Thermoacoustic engines convert heat into acoustic pressure waves with no moving parts; this inherently results in high reliability, low maintenance and low manufacturing costs. Significant increases in the performance of these devices have enabled rivalry with more mature energy conversion methods in both efficiency and power output. This optimal production of acoustic power can be ultimately used to achieve cryogenic temperatures in thermoacoustic refrigerators, or can be interfaced with reciprocating electro-acoustic power transducers to generate electricity. This thesis describes the design, fabrication and testing of a Thermoacoustic Power Converter. The system interfaces a thermoacoustic-Stirling heat engine with a pair of linear alternators to produce 100 watts of electricity from a heat input. It operates with helium at 450 psig internal pressure and a hot side temperature of 1200F. Through thermoacoustic phenomena, these conditions sustain a powerful pressure wave at a system specific 100 Hz. This pressure wave is used to drive the two opposed linear alternators in equal and opposite directions to produce a single phase AC electrical output at that same system frequency. The opposing motion of the two alternators enables a vibration-balanced system. The engine has created 110 watts of acoustic power and the complete Thermoacoustic Power Converter system has produced 70 watts of AC electricity. Compensating for some heat leaks, the converter reaches 26.3% heat to acoustic power efficiency and 16.8% heat to electric efficiency when those maximum values are achieved. This conversion of heat to acoustic power is 40% of the Carnot thermodynamic efficiency limit.

Lean, premixed combustion offers a practical approach for reducing nitrogen oxide (NOx) emissions, but increases the risk of lean blowout (LBO) in gas turbines. Active control techniques are therefore sought which can stabilize a lean flame and prevent LBO. The present work has resulted in the development of flame detection, dynamic modeling, blowout margin estimation, and actuation and control techniques. The flame s acoustic emissions were bandpass filtered at select frequencies to detect localized extinction events, which were found to increase in number near LBO. The lean flame was also found to intermittently burst into a transient tornado configuration in which the flame s inner recirculation zone would collapse. The localized extinctions were dynamically linked to the tornado bursts using a linear, first order model. The model was subsequently applied to predict tornado bursts based on optically detected localized extinction events. It was found that both localized extinctions and tornado bursts are by themselves Poisson processes; the exponential distribution of their spacing times could be used to determine blowout probability. Blowout mitigation was achieved by redistributing the fuel flow between the annular swirlers and central preinjection pilot, both of which were premixed. Rule-based and lead-lag control architectures were developed and validated.

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A new approach was developed to determine the operational stability margin of a laboratory scale combustor. Applying modern and robust techniques and tools from Dynamical System Theory, the approach was based on three basic steps. In the first step, a gray-box thermoacoustical model for the combustor was derived. The second step consisted in applying System Identification techniques to experimental data in order to validate the model and estimate its parameters. The application of these techniques to experimental data under different operating conditions allowed us to determine the functional dependence of the model parameters upon changes in an experimental control parameter. Finally, the third step consisted in using that functional dependence to predict the response of the system at different operating conditions and, ultimately, estimate its operational stability margin. The results indicated that a low-order stochastic non-linear model, including two excited modes, has been identified and the combustor operational stability margin could be estimated by applying a continuation method.

A fast response method for detection of light-off in gaseous flames and liquid spray flames has been developed. The method used chemiluminescent signals from the 2Σ - ²π OH system centered at 309 nm and the ²Δ - ²π CH system centered at 430 nm to indicate the presence of a flame. Spectral scans (performed on gaseous methane, liquid hexane and liquid Jet-A aircraft fuel) from 280 nm to 610 nm indicated that these two species produced the strongest signals available for flame detection. As their light is emitted in the ultraviolet spectrum, using the OH and CH radicals will potentially provide a good signal-to-noise ratio since, in combustion chambers, most of the broadband background emissions are in the infrared and visible wavelengths. These scans also showed that the hexane and Jet-A gave OH and CH signals of approximately equal intensity. The transient histories of OH and CH were investigated by performing light-off ignition tests and intermittent light-off ignition tests. These various flame conditions showed that both signals were good indicators of flame presence, showing on average, a response time of better than 3 milliseconds. It was found that when the Hydrogen to Carbon ratio of the fuel was decreased, the CH signal strength increased as a percentage of OH signal intensity. Finally, the output signal intensity was found to be sensitive to both the flame image magnification and to the part of the flame observed. / Master of Science

A "leveling off" trend in NOx emissions with high amounts of steam addition has been observed in industrial gas turbine diffusion flame combustors. Experiments were performed to try to reproduce this trend in a laminar, opposed flow diffusion flame burner. Experiments were performed with Cli4, C2H4, CO, COIH2 (1:1), and COIH2 (1:2) as fuels. Both hydrocarbon fuels and non-hydrocarbon fuels were tested to study the contribution of the Fenimore mechanism to the "leveling off" trend. Probe sampling with chemiluminescent analysis was used to fmd NOx concentrations; Pt/PtRh thermocouples corrected for radiation losses were used to measure flame temperatures. The experiments reproduced the "leveling off" of NOx emissions, but a "leveling off" of temperatures also occurred. There were no significant differences in the results from the hydrocarbon and non-hydrocarbon fuels. The "leveling off" of NOx emissions is attributed to the "leveling off" of temperatures in the burner. It is not necessary to invoke the Fenimore mechanism to explain this trend. At least 55% of the NOx was eliminated from the flames using steam injection, which implies that at least 55% of the NOx was formed by the Zeldovich mechanism Evidence of Fenimore NO was provided by the fact that the existence of hydrocarbon coking on the fuel nozzle encouraged NOx production in all flames. / Master of Science

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master in Science in Engineering, Johannesburg September 1998 / Combustion modelling of utility furnace chambers provides a cost efficient means to extrapolate the combustion behaviour of pulverised fuel (pf) as determined from drop tube furnace (DTF) experiments to full scale plant by making use of computational fluid dynamics (CFD). The combustion model will be used to assimilate essential information for the evaluation and prediction of the effect of • changing coal feedstocks • proposed operational changes • boiler modifications. TRI comrnlssloned a DTF in 1989 which has to date been primarily used for the comparative characterisation of coals in terms of combustion behaviour. An analysis of the DTF results allows the determination of certain combustion parameters used to define a mathematical model describing the rate at which the combustion reaction takes place. This model has been incorporated into a reactor model which can simulate the processes occurring in the furnace region of a boiler, thereby allowing the extrapolation of the DTF determined combustion assessment to the full scale. This provides information about combustion conditions in the boiler which in turn are used in the evaluation of the furnace performance. Extensive furnace testwork of one of Eskom's wall fired plant (Hendrina Unit 9) during 1996, intended to validate the model for the ar plications outlined above, included the measurement {If : • gas temperatures • O2, C02, CO, NOx and S02 concentrations • residence time distributions • combustible matter in combustion residues extracted from the furnace • furnace heat fluxes. The coal used during the tests was sampled and subjected to a series of chemical and other lab-scale analyses to determine the following: • physical properties • composition • devolatilisation properties " combustion properties The same furnace was modelled using the University of Stuttgart's AIOLOS combustion code, the results of Which are compared with the measured data. A DTF derived combustion assessment of a coal sampled from the same site but from a different part of the beneficiation plant, which was found to burn differently, was subsequently used in a further simulation to assess the sensitivity of the model to char combustion rate data. The results of these predictions are compared to the predictions of the validation simulation. It was found that the model produces results that compare well with the measured data. Furthermore. the model was found to be sufficiently sensitive to reactivity parameters of the coal. The model has thereby demonstrated that it can be used in the envisaged application of extrapolating DTF reactivity assessments to full scale plant. In using the model, it has become apparent that the evaluations of furnace modifications and assessments of boiler operation lie well within the capabilities of the model. / MT2017

Lean combustion is increasingly employed in both ground-based gas turbines and aircraft engines for minimizing NOx emissions. Operating under lean conditions increases the risk of Lean Blowout (LBO). Thus LBO proximity sensors, combined with appropriate blowout prevention systems, have the potential to improve the performance of engines. In previous studies, atmospheric pressure, swirl flames near LBO have been observed to exhibit partial extinction and re-ignition events called LBO precursors. Detecting these precursor events in optical and acoustic signals with simple non-intrusive sensors provided a measure of LBO proximity. This thesis examines robust LBO margin sensing approaches, by exploring LBO precursors in the presence of combustion dynamics and for combustor operating conditions that are more representative of practical combustors, i.e., elevated pressure and preheat temperature operation. To this end, two combustors were used: a gas-fueled, atmospheric pressure combustor that exhibits pronounced combustion dynamics under a wide range of lean conditions, and a low NOx emission liquid-fueled lean direct injection (LDI) combustor, operating at elevated pressure and preheat temperature. In the gas-fueled combustor, flame extinction and re-ignition LBO precursor events were observed in the presence of strong combustion dynamics, and were similar to those observed in dynamically stable conditions. However, the signature of the events in the raw optical signals have different characteristics under various operating conditions. Low-pass filtering and a single threshold-based event detection algorithm provided robust precursor sensing, regardless of the type or level of dynamic instability. The same algorithm provides robust event detection in the LDI combustor, which also exhibits low level dynamic oscillations. Compared to the gas-fueled combustor, the LDI events have weaker signatures, much shorter durations, but considerably higher occurrence rates. The disparity in precursor durations is due to a flame mode switch that occurs during precursors in the gas-fueled combustor, which is absent in the LDI combustor. Acoustic sensing was also investigated in both the combustors. Low-pass filtering is required to reveal a precursor signature under dynamically unstable conditions in the gas-fueled combustor. On the other hand in the LDI combustor, neither the raw signals nor the low-pass filtered signals reveal precursor events. The failure of acoustic sensing is attributed in part to the lower heat release variations, and the similarity in time scales for the precursors and dynamic oscillations in the LDI combustor. In addition, the impact of acoustic reflections from combustor boundaries and transducer placement was addressed by modeling reflections in a one-dimensional combustor geometry with an impedance jump caused by the flame. Implementing LBO margin sensors in gas turbine engines can potentially improve time response during deceleration transients by allowing lower operating margins. Occurrence of precursor events under transient operating conditions was examined with a statistical approach. For example, the rate at which the fuel-air ratio can be safely reduced might be limited by the requirement that at least one precursor occurs before blowout. The statistics governing the probability of a precursor event occurring during some time interval was shown to be reasonably modeled by Poisson statistics. A method has been developed to select a lower operating margin when LBO proximity sensors are employed, such that the lowered margin case provides a similar reliability in preventing LBO as the standard approach utilizing a more restrictive operating margin. Illustrative improvements in transient response and reliabilities in preventing LBO are presented for a model turbofan engine. In addition, an event-based, active LBO control approach for deceleration transients is also demonstrated in the engine simulation.

This thesis describes an investigation on the blowout phenomenon in gas turbine combustors. The combustor primarily used for this study was a swirl- and dump-stabilized, atmospheric pressure device, which did not exhibit dynamic combustion instabilities. The first part of the thesis work concentrated on finding a sensing methodology to be able to predict the onset of approach of combustor blowout using optical methods. Temporary extinction-reignition events that occurred prior to blowout were found to be precursor events to blowout. A threshold based method was developed to identify these events in the time-resolved sensor output. The number and the average length of each event were found to increase as the LBO limit (fuel-air ratio) is approached. This behavior is used to predict the proximity to lean blowout. In the second part of this study, the blowout sensor was incorporated into a control system that monitored the approach of blowout and then actuated an alternate mechanism to stabilize the combustor near blowout. Enhanced stabilization was achieved by redirecting a part of the main fuel to a central preinjection pilot injection. The sensing methodology, without modification, was effective for the combustor with pilot stabilization. An event based control algorithm for controlling the combustor from blowing out was also developed in this study. The control system was proven to stabilize the combustor even when the combustor loading was rapidly changed. The final part of this study focused on understanding the physical mechanisms behind the precursor events. High speed movies of flame chemiluminescence and laser sheet scattering from oil droplets seeded into the reactants were analyzed to explain the physical processes that cause the extinction and the reignition of the combustor during a precursor event. A physical model for coupling of the fluid dynamics of vortex breakdown and combustion during precursor and blowout events is proposed. This model of blowout phenomenon, along with the sensing and control strategies developed in this study could enable the gas turbine combustor designers to design combustors with wider operability regimes. This could have significant payoffs in terms of reduction in NOx emissions from the combustor.