Hyungrok Do

Plasma-Discharge Stabilization of Jet Diffusion Flames

The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge is found to be higher than that for the DBD. Furthermore, this pulsed discharge causes the most significant improvement in the flame stability. The optimal placement of the discharge electrodes is investigated, and it is found that there is a close relation between this placement and the emission spectra, suggesting use of the emission spectra as a possible indicator of fuel/air mixture fraction. The optimal placement is mapped into mixture-fraction space by use of a fully premixed flame experiment of known mixture fraction. The result shows that the mixture fraction, which corresponds to the optimal placement, is much leaner than that of a conventional lifted jet flame

On the role of oxygen in dielectric barrier discharge actuation of aerodynamic flows

Phase-locked particle image velocimetry is used to study the mechanism of induced flow in the near field of a rf dielectric barrier discharge actuator mounted in the separated flow region of a bluff body. Flow actuation is found to be asymmetric, with suction toward the buried downstream electrode when it is biased positively relative to the upstream exposed electrode. Lesser flow is seen on the reverse voltage swing, where the buried electrode should attract positive ions. This phenomenon is enhanced when oxygen is added to the flow, suggesting that oxygen negative ions, possibly O2−, play a dominant role in plasma actuation.

Dielectric barrier discharge control of a turbulent boundary layer in a supersonic flow

We demonstrate effective manipulation of a turbulent boundary layer at Mach 4.7 conditions using a surface dielectric barrier discharge (DBD) actuator. The freestream conditions of low static pressure (1 kPa) and temperature (60 K) are conducive to the visualization of flow features using Rayleigh scattering from condensed CO2 particles. The boundary layer thinning is observed when spanwise momentum is induced by the low power (6.8 W), low frequency (28 kHz) single actuator pair oriented parallel to the freestream flow.

Jet Flame Ignition in a Supersonic Crossflow Using a Pulsed Nonequilibrium Plasma Discharge

A short-pulse repetitive discharge is used to ignite hydrogen jet flames in supersonic crossflows. Nonequilibrium plasma is produced by repetitive pulses of 7-kV peak voltage, 20-ns pulsewidth, and 50-kHz repetition rate. Sonic or subsonic hydrogen jets are injected into a pure-oxygen supersonic free-stream flow of Mach numbers M = 1.7-2.3. The fuel injection nozzles and electrodes are mounted flush with the surface of a flat plate that is oriented to be parallel to the flow to minimize stagnation pressure losses associated with generated shock waves. A configuration combining an upstream subsonic oblique jet and a downstream sonic transverse jet serves to provide an adequate flow condition for jet flame ignition. The flow pattern and shock waves induced by the dual hydrogen jets are characterized by Schlieren imaging. Planar-laser-induced fluorescence and emission spectroscopy are employed for imaging the distribution of OH radicals. The OH fluorescence image of the region in the vicinity of the discharge confirms jet flame ignition by the plasma.

Bluff Body Flow Separation Control using Surface Dielectric Barrier Discharges

5 . In this flow regime, the separation point behind the bluff body can be moved downstream in the presence of the AC DBD. However, it is found that, as the flow speed increases, the separation delay effect decreases. The flow alteration effect also decreases with rougher surfaces and higher Rex. Finally, skewed electrodes and reversed electrodes relative to the flow direction are utilized as two different electrode configurations. It is seen that the skewed electrode provides a positive effect on separation delay while the reversed electrode induces earlier flow separation.

Ethylene Flame Dynamics and Inlet Unstart in a Model Scramjet

Ethylene flame dynamics in a model scramjet installed in an open test section of an arc-heated hypersonic wind tunnel are experimentally investigated with freestreams of Mach 4.5–9. An ethylene fuel jet is injected into the model scramjet that is autoignited by high-enthalpy flows compressed and decelerated by a train of shock waves and boundary layers developing in the scramjet isolator/combustor. The ethylene flame behavior is captured by a high-speed movie camera through optical access windows on the model and characterized by surface pressure/temperature traces recorded at five different locations along the flowpath through the model scramjet. Downstream pressure buildup due to combustion heat release, shock-induced compression, and mass loading via the fuel jet triggers the flame propagation toward upstream, stabilizes the flame at a location in the combustor, or pushes the flame further upstream to cause inlet unstart. In particular, flame dynamics while the scramjet undergoes the inlet unstart with...

The manipulation of an unstarting supersonic flow by plasma actuator

The manipulation of an unstarting supersonic flow is demonstrated using a dielectric barrier discharge (DBD). Experiments are carried out in a Mach 4.7 model inlet flow. Flow features, such as boundary layers and shockwaves at low freestream static pressure (1 kPa) and temperature (60 K) are visualized with Rayleigh scattering from condensed CO2 particles. Flow unstart, initiated by mass injection, is studied for three model inlet flow configurations, distinguished by the initial conditions (untripped or tripped, plasma actuated or not) of the boundary layers. Unstart in the presence of thick, tripped boundary layers is characterized by the formation of an oblique unstart shock just upstream of a separating and propagating boundary layer. The presence of plasma actuation of this tripped boundary layer seems to arrest the boundary layer separation and leads to the formation of a quasi-stationary pseudo-shock, delaying unstart. The flow generated with DBD actuation is more characteristic of what is seen when unstart is generated in a model flow in which thin boundary layers grow naturally. Planar laser Rayleigh scattering visualizations suggest that the DBD actuation thins the tripped boundary layer over the exposed electrode region.

Cross-talk in multiple dielectric barrier discharge actuators

Phase locked particle image velocimetry (PIV) is used to study flow alteration by dielectric barrier discharge actuators mounted on a bluff body. Images from these PIV experiments confirm the importance of negative ions in the actuation process and demonstrate cross-talk interactions between adjacent actuators.

A Study of Plasma-Stabilized Diffusion Flames at Elevated Ambient Temperatures

We report on a study of the use of repetitive ultrashort pulsed plasma discharges in stabilizing a lifted methane jet diffusion flame in elevated temperature (855 K-975 K) vitiated coflow. CH chemiluminescence images are used to record the flame liftoff height, which serves as a measure of the flame stability. The results show that, for the same reduced electric field (E /n), the stabilizing ability of the discharge in the investigated temperature range diminishes with increasing coflow temperature. Based on the results of a zero-dimensional transient chemistry simulation, it is conjectured that the reduced propensity for stabilization at elevated temperature is caused by the rapid depletion of H2 and CO formed in the postplasma gas, under high ambient temperature conditions.

Development and testing of the ACT-1 experimental facility for hypersonic combustion research

A new pulsed-arc-heated hypersonic wind tunnel facility, designated as ACT-1 (Arc-heated Combustion Test-rig 1), has been developed and built at the University of Notre Dame in collaboration with the University of Illinois at Urbana-Champaign and Alta S.p.A. The aim of the design is to provide a suitable test platform for experimental studies on supersonic and hypersonic turbulent combustion phenomena. ACT-1 is composed of a high temperature gas-generator system and a model scramjet combustor that is installed in an open-type vacuum test section of the wind tunnel facility. The gas-generator is designed to produce high-enthalpy (stagnation temperature = 2000 K–3500 K) hypersonic flows for a run time up to 1 s. The supersonic combustor section is composed of a compression ramp (scramjet inlet), an internal flow channel of constant cross-section, a fuel jet nozzle, and a flame holder (wall cavity). The facility allows three-way optical accesses (top and sides) into the supersonic combustor to enable various advanced optical and laser diagnostics. In particular, planar laser Rayleigh scattering (PLRS), high-speed schlieren imaging and OH-planar laser induced fluorescence (OH-PLIF) have successfully been implemented to visualize the turbulent flows and flame structures at high speed flight conditions.