Igor Adamovich

Mach 5 Bow shock Control by a Nanosecond Pulse Surface DBD 1

The present work demonstrates feasibility of Mach 5 bow shock control by low-temperature, nanosecond pulse, surface dielectric barrier discharge operated both single-pulse and at a high pulse repetition rate (100 kHz). A diffuse nanosecond pulse discharge was generated in a DBD actuator on a surface of a cylinder model placed in air flow in a supersonic test section. Plasma temperature inferred from N2 emission spectra is only a few tens of degrees higher than flow stagnation temperature, T=340±30 K. Phase-locked schlieren images are used to detect compression waves generated by individual nanosecond discharge pulses near the actuator surface. The compression wave propagates upstream toward the baseline bow shock standing in front of the cylinder model. Interaction of the compression wave and the bow shock causes its displacement in the upstream direction, thereby increasing shock stand-off distance by up to 25%. The compression wave speed behind the bow shock and the perturbed bow shock velocity are inferred from the schlieren images. The effect of compression waves generated by nanosecond discharge pulses on shock stand-off distance demonstrated in a single-pulse regime (at pulse repetition rates of a few hundred Hz) and in a quasi-continuous model (at a pulse repetition rate of 100 kHz). The results demonstrate feasibility of hypersonic flow control by low-temperature, repetitive nanosecond pulse discharges.

Spatially and Temporally Resolved Atomic Oxygen Measurements in Short Pulse Discharges by Two Photon Laser Induced Fluorescence

Two Photon Laser Induced Fluorescence (TALIF) is used to measure absolute oxygen atom concentrations as a function of time in O2/He, O2/N2, and methane/air plasmas produced with a 20 nanosecond duration, 20 kV pulsed discharge. While the pulser is capable of repetition rates as high as 50 kHz, the data reported here was purposefully obtained at much lower repetition rate, 10 Hz, in order to limit the number of pulses experienced by the flowing, room temperature gas sample during its resonance time within the plasma to one, or at most two. While not directly measured, the low repetition rate insures negligible temperature rise due to direct plasma heating. Relative atomic oxygen concentration data are put on an absolute scale by means of a calibration procedure in which the observed signal level is compared to that of reference TALIF spectra of atomic xenon, obtained under known conditions of pressure and temperature, and identical optical and spectroscopic conditions. Calibrated TALIF spectra show that a single discharge pulse creates initial atomic oxygen concentrations in the range (2.0 – 3.0) x 10 14 cm -3 for air, 10% O2 in helium and 10% O2 in nitrogen mixtures. Peak atomic oxygen concentration is a factor of approximately two lower in fuel lean (φ=0.5) methane/air mixtures. In pure helium buffer, the initially formed atomic oxygen decays monotonically, with decay time consistent with formation of ozone and oxygen. In all nitrogen containing mixtures, atomic oxygen concentrations are found to initially increase, for time scales on the order of 10-100 microseconds, due presumably to additional O2 dissociation caused by collisions with electronically excited nitrogen. The long time scale decay in O2/N2 mixtures occurs on a time scale, which is similar to that of O2/He, whereas in the methane/air mixture, the decay rate is greater by a factor of approximately five.

Computational and Experimental Analysis of Mach 5 Air Flow over a Cylinder with a Nanosecond Pulse Discharge

Abstract : A computational study is performed for Mach 5 air flow over a cylinder with a dielectric barrier discharge actuator set into the cylinder surface. The actuator is pulsed at nanosecond time scales, which rapidly adds energy to the flow, thereby creating a shock wave that travels away from the pulse source. As the shock wave travels upstream, it interacts with the standing bow-shock and momentarily increases the bow-shock standoff distance. This phenomenon is also observed in phase-locked schlieren photography captured duriing the experiment. The focus of this paper is to reproduce flow phenomena observed in the experiment using high-fidelity computations in order to provide additional insight into the shock-shock interaction, the effect the dielectric barrier discharge pulse has on the surface properties of the cylinder, and develop a reduced-order phenomenological model representative of the nanosecond pulse discharge system. Experimental and high-fidelity modeling studies of the nanosecond pulse dielectric barrier discharge plasma actuators are known to operate with relatively low temperatures. This work explores the possibility that the induced compression wave is generated by rapid thermalization of the discharge which results in a local temperature rise occurring on longer time scales. Two-dimensional simulations are performed and provide many useful details about the discharge event while comparing with many measurements captured by the experiment.

Experimental Study of a Fast Ionization Wave Discharge at High Pulse Repetition Rates

The paper presents preliminary results of studies of fast ionization wave (FIW) discharges generated at high pulse repetition rates. The experiments were conducted in quiescent air in a 2 cm x 2 cm rectangular cross section discharge cell 20 cm long. The discharge cell can be modified to admit a 2 cm x 20 cm cross section flow transverse to the ionization wave propagation. The experiments demonstrated that the discharge produced by high-voltage, nanosecond pulse duration, high repetition rate pulse bursts develops as a series of repetitive fast ionization waves propagating across the entire discharge section, except at the conditions when plasma life time may considerably exceed time delay between two consecutive pulses. Wave speed and amplitude, as well as pulse energy coupled to the FIW discharge have been measured at pressures of 1 to 10 torr and pulse repetition rates of 1 to 100 kHz. The results also show that a dual polarity (+16 kV / -16 kV) FIW discharge at a pulse repetition rate of ν=10 kHz generates a uniform plasma in room air at pressures of up to P=20 torr, across distances of up to 20 cm. The results suggest that FIW discharges can be used for generating volume-filling, stable, nonequilibrium plasmas in high-speed gas flows.

Picosecond CARS Measurements of Vibrational Distribution Functions in a Nonequilibrium Mach 5 Flow

The design and implementation of a new picosecond Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy instrument for measurement of nitrogen Vibrational Distribution Function (VDF) in a highly nonequilibrium Mach 5 flow is described. First level vibrational temperatures of the order of 2000 K are achieved in the 300 Torr nonself-sustained plasma wind tunnel plenum, generated by a high E/n (300 Td) nanosecond pulsed discharge, which provides ionization, in combination with an orthogonal low E/n (~10-30 Td) DC sustainer discharge, which efficiently loads the nitrogen vibrational mode. It is also shown that operation with the nanosecond pulsed plasma alone results in significant vibrational energy loading, with Tv of the order of 1100 K. Downstream injection of CO2, NO, and H2 results in vibrational relaxation, demonstrating the ability to further tailor the vibrational energy content of the flow.

MITIGATION OF OXYGEN ATTACHMENT IN HIGH PRESSURE AIR PLASMAS BY VIBRATIONAL EXCITATION

We present a series of measurements of the temporal evolution of electron density in high pressure, room temperature pulsed e-beam generated molecular plasmas. We show that vibrational excitation, to Tvib of order 2,000 – 3000 K, results in nearly complete mitigation of direct O2 attachment, which is the principal free electron loss process under equilibrium conditions. Spatially and temporally resolved temperature measurements, performed using spectrally filtered pure rotational Raman scattering, indicate heavy species rotational/translational temperature is only slightly increased as a result of vibrational excitation, to approximately 350 K. Kinetic modeling of two limiting cases, accelerated detachment and inhibited attachment, suggests that two non-equilibrium mechanisms may be playing a role simultaneously: i), detachment enhancement by collision of O2 - (or other negative) ions with vibrationally excited neutrals, and ii), attachment inhibition due to electron heating by superelastic collisions with vibrationally excited neutrals.

Measurements of N 2 Vibrational Distribution Function in Pulsed Nanosecond Nonequilibrium Discharge by Spontaneous Raman Scattering

Spontaneous Raman spectroscopy has been used, in conjunction with a master equation kinetic model, to study vibrational energy loading and relaxation in nitrogen (P=100 torr), in a pin-topin, nanosecond pulsed electric discharge. A highly non-equilibrium vibrational distribution (up to v=12 significantly populated) was observed for time delays after the pulse ranging from 135 nsec to 10 msec. Approximately 35% of the total discharge energy was found to couple directly to the N2 vibrational mode via direct electron impact, a result found to be in good agreement with discharge model predictions. Following the discharge pulse, the total vibrational quanta in all observed levels (v=0-12) was seen to rise by a factor of approximately 70 percent, indicating further energy coupling into N2 vibrations post-discharge, consistent with previous Coherent Anti-Stokes Raman Spectroscopy (CARS) studies performed with the same discharge geometry. As a first attempt to account for this, a simple phenomenological E-V coupling model was incorporated into the master equation model. Significantly better overall agreement with the data, including the temporal evolution of the vibrational distribution function and total vibrational quanta was observed when a 30% energy defect into the N2(X,v) vibrational mode, during E-V transfer from quenching and pooling processes (involving the N2(A) and N2(C) excited electronic states), was assumed.

MHD Flow Control and Power Generation in Low-Temperature Supersonic Flows

The paper presents results of cold MHD flow deceleration and MHD power generation experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M=3 nitrogen and air flows. 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%. No discharge polarity effect on the static pressure was detected in the absence of the magnetic field. The fraction of the discharge input power going into Joule heat in nitrogen and dry air, inferred from the present experiments, is low, α=0.1, primarily because energy remains frozen in the vibrational energy mode of nitrogen. 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. This result provides first direct evidence of cold supersonic flow deceleration by Lorentz force. The experiments have also shown that at the present conditions, electric current in the MHD power generation regime is very low, of the order of 1 mA. This is entirely due to the bottleneck effect of the non-self-sustained discharge cathode layer, at the conditions when the MHD open voltage is significantly lower than the cathode voltage fall. Adding an easily ionizable species to the flow (C9H15N at 0.1% level) did not result in the flow conductivity increase, both because of seed condensation in the cold supersonic flow and a low seed fraction. Kinetic modeling calculations of the pulser-sustainer MHD discharge demonstrate that (i) low open voltages reduce MHD currents by more than two orders of magnitude, and (ii) this effect cannot be circumvented by seeding the flow at feasible levels or by using electrodes with high secondary emission coefficient. Based on the results of the present experiments and modeling calculations, the only feasible approach to extracting MHD power at low flow temperatures is to increase the MHD open voltage to the value comparable with the discharge cathode voltage fall.

Hydroxyl Radical Kinetics In Repetitively Pulsed Hydrogen-Air Nanosecond Plasmas

Absolute hydroxyl radical (OH) concentration is determined in stoichiometric hydrogen-air mixtures at P = 54-94 torr and initial temperature of T = 100°C-200°C, which are both functions of time, after the application of a single approximately 25-ns-duration approximately 20-kV discharge pulse and 60 μs after the final pulse of a variable-length burst of pulses, using single-photon laser-induced fluorescence (LIF). Relative LIF signal levels are put on an absolute number density scale by means of calibration with a standard atmospheric-pressure near-adiabatic Hencken flat-flame burner. By obtaining OH LIF data in both the plasma and the flame and correcting for differences in the collisional quenching and vibrational energy transfer rates, absolute OH number density has been determined. For a single discharge pulse, the absolute OH temporal profile is found to rise rapidly during the initial ~0.1 ms after discharge initiation and decay relatively slowly, with a characteristic time scale of ~1 ms. In repetitive burst mode, the absolute OH number density is observed to rise rapidly during the first approximately ten pulses (0.25 ms) and then level off to a near steady-state plateau. In all cases, a large secondary rise in OH number density is also observed, which is clearly indicative of ignition, with ignition time ranging from 5 to 10 ms, for initial temperatures of 100 °C and 200°C and pressures in the range of 54-94 torr. Plasma kinetic modeling predictions capture this trend quantitatively, using both a full 22-hydrogen-air-chemical-reaction set and a reduced 9-reaction set.

Scaling of an Electric Discharge Excited Oxygen-Iodine Laser

Electric discharge excited oxygen-iodine laser apparatus has been successfully scaled to increase the electric discharge volume and power, the laser mixture flow rate, and the gain path in the M=3 laser cavity. Singlet delta oxygen (SDO) generator discharge power has been increased up to at least 4.5 kW, laser mixture flow rate up to approximately 0.5 mole/sec, and gain path up to 10 cm. The steady-state run time of the new scaled-up laser apparatus at these conditions is up to 10 sec. Two different discharge configurations have been used to generate singlet delta oxygen, crossed nanosecond pulser / transverse DC sustainer discharge and capacitively coupled transverse RF discharge. Flow temperature downstream of the discharge, singlet delta oxygen yield, and laser gain have been measured in a wide range of discharge powers, nitric oxide mole fractions in the main oxygen-helium flow, and oxygen percentage in the mixture, at discharge pressures ranging from 60 to 86 torr. The results demonstrate that SDO yield increases with the discharge power for both discharge configurations, although highest yields achieved so far remain rather low, 3.6-3.7%, due to fairly low energy loading per oxygen molecule in the discharge. Small signal gain measured in the M=3 cavity of the new laser apparatus is up to 0.116%/cm (2.3% gain per double pass), at the flow temperature of T=125 K. Laser gain remains steady during operation and decreases along the cavity, although the flow temperature along the cavity remains nearly constant. Iodine vapor flow rate critically affects gain when all other discharge and flow parameters are kept the same. The optimum iodine flow rate appears to increase with the discharge power, which suggests that greater amounts of iodine vapor in the flow are needed to optimize gain at higher powers.