Distribution Of OH Research Articles

Laser-based investigations of flow fields and OH distributions in impinging flames of domestic cooker-top burners

A thermal performance of impinging flames of widely-used cooker-top burners depends on a complex interaction between flow field and combustion. The critical situation is placed on two typical cooker-top burners having almost the same configurations, but yielding about 10% point difference in thermal efficiency. Therefore, in situ flame measurements of averaged velocity using particle image velocimetry (PIV) and image of averaged OH radicals using planar laser-induced fluorescence (OH-PLIF) are required to obtain insight into the coupling mechanisms during the combustion and heat transfer, and to develop a comprehensive experimental data base to support a phenomenological understanding of the differences in flames of the two burners. A combined study of flow field and combustion forms the basis of a much better understanding of the casual links that couple fluid mechanics, combustion and heat transfer in the two burners. This can provide guidelines for efficient burner design to meet future regulatory requirements.

Quasiclassical trajectory study of the postquenching dynamics of OH AΣ2+ by H2/D2 on a global potential energy surface

We report full-dimensional, electronically adiabatic potential energy surfaces (PESs) for the ground state (1A(')) and excited state (2A(')) of OH(3). The PESs are permutationally invariant fits to roughly 23,000 electronic energies (MRCI + Q/aVTZ). Classical trajectory calculations of the postquenching dynamics of OH A (2)Σ(+) are carried out on the 1A(') PES for H(2) and D(2), at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to "adiabatic" and "diabatic" models of the dynamics, respectively. Branching ratios of reactive to nonreactive channels from separate C(2v), C(∞v), and C(s) symmetries of CoIs are calculated, as are final rovibrational state distributions of OH and H(2) products. The rovibrational distributions of the OH and D(2) products, the D/H-atom translational energy distribution are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The vibrational state distributions of H(2)O and HOD (although not measured by experiment) are also presented.

Herschel/HIFI observations of interstellar OH+and H2O+towards W49N: a probe of diffuse clouds with a small molecular fraction

We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1 - 0 transition of OH+ and the 1115 GHz 1(11) - 0(00) transition of ortho-H2O+. The resultant spectra show absorption by ortho-H2O+, and strong absorption by OH+, in foreground material at velocities in the range 0 to 70 km/s with respect to the local standard of rest. The inferred OH+/H2O+ abundance ratio ranges from ~ 3 to ~ 15, implying that the observed OH+ arises in clouds of small molecular fraction, in the 2 - 8% range. This conclusion is confirmed by the distribution of OH+ and H2O+ in Doppler velocity space, which is similar to that of atomic hydrogen, as observed by means of 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH+/H abundance ratio of a few E-8 suggests a cosmic ray ionization rate for atomic hydrogen of (0.6 - 2.4) E-16 s-1, in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H3+ and other species.

Experimental study of flame-hole reignition mechanisms in a turbulent non-premixed jet flame using sustained multi-kHz PIV and crossed-plane OH PLIF

The dynamics of flame-hole reignition were studied experimentally in a turbulent non-premixed CH4/H2/N2 jet flame at Red=22,800 (flame ‘DLR-B’ from the TNF workshop). Simultaneous measurements of the OH combustion radical and velocity field were performed using planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV) at a sustained rate of 10kHz. The dynamics of the reignition process were tracked through time and two reignition mechanisms were identified. Particular care was taken to reduce the influence of out-of-plane motion on the analyzed events by simultaneously measuring the OH distribution in crossed planes. Flame-holes reignited due to both edge-flame propagation and turbulent transport of burning flame segments. However, the edge-flame propagation mechanism was dominant and accounted for over 90% of the flame-hole reignition rate on average. Furthermore, the presence of large scale turbulent structures adjacent to a flame-hole did not necessarily result in reignition due to turbulent transport. Instead, the edge-flames propagated around the perimeter of such structures, indicating intervening regions of well mixed gas. The range of measured edge-flame propagation speeds agreed well that of highly-preheated premixed flames, with a mode of approximately 4m/s and a mean of approximately 7m/s.

Spectroscopic study of an atmospheric pressure dc glow discharge with a water electrode in atomic and molecular gases

The positive column of dc excited atmospheric pressure glow discharges in a metal pin–water electrode system is investigated in air, N2, He, Ar, N2O, CO2 and He–N2 mixtures. The electric field and the plasma temperatures in the positive column are measured and the effect of the filling gas on the optical emission is examined. Estimates of the electron temperature and density and the observed emission properties indicate that the formation processes of the excited dissociation fragments of H2O are due to recombination processes rather than direct electron excitation. Deviation from a Boltzmann rotational population distribution of OH(A) is observed in all gases due to the formation process of OH(A). In the case of He the non-Boltzmann behaviour is more pronounced which can be caused by the lower gas temperature as a higher gas temperature leads to more thermalization of the rotational population distribution of OH(A). The results in this paper suggest that vibrational energy transfer contributes to the non-Boltzmann distribution of OH(A). The temperature corresponding to the small rotational numbers of OH(A) can be used as an estimate of the gas temperature.

Numerical Investigation of Transverse Hydrogen Jet into Supersonic Crossflow Using Detached-Eddy Simulation

A three-dimensional unsteady reacting flowfield that is generated by transverse hydrogen injection into a supersonic mainstream is numerically investigated using detached-eddy simulation and a finite-rate chemistry model. Grid refinement with the grid-convergence-index concept is applied to the instantaneous flowfield for assessing the grid resolution and solution convergence.Validation is performed for the jet penetration height, and the predicted result is in good agreement with experimental trends. The results indicate that jet vortical structures are generated as the interacting counter-rotating vortices become alternately detached in the upstream recirculation region. Although the numerical OH distribution reproduces the experimental OH–planar-laser-induced fluorescence well, there are some disparities in the ignition delay times due to the restricted availability of experimental and numerical data. The effects of the turbulence model on combustion are identified by a comparative analysis of the Reynolds-averaged Navier–Stokes and detached-eddy simulation approaches. Their effects are quantified by the production ofH2O, which is the primary species of hydrogen combustion.

Laser-based investigations in gas turbine model combustors

Dynamic processes in gas turbine (GT) combustors play a key role in flame stabilization and extinction, combustion instabilities and pollutant formation, and present a challenge for experimental as well as numerical investigations. These phenomena were investigated in two gas turbine model combustors for premixed and partially premixed CH4/air swirl flames at atmospheric pressure. Optical access through large quartz windows enabled the application of laser Raman scattering, planar laser-induced fluorescence (PLIF) of OH, particle image velocimetry (PIV) at repetition rates up to 10 kHz and the simultaneous application of OH PLIF and PIV at a repetition rate of 5 kHz. Effects of unmixedness and reaction progress in lean premixed GT flames were revealed and quantified by Raman scattering. In a thermo-acoustically unstable flame, the cyclic variation in mixture fraction and its role for the feedback mechanism of the instability are addressed. In a partially premixed oscillating swirl flame, the cyclic variations of the heat release and the flow field were characterized by chemiluminescence imaging and PIV, respectively. Using phase-correlated Raman scattering measurements, significant phase-dependent variations of the mixture fraction and fuel distributions were revealed. The flame structures and the shape of the reaction zones were visualized by planar imaging of OH distribution. The simultaneous OH PLIF/PIV high-speed measurements revealed the time history of the flow field–flame interaction and demonstrated the development of a local flame extinction event. Further, the influence of a precessing vortex core on the flame topology and its dynamics is discussed.

LBA observations of the maser cluster OH 330.953−0.182

The maser spot distribution of OH 330.953-0.182 provides a valuable probe of its massive star-forming host. We present observations with the Long Baseline Array of the Australia Telescope National Facility, of both the 1665- and 1667-MHz hydroxyl ground-state transitions, yielding a series of maps in both right- and left-hand circular polarization at velocity spacing 0.09 km s -1 with 0.1 arcsec spatial resolution. A total of 53 maser spots are spread over a remarkably large region exceeding 6 arcsec diameter. At the minimum distance estimate of 5.6 kpc, 6 arcsec corresponds to 163 mpc, much larger than the extents of previously mapped individual sites, which rarely exceed 40 mpc. Seven locations show Zeeman pairs, one of them at both 1665 and 1667 MHz corroborating the same magnetic field estimate. All measured magnetic fields are towards us (negative), and range from -3.7 to -5.8 mG. Comparisons of the morphology and kinematics of the 1665- and 1667-MHz maser spots have been made with those of the OH 1720-MHz transition, the excited state of OH at 6035 MHz, methanol at 6668 MHz, water at 22 GHz and with extensive H II emission, all of which contribute to a better understanding of the region. We interpret the overall complexity of the region as evidence for several distinct sites of excitation, similar to the remarkable W51 complex, and in contrast to the simple morphologies of previously studied single maser sites such as OH 300.969+1.147 and W3(OH).

Crossed-Beam Investigation of O(3P) + C2H5 → C2H4 + OH

The reaction dynamics of ground-state atomic oxygen [O((3)P)] with an ethyl radical (C(2)H(5)) in the gas phase was investigated using high-resolution laser spectroscopy in a crossed-beam configuration. An exothermic channel of O((3)P) + C(2)H(5) --> C(2)H(4) + OH was identified, and the nascent distributions of OH (X (2)Pi: upsilon'' = 0, 1) showed significant internal excitations with an unusual bimodal feature of low and high rotational N''-components with neither spin-orbit nor Lambda-doublet propensities. On the basis of the ab initio and statistical calculations, the reaction mechanism can be rationalized by two competing mechanisms: abstraction vs addition. The low N''-components with significant vibrational excitation can be described in terms of the direct abstraction process as a major channel. The extraordinarily hot rotational distribution of high N''-components implies that a portion of the fraction proceeds through the indirect short-lived addition-complex forming process. From the comparative analysis of the reactions of O((3)P) + several hydrocarbon molecules and radicals, the reactivity and mechanistic characteristics of the title reaction are discussed.

Numerical Analysis Model of Galvanic Corrosion with Ion Movement and Reactions

Numerical analysis models for galvanic corrosion have been developed in recent years, but most of the ordinary models can simulate only current density and potential distribution in an electrolyte solution. In order to make clear corrosion mechanism, more information such as pH, ion and corrosion product distributions are necessary. However, it is difficult to compute these phenomena by mathematical model because various kinds of ions and complex reactions exist in corrosion progress. Calculating ion movement with satisfying electroneutrality is especially difficult.We have developed a new numerical analysis model for galvanic corrosion that can calculate ions movement and reactions. In this model, ion density distribution is corrected by solving Poisson's equation to satisfy electroneutrality. Reactions are calculated based on chemical equilibrium.Galvanic corrosion of a model Fe/Zn couple in a NaCl solution was calculated with this model. The difference of ion and corrosion product distributions in 50 ppm and 50000 ppm NaCl solutions was discussed. Corrosion product distribution obtained by this numerical analysis model agreed well with OH distribution of corrosion product measured by FT-IR method qualitatively.

Electronic quenching of OH(A) by water in atmospheric pressure plasmas and its influence on the gas temperature determination by OH(A–X) emission

In this paper it is shown that electronic quenching of OH(A) by water prevents thermalization of the rotational population distribution of OH(A). This means that the observed ro-vibrational OH(A–X) emission band is (at least partially) an image of the formation process and is determined not only by the gas temperature. The formation of negative ions and clusters for larger water concentrations can contribute to the non-equilibrium. The above is demonstrated in RF excited atmospheric pressure glow discharges in He–water mixtures in a parallel metal plate reactor by optical emission spectroscopy. For this particular case a significant overpopulation of high rotational states appears around 1000 ppm H2O in He. The smallest temperature parameter of a non-Boltzmann (two-temperature) distribution fitted to the experimental spectrum of OH(A–X) gives a good representation of the gas temperature. Only the rotational states with the smallest rotational numbers (J ⩽ 7) are thermalized and representative for the gas temperature.

Is the Rotational Temperature of OH(A–X) for Discharges in and in Contact with Liquids a Good Diagnostic for Determining the Gas Temperature?

AbstractIn this paper the rotational temperature of OH(A–X) and rotational population distribution of OH(A) are investigated for streamer discharges in bubbles and glow discharges with liquid electrodes, both at atmospheric pressure. The influence of the filling gas is investigated in detail and the non‐Boltzmann nature of the rotational population distributions is discussed. It is shown that the rotational population distribution of OH(A) is even at atmospheric pressure an image of the formation process or is at least influenced by it. As a consequence the rotational temperature is in this case not a good estimate of the gas temperature as the rotational population distribution is not an image of a kinetic temperature. In some cases rotational states with small rotational numbers offer a possibility to obtain the gas temperature. The influence of these results on the determination of gas temperatures in the field of liquid plasmas is discussed.magnified image

Development of Laser Ionization Mass Spectrometer for detection of unstable species in flames

Results are presented here on the detection of unstable species formed in flames using a novel Laser Ionization Mass Spectrometry (LIMS) system. The chemical species generated in atmospheric pressure flames were directly introduced into a mass spectrometer operated at high vacuum conditions using a newly developed interface, in which the ionization was induced by laser irradiation. Optimum conditions for the experimental conditions were investigated in order to obtain mass spectra with adequate signal to noise (S/N) ratios. In addition, the distribution profiles of various species in the flame was measured and visualized. The distribution of OH profiles was also measured under the same conditions using Planer Laser Induced Fluorescence (PLIF) diagnostics. The results showed that the profiles of OH distribution by LIMS were in good agreement with those obtained using PLIF diagnostics.

Novel technique for the determination of hydroxyl distributions in fused silica

Bound OH- is one major unwanted absorbing impurity causing significant attenuation for certain wavelengths of laser radiation when propagating through fused silica. A wavelength tunable laser calorimeter (LCA) is used to detect dopants and other impurities in optical material. Also, a technique for photothermal absorption measurements is combined with the LCA set-up allowing for a lateral scan of the sample retaining an absolute calibration. Regarding inhomogeneities within the distribution of impurities the combination of both techniques provides a highly sensitive method to investigate in the lateral distribution of OH- content in fused silica.

Validation of a Grid Independent Spray Model and Fuel Chemistry Mechanism for Low Temperature Diesel Combustion

Spray and combustion submodels used in a Computational Fluid Dynamics (CFD) code, KIVACHEMKIN, were validated for Low Temperature Combustion (LTC) in a diesel engine by comparing measured and model predicted fuel spray penetrations, and in-cylinder distributions of OH and soot. The conditions considered were long ignition delay, early and late fuel injection cases. It was found that use of a grid independent spray model, called the GASJET model, with an improved n-heptane chemistry mechanism can well predict the heat release rate, not only of the main combustion stage, but also of the cool flame stage. Additionally, the GASJET model appropriately predicts the distributions of OH and soot in the cylinder even when the resolution of the computational mesh is decreased by half, which significantly reduces the required computational time.

Combustion of hydrogen in an experimental trapped vortex combustor

Combustion performances of pure hydrogen in an experimental trapped vortex combustor have been tested under different operating conditions. Pressure fluctuations, NOx emissions, OH distributions, and LBO (Lean Blow Out) were measured in the tests. Results indicate that the TVC test rig has successfully realized a double vortex construction in the cavity zone in a wide range of flow conditions. Hydrogen combustion in the test rig has achieved an excellent LBO performance and relatively low NOx emissions. Through comparison of dynamic pressure data, OH fluctuation images, and NOx emissions, the optimal operating condition has been found out to be Φp =0.4, fuel split =0.4, and primary air/fuel premixed.

Desorption of hydroxyl radicals in the vacuum ultraviolet photolysis of amorphous solid water at 90 K

We have studied the desorption dynamics of OH radicals from the 157 nm photodissociation of amorphous solid water (ASW) as well as H(2)O(2) deposited on an ASW surface at 90 K. The translational and internal energy distributions of OH were measured using resonance-enhanced multiphoton ionization methods. These distributions are compared to reported molecular dynamics calculations for the condensed phase photodissociation of water ice and also reported results for the gas phase photodissociation of H(2)O at 157 nm. We have confirmed that OH radicals are produced from two different mechanisms: one from primary photolysis of surface H(2)O of ASW, and the other being secondary photolysis of H(2)O(2) photoproducts on the ASW surface after prolonged irradiation at 157 nm.

Large Eddy Simulations of forced ignition of a non-premixed bluff-body methane flame with Conditional Moment Closure

Large Eddy Simulations (LES) of forced ignition of a bluff-body stabilised non-premixed methane flame using the Conditional Moment Closure (CMC) turbulent combustion model have been performed. The aim is to investigate the feasibility of the use of CMC/LES for ignition problems and to examine which, if any, of the characteristics already observed in related experiments could be predicted. A three-dimensional formulation of the CMC equation was used with simple and detailed chemical mechanisms and sparks with different parameters (location, size) were used. It was found that the correct pattern of flame expansion and overall flame appearance were predicted with reasonable accuracy with both mechanisms, but the detailed mechanism resulted in expansion rates closer to the experiment. Moreover, the distribution of OH was predicted qualitatively accurately, with patches of high and low concentration in the recirculation zone during the ignition transient, consistent with experimental data. The location of the spark relative to the recirculation zone was found to determine the pattern of the flame propagation and the total time for the flame stabilisation. The size was also an important parameter, since it was found that the flame extinguishes when the spark is very small, in agreement with expectations from experiment. The stabilisation mechanism of the flame was dominated by the convection and sub-grid scale diffusion of hot combustion products from the recirculation zone to the cold gases that enter the burner, as revealed by analysis of the CMC equation.

Oxygen/Hydrogen-Planar-Laser-Induced Fluorescence Measurements and Accuracy Investigation in High-Pressure Combustion

DOI: 10.2514/1.39013 Inflow species concentration measurements in reacting flows at high pressures, based on nonintrusive methods, have been acquired so far for isolated conditions in a range of experimental devices and by using a variety of methods. Furthermore, extensive assessments of the uncertainties associated with the measurement techniques are lacking. In general, these methods have been based on calibrations determined from assumptions that were not sufficiently quantified to provide a detailed range of the uncertainties associated with these measurements. This work quantifies the uncertainties associated with OH measurement in an oxygen–hydrogen system produced by a shear, coaxial injector typical of those used in rocket engines. Planar OH distributions are obtained for a range of pressures from 10 to 53 bar, providing instantaneous and averaged distribution in a unified study, using the same experimental setup and maintaining the rest of the parameters the same. The uncertainty of 18 different parameters was evaluated and the overall root mean square error was found as 21.9, 22.8, 22.5, and 22.9% at 10, 27, 37, and 53 bar, respectively.

Simulating low temperature diesel combustion with improved spray models

Current spray models based on the Lagrangian-droplet and Eulerian-fluid (LDEF) method in the KIVA-3V code are strongly mesh dependent due to errors in predicting the droplet–gas relative velocity and errors in describing droplet–droplet collision and coalescence processes. To reduce the mesh dependence, gas-jet theory is introduced to predict the droplet–gas relative velocity, and a radius of influence (ROI) of collision methodology is established for each gas phase cell to estimate the collision probability for each parcel in the cell. Spray and combustion processes in a low temperature combustion diesel engine under early and late injection strategies with a fine mesh were predicted using the conventional LDEF model and compared with the measurements of soot, OH, fuel liquid and vapor distributions obtained by laser based diagnostics including, PLIF, LII, and Mie scattering. Then, the KIVA-3V code implemented with the improved spray model based on the gas-jet model and modifications of the spray models was utilized to simulate the processes on a relatively coarse numerical mesh. Comparison of the simulations between the fine and coarse meshes shows that the improved spray model can greatly reduce the mesh dependence for low temperature combustion diesel engine CFD simulations.