This thesis studies how plasmas created by nanosecond repetitive pulsed discharges (NRPD) can affect and improve the combustion characteristics in a high-speed fluid flow that simulates scramjet conditions. This is done by creating a computational code that incorporates the effects of plasmas, high-speed fluid dynamics, combustion chemistry, and photoionization. Many physical effects across multiple temporal and spatial scales appear, and creating a code that efficiently and accurately models these effects was the biggest contribution of this research. A new chemical mechanism has been created that incorporates high energy states for nitrogen and oxygen. This code was applied to examine how NRPD is affected by high-speed fluid flows and different electrode geometries. In quiescent simulations, the multiple pulses couple with each other increasing the overall temperature, which can lead to ignition due to the plasma added. When there is a freestream flow the convection of the previous pulses plasma can prevent coupling between the pulses. Without modification to pulse characteristics (increase in frequency, intensity, or length), combustion may not be achieved. Next, a more applied study of a three-dimensional scramjet is conducted to examine how the plasma affects the flow by the scramjet geometry and conditions. These larger simulations add effects from turbulence by implementing an LES-EDC model. These simulations show how plasmas generated by NRPD can affect the fluid flow inside a scramjet combustor cavity. / Doctor of Philosophy / Chemical kinetics is often too slow compared to turbulent mixing in high-speed propulsion, limiting the effectiveness of conventional flame stabilization devices. This project investigates how a non-equilibrium plasma can support combustion in a turbulent supersonic combustor at Mach 2. Plasma can support both vibrational-electronic energy exchanges and radical branching boosting the ignition time-scales up to the microsecond range. This thesis centers around the development of a CFD model that incorporates the effects of high-speed convection, photoionization, and plasma effects by using the drift-diffusion equation. In addition, a novel chemistry model has been developed to model the ultra-fast chemistry of triplet nitrogen states, these states appear in the plasmas that are studied. A verification and validation process is conducted on the code and its various components. This code is then used to study how nanosecond repetitive pulsed discharges (NRPD), which are an efficient way to create plasmas, are affected by scramjet flow conditions. The results in this thesis show that these plasmas can increase the temperature and improve the conditions for combustion. Two major studies have been done in this thesis with this code. First, the physics of the energy transfer is studied for the NRPD in a computational domain containing differently shaped electrodes (both flat and curved electrodes). The flat electrodes provide the strongest energy transfer to the plasma and the fluid. Next, large-scale simulations on three-dimensional scramjet geometries are performed and compared to the experiments. The effect of the electrode placement in the cavity is discussed.

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Gas turbine engines are an efficient and flexible way of power generation and aircraft propulsion. Even though different combustion systems can be implemented in these engines, more stringent regulations on pollutant emissions have been imposed throughout the years, especially in regard to nitrogen oxides (NOx). A very promising technology to reduce NOx emissions is lean premixed combustion (LPC), however, it is plagued by intense flame dynamics. Thermoacoustic instabilities, lean blow-off and lean instabilities are examples of dynamical phenomena that are detrimental to the gas turbines. In view of this, the present thesis presents the experimental investigation of the response of lean premixed swirl flames to acoustic perturbations at atmospheric and elevated pressures. The results of this investigation may be used to understand the thermoacoustic instabilities and further could be helpful in their prediction. Moreover, this work addresses the effects of non-thermal plasma discharges on the lean blow-off and stability limits of premixed swirl flames at elevated pressures. For the analysis of the flame response to acoustic fluctuations, the flame transfer functions, the flame dynamics, phase-locked velocity fields, and phase-locked measurements of flame curvature are collected through heat release and velocity fluctuations measurements, phase-locked images of the flame, particle image velocimetry, and planar laser-induced fluorescence, respectively. For the analysis of the effects of plasma discharges on the stability limits, electrical measurements and direct imaging of the flame are performed. The results include the development of an empirical relation based on the laminar burning velocity and on the circulation of the acoustically generated vortex to predict the response of the flame to acoustic fluctuations in different operating conditions. Moreover, the results show that the pressure has a strong impact on the response of lean premixed swirl flames to acoustic oscillations and on the flame-plasma interactions. Therefore, extrapolating results obtained at atmospheric conditions to elevated pressures may result in erroneous conclusions. Furthermore, it is shown that non-thermal plasma discharges can effectively extend the stability limits of lean premixed swirl flames at elevated pressures, underlining the potential of these discharges at conditions relevant for gas turbines.

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Gas turbines are important for power generation and aircraft engines. Over thepast century, there has been improvements in components of the gas turbine such ascompressors, turbines and nozzles, but very little progress has been made in combustor technology. The combustion still occurs at constant pressure and the only changes made are in terms of its design and mixing of fuel and air streams. These design changes have only allowed minimal improvements in gas turbine efficiency. To achievea substantative improvement in efficiency, it is required to make a technology change such as the introduction of constant volume combustion.<div><br></div><div>In this work, one such constant volume combustion device in the form of wave rotor combustion is studied and further developed for use in gas turbines. Wave rotors are periodic-flow devices that provide dynamic pressure exchange and efficient energy transfer through internal pressure waves generated due to fast opening and closing of ports. In addition, there is also confined high speed turbulent deflagration. If the blades are curved, then the flow undergoes angular momentum change from inlet to outlet, generating shaft work. This will allow maximum extraction of work potential from the wave rotor. In addition, an attempt is made to check the applicability of plasma assisted ignition for wave rotors. </div><div><br></div><div>A computational tool is developed to understand physics of non-axial channelwave rotors. The governing equations are formulated in one dimension through a passage average approach. Shaft work is estimated using conservation of angular momentum and enrgy to verify the working of numerical model. The model shows increase in shaft work with increase in blade curvature, but as the angle is increased, the possibility of ignititing the reacting mixture becomes difficult since it is hard tomove the mixture towards the ignition port. </div><div><br></div><div>An alternate ignition source using plasma discharges is investigated through experiments. Two experiments are developed, one to make ultrafast measurements of plasma properties such as gas heating and lifetime of electronically excited molecules, and a second experiment to understand ignition characteristics of a pin to ring electrode configuration. The experiments show that excited nitrogen which reacts with molecular oxygen to form atomic oxygen is short lived and forms oxygen atoms extremely rapidly. This rapid formation of oxygen atoms assists in fast ignition. The ignition experiment with pin to pin electrode showed that even though there is fast ignition, the propagation speed does not change significantly with pulse repetition frequency. Ignition with pin to ring electrode showed fast ignition and increase inflame speed with pulse repetition frequency. Results show that plasma discharge can be used as an ignition source for wave rotors but will need further investigation.</div><div><br></div><div>The development of computational tool and plasma discharge experiments has provided a solid base for future efforts in wave rotor combustion and design of full scale non-axial wave rotor combustor experiment.</div>

Nanosecond repetitively pulsed (NRP) discharges are being increasingly used in various applications, in particular in plasma-assisted combustion and aerodynamic flow control. First, we studied the thermal and hydrodynamic effects of NRP discharges using quantitative Schlieren measurements and numerical analyses in atmospheric pressure air. The time resolved images show the expansion of the heated gas channel starting from as early as 50 ns after the discharge and the shock-wave propagation from about 500 ns. Gas density profiles simulated in 1-D cylindrical coordinates are used to reconstruct numerical Schlieren images for comparison with experimental ones. We propose an original method to determine the initial gas temperature and the fraction of energy transferred into fast gas heating, using a comparison of the contrast profiles obtained from experimental and numerical Schlieren images. The results show that a significant fraction of the electric energy is converted into gas heating within a few tens of ns. The values range from 25 % at a reduced electric field of 164 Td in air at 300 K to about 75 % at 270 Td in air preheated to 1000 K, which supports the fast heating processes via dissociative quenching of N2(B, C) by molecular oxygen. Second, we provide a database to test the kinetic modeling of lean mixture ignition by NRP discharges. We characterize the initial spark radius and the ignition kernel development at pressures up to 10 bar. Comparisons with a conventional igniter show that better results are obtained with NRP discharges in terms of flame propagation speed, especially at high pressure. The flame speed increases by up to 20 % at 10 bar due to the increased wrinkling of the flame front induced by NRP discharges. Finally, we investigate the dynamic response of a flame to actuation by NRP discharges in a 12-kW bluff-body stabilized burner. The results show a significant reduction in flame lift-off height, within 5 ms after applying the NRP discharges. The mechanism is attributed to the entrainment of the OH radicals and heat towards the shear layer of incoming fresh gases. This opens up new applications in the control of combustion instabilities.

The effect of temperature on fuel-air ignition and combustion (thermal effects) have been widely studied and well understood. However, a comprehensive understanding of nonequilibrium plasma effects (in situ generation of reactive species and radicals combined with gas heating) on the combustion process is still lacking. Over the past decade, research efforts have advanced our knowledge of electron impact kinetics and low temperature chain branching in fuel-air mixtures considerably. In contrast to numerous experimental investigations, research on modeling and simulation of plasma assisted combustion has received less attention. There is a dire need for development of self-consistent numerical models for construction and validation of plasma chemistry mechanisms. High-fidelity numerical models can be invaluable in exploring the plasma effects on ignition and combustion in turbulent and high-speed flow environments, owing to the difficulty in performing spatially resolved quantitative measurements. In this work, we establish a multi-scale modeling framework to simulate the physical and chemical effects of nonequilibrium, nanosecond plasma discharges on reacting flows. The model is capable of resolving electric field transients and electron impact dynamics in sub-ns timescales, as well as calculating the cumulative effects of multiple discharge pulses over ms timescales. Detailed chemistry mechanisms are incorporated to provide deep insight into the plasma kinetic pathways. The modeling framework is utilized to study ignition of H₂-air mixtures subjected to pulsed, nanosecond dielectric barrier discharges in a plane-to-plane geometry. The key kinetic pathways responsible for radicals such as O, H and OH generation from nanosecond discharges over multiple voltage pulses (ns-ms timescales) are quantified. The relative contributions of plasma thermal and kinetic effects in the ignition process are presented. The plasma generated radicals trigger partial fuel oxidation and heat release when the temperature rises above 700 K, after which the process becomes self-sustaining leading to igntion. The ignition kernel growth is primarily due to local plasma chemistry effects rather than flame propagation, and heat transport does not play a significant role. The nanosecond pulse discharge plasma excitation resulted in nearly simultaneous ignition over a large volume, in sharp contrast to hot-spot igniters. Next, the effect of nanosecond pulsed plasma discharges on the ignition characteristics of nC₇H₁₆ and air in a plane-to-plane geometry is studied at a reduced pressure of 20.3 kPa. The plasma generated radicals initiate and significantly accelerate the H abstraction reaction from fuel molecules and trigger a “self-accelerating” feedback loop via low-temperature kinetic pathways. Application of only a few discharge pulses at the beginning reduces the initiation time of the first-stage temperature rise by a factor of 10. The plasma effect after the first stage is shown to be predominantly thermal. A novel plasma-flame modeling framework is developed to study the direct coupling of steady, laminar, low-pressure, premixed flames to highly non-equilibrium, nanosecond-pulsed plasma discharges. The simulations are performed with and without a burst of 200 nanosecond discharge pulses to quantify the effect of non-equilibrium plasma on a pre-existing lean premixed H₂/O₂/N₂ (ϕ = 0.5) flame at 25 torr. Simulation results showed a significant increase in O and H densities due to plasma chemistry, with peak values increasing by a factor of 6 and a factor of 4, respectively. It is demonstrated that Joule heating alone cannot move the temperature and species profiles as far upstream (i.e. closer to the burner surface) as the pulsed plasma source of the same total power. LES (large eddy simulation) of ignition and combustion of H₂ jets injected into a supersonic O₂ crossflow is performed. Nanosecond plasma discharges are studied for their potential to produce radicals and impact on the flame-holding process. The plasma has a significant effect on the O atom distribution near the discharge domain as well as in the leeward side of the second jet. The other species distributions, however, remained unchanged with or without plasma. We believe the reason for this behavior was the high jet momentum ratios considered in the present study. The plasma generated radicals were unable to have an effect on the flame development downstream because of the strong penetration of the cold fuel jet.

Aujourd’hui, il semble que la méthode la plus prometteuse pour intensifier la combustion repose sur l’excitation hors-équilibre du mélange gazeux, qui permet ainsi de modifier la cinétique chimique. Pour obtenir une excitation efficace des degrés de liberté électronique et vibrationnel des molécules, l’utilisation d’une alimentation pulsée associée à une énergie des électrons adéquate est proposée. Ce travail de thèse porte sur l’application de générateur d’impulsion électrique haute fréquence capable de délivrer des pulses de 20 kilovolts pendant 20 nanosecondes ayant des temps de montée de l’ordre de 5 nanosecondes en combustion. Cette étude s’articule autour de deux grands axes de recherche. Le premier est la génération, à pression atmosphérique et à pression réduite, d’espèces excitées (oxygène électroniquement excité O2(a1?g), O2(b1Sg+) et oxygène atomique excité) ainsi que leur caractérisation par spectroscopie d’émission. Le second axe de recherche concerne leur utilisation pour l’intensification de la combustion. La première partie expérimentale s’est focalisée sur la génération d’oxygène électroniquement excité par plasma décharge à barrière diélectrique et décharge croisée dans des mélanges O2/He et O2/Ar. La décharge croisée est une cellule à barrière diélectrique pulsée qui est croisée avec une composante continue (DC). Dans ce cas, l’étape d’ionisation est réalisée par la DBD pulsée alors que la composante continue supporte le courant électrique entre chaque pulse. Le gaz produit par cette installation est étudié de manière exhaustive par spectroscopie d’émission. Ce travail, indispensable pour caractériser l’installation et obtenir les conditions initiales nécessaires pour les calculs de flamme, repose sur différents spectromètres et caméras. La première des applications potentielles dans le domaine de la combustion concerne l’intensification de la combustion par activation de l’oxygène. La conception de prototypes de brûleurs hybrides, intégrant un réacteur plasma nous a permis de valider cette technique d’intensification de la combustion et de la comparer à une flamme classique sans plasma. Finalement, la modélisation des paramètres fondamentaux d’une flamme de prémélange et de diffusion est entreprise par le logiciel CHEMKIN. L’effet promoteur de l’oxygène excité sur une flamme d’hydrogène a pu être modélisé. / Nowadays it seems that the most promising method for accelerating combustion is the non-equilibrium excitation of the gas mixture components, which allows one to affect the chemical reaction kinetics. To enable more efficient excitation of the electronic and vibrational degrees of freedom, one should use short-duration (nanosecond) pulses with a high reduced electric field. The present work focuses on the application of high frequency high voltage pulse discharges capable of delivering an electric pulse of 20 kV during 20 ns with controlled voltage rise time of 5 ns and at a frequency up to 25 kHz in combustion. This study articulates around two major research axis; that of the generation of excited species and particularly the feasibility to produce excited oxygen species in its singlet electronic states O2(a1?g) and O2(b1Sg+) by a non-thermal electric discharge, at reduced pressure until atmospheric pressure and its characterization by emission spectroscopy. The second research axis concerns their use for the intensification of combustion. The experimental part of the study concerns investigation of singlet oxygen production in the application of a dielectric barrier discharge in O2/He and O2/Ar binary mixtures. The second discharge is a special crossed discharge plasma-chemical reactor that has been developed. This crossed discharge consists of a hybrid discharge in which short high voltage pulses produce ionization while a comparatively low electric field supports the electric current between ionizing pulses. The gas produced by this installation is intensively studied by emission spectroscopy. This work, indispensable to characterize the installation and to obtain initial conditions necessary for flame calculations, relies on different spectrometers and intensified camera. The first potential in the combustion field is to significantly improve combustion efficiency and reduce pollutant emissions using oxidizer “activation”. Conception and development of hybrid plasma burner prototypes, integrating crossed discharge plasma reactor allows us to validate this application by comparing with a classical flame without plasma activation. Finally, modelling of premixed flame fundamental parameters is undergone with CHEMKIN software. The promoting effect of excited oxygen on hydrogen flame has been characterized.