Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERONAUTICS. / Bibliography: p. 235-239. / by Jere Schenck Meserole, Jr. / Ph.D.

Nos últimos 15 anos houve um grande aumento na presença de turbinas a gás no parque gerador de eletricidade brasileiro. O Brasil tem predominantemente climas tropicais e subtropicais com temperaturas oscilando entre 20 e 35C na maior parte do ano. A máxima potência que pode ser gerada por uma turbina a gás aumenta com a redução da temperatura do ar de entrada na turbina. Decorre daí o interesse na aplicação de sistemas de resfriamento do ar de entrada de turbinas. Dentre os sistemas de resfriamento, os de aplicação mais simples são o de resfriamento por meio evaporativo rígido e o de resfriamento por ciclo de compressão com acionamento elétrico. Não há na literatura um estudo sistemático da aplicação de sistemas de resfriamento de ar de entrada de turbinas a gás para operação no Brasil. Este trabalho estuda a aplicação dos dois tipos mais simples de sistemas de resfriamento de ar de entrada em turbinas operando ou a serem instaladas no território brasileiro. Um modelo para simulação da resposta de turbinas a gás às variações nas condições climáticas do ar de entrada (temperatura, umidade e pressão atmosférica) é desenvolvido. O modelo necessita como parâmetros somente de dados publicados em catálogo pelo fabricante da turbina. A simulação é feita para 27 localidades brasileiras comparando a operação de um mesmo tipo de turbina sem resfriamento e com os dois tipos de resfriamento. O dados climáticos usados são dos tipos anos metereológicos típicos e anos teste de referência. O modelo de turbina desenvolvido simula de maneira satisfatória as curvas de uma turbina comercial do tipo heavy duty. Um aumento de energia anual gerada de até 4,2% foi observado para o sistema de resfriamento por meio evaporativo rígido. O aumento de energia no resfriamento evaporativo depende da depressão de bulbo úmido média do local de instalação da turbina. Para o resfriamento por ciclo de compressão com acionamento elétrico o aumento observado foi de até 11,2%. O aumento de energia para este tipo de sistema depende da temperatura de bulbo seco média do local. / In the last 15 years an ever increasing presence of gas turbines was felt in electrical power generation in Brazil. Tropical and subtropical climates dominate most of the country, with temperatures ranging from 20 to 35C during most of the year. The maximum power that can be generated by a gas turbine increases at lower inlet air temperatures. Consequently, there is great interest in applying inlet air cooling systems in gas turbines. Among the inlet air cooling systems, the evaporative cooling by rigid wet media and the compression thermal cycle with electrical power chiller systems are the ones with most straightforward implementation. There is no systematic study of the application of gas turbine inlet air cooling systems for turbines operating in Brazil. This thesis studies the application of the two methods of gas turbine inlet air cooling mentioned above in turbines operating or to be installed in Brazil. A model to simulate the response of gas turbines to changes in the inlet air (temperature, humidity and pressure) is developed. The model uses turbine catalogue data as parameters. The simulation is performed for 27 Brazilian locations, comparing the operation of a model of turbine operating with and without cooling systems, for both types of cooling systems. Typical meteorological year and test reference year data are used in the study. The turbine model developed reproduces the turbines data curves with satisfactory accuracy. An annual increase in energy generation of up to 4,2% was observed for evaporative cooling. The energy gain for evaporative cooling depends on the annual mean wet bulb depression of the local. The compression thermal cycle increases the annual energy generation by up to 11,2%. The energy increase in this type of system depends on the mean dry bulb temperature of the local.

This thesis describes an experimental program covering the development of certain simple combustion chamber modifications to alleviate smoke emissions from the Allison T56 turboprop engines operated by the Royal Australian Air Force. The work includes a literature survey, smoke emission tests on two variants of the T56 engine, flow visualisation studies of the combustion system in a water tunnel and combustion rig tests of a standard combustor and four possible modifications. The rig tests showed that reductions in smoke emissions of 80% were possible by simple modifications that reduced the primary zone equivalence ratio and improved mixing in that zone.

This thesis explores the effects of operating parameters on the location and shape of lifted flames in a Low Swirl Burner (LSB). In addition, it details the development and analysis of a CH PLIF imaging system for visualizing flames in lean combustion systems. The LSB is studied at atmospheric pressure using LDV and CH PLIF. CH* chemiluminescence is used for high pressure flame imaging. A four-level model of the fluorescing CH system is developed to predict the signal intensity in hydrocarbon flames. Results from imaging an atmospheric pressure laminar flame are used to validate the behavior of the signal intensity as predicted by the model. The results show that the fluorescence signal is greatly reduced at high pressure due to the decreased number of CH molecules and the increased collisional quenching rate. This restricts the use of this technique to increasingly narrow equivalence ratio ranges at high pressures. The limitation is somewhat alleviated by increasing the preheat temperature of the reactant mixture. The signal levels from high hydrogen-content syngas mixtures doped with methane are found to be high enough to make CH PLIF a feasible diagnostic to study such flames. Finally, the model predicts that signal levels are unlikely to be significantly affected by the presence of strain in the flow field, as long as the flames are not close to extinction. The results from the LSB flame investigation reveal that combustor provides reasonably robust flame stabilization at low and moderate values of combustor pressure and reference velocities. However, at very high velocities and pressures, the balance between the reactant velocity and the turbulent flame speed shifts in favor of the former resulting in the flame moving downstream. The extent of this movement is small, but indicates a tendency towards blow off at higher pressures and velocities that may be encountered in real world gas turbine applications. There is an increased tendency of relatively fuel-rich flames to behave like attached flames at high pressure. These results raise interesting questions about turbulent combustion at high pressure as well as provide usable data to gas turbine combustor designers by highlighting potential problems.

The increasing price of fuel and energy, combined with environmental laws and regulations, have led many different energy producers to integrate renewable, clean energy sources with non-renewable ones, forming the idea of energy hubs. Energy hubs are systems of technologies where different energy forms are conditioned and transformed. These energy hubs offer many advantages compared to traditional single-energy sources, including increased reliability and security of meeting energy demand, maximizing use of energy and materials resulting in increasing the overall system efficiency. In this thesis, we consider an energy hub consisting of natural gas (NG) turbines for the main source of energy— electricity and heat— combined with two renewable energy sources—wind turbines and PV solar cells. The hub designed capacity is meant to simulate and replace the coal-fired Nanticoke Generating Station with NG-fired power plant. The generating station is integrated with renewable energy sources, including wind and solar. The hub will also include water electrolysers for hydrogen production. The hydrogen serves as an energy storage vector that can be used in transportation applications, or the hydrogen can be mixed into the NG feed stream to the gas turbines to improve their emission profile. Alkaline electrolysers’ technology is fully mature to be applied in large industrial applications. Hydrogen, as an energy carrier, is becoming more and more important in industrial and transportation sectors, so a significant part of the thesis will focus on hydrogen production and cost. In order to achieve the goal of replacing the Nanticoke Coal-fired Power Plant by introducing the energy hub concept, the study investigates the modeling of the combined system of the different technologies used in terms of the total energy produced, cost per kWh, and emissions. This modeling is done using GAMS® in order to make use of the optimization routines in the software. The system is modeled so that a minimum cost of energy is achieved taking into account technical and thermodynamic constrains. Excess energy produced during off-peak demand by wind turbines and PV solar cells is used to feed the electrolyser to produce H2 and O2. Through this method, a significant reduction in energy cost and greenhouse gas (GHG) emissions are achieved, in addition to an increased overall efficiency.

Combustor blowout is a very serious concern in modern land-based and aircraft engine combustors. The ability to sense blowout precursors can provide significant payoffs in engine reliability and life. The objective of this work is to characterize the blowout phenomenon and develop a sensing methodology which can detect and assess the proximity of a combustor to blowout by monitoring its acoustic signature, thus providing early warning before the actual blowout of the combustor. The first part of the work examines the blowout phenomenon in a piloted jet burner. As blowout was approached, the flame detached from one side of the burner and showed increased flame tip fluctuations, resulting in an increase in low frequency acoustics. Work was then focused on swirling combustion systems. Close to blowout, localized extinction/re-ignition events were observed, which manifested as bursts in the acoustic signal. These events increased in number and duration as the combustor approached blowout, resulting an increase in low frequency acoustics. A variety of spectral, wavelet and thresholding based approaches were developed to detect precursors to blowout. The third part of the study focused on a bluff body burner. It characterized the underlying flame dynamics near blowout in greater detail and related it to the observed acoustic emissions. Vorticity was found to play a significant role in the flame dynamics. The flame passed through two distinct stages prior to blowout. The first was associated with momentary strain levels that exceed the flames extinction strain rate, leading to flame holes. The second was due to large scale alteration of the fluid dynamics in the bluff body wake, leading to violent flapping of the flame front and even larger straining of the flame. This led to low frequency acoustic oscillations, of the order of von Karman vortex shedding. This manifested as an abrupt increase in combustion noise spectra at 40-100 Hz very close to blowout. Finally, work was also done to improve the robustness of lean blowout detection by developing integration techniques that combined data from acoustic and optical sensors.

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.

Turbulent combustion processes are inherently unsteady and, thus, a source of acoustic radiation, which occurs due to the unsteady expansion of reacting gases. While prior studies have extensively characterized the total sound power radiated by turbulent flames, their spectral characteristics are not well understood. The objective of this research work is to measure the flow and acoustic properties of an open turbulent premixed jet flame and explain the spectral trends of combustion noise. The flame dynamics were characterized using high speed chemiluminescence images of the flame. A model based on the solution of the wave equation with unsteady heat release as the source was developed and was used to relate the measured chemiluminescence fluctuations to its acoustic emission. Acoustic measurements were performed in an anechoic environment for several burner diameters, flow velocities, turbulence intensities, fuels, and equivalence ratios. The acoustic emissions are shown to be characterized by four parameters: peak frequency (Fpeak), low frequency slope (beta), high frequency slope (alpha) and Overall Sound Pressure Level (OASPL). The peak frequency (Fpeak) is characterized by a Strouhal number based on the mean velocity and a flame length. The transfer function between the acoustic spectrum and the spectrum of heat release fluctuations has an f^2 dependence at low frequencies, while it converged to a constant value at high frequencies. Furthermore, the OASPL was found to be characterized by (Fpeak mfH)^2, which resembles the source term in the wave equation.

Modeling the Response of Premixed Flames to Flow Disturbances Preetham 178 pages Directed by Dr. Tim Lieuwen Low emissions combustion systems for land based gas turbines rely on a premixed or partially premixed combustion process. These systems are exceptionally prone to combustion instabilities which are destructive to hardware and adversely affect performance and emissions. The success of dynamics prediction codes is critically dependent on the heat release model which couples the flame dynamics to the system acoustics. So the principal objective of the current research work is to predict the heat release response of premixed flames and to isolate the key non-dimensional parameters which characterize its linear and nonlinear dynamics. Explicit analytical solutions of the G- equation are derived in the linear and weakly nonlinear regime using the Small Perturbation Method (SPM). For the fully nonlinear case, the flame-flow interaction effects are captured by developing an unsteady, compressible, coupled Euler-G-equation solver with a Ghost Fluid Method (GFM) module for applying the jump conditions across the flame. The flame s nonlinear response is shown to exhibit two qualitatively different behaviors. Depending on the operating conditions and the disturbance field characteristics, it is shown that a combustor may exhibit supercritical bifurcations leading to a single stable limit cycle amplitude or exhibit sub-critical bifurcations wherein multiple stable solutions for the instability amplitude are possible. In addition, this study presents the first analytical model which captures the effects of unsteady flame stretch on the heat release response and thus extends the applicability of current models to high frequency instabilities, such as occurring during screech. It is shown that unsteady stretch effects, negligible at low frequencies (100 s of Hz) become significant at screeching frequencies (1000 s of Hz). Furthermore, the analysis also yields insight into the significant spatial dependence of the mean and perturbation velocity field induced by the coupling between the flame and the flow field. In order to meaningfully compare the heat release response across different flame configurations, this study has identified that the reference velocity (for defining the transfer function) should be based on the effective normal velocity perturbing the flame and the Strouhal number should be based on the effective residence time of the flame wrinkles.

Film cooling effectiveness on a gas turbine blade tip on the near tip pressure side and on the squealer cavity floor is investigated. Optimal arrangement of film cooling holes, effect of a full squealer and a cutback squealer, varying blowing ratios and squealer cavity depth are also examined on film cooling effectiveness. The film-cooling effectiveness distributions are measured on the blade tip, near tip pressure side and the inner pressure and suction side rim walls using a Pressure Sensitive Paint (PSP) technique. A blowing ratio of 1.0 is found to give best results on the pressure side whereas the other tip surfaces give best results for blowing ratios of 2. Film cooling effectiveness tests are also performed on the span of a fully-cooled high pressure turbine blade in a 5 bladed linear cascade using the PSP technique. Film cooling effectiveness over the entire blade region is determined from full coverage film cooling, showerhead cooling and from each individual row with and without an upstream wake. The effect of superposition of film cooling effectiveness from each individual row is then compared with full coverage film cooling. Results show that an upstream wake can result in lower film cooling effectiveness on the blade. Effectiveness magnitudes from superposition of effectiveness data from individual rows are comparable with that from full coverage film cooling. Internal heat transfer measurements are also performed in a high aspect ratio channel and from jet array impingement on a turbulated target wall at large Reynolds numbers. For the channel, three dimple and one discrete rib configurations are tested on one of the wide walls for Reynolds numbers up to 1.3 million. The presence of a turbulated wall and its effect on heat transfer enhancement against a smooth surface is investigated. Heat transfer enhancement is found to decrease at high Re with the discrete rib configurations providing the best enhancement but highest pressure losses. Experiments to investigate heat transfer and pressure loss from jet array impingement are also performed on the target wall at Reynolds numbers up to 450,000. The heat transfer from a turbulated target wall and two jet plates is investigated. A target wall with short pins provides the best heat transfer with the dimpled target wall giving the lowest heat transfer among the three geometries studied.