Effects of water vapor addition on the laminar burning velocity of oxygen-enriched methane flames

The effects of steam addition on the laminar burning velocity of premixed oxygen-enriched methane flames are investigated at atmospheric pressure. Experiments are carried out with an axisymmetric burner on which laminar conical flames are stabilized. A newly devised steam production system is used to dilute the reactants with water vapor. The oxygen-enrichment ratio in the oxidizer, defined as O₂/(O₂ + N₂) (mol.), is varied from 0.21 (air) to 1.0 (pure oxygen). The equivalence ratio ranges from 0.5 to 1.5 and the steam molar fraction in the reactive mixture is varied from 0 to 0.50. For all compositions examined, the reactive mixture is preheated to a temperature T_u = 373 K. Laminar flame speeds are determined with the flame area method using a Schlieren apparatus. The deviations induced by stretch effects due to aerodynamic strain and flame curvature are assessed using Particle Imaging Velocimetry measurements and flame images, and these data are used to estimate the uncertainty of the flame speed measurements. The experiments are completed by numerical simulations conducted with the PREMIX code using different detailed kinetic mechanisms. It is shown that the laminar flame speed of CH₄/O₂/N₂/H₂O_(v) mixtures features a quasi-linear decrease with increasing steam molar fraction, even at high steam dilution rates. Numerical predictions are in good agreement with experimental data for all compositions explored, except for low dilution rates X_H2O<0.10 in methane–oxygen mixtures, where the flame speed is slightly underestimated by the calculations. It is also shown that steam addition has a non-negligible chemical impact on the flame speed for methane–air flames, mainly due to water vapor high chaperon efficiency in third-body reactions. This effect is however strongly attenuated when the oxygen concentration is increased in the reactive mixture. For highly oxygen-enriched flames, steam can be considered as an inert diluent.     

Combustion of butanol isomers – A detailed molecular beam mass spectrometry investigation of their flame chemistry

The combustion chemistry of the four butanol isomers, 1-, 2-, iso- and tert-butanol was studied in flat, premixed, laminar low-pressure (40 mbar) flames of the respective alcohols. Fuel-rich (? = 1.7) butanol–oxygen–(25%)argon flames were investigated using different molecular beam mass spectrometry (MBMS) techniques. Quantitative mole fraction profiles are reported as a function of burner distance. In total, 57 chemical compounds, including radical and isomeric species, have been unambiguously assigned and detected quantitatively in each flame using a combination of vacuum ultraviolet (VUV) photoionization (PI) and electron ionization (EI) MBMS.Synchrotron-based PI-MBMS allowed to separate isomeric combustion intermediates according to their different ionization thresholds. Complementary measurements in the same flames with a high mass-resolution EI-MBMS system provided the exact elementary composition of the involved species. Resulting mole fraction profiles from both instruments are generally in good quantitative agreement.In these flames of the four butanol isomers, temperature, measured by laser-induced fluorescence (LIF) of seeded nitric oxide, and major species profiles are strikingly similar, indicating seemingly analog global combustion behavior. However, significant variations in the intermediate species pool are observed between the fuels and discussed with respect to fuel-specific destruction pathways. As a consequence, different, fuel-specific pollutant emissions may be expected, by both their chemical nature and concentrations.The results reported here are the first of their kind from premixed isomeric butanol flames and are thought to be valuable for improving existing kinetic combustion models.     

Laser raman measurements of temperature and species concentration in swirling lifted hydrogen jet diffusion flames

Simultaneous spatially and temporally resolved point measurements of temperature, mixture fraction, major species (H₂, H₂O, O₂, N₂), and minor species (OH) concentrations are performed in unswirled (S_g = 0), low swirl (S_g = 0.12), and high swirl (S_g = 0.5) lifted turbulent hydrogen jet diffusion flames into still air. Ultraviolet (UV) Raman scattering and laser-induced predissociative fluorescence (LIPF) techniques are combined to make the multi-parameter measurements using a single KrF excimer laser. Experimental results are compared to the fast chemistry (equilibrium) limit, to the mixing without reaction limit, and to simulations of steady stretched laminar opposed-flow flames. It is found that in the lifted region where the swirling effects are strong, the measured chemical compositions are inconsistent with those calculated from stretched laminar diffusion flames or stretched partially premixed flames. Sub-equilibrium values of temperature, sub-flamelet values of H₂O, and super-flamelet values of OH are found in an intermittent annular turbulent brush of the swirled flame but not in the unswirled flame. Farther downstream of the nozzle exit (x/D ≥ 50), swirl has little effect on the finite-rate chemistry.     

Chemical effects on molecular species concentrations in turbulent fires

The measurement and modeling of molecular species concentrations in turbulent pool and buoyant jet flames is described. The experimental parameters included burner diameter (2.8 mm jet nozzle, 190, 381, and 762 mm pools), theoretical combustion heat release rate (10–283 kW), lip size (0–25 mm), and fuel (CH₄, C₃H₈). Time-averaged species concentrations were obtained through axial and radial sampling probe traverses. A novel sampling probe was developed which provides a constant mass flow of flame gases that is not biased toward either hot or cold gas eddies.Local concentrations of major gas species (fuel, O₂, CO, CO₂, H₂O, N₂) in the fire are correlated by the mixture fraction, which is the fraction of atomic species present which originated in the supplied fuel. The correlation appears to be independent of pool diameter, lip size, and heat release rate. These turbulent correlations differ from the corresponding curves for laminar flames primarily due to composition broadening resulting from time average measurements of widely fluctuating components. We obtained higher than expected concentrations of CO and CO₂ in centerline measurements near the fuel source. An attempt is made ot explain these findings based on non-equal species diffusivity and local radiative extinction.The correlations obtained in this work form the basis for two closely related models: (1) for predicting mean species concentrations in turbulent flames by weighting laminar data with an assumed pdf of the mixture fraction, and (2) the chemical scaling of turbulent pool fires using Froude modeling principles. These applications are briefly discussed.     

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

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.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

The effect of hydrogen addition on premixed laminar acetylene–hydrogen–air and ethanol–hydrogen–air flames

In the present work, the laminar premixed acetylene–hydrogen–air and ethanol–hydrogen–air flames were investigated numerically. Laminar flame speeds, the adiabatic flame temperatures were obtained utilizing CHEMKIN PREMIX and EQUI codes, respectively. Sensitivity analysis was performed and flame structure was analyzed. The results show that for acetylene–hydrogen–air flames, combustion is promoted by H and O radicals. The highest flame speed (247 cm/s) was obtained in mixture with 95% H₂–5% C₂H₂ at λ = 1.0. The region between 0.95 < X_H2 < 1.0 was referred to as the acetylene-accelerating hydrogen combustion since the flame speed increases with increase the acetylene fraction in the mixture. Further increase in the acetylene fraction decreases the H radicals in the flame front. In ethanol–hydrogen–air mixtures, the mixture reactivity is determined by H, OH and O radicals. For X_H2 < 0.6, the flame speed in this regime increases linearly with increasing the hydrogen fraction. For X_H2 > 0.8, the hydrogen chemistry control the combustion and ethanol addition inhibits the reactivity and reduces linearly the laminar flame speed. For 0.6 < X_H2 < 0.8, the laminar flame speed increases exponentially with the increase of hydrogen fraction.     

A numerical study of laminar flame speed of stratified syngas/air flames

Numerical simulations are performed to study the flame propagation of laminar stratified syngas/air flames with the San Diego mechanism. Effects of fuel stratification, CO/H₂ mole ratio and temperature stratification on flame propagation are investigated through comparing the distribution of flame temperature, heat release rate and radical concentration of stratified flame with corresponding homogeneous flame. For stratified flames with fuel rich-to-lean and temperature high-to-low, the flame speeds are faster than homogeneous flames due to more light H radical in stratified flames burned gas. The flame speed is higher for case with larger stratification gradient. Contrary to positive gradient cases, the flame speeds of stratified flames with fuel lean-to-rich as well as with temperature low-to-high are slower than homogeneous flames. The flame propagation accelerates with increasing hydrogen mole ratio due to higher H radical concentration, which indicates that chemical effect is more significant than thermal effect. Additionally, flame displacement speed does not match laminar flame speed due to the fluid continuity. Laminar flame speed is the superposition of flame displacement speed and flow velocity.     

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

Experimental studies on dynamics of methane–air premixed flame in meso-scale diverging channels

The propagation characteristics of a laminar premixed flame front in meso-scale straight and diverging channels of 5°, 10° and 15° with inlet dimension of 25 mm × 2 mm are reported in this paper. The downstream part of the channels was heated with an external heat source, to maintain a positive wall temperature gradient along the direction of fluid flow. These investigations show that planar flames are observed near flash back limits. Negatively stretched flames were observed for moderate flow rates and rich mixtures and for high flow rates, flames were positively stretched. These flames were either symmetric or asymmetric in nature. Partially stable flames were observed at high velocities for rich mixtures, whereas for lean mixture partially stable flames were observed for all flow rates. All the divergent channels showed an improvement in high velocity limits compared with the straight channel for the same mixture. Planar flames observed in the experiments helped in determining the laminar burning velocities for these mixtures at different preheat temperatures. A co-relation of laminar burning velocity with mixture preheat temperature is also obtained for a stoichiometric methane–air mixture. This co-relation S_u/S_u,o = (T_u/T_u,o)^1.558 is in good agreement with the earlier co-relations.     

Experimental study of industrial gas turbine flames including quantification of pressure influence on flow field,fuel/air premixing and flame shape

A commercial swirl burner for industrial gas turbine combustors was equipped with an optically accessible combustion chamber and installed in a high-pressure test-rig. Several premixed natural gas/air flames at pressures between 3 and 6 bar and thermal powers of up to 1 MW were studied by using a variety of measurement techniques. These include particle image velocimetry (PIV) for the investigation of the flow field, one-dimensional laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature, planar laser induced fluorescence (PLIF) of OH for the visualization of the flame front, chemiluminescence measurements of OH^* for determining the lift-off height and size of the flame and acoustic recordings. The results give insights into important flame properties like the flow field structure, the premixing quality and the turbulence–flame interaction as well as their dependency on operating parameters like pressure, inflow velocity and equivalence ratio. The 1D Raman measurements yielded information about the gradients and variation of the mixture fraction and the quality of the fuel/air mixing, as well as the reaction progress. The OH PLIF images showed that the flame was located between the inflow of fresh gas and the recirculated combustion products. The flame front structures varied significantly with Reynolds number from wrinkled flame fronts to fragmented and strongly corrugated flame fronts. All results are combined in one database that can be used for the validation of numerical simulations.     

Structure of laminar premixed flames of methane near the auto-ignition limit

Auto-ignition and flame propagation are the two different controlling mechanisms for stabilizing the flame in secondary stage combustion in hot vitiated air environment and at elevated pressure. The present work aims at the investigation of the flame stabilization mechanism of flames developing in such an environment. In order to better understand the structure of turbulent flames at inlet temperature well above the auto-ignition temperature, the behavior of laminar flames at those conditions needs to be analyzed. As an alternative to challenging and expensive measurements at high temperature and pressure, the behavior of laminar flames at such conditions can be predicted from theory using mathematical simulation. In the present work, the laminar burning velocities and flame structures of premixed stoichiometric methane/air mixtures for inlet temperatures from 300 to 1450 K and absolute pressures from 1 to 8 bar have been calculated using a freely propagating laminar, one dimensional, planar flame model. The prediction shows that at inlet temperatures below the auto-ignition temperature, the predicted laminar burning velocity which corresponds to the unburned mixture velocity in order to create a steady laminar flame decreases with increase in pressure. When the inlet temperature of the mixture goes well beyond the auto-ignition temperature of the mixture, however, the unburned mixture velocity increases steeply at higher pressure level, because of a complete transition of the flame structure.     

Structure and reaction zones of hydrogen – Carbon-monoxide laminar jet diffusion flames

In this study, experimental and numerical investigations of laminar jet diffusion flames using carbon-monoxide – hydrogen mixtures are carried out. Using a simple experimental setup, high definition direct flame photographs and shadowgraphs are captured, and radial temperature profiles at two axial locations are measured. Numerical simulations of carbon-monoxide – hydrogen jet diffusion flames have been carried out using a comprehensive computational model, along with simplified detailed chemical kinetics mechanism having 14 species and 38 reactions, and an optically thin approximation based radiation sub-model. Validation of the numerical model is carried out by comparing the measured and predicted temperature profiles, and experimental shadowgraph images with second derivative of the predicted density field. Results from the numerical simulations provide insights to the structures, species and thermal fields of flames for varying hydrogen content in the fuel mixture. It is observed that the axial extent of the maximum temperature zone tends to move towards the burner exit as the percentage of hydrogen in the fuel increases. It is also observed that the maximum mass fraction of carbon-dioxide decreases and those of OH and water vapour increase with increasing percentage of hydrogen in the fuel. Radial distributions of important species are presented for varying hydrogen content in the fuel mixture, which clearly illustrate the structure of the flame. Radial profiles of net reaction rates of major species and net rates of few important reactions are presented. As hydrogen is added, the reaction zone moves out in the radial direction, increasing the radius of the flame.     

Characterization of biogas-hydrogen premixed flames using Bunsen burner

Experimental study is conducted to clarify the effects of hydrogen addition to biogas and hydrogen fraction in the biogas-H₂ mixture on the stability, thermal and emission characteristics of biogas-H₂-air premixed flames using a 9 mm-ID-tube Bunsen burner. Variation in biogas composition is allowed to range from BG₆₀ (60%CH₄–40%CO₂), down to BG₅₀ (50%CH₄–50%CO₂) and to BG₄₀ (40%CH₄–60%CO₂). For each biogas, the fraction of hydrogen in the biogas-H₂ mixture is varied from 10% to 50%. The results show that upon hydrogen addition and increasing hydrogen fraction in the fuel mixture, there are corresponding changes in flame stability, laminar burning velocity, flame tip temperature and CO pollutant emission.     

An a priori model for the effective species Lewis numbers in premixed turbulent flames

An a priori model for the effective species Lewis numbers in premixed turbulent flames is presented. This a priori   analysis is performed using data from a series of direct numerical simulations (DNS) of lean (?=0.4$?=0.4$) premixed turbulent hydrogen flames, with Karlovitz number ranging from 10 to 1562 (Aspden et al., 2011). The conditional mean profiles of various species mass fraction versus temperature are evaluated from the DNS and compared to unstretched laminar premixed flame profiles. The turbulent flame structure is found to be different from the laminar flame structure. However, the turbulent flame can still be mapped onto a laminar flame with an appropriate change in the Lewis numbers of the different species. A transition from “laminar” Lewis numbers to unity Lewis numbers as the Karlovitz number increases is clearly captured. A model for those effective Lewis numbers with respect to the turbulent Reynolds number is developed. This model is derived from a Reynolds-averaged Navier–Stokes (RANS) formulation of the reactive scalar and temperature balance equations. The dependency of the effective Lewis numbers on the Karlovitz number instead of the Reynolds number is discussed in this paper. Unfortunately, given that the ratio of the integral length to the laminar flame thickness is fixed throughout this series of DNS, a change in the Karlovitz number is equivalent to a change in the Reynolds number. Incorporating these effective Lewis numbers in simulations of turbulent flames would have several impacts. First, the associated laminar flame speed and laminar flame thickness vary by a factor of two through the range of obtained effective Lewis numbers. Second, the turbulent premixed combustion regime diagram changes because a unique pair of laminar flame speed and laminar flame thickness cannot be used, and a dependency on the effective Lewis numbers has to be introduced. Finally, a turbulent flame speed model that takes into account these effective Lewis numbers is proposed.     

A comparative experimental and computational study of methanol, ethanol, and n-butanol flames

Laminar flame speeds and extinction strain rates of premixed methanol, ethanol, and n-butanol flames were determined experimentally in the counterflow configuration at atmospheric pressure and elevated unburned mixture temperatures. Additional measurements were conducted also to determine the laminar flame speeds of their n-alkane/air counterparts, namely methane, ethane, and n-butane in order to compare the effect of alkane and alcohol molecular structures on high-temperature flame kinetics. For both propagation and extinction experiments the flow velocities were determined using the digital particle image velocimetry method. Laminar flame speeds were derived through a non-linear extrapolation approach based on direct numerical simulations of the experiments. Two recently developed detailed kinetics models of n-butanol oxidation were used to simulate the experiments. The experimental results revealed that laminar flame speeds of ethanol/air and n-butanol/air flames are similar to those of their n-alkane/air counterparts, and that methane/air flames have consistently lower laminar flame speeds than methanol/air flames. The laminar flame speeds of methanol/air flames are considerably higher compared to both ethanol/air and n-butanol/air flames under fuel-rich conditions. Numerical simulations of n-butanol/air freely propagating flames, revealed discrepancies between the two kinetic models regarding the consumption pathways of n-butanol and its intermediates.     

Hydrogen addition effect on laminar burning velocity,flame temperature and flame stability of a planar and a curved CH4–H2–air premixed flame

The effects of hydrogen fraction on laminar burning velocity, flame stability (Markstein number) and flame temperature of methane–hydrogen–air flame at global equivalence ratios of 0.7, 1.0 and 1.2 have been investigated numerically based on the full chemistry and the detailed molecular species transport. The effect of stretch rate on combustion characteristics is examined using an opposed-flow planar flame model, while the effect of flame curvature is identified by comparing a tubular flame to the opposed-flow planar flame. The difference in response on hydrogen fraction between the planar and curved flames has been observed. The results show when hydrogen fraction increases, the flame temperature and laminar burning velocity increases, and this effect is more significant at a large stretch rate; while Markstein length decreases. At a fixed stretch rate of 400 s⁻¹, under which the flame approaches extinction limit, the flame temperature of the tubular flame is considerably higher than that of the planar opposed flow flame, which results most likely from the contribution of the positive flame curvature to the first Damkohler number.     

Kinetic modeling study of ethanol and dimethyl ether addition to premixed low-pressure propene–oxygen–argon flames

The chemical composition of flames of mixed hydrocarbon–oxygenate fuels was examined systematically for a series of laminar, premixed low-pressure propene–oxygen–argon flames blended with ethanol or dimethyl ether (DME). All flames were established at a carbon-to-oxygen ratio of C/O = 0.5 at 40 mbar. Propene was replaced incrementally by either additive, so that the entire range from pure propene to pure ethanol or pure DME was accessible. Experimental results have been reported previously (J. Wang et al., J. Chem. Phys. A 112 (2008) 9255–9265), including temperature profiles measured with laser-induced fluorescence (LIF) and quantitative mole fraction profiles for a large number of species obtained from molecular-beam mass spectrometry (MBMS), using electron ionization (EI) and vacuum-ultraviolet (VUV) photoionization (PI). The effects of oxygenate addition to the propene base flame were seen to result in interesting differences, especially regarding trends to form aldehydes. The entire flame series is now analyzed with a comprehensive kinetic model that combines the chemistries of propene, ethanol, and DME combustion. The flames of pure fuels are also compared with the predictions of different detailed mechanisms taken from the literature. Quantitative comparison of C₁- to C₆-species from this model with the measurements is provided. Major trends of propene replacement by the oxygenates are reproduced in quantitative agreement with the experiments, enabling a more detailed understanding of the combined reaction sequences in such fuel blends.