R.S. Barlow

Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames

Measurements of temperature, the major species (N2, O2, CH4, CO2, H2O, CO, and H2), OH, and NO are obtained in steady laminar opposed-flow partially premixed flames of methane and air, using the non-intrusive techniques of Raman scattering and laser-induced fluorescence. Flames having fuel-side equivalence ratios of φ = 3.17, 2.17, and 1.8 are stabilized on a porous cylindrical burner (Tsuji burner) in a low-velocity flow of air. Results are compared with calculations using a version of the Sandia laminar flame code that is formulated for the Tsuji geometry and includes an optically thin treatment of radiation. Because velocity profiles are not measured, the strain rate in each calculation is adjusted to match the measured profile of the mixture fraction. Measured profiles of temperature and species mass fractions are then compared with results of calculations using the GRI-Mech 2.11 and 3.0 chemical mechanisms, as well as a detailed mechanism from Miller. All three mechanisms give agreement with experimental results for the major species that is generally within experimental uncertainty. With regard to NO formation, the relative performance of the three mechanisms depends on the fuel-side equivalence ratio. GRI-Mech 2.11 gives reasonably good agreement with measured NO levels in lean and near-stoichiometric conditions, but it under predicts NO levels in fuel-rich conditions. GRI-Mech 3.0 significantly over predicts the peak NO levels the φ = 3.17 and 2.17 flames, but it yields relatively good agreement with measurements in the φ = 1.8 flame. The Miller mechanism gives good agreement with measured NO levels in the φ = 3.17 flame, but it progressively over predicts peak NO levels in the leaner flames. Comparisons of adiabatic and radiative calculations show that radiation can have a significant effect on the width and structure of partially premixed flames, as well as on the levels of NO produced.

Simultaneous Laser Raman-rayleigh-lif Measurements and Numerical Modeling Results of a Lifted Turbulent H2/N2 Jet Flame in a Vitiated Coflow

An experimental and numerical investigation is presented of a lifted turbulent H 2 /N 2 jet flame in acollow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H 2 /N 2 ( Re =23,600, H/d =10) in a coflow of hot combustion products from a lean H 2 /Air flame (=0.25, T =1045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (joint scalar probability density function and eddy dissipation concept) are used in conjunction with various turbulence models to predict the liftoff height ( H PDF / d =7, H EDC / d =8.5). Kalghatgis classic phenomenological theory, which is based on scaling arguments, yields a reasonbly accurate prediction ( H K / d =11.4) of the liftoff height for the present flame. The vitiated coflow admits the possibility of autoignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small-scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.

The structure of turbulent nonpremixed flames revealed by Raman-Rayleigh-LIF measurements

Abstract This paper reviews recent advances in our understanding of the structure of turbulent nonpremixed flames due to extensive data acquired from single-point and planar imaging experiments using the Raman, Rayleigh, and LIF diagnostic methods. These techniques, used either separately or jointly, have become standard tools in combustion research. Flames with simple streaming flows as well as complex flows with recirculating zones are discussed for a variety of fuel mixtures and a range of turbulent mixing rates. The chemistry—turbulence interaction and other related issues like local flame extinction and the bimodality of the approach toward blowoff are discussed. Additional single-point data are also presented illustrating the effects of partially premixing the fuel with air, diluting it with nitrogen or adding methane to a mixture of nonhydrocarbon fuels. The bimodality of the conditional pdfs of various reactive scalars as the flames approach blowoff, and the start of occurrence of localized extinction, are correlated with two simple parameters: (a) the stoichiometric mixture fraction, ξ s , and (b) the reaction zone width, Δξ R . The latter parameter may be easily determined from standard laminar flame calculations for a given fuel mixture.

Structure of turbulent non-premixed jet flames in a diluted hot coflow

Moderate and intense low oxygen dilution combustion is a newly implemented and developed concept to achieve high thermal efficiency and fuel savings while maintaining emission of pollutants at very low levels. It utilizes the concept of heat and exhaust gas recirculation to achieve combustion at a reduced temperature, a flat thermal field, and low turbulence fluctuations. An experimental burner is used in this study to simulate the heat and exhaust gas recirculation applied to a simple jet in a hot coflow. Temporally and spatially resolved measurements of reactive scalars are conducted on three different turbulent nonpremixed flames of a H 2 /CH 4 fuel mixture at a fixed-jet Reynolds number, and different oxygen levels in the hot oxidant stream. The data were collected using the single-point Raman-Rayleigh-laser-induced fluorescence technique. The results show substantial variation in the flame structure and appearance with the decrease of the oxygen level. By reducing the oxygen level in the hot coflow, the flame becomes less luminous, the temperature increase in the reaction zone can get as low as 100 K, and the levels of CO and OH are substantially lowered. The levels of NO also decrease with decreasing the oxygen levels and at 3% by mass, it is less that 5 ppm. For this case, a widely distributed NO profile is found which is not consistent with profiles for other oxygen levels.

Instantaneous and Mean Compositional Structure of Bluff-Body Stabilized Nonpremixed Flames

Abstract Turbulent nonpremixed flames stabilized on an axisymmetric bluff-body burner are studied with fuels ranging from simple H 2 /CO to complex H 2 /CH 4 and gaseous methanol. The fuel-jet velocity is varied to investigate the Damkohler number effects on gas emissions, localized extinction (LE) in the neck zone, and the structure of the recirculation zone dependency on the flow field. Simultaneous, single-point measurements of temperature, major species, OH, and NO are made using the Raman/Rayleigh/Laser induced fluorescence (LIF) technique. The data are collected at different axial and radial locations along the full length of most flames and are presented in the form of ensemble means, root-mean-square (rms) fluctuations, scatter plots, and probability density functions (PDF). It is found that up to three mixing layers may exist in the recirculation zone, one on the air side of the outer vortex, one between the inner and the outer vortices, and one between the fuel jet and the inner vortex. With increasing jet momentum flux, the average mixture in the outer vortex loses its strength and the stoichiometric contour shifts closer to the fuel jet. The decay rate of the mixture fraction on the centerline exhibits similar trends to the ordinary jet flame downstream of the recirculation zone whereas different trends are found inside the recirculation zone. The laminar flame computations with constant mass diffusivities and Lewis number (Le) = 1 are found to be better guides for the measured temperature and stable species mass fraction in the turbulent flames. The measured peak mass fractions of CO and H 2 are similar to those reported earlier for pilot-stabilized flames of similar fuels. Compared with laminar flame compositions with equal diffusivities and Le = 1.0, measured CO may be in superflamelet concentration. Hydroxyl radical and H 2 are found not to be in superflamelet levels contrary to earlier findings in piloted flames. The start of LE and the bimodality of the conditional PDF are consistent with those reported earlier for piloted flames of similar fuels.

Raman/Rayleigh/LIF Measurements in a Turbulent CH4/H2/N2 Jet Diffusion Flame: Experimental Techniques and Turbulence-Chemistry Interaction

Abstract Single-shot Raman, Rayleigh, and laser-induced fluorescence (LIF) measurements have been performed simultaneously in a turbulent jet flame (Re = 15,200) in order to determine joint probability density functions of temperature, mixture fraction, major species (CH4, H2, O2, N2, H2O, CO2, CO), and minor species (OH, NO) mass fractions. The flame is used as a “standard flame” within the TNF Workshop, and therefore, comprehensive and accurate data sets are required to allow a quantitative comparison with predictions from computational fluid dynamics (CFD) calculations. In the first part of the paper, the results obtained at the Sandia National Laboratories are compared with those from Raman/Rayleigh measurements performed earlier at the Deutsches Zentrum fur Luft-und Raumfahrt, or German Aerospace Center (DLR). The agreement between the different data sets is generally good, and most of the observed deviations are within experimental uncertainties. The remaining discrepancies are discussed. The main issues of the second part are the characterization of the thermochemical state of the flame and the study of the influence of increasing the Reynolds number to Re = 22,800. Effects of finite-rate chemistry, local flame extinction, NO formation, and nonequilibrium concentrations are discussed.

Effect of Damköhler number on superequilibrium OH concentration in turbulent nonpremixed jet flames

Abstract Simultaneous, spatially resolved measurements of mixture fraction and absolute hydroxyl radical concentration are obtained for the first time in nonpremixed, turbulent, hydrogenair flames. This is accomplished by combining spontaneous Raman scattering with linear, laser-induced fluorescence (LIF). The Raman scattering data define the instantaneous, local collisional quenching environment of the OH molecules, allowing quenching corrections to be applied for each laser shot and making the linear LIF measurements quantitative. The effect of Damkohler number on OH superequilibrium is determined by performing measurements at selected locations in two argon-diluted hydrogen flames (Reynolds numbers 8,500 and 17,000). Results demonstrate that departures from chemical equilibrium in these flames are a consequence of the fact that time scales for turbulent transport are competitive with time scales for three-body radical recombination reactions. Due to the slow characteristic time for the radical recombination, convective histories, as well as instantaneous local conditions, determine hydroxyl concentrations. Damkohler numbers are not sufficiently low for the rapid bimolecular reactions to be strongly affected. Comparison of turbulent flame data with results from strained laminar flame calculation incicates that partial equilibrium of the bimolecular reactions is a good approximation near the stoichiometric mixture fraction. Comparisons of the experimental data with predictions by Monte Carlo simulations using a partial equilibrium chemistry model show good overall agreement. However, simulations predict a smaller variance of OH concentration than is measured for a given value of mixture fraction, lower OH concentrations at off-stoichiometric conditions, and a more rapid decay toward equilibrium with streamwise distance.

Raman/Rayleigh/LIF measurements of nitric oxide formation in turbulent hydrogen jet flames☆

Abstract Simultaneous, time-resolved measurements of NO, major species, mixture fraction, temperature, and OH are obtained in nonpremixed turbulent hydrogen jet flames, using the combination of spontaneous Raman scattering, Rayleigh scattering, and laser-induced fluorescence. Results are presented for an undiluted hydrogen flame at Reynolds number 10,000 and for flames with 20 percent and 40 percent helium dilution. Scatter plots and conditional averages of NO mole fraction show gradual increases with streamwise distance. The width in mixture fraction coordinates of the zone of high NO mole fractions decreases with increasing streamwise distance. At a given mixture fraction, the fluctuation in NO mole fraction relative to the mean is greatest near the nozzle and decreases toward the flame tip. These effects are related to the streamwise evolution of turbulent flame structure from thin reaction layers near the nozzle to broad, near-equilibrium zones at the flame tip. Dilution with helium significantly reduces the measured NO levels and causes a slight shift of the curves of conditional average NO mole fraction toward the fuel-lean side of the stoichiometric mixture fraction. Favre averages and ensemble averages show significant bias, relative to conditional averages, in the relationship between NO mole fraction and mixture fraction. This suggests that the “rich shift” observed in some sampling-probe studies may results from the averaging process itself.

Raman-LIF measurements of temperature, major species, OH, and NO in a methane-air Bunsen flame

Nonintrusive measurements of temperature, the major species (N2, O2, H2, H2O, CO2, CO, CH4), OH, and NO in an atmospheric pressure, laminar methane-air Bunsen flame were obtained using a combination of Raman-Rayleigh scattering and laser-induced fluorescence. Radial profiles were measured at three axial locations for an equivalence ratio of 1.38. Measurements along the centerline of the flame, for equivalence ratios of 1.38, 1.52, and 1.70, were also obtained. The measurements indicate that the inner unburned fuel-air mixture experiences significant preheating as it travels up into the conical flame zone surrounding it. Consequently, the centerline axial temperatures were typically 100–150 K higher than predicted by adiabatic equilibrium for reactants at an initial temperature of 300 K. Because the amount of preheating increases with the equivalence ratio (due to the increased inner flame height), the maximum temperatures (2000 K) in a Bunsen flame were rather insensitive to the stoichiometry. We observed a 20% reduction of the maximum NO concentrations (80 ppm) in a Bunsen flame by increasing the equivalence ratio from 1.38 to 1.70. We also find that using a one-dimensional premixed laminar flame model incorporating finite-rate chemistry, satisfactorily predicts properties such as the temperature, CO, OH, and NO concentrations at the inner flame.

Experiments on the scalar structure of turbulent CO/H2/N2 jet flames

Abstract Scalar and velocity measurements are reported for two turbulent jet flames of CO/H2/N2 (40/30/30 volume percent) having the same jet Reynolds number of 16,700 but different nozzle diameters (4.58 mm and 7.72 mm). Simultaneous measurements of temperature, the major species, OH, and NO are obtained using the combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence. Three-component laser-Doppler velocimetry measurements on the same flames were performed at ETH Zurich and are reported separately. This paper focuses on the scalar results but includes some limited velocity data. Axial profiles of mixture fraction, major species mole fractions, and velocity in these two flames are in close agreement when streamwise distance is scaled by nozzle diameter. However, OH mole fractions are lower and NO mole fractions are higher near the stoichiometric flame length in the larger flame due to the lower scalar dissipation rates and longer residence times. Turbulent flame measurements are compared with steady strained laminar flame calculations. Laminar calculations show remarkably close agreement with measured conditional means of the major species when all diffusivities are set equal to the thermal diffusivity. In contrast, laminar flame calculations that include the normal Chemkin treatment of molecular transport are clearly inconsistent with the measurements. These results suggest that turbulent stirring has a greater influence than molecular diffusion in determining major species concentrations at the flow conditions and locations considered in the present experiments, which begin at an axial distance of 20 nozzle diameters. Analysis of the conditional statistics of the differential diffusion parameter supports this conclusion, though some evidence of differential diffusion is observed. With regard to validation of turbulent combustion models, this data set provides a target that retains the geometric simplicity of the unpiloted jet flame in coflow, while including a chemical kinetic system of intermediate complexity between hydrogen flames and the simplest hydrocarbon flames. Aspects of the measurements, including Favre-averaged profiles, conditional statistics, mixture fraction pdf’s, and departures from partial equilibrium, are presented and discussed in terms or their relevance to the testing of turbulent combustion submodels. The complete data are available on the World Wide Web for use in model validation studies.