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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Influence of temperature and hydroxyl concentration on incipient soot formation in premixed flames

Critical equivalence ratios phi(c) have been measured as a function of temperature (1600-1880 K) for premixed flames at atmospheric pressure. The five fuels studied are methane, ethane, propane, ethylene, and acetylene. The flames were stabilized on a flat flame burner and the temperatures were measured using sodium D-line reversal. A linear relationship is found between In phi(c) and 1/T for each fuel. Based on a global kinetic model in which soot precursors are formed by fuel pyrolysis and oxidized by OH, a predictive correlation has been developed which shows the influence of temperature, OH concentration, and C/H ratio on sooting tendency. This correlation describes all of the measured phi(c) versus temperature data, suggesting that the overall mechanism of soot formation is similar among aliphatic fuels.

Harris, M. M.↗
NASA NTRS1987

Field test to intercompare carbon monoxide, nitric oxide and hydroxyl instrumentation at Wallops Island, Virginia

Documentation of the first of three instrument intercomparisons conducted as part of NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (GTE/CITE-1) is given. This ground-based intercomparison was conducted during July 1983 at NASA Wallops Flight Facility. Instruments intercompared included one laser system and three grab-sample approaches for CO; two chemiluminescent systems and one laser-induced fluorescent (LIF) technique for NO; and two different LIF systems and a radiochemical tracer technique for OH. The major objectives of this intercomparison was to intercompare ambient measurements of CO, NO, and OH at a common site by using techniques of fundamentally different detection principles and to identify any major biases among the techniques prior to intercomparison on an aircraft platform. Included in the report are comprehensive discussions of workshop requirements, philosophies, and operations as well as intercomparison analyses and results. In addition, the large body of nonintercomparison data incorporated into the workshop measurements is summarized. The report is an important source document for those interested in conducting similar large and complex intercomparison tests as well as those interested in using the data base for purposes other than instrument intercomparison.

Gregory, Gerald L.↗
NASA NTRS1987

Operational overview of NASA GTE/CITE 1 airborne instrument intercomparisons - Carbon monoxide, nitric oxide, and hydroxyl instrumentation

An overview of the airborne intercomparisons of CO, NO, and OH instrumentation is presented in this first paper of the series on the NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (GTE/CITE 1). This paper provides the reader with background information about several important characteristics of the project. These include the overall objectives and approach, the measurements taken, the intercomparison protocol, aircraft platform, profiles of each aircraft flight, and the participants. A synopsis of the overall results of the CO, NO, and OH instrument intercomparisons is also included. Companion papers discuss the detailed results of the CO and NO intercomparison tests as well as pertinent scientific findings.

Beck, Sherwin M.↗
NASA NTRS1988

Ground-based column abundance measurements of atmospheric hydroxyl

The preliminary results of ground-based OH column abundance measurements from Truk, Federated States of Micronesia, are contained. These are the first OH column measurements from the tropics, and constitute a signficcant contribution to the OH data base. Comparisons of tropical OH behavior with the extensive mid-latitude observations serve as a critical test of the current understanding of the HO (sub x) photochemistry and its relationship to the other major chemical families. The quasi-biennial oscillation (QBO) in tropical stratospheric winds exerts a major influence on the Hadley cell vertical transport. Related QBOs in total O3 and in stratospheric H2O were identified, but QBO effects on other stratospheric species are still unknown. The solar tide in the tropics produces a diurnal surface pressure variation of 2 to 3 mb; its effect on OH photochemistry in the stratosphere may be significant.

Burnett, Clyde R.↗
NASA NTRS1987

Feasibility of hydroxyl concentration measurements by laser-saturated fluorescence in high-pressure flames

The effect of pressure on the laser-saturated fluorescence method for measuring OH concentration in high-pressure flames is studied using calculations for the burned-gas region of a stoichiometric H2-O2 flame at 2000 K. A numerical model of the excitation dynamics of OH is developed to explore the validity of the balanced cross-rate model at higher pressures. It is shown that depopulation of the laser-coupled levels is sensitive to collisions which depopulate v-double-prime (VDP) = 0 and to rate coefficients for rotational transfer in the ground state which are smaller than those in the excited state. In particular, it is shown that the depopulation of VDP = 0, and hence the laser-coupled levels, depends on the probability of electronic quenching to vibrational levels for which VDP is greater than 0 and vibrational relaxation to VDP = 0.

Carter, Campbell D.↗
NASA NTRS1989

Periodic behaviors in the observed vertical column abundances of atmospheric hydroxyl

OH vertical column abundances measured at Fritz Peak Observatory, Colorado, from 1977 through 1988 reveal semi-annual and annual cycles which are amplitude modulated over the 11-year data base. The modulation of the OH seasonal behavior is in phase with solar activity as described by the sunspot number index. No mechanism for the observed covariance of this OH behavior with solar activity has yet been identified.

Burnett, Elizabeth Beaver↗
NASA NTRS1991

Photodissociation of the hydroxyl radical (OH) at 157 nm

The photodissociation of the OH radical was studied at 157 nm via the detection of the product H atoms with the resonance fluorescence technique. OH radicals were produced in a fast-flow cell from the reaction between H and NO2 and subsequently photodissociated by an excimer laser operating on the F2 emission. The quantum yield for photodissociation of OH was measured to be 1.10 + or - 0.28. The photodissociation cross section was calculated to be 6.6 x 10 to the -18th sq cm (+ or - 25 percent).

Cody, R. J.↗
NASA NTRS1990

Tropospheric hydroxyl concentrations and the lifetimes of hydrochlorofluorocarbons (HCFCs)

Three dimensional fields of modeled tropospheric OH concentrations are used to calculate lifetimes against destruction by OH for many hydrogenated halocarbons, including the CFC alternatives. The OH fields were taken from a 3-D chemical transport model (Spivakovsky et al. 1989) that accurately simulates the global measurements of methyl chloroform (derived lifetime of 5.5 years). The lifetimes of various hydro-halocarbons are shown to be insensitive to possible spatial variations and seasonal cycles. It is possible to scale the HCFC lifetimes to that of methyl chloroform or methane by using the ratio of the rate coefficients for reaction with OH at an appropriate temperature, about 277 K.

Prather, Michael J.↗
NASA NTRS1992

Global average concentration and trend for hydroxyl radicals deduced from ALE/GAGE trichloroethane (methyl chloroform) data for 1978-1990

An optimal estimation inversion scheme is utilized with atmospheric data and emission estimates to determined the globally averaged CH3CCl3 tropospheric lifetime and OH concentration. The data are taken from atmospheric measurements from surface stations of 1,1,1-trichloroethane and show an annual increase of 4.4 +/- 0.2 percent. Industrial emission estimates and a small oceanic loss rate are included, and the OH concentration for the same period (1978-1990) are incorporated at 1.0 +/- 0.8 percent/yr. The positive OH trend is consistent with theories regarding OH and ozone trends with respect to land use and global warming. Attention is given to the effects of the ENSO on the CH3CCl3 data and the assumption of continuing current industrial anthropogenic emissions. A novel tropical atmospheric tracer-transport mechanism is noted with respect to the CH3CCl3 data.

Prinn, R.↗
NASA NTRS1992

Saturated fluorescence measurements of the hydroxyl radical in laminar high-pressure C2H6/O2/N2 flames

Saturation of a transition of the OH molecule in high-pressure flames is demonstrated by obtaining saturation curves in C2H6/O2/N2 laminar flames at 1, 6.1, 9.2, and 12.3 atm. Quantitative fluorescence measurements of OH number density at pressures to 12.3 atm are presented. To assess the efficacy of the balanced cross-rate model for high-pressure flames, laser-saturated fluorescence measurements, which were calibrated in an atmospheric-pressure flame, are compared with absorption measurements at 3.1 and 6.1 atm. At 3.1 atm the absorption and fluorescence measurements compare well. At 6.1 atm, however, the concentrations given by lasre-saturated fluorescence are about 25 percent lower than the absorption values, indicating some depletion of the laser-coupled levels beyond that at atmospheric pressure. By using a reasonable estimate for the finite sensitivity to quenching, it is anticipated that fluorescence measurements that are calibrated at 1 atm can be applied to flames at about 10 atm with absolute errors within +/- 50 percent.

Carter, Campbell D.↗
NASA NTRS1992

Hydroxyl X2Pi pure rotational transitions

We present a list of frequencies, term values, Einstein A values, and assignments for the pure rotational transitions of the X2Pi state of the OH molecule. This list includes transitions from 3 to 2015/cm for Delta-v = 0, v-double-prime = 0-4, and J-double-prime = 0.5-49.5. The A values were computed using recent advances in calculating wave functions for a coupled system and an experimentally derived electric dipole moment function (Nelson et al., 1990) which exhibits curvature.

Goorvitch, D.↗
NASA NTRS1993

Velocity distributions of hydrogen atoms and hydroxyl radicals produced through solar photodissociation of water

The velocity distributions of H and OH fragments produced through solar photodissociation of gaseous H2O molecules under collisionless conditions are presented. The calculations are carried out using: the most recently available absolute partial cross sections for the production of H and OH through photodissociation of H2O from its absorption onset at 1860 A down to 500 A; the newly available vibrational and rotational energy distributions of both the excited and ground state OH photofragments; the calculated cross sections for the total dissociation processes; and the integrated solar flux in 10 A increments from 500 to 1860 A in the continuum regions and the specific wavelength and flux at the bright solar lines. The calculated results show that the H atoms and the OH radicals produced exhibit multiple velocity groups. Since most current cometary modeling uses a single velocity of 20 km/sec associated with the photodissociation of H2O, the present results may be useful in interpreting the many peaks observed in the velocity distributions of the H Lyman alpha and H alpha of comets.

Wu, C. Y. R.↗
NASA NTRS1993

Detection of the hydroxyl radical in the Saturn magnetosphere

The detection of neutral OH molecules near the orbit of Tethys is reported. The findings suggest that neutral OH is one of the dominant species in Saturn's inner magnetosphere, implying a source rate for H2O 20 times greater than current theoretical estimates. One possible explanation is that the micrometeorite erosion rates of the inner satellites are significantly higher than expected.

Shemansky, D. E.↗
NASA NTRS1993

Rates of hydroxyl radical reactions with some HFCs

Relative rate constants for OH reactions with some HFCs have been determined at 298 K by a technique which measures the loss of HFC greater than OH. The following ratios were determine: k(152a)/k(CH4) = 5.2 +/- 0.5, k(CH4)/k(125) = 3.9 +/- 0.5, k(CH4)/k(134a) = 2.1 +/- 0.2, k(134a)/k(125) = 2.0 +/- 0.2, and k(C2H6)/k(152a) = 6.2 +/- 1.0. These results are in good agreement with literature values for the absolute rate constants except for HFC 134a, where a slower rate constant is indicated.

Demore, William B.↗
NASA NTRS1994

The active titration method for measuring local hydroxyl radical concentration

We are developing a method for measuring ambient OH by monitoring its rate of reaction with a chemical species. Our technique involves the local, instantaneous release of a mixture of saturated cyclic hydrocarbons (titrants) and perfluorocarbons (dispersants). These species must not normally be present in ambient air above the part per trillion concentration. We then track the mixture downwind using a real-time portable ECD tracer instrument. We collect air samples in canisters every few minutes for roughly one hour. We then return to the laboratory and analyze our air samples to determine the ratios of the titrant to dispersant concentrations. The trends in these ratios give us the ambient OH concentration from the relation: dlnR/dt = -k(OH). A successful measurement of OH requires that the trends in these ratios be measureable. We must not perturb ambient OH concentrations. The titrant to dispersant ratio must be spatially invariant. Finally, heterogeneous reactions of our titrant and dispersant species must be negligible relative to the titrant reaction with OH. We have conducted laboratory studies of our ability to measure the titrant to dispersant ratios as a function of concentration down to the few part per trillion concentration. We have subsequently used these results in a gaussian puff model to estimate our expected uncertainty in a field measurement of OH. Our results indicate that under a range of atmospheric conditions we expect to be able to measure OH with a sensitivity of 3x10(exp 5) cm(exp -3). In our most optimistic scenarios, we obtain a sensitivity of 1x10(exp 5) cm(exp -3). These sensitivity values reflect our anticipated ability to measure the ratio trends. However, because we are also using a rate constant to obtain our (OH) from this ratio trend, our accuracy cannot be better than that of the rate constant, which we expect to be about 20 percent.

Sprengnether, Michele↗
NASA NTRS1995

Planar Imaging of Hydroxyl in a High Temperature, High Pressure Combustion Facility

An optically accessible flame tube combustor is described which has high temperature, pressure, and air flow capabilities. The windows in the combustor measure 3.8 cm axially by 5.1 cm radially, providing 67 percent optical access to the square cross section flow chamber. The instrumentation allows one to examine combusting flows and combustor subcomponents, such as fuel injectors and air swirlers. These internal combustor subcomponents have previously been studied only with physical probes, such as temperature and species rakes. Planar laser-induced fluorescence (PLIF) images of OH have been obtained from this lean burning combustor burning Jet-A fuel. These images were obtained using various laser excitation lines of the OH A yields X (1,0) band for two fuel injector configurations with pressures ranging from 1013 kPa (10 atm) to 1419 kPa (14 atm), and equivalence ratios from 0.41 to 0. 59. Non-uniformities in the combusting flow, attributed to differences in fuel injector configuration, are revealed by these images.

Hicks, Yolanda R.↗