Nanosecond Repetitively Pulsed Discharges Research Articles

BOATS: Bayesian Optimization for Active control of ThermoacousticS

This investigation presents novel adaptive control algorithms specifically designed to address and mitigate thermoacoustic instabilities. Gas turbines are limited in their operational range due to thermoacoustic instability. Two control strategies are available to alleviate this issue: active and passive. Active control strategies have a wider flexibility than passive control strategies because they can adapt to the operating conditions of the gas turbine. However, optimizing the control parameters remains a challenge, especially if additional constraints have to be fulfilled, such as e.g. pollutant emission levels. To address this issue, we propose three adaptive control strategies based on Bayesian optimization. The first and foundational algorithm is the safeOpt algorithm, and the two adaptations that have been made are stageOpt and shrinkAlgo. The algorithms facilitate safe exploration within the control parameter space, ensuring compliance with the constraint function, while simultaneously optimizing the objective function. The Gaussian Process Regressor (GPR) is employed to approximate both the objective and constraint functions, with continuous updates occurring during iterations. The algorithms also enable the transfer of knowledge obtained from one operating point to another, thereby reducing the number of iterations needed to reach the optimal point. We demonstrate the effectiveness of the algorithms both numerically and through two distinct experimental validations. In the numerical demonstration, we employ a low-order thermoacoustic network model to simulate a single-stage combustor setup equipped with loudspeaker actuation and a gain-delay (n−τ) controller for active stabilization. In the first experimental validation, we optimize the control parameters of a single-stage turbulent combustor with loudspeaker actuation and a gain-delay controller. For the second experimental validation, we apply the framework to a sequential combustor configuration utilizing nanosecond repetitively pulsed discharges (NRPD) as the control actuator. This demonstrates the framework’s adaptability to various control actuation methods in turbulent combustors where control parameter optimization is required.

Dynamic response of nanosecond repetitively pulsed discharges to combustion dynamics: regime transitions driven by flame oscillations

When using nanosecond repetitively pulsed discharges to actuate on dynamic combustion instabilities, the environment the discharge is created in is unsteady and changing on the timescale of the combustion processes. As a result, individual discharge pulses are triggered in a background gas that evolves at the timescale of combustion dynamics, and pulse-to-pulse variations may be observed during the instability cycle. Prior work has studied nanosecond pulsed discharges in pin-to-ring configurations used to control instabilities in lean-operating swirl-stabilized combustors, and observed variable discharge behavior. The focus of this work is on characterizing how the pulse-to-pulse discharge morphology, energy deposition, and actuation authority, evolve during the combustion instability cycle. This has important implications for designing effective plasma-assisted combustion control schemes. The discharge is observed in two distinct modes, a streamer corona and a nanosecond spark, with the occurrence of each regime directly linked to the phase of the combustor instability. Variation of pulse repetition frequency affects the total fraction of pulses in each mode, while variation of voltage affects the onset of the nanosecond spark mode. The transitions are described in terms of ratios of the relevant combustion and plasma timescales and the implications of this coupled interaction on the design of an effective control scheme is discussed.

Heat Release in a Nanosecond Repetitively Pulsed Discharge in an Ar–He Mixture at the Atmospheric Pressure

Heat Release in a Nanosecond Repetitively Pulsed Discharge in an Ar–He Mixture at the Atmospheric Pressure

Numerical study of nitrogen oxides chemistry during plasma assisted combustion in a sequential combustor

Plasma Assisted Combustion (PAC) is a promising technology to enhance the combustion of lean mixtures prone to instabilities and flame blow-off. Although many PAC experiments demonstrated combustion enhancement, several studies report an increase in NOx emissions. The aim of this study is to determine the kinetic pathways leading to NOx formation in the second stage of a sequential combustor assisted by Nanosecond Repetitively Pulsed Discharges (NRPDs). For this purpose, Large Eddy Simulation (LES) associated with an accurate description of the combustion/NOx chemistry and a phenomenological model of the plasma kinetics is used. Detailed kinetics 0-Dimensional reactors complement the study. First, the LES setup is validated by comparison with experiments. Then, the NOx chemistry is analyzed. For the conditions of operation studied, it is shown that the production of atomic nitrogen in the plasma by direct electron impact on nitrogen molecules increases the formation of NO. Then, the NO molecules are transported through the turbulent flame without being strongly affected. This study illustrates the need to limit the diatomic nitrogen dissociation process in order to mitigate harmful emissions. More generally, the very good agreement with experimental measurements demonstrates the capability of LES combined with accurate models to predict the NRPD effects on both turbulent combustion and NOx emissions.

Enhancement of lean blowout limits of swirl stabilized NH3-CH4-Air flames using nanosecond repetitively pulsed discharges at elevated pressures

This study experimentally examines the impact of nanosecond repetitively pulsed (NRP) discharges on lean premixed methane-ammonia-air swirl flames at pressures up to 4 bar. The results reveal the extension of flame blowout limit by 10–12 % under atmospheric conditions and 5–6 % at 4 bar, when NRP discharges are applied. This decrease in the plasma effect could be attributed to the detrimental impact of strong shock waves at high pressure, destabilizing the flame. NH3-CH4-air flames shows higher lean blowout limits in comparison to CH4-air flames. NRP discharges exhibit consistent impact on the lean blowout limit enhancement across ammonia fractions (20–60 %), irrespective of pressure conditions. These results are obtained for a ratio of NRP discharge power to flame thermal power of 0.63 % and constant pulse repetition frequency of 30 kHz.

A numerical investigation of plasma-assisted ignition by a burst of nanosecond repetitively pulsed discharges

Nanosecond repetitive pulse discharges (NRP) are a promising technology to promote the ignition of a lean reactive mixture. The success or failure of ignition results from complex interaction phenomena between the plasma generated by the discharge and the properties of the turbulent flow. This work presents two approaches for modeling plasma-assisted ignition scenarios resulting from NRP discharges. The first consists of a low-order model based on dimensionless analysis, while the second involves a Large Eddy Simulation (LES) of the reactive turbulent flow that requires high-performance computing. Both methods are presented and successfully applied to an experimental setup, which was specifically designed to study the influence of flow operating conditions and electrical discharge properties on plasma-assisted flame ignition.The simple dimensionless analysis can identify coupled and decoupled regimes, which always lead to ignition success or failure, respectively. However, the model is too crude to predict the fine interactions between plasma kinetics, combustion chemistry, and flow dynamics that control the formation of the reactive core in partially coupled regimes. The LES provides a means to capture these complex interaction mechanisms. A quantitative analysis of the reactive kernel growth has been performed by numerically reconstructing the OH-PLIF signal measured by the camera. A very good agreement is observed, validating the numerical strategy for plasma assisted combustion.

Thermoacoustic stabilization of a sequential combustor with ultra-low-power nanosecond repetitively pulsed discharges

This study demonstrates the stabilization of a sequential combustor with Nanosecond Repetitively Pulsed Discharges (NRPD). Sequential combustors offer advantages such as enhanced fuel flexibility and broader operational range compared to the conventional combustors. However, thermoacoustic instabilities limit their potential. While passive control strategies like Helmholtz dampers have been utilized, active control methods have been hindered by the lack of robust actuators for harsh conditions with sufficient control authority. This study demonstrates successful suppression of instabilities using NRPD in a lab-scale atmospheric sequential combustor, even at low plasma power (1.1 W, about 1.5×10−3 percent of thermal power of 73.4 kW). We also examine the effect of pulse repetition frequency and plasma generator voltage on NO emissions and the sequential flame topology. Notably, higher plasma power can further enhance suppression, albeit with a slight increase in NO emissions. However, the highest plasma power in our study (81 W, 1.1×10−1 percent of flame thermal power) shows a slight improvement in acoustic suppression while significantly elevating NO emissions. Intriguingly, specific plasma parameter combinations can excite another acoustic mode, necessitating further exploration for optimized control. This pioneering study highlights the potential of ultra-low-power plasma-based thermoacoustic control in sequential combustors, paving the way for enhanced stability and performance.

Ignition detection with the breakdown voltage measurement during nanosecond repetitively pulsed discharges

Nanosecond Repetitively Pulsed Discharge (NRPD) is a promising ignition concept for introducing diesel-like process parameters for hard-to-ignite renewable fuels in Spark Ignition (SI) engines. Knowing whether an ignition event initiated by a series of nanosecond electrical discharges was successful or not gives the possibility of using this information for closed-loop ignition control. This paper presents a methodology for detecting successful ignition under NRPD ignition.After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse is proportional to the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. Lower breakdown voltages of the control pulse are present if a robust early flame kernel is present. The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to place additional ignition events, if necessary.This technique was experimentally analyzed in a Constant Volume ignition Cell (CVC) and in a Rapid Compression Expansion Machine (RCEM). In the CVC at the ignitability limit, lower breakdown voltages of the control pulse are mostly measured when no pressure rise is measured. In the RCEM, the heat release rate is analyzed with a two-zone thermodynamic model, and the early flame kernel formation is monitored with Schlieren imaging. Some overlap exists in the control pulses' breakdown voltages for the ignition and quenching experiments; nevertheless, the Schlieren videos outline that the overlapping cases have a similar flame kernel formation, and the difference arises thereafter.

Efficiency analysis of ignition by Nanosecond Repetitively Pulsed discharges using a low-order model

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.

Nanosecond repetitively pulsed discharges and conventional sparks of ammonia-air mixtures in a fan-stirred cruciform burner: Flammability limits and ignition transition

Ammonia, an efficient hydrogen carrier, is crucial for achieving net-zero emissions. However, its low reactivity, as manifested through small unstretched laminar flame speed (SL,0 ∼ 7 cm/s) and large laminar flame thickness (δL ∼ 2.85 mm) at atmospheric pressure and stoichiometric conditions together with narrow flammability limits, makes it difficult for initiation. As such, how to measure accurately ammonia minimum ignition energies under laminar and turbulent conditions (MIEL and MIET) and identify ammonia flammability limits are important to understand fundamental challenges and restricted usage for practical applications. We apply both nanosecond-repetitively-pulsed-discharges (NRPD) and conventional sparks (CS) via the same stainless-steel electrodes of 1-mm diameter with sharp ends at a fixed gap of 2 mm in the same fan-stirred cruciform burner to identify flammability limits of ammonia/air mixtures for spherical flame initiation. The burner is capable of generating near-isotropic turbulence having roughly equal magnitudes of r.m.s. turbulent fluctuating velocities in all three directions (u′) with negligible mean velocities. We find that NRPD operated at a pulse repetition frequency of 40 kHz can promote ignition or decrease the MIE on fuel lean and fuel rich sides as compared to that of CS. However, even using 2,000 pulses with a total ignition energy of 4.4 J for NRPD, no self-sustained flame propagation can be observed at ϕ = 0.65 and/or ϕ = 1.44 that marks lean and/or rich flammability limits for spherical flame initiation. Moreover, we also find a turbulent ignition transition for the stoichiometric ammonia/air mixture, of which the increasing slopes of MIET/MIEL versus u′/SL change drastically from gradually to exponentially at a critical value of (u′/SL)c ≈ 13 for both NRPD and CS. Finally, these results should be useful for future practical applications of premixed ammonia/air combustion.

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

In this work, the use of Nanosecond Repetitively Pulsed (NRP) discharges is investigated numerically to improve the stabilisation of the flame in a swirl burner. The studied configuration is the PACCI burner test rig set up at KAUST, in which it was experimentally demonstrated that NRP discharges can effectively enhance flame stability. Simulations are performed with the reactive compressible Navier–Stokes solver AVBP, using a recently developed phenomenological model for plasma-assisted combustion of methane-air premixed flame. The ability of the numerical model to reproduce the main flow behaviours is first assessed by comparison with PIV measurements in cold flow conditions. Then, a lean atmospheric pressure case (ϕ=0.67) without plasma actuation is simulated and compared with OH* chemiluminescence image from experiment. Both numerical and experimental results reveal an unstable turbulent flame, with intermittent attachment to the burner exit. Finally, two operating conditions with NRP discharges are simulated. The NRP discharges correspond to 10 kV pulses applied at a frequency of 20 and 30 kHz, respectively corresponding to a discharge power 0.72 and 1.17% of the thermal flame power. First, a significant stabilisation effect is observed when applying the discharges, which efficiently mitigate the flame lifting. This effect is quantitatively demonstrated by tracking the flame centre of gravity, which shows that the flame moves upstream under the influence of the plasma, in good agreement with experimental observations. Results also show that a net power gain is obtained by applying NRP discharges to combustion by comparing the thermal flame power to the plasma power. In particular, an increase of plasma actuation efficiency from 4 to 6 as been observed in the cases studied by increasing the plasma power.

Combustion performance of plasma-stabilized lean flames in a gas turbine model combustor

This work presents an experimental study of the stabilization of lean methane-air flames by nanosecond repetitively pulsed (NRP) discharges. The experimental facility consists of a gas turbine model combustor with a Lean-Premixed-Prevaporized injector. The working pressure is 1 atm. The fuel is injected through two stages, each stabilized by swirl. The main stage consists of a multipoint annular injection. This facility is representative of a single sector of a gas turbine combustion chamber. The NRP discharges significantly extend the lean blow-off limit for a wide range of operating conditions, down to an equivalence ratio of 0.16. Lean flame stabilization is demonstrated for flame thermal powers up to 100 kW, with an electric power of less than 0.2% of the flame thermal power. We also observe plasma-assisted lean flames emiting less NOX than the leanest stable flames without plasma. Finally, by exploring the application of various pulse patterns instead of applying the discharges continuously at a constant repetition frequency, the plasma-to-flame power ratio required to stabilize a lean flame is decreased to 0.06% and the pollutant emissions can be further decreased.

Nanosecond plasma actuation by a bending actuator mounted on a sharp edge in quiescent air

An experimental investigation of the actuation characteristics of a repetitive nanosecond pulsed dielectric barrier discharge from bending plasma actuators mounted on a sharp edge is conducted. Four bending actuators with different bending angles are tested and compared with a planar actuator in quiescent air using schlieren imaging and electrical measurements. The results show that when fed by the same pulse, the plasma morphology, current waveform, and energy consumption of the bending and planar actuators are very similar. However, the thermal perturbations and induced flows differ significantly. In this experiment, at a low load voltage (V = 10–14 kV), when the plasma discharge is in the diffuse mode, the bending actuator can induce a stronger vortex and near-wall jet than the planar actuator, and as the bending angle decreases, the strength of the induced jet increases rapidly. At a higher load voltage (V = 18–20 kV), when the discharge is in the constricted mode, the plasma filaments produce hot plumes with both the bending and planar actuators, but the hot plumes from the bending actuator are injected into the air with a larger incidence. During the streamer-to-filament transition (V = 16 kV), induced flows from small-bending-angle (30° and 60°) actuators are characterized by a thickened near-wall jet, while those from the planar actuator are characterized by hot plumes.

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

Plasma-assisted combustion has the potential to improve flame stability and combustion performance. Among all the different types of plasma source, nanosecond repetitively pulsed (NRP) discharges have received a lot of attention since they can produce nonequilibrium plasma containing excited species, ionized species, and radicals. These species can then accelerate combustion processes by opening new kinetic pathways. NRP discharges can also affect combustion through thermal and hydrodynamic pathways. This study investigates the hydrodynamic effect of NRP discharges produced throughout the surface of a lean premixed methane-air flame at atmospheric pressure. Time-resolved imaging was used to study the effect of the NRP discharges pulse repetition frequency (PRF). The imaging showed that the flame stabilizes further upstream as the PRF increases and that the flame remains steady and does not relax between consecutive discharges. The flame relaxation was also studied using time-resolved imaging after abruptly turning off the NRP discharges. This revealed that the flame takes about 50ms to completely relax to its unactuated position. Particle image velocimetry showed that the NRP discharges substantially reduce the flow velocity in their vicinity, which causes the flame to move upstream and stretch. The induced stretch was found to increase flame speed though nonequidiffusive effects. The experiment was repeated with a lean premixed propane-air flame. In those conditions, the flame speed was unaffected by the NRP discharges, as expected for a mixture that is less sensitive to nonequidiffusive effects. Hence, the hydrodynamic effect of NRP discharges appears to dominate over the thermal and kinetic pathways for the conditions investigated.

Streamer dynamics and periodical discharge regime transitions under repetitive nanosecond pulses with airflow

Although the nanosecond repetitively pulsed (NRP) discharge normally stabilizes into one of three regimes (corona/glow/spark) in a pulse train, another nonintuitive instability recently proved that it could periodically swing between corona and spark regimes characterized by repeated spark quenches and reestablishments (Zhao et al 2022 Plasma Sources Sci. Technol. 31 045005). In this paper, we have further investigated the suitability of NRP discharge regime transitions for different pulsed power supplies and revealed dramatic effects of the gas flow on streamer dynamics that possibly lead to spark quenches. Pulse-sequence and temporally resolved electrical and optical diagnostics were implemented to capture discharge evolutions in long pulse trains. Periodical discharge regime transitions under long-term repetitive nanosecond pulses are prevalent under a transmission line transformer pulser and a commercially available FID pulser with parameter constraints. A minimum deposited energy per spark is required for the successive spark pattern. The spark channel before its quench statistically prefers to deviate upstream rather than following the straight axis or intuitively bending downstream to search for more remnants. Before spark quenches, the initial streamer already either exhibits a large radial ‘detour’ or propagates with a zig-zag profile along the periphery of previous spark regions. The periodical discharge regime transition and effects of the gas flow are qualitatively explained based on the plasma–source coupling, evolutions of dominant negative ion composition, and 3D streamer simulation. Periodical NRP spark quenches are probably initiated with the streamer ‘detour’ and then accelerated by the thermal-ionization feedback instability. Inhomogeneous residual charge distribution and accumulations of complex negative ions with high electron bound energies may facilitate the following discharge to search for the gas inlet. In-depth understanding of NRP discharge instabilities could be reached, which are fundamentally governed by residual charge transport and energy relaxation.

Plasma thermal-chemical instability of low-temperature dimethyl ether oxidation in a nanosecond-pulsed dielectric barrier discharge

Plasma stability in reactive mixtures is critical for various applications from plasma-assisted combustion to gas conversion. To generate stable and uniform plasmas and control the transition towards filamentation, the underlying physics and chemistry need a further look. This work investigates the plasma thermal-chemical instability triggered by dimethyl-ether (DME) low-temperature oxidation in a repetitive nanosecond pulsed dielectric barrier discharge. First, a plasma-combustion kinetic mechanism of DME/air is developed and validated using temperature and ignition delay time measurements in quasi-uniform plasmas. Then the multi-stage dynamics of thermal-chemical instability is experimentally explored: the DME/air discharge was initially uniform, then contracted to filaments, and finally became uniform again before ignition. By performing chemistry modeling and analyzing the local thermal balance, it is found that such nonlinear development of the thermal-chemical instability is controlled by the competition between plasma-enhanced low-temperature heat release and the increasing thermal diffusion at higher temperature. Further thermal-chemical mode analysis identifies the chemical origin of this instability as DME low-temperature chemistry. This work connects experiment measurements with theoretical analysis of plasma thermal-chemical instability and sheds light on future chemical control of the plasma uniformity.

Study of Flame Kernel Development at High EGR and High Flow Speed Using Conventional Spark Igniter and Non-Thermal Plasma Under Gasoline Engine Relevant Conditions

ABSTRACT As a novel ignition approach, nanosecond repetitively pulsed discharge (NRPD)-based non-thermal plasma offers significant benefits such as low energy consumption, and lean operating conditions. However, there is no investigation conducted on the early flame kernel formation and development induced by non-thermal plasma. Therefore, in this paper, experiments of conventional spark and non-thermal plasma ignition systems in constant volume combustion chamber (CVCC) are conducted under gasoline engine relevant conditions: wide initial ambient pressures (6.5, 8.3, 11.3 bar), a range of equivalence ratios (0.7–1.0), EGR rates (10−25%), and cross flow speeds (0–30 m/s). The discharge energy of non-thermal plasma is of around 210 mJ, while a single spark event can generate energy from 65 mJ to 83 mJ depending on ambient conditions. A consecutive spark strategy is adopted to guarantee comparable total input energy to non-thermal plasma. The ignition delay and combustion phase obtained from the chamber pressure history are calculated. In the meanwhile, flame kernel radius, flame propagation rate, and flame front length ratio via schlieren images are quantified and analyzed. Results showed that the flame initiated by non-thermal plasma can maintain a robust flame kernel and propagates fast. Under high-speed cross flow and high EGR rate conditions, non-thermal plasma can maintain a robust flame kernel at early stage supporting the initial flame kernel to survive and improve its ignition probability. Considering the effects of the EGR ratio and high-speed cross flow on flame kernel development, non-thermal plasma ignition can efficiently enhance the flame propagation and ignition probability. It is also concluded that non-thermal plasma can successfully ignite under lean (equivalence ratio between 0.7–1.0), high-diluted mixture (25% EGR), and high-speed cross flow of 30 m/s within the range of current study.

Effect of plasma-flow coupling on the ignition enhancement with non-equilibrium plasma in a sequential combustor

The effect of the regime of nanosecond repetitively pulsed discharges (NRPDs) on ignition and stabilization of a natural-gas/hydrogen/air flame in the sequential stage of a lab-scale atmospheric pressure sequential combustor is investigated experimentally. Electrical parameters of the NRPDs are characterized by measuring voltage, current, and deposited energy. Fast gas heating (FGH) of the nanosecond discharges is measured in a single pulse regime and validated by means of 0D kinetic modelling. It was found that the conventional scheme for energy release from internal degrees of freedom adequately describes the dynamics of FGH in vitiated hot environment diluted with air. Short-gated ICCD imaging and spatially-resolved emission spectroscopy are used to identify the coupling between the NRPDs and the vitiated hot flow. The effectiveness of the NRPDs actuation is assessed through the chemiluminescence images of the sequential flame. The distance of the center of gravity of the sequential flame to the outlet of the mixing channel is evaluated, with and without plasma actuation. The effect of fuel reactivity on plasma effectiveness is also studied by varying the fraction of hydrogen in the fuel blend of the second stage of the combustor. The results show that the glow NRPDs regime allows strengthening the flame anchoring for the most reactive blends considered in this work, while the spark NRPDs is required for the ignition and prevention of lean blow-out of the flame for the least reactive fuel blends which exhibit low fractions of hydrogen.

Investigation of the impact of NRP discharge frequency on the ignition of a lean methane-air mixture using fully coupled plasma-combustion numerical simulations

The ignition of an atmospheric pressure laminar premixed methane/air mixture by Nanosecond Repetitively Pulsed (NRP) discharges in a pin-pin configuration is studied using fully coupled plasma-combustion numerical simulations. These simulations are performed using the AVIP code specifically developed for low temperature plasma modeling and coupled to the combustion code AVBP. A reduced chemical scheme for plasma-assisted combustion previously derived and validated is used to investigate the effect of the frequency of NRP discharges and the benefits of their chemical enhancement. It is observed that the induced shock wave produced by strong discharges is of major importance for ignition and can lead to quenching of the ignition kernels through strong induced recirculation of gases. Increasing the frequency of the discharges reduces this effect by depositing less energy at each discharge and accumulating energy more homogeneously between the electrodes, leading to a faster and more stable ignition. The minimum energy necessary to ignite decreases with increasing frequency and at the highest studied frequency (100 kHz) ignition has been achieved with 30% less energy than with a single-pulse discharge.

Numerical analysis of turbulent flame enhancement by nanosecond repetitively pulsed plasma discharges

Experimental studies of plasma-assisted combustion have shown that Nanosecond Repetitively Pulsed (NRP) discharges are a very efficient way to improve flame stability in lean regimes. These discharges generate a non-equilibrium plasma that induces a local heating and an important production of active species sufficient to enhance the combustion. The aim of this article is to understand the physical and chemical mechanisms involved in plasma-assisted turbulent combustion. High-performance computations of a lean bluff-body turbulent premixed methane-air flame, experimented at EM2C laboratory, are conducted for that purpose. Simulations are performed by combining a semi-empirical plasma model with an analytical combustion mechanism. Without electrical discharges, only a weak reaction zone behind the bluff body is observed. However, when the flame is assisted with the plasma, the flame power and surface increase significantly. A chemical analysis performed at the flame basis shows that the heat and the radical O produced by NRP discharges induce the dissociation of main burnt gases species H2O and CO2 into radicals OH, H, CO, O and species H2, O2. The species with long lifespan OH, H2 and O2 are convected from the center of the bluff-body recirculation zone to the flame front where they are consumed increasing the chemical reactivity. This causes a local increase of heat release rate that attaches the turbulent flame front which can develop downstream.