OH Concentration Measurements Research Articles

The chemical kinetics and thermodynamics of sodium species in oxygen-rich hydrogen flames

Measurements of sodium and OH concentrations in ten oxygen-rich H2/O2/N2 flames by respective saturated and low-power laser-induced fluorescence techniques have led to a much improved understanding of the complex flame chemistry of sodium in such oxygen-rich media. Previous interpretations have been shown to be largely incomplete or in error. The one-dimensional flame downstream profiles indicate that the amount of free sodium approximately tracks the decay of H atom and as the flame radicals decay sodium becomes increasingly bound in a molecular form. A detailed kinetic model indicates that the sodium is distributed between NaOH, which is dominant, and NaO2. Concentrations of NaO are very small and NaH negligible. The actual distribution is controlled by the temperature, the oxygen concentration, and the degree of nonequilibration of the flames’ basic free radicals. Na, NaO, NaO2, and NaOH are all coupled to one another by fast reactions which can rapidly interconvert one to another as flame conditions vary. NaO2 plays an indispensable role in providing alternate efficient channels by which NaOH can be produced. Its contribution becomes increasingly important at lower temperatures where the flux through the NaO2 intermediate becomes dominant over that for the direct reaction between Na and H2O. As a consequence, the ratio of NaOH to Na can become enhanced by up to two orders of magnitude at lower temperatures over what might have been expected from the Na+H2O direct reaction alone. The dissociation energy D°0(Na–O2) is established to be 39±5 kcal mol−1 and the value of the rate constant for the Na+O2+M reaction of 2×10−28 T−1 cm6 molecule−2 s−1 for the flame gases. The sodium distribution within the highest temperature, low-O2 flame, in which NaOH is dominant and equilibrated, supports a value of D°0(Na–OH) of 78.9±2 kcal mol−1. The rate constants for several reactions of Na, NaOH, NaO2, and NaO with flame species have been established approximately. An analysis of the total kinetic scheme shows that the chemical fluxes are carried predominantly by four reactions only. These considered alone, reproduce the data surprisingly well. An analysis of the implications of the corresponding large rate constants for the termolecular reaction of the other alkali metals with oxygen suggests that these will undoubtedly show to varying degrees similar behavior to sodium. Values for the bond dissociation energies of the other alkali dioxides are discussed. It appears that in practical combustion systems, even at low temperatures, the conversion of alkali metals to the corresponding hydroxide will not be kinetically constrained and its concentration will be at or in excess of the expected equilibrium value.

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.

Fluorescence measurements of OH in a turbulent flame

Laser-induced fluorescence spectroscopy has been used to measure OH concentration probability density functions (PDFs) in a turbulent premixed flame. The methane-air flame is stabilized on a rod flame holder located downstream of a turbulence producing grid. Measurements in both the streamwise and transverse directions have been made for a variety of flow conditions. The PDFs display a single peaked behavior throughout the flame, with the broadest structure in the center of the flame. A simple limit analysis shows that the observed behavior of the PDF is consistent with expected behavior for an intermediate radical specie over the range of flame, optical, and turbulence length scales present in our experiment. HE study of turbulent premixed flames traditionally has been limited by a lack of experimental data that could be of help in closing the governing equations. In recent years, much work has focused on the use of probability density distribution functions (PDFs) of the dependent variables in an attempt to close the turbulent transport equations in a way amenable to experimental verification. It has been shown that a knowledge of PDFs can often greatly simplify the closure problem and lead to practical numerical schemes for predicting flame behavior. Until now, work has focused on the passive scalars tem- perature or reaction progress variable. Little has been done to incorporate more realistic chemical kinetics into an ex- perimentally verifiable theory. Since we have the capability of making spatially and temporally resolved OH concentration measurements using laser-induced fluorescence spectroscopy, we felt that measurement of OH PDFs in a turbulent premixed flame would provide both useful data and a feasibility test for the experimental technique. Concurrently with making the measurements, we have been working on a theoretical model of our flow. To be reported on in detail elsewhere, the model treats reaction kinetics in a two- step mechanism: R-+I and /HP, where reactants R form intermediates / which react to form products P. The con- servation equations are then, cast in a form which leads to integrodifferential transport equations for the probability density of each specie. The reactant, intermediate, and product pools are chosen based on the kinetic modeling of Guirgius et al., 1 who show that OH can be used as a representative specie for the intermediate pool. Thus a measurement of OH PDFs would provide useful data for direct verification of an analytic model. Laser-induced fluorescence spectroscopy (LIFS) involves the selective excitation of a chemical specie of interest and detection of the resulting fluorescence signal.2 Under suitable conditions the fluorescence signal resulting from a molecule such as OH can be shown to remain linearly related to con- centration over the temperature and mixture variation found in a turbulent flame. Because the focal volume formed at the intersection of the laser beam and collection optics can be quite small, spatial resolution on the order of 0.3 mm is easy to obtain. Furthermore, excitation with a pulsed laser of 1 />ts

Laser-saturated fluorescence measurements of OH concentration in flames

A laser-saturated fluorescence technique which can be used to measure accurate OH radical concentrations over wide ranges of flame pressure, composition, and temperature has been developed. The balanced cross-rate model is used to analyze the fluorescence data. The basic premise of the model is that the population of the upper and lower rotational levels which are coupled by the laser is approximately constant during the laser pulse. Using the balanced cross-rate model to calculate concentrations, OH number densities calculated from saturated fluorescence agree to within 20% with number densities calculated from independent absorption measurements over a wide range of flame conditions. In addition, the ratio of number densities calculated from saturated fluorescence and absorption is constant over an order of magnitude range of flame pressure, demonstrating the insensitivity of the saturated fluorescence signal to collisional quenching rates. The estimated OH detection limit of the fluorescence system is 10 12–10 13 molecules/cm 3.

The calculation of mean radical concentrations in turbulent diffusion flames

A prediction model for turbulent diffusion flames is presented and applied to H 2air-diffusion flames and in particular to the calculation of mean radical concentrations. The fast shuffle reactions in the H 2air reaction system are assumed to be in partial equilibrium, whereas the three-body recombination reactions are treated kinetically. These assumptions lead to a two variable formalism which allows the calculation of nonequilibrium values of radicals. It is shown that these values differ considerably from calculations obtained with equilibrium assumptions. The results of the calculations compare favourably with measurements of mean OH concentrations by El-Dib [9].

Measurements of temperatures and OH-concentrations in a lean methane-air flame using high-resolution laser-absorption spectroscopy

Line profile measurements of selected spectral lines in the 1-0 band of the A 2Σ +- X 2π system of OH near 2830 Å, using a tunable, frequency-doubled dye laser, have been made to determine simultaneously the translational temperature and hydroxyl concentration in a laminar, flat methane-air flame. The rotational temperature was determined from the ratio of two line-absorption coefficients. The rotational degree of freedom was found to be in equilibrium with the translational degree of freedom. Values deduced for the collision broadening parameter in the 1-0 band depend on OH-concentration, ground state rotational quantum number and, possibly, on the vibrational quantum number.

Pulsewidth dependence of ozone interference in the laser fluorescence measurement of OH in the atmosphere

By varying the pulsewidth of the output from two dye lasers, we have verified experimentally that the steady-state interference level of OH due to laser-induced dissociation of ozone decreases linearly with decreasing pulsewidth of the exciting radiation. At low pressures, further reduction in the interference level due to the transient nature of OH formation processes was also observed. These results should greatly facilitate measurements of OH concentrations in the atmosphere.

Simultaneous determination of temperature and OH-concentration in flames using high-resolution laser-absorption spectroscopy

The development of tunable cw dye lasers with an extremely narrow bandwidth of <20 MHz and high stability in frequency and amplitude, which operate in the wavelength region from 2800 to 3150 Å by frequency doubling, has made possible the quantitative proof of the presence of OH with high-resolution laser-absorption spectroscopy. A method is presented, which allows the simultaneous measurement of temperature and OH-concentration by tuning across a suitable rotational line.

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.

The concentration of hydroxyl and of oxygen atoms in gases from lean hydrogen-air flames

Measurements of OH concentrations in flame gases from lean hydrogen-air flames held on porous burners are reported. [ OH] is always found to be in excess of equilibrium near the flame and to decay downstream. The decay is formally second order in OH and the rate constant is relatively insensitive to pressure, temperature and composition, and has an average value of about 10 −13 cm 3/molecule sec. The effects of the addition of small amounts of nitric oxide have been studied. Data are presented on the relationship between the intensity of light emitted from nitric oxide containing nearly stoichiometric flames and [ OH], and it is argued that these provide strong evidence that the nitric oxide test for O atoms is quantitative. The light-producing reaction is probably NO+ O→ NO 2+hv with a rate constant, k 10, equal to 2 × 10 −18 cm 3 / molecule sec. By means of this test it is shown that under all of the conditions studied oxygen atoms and hydroxyl radicals are equilibrated according to the reaction 2 OH⇌ H 2 O+ O