Walter R. Lempert

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

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)

Characterization of a surface dielectric barrier discharge plasma sustained by repetitive nanosecond pulses

The present work discusses experimental characterization of a surface Dielectric Barrier Discharge (DBD) plasma sustained by repetitive, high-voltage, nanosecond duration pulses. The measurements have been conducted in quiescent room air. Current, voltage, instantaneous power, and coupled pulse energy in the surface DBD actuator powered by high voltage nanosecond pulses have been measured for different pulse peak voltages, pulse repetition rates, and actuator lengths. Pulse energy per unit length is controlled primarily by the pulse peak voltage and is not affected by the actuator length. The results show that the actuator can be scaled to a length of at least 1.5 m. Images of the plasma generated during the nanosecond pulse discharge development have been taken by an ICCD camera with nanosecond gate. The results show that the plasma remains fairly uniform in the initial phase of discharge development and becomes highly filamentary at a later stage. Although the negative polarity nanosecond pulse discharge generates uniform plasma at low pulse repetition rates (~100 Hz), the plasma becomes strongly filamentary as the pulse repetition rate is increased beyond ~1 kHz. Phase-locked schlieren images have been used to visualize compression waves generated by the repetitively pulsed plasma and to measure the compression wave propagation speed. Density gradient in the compression waves generated by the nanosecond pulse discharge has been inferred from the schlieren images using calibration by a pair of wedged mirrors. The results demonstrate that compression waves generated by discharge filaments have higher amplitude and higher speed, compared to those produced in a diffuse discharge. Purely rotational CARS thermometry has been used to measure the temperature in a repetitive nanosecond pulse discharge filament, stabilized by using a sharp point floating electrode. The temperature rise in the filament, inferred from the CARS measurements, approximately ΔT=40 K, is significantly lower compared to the temperature rise in the filament inferred from the UV/visible emission spectroscopy measurements at the same conditions, ΔT=350 K. Comparison of the experimental density gradient in a compression wave generated by a nanosecond pulse discharge filament with modeling calculations suggests that the temperature inferred from the emission spectroscopy is more accurate.

Energy coupling to the plasma in repetitive nanosecond pulse discharges

A new analytic quasi-one-dimensional model of energy coupling to nanosecond pulse discharge plasmas in plane-to-plane geometry has been developed. The use of a one-dimensional approach is based on images of repetitively pulsed nanosecond discharge plasmas in dry air demonstrating that the plasma remains diffuse and uniform on a nanosecond time scale over a wide range of pressures. The model provides analytic expressions for the time-dependent electric field and electron density in the plasma, electric field in the sheath, sheath boundary location, and coupled pulse energy. The analytic model predictions are in very good agreement with numerical calculations. The model demonstrates that (i) the energy coupled to the plasma during an individual nanosecond discharge pulse is controlled primarily by the capacitance of the dielectric layers and by the breakdown voltage and (ii) the pulse energy coupled to the plasma during a burst of nanosecond pulses decreases as a function of the pulse number in the burst. T...

Singlet oxygen generation in a high pressure non-self-sustained electric discharge

This paper presents results of singlet oxygen generation experiments in a high-pressure, non-self-sustained crossed discharge. The discharge consists of a high-voltage, short pulse duration, high repetition rate pulsed discharge, which produces ionization in the flow, and a low-voltage dc discharge which sustains current in a decaying plasma between the pulses. The sustainer voltage can be independently varied to maximize the energy input into electron impact excitation of singlet delta oxygen (SDO). The results demonstrate operation of a stable and diffuse crossed discharge in O2–He mixtures at static pressures of at least up to P0 = 380 Torr and sustainer discharge powers of at least up to 1200 W, achieved at P0 = 120 Torr. The reduced electric field in the positive column of the sustainer discharge varies from E/N = 0.3 × 10 −16 to 0.65 × 10 −16 Vc m 2 , which is significantly lower than E/N in self-sustained discharges and close to the theoretically predicted optimum value for O2(a 1 �) excitation. Measurements of visible emission spectra O2(b 1 � → X 3 �) in the discharge afterglow show the O2(b 1 �) concentration to increase with the sustainer discharge power and to decrease as the O2 fraction in the flow is increased. Rotational temperatures inferred from these spectra in 10% O2–90% He flows at P0 = 120 Torr and mass flow rates of ˙ m = 0.73–2.2 g s −1 are 365–465 K. SDO yield at these conditions, 1.7% to 4.4%, was inferred from the integrated intensity of the (0,0) band of the O2(a 1 � → X 3 �) infrared emission spectra calibrated using a blackbody source. The yield remains nearly constant in the discharge afterglow, up to at least 15 cm distance from the discharge. Kinetic modelling calculations using a quasi-one-dimensional nonequilibrium pulser–sustainer discharge model coupled with the Boltzmann equation for plasma electrons predict gas temperature rise in the discharge in satisfactory agreement with the experimental measurements. However, the model overpredicts the O2(a 1 �) yield by a factor of 2–2.5, which suggests that the model’s description of nonequilibrium O2–He plasma kinetics at high pressures is not quite adequate. (Some figures in this article are in colour only in the electronic version)

Low-temperature supersonic boundary layer control using repetitively pulsed magnetohydrodynamic forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short-pulse duration, high-voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization results show that as the Reynolds number increases, the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using laser differential interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0∼200–250Torr. A crossed discharge (pulser+dc sustainer) in M=3 flows of air and nitrogen produced a stable, diffuse, and uniform plasma, with the time-average dc current up to 1.0A in nitrogen and up to 0.8A in air. The electrical conductivity and the Hall parameter in these f...

Ultrahigh-frame-rate OH fluorescence imaging in turbulent flames using a burst-mode optical parametric oscillator

Burst-mode planar laser-induced fluorescence (PLIF) imaging of the OH radical is demonstrated in laminar and turbulent hydrogen-air diffusion flames with pulse repetition rates up to 50 kHz. Nearly 1 mJ/pulse at 313.526 nm is used to probe the OH P(2)(10) rotational transition in the (0,0) band of the A-X system. The UV radiation is generated by a high-speed-tunable, injection-seeded optical parametric oscillator pumped by a frequency-doubled megahertz-rate burst-mode Nd:YAG laser. Preliminary kilohertz-rate wavelength scanning of the temperature-broadened OH transition during PLIF imaging is also presented for the first time (to our knowledge), and possible strategies for spatiotemporally resolved planar OH spectroscopy are discussed.

Picosecond CARS measurements of nitrogen vibrational loading and rotational/translational temperature in non-equilibrium discharges

Picosecond Coherent Anti-Stokes Raman Spectroscopy is used to study vibrational energy loading and relaxation kinetics in nitrogen and air nsec pulsed non-equilibrium plasmas, in both plane-to-plane and pin-to-pin geometries. In 10 kHz repetitively pulsed plane-to-plane plasmas, up to ~50% of coupled discharge power is found to load vibrations, in good agreement with a master equation kinetic model. In the pin-to-pin geometry, ~33% of total discharge energy in a single pulse in air at 100 torr is found to couple directly to nitrogen vibrations by electron impact, also in good agreement with model predictions. Post-discharge, the total quanta in vibrational levels v=0-9 is found to increase by a factor of ~2 in air and by a factor of ~4 in nitrogen, respectively, a result in direct contrast to modeling results which predict the total number of quanta to be essentially constant until ultimately decaying by V-T relaxation and mass diffusion. More detailed comparison between experiment and model show that the vibrational distribution function (VDF) predicted by the model during, and directly after, the discharge pulse is in good agreement with that determined experimentally. However, for time delays exceeding ~10 μsec, the experimental VDF shows populations of vibrational levels v≥2 greatly exceeding modeling predictions, which predicts their monotonic decay due to net downward V-V transfer and corresponding increase in v=1 population. This is at variance with the experimental results, which show an increase in the populations of levels v=2 and v=3, reaching a maximum at t~50-100 μsec after the discharge pulse, and relatively steady v=4-9 populations at t~10-100 μsec. It is concluded that a collisional process is feeding high vibrational levels at a rate which is comparable to the rate at which population of the high levels is lost due to net downward V-V energy transfer. A likely candidate for the source of additional vibrational quanta is quenching of metastable electronic states of nitrogen to highly excited vibrational levels of the ground electronic state.

Repetitively Pulsed Nonequilibrium Plasmas for Magnetohydrodynamic Flow Control and Plasma-Assisted Combustion

This paper demonstrates significant potential of the use of high-voltage, nanosecond pulse duration, high pulse repetition rate discharges for aerospace applications. The present results demonstrate key advantages of these discharges: 1) stability at high pressures, high flow Mach numbers, and high-energy loadings by the sustainer discharge, 2) high-energy fractions going to ionization and molecular dissociation, and 3) targeted energy addition capability provided by independent control of the reduced electric field of the direct current sustainer discharge. These unique capabilities make possible the generation of stable, volume-filling, low-temperature plasmas and their use for high-speed flow control, nonthermal flow ignition, and gasdynamic lasers. In particular, the crossed pulsersustainerdischargewasusedformagnetohydrodynamic flowcontrolincoldM � 3 flows,providing firstevidenceof cold supersonic flow deceleration by Lorentz force. The pulsed discharge (without sustainer) was used to produce plasma chemical fuel oxidation, ignition, and flameholding in premixed hydrocarbon–air flows, in a wide range of equivalence ratios and flow velocities and at low plasma temperatures, 150–300 � C. Finally, the pulser-sustainer discharge was used to generate singlet oxygen in an electric discharge excited oxygen–iodine laser. Laser gain and output power are measured in the M � 3 supersonic cavity.

Filtered Rayleigh scattering measurements in supersonic/hypersonic facilities

Preliminary measurements are presented of flow field properties in Mach 3 and Mach 5 flows using filtered Rayleigh scattering. Filter properties have been characterized by high resolution spectroscopy in order to optimize the selection of laser frequency and filter operating conditions, as well as for the development of an accurate filter modeling program. An optimized filter is used the background suppression feature of this technique to image the boundary layer structure in a Mach 3 high Reynolds number facility and the shock structure in a Mach 5 overexpanded jet. This had been achieved using a visible laser source. By frequency scanning the laser, time-averaged velocity measurements in the Mach 3 and Mach 5 flows are made. Data acquisition at 10 torr and below indicates that this approach can be extrapolated for use in hypersonic flow facilities and is applicable as an in-flight optical air data device for hypersonic vehicles.

Low-temperature M=3 flow deceleration by Lorentz force

This paper presents results of cold magnetohydrodynamic (MHD) flow deceleration experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M=3 nitrogen and air flows in the presence of transverse direct current electric field and transverse magnetic field. MHD effect on the flow is detected from the flow static pressure measurements. Retarding Lorentz force applied to the flow produces a static pressure increase of up to 17%–20%, while accelerating force of the same magnitude results in static pressure increase of up to 5%–7%. The measured static pressure changes are compared with modeling calculations using quasi-one-dimensional MHD flow equations. Comparison of the experimental results with the modeling calculations shows that the retarding Lorentz force increases the static pressure rise produced by Joule heating of the flow, while the accelerating Lorentz force reduces the pressure rise. The effect is produced for two possible combinations of the magne...