Major Stable Species Research Articles

Non-Premixed Filtered Tabulated Chemistry for LES: Evaluation on Sandia Flames D and E

The non-premixed filtered tabulated chemistry for large eddy simulations employs numerical filtering to resolve a thin flame front on practical LES numerical grids. The flame structure is modified to be coherent with the domain discretization. The first turbulent combustion application of the non-premixed filtered tabulated chemistry approach is presented. A keen comparison of the flamelet filtering transformation in the premixed and non-premixed regimes is carried out. Three distinctive features are outlined: the flame thickness variation, the filtered manifold transformation, and the model activation dependence on the chosen diffusion flamelet configuration for a non-premixed filtered approach. The model performance is assessed on two real turbulent flame configurations, Sandia flames D and E, employing a three-dimensional tabulation strategy, where the numerical grid is coupled with the model by the third parameter, i.e., the computational cell size. The repercussions of the above cited aspects are carefully assessed. The results demonstrate that the formalism coupling with an SGS modeling function can adequately describe wrinkled flame front effects. The predictions for both the major stable species and the minor ones accurately correspond with the underlying physics. It turns out that there is a substantial variation of the filter effect as a function of the strain rate of the flame and the considered species. The varying filter sensitivity along the manifold influences the response of the model correction terms and the retrieved variables. The non-premixed FTACLES formalism possibilities and conditions for the model’s utilization and optimal performance are clearly stated, to confirm the idea that SGS closure in diffusive combustion can be derived based on filtering arguments, and not only based on statistical approaches.

Analysis of kinetic models for rich to ultra-rich premixed CH4/air weak flames using a micro flow reactor with a controlled temperature profile

Aromatic formation in methane combustion is still not well understood. The first step to build kinetic models for aromatic formation is to understand fuel-rich oxidation and formation of aromatic precursors like acetylene and ethylene in varied temperature and equivalence ratio conditions. This understanding is also useful to optimize partial oxidation of methane in fuel-rich conditions. With this view, the current work presents investigation of fuel-rich but non-sooting premixed methane/air flames for equivalence ratios between 1.7 and 6.0 using a micro flow reactor with a prescribed temperature gradient from 300 K up to maximum temperature of 1200 K. Weak flames which represent the ignition behavior of fuel were studied. Concentrations of major stable species, CH4, CO, CO2, C2H6, C2H4 and C2H2 were measured using a gas chromatograph with a thermal conductivity detector (GC-TCD). Computations with 1-D reactive flow model using existing kinetic models, GRI 3.0, SD 2016 (San Diego mech), KAUST (USC II) and ARAMCO 1.3 were performed. Change in weak flame structure with change in equivalence ratio was studied. KAUST (USC II) predicted the weak flame positions very well at both equivalence ratios of 2.0 and 6.0, but species mole fractions were not so well predicted. It was found that models with higher rates of CH3+O2 <=> CH2O+OH (SD 2016) and CH3+HO2 <=> CH3O+OH (GRI 3.0) predicted upstream weak flame positions and thus higher reactivity. However, SD 2016 and GRI 3.0 predicted species mole fractions better, particularly at higher equivalence ratios. All the mechanisms used in the current work underpredicted the mole fractions of C2H2 by factors of four to five at all equivalence ratios addressed here. Reaction path analysis showed significant differences among kinetic models. Particularly, consumption pathways of CH3 vary greatly among kinetic models leading to different reactivity. Flexibility and applicability of existing mechanisms to predict the current fuel-rich to ultra-fuel-rich CH4 combustion was discussed. HO2 radicals, which are generally important at high pressures are found to be significant in current fuel-rich conditions.

Species and temperature measurements of methane oxidation in a nanosecond repetitively pulsed discharge.

Speciation and temperature measurements of methane oxidation during a nanosecond repetitively pulsed discharge in a low-temperature flow reactor have been performed. Measurements of temperature and formaldehyde during a burst of pulses were made on a time-dependent basis using tunable diode laser absorption spectroscopy, and measurements of all other major stable species were made downstream of a continuously pulsed discharge using gas chromatography. The major species for a stoichiometric methane/oxygen/helium mixture with 75% dilution are H(2)O, CO, CO(2), H(2), CH(2)O, CH(3)OH, C(2)H(6), C(2)H(4) and C(2)H(2). A modelling tool to simulate homogeneous plasma combustion kinetics is assembled by combining the ZDPlasKin and CHEMKIN codes. In addition, a kinetic model for plasma-assisted combustion (HP-Mech/plasma) of methane, oxygen and helium mixtures has been assembled to simulate the measurements. Predictions can accurately capture reactant consumption as well as production of the major product species. However, significant disagreement is found for minor species, particularly CH(2)O and CH(3)OH. Further analysis revealed that the plasma-activated low-temperature oxidation pathways, particularly those involving CH(3)O(2) radical reactions and methane reactions with O((1)D), are responsible for this disagreement.

New Insights into Low-Temperature Oxidation of Propane from Synchrotron Photoionization Mass Spectrometry and Multiscale Informatics Modeling.

Low-temperature propane oxidation was studied at P = 4 Torr and T = 530, 600, and 670 K by time-resolved multiplexed photoionization mass spectrometry (MPIMS), which probes the reactants, intermediates, and products with isomeric selectivity using tunable synchrotron vacuum UV ionizing radiation. The oxidation is initiated by pulsed laser photolysis of oxalyl chloride, (COCl)2, at 248 nm, which rapidly generates a ∼1:1 mixture of 1-propyl (n-propyl) and 2-propyl (i-propyl) radicals via the fast Cl + propane reaction. At all three temperatures, the major stable product species is propene, formed in the propyl + O2 reactions by direct HO2 elimination from both n- and i-propyl peroxy radicals. The experimentally derived propene yields relative to the initial concentration of Cl atoms are (20 ± 4)% at 530 K, (55 ± 11)% at 600 K, and (86 ± 17)% at 670 K at a reaction time of 20 ms. The lower yield of propene at low temperature reflects substantial formation of propyl peroxy radicals, which do not completely decompose on the experimental time scale. In addition, C3H6O isomers methyloxirane, oxetane, acetone, and propanal are detected as minor products. Our measured yields of oxetane and methyloxirane, which are coproducts of OH radicals, suggest a revision of the OH formation pathways in models of low-temperature propane oxidation. The experimental results are modeled and interpreted using a multiscale informatics approach, presented in detail in a separate publication (Burke, M. P.; Goldsmith, C. F.; Klippenstein, S. J.; Welz, O.; Huang H.; Antonov I. O.; Savee J. D.; Osborn D. L.; Zádor, J.; Taatjes, C. A.; Sheps, L. Multiscale Informatics for Low-Temperature Propane Oxidation: Further Complexities in Studies of Complex Reactions. J. Phys. Chem A. 2015, DOI: 10.1021/acs.jpca.5b01003). The model predicts the time profiles and yields of the experimentally observed primary products well, and shows satisfactory agreement for products formed mostly via secondary radical-radical reactions.

INTERSTELLAR SIMULATIONS USING A UNIFIED MICROSCOPIC-MACROSCOPIC MONTE CARLO MODEL WITH A FULL GAS-GRAIN NETWORK INCLUDING BULK DIFFUSION IN ICE MANTLES

We have designed an improved algorithm that enables us to simulate the chemistry of cold dense interstellar clouds with a full gas-grain reaction network. The chemistry is treated by a unified microscopic-macroscopic Monte Carlo approach that includes photon penetration and bulk diffusion. To determine the significance of these two processes, we simulate the chemistry with three different models. In Model 1, we use an exponential treatment to follow how photons penetrate and photodissociate ice species throughout the grain mantle. Moreover, the products of photodissociation are allowed to diffuse via bulk diffusion and react within the ice mantle. Model 2 is similar to Model 1 but with a slower bulk diffusion rate. A reference Model 0, which only allows photodissociation reactions to occur on the top two layers, is also simulated. Photodesorption is assumed to occur from the top two layers in all three models. We found that the abundances of major stable species in grain mantles do not differ much among these three models, and the results of our simulation for the abundances of these species agree well with observations. Likewise, the abundances of gas-phase species in the three models do not vary. However, the abundances of radicals in grain mantles can differ by up to two orders of magnitude depending upon the degree of photon penetration and the bulk diffusion of photodissociation products. We also found that complex molecules can be formed at temperatures as low as 10 K in all three models.

The combustion properties of 1,3,5-trimethylbenzene and a kinetic model

Trimethylbenzenes have been suggested as useful components for the formulation of simple hydrocarbon mixtures that quantitatively emulate the gas-phase combustion behaviour of real liquid transportation fuels as model or surrogate fuels. To facilitate this application, various combustion properties of 1,3,5-trimethylbenzene (mesitylene) have been characterised experimentally and a new chemical kinetic model for its combustion constructed. Experimental determinations of 1,3,5-trimethylbenzene reflected shock ignition delay, laminar burning velocities and high-pressure flow reactor oxidative reactivity profiles are presented. These data allow for the testing of a detailed kinetic model, developed by direct analogy to, and incorporating as a subcomponent, a recent comprehensively tested kinetic model for toluene oxidation [Metcalfe WK, Dooley S, Dryer FL. Energy Fuels 2011; 25: 4915–4936]. Model calculations are also compared against data pertinent to 1,3,5-trimethylbenzene combustion phenomena from the published literature. The modelling approach allows for the accurate reproduction of the global combustion phenomena of ignition delay, burning velocity, diffusive and premixed strained extinction limits and flow reactor reactivity, with some noted shortcomings. Analyses of the constructed model suggest that the mechanism of 1,3,5-trimethylbenzene combustion occurs through the formation of 3,5-dimethylbenzaldehyde and 1,2-bis(3,5-dimethylphenyl) ethane as the major stable intermediate species, with relative proportions depending on the conditions of the particular reacting environment.

Soot surface growth and oxidation at pressures up to 8.0 atm in laminar nonpremixed and partially premixed flames

The flame structure and soot particle surface reaction properties, including growth and oxidation, of laminar jet nonpremixed flames were studied experimentally at pressures of 1.0–8.0 atm. Ethylene–helium mixtures were used in an oxygen/helium coflow at normal temperature (300 K) in order to minimize the effects of buoyancy. The following properties along the axis of flames were measured as a function of distance from the burner exit: soot concentrations by laser extinction, soot temperatures by multiline emission, soot structure by thermophoretic sampling and analysis using transmission electron microscopy (TEM), concentrations of major stable gas species by isokinetic sampling and gas chromatography, concentrations of radical species (H, OH, O) by Li/LiOH atomic absorption, and flow velocities by laser velocimetry. The measurements were analyzed to determine local flame properties in order to find soot surface growth and oxidation rates. The measurements of soot surface growth rates (corrected for soot surface oxidation) were found to be consistent with earlier measurements at atmospheric and subatmospheric pressures involving laminar premixed and diffusion flames fueled with a variety of hydrocarbons. The growth rates from all the available flames were in good agreement with each other and with existing hydrogen-abstraction/carbon-addition (HACA) soot surface growth mechanisms available in the literature. Measurements of early soot surface oxidation rates at pressures of 1.0–8.0 atm (corrected for soot surface growth and prior to consumption of 70% of the maximum mass of the primary soot particles) were found to be consistent with earlier measurements at atmospheric and subatmospheric pressures. The oxidation rates of up to 8 atm in flame environment could be explained by reaction with OH, having a collision efficiency of 0.12.

Study on Unsteady NOx Formation Characteristics in Diffusion Flame and Verification of Combination Method Predicting NOx Emission of Turbulent Flame (Extension to Diluted Fuel and Preheated Oxidizer Conditions)

In order to predict accurately the NOx emission of turbulent diffusion flame, we proposed the combination method based on the laminar flamelet model. In this method, the detailed kinetics calculation is separated from the turbulent flow calculation, and then they are recombined in terms of recombination parameters. We examined whether this method could be extended to diluted fuel and preheated oxidizer conditions. With the aim to find out the recombination parameters and to prove the validity of them for these conditions, we carried out the numerical calculation for the unsteady combustion characteristics of counterflow diffusion flame, in which the spout velocities of counterflow varied in a cyclic. The obtained results are as follows : It is necessary for extending this method to change the definition of the mixture fraction Z in consideration of fuel dilution. The production/consumption rates of major stable species are mostly determined by the scalar dissipation rate at flame surface SDRq, and the production rates of intermediate products and nitric oxides are mostly determined by SDRq and the maximum flame temperature Tmax. Consequently, it is verified that SDRq and Tmax are appropriate as the recombination parameters for diluted fuel and preheated oxidizer conditions.

Soot surface growth and oxidation in laminar diffusion flames at pressures of 0.1–1.0 atm

The flame and soot structure and the soot surface growth and oxidation properties of round laminar jet diffusion flames were studied experimentally at pressures of 0.1–1.0 atm. Measurements were made along the axes of flames fueled with acetylene–nitrogen mixtures and burning in coflowing air with the reactants at normal temperature (300 K). The following properties were measured as a function of distance from the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of major stable gas species (N2, Ar, H2O, H2, O2, CO, CO2, CH4, C2H2, C2H4, C2H6, C3H6, C3H8, and C6H6) by isokinetic sampling and gas chromatography, concentrations of some radial species (H, OH, O) by deconvoluted Li/LiOH atomic absorption, and flow velocities by laser velocimetry. These measurements yielded local soot surface growth and oxidation rates, as well as local flame properties that are thought to affect these rates. Present measurements of soot surface growth rates (corrected for soot surface oxidation) in laminar diffusion flames at various subatmospheric pressures were consistent with earlier measurements of soot surface growth rates in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure; in addition, rates from all the available flames were in good agreement with each other and with existing Hydrogen-Abstraction/Carbon-Addition (HACA) soot surface growth mechanisms in the literature for values of steric factors in these mechanisms on the order of unity, as expected. Similarly, present measurements of early soot surface oxidation rates (corrected for soot surface growth and prior to consumption of 70% of the maximum mass of the primary soot particles) for fuel-rich and near-stoichiometric conditions in laminar diffusion flames at pressures of 0.1–1.0 atm were consistent with earlier measurements of rates of early soot surface oxidation in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure; in addition, rates from all available flames could be explained by reaction with OH, as proposed by K.G. Neoh et al. (in: Particulate Carbon, Plenum, New York, 1980, p. 261), having a collision efficiency of 0.13 and supplemented to only a minor degree by direct soot surface oxidation by O2.

03/02039 Modeling and experimental study on an innovative passive cooling system-NVP system: Kang, Y. et al. Energy and Buildings, 2003, 35, (4), 417–425

The flame and soot structure and the soot surface growth and oxidation properties of round laminar jet diffusion flames were studied experimentally at pressures of 0.1–1.0 atm. Measurements were made along the axes of flames fueled with acetylene–nitrogen mixtures and burning in coflowing air with the reactants at normal temperature (300 K). The following properties were measured as a function of distance from the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of major stable gas species (N2, Ar, H2O, H2, O2, CO, CO2, CH4, C2H2, C2H4, C2H6, C3H6, C3H8, and C6H6) by isokinetic sampling and gas chromatography, concentrations of some radial species (H, OH, O) by deconvoluted Li/LiOH atomic absorption, and flow velocities by laser velocimetry. These measurements yielded local soot surface growth and oxidation rates, as well as local flame properties that are thought to affect these rates. Present measurements of soot surface growth rates (corrected for soot surface oxidation) in laminar diffusion flames at various subatmospheric pressures were consistent with earlier measurements of soot surface growth rates in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure; in addition, rates from all the available flames were in good agreement with each other and with existing Hydrogen-Abstraction/Carbon-Addition (HACA) soot surface growth mechanisms in the literature for values of steric factors in these mechanisms on the order of unity, as expected. Similarly, present measurements of early soot surface oxidation rates (corrected for soot surface growth and prior to consumption of 70% of the maximum mass of the primary soot particles) for fuel-rich and near-stoichiometric conditions in laminar diffusion flames at pressures of 0.1–1.0 atm were consistent with earlier measurements of rates of early soot surface oxidation in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure; in addition, rates from all available flames could be explained by reaction with OH, as proposed by K.G. Neoh et al. (in: Particulate Carbon, Plenum, New York, 1980, p. 261), having a collision efficiency of 0.13 and supplemented to only a minor degree by direct soot surface oxidation by O2.

Soot surface oxidation in hydrocarbon/air diffusion flames at atmospheric pressure

Soot surface oxidation was studied experimentally in laminar hydrocarbon/air diffusion flames at atmospheric pressure. Measurements were carried out along the axes of round fuel jets burning in co-flowing dry air considering acetylene-nitrogen, ethylene, propylene-nitrogen, propane and acetylene-benzene-nitrogen in the fuel stream. Measurements were limited to the initial stages of soot oxidation (carbon consumption less than 70%) where soot oxidation occurs at the surface of primary soot particles. The following properties were measured as a function of distance above the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of major stable gas species (N 2, H 2O, H 2, O 2, CO, CO 2, CH 4, C 2H 2, C 2H 4, C 2H 6, C 3H 6, C 3H 8, and C 6H 6) by sampling and gas chromatography, concentrations of some radical species (H, OH, O) by deconvoluted Li/LiOH atomic absorption and flow velocities by laser velocimetry. For present test conditions, it was found that soot surface oxidation rates were not affected by fuel type, that direct rates of soot surface oxidation by O 2 estimated from Nagle and Strickland-Constable (1962) were small compared to observed soot surface oxidation rates because soot surface oxidation was completed near the flame sheet where O 2 concentrations were less than 3% by volume, and that soot surface oxidation rates were described by the OH soot surface oxidation mechanism with a collision efficiency of 0.14 and an uncertainty (95% confidence) of ±0.04 when allowing for direct soot surface oxidation by O 2, which is in reasonably good agreement with earlier observations of soot surface oxidation rates in both premixed and diffusion flames at atmospheric pressure.

High pressure, high temperature shock tube studies of ethane pyrolysis and oxidation

A unique high-pressure single pulse shock tube has been used to obtain the first experimental data for ethane oxidation and pyrolysis at very high pressures and temperatures. Experiments were performed at two nominal reaction pressures of 340 bar and 613 bar in the temperature range 1050 K to 1450 K. The major stable species were identified and their concentrations determined using gas chromatography. Several minor species, with up to four carbon atoms and including oxygenates, were also observed in the oxidation studies. Three models based on literature mechanisms for hydrocarbon oxidation were used to simulate the experimental data. All of the models simulate the pyrolysis data well although only one of the models was capable of accurately describing the oxidation data.

High-pressure single-pulse shock tube investigation of rich and stoichiometric ethane oxidation

A high-pressure, single-pulse shock tube was used to study, the oxidation of ethane/oxygen mixtures, =1 and =5, between 1000 and 1400 K at reaction pressures of 40 bar. Gas chromatographic techniques were used to quantify the major stable species formed in the reactions. The experimental data were simulated using two models for ethane combustion that had previously been tested against very high pressure (300–600 bar) oxidation and pyrolysis experimental data obtained in the same apparatus. Good agreement with the stoichiometric data was obtained: however, neither model was capable of simulating the =5 data. Several modifications to one of the models were made, and these modifications are discussed. The final version of the model provides much more accurate predictions of the experimental data and forms the first step in developing a comprehensive model for ethane oxidation that has been tested against experimental data covering a pressure range from 1 to 600 bar.

The behavior of inorganic material in biomass-fired power boilers: field and laboratory experiences

This paper highlights some of the major findings of the Alkali Deposits Investigation, a collaborative effort to understand the causes of unmanageable ash deposits in biomass-fired electric power boilers. A group of interested industrial institutions and the US DOE Energy Efficiency and Renewable Energy Office's Biomass Power Program through the National Renewable Energy Laboratory jointly sponsored the project. The industries contributed both funding and, in most cases, use of facilities to the project and included Mendota Biomass Power and Woodland Biomass Power (both associated with Thermo Electron Energy Systems), CMS Generation Operating (formerly Hydra-Co Operations), Wheelabrator, Shasta and Hudson Energy, Sithe Energy, Delano Energy, the Electric Power Research Institute, Foster Wheeler Development, and Elkraft Power of Denmark. Research contracts with Thomas R. Miles Consulting Design Engineers, Sandia National Laboratories, and The National Renewable Energy Laboratories provided the government portion of the funding. In addition, the University of California at Davis and the Bureau of Mines performed significant work in close collaboration with the other researchers. This summary highlights the major findings of the project more thoroughly discussed in a recent report [2]. We highlight fuel properties, bench-scale combustion tests, a framework for considering ash deposition processes, pilot-scale tests of biomass fuels, and field tests in commercially operating biomass power generation stations. Detailed chemical analyses of 11 biomass fuels representing a broad cross-section of commercially available fuels reveal their properties that relate to ash deposition tendencies. The fuels fall into three broad categories: (1) straws and grasses (herbaceous materials); (2) pits, shells, hulls and other agricultural by-products of a generally ligneous nature; and (3) woods and recycle fuels of commercial interest. Woods and wood-derived products represent the most commonly used biomass fuels. Herbaceous fuels contain silicon and potassium as their principal ash-forming constituents. They are also commonly high in chlorine relative to other biomass fuels. These properties portend potentially severe ash deposition problems at high or moderate combustion temperatures. The primary sources of these problems are shown to be: (1) the reaction of alkali with silica to form alkali silicates that melt or soften at low temperatures (can be lower than 700°C, depending on composition), and (2) the reaction of alkali with sulfur to form alkali sulfates on combustor heat transfer surfaces. Alkali material plays a central role in both processes. The mobility of alkali material, defined as its ability to come in physical contact with other materials, is measured using chemical extractive techniques. Potassium is the dominant source of alkali in most biomass fuels. The analyses below indicate that essentially, all of the biologically occurring alkali, in particular potassium, has high mobility. The non-biologically occurring alkali is present as soil contaminants and additives to the fuels, such as clay fillers used in paper production. This non-biologically occurring alkali exhibits far lower mobility than the biological fraction. The relative amounts of biologically vs. non-biologically occurring material depend on fuel type and fuel handling. In the fuels investigated here, the dominant form of alkali was biologically occurring potassium. Some traditional indicators of deposit behavior, most notably ash fusion temperatures, poorly predict ash behavior compared with a more mechanistic interpretation of the data. Many of the agricultural by-products also contain high potassium concentrations with equally high potassium mobility. Some woods, on the other hand, contain far less ash overall, differing by as much as a factor of 40 from high-ash straws, for example. In addition, the ash-forming constituents contain greater amounts of calcium with

Reduction of NOx Formation by Water Sprays in Strained Two-Stage Flames

Staged combustion can be employed to reduce the formation of CO and NOx, stabilize the flame, decrease the flame temperature, and create better working conditions in gas turbine combustors. To help understand influences of partial premixing and addition of water on NOx formation, we study two-stage flames in a counterflow spray burner. This paper reports experimental and theoretical results concerning two-stage combustion in which one feed stream is composed of a fuel-rich mixture of methane and air and the other is air. Water sprays are added to the air stream. This two-phase laminar counterflow configuration exhibits a green premixed flame, a blue diffusion flame, and a vaporization plane. All three are flat and parallel. The separation distances between them decrease with increasing equivalence ratio and strain rate. Flow visualization is provided through illumination by an argon ion laser sheet, velocity fields and spray structure are measured by a phase-doppler particle analyzer, concentration fields of major stable species are measured by gas chromatography of samples withdrawn from the flame, and temperature fields are measured by a thermocouple. Numerical integrations that employ a recent chemical-kinetic data base are performed to model the flame structure and NOx formation. Comparisons of experimental results with numerical predictions are made to test agreement. This work provides information on hydrocarbon combustion in both premixed flames and diffusion flames, indicates how NOx is formed in fuel-rich flames, and suggests how the pollutants can be reduced.

Surface kinetics for catalytic combustion of hydrogen-air mixtures on platinum at atmospheric pressure in stagnation flows

Experimental studies of the combustion of premixed hydrogen-air mixtures impinging on the surface of a heated platinum plate at normal atmospheric pressure were performed and employed to draw inferences concerning surface reaction mechanisms and rate parameters applicable under practical conditions of catalytic combustion. Plate and gas temperatures were measured by thermocouples, and concentration profiles of major stable species in the gas were measured by gas-chromatographic analyses of samples withdrawn by quartz probes. In addition, ignition and extinction phenomena were recorded and interpreted with the aid of a heat balance at the surface and a previous flow-field analysis of the stagnation-point boundary layer. From the experimental and theoretical results, conclusions were drawn concerning the surface chemical-kinetic mechanisms and values of the elementary rate parameters that are consistent with the observations. In particular, the activation energy for the surface oxidation step H + OH → H 2O is found to be appreciably less at these high surface coverages than in the low-coverage limit.

Comparison of Experimental and Computed Species Concentration and Temperature Profiles in Laminar, Two-Dimensional Methane/Air Diffusion Flames

Abstract Experimental concentration measurements of the major stable species and five radical species (OH., H atom, O atom, CH., and CH3) obtained on a rectangular Wolfhard-Parker slot burner are compared with a detailed computation of the chemical structure of an axisymmctric laminar, CH4/air diffusion flame burning at atmospheric pressure. In order to examine these CH4/air flames with different geometries and different sizes, the species profiles are plotted as functions of the local mixture fraction, and the scalar dissipation rate has been matched in a region around the stoichiometric surface. The overall agreement in the absolute concentrations, the shape of the profiles, and their location in terms of the local mixture fraction is good to excellent for the stable species (except for O2) and for the most abundant radicals OH, H atom, and O atom. For example, the calculated OH- maximum concentration is in much better agreement with the experimental results than are full equilibrium and partial equilib...

Comparisons of the structure of stoichiometric CH 4N 2OAr and CH 4O 2Ar flames by molecular beam sampling and mass spectrometric analysis

Measurements are reported of the profiles of composition and temperature in laminar premixed flat flames of CH 4N 2OAr and CH 4O 2Ar. Measurements were made in near stoichiometric mixtures at 30 torr by molecular beam sampling and mass spectrometric sample analysis. All major stable species and many important unstable species were measured by this technique, many species being identified in the flame with N 2O as oxidizer for the first time. Calibration of the concentration profiles was accomplished by the use of calibration gases for stable species and by comparison of the mass spectrometer signal in the well-characterized CH 4O 2Ar flame with signals in the CH 4N 2OAr flame and by partial equilibrium for the hydrogenoxygen system. The measurements have identified the presence of NCO, HCN, and HNCO as reaction intermediates and the importance of these species in the reaction mechanism is discussed.

Measurements and modeling of light hydrocarbons in rich C 2H 4 combustion in a jet-stirred reactor

Premixed fuel and air were burned at atmospheric pressure and 1750 K in a jet-stirred combustor. A water-cooled stainless-steel probe was used to sample stable species over an equivalence ratio range of 1.3–2.0. In general, the major stable species measured showed good agreement with a model consisting of a single well-stirred reactor and a reaction mechanism proposed by Glarborg, Miller, and Kee. Radical reactions occurring in the sampling probe were also modeled using a measured probe temperature profile. At low equivalence ratios (1.3–1.5), where the concentration of stable species are small relative to the concentrations of radical species (H, OH, O, CH3, etc.), radical reactions in the probe produce large changes in the measured species concentrations. At higher equivalence ratios (above 1.5), the effects of probe quenching are diminished due to the small radical concentrations relative to the concentrations of the major stable species.

Measurement of stable species present during filament-assisted diamond growth

We have measured mole fractions of two of the major stable species at the surface of a silicon substrate during filament-assisted diamond growth as a function of the filament-to-substrate distance. Input gases were methane and hydrogen. A quartz probe withdrew gases at the growing surface, and the gases were sampled with an on-line mass spectrometer. Close to the filament the methane is largely consumed, with most of the remaining gas phase carbon in the form of acetylene. Mass spectral results are compared to compositions calculated with a detailed chemical kinetics model. Our initial analysis suggests that diamond growth comes mainly from reaction of acetylene, ethylene, methane, or methyl radical.