Naibo Jiang

Plasma assisted ignition and high-speed flow control: non-thermal and thermal effects

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 300K. 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, ∼10W 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 ∼1000K/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. (Some figures in this article are in colour only in the electronic version)

Molecular Tagging Velocimetry Measurements in Supersonic Microjets

The application of acetone-based molecular tagging velocimetry (MTV) is demonstrated in sonic and supersonic jets produced by a 1-mm-exit-diameter nozzle. Measurements are performed in the static pressure range 1.3‐53 mbar, with spatial resolution of approximately 10 πm. The statistical uncertainty (2ae) in velocity is found to be of order 6 ‐10 m/s, approximately independent of e owe eld pressure. Acetone laser-induced e uorescence temporal decay curves were also obtained, with 1/ e lifetime found to range from » 200 ns at 1.3 mbarto less than 50 ns at 24 mbar. These relatively short lifetimes were nonetheless sufe cient to obtain MTV data over the entire pressure range.

Low-temperature supersonic boundary layer control using repetitively pulsed magnetohydrodynamic forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short-pulse duration, high-voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization results show that as the Reynolds number increases, the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using laser differential interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0∼200–250Torr. A crossed discharge (pulser+dc sustainer) in M=3 flows of air and nitrogen produced a stable, diffuse, and uniform plasma, with the time-average dc current up to 1.0A in nitrogen and up to 0.8A in air. The electrical conductivity and the Hall parameter in these f...

Narrow-linewidth megahertz-rate pulse-burst laser for high-speed flow diagnostics

A second-generation pulse-burst laser system for high-speed flow diagnostics is described in detail. The laser can produce a burst of high-energy pulses (of the order of hundreds of millijoules per pulse) with individual pulse durations of less than 10 ns and pulse separations as short as 1 micros. A key improvement is the addition of a phase-conjugate mirror, which effectively isolates the high-intensity, short-duration pulses from the low-intensity, long-duration background illumination. It allows for more-efficient amplification and harmonic generation, with efficiencies exceeding 50% for second-harmonic and 40% for third-harmonic generation. Characteristics of the laser system, including gain narrowing, pulse-burst energy distribution, pulse narrowing, and overall pulse-burst energy, are described. In addition, the applicability of the laser for spectroscopic-based flow diagnostics is demonstrated through the presentation of megahertz-rate planar Doppler velocimetry results.

Ultrahigh-frame-rate OH fluorescence imaging in turbulent flames using a burst-mode optical parametric oscillator

Burst-mode planar laser-induced fluorescence (PLIF) imaging of the OH radical is demonstrated in laminar and turbulent hydrogen-air diffusion flames with pulse repetition rates up to 50 kHz. Nearly 1 mJ/pulse at 313.526 nm is used to probe the OH P(2)(10) rotational transition in the (0,0) band of the A-X system. The UV radiation is generated by a high-speed-tunable, injection-seeded optical parametric oscillator pumped by a frequency-doubled megahertz-rate burst-mode Nd:YAG laser. Preliminary kilohertz-rate wavelength scanning of the temperature-broadened OH transition during PLIF imaging is also presented for the first time (to our knowledge), and possible strategies for spatiotemporally resolved planar OH spectroscopy are discussed.

MHz-rate nitric oxide planar laser-induced fluorescence imaging in a Mach 10 hypersonic wind tunnel

Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging at repetition rates as high as 1 MHz is demonstrated in the NASA Langley 31 in. Mach 10 hypersonic wind tunnel. Approximately 200 time-correlated image sequences of between 10 and 20 individual frames were obtained over eight days of wind tunnel testing spanning two entries in March and September of 2009. The image sequences presented were obtained from the boundary layer of a 20° flat plate model, in which transition was induced using a variety of different shaped protuberances, including a cylinder and a triangle. The high-speed image sequences captured a variety of laminar and transitional flow phenomena, ranging from mostly laminar flow, typically at a lower Reynolds number and/or in the near wall region of the model, to highly transitional flow in which the temporal evolution and progression of characteristic streak instabilities and/or corkscrew-shaped vortices could be clearly identified.

Review of ultra-high repetition rate laser diagnostics for fluid dynamic measurements

Recent advances in ultra-high repetition rate (100?kHz and above) laser diagnostics for fluid dynamic measurements are reviewed. The development of the pulse burst laser system, which enabled several of these advances, is described. The pulse burst laser system produces high repetition rate output by slicing the output of a low power continuous wave laser and passing the resulting burst of pulses through a series of pulsed Nd:YAG amplifiers. Several systems have been built with output approaching 1.0 J/pulse over bursts of up to 100 pulses generated at between 50 and 1000?kHz. Combined with the capabilities of several types of commercially available high-speed cameras, these systems have been used to make a wide variety of high repetition rate and 3D flow measurements. Several examples of various high repetition rate laser diagnostics are described, including flow visualization, filtered Rayleigh scattering, planar Doppler velocimetry, particle image velocimetry, planar laser induced fluorescence, molecular tagging velocimetry and 3D flow visualization.

Advances in generation of high-repetition-rate burst mode laser output

It is demonstrated that the incorporation of variable pulse duration flashlamp power supplies into an Nd:YAG burst mode laser system results in very substantial increases in the realizable energy per pulse, the total pulse train length, and uniformity of the intensity envelope. As an example, trains of 20 pulses at burst frequencies of 50 and 20 kHz are demonstrated with individual pulse energy at 1064 nm of 220 and 400 mJ, respectively. Conversion efficiency to the second- (532 nm) and third- (355 nm) harmonic wavelengths of ~50% and 35-40%, respectively, is also achieved. Use of the third-harmonic output of the burst mode laser as a pump source for a simple, home built optical parametric oscillator (OPO) produces pulse trains of broadly wavelength tunable output. Sum-frequency mixing of OPO signal output at 622 nm with residual output from the 355 nm pump beam is shown to produce uniform bursts of tunable output at ~226 nm, with individual pulse energy of ~0.5 mJ. Time-correlated NO planar laser induced fluorescence (PLIF) image sequences are obtained in a Mach 3 wind tunnel at 500 kHz, representing, to our knowledge, the first demonstration of NO PLIF imaging at repetition rates exceeding tens of hertz.

Low-Temperature Supersonic Boundary Layer Control Using Repetitively Pulsed MHD Forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short pulse duration, high voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5 T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization shows that side wall boundary layers in the supersonic test section are considerably thicker near the center plane of the flow. The results also show that as the Reynolds number increases from Rex=2.7·10 5 to 8.1·10 5 , the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using Laser Differential Interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0~200-250 torr. Operation of a crossed discharge (pulser + DC sustainer) in M=3 flows of air and nitrogen demonstrated that such a discharge produces a stable, diffuse, and uniform plasma. The time-average DC current achieved in such discharges is up to 1.0 A in nitrogen (conductivity of σ=0.073 mho/m) and up to 0.8 A in air (σ=0.072 mho/m). The electrical conductivity and the Hall parameter in nitrogen and air flows are inferred from the current voltage characteristics of the sustainer discharge. LDI measurements detected MHD effect on the ionized boundary layer density fluctuations at these conditions. Retarding Lorentz force applied to M=3 nitrogen, air, and N2-He flows produces an increase of the density fluctuation intensity by up to 2 dB (about 25%), compared to the accelerating force of the same magnitude. The effect is demonstrated for two possible combinations of the magnetic field and current directions producing the same Lorentz force direction (both for accelerating and retarding force). Comparison with the LDI spectra measured with no MHD force applied showed that the effect on the density fluctuations is produced only by the retarding Lorentz force, while the Joule heat effect appears insignificant.

Development of Megahertz-Rate Planar Doppler Velocimetry for High Speed Flows

A pulse burst laser and either one or two high-speed charge-coupled-device cameras were used to perform onecomponent time-resolved planar-Doppler-velocimetry (PDV) measurements in a rectangular Mach 2.0 jet. The measurements were carried out on a streamwise plane passing through the jet centerline and covering approximately 6‐12 jet heights downstream of the jet exit. The pulse burst laser operated at 0.532-µ mw avelength and produced 28 pulses at 250 kHz with approximately 9 mJ/pulse energy. Velocity image sequences consisting of 28 frames showed dynamics of the velocity field over a time span of 108 µs (approximately 4.5 convective timescales). A typical sequence of images is presented, which demonstrates the process of entrainment of low-speed fluid into the high-speed region of the jet. Mean and standard deviation statistics of the velocity calculations produced expected trends and showed good agreement between the single- and two-camera experiments. An error analysis revealed speckle as the predominant source of noise, as in a conventional PDV technique. At a transmission ratio of 0.5, the estimated total error is 13 m/s for the single-camera experiment and 15 m/s for the two-camera experiment. I. Introduction P LANAR Doppler velocimetry is a powerful optical diagnostic technique that can be used to measure all three components of instantaneous velocity over a two-dimensional plane within a flowfield with high spatial resolution. This is accomplished by using an atomic or molecular vapor filter to measure the frequency shift of light as it is scattered by particles contained in the flowfield. The �