Bulletin of the American Physical Society
2006 59th Annual Gaseous Electronics Conference
Tuesday–Friday, October 10–13, 2006; Columbus, Ohio
Session CT1: Plasma Aerodynamics and Propulsion I |
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Chair: J. William Rich, Ohio State University Room: Holiday Inn Salon CD |
Tuesday, October 10, 2006 10:00AM - 10:30AM |
CT1.00001: Repetitively Pulsed Nonequilibrium Plasmas for Plasma-Assisted Combustion, Flow Control, and Molecular Lasers Invited Speaker: The paper presents results of three experiments using high voltage, short pulse duration, high repetition rate discharge plasmas. High electric field during the pulse (E/N$\sim $500-1000 Td) allows efficient ionization and molecular dissociation. Between the pulses, additional energy can be coupled to the decaying plasma using a DC field set below the breakdown threshold. While the DC sustainer discharge adds 90-95{\%} of all the power to the flow, it does not produce any additional ionization. The pulser and the sustainer discharges are fully overlapped in space. Low duty cycle of the pulsed ionizer, $\sim $1/1000, allows sustaining diffuse and uniform pulser-sustainer plasmas at high pressures and power loadings. The first experiment using the pulsed discharge is ignition of premixed hydrocarbon-air flows, which occurs at low pulsed discharge powers, $\sim $100 W, and very low plasma temperatures, 100-200$^{0}$ C. The second experiment is Lorentz force acceleration of low-temperature supersonic flows. The pulsed discharge was used to generate electrical conductivity in M=3 nitrogen and air flows, while the sustainer discharge produced transverse current in the presence of magnetic field of B=1.5 T. Retarding Lorentz force applied to the flow produced a static pressure increase of up to 15-20{\%}, while accelerating force of the same magnitude resulted in static pressure rise of up to 7-8{\%}, i.e. a factor of two smaller. The third experiment is singlet delta oxygen (SDO) generation in a high-pressure pulser-sustainer discharge. SDO yield was inferred from the integrated intensity of SDO infrared emission spectra calibrated using a blackbody source. The measured yield exceeds the laser threshold yield by about a factor of three, which makes possible achieving positive gain in the laser cavity. The highest gain measured so far is 0.03{\%}/cm. [Preview Abstract] |
Tuesday, October 10, 2006 10:30AM - 11:00AM |
CT1.00002: Aerodynamic Effects in Weakly Ionized Gas: Phenomenology and Applications Invited Speaker: Successful application of gas discharges in aerodynamics requires their efficient generation, sustaining and control at supersonic or hypersonic flow conditions. Wall-free plasma formations that meet the requirements may then act as time-controlled and space-localized actuators to modify the flow. Potential candidates for this challenging task are plasmas contained in open or linear-cavity microwave field structures. We present and discuss direct observations of aerodynamic effects activated or modified by wall-free discharges. Further, we compare two generic types of wall-free discharges. First group, applicable for inlet-type structures, consists of a periodic series of microwave-induced plasmoids generated in a linear cavity, using the outgoing wave from a microwave antenna and the reflected wave from a nearby on-axis concave reflector. The plasmoids are spaced at half-wavelength separations according to the standing-wave pattern. The plasmoids are enhanced by an ``effective focusing'' in the near field of the antenna (Fresnel region) as a result of diffraction effects and mode structure. Second group, applicable to supersonic and hypersonic boundary layers, are the surface microwave discharges enhanced by a structure of Hertz dipoles. Standard microwave discharge phenomenology, such as microwave breakdown, mode structure and plasma parameters, is revisited to present a quantitative interpretation of the observed effects. Special attention is given to complex phenomena specific to flow-plasma interaction (double electric layers, ionization waves, instabilities), which provide the physical basis for localized heating in the aerodynamic flow. [Preview Abstract] |
Tuesday, October 10, 2006 11:00AM - 11:15AM |
CT1.00003: Modeling of Asymmetric Dielectric Barrier Discharge Plasma Actuators for Flow Control Sergey Macheret, Alexander Likhanskii, Mikhail Shneider, Richard Miles Asymmetric dielectric barrier discharge (DBD) plasma actuators have been demonstrated to be effective in low-speed flow control. However, understanding of their physics is currently insufficient. We have developed a comprehensive kinetic model for asymmetric DBD actuators in air. Modeling showed that charging of the dielectric during the avalanche ionization lasts a small fraction of the cycle but plays crucial role. The tangential force on the gas is shown to be directed downstream in both cathode and anode half-cycles, with critical role of negative ions in the cathode half-cycle and of positive ions in the anode half-cycle. The motion of positive ions toward the exposed electrode in the cathode half-cycle considerably decreases the integrated downstream force. Based on the detailed understanding of DBD actuator operation, an optimal voltage waveform is proposed, consisting in high repetition rate nanosecond pulses of negative voltage in combination with positive dc bias applied to the exposed electrode. Computations show that repetitive-pulse waveform can induce gas velocities similar to those in conventional sine-voltage DBD actuators at considerably lower voltages and smaller plasma sizes. Application of repetitive-pulse waveform with several kilovolt peak voltages is predicted to generate wall jet velocities at least an order of magnitude higher than those in conventional DBD actuators. [Preview Abstract] |
Tuesday, October 10, 2006 11:15AM - 11:30AM |
CT1.00004: Modeling of plasma-assisted combustion in premixed air-fuel supersonic flows Anatoly Napartovich, Igor Kochetov, Sergey Leonov Numerical model was developed combining traditional approach of thermal combustion chemistry with advanced description of the plasma kinetics based on solution of electron Boltzmann equation. This approach allows us to describe self-consistently strongly non-equilibrium electric discharge in chemically unstable gas. A comparison is made between plasma-assisted and thermal ignitions for the hydrogen/air and ethylene/air mixtures. A pseudo-one-dimensional plug flow model was developed to calculate gas flow evolution in plane duct. Numerical simulations predicted a notable reduction of the ignition length and the energy input in the discharge required for the ignition of the pre-mixed fuel. In particular, for the hydrogen/air mixture in the duct of length 80 cm, the inlet static gas temperature 700 K and static gas pressure 1 bar the minimum reduced energy input in the dc glow discharge is 150~J/g, while for the thermal ignition it is as twice as high. The numerical simulation of dc discharge-initiated combustion of a hydrogen-air mixture in a supersonic duct has shown that the effects of acceleration is not very sensitive to the fuel/oxidant ratio and gradually decreases with fuel dilution. It is shown that the ethylene/air mixture can be ignited by the glow discharge at the reduced energy input 1.6 times greater than the hydrogen/air mixture. [Preview Abstract] |
Tuesday, October 10, 2006 11:30AM - 11:45AM |
CT1.00005: MHD Flow Control and Power Generation in Low-Temperature Supersonic Flows Igor Adamovich, Munetake Nishihara 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, $\alpha $=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. [Preview Abstract] |
Tuesday, October 10, 2006 11:45AM - 12:00PM |
CT1.00006: Ignition of Gaseous and Liquid Hydrocarbon Fuels by Repetitively Pulsed, Nanosecond Pulse Duration Plasma Igor Adamovich, Ainan Bao, Yurii Utkin, Saurabh Keshav The paper presents results of plasma assisted combustion experiments in premixed hydrocarbon-air flows excited by a low-temperature, transverse, repetitively pulsed discharge plasma. The experiments have been conducted with methane, ethylene, methanol, and ethanol fuels in a wide range of equivalence ratios and flow velocities. The plasma was generated by high-voltage (16-18 kV), short pulse duration (20-30 nsec), high repetition rate (up to 50 kHz) pulses. The high reduced electric field during the pulse allows efficient electronic excitation and molecular dissociation. The low duty cycle of the discharge, $\sim $1/1000, greatly improves its stability and helps sustaining diffuse and volume filling plasma. In a wide range of conditions, generating the plasma in premixed air-fuel flows resulted in flow ignition and flameholding. Plasma assisted ignition occurred at a low discharge powers, $\sim $100 W ($\sim $1{\%} of heat of reaction), and very low flow temperatures, 100-200$^{0}$ C. The reacted fuel fraction, measured by the FTIR absorption spectroscopy, is up to 85-95{\%}. Plasma temperature was inferred from nitrogen second positive band system emission spectra and calibrated using thermocouple measurements in flows preheated by an in-line flow heater (without plasma). [Preview Abstract] |
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