Optimal Composition of the Ar‒He Mixture of Atmospheric Pressure for Production of Metastable Argon Atoms in a Nanosecond Repetitively Pulsed Discharge

Dynamic response of a weakly turbulent lean-premixed flame to nanosecond repetitively pulsed discharges

Large-Eddy Simulation of swirled flame stabilisation using NRP discharges at atmospheric pressure

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.

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.

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 .

Quantitative comparison of the benefits of cracking and nanosecond repetitive pulsed discharges on the lean blow-off, emissions, and topology of ammonia premixed swirl flames

Plasma-assisted combustion is an interesting method to promote the ignition of a lean reactive mixture instead of conventional spark plugs. Especially, Nanosecond Repetitively Pulsed (NRP) discharges technique is an energy-efficient way to initiate and control combustion processes. Ignition success or failure results from the competition between the discharge energy accumulation, the gas residence time in the discharge region, and the combustion chemistry. Plasma-assisted ignition is modeled here by a Perfectly-Stirred Reactor (PSR) whose volume represents the one surrounding the interelectrode region. NRP discharges are applied inside the reactor to initiate the combustion of the injected mixture. This model numerically identifies a plasma-assisted ignition efficiency map in terms of the amount of energy per pulse and the pulse repetition frequency, at low CPU cost. A criterion based on the residence time, interpulse time, and chemical time is introduced to predict the successful formation of a reactive kernel. This criterion is successfully validated by analyzing the plasma-assisted PSR solutions.

Numerical investigations of turbulent premixed flame ignition by a series of Nanosecond Repetitively Pulsed discharges

Summary form only given. Nanosecond Repetitively Pulsed (NRP) discharges were produced by short duration (15 ns), high-voltage (0-30 kV) pulses at Pulse Repetition Frequency (PRF) of 1-100 kHz with a FID Technologies pulser. NRP discharges were generated between two pin-electrodes separated by a gap distance of 1 cm, perpendicular to the gas flow. Two quartz windows allow optical access to the discharge zone. We first studied how PRF and applied voltage affect the regimes of operation of the NRP discharge in water vapor. The results were obtained with water vapor at a temperature of 450 K and flow velocity of 0.15 m/s. At low PRF, spark breakdown occurs for an electric field of about 30 kV/cm. The breakdown voltage decreases with the applied discharge PRF with an asymptote at about 13 kV/cm at 100 kHz. Corona breakdown occurs at about 20 kV/cm at low PRF and decreases at about 6 kV/cm at 100 kHz. There seems to be a narrow region with gap filling glow discharge, but additional measurements are needed to actually confirm the glow nature of the discharge. We investigated on the nature of the discharge with respect to the electrode distance and the water vapor temperature. We will also determine the temperature of the emitting species (OH, I) by optical emission spectroscopy. We have performed preliminary emission spectroscopy measurements in the spark regime, at about 200 Td. They show the presence of OH, O and H, thus indicating that the discharge dissociates water vapor.

This paper presents quantitative experimental data generated for the validation of plasma-assisted combustion (PAC) simulations. These data are then used to validate the phenomenological model of Castela et al. They are also useful to test other PAC models. In the experiment presented here, Nanosecond Repetitively Pulsed (NRP) discharges are applied to a lean-premixed turbulent methane-air flame initially near the lean blow-off limit. The discharges significantly enhance the combustion and stabilize the flame after a few pulses. Electrical and optical diagnostics are employed to extensively quantify the transient and steady state of the plasma-stabilization process. The flame shape is characterized by OH* chemiluminescence imaging. In the discharge region, OH density profiles are obtained by 1D laser-induced fluorescence, and the gas temperature is measured by optical emission spectroscopy measurements. The local gas temperature increases by 1250 K, and the OH number density rises sevenfold when NRP discharges are applied. These results evidence the cumulative thermal and chemical effects of NRP discharges, which are especially challenging to replicate numerically. A Large Eddy Simulation (LES) of the experiment is performed. Combustion chemistry is modeled by an analytically reduced mechanism, while the plasma discharge is described by the low-CPU cost phenomenological model of Castela et al., which aims to capture the main thermal and chemical effects induced by the discharges. The model of Castela et al. is validated in the burnt gases by the remarkable agreement between the simulations and the experiments regarding the flame shape, the local gas temperature, and the OH number density. More generally, this work demonstrates the relevance of simplified plasma models in LES solvers to simulate complex plasma-assisted burners.

Nanosecond Repetitively Pulsed (NRP) discharges are an energy-efficient and promising technique to stabilize lean premixed flames. High-performance computing is an attractive and complementary approach to experiments to understand the effects of NRP discharges on the flame stability limit. The high computational cost of the complex plasma kinetics has led to the development of the phenomenological model of Castela et al. which aims to capture the main thermal and chemical effects induced by the discharges. Although it has been successfully applied and validated in many cases of flame ignition and stabilization by NRP discharges, limitations exist, such as the possible significance of the dissociation of species other than O 2 . Recently, Blanchard et al. proposed a new phenomenological model based on physics-based analytical closures of the plasma chemical rates. This article aims to assess the impact of Blanchard’s model on numerical predictions in a 3-D configuration, the Mini-PAC bluff-body burner, by validating and comparing it against experimental data and Castela’s model. Simulations are performed by combining the low-CPU cost plasma discharge model with an analytical reduced combustion mechanism. Both discharge models are comparable and agree with the measurements in the steady-state pulsing and flame-growing regimes. Some differences can be observed in transient regimes. Nevertheless, Blanchard’s model is more detailed than Castela’s, while having a similar CPU cost.

Hydrodynamic effect of nanosecond repetitively pulsed discharges produced throughout a laminar stagnation flame

High-pressure premixed lean combustion is widely used in gas turbine engines to reduce pollutant emissions. One of the main challenges associated with this technology is the flame stabilization under lean conditions. Non-thermal plasma discharges have been extensively investigated as a way of stabilizing premixed flames in extremely-lean conditions at atmospheric pressure <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> . However, there is still a lack of understanding about the ability of this type of discharges to stabilize swirl flames at elevated pressures. This study investigates the influence of nanosecond repetitively pulsed (NRP) plasma discharges on the stability limits of premixed methane-air swirl flames at pressures up to 5 bar. The effects of two discharge regimes, NRP glows and NRP sparks, were analyzed. The voltage and the current of the discharge were measured and the influence of the discharge on the flame stabilization was assessed through direct images of the flames at 60 Hz. Carbon oxide (CO) emissions were also measured with a flue gas analyzer, with and without plasma discharges. Results demonstrated that NRP discharges improved stabilization of premixed swirl flames at elevated pressures even if the ratio of NRP discharge power to flame thermal power equal was 0.7% or less. It was also observed that the peak voltage required to maintain this very low power ratio did not increase linearly with increasing pressure. This was contrary to expectations because increasing pressure should result in a linearly decreasing reduced electric field. Moreover, the relative effectiveness of NRP glow and the NRP spark discharges to extend the flame stability limits changed by increasing pressure. Furthermore, the CO concentration in flue gases was reduced when NRP discharges were used to stabilize the flames. Improved flame stability and reduced CO emissions demonstrated the strong potential of utilizing NRP discharges in gas turbines.

Plasma enhanced auto-ignition in a sequential combustor

Characterization of plasma properties that underpin kinetic processes in nanosecond repetitively pulsed discharges (NRPDs) is necessary to understand and manipulate the behavior of these discharges for a wide variety of applications. Here, the neutral gas temperature in N2 and 50% N2/50% Ar NRPDs during the discharge is determined by characterizing the rotational temperature of rovibrational spectra from the N2 2nd positive system. At the conditions investigated, it is shown that the timescale for rotational–translational relaxation is shorter than the effective lifetime of the N2(C) state, thereby, rendering the rotational temperature measurements a reasonable representation of the background gas temperature. The measurements show that the translational temperature of ground state nitrogen molecules does not increase significantly above ambient temperature during the discharge generated at a constant pressure of 20 Torr, 10 kHz pulse repetition frequency, and pulse energy of 50 μJ. An absorption based detection technique with a 2 ns time-resolution used to measure the translational temperature history of the metastable argon atoms (Ar(1s3)) in the N2/Ar NRPDs shows that the Ar(1s3) atoms and the neutral background gas are in thermal non-equilibrium during the discharge. Furthermore, the addition of nitrogen gas is shown to significantly reduce the translational energy enhancement of the metastable argon atoms produced in the N2/Ar discharges compared to that in pure argon discharges.