Deanna A. Lacoste

Plasma-Enhanced Combustion of a Lean Premixed Air- Propane Turbulent Flame using a Nanosecond Repetitively Pulsed Plasma

A Nanosecond Repetitively Pulsed Plasma (NRPP) generator capable of delivering an electric pulse of 10 kV during 10 ns at a frequency of up to 30 kHz has been used to stabilize and improve the efficiency of a 25 kW lean premixed propane/air flame at atmospheric pressure. We show that, when placed in the recirculation zone of the flame, the plasma significantly increases the heat release and the combustion efficiency, thus allowing to stabilize the flame under lean conditions where it would not exist without plasma. Stabilization is obtained with a very low level of plasma power of about 75 W, or 0.3 % of the maximum power of the flame. In addition, we find that at high flow rates, where the flame should normally extinguish, the NRPP allows the existence of an intermittent Vshaped flame with significant heat release, and at even higher flow rates the existence of a small dome-shaped flame confined near the electrodes that can serve as a pilot flame to reignite the combustor. Optical emission spectroscopy measurements are presented to determine the temperature of the plasma-enhanced flame and to identify the active species produced by the plasma, namely O, H and OH.

Influence of the Repetition Rate of a Nanosecond Pulsed Discharge on the Stabilization of a Turbulent Lean Premixed Flame

A Nanosecond Repetitively Pulsed Plasma (NRPP) generator capable of delivering an electric pulse of 10 kV during 10 ns at a frequency of up to 30 kHz has been used to stabilize and improve the efficiency of a 25 kW lean premixed propane/air flame at atmospheric pressure. We had previously shown that, when placed in the recirculation zone of the flame, the plasma significantly increases the heat release and the combustion efficiency, thus allowing to stabilize the flame under lean conditions where it would not exist without plasma. In the present work we study the influence of the discharge’s repetition rate on the operating regimes of the burner and on both thermal and chemical effects. We present temperature and electron number density measurements based on the Stark-broadening effect of the Hβ Balmer line in the region where the discharge is created.

Two Photon Absorption Laser Induced Fluorescence Study of Repetitively Pulsed Nanosecond Discharges in Atmospheric Pressure Air

Nanosecond repetitive discharges generated by high voltage pulses in a pin-to-pin electrode configuration in atmospheric pressure air are currently used to stabilize lean flames, for the purpose of reducing pollutant concentrations. The goal of this work is to gain an understanding of the plasma-flame stabilization mechanism. Two-photon absorption laser induced fluorescence was employed here for the measurement of atomic oxygen, that is considered to be the key species for stabilization mechanism. Time resolved measurements of the atomic oxygen number density during plasma formation and decay were performed. The hypothesis that the plasma creates reactive O via a two-step mechanism is partially demonstrated.

Measurements and Simulations of the Ionic wind Produced by a DC Corona Discharge in Air, Helium and Argon

A parametric study of the ionic wind produced by a DC corona discharge between two parallel wires in atmospheric air is presented. The experiment intends to examine the effect of the gap distance between the electrodes, the wire diameters, the material and the polarity of the electrodes, the type of gas, the voltage and the current. The current is steady for the positive discharge and pulsed (Trichel pulses) in the negative one. The state of the surface of the electrodes (but not their material) does have an effect on the ionic wind. We verified the analytic prediction that the ionic wind varies as the square root of the current. In our experimental ranges, to maximize the ionic wind, the distance must be the smallest, the electrode diameter dissymmetry the largest. A model is proposed to simulate the ionic wind for our experimental setup. The results with simplified air chemistry are in good agreement with the measurements.

Investigation of water dissociation by Nanosecond Repetitively Pulsed Discharges in superheated steam at atmospheric pressure

Nanosecond Repetitively Pulsed (NRP) discharges in atmospheric pressure water vapor at 450 K are studied with time-resolved optical emission spectroscopy (OES). A 20-ns highvoltage pulse is applied across two pin-shaped electrodes at a frequency of 10 kHz, with an energy of 2 mJ per pulse. Emission of OH(A-X) as well as atomic states of O and H are observed. The emission of these species increases during the 20-ns pulse, then decreases. Then, after about 150 ns, we observe again a strong increase of emission of these species. To determine the gas temperature, we add a small amount (1%) of molecular nitrogen to the ow of water vapor. The rotational temperature measured from N 2(C 3 u - B 2 g) second positive system of N2 is measured and compared with the rotational temperature measure with the OH(A-X) transition. The electron density is obtained by the Stark broadening of the H emission line at 486 nm. The electron number density increases to about 6 10 15 cm 3 during the pulse, then decays to 10 14 cm 3 after 150 ns. But then, a surprising behavior occurs: the Full-Width at Half-Maximum (FWHM) of the H emission line increases again sharply, with no electric eld applied, up to 5 nm, and then decays slowly to 1 nm over the next microsecond.

ION WIND EFFECTS IN A POSITIVE DC CORONA DISCHARGE IN ATMOSPHERIC PRESSURE AIR

Two types of electrical discharges in atmospheric pressure air plasmas are investigated. The first dis- charge investigated, a positive DC corona operating in ambient air, is used to accelerate an air flow using the ion wind effect. The flow velocity profile is measured by Laser Doppler Velocimetry (LDV) and is compared with the results of a simplified model. In the second part of this paper, we give a brief progress report of our studies of high voltage nanosecond discharges repetitively pulsed at frequencies of up to 30 kHz. These discharges are applied to atmospheric pressure air at temperatures in the range 300-1000 K, in order to determine the recombination of elec- trons as a function of the gas temperature.

Temporal and Spatial Evolution of OH Concentration in a Lean Premixed Propane-Air Flame Assisted by Nanosecond Repetitively Pulsed Discharges

Planar laser-induced fluorescence (LIF) has been used to study the evolution of the OH concentration in a weakly turbulent lean premixed flame both spatially and temporally, when applying Nanosecond Repetitively Pulsed (NRP) discharges. A 2-kW turbulent leanpremixed propane-air flame is stabilized by a cylindrical bluff-body. The discharge is created by voltage pulses of amplitude 7 kV, duration 10 ns, applied at a frequency of 30 kHz between two pin electrodes placed in the recirculation zone downstream of the bluffbody. The average electric power deposited by the plasma is up to 20 W, typically less than 1% of the thermal power of the flame. LIF images of the recirculation zone are recorded starting from 15 microseconds after discharge initiation. CH* chemiluminescence images of the flame are also recorded to trace the location of the flame. The results show that OH is produced in the plasma region and is then convected towards the shear layer. The key mechanism of the reduction of flame lift-off height by NRP discharges is the continuous ignition of the fresh combustible mixture in the shear layer, where it comes in contact with the active species produced by the NRP discharges.

Dissociative quenching and ultrafast heating following nanosecond repetitively pulsed discharges in air

Summary form only given. Plasma-discharges using nanosecond repetitive high voltage pulses has been very effective for the stabilization of lean flame1. To explain the chemical mechanisms of active species production by pulsed discharges, a two-step mechanism was proposed to explain the production of high densities of atomic oxygen. This mechanism first creates excited electronic states of nitrogen, which then dissociate molecular oxygen through quenching reactions2. In this paper, we investigated the temperature of the gas during and after the discharge using Optical Emission Spectroscopy on the second positive system of nitrogen, and simulated spectra from SPECAIR. The spatial profiles of excited nitrogen species densities in the discharge were determined using Abel-inverted spectra of the first and second positive system of nitrogen. The time evolution of the absolute density of N2(B) and N2(C) was also determined and the quenching rates of N2(B) and N2(C) by collisions with O 2 were found to be 2.5 (±0.5) ×10−10 cm−3.s−1 and 5.2 (±0.5) ×10−10 cm−3.s −1 at 2000 K.

Investigation of the Hydrodynamic Expansion Following a Nanosecond Repetitively Pulsed Discharge in Air

We report on an experimental study of the hydrodynamic expansion following a Nanosecond Repetitively Pulsed (NRP) discharge in atmospheric pressure air at 300 and 1000 K. The discharge is created by voltage pulses of amplitude 10 kV, duration 10 ns, applied at a frequency of 1-10 kHz between two pin electrodes. The electrical energy of each pulse is of the order of 1 mJ. We recorded single-shot schlieren images starting from 50 nanosecond after the discharge. The time-resolved images show the shock-wave propagation and the expansion of the heated gas channel. The temporal evolution of the gas temperature behind the shock front is estimated from the measured shock-wave velocity by using the Rankine-Hugoniot relations. The results show that the gas heats up by almost 1100 K within 50 ns after the pulse. This fast gas heating is consistent with a two-step mechanism involving electron-impact excitation of N 2 followed by the dissociative quenching of the excited electronic states of N 2 by O 2 .

Nanosecond Repetitively Pulsed Plasmas in Preheated Air at Atmospheric Pressure - The Diffuse Regime

Many applications for atmospheric-pressure air plasmas require non-thermal largevolume low-power plasmas with high chemical reactivity at low gas temperature. The Nanosecond Repetitively Pulsed (NRP) method can generate such non-thermal plasmas for power budgets lower than those of traditional generation methods by several orders of magnitude. A diffuse non-thermal plasma regime in air at atmospheric pressure from 3001000 K has been generated using the NRP method. This diffuse plasma develops through an initial cathode-directed streamer, followed by a return wave of potential redistribution. Furthermore, at a given gas temperature, there is a minimum gap distance required for the existence of the diffuse regime. The sequence of events observed in the formation of the diffuse plasma lead us to conclude that it is an “imminent” glow discharge. The applied electric field is maintained long enough (~10 ns) to initiate a streamer that interacts with the cathode to produce a return wave. However, the field is switched off before the development of significant ion-electron emission, which requires tens of nanoseconds. This limits the formation of the cathode fall, thus avoiding the imminent creation of a glow discharge. The non-uniform electric field generated by the pin-pin geometry can explain the minimum gap distance requirement. Using an approximate analytic expression for the Laplacian field, it can be shown that the gap distance primarily affects the field in the middle of the gap, whereas the radius of curvature of the electrodes primarily affects the field near the tips of the electrodes. Thus, the discharge gap can be divided into high-field regions near the electrodes where strong ionization occurs and a low-field weakly ionizing region between the electrodes, which can inhibit the spread of ionization instability and prevent the diffuse-tofilamentary regime transition. As the gas temperature is decreased, the field in strongly ionizing regions must be increased to maintain sufficient ionization, but the gap distance must be simultaneously increased for the weakly ionizing region to remain a buffer against the spread of the ionization instability.