist between the in-plane two-dimensional curvature and the three-dimensional progress-variable gradient. OH mole fraction is also correlated with the progress-variable gradient. Local flame orientation in three-dimensional space is close to an isotropic distribution, which is attributed to flame-surface wrinkling being strongly nonpassive. More backward-facing flame fronts are formed nearer the unburnt than the burnt side of the turbulent flame brush, suggesting the importance of large-scale flame-front bulges in turbulent flame propagation. Higher progress-variable dissipation rates are measured than for the unstretched laminar flame. By comparison with previously published data for turbulent hydrocarbon/air premixed flames, the present dissipation-rate measurements suggest that chemical reactions do not play an important role in the destruction or generation of progress-variable fluctuations. Discrepancies are observed even in the qualitative trends of some statistics of the progress-variable dissipation rate when compared with DNS data modeled with detailed chemistry.

Stabilization mechanisms of lifted laminar flames in axisymmetric jet flows

Abstract Stabilization mechanisms of lifted laminar propane flames are investigated in an axisymmetric jet flow configuration. Detailed mixing and flow fields upstream of the flame lift-off heights measured by Chung and coworkers 28 , 29 , 30 are calculated on a nonreacting flow basis. The local stoichiometric axial velocity, Ust, and scalar dissipation rate, χst, are obtained at points that are upstream of the stabilization locations by a redirection region length. Variation is investigated with jet exit velocity, nozzle diameter, coflow air velocity as well as partial jet premixing with air. It is found that Ust decreases consistently with increasing χst when the centerline mixture fraction is higher than that of the rich flammability limit. Beyond this threshold, a rich-joined flame is formed at downstream locations with Ust independent of the local scalar dissipation rate. Instead, Ust decreases with increasing ξst, the stoichiometric mixture fraction of the jet flow. For lifted flames stabilized close to the burner exit with an edge-flame appearance, the maximum attainable mixture fraction gradient approaches that calculated for extinction of stretched diffusion flames in the counterflow geometry. The flame propagation velocity remains positive. The trend of decreasing Ust with increasing χst is also found for numerical simulations with a unity Lewis number [18] and for experimental data of lifted methane flames [19] . These results corroborate the triple flame stabilization concept. An empirical formula for the triple flame propagation velocity is proposed with a nonlinear dependency on local scalar dissipation rate. In addition, different flame stabilization mechanisms previously proposed for lifted turbulent diffusion flames are reconciled for lifted laminar flames, depending on the local flow/mixing conditions. Appropriate flame stability and blow-out criteria are derived and these predict the flame lift-off height and blow-out velocity accurately. Implications for flame stabilization in turbulent jet flows with such triple flame structures are also discussed.

Experimental investigation of turbulent scalar flux in premixed stagnation-type flames

Abstract Joint velocity/scalar imaging measurements are performed in turbulent premixed stagnation-type flames to characterize the turbulent scalar flux, ρ u ″c″ . Simultaneous two-dimensional measurements of the flow field and relative OH concentration are obtained by particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), respectively. Conditional processing of the flow-field based on qualitative OH concentration provides ensemble averaged product and reactant velocities which may be used to infer the behavior of the turbulent scalar flux according to the Bray-Moss-Libby (BML) formalism. Stagnation-type flames, being subject to an external mean pressure gradient because of the impinging flow, are examined to test the effect of external pressure gradients on the behavior of the scalar flux. The results indicate that external pressure gradients affect the scalar flux by either inhibiting or enhancing the expansion of combustion products away from the instantaneous flame front, depending on orientation of the pressure gradient relative to the flame brush. Additionally, experimental results are used to derive a modified version of the Bray number which is able to predict the behaviour of the scalar flux as gradient or counter-gradient for all flames for which data are available.

Investigation of flame broadening in turbulent premixed flames in the thin-reaction-zones regime

A two-plane two-dimensional Rayleigh thermometry technique is applied to investigate, the three-dimensional flame structure in highly stretched turbulent premixed flames beyond the flamelet regime. Experimental data of the preheat zone thickness, σ F , and flame temperature conditioned at the maximum temperature gradient, T o , are reported. Broadening of the preheat zone thickness is observed and found to be correlated well with the increase of turbulence intensity. This suggests a pure hydrodynamic straining effect in a chemically inert preheat zone in the thin-reaction-zones regime, which is consistent with the rate-ratio asymptotic analysis. Although a general decrease of T o is also found with increasing flame stretch, this temperature drop is subject to the nonadiabatic condition in the postflame region. Flame extinction would be expected when T o is approaching the ignition temperature ≥900 K.

Measurements of scalar dissipation in turbulent hydrogen diffusion flames and some implications on combustion modeling

One-dimensional simultaneous measurements of the species mass fractions of O 2 , H 2 , H 2 O, N 2 and OH radical as well as flame temperature have been carried out in hydrogen jet diffusion flames diluted with Ar using the line-Raman/Rayleigh/LIPF-OH technique. Statistical information about the mixture fraction Z and scalar dissipation rate X can be obtained for evaluation of current finite-chemistry combustion models for the turbulent nonpremixed flames. At an upstream position of x/d = 8, the flamelet approach is found to be qualitatively valid, in agreement with a larger Favre mixture fraction fluctuation than the reaction zone thickness in the Z-space. Transient effect, however, can be important in view of H 2 and O 2 superequilibrium. Preferential diffusion effects are negligible due to a relatively large Peclet number. Joint correlations between a reactive scalar Twith X and the conserved scalar Z with X are found to be different from those reported in nonreacting flows. The observed distinct features can be explained based on the flamelet concept so that statistical independence is argued to be a good approximation. There is evidence showing that large-scale turbulent motion dominates scalar transport in the connected reaction zone regime. Thus, flamelet approach is also favored. Some modeling assumptions used in the flamelet model and the CMC method are discussed. Model refinements based on the present experimental data are suggested as well.

Simultaneous 2-D Imaging Measurements of Reaction Progress Variable and OH Radical Concentration in Turbulent Premixed Flames: Instantaneous Flame-Front Structure

Improved experimental techniques for the joint two-dimensional measurement of reaction progress variable and OH mole fraction, detailed in a companion paper (Chen and Bilger, 2001), are applied here to the investigation of the instantaneous flame-front structure in turbulent premixed flames. The flames are of natural-gas/air mixtures and are pilot-stabilized on the Bunsen-type burner used by Frank et al. (1999). Multiple joint image pairs show distortion and folding of the preheat zone in the reaction progress variable images at the higher turbulence levels in the range investigated. A modified interaction length scale, ϱm, is defined to describe this phenomenon which occurs when ϱm > δth, where δth is the thermal flame front thickness of an unstretched laminar flame determined from the maximum temperature gradient. Significant departures from flamelet structure are found for spatial derivatives of the progress variable and for OH concentrations. It appears that these quantitative effects persist for ϱm < δth and that a new regime of “flame fronts with complex strain” exists where δth > η and ϱm < δth with η being the Kolmogoroff scale. In this regime, the effects of the turbulence are not sufficient to distort the lamellar-like appearance of the flame front. At scales just larger than the Kolmogoroff scale, strong spatial and temporal variations in the strain-rate tensor occur. These are evidently sufficient to cause significant departure of the quantitative structure of the front from that found for laminar flames subjected to the simple straining used in laminar flamelet modelling of turbulent premixed combustion.

Simultaneous 2-D Imaging Measurements of Reaction Progress Variable and OH Radical Concentration in Turbulent Premixed Flames: Experimental Methods and Flame Brush Structure

Experimental methods are detailed for comprehensive further investigation of the turbulent premixed flames of natural-gas/air mixtures stabilized on a Bunsen-type burner already studied by joint imaging of OH and velocity (Franker et.al, 1999). Simultaneous two-dimensional measurements of reaction progress variable and OH mole fraction are made from planar imaging of Rayleigh scattering and laser-induced fluorescence of OH. Image in-plane and out-of-plane spatial resolution of the order of 100 μm has been achieved. Care is taken to ensure that the instantaneous flame-front structure can be adequately resolved, and the measured scalar structure in laminar flames agrees well with flame calculations with a C-3 mechanism. Mean velocity and turbulence intensity profiles are presented for the non-reacting flows together with measurements of longitudinal and transverse correlation functions, their associated length scales, and the dissipation rate of the turbulence kinetic energy. In this paper, we report the mean structure of the turbulent flame brush in terms of Reynolds- and Favre-averaged profiles of the reaction progress variable and its standard deviation, and selected probability density functions. The results are compared with those derived from the OH images using the thin flamelet assumption. The mean and standard deviation of OH mole fraction are also presented. Preliminary conclusions are drawn about the relationship of the mean flame brush structure to the turbulence and the admixture of co-flowing air. The validity of the thin-flamelet assumption appears to be questionable for the lean flames investigated. Results for the structure of the instantaneous flame fronts are reported in a companion paper (Chen and Bilger, 2001).

Geometric interpretation of fractal parameters measured in turbulent premixed Bunsen flames

Abstract Fractal analyses have been conducted to investigate flame-front wrinkling of turbulent premixed Bunsen flames. Emphasis is placed on the geometric interpretation of measured fractal parameters and their relationship with parameters used in current combustion models. The outer cutoff scale is found to be four times the integral length scale of flame-front wrinkling defined in the Bray–Moss–Libby model. A revised Gibson length scale is derived by taking into account the curvature effect. Dependency of the revised Gibson scale on the Karlovitz number agrees better with the bulk of available data for the inner cutoff scale than the original definition. The term “self-similarity dimension” is proposed for the fractal dimension measured at finite spatial resolution to highlight the self-similarity feature of local flame fronts. The self-similarity dimension is found to increase linearly with time, and correlates well with the turbulent flame brush thickness.

Highly strained turbulent rich methane flames stabilized by hot combustion products

Abstract The structure of rich turbulent methane flames, with an equivalence ratio of 2.0, at high strain rates has been investigated based on simultaneous and instantaneous line profile measurements of the species mass fractions of CH4, O2, N2, H2O, CO2, CO, H2 and OH radical, together with gas temperature. The flames are stabilized by the hot combustion products from a stoichiometric large pilot flame surrounding a Bunsen burner. Two flames have been investigated at overall stretch rates of 2500 s−1 and 4167 s−1. One-dimensional combined UV Raman, Rayleigh, and laser-induced predissociation fluorescence (LIPF) technique has been applied. Measurements at different axial positions enable the flame structure to be studied at different turbulence levels. The instantaneous species mass fractions and temperature profiles are conditioned on both mixture fraction and a reaction progress variable, namely the H2O mass fraction, allowing a proper description of the partially premixed flame structure. The components of the scalar dissipation rate of both mixture fraction and progress variable in the radial direction are also measured. The flames are stabilized at rich conditions by radicals and heat transferred from the stoichiometric pilot flame. Stretch effects are found to be dominant in these flames, leading to local flame structures that correspond to flames in the distributed-reaction-zones regime. The pdf of the progress variable is found to be monomodal in all cases. The conditional mean scalar dissipation rate profile in mixture fraction space exhibits a local minimum at the reaction zone due to differential polynary diffusion effects. The scalar dissipation rates of the progress variable are of the same order of magnitude as those of the mixture fraction, when these are appropriately normalized. This indicates that molecular mixing in both variables is equally important. This also suggests that a two-variable formulation is necessary to model these flames.