OH Concentration Profiles Research Articles

The temporal development of OH-concentration profiles in ignition kernels studied by single-pulse laser induced fluorescence

A freqency doubled dye laser has been used to excite fluorescence from OH-radicals in an ignition kernel in an atmospheric methane/air mixture. The fluorescence was imaged onto a gated and intensified diode array camera. Radial profiles with a spatial resolution of 0.05 mm were obtained from 10 μs to 3 ms after the spark. A transistorized coil ignition unit was used. The OH-concentration profiles grew smoothly during the first ms of the glow discharge phase. Due to the low energy in the initial breakdown the OH-profiles were not distorted by a spark channel and shock wave. At about 1 ms the flame front was seen to separate from the continuing glow discharge. The influence of the gas mixture stoichiometry on the OH-profiles was studied.

The Reaction of NH2‐Radicals with Electronically Excited Molecular Oxygen O2 (1Δg)

AbstractThe reaction: has been studied in the gas phase in an isothermal discharge flow system with He as the main carrier gas. The pressure was varied between 2.6 and 5.2 mbar and the temperature between 295 and 353 K. The O2(1Δg) was produced in a chemical reactor and its absolute concentration was measured by photoionisation. The fast reaction F + NH3 → NH2 + HF was used as the NH2‐source. The pseudo‐first order NH2‐decay and the OH and NH concentration profiles were obtained by laser induced fluorescence (LIF). This technique was found to be not sensitive enough to follow the profiles of HNO formed in this reaction. — The rate independent of temperature and pressure in the above range is: Computer simulation together with the product measurements show that OH and HNO are the primary products of reaction (1).

Optical fibres: study of the incorporation of OH groups in a CVD silica preform

Synthetic fused silica for optical fibres is currently produced by vapour phase oxidation of silicon tetrachloride in a plasma torch, according to the equation: SiC1 4( g) + O 2( glass) +2 C1 2( g). The resulting soot particles form a deposit which is immediately vitrified onto the edge of a rotating substrate to give a cylindrical preform. We have made spontaneous Raman scattering suitable for a punctual determination of very low OH concentrations inside samples cut out of the preform, the limit of detection being estimated at 0.3 ppm (weight of OH groups/weight of silica). A thermodynamic study of the OH content at the heart of the preform has shown that the surrounding air enters the reaction zone. On the other hand, the outer zone is deposited in thin successive layers which makes possible the incorporation of OH groups by diffusion of atmospheric damp. Thermodynamic and diffusional calculations allow a good prediction to be made of the OH concentration profiles inside the preforms.

High temperature study of the reactions of O and OH with NH3

AbstractMixtures of NH3 and N2O dilute in Ar were heated behind incident shock waves in the temperature range 1750–2060 K. A cw ring dye laser, tuned to the center of an OH absorption line in the ultraviolet, was used to monitor OH concentration profiles by absorption spectroscopy. Infrared emission was used to follow N2O (at 4.5 μm) and NH3 (at 10.5 μm) concentration—time histories. The early‐time NH3 and OH concentration profiles were sensitive to the rate constants of the reactionsmagnified imageleading to the following best‐fit expressions for k2 and k3:k2 = 1013.34±0.3 exp(−4470/T) and k3 = 1013.91±0.2 exp(‐4230/T) cm3 mol−1 s−1. The results of this study combined with previous low‐temperature data suggest a significant non‐Arrhenius behavior for both k2 and k3.

Hydroxyl concentration measurements in the NH 3NOO 2 reaction in postflame gases

Hydroxyl and stable species concentration profiles associated with the mixing of ammonia with lean combustion products containing nitric oxide have been measured. These measurements were then compared to a numerical calculation of the mixing and reaction process between ammonia and nitric oxide including elementary chemical kinetics.The comparison between measured and calculated profiles shows good agreement. The OH concentration is maintained in a reaction mechanism dominated by H 2 O 2 reactions so that the concentration profiles of OH are similar with and without ammonia addition. The NH concentration was estimated experimentally to be less than 10 ppm and the NH 2 concentration to be below 500 ppm. Since both mixing and reaction are important in this NO reduction process, mixing low-temperature ammonia containing gases with combustion products containing NO at temperatures above the usual range for homogeneous NO reduction still leads to considerable NO reduction efficiency.

Laser Fluorescence Measurements of the OH Concentration in a Combustion Boundary Layer†

Abstract A laser induced fluorescence technique was used to measure the OH radical concentration in the boundary layer combustion of a lean hydrogen-air mixture (π = 0.2) flowing over a heated (1170 K.) platinum plate. The highly nonequilibrium OH concentration profile was observed 5 to 50 mm from the leading edge of the plate and to within 0.25 mm of the surface. The maximum OH concentration occured near the leading edge of the platinum plate and was ~200 times its equilibrium value.

OH Concentration in an Atmospheric-Pressure Methane-Air Flame from Molecular-Beam Mass Spectrometry and Laser-Absorption Spectroscopy

Abstract The concentration of lhe OH radical in a stoichiometric methane-air flat flame at atmospheric pressure was measured with both laser-absorption spectroscopy and molecular-beam mass spectrometry (MBMS). The nonequilibrium peak OH concentrations and the OH decay rate measured from the two techniques were in good agreement. The OH profile from the MBMS measurements, however, was shifted downstream from the absorption measurements by the MBMS sampling process. A comparison of temperature profiles from thermocouple measurements and from a molecular-beam time-of-flight technique exhibited a similar downstream shift. The MBMS measurements effectively sampled the gas properties approximately five orifice diameters ahead of the sampling-probe tip. Perturbation of the OH concentration profile using various sampling probes indicate the importance of minimizing the length of the sampling-orifice channel to reduce composition relaxation during sampling.

Laser absorption measurements of OH, NH, and NH2 in NH3/O2 flames: Determination of an oscillator strength for NH2

We have used laser diagnostics to probe atmospheric pressure ammonia–oxygen flames. Absorption from a tunable dye laser was used to measure concentration profiles of OH, NH, and NH2 radicals at fuel equivalence ratios φ of 1.28, 1.50, and 1.81. The absolute concentrations of OH and NH and the product of the NH2 concentration and the NH2 oscillator strength were derived as a function of height above the flat flame burner. These measurements indicate that the reaction NH2+OH?NH+H2O is equilibrated near the flame front, as well as in the post flame region. It was possible to use these measurements to derive an oscillator strength fi = (2.04±0.44)×10−4 for the PQ1,7 line in the (0,9,0)←(0,0,0) vibrational band of the A 2A1←X 2B1 transition of NH2. Rotational temperatures of OH were obtained from absorption measurements on a variety of rotational lines. These temperatures suggest the possibility of excess rotational energy in OH in the flame front regions of the φ = 1.28 and φ = 1.50 flames.

Concentration profiles of NH and OH in a stoichiometric CH 4/N 2O flame by laser excited fluorescence and absorption

The concentration profiles of NH and OH were measured in an atmospheric pressure, premixed, laminar CH 4/N 2O flame over a porous-plug, flat flame burner. Relative profiles were obtained by laser excited fluorescence for both compounds and normalized by laser absorption at a single point for NH. The OH concentration was high enough to allow absorption measurements at several points for comparison with fluorescence results. The steeply rising portions of the profiles occur at about the same height above the burner, but the NH is confined to the primary reaction zone, while the OH decay extends into the burnt gases. The OH decay was analyzed using a simple rate law. The results and comparisons with previous results in CH 4/air flames are discussed briefly. Brief studies of the peak NH concentration in CH 4/air, CH 4/NO, H 2/NO and H 2/N 2O flames were made in order to elucidate the flame chemistry.

The effect of surface chemistry on the development ofthe [OH] in a combustion boundary layer

The development of a combustion boundary layer formed by flowing a lean hydrogen-air mixture (=0.1) over a high temperature (1170K) flat plate has been investigated. Two different plate surface materials were studied: platinum and quartz. The OH concentration and temperature were measured respectively by laser induced fluorescence and Rayleigh scattering. The highly non-equilibrium OH concentration profiles were compared with the results of a numerical boundary layer model. The effect of surface chemistry was evaluated with three general surface chemistry boundary conditions: noncatalytic, surface oxidation, and surface oxidation with radical recombination. Near the leading edge of the flat plate the [OH] gradient obtained from the measured profiles for the platinum and quartz surfaces indicated a net OH production. Farther downstream the [OH] profiles were substantially different, with the quartz surface behaving similar to the noncatalytic boundary condition and the platinum surface behaving similar to the boundary condition for surface oxidation with radical recombination.

Use of laser-induced fluorescence of OH to study the perturbation of a flame by a probe

The paper presents the first use of laser-induced fluorescence (LIF) of OH to overcome a problem irresolvable otherwise (owing to the spatial resolution needed). Previous work had shown a discrepancy between OH concentration profiles in a low pressure flame measured by mass spectroscopy and optical absorption, especially in the preheated zone. The first part of this work is devoted to a remake of the measurement by absorption of a pulsed dye laser beam tuned on a transition of OH, and to a LIF measurement of the ‖OH‖ profile. They both confirm the previous conventional absorption measurements. A detailed study by LIF of ‖OH‖ perturbation by the probe is then presented. It leads to the following conclusions: The quartz cone of a mass spectrometer sampling probe reduces the OH concentration in the preheating zone of the low pressure O 2 −C 3 H 8 , flame investigated. The extension of the perturbation is comparable to the probe penetration into the reactive zone of the flame. Therefore, the effect is attributed to a perturbation of the diffusion field by the probe. The OH concentration measured by mass spectrometry is the actual value in the sample. As predicted by the literature, the extension of the sampling zone is two diameters of the probe nozzle.

Direct measurement of OH local concentration in a flame from the fluorescence induced by a single laser pulse

An alternative to the saturation approach is proposed to avoid the quenching dependence of the fluorescence intensity. The method allows direct absolute measurement of the local number density from a single laser shot. A mean quenching cross section of 5 x 10(-19) m(2) is deduced from the experiments, and a OH concentration profile through the flame is given.

Radical Overshoot in the Reaction Zone of Hydrocarbon-Air Flame

The concentration profiles of NOx and OH are measured in a CH4-air flat flame at atmospheric pressure. The mole fraction of oxygen atoms Po in the reaction zone is obtained using partial equilibrium of reaction (4), O+H2O↔2OH, and it has been verified that a large excess amount of O-atoms exists in the reaction zone. It is also shown that the measured values of O-atom concentration are predictable from a relation between the amounts of prompt NO and saturation current using the rate constants k1=4×1011exp (-13600/RT) and k8=5.75×1011exp (-6000/RT) cc/mole sec-1 for the formation of prompt NO, CH+N2→HCN+N→2NO (1), and chemi-ions, CH+O→HCO++e- (8), respectively.

High-temperature oxidation of hydrogen by nitrousoxide in shock waves

High-temperature ignition reactions have been studied behind incident shock waves in equimolar N 2 O/H 2 mixtures highly diluted with argon. The infrared emission of N 2 O molecules at 4.5 microns behind a shock front was utilized to follow the current concentrations of nitrous oxide, and the transient OH-concentration profiles were determined by monitoring the ultraviolet emission intensity. The experimental data indicate a complicated reaction mechanism in two distinct stages, respectively, at relatively low and high temperatures. In the temperature range of 1400°–2300°K, an induction period is observed in N 2 O and N 2 O/H 2 systems, the induction times being significantly shorter for the latter case under similar thermal conditions. The importance of chain-branching reaction steps, involving atomic oxygen and hydrogen, is emphasized, and the induction times are evaluated quantitatively. At temperatures above 2000°K, the N 2 O-concentration profile features can be satisfactorily interpreted, based on a mechanism which contains the currently accepted data on elementary steps of the N 2 O/H 2 reaction, whereas OH emission differs significantly from the assumed mechanism.

Shock-tube study of the initiation process in the hydrogen-oxygen reaction

The hydrogen-oxygen initiation process was studied behind incident shock waves over the temperature range 1200–1800°K. Experimentally, OH concentration profiles were measured by ultraviolet absorption spectroscopy, and rate information was obtained through analysis of induction time and exponential growth parameter data. Analysis of the data indicated that the reaction H2+O2→k02OH is the initiation reaction with the rate coefficient h0=170×1010exp(−48150/RT)litersmole−1sec−1 This rate coefficient expression is considered to be correct to within a factor of 3.

Kinetics of the Nitrous Oxide–Hydrogen Reaction

The reaction of hydrogen with nitrous oxide was studied behind incident shock waves over the temperature range 1700°–2600°K. Experimentally, the concentration profiles of OH and the concentration of NO after the decomposition of N2O were measured by uv absorption. The OH profiles have qualitatively similar features to those determined in the H2–O2 reaction. The concentration of NO after the disappearance of N2O remains constant for several hundred microseconds. Interpretation of the experiments was based on a mechanism which contains the currently accepted data on the H2–O2 reaction and four additional reactions. Computer calculations show that the rate constants for the reaction O+N2O→ lim k32NO as cited in the current literature are too low. The magnitude of k3 determined in this work is consistent with published data on the reverse reaction, and k3 = 6.3 × 1014exp(− 26 700 / RT) cm3mole−1·sec−1 is believed to be correct to within a factor of 3 between 1700° and 4000°K. The present investigation also permitted estimation of the constant for the reaction H+N2O→ lim k5OH+N2. If the literature rate constants for the H2–O2 system are assumed to be correct the best fit to our data is achieved with k5 = 4.0 × 1013exp(− 12 000 / RT). However, a number of inconsistencies remain in the lower temperature range which cannot be reconciled within the framework of the assumed mechanism. These are similar to difficulties which were reported by others on the H2–O2 reaction.

Post-induction kinetics in shock-initiated H 2−O 2 reactions

The course of the shock-initiated hydrogen-oxygen reaction after the induction period, during the transition to partial equilibrium, has been investigated by computer integration of the rate equations, and interpretations of particular aspects are developed. H2:O2 ratios far from stoichiometric are considered. An earlier suggestion by one of the authors that partial equilibrium conditions with no recombination represent a practical upper limit to the transient atom and radical concentrations is contradicted, and attention is focused on the pronounced “spike” in oxygen-atom concentration that modern rate-coefficient data predict to occur in fuel-rich mixtures at the time of maximum reaction rates and most rapid O2 depletion. Determination of the time duration and amplitude of O “spikes” in rich mixtures, which we have not yet accomplished, offers the possibility of direct information on the rate coefficients for the reactions H + O 2 → k a O H + O O + H 2 → k b O H + H Failure of measured OH-concentration profiles to exhibit such “spikes” in rich mixtures over the range 1160°≤T≤1884°K places an upper bound on the ratio ka/kc O H + H 2 → k c H 2 O + H