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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>

The thesis, entitled 'Investigation of the conversion of fuels in the presence of solid oxygen carriers and the development of a plasma-assisted chemical looping system for H2 production', presents the work of Yaoyao Zheng in the Department of Engineering, University of Cambridge, for the degree of Doctor of Philosophy. The thesis focused on chemical looping conversion of fuels, which employ oxygen carriers to supply oxygen, followed by the regeneration of the reduced oxygen carriers in air. Combustion of a Polish coal-derived char was first carried out in a fluidised bed reactor in the presence of Fe2O3 or ZrO2-supported Fe2O3. CO2 was introduced to the fluidised bed, to allow the char to be gasified in situ, prior to the reaction with the oxygen carriers. The presence of Fe2O3 did not alter the gasification step, given that the gasification of the char was free of external mass transfer limitation. A numerical model was developed to describe the gasification behaviour, as well as predicting the effect of CO on gasification. The inhibition effect of CO on char gasification was found more significant than expected. Combustion of biomass (wood pellets), by Fe2O3 or mayenite-supported CuO was studied in a fluidised bed. This was to understand how efficient the wood pellets were combusted by the oxygen carriers, as well as the distribution of the products. A tar measurement system, based on a plasma reactor, was first developed. With the developed measuring system, it was found that both Fe2O3 and mayenite-supported CuO were efficient for combusting wood pellets. Particularly, the CLOU nature of CuO makes mayenite-supported CuO a promising candidate for direct combustion, without introducing any reactive gaseous oxidant. The final part of the dissertation was focused on developing a plasma-assisted chemical looping system for H2-rich gas production (PCLH) from CH4 at mild temperatures (~ 673 K). SrFeO3-, Fe2O3, and Ni-doped SrFeO3- and Fe2O3 were investigated as the packing material. Total combustion of CH4 was observed in SrFeO3-. The addition of Ni onto SrFeO3- significantly improved the selectivity towards H2; whilst it was only active in the fresh cycle. Fe2O3 was found to be inert for converting CH4; however, the addition of Ni to form NiO/Fe2O3 dramatically improved H2 production and the reactivity maintained high for three redox cycles. The energy cost of such PCLH was comparable to that of water electrolysis.

Acrolein is an indispensable chemical intermediate with a rising demand in recent years. The concern of the increase of propylene prices due to the shrinking supply of nonrenewable crude oil makes the acid-catalyzed gas-phase glycerol dehydration to acrolein a prime candidate for research. Our analysis showed that the sustainable acrolein production from glycerol was both technically and economically viable. Alumina2700® (Al) and Silica1252® (Si) loaded with silicotungstic acid (HSiW) possessed distinct features while provided equally good acrolein yield (73.86mol% and 74.05mol%, respectively) optimally. Due to the unique non-equilibrium characteristics, non-thermal plasma (NTP) could promote a variety of chemical reactions; however, its application in a dehydration process remained blank. This study used the reaction of glycerol dehydration to acrolein to probe whether NTP could 1) improve acrolein yield during dehydration, 2) suppress the coke formation and regenerate the catalyst, and 3) modify the properties of the catalyst. The dielectric barrier discharge configuration was used to generate NTP; various NTP field strengths and also their interaction with temperature and the catalyst were investigated. The results showed that NTP improved the glycerol conversion and that NTP with a proper field strength increased acrolein selectivity. The optimal acrolein yields of 83.6 mol% and 83.1 mol% were achieved with 3.78 kV/cm NTP and 4.58 kV/cm NTP at 275°C for HSiW-Al and HSiW-Si, respectively. The application of NTP-O2 (5% oxygen in argon, 4.58 kV/cm) during glycerol dehydration significantly suppressed coke formation on HSiW-Si. NTP-O2 could regenerate the deactivated HSiW-Si at low temperatures by removing both soft and hard coke at various rates. NTP-O2 with higher field strength, at medium operation temperature (150ºC) and in argon atmosphere was more effective for coke removal/catalyst regeneration. Applying NTP to the catalyst fabrication showed some capabilities in modifying catalyst properties, including enlarging surface area, preserving mesopores, increasing acid strength and Brønsted acidity. NTP with argon as the discharge gas performed better in these modifications than NTP with air as the discharge gas.