Jacob B. Schmidt

Ultrafast-time-gated ballistic-photon imaging and shadowgraphy in optically dense rocket sprays

Time-gated ballistic-photon imaging is a form of shadowgraphy in which an ultrashort, optical-Kerr-effect (order 2 ps) time gate is used to enhance the relative intensity of ballistic versus multiply scattered photons. In the current work, this technique is adapted for what is believed to be the first time for use in the moderately dense environment (optical density approximately 1.5 to 2) of a high-speed 5 to 15 mm diameter rocket spray to improve image contrast and observe liquid-breakup phenomena. Unlike coherence gating, which is another form of ballistic imaging, the time-gating approach allows sufficient signal levels from ballistic and near-ballistic photons to enable time-resolved single-shot imaging. Direct comparisons with non-time-gated shadowgraphy indicate that the two techniques are sensitive to different features of the flowfield, with regions composed of a dense field of droplets being highly attenuated in conventional shadowgrams but appearing transparent to ballistic photons. This enables significant image contrast enhancement (approximately 6.6:1) of liquid-core structures and facilitates improved understanding of the primary and secondary breakup processes in sprays of moderate optical density.

Spatially and temporally resolved temperature and shock-speed measurements behind a laser-induced blast wave of energetic nanoparticles

Spatially and temporally resolved temperature measurements behind an expanding blast wave are made using picosecond (ps) N2 coherent anti-Stokes Raman scattering (CARS) following laser flash heating of mixtures containing aluminum nanoparticles embedded in ammonium-nitrate oxidant. Production-front ps-CARS temperatures as high as 3600 ± 180 K—obtained for 50-nm-diameter commercially produced aluminum-nanoparticle samples—are observed. Time-resolved shadowgraph images of the evolving blast waves are also obtained to determine the shock-wave position and corresponding velocity. These results are compared with near-field blast-wave theory to extract relative rates of energy release for various particle diameters and passivating-layer compositions.

Femtosecond, two-photon-absorption, laser-induced-fluorescence (fs-TALIF) imaging of atomic hydrogen and oxygen in non-equilibrium plasmas

Femtosecond, two-photon-absorption laser-induced fluorescence (fs-TALIF) is employed to measure space- and time-resolved distributions of atomic hydrogen and oxygen in moderate-pressure, non-equilibrium, nanosecond-duration pulsed-discharge plasmas. Temporally and spatially resolved hydrogen and oxygen TALIF images are obtained over a range of low-temperature plasmas in mixtures of helium and argon at 100 Torr total pressure. The high-peak-intensity, low-average-energy fs pulses combined with the increased spectral bandwidth compared to traditional ns-duration laser pulses provide a large number of photon pairs that are responsible for the two-photon excitation, which results in an enhanced TALIF signal. Krypton and xenon TALIF are used for quantitative calibration of the hydrogen and oxygen concentrations, respectively, with similar excitation schemes being employed. This enables 2D collection of atomic-hydrogen and -oxygen TALIF signals with absolute number densities ranging from 2 × 1012 cm−3 to 6 × 1015 cm−3 and 1 × 1013 cm−3 to 3 × 1016 cm−3, respectively. These 2D images are the first application of TALIF imaging in moderate-pressure plasma discharges. 1D self-consistent modeling predictions show agreement with experimental results within the estimated experimental error of 25%. The present results can be used to further the development of higher fidelity kinetic models while quantifying plasma-source characteristics.

Femtosecond, two-photon laser-induced-fluorescence imaging of atomic oxygen in an atmospheric-pressure plasma jet

Femtosecond, two-photon-absorption laser-induced-fluorescence (fs-TALIF) spectroscopy is employed to measure space- and time-resolved atomic-oxygen distributions in a nanosecond, repetitively pulsed, externally grounded, atmospheric-pressure plasma jet flowing helium with a variable oxygen admixture. The high-peak-intensity, low-average-energy femtosecond pulses result in increased TALIF signal with reduced photolytic inferences. This allows 2D imaging of absolute atomic-oxygen number densities ranging from 5.8???×???1015 to 2.0???×???1012cm?3 using a cooled CCD with an external intensifier. Xenon is used for signal and imaging-system calibrations to quantify the atomic-oxygen fluorescence signal. Initial results highlight a transition in discharge morphology from annular to filamentary, corresponding with a change in plasma chemistry from ozone to atomic oxygen production, as the concentration of oxygen in the feed gas is changed at a fixed voltage-pulse-repetition rate. In this configuration, significant concentrations of reactive oxygen species may be remotely generated by sustaining an active discharge beyond the confines of the dielectric capillary, which may benefit applications that require large concentrations of reactive oxygen species such as material processing or biomedical devices.

Point-to-plane pulsed discharge initiated flame structure modification in propane–air flame

The effect of a point-to-plane pulsed discharge on a propane/air flame has been investigated by phaselocked, simultaneous measurements of the change in gas temperature and OH planar Laser-InducedFluorescence (PLIF). Phase locked simultaneous measurement of gas temperature through spontaneous Raman scattering and OH PLIF with the variation of pulsed plasma energy and plasma generation location with respect to the flame holder and flame reaction zone have been performed. A fast rise time (15 ns) and a slower rise time (150 ns) high voltage pulsers are used to produce OH radical densities 50% greater than the ambient flame produced OH radicals in both lean and rich pre-mixed flames. The excess OH radical densities were found decay to 50% level with time constants greater than 100 μs in the burnt gas regions with gas temperatures greater than 1000 K. The flame perturbation was dependent on the pulse repetition rates as well as on the pulse rise time for similar energy deposition per pulse.

Single-shot thermometry and OH detection via femtosecond fully resonant electronically enhanced CARS (FREE-CARS).

Femtosecond time-resolved, fully resonant electronically enhanced coherent anti-Stokes Raman scattering (FREE-CARS) spectroscopy, incorporating a two-color excitation scheme, is used to demonstrate selective and sensitive gas-phase detection of the hydroxyl (OH) radical in a reacting flow. Spectral resolution of the emitted FREE-CARS signal allows simultaneous detection of temperature and relative OH mole fraction under single-laser-shot conditions in a laminar ethylene-air flame. By comparison to previously reported OH concentration and temperature measurements, we demonstrate excellent single-shot temperature accuracies (∼2% deviation from adiabatic flame temperature) and precisions (∼2% standard deviation), with simultaneous relative OH concentration measurements that demonstrate high detection sensitivity (100-300 ppm).

Two-color vibrational, femtosecond, fully resonant electronically enhanced CARS (FREE-CARS) of gas-phase nitric oxide

A resonantly enhanced, two-color, femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) approach is demonstrated and used to explore the nature of the frequency- and time-dependent signals produced by gas-phase nitric oxide (NO). Through careful selection of the input pulse wavelengths, this fully resonant electronically enhanced CARS (FREE-CARS) scheme allows rovibronic-state-resolved observation of time-dependent rovibrational wavepackets propagating on the vibrationally excited ground-state potential energy surface of this diatomic species. Despite the use of broadband, ultrafast time-resolved input pulses, high spectral resolution of gas-phase rovibronic transitions is observed in the FREE-CARS signal, dictated by the electronic dephasing timescales of these states. Analysis and computational simulation of the time-dependent spectra observed as a function of pump-Stokes and Stokes-probe delays provide insight into the rotationally resolved wavepacket motion observed on the excited-state and vibrationally excited ground-state potential energy surfaces of NO, respectively.

Femtosecond TALIF Imaging of Atomic Hydrogen in Pulsed, Non-Equilibrium Plasmas

In recent decades significant interest has been paid to investigate the applications of non-equilibrium plasma discharges to a variety of practical combustion related applications. However in order to develop a more fundamental understanding of such phenomena, additional insight into important kinetic processes is of great importance. A two-photon absorption laser-induced fluorescence (TALIF) technique is developed utilizing wide-bandwidth, short-time duration, femtosecond (fs) laser pulses. Due to its increased bandwidth and short pulse duration, fs-TALIF has reduced impact from photo-dissociation and increased signal compared to traditional nanosecond TALIF schemes. This fs-TALIF technique is used to image key atomic species within nanosecond pulsed, non-equilibrium discharges at moderate pressures. These two dimensional results provides both spatial and temporal information that can be used in more predictive plasma kinetics models. 1 Research Engineer, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Member 2 Senior Research Scientist, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Associate Fellow 3 Senior Research Scientist & CEO, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Associate Fellow 4 Research Associate, The Ohio State University, Columbus, OH 43210, AIAA Member 5 Professor, The Ohio State University, Columbus, OH 43210, AIAA Associate Fellow 6 Principal Research Chemist, Air Force Research Laboratory, WPAFB, OH 45433, AIAA Associate Fellow

Transition from stable laminar to highly unstable behavior in premixed propane/air flame with sub-breakdown electric field

The effects of chemi-ion current induced transition of a stable pre-mixed laminar flame to highly unstable behavior have been investigated using kHz repitition rate flame emission, single-shot OH PLIF, OH chemiluminescence, and gas temperature measurements. Random fluctuations of flame structures are observed when the sub-breakdown voltage pulse duration exceeds 20 milliseconds for the flame conditions with total flow velocity of 2.5 meters/sec.

Femtosecond, fully resonant electronically enhanced CARS (FREE-CARS) for simultaneous single-shot thermometry and detection of minor combustion species

Femtosecond time-resolved, fully resonant electronically enhanced coherent anti-Stokes Raman scattering (FREECARS) spectroscopy, incorporating a two-color ultraviolet excitation scheme, is used to demonstrate chemically selective and sensitive detection of gas-phase species, including nitric oxide (NO) and the hydroxyl (OH) radical. The observed time-dependent, spectrally resolved CARS signal contains rich structure that depends both on the rovibronic states accessed within the bandwidth of the initial (pump) excitation pulse and the Raman-active rovibrational levels within the vibrationally excited ground electronic state that are accessed following interaction with the second (Stokes) excitation pulse. By comparing experimental spectra to computational simulations, therefore, this approach also allows simultaneous determination of local temperature associated with the thermal distribution of initial states under singlelaser- shot conditions. For OH radical detected in a reacting flow, spectral resolution of the emitted FREE-CARS signal allows simultaneous single-shot detection of relative OH mole fraction and temperature in a laminar ethylene–air flame at 1-kHz repetition rates. By comparison to previously reported OH concentration and temperature measurements, we demonstrate excellent single-shot temperature accuracies (~2% deviation from adiabatic flame temperature) and precisions (~2% standard deviation), with simultaneous relative OH concentration measurements that demonstrate high detection sensitivity (100–300 ppm).