N. H. Heberle, G. P. Smith, J. B. Jeffries, and D. R. Crosley, "Simultaneous Laser-Induced Fluorescence and Rayleigh Scattering Measurements of Structure in Partially Premixed Flames," Appl. Phys. B 71, 733 (2000).
Laser-induced fluorescence and Rayleigh scattering measurements were made in well-stabilized, laminar partially premixed Bunsen-type methane/air flames. Simultaneous Rayleigh scattering (temperature) and laser-induced fluorescence (flame radical concentration) enable precise determination of the position of CH and OH radical structures in the gradient of the flame temperature. The OH and CH structures in the straight walls of the premixed inner flame cone are well described by models incorporating detailed flame chemistry and one-dimensional transport. Near the tip of the inner cone in regions of increased flame stretch, at richer stoichiometries of the premixed portion of the flames, and in flames perturbed by a metal insert, the measured structure of CH and OH deviates from this simple description. CH concentrations predicted by the model depend on flow rate, thus suggesting the importance of strain rate to prompt NOx formation in these flames.
R.J.H. Klein-Douwel, J.B. Jeffries, J. Luque, G.P. Smith, and D.R. Crosley, "Laser Induced Fluorescence of Formaldehyde Hot Bands in Flames," Appl. Opt. 39, 3712 (2000).
Laser-induced fluorescence and excitation spectra of formaldehyde in the A-X 410 band at 370 nm are recorded in the primary flame front of a Bunsen flame. An examination of partition functions shows this excitation can minimize temperature bias for formaldehyde in situ diagnostic measurements.
D.M. Golden and G.P. Smith, "Reaction of OH + NO2 + M: A New View," J. Phys. Chem. 104, 3991 (2000).
A re-examination of data and theoretical computations for the title reaction leads to the conclusion that both HONO2 (nitric acid) and HOONO (pernitrous acid) can be formed. We describe hindered-Gorin RRKM calculations that fit most of the extant data and explain differences among the studies. We conclude that the rate constant for nitric acid formation at low temperatures could be considerably (~20%) lower than the data for loss of OH in the presence of NO2 would indicate. We have examined atmospheric consequences of this conclusion on ozone and nitrogen oxide concentrations through the use of box models. We include a tentative recommendation for the rate constants for the individual steps as functions of temperature and density in the NASA format.
J. Luque, J.B. Jeffries, G.P. Smith, D.R. Crosley, K.T. Walsh, M.B. Long, and M.D. Smooke, "CH(A-X) and OH(A-X) Optical Emission in an Axisymmetric Laminar Diffusion Flame," Combustion and Flame 122, 172 (2000).
Steady state concentrations of electronically excited CH(A) and OH(A) are extracted from previous quantitative measurements of optical emission from an asymmetric laminar diffusion flame (Walsh, Long, Tanoff, and Smooke, Proc. Combust. Inst. 27: 615 (1998).). The flame is modeled with a two-dimensional transport and detailed chemistry explicitly augmented with the reactive, radiative, and energy transfer collisional processes to produce and remove electronically excited CH(A) and OH(A). Computations predict concentrations of CH(A) and OH(A) which agree with the measurement within a factor of 6 or better, a significant improvement compared to the earlier report.
P. A. Berg, D. A. Hill, A. R. Noble, G. P. Smith, J. B. Jeffries, and D. R. Crosley, "Absolute CH Concentration Measurements in Low Pressure Methane Flames," Combustion and Flame 121, 223 (2000).
Absolute CH concentration profiles were measured for rich and lean premixed, low-pressure, laminar-flow methane flames, using laser-induced fluorescence calibrated with a Rayleigh scattering technique. Agreement between the measured concentrations and GRI-Mech 3.0/Premix model predictions varies with the flame stoichiometry: measured and predicted maximum CH concentrations and position agree well for a near-stoichiometric flame, but the predicted values are approximately 20% too large for rich flames and too small for the lean flame. The model, constrained by a measured temperature profile, predicts the CH peak to be too far from the burner for the rich flames and too close for the lean flame. Measured OH concentrations give good agreement with model results for all flame stoichiometries.
G P. Smith, J. Luque, J. B. Jeffries, and D. R. Crosley, "Rate Constants for Flame Chemiluminescence," Paper 25, 2nd Joint Meeting US Sections of the Combustion Institute, Oakland CA, Mar. 2001.
Absolute excited state concentrations of OH(A), CH(A), and C2(d) were determined in three low pressure premixed methane-air flames. Two dimensional images of chemiluminescence from these states were recorded by a filtered CCD camera, processed by Abel inversion, and calibrated against Rayleigh scattering. Using a previously validated 1-D flame model with known chemistry and excited state quenching rate constants, rate constants are extracted for the reactions CH + O2 ® OH(A) + CO and C2H + O ® CH(A) + CO at flame temperatures. Variations of flame emission intensities with stoichiometry agree well with model predictions.
G. P. Smith, "Diagnostics for Detailed Kinetic Modeling," in Applied Combustion Diagnostics, K. Kohse-Höinghaus and J. B. Jeffries, ed., Taylor and Francis, N. Y., p. 501-517, 2001.
Optimization and validation of combustion chemistry mechanisms requires quantitative data on key species within complex reactive networks for comparison to kinetic model predictions. Uncertainty limits on the individual rate constants produce a parameter space of possible mechanisms still too imprecise for accurate prediction of combustion properties such as flame speed or ignition delay, thus requiring additional system data. Low pressure and counterflow flames, mixtures in shock tubes, and flow or well-stirred reactors are examples of such experimental environments. The usefulness of the resulting data depends critically on three factors: a minimum dependence of the mathematical model on uncertain parameters or assumptions, a maximum dependence of the observables on the kinetics parameters one wishes to test or optimize, and a sufficient accuracy in the observation to provide a sensitive test. These conditions should always be tested, using sensitivity analysis of the model calculations, and the concept of model error bars can be useful in this regard. Examples from a low pressure flame used to study NO reburn illustrate these considerations. Observations of species concentrations, spatial profiles, and relative ratios (between species or experiments) are considered with respect to their sensitivity to the kinetics. Handwaving judgments about model agreement should be eliminated. Some experiments will be too dependent on modeling uncertainties (e.g., temperature history or flow rate). Some potential observations will have little kinetic relevance or be redundant. The use of a kinetic uncertainty index for different species derived from sensitivity analysis can suggest valuable measurements, and provide guidance regarding the required accuracy of the diagnostic technique and measurement. Correlation of sensitivities can identify surrogate or redundant measurements.
J. Luque, J. B. Jeffries, G. P. Smith, and D. R. Crosley, "Predissociation of CH B v’=0,1 Levels Studied by Cavity Ringdown Absorption Spectroscopy," Chem. Phys. Lett., 126, 1725 (2001).
Cavity ring-down absorption spectroscopy is applied to study predissociation of the CH B2S+ v’=0 N’=17-19 and v’=1 N’=9-11 rotational levels in a low pressure methane/air flame. Collision free lifetimes from broadened spectral lineshapes are 45 ± 6 ps and 10.0 ± 1.5 ps for CH B2S+ v’=0, N’=18, 19 and 75 ± 15 ps, 15 ± 2 ps and 5 ± 1 ps for CH B2S+ v’=1, N’=9-11 These lifetimes are in good agreement with previous calculations by Wyel theory using potentials calculated by the iterative Rydberg-Klein-Dunham method (N. Elander et al., Physica Scripta, 20, 631 (1979)). Experimental results suggest that the F2 spin-orbit levels might predissociate slightly faster than F1 levels.
J. Luque, J. B. Jeffries, G. P. Smith, and D. R. Crosley, "Quasi-Simultaneous Detection of CH2O and CH by Cavity Ring-Down Absorption and Laser-Induced Fluorescence in a Methane/Air Low Pressure Flame," Appl. Phys. B, 73, 731 (2001).
The combination of two-dimensional, planar laser-induced fluorescence (PLIF) and cavity ring-down (CRD) absorption spectroscopy is applied to map quantitatively the spatial distributions of CH2O and CH in a methane/air flame at 25 Torr. Both species are detected in the same spectral region using the overlapping CH2O A 1A2 - X 1A1 410 and CH B-X(1,0) bands. The combination of diagnostic techniques exploits the spatial resolution of LIF and the quantitative CRD absorption measure of column density. The spatially resolved PLIF provides the distribution of absorbers and line-of-sight CRD absorption the absolute number density needed for quantitative concentration images. The peak CH2O concentration is (3.5 ± 1.4)x1014 cm-3 or 1450 ± 550 ppm at 1000 K. The lack of precise absorption cross section data produces these large error limits. Although a flame model predicts lower amounts, these large uncertainties limit this measurement’s usefulness as a test of the flame chemistry.
J. Luque, J. B. Jeffries, G. P. Smith, D. R. Crosley, and J. J. Scherer, "Combined Cavity Ringdown Absorption and LIF Imaging Measurements of CN(B-X) and CH(B-X) in Low Pressure CH4-O2-N2 and CH4-NO-O2-N2 Flames," Combustion and Flame, 126, 1725 (2001).
A combined cavity ringdown absorption spectroscopy(CRD) and laser-induced fluorescence(LIF) imaging method is used to study CN and CH absolute concentration profiles in low pressure premixed flames featuring prompt NO and reburn chemistry. For methane-air flames without and with seeded NO, the absolute concentrations and the shapes and peak positions of CN and CH above the burner compare favorably to model predictions and validate the chemical mechanism. CRD absorption of CN provides part per billion detection sensitivity. The CH results agree with previous LIF measurements calibrated with Rayleigh scattering, after correcting CRD for laser linewidth effects and accounting for the spatial inhomogeneities of the CH distribution in the flame.
G. P. Smith, M. K. Dubey, D. E. Kinnison, and P. S. Connell, "Assessing Effects of Rate Parameter Changes on Ozone Models Using Sensitivity Analysis," J. Phys. Chem., 105, 1449 (2001).
Effects of recommended rate parameter changes in the NASA JPL-2000 evaluation from JPL-94 values on local ozone concentrations in a 2-D model are predicted using local sensitivity analysis results from the LLNL 2-D diurnally averaged model. Ozone decreases of 5% in the middle stratosphere, and 10% increases near the tropopause and upper troposphere are indicated. Altered NOx kinetics are largely responsible for these changes, and increased model NOx levels and ozone depletion from stratospheric aircraft are also expected according to sensitivity analysis. Effects of specific changes, such as the nitric acid formation rate, are examined. New error bars on rate parameters in the evaluation are propagated by the sensitivity coefficients to derive revised kinetics uncertainties for the model ozone calculations at several altitudes, latitudes, and seasons. Middle-upper stratospheric ozone uncertainties of 12% from the catalytic photochemistry are indicated, increasing in the lower stratosphere.
R.J.H. Klein-Douwel, J.B. Jeffries, J. Luque, G.P. Smith, and D.R. Crosley, "CH and Formaldehyde Structures in Partially Premixed Methane Air Coflow Flames," Combust. Sci. & Technol., 167, 291 (2001).
CH and CH2O inner cone structures in partially premixed methane/air Bunsen flames are examined with 2-D laser-induced fluorescence (LIF) imaging for stoichiometries 1.36 £ F £ 3.0. Chosen LIF excitation strategies minimize the temperature dependent partition function variation for CH2O, and maintain CH signal strength while eliminating Rayleigh scattering background. The formaldehyde structure appears inside CH in the inner flame cone for moderately fuel rich stoichiometries typical of appliance flames. CH LIF becomes too weak to distinguish from background at richer stoichiometries (F =2.7). A distinct formaldehyde inner cone structure persists even for very rich F , with an increasing width. A simple 1-D model replicates the variation in the relative concentrations of CH and formaldehyde in the inner cone. Predicted absolute CH agrees within a factor of two with the measured value. Exhaust probe measurements show that metal inserts reduce NO and increase CO emissions. LIF images of CH and CH2O taken for these perturbed flames reveal the CH2O structures are spatially expanded by the inserts.
G. P. Smith, J. Luque, C. Park, J. B. Jeffries, and D. R. Crosley, "Low Pressure Flame Determinations of Rate Constants for OH(A) and CH(A) Chemiluminescence," Combust. Flame, 131, 59 (2002).
Absolute excited state concentrations of OH(A), CH(A,B), and C2(d) were determined in three low pressure premixed methane-air flames. Two dimensional images of chemiluminescence from these states were recorded by a filtered CCD camera, processed by Abel inversion, and calibrated against Rayleigh scattering. Using a previously validated 1-D flame model with known chemistry and excited state quenching rate constants, rate constants are extracted for the reactions CH + O2 ® OH(A) + CO and C2H + O ® CH(A,B) + CO at flame temperatures. Variations of flame emission intensities with stoichiometry agree well with model predictions.
R. Robertson and G. P. Smith, "Photolytic Measurement of the O + OH Rate Constant at 295K," Chem. Phys. Lett., 358, 157 (2002).
The photolysis of ozone in 40 torr nitrogen with small amounts of water was used to measure the O + OH rate constant at 295K. A value of 3.17 ± 0.51 (1-s) x 10-11 cm3/s was determined by kinetic modeling of OH decays monitored by laser induced fluorescence, in agreement with current recommendations.
J. Luque, R. J. H. Klein-Douwel, J. B. Jeffries, G. P. Smith, and D. R. Crosley, "Quantitative Laser-Induced Fluorescence of CH in Atmospheric Pressure Flames", Appl. Phys. B, 75, 779 (2002).
Absolute number densities of CH radical were determined in a partially premixed methane/air flame (equivalence ratio is 1.36) at atmospheric pressure by exciting the CH B-X(1,0) transition using a quasi-linear laser-induced fluorescence scheme. The peak number density is (1.0 ± 0.4)x1013 cm-3 or 2.4 ± 1 ppm at 1900 K, with a flame front width of 250 mm (FWHM). Rotational energy transfer must be considered for correct laser-induced fluorescence signal interpretation. Competition between optical pumping and rotational relaxation in both excited and ground states produces a signal that varies almost linearly with laser pulse energy even for large pumping rates. For these conditions, the population of the initial ground state rotational level is depleted by optical pumping, and rotational energy transfer collisions rapidly repopulate the level during the laser pulse. Deviations from linear behavior are less than 20%. Effects of spatial resolution and polarization of the fluorescence on the absolute measurements are also discussed.
G. P. Smith, "Evidence of NCN as a Flame Intermediate for Prompt NO," Chem. Phys. Lett., 367, 541 (2002).
NCN is detected by laser induced fluorescence of the A-X transition at 329 nm in low pressure methane-air and methane-nitrous oxide flames. The excitation wavelengths, signal intensities, spatial height profiles above the burner, fluorescence quenching lifetimes, and highly resonant fluorescence wavelength are all consistent with this assignment. The spectral features require the presence of hydrocarbon fuel and nitrogen, and do not gain added intensity under reburn conditions of NO seeding. The resulting arguments and evidence favoring NCN as the initial intermediate in prompt NO formation are discussed.
G. P. Smith, "Rate Theory of Methyl Recombination at the Low Temperatures and Pressures of Planetary Atmospheres," Chem. Phys. Lett., 376, 381 (2003).
The recombination of methyl radicals in low pressures of hydrogen, helium, and nitrogen to form ethane controls the concentrations observed for methyl and the photochemical synthesis of higher hydrocarbons above the Jovian planets. Few measured or theoretical rate constants are available to provide reliable model predictions. RRKM and master equation calculations are reported here, using 3 levels of transition state detail, to describe existing data and provide consistent and reliable expressions for this rate constant at 65K-300K and any pressure. This gives k¥ = 3.59 x 10-10T -.262e -37/T cm3/molec/s and k0(H2) = 3.32 x 10-15 T -4.28e-131/T cm6/molec2/s.
J. Luque, P. Berg, J. B. Jeffries, G. P. Smith, D. R. Crosley, and J. J. Scherer, "Cavity Ringdown Absorption and Laser Induced Fluorescence for Quantitative Measurements of CH Radicals in Low Pressure Flames," Appl. Phys. B, 78, 93 (2004).
The absolute, quantitative spatially-resolved distribution of CH radicals was measured in the reaction zone of a low-pressure methane/air flame (25 Torr) using a combination of laser-induced fluorescence (LIF) and cavity ring-down (CRD) absorption spectroscopy operating on the A 2D - C 2P (0,0) transition. The spatially resolved 1-D image of LIF provides a direct measure of the CH distribution along the path of the laser beam in the CRD cavity. The temperature distribution was determined from measurements on a pair of rotational transitions. A series of LIF line images and CRD absorption measurements taken at various burner heights are combined to form a quantitative 2-D image of the CH distribution. This is used to interpret the CRD measurements along this inhomogeneous path. The 10 ppm peak CH concentration measured here on the centerline of the flame is in good agreement (within 15%) with earlier CH A-X LIF measurements calibrated by Rayleigh and Raman scattering. A 1-D LIF image collected simultaneously with CRD absorption was also used to quantify and optimize the spatial resolution of the CRD measurement.
G. P. Smith, C. Park, and J. Luque, "A Note on Chemiluminescence in Low Pressure Hydrogen and Methane-Nitrous Oxide Flames," Combust. Flame, 140, 385 (2005).
Absolute OH(A) and CH(A) concentrations were determined in low pressure H2–air and CH4–N2O flames respectively by measuring absolute chemiluminescence yields at 310 nm and 430 nm. From spatial profiles and intensities in these and other flames, we deduce that two reactions are responsible in each case, and derive rate constants for all.
G. P. Smith, C. Park, J. Schneiderman, and J. Luque, "C2 Swan Band Laser Induced Fluorescence and Chemiluminescence in Low Pressure Hydrocarbon Flames," Combust. Flame, 141, 66 (2005).
Laser induced fluorescence excitation of the C2 (d-a) 2,0 band at 438 nm was used to determine spatial profiles and relative lower state concentrations in several low pressure hydrocarbon-air premixed flames, to test fuel-rich flame kinetics. Quenching loss rate constants are derived from measured fluorescence decay rates. Quantitative Swan band chemiluminescence intensities, coupled with computer modeling of the flame kinetics and these excited state loss rates, then lead to recommended rate constant values for the excited C2(d) state chemiluminescence production kinetics.
G. P. Smith, and R. A. Copeland, "Comment on 'Are Vibrationally Excited Molecules a Clue for the O3 Deficit Problem and HOX Dilemma in the Middle Atmosphere?'" J. Phys. Chem.A, 109, 2698 (2005).
G. P. Smith and D. Nash, "Local Sensitivity Analysis for Observed Hydrocarbons in a Jupiter Photochemistry Model," Icarus 182, 181 (2006).
A box model sensitivity analysis was applied to output from a version of the 1-D JPL/Caltech KINETICS photochemistry-transport model of Jupiter's atmosphere. Results quantify the controlling chemical reaction parameters for the variety of observable hydrocarbons, and suggest changes to explore and new observations and rate measurements to pursue. High sensitivities are found to photolysis steps, and to several hydrogen atom recombination steps and product branches. Complexity ranges from the relatively simple scheme seen for the methyl radical, to the rich variety of reactions tested by diacetylene.
G. P. Smith, M. Frenklach, R. Feeley, A. Packard, and P. Seiler, " A System Analysis Approach for Atmospheric Observations and Models: the Mesospheric HOx Dilemma," J. Geophys. Res. 111, D23301 (2006).
A systematic consistency analysis and optimization procedure is applied to models of representative ozone, OH, and HO2 observations in the mesosphere and upper stratosphere. The approach considers both measurement and rate parameter uncertainties. The results show some data point inconsistencies and the inability of the accepted photochemical mechanism to predict observations without unfavored large alterations of many rate constants from their consensus values. Optimization results do favor larger rate constants for OH + O, and photolytic ozone and OH production.
R. Robertson and G. P. Smith, " Temperature Dependence of O + OH at 136-377 K Using Ozone Photolysis," J. Phys. Chem. A 110, 6673 (2006).
Ozone was photolyzed at 248 nm in 40 torr nitrogen with small amounts of water or hydrogen added in a cooled or heated flow cell, to measure the O + OH rate constant at 136-377 K. Rate constant values were determined by kinetic modeling of the OH decays in excess O as monitored by laser induced fluorescence, and are in reasonable agreement with current recommendations. Results may be summarized by the expression k = 11.2 x 10-11 T-.32 e177/T cm3/molecule/s.
G. P. Smith and R. Robertson, "Search for HO3 Formation at 144 K," J. Phys. Chem. A, submitted, 2006.
Time dependent behavior of OH in O2 and N2 at 144 K was measured by laser induced fluorescence after formation by laser photolysis of trace O3 and reaction with H2. The absence of a decay in O2 attributable to HO3 formation sets an upper limit of 3.5 kcal/mole on its stability, and 3x10-35 cm6/molecule2/s on the OH + O2 + M ® HO3 + M rate constant at 144 K.