OH Mole Fraction Research Articles

Hydrogen enrichment for improved lean flame stability

The stability characteristics of a premixed, swirl-stabilized flame were studied to determine the effects of hydrogen addition on flame stability under fuel-lean conditions. The burner configuration consisted of a centerbody with an annular, premixed methane/air jet introduced through five, 45° swirl vanes. Flame stability was studied over a range of operating conditions. Under fuel-rich conditions the flame was lifted from the burner surface due to the mixing with entrained ambient air that was needed to form a flammable mixture. As the fuel/air mixture ratio was decreased toward stoichiometric, the resulting increase in flame speed allowed the flame to propagate upstream through the low-velocity wake region and attach to the centerbody face. The maximum blowout velocity occurred at stoichiometric conditions, and decreased as the mixture became leaner. OH PLIF measurements were used to study the behavior of OH mole fraction as the lean stability limit was approached. Near the lean stability limit the overall OH mole fraction decreased, the flame decreased in size and the high OH region took on a more shredded appearance. The addition of up to 20% hydrogen to the methane/air mixture resulted in a significant increase in the OH concentration and extended the lean stability limits of the burner.

OH/F substitution in topaz studied by Raman spectroscopy

The Raman band related to the stretching mode of hydroxyl (OH - ) centered at about 3650 cm - 1 is investigated in several natural specimens of topaz single crystals with a composition Al 2 SiO 4 (OH x F ( 1 - x ) 2 within a wide range of OH mole fractions (x). The analysis of the OH band shape shows that its asymmetry is due to an unresolved splitting in two peaks, centered at 3639 and 3647 cm - 1 , labeled as OH A and OH B , respectively. Although the asymmetry is drastically different for OH-rich and OH-poor topaz, we show that this is only caused by the change of the OH A /OH B intensity ratio with the total OH concentration. In order to explain the splitting of the OH band, we suggest a model which involves a reduction of the local symmetry from D 1 6 2 h to C 9 2 v , resulting in two types of physically nonequivalent F sites (A and B) where the OH/F substitution can occur. Both sites have C 1 site symmetries and a multiplicity of 4.

Demonstration of temperature and OH mole fraction diagnostic in SiH 4/H 2/O 2/Ar flames using narrow-line UV OH absorption spectroscopy

Results of an experimental study of the application of frequency-modulated UV laser absorption spectroscopyto silica (SiO2) particle-forming flames (SiH4/H2/O2/Ar) are presented. An argon-ion pumped ring-dye laser system in the rapid wavelength scanning configuration was used to obtain multiple line shape prifoles of the R1(7) and R1(11) transitions in the A2Σ+«X2IIi (0,0) band of the OH spectrum (vo=32,625,547 and 32,625.072 cm−1, respectively). Temperature and OH mole fraction were determined by a best fit a convolved Voigt absorption profile to the data. Measurements were made in the multiphase regions of silane/hydrogen/oxygen/argon flames, verifying the applicability of the diagnostic approach to combustion synthesis systems. Absorption measurements were taken over a range of particle environments found at increasing heights above the burner surface (5–20 mm) and equivalence ratios (φ=1.0 and 1.2). The experimental data were compared with thermocouple measurements, equilibrium, and one-dimensional modeling simulations. The results of the study successfully demonstrate OH UV absorption spectroscopy as a highly sensitive and accurate (uncertainties less than±10% in the current work) diagnostic approach for in situ measurements of temperature and Oh mole fractions in combustion synthesis flames.

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).

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.

Experiments on the scalar structure of turbulent CO/H 2/N 2 jet flames

Scalar and velocity measurements are reported for two turbulent jet flames of CO/H 2/N 2 (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.

Intermediate species profiles in low-pressure methane/oxygen flames inhibited by 2-H heptafluoropropane: comparison of experimental data with kinetic modeling

Experimental profiles of the intermediate species H, OH, CH, CF, CF 2, and CHF are obtained in a 10 torr premixed flat flame of methane/oxygen in a 1:2 molar ratio, inhibited by a 4% mole fraction of 2-H heptafluoropropane (HFP, CF 3CHFCF 3). These data are compared to calculations using a recently published kinetic mechanism describing the consumption of this fire suppression agent. The profiles in the flame inhibited by HFP are compared to previously published data for flames containing CHF 3 and CH 2F 2 under the same conditions of stoichiometry and flux of fluorine atoms. The species profiles relative to the flames containing the fluoromethanes are accurately predicted and atmospheric pressure flame speeds are fairly well predicted by the kinetic mechanism. Under equal fluorine loadings, profiles of temperature and of H and OH mole fraction are virtually identical between the flames containing HFP and CHF 3. The flame inhibited by HFP, however, has approximately twice as much CH∗ emission as the flame containing CHF 3. The kinetic model predicts that thermal decomposition, rather than H atom abstraction, is the primary destruction mechanism for HFP under the conditions studied.

On the Adequacy of Certain Experimental Observables as Measurements of Flame Burning Rate

This work presents detailed chemical kinetic computations and experimental measurements of a premixed stoichiometric N 2-diluted methane-air flame in two-dimensional unsteady vortical flow, which are used to investigate the utility of several experimental observables as measurements of local burning and heat release rates. The computed mole fraction of HCO is found to have excellent correlation with flame heat release rate over the whole range of unsteady curvature and strain-rate investigated, for the flame under consideration. HCO planar laser induced fluorescence (PLIF) imaging is discussed and demonstrated in a V-flame experiment. On the other hand, we find the utility of peak dilatation rate as an indicator of heat release rate to be dependent on the unsteady strain-rate and flame curvature environment, and the associated modification in diffusional thermal fluxes within the flame. The integrated dilatation rate is found to be more robust under unsteady strain-rate, but still questionable in regions of high flame curvature. We also study the utility of a particular formulation for CO ∗ 2 chemiluminescence, OH, and CH PLIF imaging, as well as OH∗, C ∗ 2, and CH∗ chemiluminescence, as measurements of flame burning and heat release rates. We generally find these measures to be inferior to HCO. Experimental results suggest that CH, OH∗, C ∗ 2, and CH∗ are not adequate indicators of local extinction; rather they provide signals of subtle shifts of hydrocarbon consumption among different chemical pathways. Moreover, numerical results suggest that both OH mole fraction and an existing CO ∗ 2 chemiluminescence model do not correlate with burning or heat release rate variations in regions of high unsteady flame curvature. The present numerical investigation uses a single flame/vortex condition and a specific 46-step C (1) chemical mechanism. The conclusions reached herein may be generalized with further studies using more detailed mechanisms over ranges of stoichiometry, dilution, and flow time and spatial scales.

Finite Rate Chemistry and NO Molefraction in Non-Premixed Turbulent Flames

Finite rate chemistry is investigated in turbulent N2-diluted H2 diffusion flames by means of laser spectroscopic methods and a numerical model of combustion. The major species occurring in these hydrogen flames or the temperature are measured with SRS (Spontaneous Raman Spectroscopy) or Rayleigh spectroscopy. Some minor species, the radical OH and the pollutant NO, are simultaneously measured with linear LIF (Laser Induced Fluorescence). Multidimensional pdfs (probability density function) can be deduced from these measurements. Use is made of a numerical model with two principal variables, mixture fraction ξ and reaction progress variable η, as a basis for discussion of the experimental results. A k-ϵ-turbulence model together with a two-dimensional presumed pdf for the coupling of turbulence and chemistry are applied. So, experimental two-dimensional pdfs as well as mean values of ξ and η as functions of the position in the flame are deduced from the simultaneous measurements. The experimental and theoretical spatial maxima of the mean OH molefraction agree well in magnitude, despite the correlation coefficient between ξ and η of the measured pdf can be as high as 0.5. The neglect of this covariance for the calculation of the presumed pdf is quantified. It results in clear deviations for the OH molefraction. The experimental NO and OH molefractions are better simulated by flame calculations carried out with the presented combustion model than by the also shown calculations based on a single variable for description of chemistry.

Coupling of diffusion flame structure to an unsteady vortical flow-field

A laminar methane-air diffusion flame is interacted with vortices of various sizes and strengths in order to better understand unsteady stretch and history effects on turbulent flames. The nitrogen-diluted fuel stream of a Wolfhard-Parker slot burner is acoustically forced, producing repeatable two-dimensional vortices that strain and curve the flame. Phase-locked, planar laser-induced fluorescence (PLIF) diagnostics are used to quantify the response of the OH-radical to the vortex-induced stretch. Acetone PLIF images are used to clarify the relationship between the vortex structure and the flame. The results show that the vortex causes significant variations in the OH layer thickness. In particular, negative strain produces a doubling of the flame thickness. Local extinction of the OH layer due to high strain is observed near the flame base. Peak OH mole fraction levels vary considerably more than adiabatic LFM predictions. In particular the peak OH decreases by a factor of 2 with downstream distance. This decrease is believed due to dilution of reactants by combustion products formed elsewhere in the flow. A simplified model is proposed, which shows the OH concentration is sensitive to product dilution through the scalar dissipation rate.

Universal Relationships in Sooting Methane-Air Diffusion Flames

The laminar flamelet concept is based on the premise that scalar properties in laminar diffusion flames are nearly universal functions of mixture fraction. It has been well-tested and proven for temperature and the major species, however, few studies have addressed its applicability for minor species especially the radical species. In this study, we present a direct numerical simulation of an axisymmetric, laminar, methane-air diffusion flame to examine these universal relationships, including the major and minor chemical species, and the radical species. The numerical model solves the axisymmetric, time-dependent, reactive-flow Navier-Stokes equations coupled with sub-models for soot formation and radiation transport, and includes a detailed reaction mechanism for methane-air combustion. Quantitative comparisons with existing experimental data show a slightly wider computed flame compared to the experimental flame, however, the peak values and radial locations of temperature and soot volume fraction line up well with the experimental measurements. To study the universal relationships, scatter plots are made for temperature and the major and minor species throughout the entire flame and compared with existing experimental measurements. Excellent agreement is obtained between the computations and experiments for temperature and mole fractions of CH4, O2, OH, H2O, O, H, CO2 and N2, as a function of mixture fraction in the fuel lean, stoichiometric and fuel rich regions of the flame. The computations underpredict the concentration of H2 and CO in the fuel rich region, however, excellent agreement is obtained in the fuel lean and stoichiometric regions. The computations also overpredict the concentration of CH3 in the stoichiometric and fuel rich regions, however, the peak concentration occurs at the same mixture fraction for both the experiments and computations. The scatter plots indicate that many of the species studied, including the minor species and radical species, can be considered universal functions of mixture fraction, however some of the species show more scatter than others. The species which showed the least scatter were those whose production rates were fastest in the stoichiometric and fuel lean regions of the flame. Those species whose production rates were highest in the very fuel rich region showed less universality with mixture fraction.

Unsteady flamelet modeling of turbulent hydrogen-air diffusion flames

The unsteady flamelet model is applied in numerical simulations of a steady, turbulent, nitrogen-diluted hydrogen-air diffusion flame. An unsteady flamelet is solved interactively with a CFD solver for the turbulent flow and the mixture fraction field. Transient effects occurring in steady jet diffusion flames are discussed in terms of the relevant timescales. It is shown that radiation can be neglected and that the flame structure is hardly influenced by transient effects for the present case. However, for predictions of slow processes, like the formation of NO, unsteady effects have to be considered. The results predicted by the model are in reasonable agreement to experimental data for temperature, major species mass fractions, OH, and NO mole fractions. On the contrary, the use of steady flamelet libraries yields good results for flame structure and even OH concentrations, but NO is overpredicted by an order of magnitude. However, reasonably well-predicted NO concentrations can also be obtained by solving an unsteady flamelet as a postprocessing mode.

The influence of heterogeneous kinetics and thermal radiation on the oxidation of graphite particles

This work presents detailed chemical kinetic computations and experimental measurements of a premixed stoichiometric N2-diluted methane-air flame in two-dimensional unsteady vortical flow, which are used to investigate the utility of several experimental observables as measurements of local burning and heat release rates. The computed mole fraction of HCO is found to have excellent correlation with flame heat release rate over the whole range of unsteady curvature and strain-rate investigated, for the flame under consideration. HCO planar laser induced fluorescence (PLIF) imaging is discussed and demonstrated in a V-flame experiment. On the other hand, we find the utility of peak dilatation rate as an indicator of heat release rate to be dependent on the unsteady strain-rate and flame curvature environment, and the associated modification in diffusional thermal fluxes within the flame. The integrated dilatation rate is found to be more robust under unsteady strain-rate, but still questionable in regions of high flame curvature. We also study the utility of a particular formulation for CO∗2 chemiluminescence, OH, and CH PLIF imaging, as well as OH∗, C∗2, and CH∗ chemiluminescence, as measurements of flame burning and heat release rates. We generally find these measures to be inferior to HCO. Experimental results suggest that CH, OH∗, C∗2, and CH∗ are not adequate indicators of local extinction; rather they provide signals of subtle shifts of hydrocarbon consumption among different chemical pathways. Moreover, numerical results suggest that both OH mole fraction and an existing CO∗2 chemiluminescence model do not correlate with burning or heat release rate variations in regions of high unsteady flame curvature. The present numerical investigation uses a single flame/vortex condition and a specific 46-step C(1) chemical mechanism. The conclusions reached herein may be generalized with further studies using more detailed mechanisms over ranges of stoichiometry, dilution, and flow time and spatial scales.

Granitic melt viscosities; empirical and configurational entropy models for their calculation

Newly measured viscosities of a low H20-content granitic melt and a low H20-content albitic melt have been combined with viscosity data from the literature to create two new models for the calculation of granitic melt viscosities at crustal pressures and temperatures between 700 and 900 °C; one model is purely empirical, and the other is based on config- urational entropy theory. When the molecular weight of the anhydrous melt is calculated on the basis of eight 0 atoms, or assumed to be 260 gram formula weight (gfw), and the total H20 concentration is expressed by its mole fraction, the viscosity, '1/,of per- and metaluminous granitic melts, -70-76 wt% SiOl, can be calculated from the following empirical equations: For X~~o ::s 0.25, log '1/= (0.0292649T - 53.198903)X~~o- 0.0129207T + 24.2973 and for X~~o> 0.25, log '1/= - 2.2977826X~:o - 0.0095llOT + 15.6293 where viscosity is in pascal-seconds, and temperature, T, is in kelvins. The first equation expresses the effects of H20 addition on melt viscosity at low H20 contents where the predominant H20 species in the melt is OH- (as demonstrated in previous studies) and small additions of H20 have large effects on melt viscosity; above X~:o ' 0.25, molecular H20 is the dominant species of H20, the addition of which has only modest effects on melt viscosities. This empirical model is applicable to granitic melts with total H20 con- tents between 0.3 and 12.3 wt%. Following configurational entropy theory, viscosities of the same hydrous granitic melts can be calculated by modeling the viscosities as ternary mixtures of various mole fractions of molecular H20, X H20,OH ~, XOH-,and silicate melt, Xsilieatemelt: In '1/ = -15.398 + (181.622/{T(Sconf(Xsilieate melt) - R(CH2oXH201n XH20 + CowXowln Xow + CsilicatemeltXsilicatemeltln Xsilicatemelt))}), where Seonf, the configurational en- tropy calculated from the viscosity measurements, is 4.567 x 10-3 J/(mol. K); R is the gas constant, 8.314 J/(mol. K); CH20is -1.899 x 10-3; COH- is -1.531 x 10-3; and Csilicatemelt is 4.913 X 10-3. Mole fractions of H20 and OH- are estimated from previously published measurements of species abundances in quenched rhyolitic glasses, and the molecular weight of the silicate melt is 260 gfw. Application of this model is limited to melts with. a maximum of approximately 6 wt% total H20 because X H20-XOH- speciation information is not available at higher H20 contents. Numerical values in the above equation are not considered to have any physical or chemical significance. Calculated granitic melt viscos- ities deviate on average from measured viscosities by -0.03 log units for the empirical model and -0.2 log units for the configurational entropy model. At H20 concentrations below 4 wt% and above 7 wt%, these models for viscosity estimation are significantly better than previous models, which can be in error by one to two orders of magnitude. Using either of these new models, the viscosities of granitic melts with 1-3 wt% H20 are calculated to be up to two orders of magnitude lower than previously thought. This results in a one to two order-of-magnitude increase in the viscosities of melt extraction from source regions, transport of melts through the crust, and Stokes settling of crystals.

Measurement of the rate coefficient of H+O 2+M→HO 2+M for M=Ar and N 2 at high pressures

OH mole fraction profiles were measured using cw UV laser absorption in shock-heated mixtures of H 2 /O 2 /Ar/N 2 between 1260 and 1375 K at 50, 68, and 115 atm. Rate coefficients for the reaction H+O 2 +M→HO 2 +M, M=Ar and N 2 were inferred by comparison of measured OH profiles with OH profiles modeled using the GRI-Mech v1.2 mechanism. The rate coefficient data were fit with pressure-dependent expressions that use k ∞ and F cent from Cobos et al. [3]. For M=Ar, the measured third-order rate coefficients are consistent with a pressure-dependent expression that uses k 0 =7×10 17 T −0.8 [cm 6 /mol 2 /s] from GRI-Mech v1.2. For M=N 2 , the present data, when combined with the lower-temperature value of Bromly et al. [15], are consistent with a pressure-dependent expression that uses k 0 =2.6×10 19 T −1.24 [cm 6 /mol 2 /s] over the temperature range of 700–1375 K. This expression yields k 0 values that are substantially higher than the GRI-Mech v1.2 recommendation but consistent with previous flow reactor, flame, and shock tube data and the review of Baulch et al. [24].

High-pressure methane oxidation behind reflected shock waves

Experiments on CH 4 oxidation behind reflected shock waves were conducted at elevated pressures, and the results compared to a detailed kinetics model. Mixtures of CH 4 and O 2 dilute in either argon or nitrogen were studied over a wide range of stoichiometry (=0.5–4.0), bath gas dilution (90.0–99.5%), pressure (9–480 atm), and temperature (1410–2040 K), corresponding to total concentrations from 5.6×10 −5 to 3.6×10 −3 mol/cm 3 . Reaction progress was monitored using narrow-line laser absorption of OH at 306 nm, infrared emission of CH 4 near 3.4 μm, and pressure measurements. The measured species time-histories and pressure traces were assembled into an extensive database of characteristic reaction times, peak OH mole fractions, and ignition delay times that can be used for comparisons with detailed kinetics mechanisms. The chemical kinetics model utilized in the present comparisons is the latest GRI mechanism, GRI-Mech 1.2. As a whole, agreement between the model predictions and the experimental measurements is good, particularly for ignition delay times. However, based on the results of certain CH 4 profiles, improvements in the model for high-pressure, fuel-rich conditions are needed. Sensitivity and species contribution analyses were used to identify the most important reactions at pressures up to 500 atm, some of which require more accurate rate coefficients. As pressure increases, the CH 3 removal pathways are altered, the CH 3 formation pathways remain the same, and reactions involving HO 2 become important.

Laser-induced fluorescence measurements of NO and OH mole fraction in fuel-lean, high-pressure (1–10 atm) methane flames: Fluorescence modeling and experimental validation

A method for quantifying laser-induced fluorescence (LIF) signals and planar laser-induced fluorescence (PLIF) images of the OH and NO radicals in high pressure flames is presented. The fluorescence signal per unit radical mole fraction is modeled as a function of temperature, pressure, overall flame stoichiometry and laser spectral bandwidth. A recently developed model ( JQSRT, 51, 511; Appl. Phys. B, 57, 249) 1,2 for electronic quenching cross-sections of OH and NO is utilized to express the fluorescence yield as a function of these parameters. The models are confirmed using single-point measurements in the burnt gas region of a flat flame burner at up to 10 atm. The measurements are performed at points in the flame where the temperature, pressure, OH and NO mole fraction are all known. For fuel-lean flames at elevated pressure, interference from the O 2 Schumann-Runge system was found with NO A ← X(0, 0) fluorescence measurements. This interference must be considered when selecting an appropriate NO transition in this type of environment.

Effect of unsteady stretch rate on OH chemistry during a flame-vortex interaction: To assess flamelet models

Some basic assumptions of flamelet models are assessed by comparing profiles of OH mole fraction measured during an unsteady flame-vortex interaction to the OH profiles computed for a steady, planar counterflow flame (SPCF) with full chemistry. It is important to make such comparisons for both the same local three-dimensional stretch rate, which is measured instantaneously at locations along the flame front, and the same heat loss, as characterized by the product temperature T 2. The fundamental experimental procedure consists of interacting a laminar, premixed flame with an impinging vortex ring of reactants. The OH flame chemistry was quantified using planar laser-induced fluorescence (PLIF) techniques, while the three-dimensional stretch rate measurements were made possible by the use of particle-imaging velocimetry (PIV) diagnostics on the repeatable, axisymmetric experiment. Whereas previous comparisons have been limited to steady counterflow flame experiments, the present study considers a flame which is unsteady, freely-propagating, curved, far from walls, and has realistic heat losses; thus, it contains the physical processes present in turbulent premixed flames. It was found that there are significant (25%) differences between measurements and steady counterflow flame computations of peak OH mole fractions and OH reaction zone widths. Even where the stretch rate was constant along the flame, the OH profiles showed variations, indicating that the OH profile is not a unique function of the instantaneous local stretch, but depends on the time history of the flowfield. Such history effects may be better modeled using unsteady counterflow flame simulations. Large differences (a factor of two on centerline) occur between the measured three-dimensional and two-dimensional stretch rates, indicating the importance of experimentally determining the full three-dimensional stretch rate for meaningful comparisons with models. Sensitivity analysis shows that heat losses must be realistically modeled, especially if flame extinction is to be simulated. The present type of comparison represents a first step in the assessment of flamelet models.

MEASUREMENT OF TEMPERATURES AND OH CONCENTRATIONS OF SOLID PROPELLANT FLAMES USING ABSORPTION SPECTROSCOPY

ABSTRACT Multi-parameters least-squares analysis of rotationally resolved absorption spectroscopy in the A2Σ − X2Π(0, 0) vibrational band system of OH from 307 to 311 nm was performed to determine the flame temperatures and OH concentrations of JA-2 double-base propellant under steady-state burning at elevated pressures with a spatial resolution of 50–100 µm. Deduced flame temperatures were 2754 ± 124 K, 2836 ± 147 K, 2852 ± 179 K, and 2873 ± 188 K at pressures of 1.60, 3.55, 5.62, and 6.99 MPa, respectively; the corresponding OH number densities were (1.09 ± 0.34) × 1017,(2.31 ± 0.48) × 1017,(3.31 ± 0.27) × 1017,and (3.44 ± 0.30) × 1017 molecules/c.c., respectively. The results at 1.60 MPa agree well with data in the literature, and deduced flame temperatures at high pressures agree well with chemical equilibrium calculations. OH mole fractions are about 40% lower than the equilibrium analysis. This work represents the first set of optical absorption measurements performed on solid propellants at pressur...

Temperature and Mass Flow Effects in 10 Torr Methane/Oxygen Flames

ABSTRACT We have investigated the effect of burner surface temperature and mass flow on OH and CH radical mole fraction profiles in 10 Torr premixed methane/oxygen flames. Experimental flames were monitored using both flame emission and laser induced fluorescence. Modeling predictions using Sandia flame codes are compared to experimental results. An inverse relationship between mass flow and flame front distance from the burner is observed and modeled. Lean methane/oxygen flames stabilize closer to the burner than rich flames when mass flow is held constant. For methane/oxygen flames at 10 Torr, the effect on the location of the flame front as judged by the OH and CH profiles of a ± 10% measurement uncertainty in the experimental gas flow or a ± 10% uncertainty in the burner surface temperature, cannot be measured within experimental precision.