Yvette Zuzeek

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.

Pure rotational CARS thermometry studies of low-temperature oxidation kinetics in air and ethene–air nanosecond pulse discharge plasmas

Pure rotational CARS thermometry is used to study low-temperature plasma assisted fuel oxidation kinetics in a repetitive nanosecond pulse discharge in ethene–air at stoichiometric and fuel lean conditions at 40 Torr pressure. Air and fuel–air mixtures are excited by a burst of high-voltage nanosecond pulses (peak voltage, 20 kV; pulse duration, ~ 25 ns) at a 40 kHz pulse repetition rate and a burst repetition rate of 10 Hz. The number of pulses in the burst is varied from a few pulses to a few hundred pulses. The results are compared with the previously developed hydrocarbon–air plasma chemistry model, modified to incorporate non-empirical scaling of the nanosecond discharge pulse energy coupled to the plasma with number density, as well as one-dimensional conduction heat transfer. Experimental time-resolved temperature, determined as a function of the number of pulses in the burst, is found to agree well with the model predictions. The results demonstrate that the heating rate in fuel–air plasmas is much faster compared with air plasmas, primarily due to energy release in exothermic reactions of fuel with O atoms generated by the plasma. It is found that the initial heating rate in fuel–air plasmas is controlled by the rate of radical (primarily O atoms) generation and is nearly independent of the equivalence ratio. At long burst durations, the heating rate in lean fuel air–mixtures is significantly reduced when all fuel is oxidized.

Pure Rotational CARS Measurements of Thermal Energy Release and Ignition in Nanosecond Pulse Burst Air and Hydrogen-Air Plasmas

Pure rotational Coherent Anti-Stokes Raman Spectroscopy (CARS), complemented by UV emission and ICCD imaging, is used to study low-temperature plasma assisted fuel oxidation kinetics and ignition in a repetitive nanosecond pulse discharge in hydrogen-air at stoichiometric and fuel lean conditions at 40 Torr pressure. Air and premixed fuel-air mixtures are excited by a burst of high-voltage nanosecond pulses (peak voltage 20 kV, pulse duration ~25 nanoseconds) at a 40 kHz pulse repetition rate and burst repetition rate of 10 Hz. The number of pulses in the burst is varied from a few pulses to a few hundred pulses. The results are compared to a new hydrogen-air plasma chemistry model which incorporates nonequilibrium plasma processes, low temperature H2 – air chemistry, non-empirical scaling of nanosecond discharge pulse energy coupled to the plasma with the pulse waveform and the number density, and quasi-one-dimensional conduction heat transfer. Experimental centerline time-resolved temperature and O2 mole fraction, determined as a function of number of pulses in a burst, are found to agree well with model predictions. The results demonstrate that the heating rate in low temperature hydrogen-air plasmas is much faster than in pure air plasmas, primarily due to energy release from the exothermic reactions of fuel with O and H atoms generated in non-equilibrium quantities in the plasma. Specifically, it is found that the initial heating rate at room temperature is controlled by the low temperature processes, O + HO2 → OH + O2 and OH + H2 → H2O + H, where HO2 is formed by three-body recombination of O and H2. At intermediate temperatures, 500 – 600 K, significant chain branching, with associated additional energy release, occurs in reactions of O with HO2, as well as in O + H2 → OH + H reaction. Both chain branching and net exothermic heat release in plasma chemical reactions becomes more pronounced at higher temperatures, eventually resulting in ignition. Sensitivity analysis also shows that generation of radicals in the plasma is key to low-temperature plasma chemical fuel oxidation and associated heat release, while ignition is primarily controlled by the well known chain branching sequence O + H2 → OH + H and H + O2 → OH + O. Rapid plasma chemical hydrogen oxidation, in ϕ = 0.5 and ϕ = 1.0 mixtures, leads to a distinct maximum in temperature which is both predicted, and observed, after approximately 700 discharge pulses (17.5 msec), concurrent with a predicted and observed rapid loss in O2. UV emission and ICCD imaging provides further evidence that plasma chemical reactions lead to volumetric ignition at pressures in the approximate range 40 - 100 Torr, and equivalence ratios in the approximate range φ = 0.3 to φ = 1. Experimental ignition delay times are found to be a strong function of pressure, but a weak function of equivalence ration, general trends which are consistent with modeling predictions.

Energy Coupling and Heat Release in Air and Ethylene-Air Nanosecond Pulse Discharge Plasmas

A new analytic model of energy coupling to repetitive nanosecond pulsed discharge plasmas has been developed and shown to agree well with previous estimates from single pulse nanosecond discharges in air. The new, quasi-one dimensional model provides accurate expressions for electric field, electron density, and coupled pulse energy, including effects of dielectric surface charge accumulation, and corresponding sheath development. Primary conclusions which result directly from this new model are: (i) the pulse 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, rather than remaining constant. This occurs primarily because of plasma temperature rise, and resultant reduction of breakdown voltage, with the result that the coupled pulse energy varies approximately proportionally to the number density. The model has been validated by a series of experimental temperature measurements in air and ethylene-air discharges, where the nanosecond discharge was operated in repetitive burst mode. Experimentally determined temperature, by pure rotational Coherent Anti-Stokes Raman Spectroscopy (CARS) and N2 emission spectroscopy, as a function of number of pulses in a burst, was found to agree well with predictions of the model. It is found that heating rate in fuel-air plasmas is much faster compared to air plasmas, primarily due to energy release in exothermic reactions of fuel with O atoms generated by the plasma. The initial heating rate in fuel-air plasmas is controlled by the rate of radical (primarily O atoms) generation and is nearly independent of the equivalence ratio. At long burst durations, heating rate in lean fuel air-mixtures is significantly reduced when all fuel is oxidized.