Добірка наукової літератури з теми "Combustion instabilitiely pulsed plasma discharges"

Thermoacoustic instabilities occur when heat release is coupled with pressure fluctuation, which may cause performance degradation of the combustor and serious structural damage. This study focued on an active control method using discharge plasma and showed experimentally that discharge plasma can make a difference in controlling the thermoacoustic instabilities in a Rijke tube. A vertically placed Rijke tube thermoacoustic system using induction heating tungsten mesh as a heat source was built. The results show that the high repetition rate discharge can effectively suppress the thermoacoustic oscillations in the Rijke tube and that they will not re-occur for some time. Additionally, their effectiveness depended more on average power than energy per pulse. Combining the collected pressure, schlieren data, and theoretical analysis, it can be suggested that the plasma discharge could heat the inlet airflow, which could influence the heat exchange and then could break thermo-acoustic coupling, and its high-frequency pressure perturbation might increase the dissipation of the energy of sound.

This paper presents an overview of experimental and numerical investigations of the nonequilibrium cold plasma generated under high overvoltage and further usage of this plasma for plasma-assisted combustion.Here, two different types of the discharge are considered: a streamer under high pressure and the so-called fast ionization wave (FIW) at low pressure.The comprehensive experimental investigation of the processes of alkane slow oxidation in mixtures with oxygen and air under nanosecond uniform discharge has been performed. The kinetics of alkane oxidation has been measured from methane to decane in stoichiometric and lean mixtures with oxygen and air at room temperature under the action of high-voltage nanosecond uniform discharge.The efficiency of nanosecond discharges as active particles generator for plasma-assisted combustion and ignition has been investigated. The study of nanosecond barrier discharge influence on a flame propagation and flame blow-off velocity has been carried out. With energy input negligible in comparison with the burner's chemical power, a double flame blow-off velocity increase has been obtained. A signicant shift of the ignition delay time in comparison with the autoignition has been registered for all mixtures.Detonation initiating by high-voltage gas discharge has been demonstrated. The energy deposition in the discharge ranged from 70 mJ to 12 J. The ignition delay time, the velocity of the flame front propagation, and the electrical characteristics of the discharge have been measured during the experiments. Under the conditions of the experiment, three modes of the flame front propagation have been observed, i.e., deflagration, transient detonation, and Chapman-Jouguet detonation. The efficiency of the pulsed nanosecond discharge to deflagration-to-detonation transition (DDT) control has been shown to be very high.

Abstract Non-equilibrium plasma discharges in spark gaps have been an increasingly studied method for alleviating cycle to cycle variation in lean and dilute combustion environments. However, ignition models that account for streamer propagation, cathode fall, and transmission line amplification over nanosecond time scales have so far not been developed. The present study develops such a model, with emphasis on the energy delivered from circuit to cylinder. Key pieces of the relevant physics and chemistry are summarized, simplified, and systematically coupled to one another. The set of parameters is limited to a handful of key observables and modeled using Modelica. Results show non-trivial behavior in the energy delivery characteristics of such discharges with important implications for ignition.

These days, various national and international research organizations are working on the development of low NOx combustors. The present work describes the experimental and numerical characterization of flow dynamics and combustion characteristics in a rectangular burner. A ring-needle type plasma actuator was developed and driven by a high voltage nanosecond pulsed generator under atmospheric conditions. Smoke flow visualizations and Proper Orthogonal Decomposition (POD) were carried out to identify the relevant flow structures. Electrical characterization of the non-reactive flow was carried out to predict the electrical power and the optimum value of the reduced electric field (EN), which is useful for the implementation of a numerical model for the study of plasma-assisted ignition. A detailed plasma kinetic mechanism integrated with all excited species was considered and validated with experimental studies. Numerical modeling of plasma ignition has been performed by coupling ZDPlasKin with CHEMKIN. Energy and power consumption for methane/air plasma actuation is higher than the air plasma actuation. This could be due to the excitation and ionization of methane that required more energy deposition and power. The mole fraction of O atoms and ozone was higher in the air than the methane/air actuation. However, O atoms were produced in a very short time interval of 10−7 to 10−6 s; in contrast, the concentration of ozone was gradually increased with the time interval and the peak was observed around 10−1 s. Plasma discharges on the methane/air mixture also produced radicals that played a key role to enhance the combustion process. It was noticed that the concentration of H species was high among all radicals with a concentration of nearly 10−1. The concentration peak of CH3 and OH was almost the same in the order of 10−2. Finally, the mixture ignition characteristics under different low inlet temperatures were analyzed for both air and methane/air plasma actuation in the presence of different plasma discharges pulses numbers. Results showed that it is possible to reach flame ignition at inlet temperature lower than the minimum required in the absence of plasma actuation, which means ignition is possible in cold flow, which could be essential to address the re-ignition problems of aeroengines at high altitudes. At Ti = 700 K, the ignition was reached only with plasma discharges; ignition time was in the order of 0.01 s for plasma discharges on methane/air, lower than in case of plasma in air, which permitted ignition at 0.018 s. Besides this, in the methane/air case, 12 pulses were required to achieve successful ignition; however, in air, 19 pulses were needed to ignite.

The properties of the synthesized nanostructured materials are determined by the methods of their preparation. The combination of electric discharges with liquid is one of the new tools for the synthesis of pure structures but the conditions for obtaining structures play an important role as in the case of traditional synthesis methods. In this work, the electrical and emission characteristics of a low-temperature direct current plasma in contact with water at currents of 0.25 and 0.80 A are studied. The values of the power (energy) of single discharges are calculated. It has been established that this type of discharge burns in a pulsed mode. The value of the discharge current affects the frequency of occurrence of discharges and the energy of an single discharge. It is shown that low-temperature underwater plasma is an effective tool for the synthesis of nanocomposites based on metal oxides, the precursors of which are metal electrodes. The emission spectroscopy method was used to study the emission spectra of underwater plasma. The sputtering of electrodes during plasma combustion, has been established. X-ray phase analysis showed that the phase composition of the obtained products is determined by the strength of the plasma current. The formation of oxides and hydroxides of Ni and Cr with different valencies of metal ions was found.

Increasing emphasis on the de-carbonization of energy production has led researchers to consider carbon-free, renewable, and green fuels such as hydrogen (H2) and ammonia (NH3). While H2 suffers from the major challenges of production difficulties, transportation, and storage, NH3 is plagued with challenges of low flame speeds causing unstable flames, high autoignition temperatures resulting in longer ignition delays, narrow flammability limits, and higher levels of NOx emission. Among the different solutions to overcome these challenges of NH3 combustion, non-equilibrium plasma-based igniters are significant owing to the promotion of localized volumetric ignition kernel development by both thermal and chemical assistance. Computational investigation of plasma-assisted combustion of ammonia-air mixtures in constant volume and constant pressure reactors are conducted, to determine the impact of operating conditions on ignition delays and NOx emissions. A mechanism has been assembled in this work using well-validated plasma reactions of NH3 with O2 and N2, alongside plasma kinetics of air from the literature. Subsequently, the newly developed mechanism was used to investigate the plasma-assisted oxidation of NH3. In particular, the impact of the reduced electric field (E/N), equivalence ratio, pressure, pulse frequency, and energy density on the ignition delays and NOx emission were investigated. A Global Pathway-based Analysis algorithm for plasma-assisted systems (PGPA) is used to analyze the nanosecond pulsed nonequilibrium plasma-assisted combustion of NH3/air mixtures. Firstly, a faster ignition and lower production of NOx are observed in the case of plasma discharges compared to thermal energy deposition, owing to the enhanced production of OH radicals and the early reforming of NH3 to produce N2 and H2 with plasma, respectively. At lower reduced electric fields (E/N), PGPA analyses elucidated the significance of gas heating due to vibrational-translational cycles of NH3 and N2 on the increased reactivity of NH3/air mixtures as compared to ignition at a higher E/N. The fuel-lean mixture is observed to exhibit higher production of NOx than stoichiometric and fuel-rich mixtures, resulting from plasma chemistry involving oxygen radical and electronic excited states of N2. Higher rates of collisional quenching at higher pressures during the inter-pulse gaps are found to result in a lesser amount of electronically excited states of N2 and O2, resulting in lower production of air-bound NOx during the pulses. Complementing combustion enhancements, the study also considers the role of plasma-assisted systems in gas reforming, thereby imparting specific desired characteristics lacking in the original mixture. For instance, plasma-assisted reforming can be utilized to control emissions by reforming specific emission precursors or by improving the gas reactivity to promote clean combustion. Natural gas associated with oil wells and natural gas fields is a significant source of greenhouse gas emissions and airborne pollutants. Flaring/burning of the associated gas removes greenhouse gases like methane (CH4) and other hydrocarbons. Our study explores the possibility of enhancing the flaring of associated gas mixtures (C1 – C4 alkane mixture) using nanosecond pulsed non-equilibrium plasma discharges. A well-studied conventional combustion chemistry for small alkanes is coupled with the plasma kinetics of CH4, C2H6, C3H8, and N2, including electron-impact excitations, dissociations, and ionization reactions. The newly developed plasma-based flare gas chemistry is then utilized to investigate repetitively pulsed nonequilibrium plasma-assisted reforming and subsequent combustion of the flare gas mixture diluted with N2 at different conditions. The results indicate an enhanced production of H2 and C2H4 in the reformed gas mixture, owing to the electron-impact dissociations of alkanes and subsequent H-abstractions and recombination reactions, thereby resulting in a mixture of CH4, H2, C2H4, C2H2, and other unsaturated C3. The reformed mixture exhibits significantly high reactivity as exhibited by their increased flame speeds and shorter ignition delays. The reformed mixture is also observed to promote increased CH4 destruction levels and complete flaring, thereby reducing the emissions of CH4 and other hydrocarbons.

Non-equilibrium plasma demonstrates great potential to control ultra-lean, ultra-fast, low-temperature flames and to become an extremely promising technology for a wide range of applications, including aviation gas turbine engines, piston engines, RAMjets, SCRAMjets and detonation initiation for pulsed detonation engines. The analysis of discharge processes shows that the discharge energy can be deposited into the desired internal degrees of freedom of molecules when varying the reduced electric field, E / n , at which the discharge is maintained. The amount of deposited energy is controlled by other discharge and gas parameters, including electric pulse duration, discharge current, gas number density, gas temperature, etc. As a rule, the dominant mechanism of the effect of non-equilibrium plasma on ignition and combustion is associated with the generation of active particles in the discharge plasma. For plasma-assisted ignition and combustion in mixtures containing air, the most promising active species are O atoms and, to a smaller extent, some other neutral atoms and radicals. These active particles are efficiently produced in high-voltage, nanosecond, pulse discharges owing to electron-impact dissociation of molecules and electron-impact excitation of N 2 electronic states, followed by collisional quenching of these states to dissociate the molecules. Mechanisms of deflagration-to-detonation transition (DDT) initiation by non-equilibrium plasma were analysed. For longitudinal discharges with a high power density in a plasma channel, two fast DDT mechanisms have been observed. When initiated by a spark or a transient discharge, the mixture ignited simultaneously over the volume of the discharge channel, producing a shock wave with a Mach number greater than 2 and a flame. A gradient mechanism of DDT similar to that proposed by Zeldovich has been observed experimentally under streamer initiation.

This paper reports on an experimental study of the influence of a nanosecond repetitively pulsed spark discharge on the stability domain of a propane/air flame. This flame is produced in a lean premixed swirled combustor representative of an aeronautical combustion chamber. The lean extinction limits of the flame produced without and with plasma are determined and compared. It appears that only a low mean discharge power is necessary to increase the flame stability domain. Lastly, the effects of several parameters (pulse repetition frequency, global flowrate, electrode location) are studied.

A simulation is developed to investigate the kinetics of nitric oxide (NO) formation in premixed methane/air combustion stabilized by nanosecond-pulsed discharges. The simulation consists of two connected parts. The first part calculates the kinetics within the discharge while considering both plasma/combustion reactions and species diffusion, advection and thermal conduction to the surrounding flow. The second part calculates the kinetics of the overall flow after mixing the discharge flow with the surrounding flow to account for the effect that the discharge has on the overall kinetics. The simulation reveals that the discharge produces a significant amount of atomic oxygen (O) as a result of the high discharge temperature and dissociative quenching of excited state nitrogen by molecular oxygen. This atomic oxygen subsequently produces hydroxyl (OH) radicals. The fractions of these O and OH then undergo Zel’dovich reactions and are found to contribute to as much as 73% of the total NO that is produced. The post-discharge simulation shows that the NO survives within the flow once produced.

Les instabilités de combustion résultant d'un couplage thermo-acoustique sont un frein au développement de nouveaux systèmes de combustion avec des émissions polluantes réduites. Dans les systèmes au sol, elles peuvent être atténuées par des amortisseurs acoustiques. Cette thèse porte sur l'analyse des amortisseurs acoustiques pour les instabilités thermo-acoustiques par divers moyens et dans diverses applications. Le travail est divisé en trois parties. Dans une première partie, les performances d'amortissement de plaques perforées traversées par un écoulement et combinées à une cavité arrière résonante sont étudiées analytiquement et expérimentalement dans les régimes linéaire et non linéaire. Leur réponse acoustique à des ondes sonores de niveau croissant est étudiée dans une configuration dédiée pour des conditions d'écoulement froid. L'impédance de l'amortisseur est déterminée à partir de trois microphones. Un manomètre différentiel mesure également la chute de pression à travers les plaques. L'absorption acoustique est analysée pour différentes porosités des plaques. On montre que la transition du régime linéaire au régime non linéaire dépend du rapport entre la perturbation de la vitesse acoustique à l'intérieur du trou et la vitesse d'écoulement à travers la perforation. Un modèle quasi-stationnaire valable aux faibles nombres de Strouhal est développé pour déterminer la réponse des plaques en régime non linéaire. On montre que l'absorption est plus faible qu'en régime linéaire et qu'un forçage acoustique élevé entraîne une chute de pression supplémentaire à travers les plaques. Les prédictions et les mesures présentent un accord raisonnable sur la bande de fréquence de 100 Hz à 1000 Hz. Les impacts de l'épaisseur de la plaque, la répartition de deux tailles de trous différentes et un chanfrein à la sortie du trou sont également étudiés. Les modèles analytiques développés donnent un relativement bon accord avec les mesures effectuées dans les régimes linéaire et non linéaire. Dans la deuxième partie, une étude analytique est menée pour modéliser l'impact d'une plaque à trous sur le champ acoustique dans une chaudière laminaire. Premièrement, les instabilités sont détectées par des microphones autour de 1000 Hz. Des calculs acoustiques sont effectués pour identifier l'origine du déclenchement de ces instabilités et identifier à quel mode acoustique, longitudinal et/ou azimutal, sont couplées les oscillations auto-entretenues. Des plaques perforées placées à différents endroits à l'intérieur d'une chaudière générique sont alors étudiées pour amortir ces oscillations. La réponse du système est d'abord analysée sans combustion. L'analyse est ensuite étendue à l'impact de la combustion et des perforations sur le champ acoustique résultant. Des critères sont déterminés pour chaque configuration indiquant les meilleurs amortisseurs possibles. Dans la troisième partie, une étude analytique complétée par une analyse numérique de la réponse acoustique d'un système composé d'un plénum, d'un tube d'injection et d'une chambre de combustion est réalisée. Cette configuration est modélisée par un système de trois cavités avec une flamme à l'intersection entre le tube d'injection et la chambre de combustion. Les instabilités thermo-acoustiques à basse fréquence correspondant à des oscillations en bloc des variables d'écoulement sont étudiées. Elles peuvent être associées à un mode de Helmholtz dont l'origine est déterminée. Une analyse modale basse fréquence démontre que les instabilités de combustion qui se produisent dans ces systèmes sont essentiellement contrôlées par le rapport entre les volumes du plénum et de la chambre de combustion et par l'impédance de sortie de la chambre. Ceci est également confirmé indépendamment par une modélisation bas ordre de la dynamique du système
Combustion instabilities resulting from a thermo-acoustic coupling represent a difficult obstacle for the design of new combustion systems with reduced pollutant emissions. In ground-based systems they are often hindered with acoustic dampers. This thesis focuses on the analysis of acoustic dampers for thermos-acoustic instabilities by various means and in various applications. The work is divided into three parts. In the first part, the damping performances of perforated plates traversed by a bias flow combined with a resonant back-cavity are studied analytically and experimentally in the linear and nonlinear regimes. Their acoustic response to sound waves of increasing level is investigated in a dedicated setup under cold flow conditions. The impedance of the damper is determined from three microphones. A differential pressure gauge also measures the pressure drop through the plates. Acoustic absorption is analyzed for different plate porosities. It is shown that transition from linear to nonlinear regime depends on the ratio between the acoustic velocity perturbation inside the hole and the bias flow velocity through the perforation. A quasi-steady model valid at low Strouhal numbers is developed to determine the plate response in the nonlinear regime. It is shown that absorption is lower than in the linear regime and that high acoustic forcing leads to an additional pressure drop through the plates. Predictions at the zero and at the forcing frequencies are compared to measurements with reasonable agreement over the frequency band from 100 Hz to 1000 Hz. The impact of plate thickness, distribution of two different hole sizes and a chamfer at the hole outlet are also studied. The analytical model that are developed give relatively good agreement with measurements made in the linear and nonlinear regimes. In the second part, an analytical study is conducted to model the impact of perforates on the acoustic field in a laminar boiler. Firstly, abnormal noise is detected by microphones with instabilities around 1000 Hz. Acoustic calculations are carried out to identify the origin of triggering of these instabilities and identify to which acoustic mode, longitudinal and/or azimuthal, the self-sustained oscillations are coupled to. Perforated plates placed at different locations inside the boiler are then envisaged to damp these oscillations. The system responses are analyzed first without combustion. The analysis is then extended to include both the impact of combustion and perforations on the resulting acoustic field. Criteria are determined for each configuration for the best damper candidates. In the third part, an analytical study completed by a numerical analysis of the acoustic response of a system composed by a plenum, an injection tube and a combustion chamber are carried out. This configuration is modeled as a system of 3 cavities with a flame at the intersection between the injection tube and the combustion chamber. Low frequency thermo-acoustic instabilities corresponding to bulk oscillations of the flow variables are investigated. They are generally associated to a Helmholtz mode of the system, but the associated cavities are rarely fully identified. A low frequency modal analysis demonstrates that bulk flow combustion instabilities taking place in these systems, are essentially controlled by the ratio of the plenum to combustion chamber volumes and by the outlet impedance of the chamber. This is also confirmed independently by a low order modeling of the system dynamics including the flame response to flow rate perturbations. Two configurations featuring intrinsically distinct dynamics when the combustion chamber can or cannot sustain an over-pressure are examined

In this Ph.D. thesis, we have carried out numerical simulations to study nanosecond repetitively pulsed discharges (NRPD) in a point-to-point geometry at atmospheric pressure in air and in H2-air mixtures. Experimentally, three discharge regimes have been observed for NRPD in air at atmospheric pressure for the temperature range Tg = 300 to 1000 K: corona, glow and spark. To study these regimes, first, we have considered a discharge occurring during one of the nanosecond voltage pulses. We have shown that a key parameter for the transition between the discharge regimes is the ratio between the connection-time of positive and negative discharges initiated at point electrodes and the pulse duration. In a second step, we have studied the dynamics of charged species during the interpulse at Tg = 300 and 1000 K and we have shown that the discharge characteristics during a given voltage pulse remain rather close whatever the preionization level (in the range 109-1011 cm��3) left by previous discharges. Then, we have simulated several consecutive nanosecond voltage pulses at Tg = 1000 K at a repetition frequency of 10 kHz. We have shown that in a few voltage pulses, the discharge reaches a stable quasi-periodic glow regime observed in the experiments. We have studied the nanosecond spark discharge regime. We have shown that the fraction of the discharge energy going to fast heating is in the range 20%- 30%. Due to this fast heating, we have observed the propagation of a cylindrical shockwave followed by the formation of a hot channel in the path of the discharge that expands radially on short timescales (t < 1 _s), as observed in experiments. Then we have taken into account an external circuit model to limit the current and then, we have simulated several consecutive pulses to study the transition from multipulse nanosecond glow to spark discharges. Finally the results of this Ph.D. have been used to find conditions to obtain a stable glow regime in air at 300 K and atmospheric pressure. Second we have studied on short time-scales (t_ 100_s) the ignition by a nanosecond spark discharge of a lean H2-air mixture at 1000 K and atmospheric pressure with an equivalence ratio of _ = 0:3. We have compared the relative importance for ignition of the fast-heating of the discharge and of the production of atomic oxygen. We have shown that the ignition with atomic oxygen seems to be slightly more efficient and has a completely different dynamics.

Les interactions entre les flammes et les phénomènes thermiques sont le fil conducteur de ce travail. En effet, les flammes produisent de la chaleur, mais peuvent aussi être affectées par des transferts ou des sources de chaleur. La Simulation aux Grandes Echelles (SGE) est utilisée ici pour étudier ces interactions, en mettant l’accent sur deux sujets principaux: le transfert de chaleur aux parois et le contrôle de la combustion. Dans un premier temps, on étudie le transfert de chaleur aux parois dans un modèle de brûleur CH4/O2 de moteur-fusée. Dans un contexte deréutilisabilité et de réduction des coûts des lanceurs, qui constituent des enjeux majeurs, de nouveaux couples de propergols sont envisagés et les flux thermiques à la paroi doivent êtreprécisément prédits. Le but de ce travail est d’évaluer les besoins et les performances des SGEpour simuler ce type de configuration et de proposer une méthodologie de calcul permettant desimuler différentes configurations. Les résultats numériques sont comparés aux donnéesexpérimentales fournies par la Technische Universität München (Allemagne). Dans un deuxième temps, le contrôle de la combustion au moyen de décharges de plasma de type NRP (en anglaisNanosecond Repetitively Pulsed) est étudié. Les systèmes de turbines à gaz modernes utilisent en effet une combustion pauvre dans le but de réduire la consommation de carburant et les émissions de polluants. Les flammes pauvres sont connues pour être sujettes à des instabilités et le contrôle de la combustion peut jouer un rôle majeur dans ce domaine. Un modèle phénoménologique qui considère les décharges de plasma comme une source de chaleur est développé et appliqué à un brûleur pauvre avec prémélange CH4/Air stabilisé par un swirler. LesSGE sont réalisées afin d’évaluer les effets des décharges NRP sur la flamme. Les résultats numériques sont comparés aux observations expérimentales faites à la King Abdulla University ofScience and Technology (Arabie Saoudite)
Interactions between flames and thermal phenomena are the guiding thread of this work. Flamesproduce heat indeed, but can also be affected by it. Large Eddy Simulations (LES) are used hereto investigate these interactions, with a focus on two main topics: wall heat transfer andcombustion control. In a first part, wall heat transfer in a rocket engine sub-scale CH4/O2 burner isstudied. In the context of launchers re-usability and cost reduction, which are major challenges,new propellant combinations are considered and wall heat fluxes have to be precisely predicted.The aim of this work is to evaluate LES needs and performances to simulate this kind ofconfiguration and provide a computational methodology permitting to simulate variousconfigurations. Numerical results are compared to experimental data provided by the TechnischeUniversität München (Germany). In a second part, combustion control by means of NanosecondRepetitively Pulsed (NRP) plasma discharges is studied. Modern gas turbine systems use indeedlean combustion with the aim of reducing fuel consumption and pollutant emissions. Lean flamesare however known to be prone to instabilities and combustion control can play a major role in thisdomain. A phenomenological model which considers the plasma discharges as a heat source isdeveloped and applied to a swirl-stabilized CH4/Air premixed lean burner. LES are performed inorder to evaluate the effects of the NRP discharges on the flame. Numerical results are comparedwith experimental observations made at the King Abdulla University of Science and Technology(Saudi Arabia)

Two types of supersonic combustion instabilities identi¦ed in a M = 2 reacting §ow at direct injection of gaseous hydrocarbon fuel and Quasi-DC plasma assistance are considered: a dynamic (global) instability triggered by a mechanism of §ow combustion plasma coupling and a fast instability of a thermoacoustic nature. The experiments were performed at SBR-50 supersonic combustion facility at variable conditions: pressure P0 = 1 4 bar; temperature T0 = 300 750 K; and fuel mass §ow rate ‘m = 1 8.5 g/s. Diagnostics included pressure measurements, ¦ltered fast camera imaging, schlieren visualization, and spectroscopic observations. The dynamic instability develops with a characteristic time of about 10 ms and is related to the interaction of the combustion-based separation zone with the re§ected shock wave and electric discharge. It is shown that this instability could be e¨ectively controlled by electrical discharge power. The thermoacoustic instability is developed with a characteristic time < 1 ms. The analysis of pressure data reveals a resonant acoustic wave presence in the combustion zone between fuel injection ports and the test section di¨user.

The central objective of this study is to investigate the effectiveness of implementing a plasma discharge to improve combustor dynamics and flame stability. Specifically, a nano-second pulsed plasma discharge (NSPD) was applied to a premixed gaseous fuel/air dump combustor for mitigation of dynamic combustion instabilities with a minimal NOX penalty. This paper addresses the scaling of this technology from ambient pressure and temperature conditions to more realistic gas turbine combustor conditions. A model combustor operating at representative conditions of O (102) m/s flow velocity, ∼ 580 K combustor inlet temperature, and ∼ 5 atm in-combustor pressure was selected to simulate a typical low-power environment of future aero engine gas turbine combustors. Fully premixed methane or propane was utilized as a fuel. Similar to a previous ambient-pressure study, a significant reduction of pressure fluctuation level was observed, by a factor of 2X to 4X over a wide range of velocity at the baseline temperature and pressure. The plasma power required for the reduction increased linearly with increasing velocity. The change of fuel from methane to propane showed that propane requires significantly (2X) higher plasma power to achieve a similar level of noise reduction. It was also observed that the lean blowout (LBO) limit was significantly extended in the presence of the plasma, however, substantial incomplete combustion occurs in the extended regime. NOX measurements showed that the incremental NOX production due to the presence of the plasma was low (∼ < 1EINOX) in general, however, it increased with decreasing velocity and pressure, and increasing temperature.

Abstract Recent interest in non-equilibrium plasma discharges as sources of ignition for the automotive industry has not yet been accompanied by the availability of dedicated models to perform this task in computational fluid dynamics (CFD) engine simulations. The need for a low-temperature plasma (LTP) ignition model has motivated much work in simulating these discharges from first principles. Most ignition models assume that an equilibrium plasma comprises the bulk of discharge kernels. LTP discharges, however, exhibit highly non-equilibrium behavior. In this work, a method to determine a consistent initialization of LTP discharge kernels for use in engine CFD codes like CONVERGE is proposed. The method utilizes first principles discharge simulations. Such an LTP kernel is introduced in a flammable mixture of air and fuel, and the subsequent plasma expansion and ignition simulation is carried out using a reacting flow solver with detailed chemistry. The proposed numerical approach is shown to produce results that agree with experimental observations regarding the ignitability of methane-air and ethylene-air mixtures by LTP discharges.

Ignition of the fuel-air mixture through compression and spark ignition are the two basic methods for igniting combustion mixtures in modern internal combustion (IC) engines. A significant environmental and economic benefit could be obtained if spark ignited (SI) engines were to be made more efficient. Higher thermal efficiencies could be obtained through operation with leaner fuel-air mixtures and through operations at higher power densities and pressures. These types of mixtures are often more difficult to ignite with traditional spark plugs. In pursuit of better ignition sources, this paper investigates a microwave plasma alternative to the traditional spark plug. Additionally, measurements of a novel pulsed microwave induced plasma ignition concept are presented. Measurements at atmospheric pressures show promising results with respect to energy and plasma formation delay. With some refinement, it is possible to produce sufficiently energetic microwave discharges, in a short enough time frame to make them feasible for ignition of SI engines in an automotive setting. These results justify further investigation to quantify the advantages such an ignition source may have to offer.

The effects of Nanosecond Repetitively Pulsed (NRP) plasma discharges on the dynamics of a swirl-stabilized lean premixed flame are investigated experimentally. Voltage pulses of 8-kV amplitude and 10-ns duration are applied at a repetition rate of 30 kHz. The average electric power deposited by the plasma is limited to 40 W, corresponding to less than 1 % of the thermal power of 4 kW released by the flame. The investigation is carried out with a dedicated experimental setup that allows for studies of the flame dynamics with applied plasma discharges. A loudspeaker is used to perturb the flame acoustically, and the discharges are generated between a central pin electrode and the rim of the injection tube. Velocity and CH* chemiluminescence signals are used to determine the flame transfer function assuming that plasma discharges do not affect the correlation between CH* emission and heat release rate fluctuations. Phase-locked images of the CH* emission were recorded to assess the effect of the plasma on the oscillation of the flame. The results show a strong influence of the NRP discharges on the flame response to acoustic perturbations, thus opening interesting perspectives for combustion control. An interpretation of the modifications observed in the transfer function of the flame is proposed by taking into account the thermal and chemical effects of the discharges. It is then demonstrated that by applying NRP discharges at unstable conditions, the oscillation amplitudes can be reduced by an order of magnitude, thus effectively stabilizing the system.

Abstract This paper presents an experimental study of lean flames stabilization with nanosecond repetitively pulsed discharges. The two-stage, swirled-stabilized, multipoint injector BIMER operates at atmospheric pressure with methane-air mixtures in the present study. It is representative in its design of a realistic lean premixed prevaporized injector of gas turbine engines operated at a lab-scale level. The lean blow-off extension with plasma is characterized. The combustion efficiency and the pollutant emissions are quantified near blow-off with and without plasma for 50-kW flames. We show that it is possible to stabilize lean flames down to an equivalence ratio of 0.3, with less than 5 ppm of NOx emitted, thanks to NRP discharges with an electric power that represents less than 0.25% of the flame thermal power. This study also clearly shows that it is necessary to account for the plasma system integration at the early stage of the combustor design to fully benefit from the plasma stabilizing effects on the flame.

Pulsed detonation engines are considered to be the promising e¨ective propulsion systems for future space missions. The ignition system has always posed problems in commercial applications. Many experimental, theoretical, and numerical studies have been performed for the past years and various ignition systems (e. g., electric discharge, microwave discharge, laser radiation) have been tested. The propulsive performance of air-breathing pulsed detonation engines (PDEs) has been theoretically and numerically studied over a wide range of system con¦gurations, operating parameters, and §ight conditions. It has been suggested that discharges which create the quickest expanding high-temperature region or discharges which occupy a large volume are optimal for ignition because they can most rapidly and reliably bring the radius of the ignition kernel to its critical value for transition into a self-propagating §ame. Signi¦cant e¨orts are being spent on acceleration of fuel combustion and rising its e©ciency. Existing studies have mainly focused on optimizing fuel injection and mixing, repetitive initiation of detonation, and integration of detonation tubes with fuel inlets. Understanding of streamer propagation mechanism is of essential importance for the studies of electrical breakdown phenomena and their related applications. In this study, a subcritical microwave streamer discharge is used to initiate ignition of air fuel mixtures. The study focuses on investigation of possibilities of the use of microwave radiation to initiate combustion and detonation of air fuel mixtures. The results of experimental and computational studies related combustion and detonation of air propane mixture are presented. To initiate the combustion and detonation, the deep subcritical streamer discharge is used. The discharge is formed by a ¦eld with the intensity smaller than the minimum pulse intensity leading to the gas breakdown. An acceleration of combustion and a uniform temperature front are obtained and the possibility of combustion of fuel-lean mixture is con¦rmed. An increase in combustion e©ciency is also observed. Streamer discharge ignition of particularly lean air fuel mixture with air-to-fuel ratio greater than the §ammability limit has been demonstrated under normal conditions. The indirect evidence suggests that the ignition by the microwave discharge is of the nonthermal nature. The advantages of igniting the fuel mixture by streamer discharge is attributed to the ultraviolet radiation emitted by oxygen atoms subjected to the discharge. The ultraviolet radiation generation causes formation of the nonequilibrium cold plasma with avalanche increase in the number of free electrons. The microwave streamer ignition can be considered for the application in internal combustion engines to replace the conventional spark ignition.