Moon Soo Bak

On the quenching of excited electronic states of molecular nitrogen in nanosecond pulsed discharges in atmospheric pressure air

Emission measurements are carried out to study the quenching of excited electronic states of nitrogen, N2∗, in nanosecond pulsed discharges in atmospheric pressure air and nitrogen. The results reveal that ground state N2 quenches N2(C) and N2(B) at rates less than dissociative quenching by ground state O2 by a factor of 4 and 2.5, respectively. Kinetic simulations with the inferred quench rates indicate that the dissociative quenching of N2∗ by O2 is responsible for 82% of atomic oxygen production while electron-impact dissociation of O2 is for 5% under these discharge conditions.

Schlieren imaging investigation of successive laser-induced breakdowns in atmospheric-pressure air

Fast Schlieren imaging was performed to visualize the interactions between previously produced laser breakdown and a subsequent laser pulse. A pair of laser pulses was used to generate successive breakdowns in the quiescent standard air, and the interval between the pulses was varied from 50 ns to 100 μs to experimentally simulate various laser repetition rates. The incident laser energies ranged from 5 mJ to 31 mJ, and the energy absorbed by the breakdown of the second laser pulse was quantified by measuring the energies before and after the breakdown. The results indicate that the second laser pulse coupled to the background gas and produced a second laser breakdown only when the pulse interval was shorter than 250 ns or longer than 15 μs. For the shorter pulse intervals, the second breakdown occurred at the edge of the first breakdown region along the laser beam path, and its effect on the perturbation of the density field was found to be small. On the other hand, for the longer pulse intervals, the second breakdown occurred at the lens focal point, and the density field perturbations caused by the first and second breakdowns seemed to interact with each other inducing the Richtmyer-Mechkov instability. As a result, more significant turbulence in the density field was observed after successive laser pulse breakdowns than was observed following a single breakdown.

Simulations of nanosecond-pulsed dielectric barrier discharges in atmospheric pressure air

This paper describes simulations of nanosecond pulse plasma formation between planer electrodes covered by dielectric barriers in air at atmospheric pressure and 340 K. The plasma formation process starts as electrons detach from negative ions of molecular oxygen that are produced from the previous discharge pulse. An ionization front is found to form close to the positively biased electrode and then strengthens and propagates towards the grounded electrode with increasing gap voltage. Charge accumulation and secondary emission from the grounded electrode eventually lead to sheath collapse. One interesting feature is a predicted reversal in gap potential due to the accumulated charge, even when there is no reversal in applied potential. The simulation results are compared to recent measurement of mid-gap electric field under the same discharge conditions [Ito et al., Phys. Rev. Lett. 107, 065002 (2011)].

Nanosecond-Pulsed Discharge Plasma Splitting of Carbon Dioxide

This paper reports on the study of repetitive nanosecond-pulsed discharge splitting of carbon dioxide (CO2) for the production of CO. Gas chromatography is used to analyze the composition of the reformed gas when CO2 is exposed to high-voltage (15 kV) very short (10 ns) electrical discharges that deposit as much as 0.4 mJ of energy at a rate of 30 kHz. Conversion rate and energy efficiency are obtained while the discharge pressure is varied between 2.4 and 5.1 atm. At the tested conditions, the maximum conversion rate and energy efficiency are found to be 7.3% and 11.5%, respectively. The energy efficiency drops slightly with increased pressure because of the decreased electric field and electron energy per molecule. An energy balance analysis of a set of CO2 plasma reactions reveals that the dominant dissociation pathway under these conditions passes through the excitation of CO2 (10.5 eV) followed by autodissociation into CO and O, which are often in excited states.

Successive laser ablation ignition of premixed methane/air mixtures

Laser ablation has been used to study successive ignition in premixed methane/air mixtures under conditions in which the flow speed leads to flame blow-out. A range of laser pulse frequencies is experimentally mimicked by varying the time interval between two closely spaced laser pulses. Emission intensities from the laser ablation kernels are measured to qualitatively estimate laser energy coupling, and flame CH* chemiluminescence is recorded in a time-resolved manner to capture the flame evolution and propagation. A comparison of the measurements is made between the two successive breakdown ignition events. It is found that the formation of the subsequent ablation kernel is almost independent of the previous one, however, for the successive breakdowns, the first breakdown and its ensuing combustion created temporal regions of no energy coupling as they heat the gas and lower the density. Flame imaging shows that the second ablation event successfully produces another flame kernel and is capable of holding the flame-base even at pulse intervals where the second laser pulse cannot form a breakdown. This study demonstrates that successive ablation ignition can allow for the use of higher laser frequencies and enhanced flame stabilization than successive breakdown ignition.

A study of flow induced by laser induced breakdown-enhanced dielectric barrier discharges in air

The flow induced by an asymmetric dielectric barrier discharge (DBD) actuator in air together with laser induced breakdown (LIB) near the exposed electrode is investigated using particle image velocimetry. In this approach, the electrodes, driven by alternating current (8 kHz, 14 kVp-p) serve primarily to accelerate the ions generated by the laser pulse (532 nm, 15 mJ per pulse, and 2 Hz). The mean velocity fields suggest that this hybrid scheme leads to a significant enhancement in the wall-jet velocity and momentum flux generated by actuation.

Plasma Actuator Control of a Lifted Ethane Turbulent Jet Diffusion Flame

A dielectric barrier discharge (DBD) actuator is used to stabilize the base of a lifted ethane turbulent jet diffusion flame by modifying the coflow velocity field. The velocity field and flame base are measured by particle image velocimetry and unfiltered flame chemiluminescence imaging, respectively. An axisymmetric DBD actuator, integrated onto the nozzle body and driven by 8 kHz, 10-12-kV peak-to-peak sinusoidal voltage, generates the directional flow that opposes the coflow that surrounds the jet nozzle. This flow induces a separation bubble that retards and diverts the surrounding fluid further away from the fuel jet. The turbulent jet diffusion flame liftoff height is found to be significantly reduced by this plasma actuation.

Experimental and numerical studies on carbon dioxide decomposition in atmospheric electrodeless microwave plasmas

Electrodeless microwave plasmas in carbon dioxide at atmospheric pressure have been studied for carbon dioxide decomposition. Plasma optical emission spectroscopy has been conducted to measure ro-vibrational temperatures of the plasma. It is found that the temperature reaches 6200 K at the plasma center and there is little difference between the trans-rotational and vibrational temperatures. Kinetic simulations considering the trans-rotational, vibrational, and electron temperatures separately are also conducted to investigate the details of the plasma decomposition of carbon dioxide. As observed in the measurements, the kinetic simulation demonstrated that the difference between the trans-rotational and vibrational temperatures is negligible, and all the carbon dioxide within the plasma is found to be decomposed into carbon monoxide and atomic oxygen, as a result of the extremely high temperatures of the plasma. The carbon monoxide and oxygen then recombine as the temperature decreases, forming mostly ca...

Laser-induced breakdown emission in hydrocarbon fuel mixtures

Time-resolved emission measurements of laser-induced breakdown plasmas have been carried out to investigate the effect that gas species might have on the kinetics, particularly in excited states, and the resulting plasma properties. For this purpose, fuel–oxygen (O2)–carbon dioxide (CO2) mixtures with either helium (He) or nitrogen (N2) balance are prepared while maintaining their atomic compositions. The fuels tested in this study are methane (CH4), ethylene (C2H4), propane (C3H8), and butane (C4H10). The breakdown is produced in the mixtures (CH4/CO2/O2/He, C2H4/O2/He, C3H8/CO2/O2/He and C4H10/CO2/O2/He or CH4/CO2/O2/N2, C2H4/O2/N2, C3H8/CO2/O2/N2 and C4H10/CO2/O2/N2) at room conditions using the second harmonic of a Q-switched Nd:YAG laser (with pulse duration of 10 ns). The temporal evolution of plasma temperature is deduced from the ratio of two oxygen lines (777 nm and 823 nm) through Boltzmann analysis, while the evolution of electron number density is estimated based on Stark broadening of the Balmer-alpha (H α ) line at 656 nm and the measured plasma temperature. From the results, the temporal evolution of emission spectra and decay rates of atomic line-intensities are found to be almost identical between the breakdown plasma in the different mixtures given balancing gases. Furthermore, the temporal evolution of plasma temperature and electron number density are also found to be independent of the species compositions. Therefore, this behavior—of the breakdown emissions and plasma properties in the different mixtures with identical atomic composition—may be because the breakdown gases reach similar thermodynamic and physiochemical states immediately after the breakdown.

A Simulation of the Effects of Varying Repetition Rate and Pulse Width of Nanosecond Discharges on Premixed Lean Methane-Air Combustion

Two-dimensional kinetic simulation has been carried out to investigate the effects of repetition rate and pulse width of nanosecond repetitively pulsed discharges on stabilizing premixed lean methane-air combustion. The repetition rate and pulse width are varied from 10 kHz to 50 kHz and from 9 ns to 2 ns while the total power is kept constant. The lower repetition rates provide larger amounts of radicals such as O, H, and OH. However, the effect on stabilization is found to be the same for all of the tested repetition rates. The shorter pulse width is found to favor the production of species in higher electronic states, but the varying effects on stabilization are also found to be small. Our results indicate that the total deposited power is the critical element that determines the extent of stabilization over this range of discharge properties studied.