Reactive species in non-equilibrium atmospheric-pressure plasmas: Generation, transport, and biological effects

Plasma assisted combustion: Dynamics and chemistry

Localized fluid flow measurements with an He-Ne laser spectrometer

A survey is given of the major developments in three-dimensional velocity field measurements using the tomographic particle image velocimetry (PIV) technique. The appearance of tomo-PIV dates back seven years from the present review (Elsinga et al 2005a 6th Int. Symp. PIV (Pasadena, CA)) and this approach has rapidly spread as a versatile, robust and accurate technique to investigate three-dimensional flows (Arroyo and Hinsch 2008 Topics in Applied Physics vol 112 ed A Schroder and C E Willert (Berlin: Springer) pp 127–54) and turbulence physics in particular. A considerable number of applications have been achieved over a wide range of flow problems, which requires the current status and capabilities of tomographic PIV to be reviewed. The fundamental aspects of the technique are discussed beginning from hardware considerations for volume illumination, imaging systems, their configurations and system calibration. The data processing aspects are of uppermost importance: image pre-processing, 3D object reconstruction and particle motion analysis are presented with their fundamental aspects along with the most advanced approaches. Reconstruction and cross-correlation algorithms, attaining higher measurement precision, spatial resolution or higher computational efficiency, are also discussed. The exploitation of 3D and time-resolved (4D) tomographic PIV data includes the evaluation of flow field pressure on the basis of the flow governing equation. The discussion also covers a-posteriori error analysis techniques. The most relevant applications of tomo-PIV in fluid mechanics are surveyed, covering experiments in air and water flows. In measurements in flow regimes from low-speed to supersonic, most emphasis is given to the complex 3D organization of turbulent coherent structures.

Tomographic PIV: principles and practice

Low-temperature plasma physics and technology are diverse and interdisciplinary fields. The plasma parameters can span many orders of magnitude and applications are found in quite different areas of daily life and industrial production. As a consequence, the trends in research, science and technology are difficult to follow and it is not easy to identify the major challenges of the field and their many sub-fields. Even for experts the road to the future is sometimes lost in the mist. Journal of Physics D: Applied Physics is addressing this need for clarity and thus providing guidance to the field by this special Review article, The 2012 Plasma Roadmap.

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The 2012 Plasma Roadmap

The paper reviews recent progress in two rapidly developing engineering applications of plasmas, plasma assisted combustion and plasma assisted high-speed flow control. Experimental and kinetic modeling results demonstrate the key role of non-thermal plasma chemistry in hydrocarbon ignition by uniform, repetitively pulsed, nanosecond pulse duration, low-temperature plasmas. Ignition delay time in premixed ethylene‐air flows excited by the plasma has been measured in a wide range of pulse repetition rates and equivalence ratios and compared with kinetic modeling calculations, showing good agreement. Comparing ignition delay time predicted by the model for plasma assisted ignition and for ignition by equilibrium heating demonstrated that chain reactions of radicals generated by the plasma reduce ignition time by up to two orders of magnitude and ignition temperature by up to 300K. These results provide additional evidence of the non-thermal nature of low-temperature plasma assisted ignition. Experiments and flow modeling show that the dominant mechanism of high-speed plasma flow control is thermal, due to heating of the flow by the plasma. Development and characterization of pulsed dc and pulsed RF localized arc filament plasma actuator arrays for control of high-speed atmospheric pressure jet flows are discussed. Actuator power is quite low, ∼10W at 10% duty cycle. Plasma emission spectra show that a greater fraction of the pulsed RF discharge power goes to heat the flow (up to 2500 ◦ C), while a significant fraction of the pulsed dc discharge power is spent on electrode and wall heating, resulting in their erosion. Rapid localized heating of the flow by the pulsed arc filaments, at a rate of ∼1000K/10 µs, results in the formation of strong compression/shock waves, detected by schlieren imaging. Effect of flow forcing by repetitively pulsed RF actuators is demonstrated in a M = 1.3 axisymmetric jet. These two case studies provide illustrative examples of isolating non-thermal (non-equilibrium plasma chemistry) and thermal (Joule heating) effects in plasmas and adapting them to develop efficient large-volume plasma igniters and high-speed flow actuators. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0963-0252/18/3/034018/pdf

Plasma assisted ignition and high-speed flow control: non-thermal and thermal effects

Xenon calibrated two photon absorption laser induced fluorescence (TALIF) is used to measure absolute atomic oxygen concentrations in air, methane–air, and ethylene–air non-equilibrium plasmas, as a function of time after initiation of a single 25 ns discharge pulse. Peak mole fraction in air at 60 torr is ∼0.5 × 10−4, with decay occurring on a time scale of ∼2 ms. Peak mole fraction in a stoichiometric methane–air mixture is found to be approximately equal to that in pure air, but the rate of decay is found to be faster by a factor of approximately two to three. In Φ = 0.5 ethylene–air, peak atomic oxygen concentration is reduced by a factor of approximately four, relative to air, and the rate of decay increased by approximately two orders of magnitude due to the greatly increased rate of reaction of atomic oxygen with ethylene, as compared to methane, at room temperature. Discharge kinetic modeling calculations, using both GRI Mech 3.0 and a more recent model of Wang et al., are shown to provide good overall agreement with all of the experimental data, as well as suggesting key processes of O atom generation and decay.

Atomic oxygen measurements in air and air/fuel nanosecond pulse discharges by two photon laser induced fluorescence

Laser induced fluorescence is used to measure absolute nitric oxide concentrations in air, methane–air and ethylene–air non-equilibrium plasmas, as a function of time after initiation of a single pulse, 20 kV peak voltage, 25 ns pulse duration discharge. A mixture of NO and nitrogen with known composition (4.18 ppm NO) is used for calibration. Peak NO density in air at 60 Torr, after a single pulse, is ~8 × 1012 cm−3 (~4.14 ppm) occurring at ~250 µs after the pulse, with decay time of ~16.5 ms. Peak NO atom mole fraction in a methane–air mixture with equivalence ratio of = 0.5 is found to be approximately equal to that in air, with approximately the same rise and decay rate. In an ethylene–air mixture (also with equivalence ratio of = 0.5), the rise and decay times are comparable to air and methane–air, but the peak NO concentration is reduced by a factor of approximately 2.5. Spontaneous emission measurements show that excited electronic states N2(C 3Π) and NO(A 2Σ) in air at P = 60 Torr decay within ~20 ns and ~1 µs, respectively. Kinetic modelling calculations incorporating air plasma kinetics complemented with the GRI Mech 3.0 hydrocarbon oxidation mechanism are compared with the experimental data using three different NO production mechanisms. It is found that NO concentration rise after the discharge pulse is much faster than predicted by Zel'dovich mechanism reactions, by two orders of magnitude, but much slower compared with reactions of electronically excited nitrogen atoms and molecules, also by two orders of magnitude. It is concluded that processes involving long lifetime (~100 µs) metastable states, such as N2(X 1Σ,v) and O2(b 1Σ), formed by quenching of the metastable N2(A 3Σ) state by ground electronic state O2, may play a dominant role in NO formation. NO decay, in all cases, is found to be dominated by the reverse Zel'dovich reaction, NO + O → N + O2, as well as by conversion into NO2 in a reaction of NO with ozone.

https://iopscience.iop.org/article/10.1088/0022-3727/42/7/075205/pdf

Nitric oxide density measurements in air and air/fuel nanosecond pulse discharges by laser induced fluorescence

The application of acetone-based molecular tagging velocimetry (MTV) is demonstrated in sonic and supersonic jets produced by a 1-mm-exit-diameter nozzle. Measurements are performed in the static pressure range 1.3-53 mbar, with spatial resolution of approximately 10 μm. The statistical uncertainty (2σ) in velocity is found to be of order 6-10 m/s, approximately independent of flowfield pressure. Acetone laser-induced fluorescence temporal decay curves were also obtained, with 1/e lifetime found to range from 200 ns at 1.3 mbar to less than 50 ns at 24 mbar. These relatively short lifetimes were nonetheless sufficient to obtain MTV data over the entire pressure range

Molecular Tagging Velocimetry Measurements in Supersonic Microjets

Recent advances in ultra-high repetition rate (100?kHz and above) laser diagnostics for fluid dynamic measurements are reviewed. The development of the pulse burst laser system, which enabled several of these advances, is described. The pulse burst laser system produces high repetition rate output by slicing the output of a low power continuous wave laser and passing the resulting burst of pulses through a series of pulsed Nd:YAG amplifiers. Several systems have been built with output approaching 1.0 J/pulse over bursts of up to 100 pulses generated at between 50 and 1000?kHz. Combined with the capabilities of several types of commercially available high-speed cameras, these systems have been used to make a wide variety of high repetition rate and 3D flow measurements. Several examples of various high repetition rate laser diagnostics are described, including flow visualization, filtered Rayleigh scattering, planar Doppler velocimetry, particle image velocimetry, planar laser induced fluorescence, molecular tagging velocimetry and 3D flow visualization.

https://iopscience.iop.org/article/10.1088/0957-0233/24/1/012002/pdf

Naibo Jiang

Papers

Plasma assisted ignition and high-speed flow control: non-thermal and thermal effects

Atomic oxygen measurements in air and air/fuel nanosecond pulse discharges by two photon laser induced fluorescence

Nitric oxide density measurements in air and air/fuel nanosecond pulse discharges by laser induced fluorescence

Molecular Tagging Velocimetry Measurements in Supersonic Microjets

Review of ultra-high repetition rate laser diagnostics for fluid dynamic measurements