Yurii Utkin

Active control of high-speed and high-Reynolds-number jets using plasma actuators

Localized arc filament plasma actuators are used to control an axisymmetric Mach 1.3 ideally expanded jet of 2.54 cm exit diameter and a Reynolds number based on the nozzle exit diameter of about 1.1×10 6 . Measurements of growth and decay of perturbations seeded in the flow by the actuators, laser-based planar flow visualizations, and particle imaging velocimetry measurements are used to evaluate the effects of control. Eight actuators distributed azimuthally inside the nozzle, approximately 1 mm upstream of the nozzle exit, are used to force various azimuthal modes over a large frequency range ( St DF of 0.13 to 1.3). The jet responded to the forcing over the entire range of frequencies, but the response was optimum (in terms of the development of large coherent structures and mixing enhancement) around the jet preferred Strouhal number of 0.33 ( f = 5 kHz), in good agreement with the results in the literature for low-speed and low-Reynolds-number jets. The jet (with a thin boundary layer, D /θ ∼ 250) also responded to forcing with various azimuthal modes ( m = 0 to 3 and m = ±1, ±2, ±4), again in agreement with instability analysis and experimental results in the literature for low-speed and low-Reynolds-number jets. Forcing the jet with the azimuthal mode m = ±1 at the jet preferred-mode frequency provided the maximum mixing enhancement, with a significant reduction in the jet potential core length and a significant increase in the jet centreline velocity decay rate beyond the end of the potential core.

Active Control of a Mach 0.9 Jet for Noise Mitigation Using Plasma Actuators

longer time before decaying gradually. The saturation and decay of the seeded perturbations moved farther upstream as their Strouhal number was increased. Seeded perturbations with higher azimuthal modes exhibited faster decay. Particle image velocimetry results showed that when exciting the jet’s preferred-mode instability at lower azimuthal modes, the jet potential core was shortened and the turbulent kinetic energy was increased significantly. At higher Strouhal numbers and higher azimuthal modes, forcing had less of an impact on the mean velocity and turbulent kinetic energy. Far-field acoustic results showed a significant noise increase (2 to 4 dB) when thejetisexcitedaroundthejet’spreferred-modeinstabilityStrouhalnumber(StDF 0:2–0:5),inagreementwiththe results in the literature. Noise reduction of 0.5 to over 1.0 dB is observed over a large excitation Strouhal number range; this reduction seems to peak around StDF 1:5 to 2.0 at a 30-deg angle, but around StDF 3:0 to 3.5 at a 90deg angle. Although forcing the jet with higher azimuthal modes is advantageous for noise mitigation at a 30-deg angleandlowerStrouhalnumbers,theeffectisnotasclearathigherforcingStrouhalnumbersandata90-degangle.

Development and use of localized arc filament plasma actuators for high-speed flow control

The paper discusses recent results on the development of localized arc filament plasma actuators and their use in controlling high-speed and high Reynolds number jet flows. Multiple plasma actuators (up to 8) are controlled using a custom-built 8-channel high-voltage pulsed plasma generator. The plasma generator independently controls pulse repetition rate (0–200 kHz), duty cycle and phase for each individual actuator. Current and voltage measurements demonstrated the power consumption of each actuator to be quite low (20 W at 20% duty cycle). Emission spectroscopy temperature measurements in the pulsed arc filament showed rapid temperature increase over the first 10–20 µs of arc operation, from below 1000 °C to up to about 2000 °C. At longer discharge pulse durations, 20–100 µs, the plasma temperature levels off at approximately 2000 °C.Modelling calculations using an unsteady, quasi-one-dimensional arc filament model showed that rapid localized heating in the arc filament on a microsecond time scale generates strong compression waves. The results of the calculations also suggest that flow forcing is most efficient at low actuator duty cycles, with short heating periods and sufficiently long delays between the pulses to allow for convective cooling of high-temperature filaments. The model predictions are consistent with laser sheet scattering flow visualization results and particle imaging velocimetry measurements. These measurements show large-scale coherent structure formation and considerable mixing enhancement in an ideally expanded Mach 1.3 jet forced by eight repetitively pulsed plasma actuators. The effects of forcing are most significant near the jet preferred mode frequency (ν = 5 kHz). The results also show a substantial reduction in the jet potential core length and a significant increase in the jet Mach number decay rate beyond the end of potential core, especially at low actuator duty cycles.

Ignition of Ethylene–Air and Methane–Air Flows by Low-Temperature Repetitively Pulsed Nanosecond Discharge Plasma

This paper presents results of low-temperature plasma-assisted combustion experiments in premixed ethylene-air and methane-air flows. The plasma was generated by high-voltage, nanosecond pulse duration, high repetition rate pulses. The high reduced electric field during the pulse allows efficient electronic excitation and molecular dissociation, thereby generating a pool of chemically active radical species. The low duty cycle of the repetitively pulsed discharge improves the discharge stability and helps sustain diffuse, uniform, and volume filling nonequilibrium plasma. Plasma temperature was inferred from nitrogen second positive band system emission spectra and calibrated using thermocouple measurements in preheated flows (without plasma). The experiments showed that adding fuel to the air flow considerably increases the flow temperature in the plasma, up to DeltaT = 250degC-350degC. On the other hand, adding fuel to nitrogen flow at the same flow and discharge conditions resulted in a much less pronounced plasma temperature rise, only by about DeltaT = 50degC. This shows that temperature rise in the air-fuel plasma is due to plasma chemical fuel oxidation reactions initiated by the radicals generated in the plasma. In a wide range of conditions, generating the plasma in air-fuel flows resulted in flow ignition, flameholding, and steady combustion downstream of the discharge. Plasma-assisted ignition occurred at low air plasma temperatures, 100degC-200 degC, and low discharge powers, ~100 W (~1% of heat of reaction). At these conditions, the reacted fuel fraction is up to 85%-95%. The present results suggest that the flow temperature rise caused by plasma chemical fuel oxidation results in flow ignition downstream of the plasma.

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.

Time-resolved measurements of ionization and vibration-to-electronic energy transfer in optically pumped plasmas

A method for direct measurements of electron number density, ionization rate, and the electron–ion recombination rate coefficient in optically pumped non-equilibrium plasmas has been developed. In this method, a pulsed, non-self-sustained discharge created by applying square-shaped, below breakdown voltage pulses to two electrodes placed outside the plasma (a Thomson probe) is used to remove electrons from the plasma. The electron number density is inferred for CO/Ar and CO/N2 optical mixtures with small amounts of O2 additive present. The results are compared with microwave attenuation measurements. The electron–ion recombination rate coefficients in CO/Ar/O2 and in CO/N2/O2 plasmas are β = (3–4) × 10−8 cm3 s−1 and β = (2–3) × 10−7 cm3 s−1, respectively. Time-dependent measurements of the electron concentration and vacuum ultraviolet radiation (CO fourth positive system) are used to study the mechanism of CO(A1Π) population in the optically pumped plasma. The experimental results are compared with kinetic modelling calculations. The results systematically show that the intensity of the CO fourth positive radiation closely follows the electron number density in the laser-excited plasma region after the Thomson probe voltage is turned on or off. This demonstrates that electrons play a major role in the excitation of the A1Π electronic state of CO and provides additional evidence that vibrational-to-electronic (V–E) energy transfer in plasmas sustained without external electric fields is mediated by collisions with electrons.

Studies of Chemi-Ionization and Chemiluminescence in Supersonic Flows of Combustion Products

A stable ethylene/oxygen/argon flame is sustained and nearly complete combustion is achieved in the combustion chamber of an M = 3 supersonic nozzle, at a stagnation pressure of P 0 =1 atm. Ultraviolet and visible emission is detected both from the combustion chamber and from the M = 3 flow of combustion products. Temperature in the combustor, inferred from the visible emission spectra, is To = 2000 ± 200 K. Electron density in M = 3 flow of combustion products has been measured using Thomson discharge n, = 1.4 ± 0.2·10 8 cm -3 , at an ionization fraction of n e /N = (0.65 ± 0.15) · 10 -9 . This corresponds to an electron density of n e0 = 2.2 ·10 9 cm -3 in the combustor. The chemi-ionization current measured in the M = 3 flow is found to be proportional to the equivalence ratio in the combustor. The time-resolved chemi-ionization current is in very good correlation with the visible emission from ethylene-air and propane-oxygen-argon flames in the combustor at unstable combustion conditions. The results show that nearly all electrons can be removed from the supersonic flow of combustion products by applying a moderate transverse electric field. No effect of electron removal on visible emission has been detected. A similar result was obtained for nitric oxide β bands and cyanogen violet band emission, when nitric oxide was injected into the combustion product flow.

Characterization of Localized Arc Filament Plasma Actuators Used for High-speed Flow Control 1

The paper discusses development and characterization of localized arc filament plasma actuators and their use to control high-speed and high Reynolds number jet flows. Multiple plasma actuators (up to 8) are controlled using a custom-built 8-channel high-voltage pulsed plasma generator. The plasma generator independently controls pulse repetition rate (0 to 200 kHz), duty cycle, and phase for each individual actuator. Current and voltage measurements demonstrated the power consumption of each actuator to be quite low (20 W at 20% duty cycle). Plasma power budget for 8 actuators is approximately 0.6% of the flow power. Emission spectroscopy temperature measurements in the pulsed arc filament showed rapid temperature increase over the first 10-20 µsec of arc operation, from below 1000 0 C to up to about 2000 0 C. At longer discharge pulse durations, 20-100 µsec, the plasma temperature levels off at approximately 2000 0 C. The pulsed plasma temperature measurements provide key input data for ongoing and future CFD modeling of a high-speed flow forcing by the actuators. Preliminary modeling calculations using an unsteady, quasi-one-dimensional arc filament model showed that rapid localized heating in the arc filament on a microsecond time scale generates strong compression waves. The results of calculations also suggest that flow forcing is most efficient at low actuator duty cycles, with short heating periods and sufficiently long delays between the pulses to allow for convective cooling of high-temperature filaments. The model predictions are consistent with laser sheet scattering flow visualization results and particle imaging velocimetry measurements. These measurements show large-scale coherent structure formation and considerable mixing enhancement in an ideally expanded Mach 1.3 jet forced by eight repetitively pulsed plasma actuators. The effects of forcing are most significant near the jet preferred mode frequency (ν=5 kHz). The results also show a substantial reduction in the jet potential core length and a significant increase in the jet Mach number decay rate beyond the end of potential core, especially at low actuator duty cycles.

Feedback Combustion Control Using Chemi-Ionization Probe in Supersonic Flow of Combustion Products

DOI: 10.2514/1.43809 The results of the present work demonstrate feasibility of feedback combustion control using a chemi-ionization current sensor placed in a supersonic flow of combustion products. Operated in the saturation regime, the ionization sensor (Thomson probe) removes nearly all electrons from the flow and therefore acts as a global chemi-ionization detector. The results show that a relation between the equivalence ratio in the combustor and the chemi-ionization current in the M 3 flow can be used to maintain the equivalence ratio at the desired value. The experiments have shown that using different target values of the chemi-ionization current in the feedback control program enforces transition from near-stoichiometric to fuel-lean and from fuel-lean to near-stoichiometric conditions in the combustor. The experiments have also demonstrated that the feedback control system can counter external perturbations, which either increase or decrease the equivalence ratio in the combustor and bring the fuel–oxidizer mixture composition back to the specified values. The combustion feedback control system based on a Thomson chemi-ionization probe is simple and straightforward and can be easily adapted for practical applications.

Feedback Combustion Control using Chemi-ionization Current 1

The present work demonstrates feasibility of feedback combustion control using a chemiionization current sensor placed in the combustion product flow. The experiments have been conducted in a small-scale combustor followed by a M=3 nozzle. Two electrodes placed in a supersonic flow downstream of the combustor, with a voltage bias applied to them, have been used to measure chemi-ionization current in the flow. Results of previous chemi-ionization current and flame emission measurements demonstrated that the current can be used as a flame indicator. The present experiments show that in lean fuel-oxidizer mixtures, the current is nearly proportional to the equivalence ratio. Chemi-ionization current signal from the combustion product flow have been used for feedback combustion control, to maintain the equivalence ratio in the combustor at the desired level and adjust it, if necessary. In particular, chemi-ionization current was used to control an actuator valve in the fuel delivery line and to vary the fuel mass flow rate. This approach has also been used to counter external perturbations used to deliberately change the equivalence ratio in the combustor. The results suggest that the present method can be used to operate a combustor at fuel lean conditions and to prevent flame extinction by increasing the fuel flow rate before the blow-off occurs. This approach can be used to develop a simple and straightforward combustion control technique.