Hydroxyl tagging velocimetry (HTV) involves tagging a flow by “writing” a line of OH molecules using a laser beam to dissociate H2O molecules and capturing an image of the line after a short delay using laser-induced fluorescence. Velocity is obtained by a time-of-flight analysis of the data. In this effort, HTV was used for obtaining both instantaneous and average velocity profiles in the flow of an augmented spark igniter. Two modes of camera readout were investigated, called conventional full-frame mode and dual image feature (DIF) mode. In DIF mode, two images are captured in quick succession and therefore the measurement has low sensitivity to vibration. Measurement uncertainty for the DIF case varied from 3% at the centerline to 10% at the edges of the profile in a 1000-m/s flow. For the full-frame case, measurement uncertainty varied from 3% at the centerline to 7% at the edges. This demonstration provides evidence that the HTV technique is well suited for obtaining velocity profiles in the challenging environment of either a rocket engine or rocket engine igniter.

An approach of reducing spontaneous Raman spectroscopy data, based on the matrix inversion method which logically removes undesired sources of the Raman scattering signals including cross-talk due to the signals from other species, is discussed. The approach contains extensive calibration of temperature-dependent system response. The Raman spectra of several cold and hot calibration flames and their resulting calibration matrix are elaborated. The structure of H₂-CO₂-air tubular flames, in terms of major species (CO₂, O₂, N₂, H₂O, and H₂) concentrations and temperature, was successfully measured with the optical technique. The Raman spectra data used in this study are taken from a uniquely designed experimental apparatus – a tubular burner and Raman spectroscopy system at Vanderbilt University. The characteristic uncertainties for chemical species are approximately ±2% by mole fraction for hot products and ±0.5% for room temperature reactants. The approach described here for hydrogen/air tubular flames may be adapted to hydrocarbon/air tubular flames when other necessary major species, for example, CH₄ and CO for methane/air tubular flame, are considered in the whole process.

Spark discharges were parametrically examined for a cylindrical air-gap electrode configuration. The aim of this study was to elucidate spark discharge characteristics, arc penetration, and exhaust...

Quantitative O-atom profiles are measured for the first time in tubular flames and used to assess the performance of chemical kinetic mechanisms in tubular flame simulations. Atomic oxygen is measured via femtosecond, Two-Photon Laser Induced Fluorescence (fs-TPLIF) to avoid photolytic interference. The fsTPLIF signal is corrected for collisional quenching from major species concentrations measured by Raman scattering. Temperature-dependent quenching rate in the form of T & minus;0 . 5 for H 2 O is applied to better represent the actual physics, and all simulations are found to agree with this method. Atomic oxygen is reported in H 2 /O 2 flames diluted with N 2 or CO 2 at 200 and 400 s & minus;1 stretch rates. The oxygen radical data is compared to predictions using three different, detailed chemical kinetic mechanisms. Predictions of profile shape vary slightly, but the peak O-atom number density is calculated within experimental uncertainty by each mechanism. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Femtosecond, Two-photon Absorption Laser Induced Fluorescence (fs-TALIF) corrected for collisional quenching with Raman scattering is used to capture spatially resolved atomic oxygen profiles in lean premixed, hydrogen cellular tubular flames. This method has allowed comparisons of number density and O-atom concentration distributions in flames of variable stretch rates in a manner similar to that previously performed on the minor flame species H and OH. As stretch rate increases, the radii of peak O-atom in the cells decrease while O-atom concentrations remain relatively unaffected. This differs from non-cellular flame data where increasing stretch rate increases minor species number densities. Three chemical mechanisms are employed to perform direct numerical simulations of the O-atom profiles in the tubular flames and are found to be in close agreement with one another. For N 2 -diluted flames, the simulations predict O-atom number densities within the uncertainty of the data for the cellular region but over-predict the O-atom number densities in the dearth region of the 2D flames. Additionally, simulated O-atom concentrations contradict the trend of the data and increase with stretch rate. Changing the diluent from N 2 to CO 2 lowers the peak concentrations of atomic oxygen as CO 2 becomes reactive at flame temperatures. This allows the CO + O ( + M ) ⇌ C O 2 ( + M ) reaction to consume atomic oxygen. Flames diluted with carbon dioxide caused the model to over-predict the O-atom concentration in these flames. This discrepancy is similar to past minor species measurements in cellular tubular flames though it does not occur in minor species profiles of non-cellular (1D), CO 2 -diluted tubular flames. The discrepancy could be caused by the simplifying relationships employed to convert the 3D geometry to 2D in the simulations.

Steady, premixed tubular flames that produce cellular flames due to low Lewis number can assist in validating diffusional properties for numerical models. The lean hydrogen-air mixture creates conditions of low Lewis number that have shown to result in cellular flames inside tubular burners due to preferential diffusion. Filtered Rayleigh scattering (FRS) is used to achieve two-dimensional temperature measurements across perpendicular planes in the cellular tubular flame. Different FRS strategies, a pulsed Nd:YAG laser with and without Fabry–Perot etalon as well as a continuous wave laser, are contrasted for consistency and uncertainty in the tubular flame measurements. The FRS temperature profiles are found to be consistent with previous spontaneous Raman scattering temperature measurements but differ from previous direct numerical simulation results.

The structure of methane/air tubular diffusion flames with 65 % fuel dilution by either CO2 or N2 is numerically investigated as a function of pressure. As pressure is increased, the reaction zone thickness reduces due to decrease in diffusivities with pressure. The flame with CO2-diluted fuel exhibits much lower nitrogen radicals (N, NH, HCN, NCO) and lower temperature than its N2-diluted counterpart. In addition to flame structure, NO emission characteristics are studied using analysis of reaction rates and quantitative reaction pathway diagrams (QRPDs). Four different routes, namely the thermal route, Fenimore prompt route, N2O route, and NNH route, are examined and it is observed that the Fenimore prompt route is the most dominant for both CO2- and N2-diuted cases at all values of pressure followed by NNH route, thermal route, and N2O route. This is due to low temperatures (below 1900 K) found in these highly diluted, stretched, and curved flames. Further, due to lower availability of N2 and nitrogen bearing radicals for the CO2-diluted cases, the reaction rates are orders of magnitude lower than their N2-diluted counterparts. This results in lower NO production for the CO2-diluted flame cases.

Experimental and numerical simulation results are reported of partially-premixed cellular tubular flames. Parametric measurements across stretch rate and equivalence ratio are taken by chemiluminescent imaging and are presented for the first time. Select hybrid cases with both cellular and non-cellular flame structures are examined with laser-induced spontaneous Raman scattering. Results are spatially resolved in two dimensions and radial interpolations of reaction and extinction zones are compared to numerical simulations using multicomponent transport and detailed chemical kinetics. Experimental cell structures and extinction zones are well predicted by numerical simulation, with discrepancies of temperature and H2O and temperature primarily observed in locations with moderate and high mole fractions of CO2. A novel cellular structure, denoted as a “split-cell” flame, is reported for the first time with both chemiluminescent imaging and Raman scattering. Results indicate that partially-premixed flames are valuable as experimental and numerical benchmarks to advance fundamental combustion research.

A parametric study of annular spark gaps, pressures, and spark discharges in flowing oxygen gas was performed with a Champion spark exciter. The range of the pressure-distance product for the experiment is from approximately 50 torr-cm to 2500 torr-cm. Measurements of breakdown voltage qualitatively trend with Paschen's curve. Spark duration remained constant until the pressure-distance product exceeded 200 torr-cm, and then steadily increased. The mean spark energy increases linearly with the pressure-distance on a log-log plot indicating that a definite power relationship exists. The distribution of sparks at low energies and low pressures is not Gaussian and has no dominant peaks. Moderate and high spark energies are bimodal, with the dominant mode near 80 mJ. As pressure increases, dominant and secondary modes approach the same probability.

Heat release rate in combustion systems must be understood in order to control thermoacoustic instabilities, flame extinction, and heat losses. Traditionally OH chemiluminescence (OH*) is used to trace heat release rate (HRR) in H2/air flames, but its accuracy as a tracer has not been assessed. Lean premixed H2/air cellular tubular flames are a good test case to evaluate HRR tracers due to the presence of highly reactive flame cells surrounded by regions of near extinction. Comparing the calculated heat release rate to OH* concentration, one finds that [OH*] profiles correlate with the regions of high reactivity (flame cells) but the correlation fails in the low reactivity regions where the HRR is much higher than the [OH*] value indicates. Alternate HRR tracers including [H] and pixel-by-pixel products of [O2]x[H], [OH]x[H2], and [O]x[H2] are analyzed with detailed numerical simulations. The chosen products derive from the main chain reaction steps that contribute to overall HRR in lean, premixed H2/air flames. Findings suggest that [H] is an accurate yet simple way of tracking HRR. Planar measurements of HRR are possible if LIF measurements of [H] are improved.

Simultaneous point measurements of temperature, mixture fraction, major species (H-2, H2O, O-2, N-2) concentrations from KrF laser-induced spontaneous Raman scattering and minor species (OH) concentrations from KrF laser-induced predissociative fluorescence (LIPF) in unswirled (S-g = 0), low swirl (S-g = 0.12), and high swirl (S-g = 0.5) lifted turbulent hydrogen jet diffusion flames into still air are reprocessed to obtain profiles of the Favre-averaged scalars and conditional moments. Large discrepancies between the Favre-averaged and ensemble-averaged temperature, H2O, and OH mole fractions are found at the lifted flame region, due to density weighting of fairly large samples of unreacted mixtures. Conditional statistics are used to reveal the reaction zone structure in mixture fraction coordinates. The cross-stream dependence of conditional means of temperature and species concentration is found to be significant in the lifted flame region of the swirled flames and decreases to negligible levels with increasing streamwise position. Comparison of the measured conditional mean variation of OH vs. H2O with a series of stretched laminar partially premixed flame and diffusion flame calculations reveals that for the unswirled flame, the differential molecular diffusion and radial dependence of conditional means are minor at x/D = 6.4 for stretch rates from a = 14,000 to 400 S-1. For the low and high swirling flames, however, the measured OH vs. H2O conditional means at the lifted flame region are not consistent with stretched laminar flame calculations. The level of partial premixing and the stretch rate decrease with increasing downstream locations. The estimated stretch rate at x/D = 53.5 is about 50-10s(-1), while at x/D = 107 the stretch rate is about 10 s(-1) with some measurements at adiabatic equilibrium. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

We report a new molecular tagging velocimetry method for use in pure nitrogen gas with a small impurity of water vapor on the order of 0.1%. This two laser method can produce a 25 mm long tag line of NH (imidogen) radicals at a standoff of 1 m which is imaged by laser-induced fluorescence after a delay of a few microseconds. The signal-to-noise ratio of the tag image can exceed 50 at standard temperature and pressure, allowing excellent spatial resolution. This new method called imidogen tagging velocimetry is useful for measuring spatially resolved velocity in flow facilities which run pure nitrogen rather than air, contain or can be seeded with trace water vapor, and in which other seeded or intrusive velocimetry methods are impractical.

Experimental and numerical simulation results are presented for the first time of H2 fueled non-premixed cellular tubular flames. Non-intrusive measurements of temperature and major species were obtained with Raman scattering. Numerical simulations were performed with a 2D fully-implicit primitive variable finite difference code that includes multicomponent transport and detailed chemical kinetics. Good agreement is found for all flame cases. The experimental cell temperatures and the shapes of the flame cells are well predicted. Less significant discrepancies are observed in the character of the extinction zones and are discussed. Overall, non-premixed cellular tubular flames are found to be valuable experimental and numerical targets for fundamental combustion research.

We report a new molecular tagging velocimetry method for use in pure nitrogen gas with a small impurity of water vapor on the order of 0.1%. This two laser method can produce a 25 mm long tag line of NH (imidogen) radicals at a standoff of 1 m which is imaged by laser-induced fluorescence after a delay of a few microseconds. The signal-to-noise ratio of the tag image can exceed 50 at standard temperature and pressure, allowing excellent spatial resolution. This new method called imidogen tagging velocimetry is useful for measuring spatially resolved velocity in flow facilities which run pure nitrogen rather than air, contain or can be seeded with trace water vapor, and in which other seeded or intrusive velocimetry methods are impractical.