Absolute OH Research Articles

Local absolute OH concentration measurement in a diffusion flame by laser induced fluorescence

Local absolute OH concentration measurement in a diffusion flame by laser induced fluorescence

Laser-Saturated Fluorescence Measurements of OH in Atmospheric Pressure CH4/O2/N2Flames Under Sooting and Non-Sooting Conditions

Abstract OH concentration measurements have been performed in rich, atmospheric pressure CH4/O2/N2 flames using laser-saturated fluorescence. Both sooting and non-sooting conditions have been investigated. OH is thought to be an important species in soot chemistry because of its role in the oxidation of soot and soot precursors. In the sooting flame, the centerline axial OH concentration drops sharply 3-4 mm above the burner surface. Visible soot emission begins only after the OH concentration decreases significantly from its peak value in the flame zone. Rotational Raman scattering was identified as a potentially significant interference in the fluorescence measurements at the low OH concentration levels (down to 5 ppm) typical of the rich flames. Laser-saturated fluorescence and optical absorption measurements of OH concentration were also performed in a lean, atmospheric pressure CH4/O2/N2 flame. A combination fluorescence-absorption calibration in the lean flame was used to determine absolute OH number densities from fluorescence measurements in the rich flame, where absorption techniques were inapplicable. Temperature measurements by two-line laser-saturated OH fluorescence in the lean flame showed excellent agreement with OH absorption and corrected thermocopule temperature measurements.

Sulfur-nitrogen interaction in stirred flames

Comprehensive nitrogen species measurements are performed for combustion gases in a premixed, atmospheric pressure, jet-stirred reactor containing 0.1 to 0.6% fuel -nitrogen and 1% fuel -sulfur. Species measured are NO, NH 3 , and HCN by gas sampling, and relative CN and NH and absolute OH by uv absorption spectroscopy. Reactor fuels are C 3 H 8 and H 2 . Nitrogen dopants are NO and CH 3 CN, and the sulfur dopant is SO 2 . Most results are for the conditions: T=1800 K, 1.1≤≤1.7, and 6≤τ≦11 ms. It is confirmed for rich propane flames that sulfur increases NO, NH 3 , and HCN yields, and decreases [OH]. Decreases in CN and NH concentrations are also noted. Kinetic interpretation shows the increase in NO, NH 3 , and HCN yields effected by the depression in [OH] and [H] by sulfur. Less NH and N are formed; therefore, less NO is reduced to N 2 . Also, a shift occurs from CN to HCN, and from NH to NH 3 . The second effect is the weakening of the NO-to-HCN recycle, as sulfur oxides apparently compete for the hydrocarbon fragments required for the recycle. Thus, under some circumstances HCN yields are depressed by sulfur. For NO doped rich H 2 flames, only 15 to 30 percent of the NO is chemically reduced to N 2 . Kinetic interpretation shows that HNO is formed from NO; then small amounts of NH and N form from the HNO. Sulfur weakens the NH and N formation, giving higher NO yield.

Determination of absolute OH and CH concentrations in a low pressure flame by laser-induced saturated fluorescence

The method of species concentration measurements by laser-induced fluorescence (LIF) in flames was further developed. The applicability of the two-level rate-equation model was examined for the experimental conditions. Inhomogeneous saturation due to the spatial intensity distribution of the laser light was corrected for using a field of local laser intensities obtained experimentally. Among other things, the accuracy of the calibration of the optical detection system was crosschecked by two methods. Absolute number densities of CH and OH could be determined in a 13 mbar C 2 H 2 −O 2 flame from measurements of the average saturation degree as a function of laser intensity. From these saturation curves approximate oscillator strengths could be obtained which provided a check of the evaluated number densities. Temperatures were determined from rotational spectra of the excited molecules. For the extension of the method to higher pressure, preliminary results obtained with a multilevel relaxation model are presented.

Flame studies of atomic hydrogen and oxygen using resonant multiphoton optogalvanic spectroscopy

Resonant multiphoton optogalvanic spectroscopy has been used to measure relative concentration profiles of atomic hydrogen and oxygen as a function of height above the burner in a lean, atmospheric-pressure hydrogen/oxygen flame. The relative concentrations were converted to absolute values by comparing them to concentrations calculated assuming partial equilibrium from an absolute OH profile measured using laser absorption spectroscopy. Very good agreement was obtained between the profiles measured with optogalvanic spectroscopy and the profiles calculated from the OH measurements for both atomic species. Detection levels of a few parts-per-million have been demonstrated for these important radicals using relatively modest ultraviolet laser energies of one hundred microjoules per pulse.

Absolute OH absorption cross sections (for lidar measurements)

Absorption cross sections are important in quantitative optical measurements of OH. We have used existing experimental data to calculate the effective cross sections for a number of transitions in the A-X system of OH, taking into account such factors as temperature and the spectral shape and width of both the absorption line and the excitation source. Details of the calculations are presented to allow easy extension to other transitions.

Single-pulse, laser-saturated fluorescence measurements of OH in turbulent nonpremixed flames

A single-pulse, laser-saturated fluorescence technique has been developed for absolute OH concentration measurements with a temporal resolution of 2 nsec, a spatial resolution of <0.1 mm(3), and an estimated accuracy of +/-30%. It has been applied in laminar, transitional, and turbulent hydrogen-air diffusion flames, providing the first reported quantitative measurements of average values, rms fluctuations, and probability density functions of OH radical concentration in nonpremixed flames.

Behavior of OH and NO^ in a Mixing-Influenced Jet-Stirred Reactor

Measurements of OH and NO^ in a premixed jet-stirred reactor are reviewed and interpreted in order to discern the influence of finite mixing and the formation of pollutants. The reactor conditions were P = 93 kPa, 4<r, < 12ms, and 1400 <T< 1950 K. Reactor fuels were CH4and C3H8 for the OH measurements, and H2/CO, CH4C3H8, andC2H2 for the NOX measurements. Absolute OH was determined from the measured integrated absorption of resolved spectral lines in the OH 2 irE + (0,0) band. Gas sampling and chemiluminescent analysis gave NOX. The OH measurements were indicative of a stirred reactor operated under the influence of a finite mixing rate. This was found by extending the established simple theories for a mixinginfluenced reactor to the case of OH behavior. With respect to NO*, its concentration was uniform in the reactor at fuel-lean conditions. At fuel-rich conditions, however, the NO^ concentration was nonuniform. The amount of NO^. formed depended on fuel type, increasing substantially as the fuel was switched from H2/CO to C2H2.

The reaction of fuel-sulfur in hydrocarbon combustion

Comprehensive sulfur species measurements were performed on combustion gases in a premixed, jet-stirred reactor containing 0.23 mole % total sulfur. Species measured were H 2 S, COS, CS 2 , C 4 H 4 S, SO 2 , O 2 , CH 4 , and C 2 H 2 by gas chromatography; relative SH and CS and absolute OH by uv absorption spectroscopy; and SO 3 by controlled condensation. Reactor fuels were C 3 H 8 , C 2 H 2 , CH 4 , H 2 , and a H 2 /CO mixture. Sulfur dopants were C 4 H 4 S, H 2 S, and SO 2 . Reactor conditions were 1600≤T≤1800 K, 0.7≤Ф≤1.7, and 3.5≤τ≤7 ms. Pressure was 0.9 atm, except for SO 3 measurements at 0.9≤P≤2.9 atm. It was confirmed that SO 2 was always the dominant product. In fuel-rich flames lacking significant hydrocarbons, H 2 S was the stable non-SO 2 product sampled—of order 5% yield. In hydrocarbon flames at richer conditions, COS and CS 2 replaced H 2 S as the non-SO 2 product. Kinetic interpretation indicated that internal equilibration of the S−H−O subsystem occurred and that H 2 S, COS, and CS 2 likely formed within the sample probe through reaction of SO. The SO 3 measurements, in conjunction with inference of O-atom concentrations from measured OH, indicated that the reaction SO 2 +O+M→SO 3 +M predicts SO 3 formation provided that a second order pressure limit of 0.3 atm is assumed.

Hydroxyl radical and atomic oxygen concentrations in high-intensity turbulent combustion

Spectroscopic measurements of OH-radical and O-atom concentrations are analyzed for high-intensity turbulent combustion of methane/air in a modified Longwell jet-stirred reactor at conditions P =0.92 atm, Φ FA =0.57 to 1,33, andm0307/V=1.89×10 −2 to 4.03 ×10 −2 g/cm 3 -s. Absolute OH concentrations were determined from the measured integrated absorption of resolved spectral lines in the OH 2 π- 2 + (0,0) band. Atomic oxygen was inferred from chemiluminescence measured at 3500due to the radiative recombination CO+O→CO 2 + hv . Thermocouple temperatures varied from 1440 to 1910°K, with the OH rotational temperature being greater by ca . 7%. Hydroxyl radical concentrations versus equivalence ratio exhibited an apparent maximum near Φ FA ≅0.85, and increased with increasingm0307/V. Partial equilibrium between OJ and O-atom was also observed in the vicinity of Φ FA ≅0.85. The peak O-atom concentration occurred at FA ≅0.8. Form0307/V=1.89×10 −2 g/cm 3 -s, the peak values for OH and O-atom were 2700 and 720 ppm (molar), respectively. Calculations of NO x formation, using the PSR model with the measurements of OH and O-atom concentrations and rotational temperature as input, fell below measurements of NO x determined by gas sampling.

Flash photolysis study of the gas phase recombination of hydroxyl radicals

Rate constants are reported for the reactions OH+OH → H2O+O (where k1 is the rate constant) and OH+OH+N2 → H2O2+N2 (where k5 is the rate constant) measured in a room temperature flash photolysis experiment. Hydroxyl radicals were produced by the photodecomposition of water vapor, and time resolved absorption spectroscopy was utilized to determine the absolute OH concentration. Specific rate constants obtained are k1=2.1±0.2×10−12 cm3/sec and k5=2.5±0.3×10−31 cm6/sec.