No abstract

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 300 K. 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, ∼10 W 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 ∼1000 K/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.

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 ͑i͒ 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 ͑ii͒ the pulse energy coupled to the plasma during a burst of nanosecond pulses decreases as a function of the pulse number in the burst. This occurs primarily because of plasma temperature rise and resultant reduction in breakdown voltage, such that the coupled pulse energy varies approximately proportionally to the number density. Analytic expression for coupled pulse energy scaling has been incorporated into the air plasma chemistry model, validated previously by comparing with atomic oxygen number density measurements in nanosecond pulse discharges. The results of kinetic modeling using the modified air plasma chemistry model are compared with time-resolved temperature measurements in a repetitively pulsed nanosecond discharge in air, by emission spectroscopy, and purely rotational coherent anti-Stokes Raman spectroscopy showing good agreement.

No abstract

Bow shock perturbations in a Mach 5 air flow, produced by low-temperature, nanosecond pulse, and surface dielectric barrier discharge (DBD), are detected by phase-locked schlieren imaging. A diffuse nanosecond pulse discharge is generated in a DBD plasma actuator on a surface of a cylinder model placed in air flow in a small scale blow-down supersonic wind tunnel. Discharge energy coupled to the actuator is 7.3–7.8 mJ/pulse. Plasma temperature inferred from nitrogen emission spectra is 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, 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 is demonstrated in a single-pulse regime (at pulse repetition rates of a few hundred Hz) and in a quasi-continuous mode (using a two-pulse sequence at a pulse repetition rate of 100 kHz). The results demonstrate feasibility of hypersonic flow control by low-temperature, repetitive nanosecond pulse discharges.