Improvements to heritage gas turbine engines will be needed in the coming years as the demand made on these systems increase. While industry continually presses for higher performance of both military and civilian aero engines, the government simultaneously raises the bar for emissions standards in the commercial sector to support public health. The next generation of aerospace gas turbine engines will be defined by their ability to operate at high power conditions while maintaining efficiency. This challenge is compounded by airlines' proposition of a return to supersonic flight- an operating regime characterized by higher total temperatures, and thus more NOx production. Lean Direct Injection (LDI) is a combustion scheme that was proposed by NASA, and inherently addresses the needs of both the private sector and the military. LDI is a liquid fueled combustor that promotes rapid mixing of fuel and air at the entrance of the combustor. Despite the benefits of LDI, it has never been implemented, nor has any other lean burning scheme been implemented in an aircraft due to the system level complications of such technology. This dissertation focuses on the dynamics of thermoacoustic instability and lean blowout (LBO), two of the major complications that industry will face when they attempt to incorporate LDI in a production engine. The present dissertation investigates these dynamics from a fundamental and applications standpoint. Fundamental insights on thermoacoustic instabilities are developed by investigating droplet dynamics in a self-excited flow field, and significant oscillations in droplet diameters are discerned. PDPA measurement will be taken to identify coupling of the fuel spray with the instability, and a phase locking algorithm will be used to develop a new spray parameter than is more indicative of combustion heat release that the standard Sauter mean diameter. Next, while varying the swirl number and the venturi geometry of the combustor, the evolution of the flow field will be characterized. An in-house innovation called the Direct Rotation Swirler (DRS) is built for this purpose. The DRS uses an active geometry to provide continuously variable swirl number modulation. The effects of these changes on lean blow out, pressure drop and NOx emissions will then be experimentally determined. Venturis were rapidly manufactured using a ii casting procedure that was developed to make venturi geometries from a commercially available ceramic at very low cost. / Doctor of Philosophy / Improvements to heritage gas turbine engines will be needed in the coming years as the demand made on these systems increase. While industry continually presses for higher performance of both military and civilian aero engines, the government simultaneously raises the bar for emissions standards in the commercial sector to support public health. The next generation of aerospace gas turbine engines will be defined by their ability to operate at high power conditions while maintaining efficiency. This challenge in compounded by airlines' proposition of a return to supersonic flight- an operating regime characterized by higher total temperatures, and thus more NOx production. Lean Direct Injection (LDI) is a combustion scheme that was proposed by NASA, and inherently addresses the needs of both the private sector and the military. LDI is a liquid fueled combustor that promotes rapid mixing of fuel and air at the entrance of the combustor. Rapid mixing yields a clean, even flame and eliminates the fuel-rich primary zone which is the heart of NOx production. Despite the benefits of LDI, it has never been implemented, nor has any other lean burning scheme been implemented in an aircraft due to the system level complications of such technology. This dissertation focuses on two of the major complications that industry will face when they attempt to incorporate LDI in a production engine. Drastically reducing the local hot spots in the primary zone is fundamentally necessary to improve pattern factor and emissions, but this change is directly at odds with two dynamic phenomenon that plague combustors. These effects are thermoacoustic instabilities, and lean blow out. Thermoacoustic instabilities are a major concern in any type of combustor and instabilities are more common and more intense in lean engines which is a significant safety risk to aircraft. A thermoacoustic instability occurs when pressure waves in an engine grow to high amplitudes. Small pressure waves are normal in any combustion process, but when the acoustic waves couple with the heat release, the waves can grow uncontrollably. The amplitudes can reach magnitudes capable of damaging the combustor or significantly reducing its cyclic life. Due to the high iv standard of safety in the aerospace industry, lean combustion will not be implemented until engines can be designed to avoid instabilities throughout the entire flight envelope. Lean blow out occurs when the fuel to air ratio of the engine becomes too low to sustain a flame. Lean blow out is a transient phenomenon that is dependent on local flame speeds, local chemical time scales and turbulence parameters. Typically, lean blow out is combated by designing a rich flame anchoring region that burns with plenty of excess fuel so that even if the fuel flow rate is reduced, a core region is still within its flammability regions. The present dissertation investigates these dynamics from a fundamental and applications standpoint. Fundamental insights on thermoacoustic instabilities are developed by investigating droplet dynamics in a self-excited flow field, and significant oscillations in droplet diameters are discerned. PDPA measurement will be taken to identify coupling of the fuel spray with the instability, and a phase locking algorithm will be used to develop a new spray parameter than is more indicative of combustion heat release that the standard Sauter mean diameter. Next, while varying the swirl number and the venturi geometry of the combustor, the evolution of the flow field will be characterized. An in-house innovation called the Direct Rotation Swirler (DRS) is built for this purpose. The DRS uses an active geometry to provide continuously variable swirl number modulation. The effects of these changes on lean blow out, pressure drop and NOx emissions will then be experimentally determined. Venturis were rapidly manufactured using a casting procedure that was developed to make venturi geometries from a commercially available ceramic at very low cost.

No description available.

Increasing concerns over the condition of our environment and its long term health have led to the development of greener combustion techniques for use in turbomachinery applications. Lean Direct Injection is an active area of research for how fuel is introduced and burned in the combustor section of a jet engine or land based liquid fuel turbine. Overall lean combustion results in lower NOx emmisions while direct injection insures shorter combustor lengths. Lean Direct Injection and other lean burning combustor designs are susceptible to thermoacoustic instabilities. The SARA or Synchronously Actuated Response Atomizer is a liquid fuel atomizer and supply system designed to allow for the active control of droplet size, cone angle, and mass flow rate. These three parameters have been shown to be important in controlling combustion quality and heat release. This research investigates the capabilities of the SARA design in a series of non-reacting tests. Static and Dynamic tests were performed on the SARA nozzle with a maximum actuation of 400 Hz. Also, a novel use of hot-film anemometry was developed to measure the dynamic flow rate fluctuations. / Master of Science

No description available.

This thesis describes experiments to investigate high-altitude relight of a lean direct injection (LDI) combustor. The features that make LDI technology less polluting in terms of NOx compared to conventional combustors are expected to impede relight performance. Therefore an improved understanding of ignition behaviour is required to ensure that stringent relight requirements can be satisfied. Realistic operating conditions are simulated in a ground-based test facility. The application of laser diagnostics presents particular difficulties due to the large quantities ofliquid fuel that impinge on the combustor walls during relight. Advances are made in the application of planar laser-induced fluorescence (PLIF) to monitor fuel placement in a combustor under these conditions. A novel apparatus is developed to deliver a laser sheet to the combustion chamber while protecting all optical surfaces from contamination. The PLIF images are compared with the cold flow field obtained from CFD modelling. These results indicate that fuel becomes trapped inside the central recirculation zone in highconcentrations. High-speed flame imaging performed simultaneously with the PLIF measurements provides important insights into the motion and breakup of flame during relight. An algorithm developed to track the flame activity reveals that the initial spark kernel is convected downstream, before breaking apart and moving upstream towards a recovery origin close to the fuel injector. Analysis of many ignition events has revealed several distinct modes of ignition failure.

This paper investigated the effects of swirl number and momentum ratio on the atomization and mixing performance of Swirl-Venturi Lean Direct Injection technology. Mie scattering of liquid water, was used to identify the location of water droplets in a cross section of the injector spray. Experiments were performed with three air swirlers with vane angles of 45, 52 and 60 degrees. The swirl number varied from 0.58 to 1.0 and air-to-liquid ratios from 15.8 to 35.6. A transition was observed in the liquid spray distribution for the 52 degree case, which unexpectedly produced twice as much signal than the 45 and 60 degree cases. The main cause of this increased signal may be due to instabilities in the flow when transitioning from low to high swirl states. The results from investigation of swirl number it was found that the spray pattern for is sensitive to swirl intensity. Two flow states were observed for a lower and higher swirl flow as well as a transition state that occurred with the lower swirl state. This work may aid in the specific inquiry of physical mechanisms relating to the effect of flow states on spray distribution. It is found that improved atomization and mixing performance are a result of increase in swirl number.

No description available.

No description available.

Thermoacoustic instabilities in modern high-performance, low-emission gas turbine engines are often observable as large amplitude pressure oscillations and can result in serious performance and structural degradations. These acoustic oscillations can cause oscillations in combustor through-flows and given the right phase conditions, can also drive unsteady heat release. This coupling has the potential to enhance the amplitude of pressure oscillations. To curb the potential harms caused by the existence of thermoacoustic instabilities, recent efforts have focused on the active suppression and even complete control of these instabilities. Intuitively, development of effective active combustion control methodologies is strongly dependent on the knowledge of the onset and sustenance of thermoacoustic instabilities. Specially, non-premixed spray combustion environment pose additional challenges due to the inherent unstable dynamics of sprays. The understanding of the manner in which the combustor acoustics affect the spray characteristics, which in turn result in heat release oscillation, is therefore, of paramount importance. The experimental investigations and the modeling studies conducted towards achieving this knowledge have been presented in this dissertation. Experimental efforts comprise both reacting and non-reacting flow studies. Reacting flow experiments were conducted on a overall lean direct injection, swirl-stabilized combustor rig. The investigations spanned combustor characterization and stability mapping over the operating regime. All experiments were performed under atmospheric pressure condition, which is considered as an obvious first step towards providing valuable insights into more intense processes in actual gas turbine combustors. The onset of thermoacoustic instability and the transition of the combustor to two unstable regimes were investigated via phase-locked chemiluminescence imaging and measurement and phase-locked acoustic characterization. It was found that the onset of the thermoacoustic instability is a function of the energy gain of the system, while the sustenance of instability is due to the in-phase relationship between combustor acoustics and unsteady heat release driven by acoustic oscillations. The presence of non-linearities in the system between combustor acoustic and heat release and also between combustor acoustics and air through-flow were found to exist. The impact of high amplitude limit-cycle pressure on droplet breakdown under very low mean airflow and the localized effects of forced primary fuel modulations on heat release were also investigated. The non-reacting flow experiments were conducted to study the spray behavior under the presence of an acoustic field. An isothermal acoustic rig was specially fabricated, where the pressure oscillations were generated using an acoustic driver. Phase Doppler Anemometry was used to measure the droplet velocities and sizes under varying acoustic forcing conditions and spray feed pressures. Measurements made at different locations in the spray were related to these variations in mean and unsteady inputs. The droplet velocities were found to show a second order response to acoustic forcing with the cut-off frequency equal to the relaxation time corresponding to mean droplet size. It was also found that under acoustic forcing the droplets migrate radially away from the spray centerline and show oscillatory excursions in their movement. Non-reacting flow experiments were also performed using Time-Resolved Digital Particle Image Velocimetry to characterize modulated sprays. Frequency response of droplet diameters were analyzed in the pulsed spray. These pilot experiments were conducted to assess the capability of the system to measure dynamic data. Modeling efforts were undertaken to gain physical insights of spray dynamics under the influence of acoustic forcing and to explain the experimental findings. The radial migration of droplets and their oscillatory movement were validated. The flame characteristics in the two unstable regimes and the transition between them were explained. It was found that under certain acoustic and mean air-flow condition, bands of high droplet densities were formed which resulted in diffusion type group burning of droplets. It was also shown that very high acoustic amplitudes cause secondary breakup of droplets. / Ph. D.

[ES] El principal desafío en los motores turbina de gas empleados en aviación reside en aumentar la eficiencia del ciclo termodinámico manteniendo las emisiones contaminantes por debajo de las rigurosas restricciones. Ésto ha conllevado la necesidad de diseñar nuevas estrategias de inyección/combustión que operan en puntos de operación peligrosos por su cercanía al límite inferior de apagado de llama. En este contexto, el concepto Lean Direct Injection (LDI) ha emergido como una tecnología prometedora a la hora de reducir los óxidos de nitrógeno (NOx) emitidos por las plantas propulsoras de los aviones de nueva generación. En este contexto, la presente tesis tiene como objetivos contribuir al conocimiento de los mecanismos físicos que rigen el comportamiento de un quemador LDI y proporcionar herramientas de análisis para una profunda caracterización de las complejas estructuras de flujo de turbulento generadas en el interior de la cámara de combustión. Para ello, se ha desarrollado una metodología numérica basada en CFD capaz de modelar el flujo bifásico no reactivo en el interior de un quemador LDI académico mediante enfoques de turbulencia U-RANS y LES en un marco Euleriano-Lagrangiano. La resolución numérica de este problema multi-escala se aborda mediante la descripción completa del flujo a lo largo de todos los elementos que constituyen la maqueta experimental, incluyendo su paso por el swirler y entrada a la cámara de combustión. Ésto se lleva a cabo través de dos códigos CFD que involucran dos estrategias de mallado diferentes: una basada en algoritmos de generación y refinamiento automático de la malla (AMR) a través de CONVERGE y otra técnica de mallado estático más tradicional mediante OpenFOAM. Por un lado, se ha definido una metodología para obtener una estrategia de mallado óptima mediante el uso del AMR y se han explotado sus beneficios frente a los enfoques tradicionales de malla estática. De esta forma, se ha demostrado que la aplicabilidad de las herramientas de control de malla disponibles en CONVERGE como el refinamiento fijo (fixed embedding) y el AMR son una opción muy interesante para afrontar este tipo de problemas multi-escala. Los resultados destacan una optimización del uso de los recursos computacionales y una mayor precisión en las simulaciones realizadas con la metodología presentada. Por otro lado, el uso de herramientas CFD se ha combinado con la aplicación de técnicas de descomposición modal avanzadas (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificación numérica de los principales modos acústicos en la cámara de combustión ha demostrado el potencial de estas herramientas al permitir caracterizar las estructuras de flujo coherentes generadas como consecuencia de la rotura de los vórtices (VBB) y de los chorros fuertemente torbellinados presentes en el quemador LDI. Además, la implementación de estos procedimientos matemáticos ha permitido tanto recuperar información sobre las características de la dinámica de flujo como proporcionar un enfoque sistemático para identificar los principales mecanismos que sustentan las inestabilidades en la cámara de combustión. Finalmente, la metodología validada ha sido explotada a través de un Diseño de Experimentos (DoE) para cuantificar la influencia de los factores críticos de diseño en el flujo no reactivo. De esta manera, se ha evaluado la contribución individual de algunos parámetros funcionales (el número de palas del swirler, el ángulo de dichas palas, el ancho de la cámara de combustión y la posición axial del orificio del inyector) en los patrones del campo fluido, la distribución del tamaño de gotas del combustible líquido y la aparición de inestabilidades en la cámara de combustión a través de una matriz ortogonal L9 de Taguchi. Este estudio estadístico supone un punto de partida para posteriores estudios de inyección, atomización y combus / [CA] El principal desafiament als motors turbina de gas utilitzats a la aviació resideix en augmentar l'eficiència del cicle termodinàmic mantenint les emissions contaminants per davall de les rigoroses restriccions. Aquest fet comporta la necessitat de dissenyar noves estratègies d'injecció/combustió que radiquen en punts d'operació perillosos per la seva aproximació al límit inferior d'apagat de flama. En aquest context, el concepte Lean Direct Injection (LDI) sorgeix com a eina innovadora a l'hora de reduir els òxids de nitrogen (NOx) emesos per les plantes propulsores dels avions de nova generació. Sota aquest context, aquesta tesis té com a objectius contribuir al coneixement dels mecanismes físics que regeixen el comportament d'un cremador LDI i proporcionar ferramentes d'anàlisi per a una profunda caracterització de les complexes estructures de flux turbulent generades a l'interior de la càmera de combustió. Per tal de dur-ho a terme s'ha desenvolupat una metodología numèrica basada en CFD capaç de modelar el flux bifàsic no reactiu a l'interior d'un cremador LDI acadèmic mitjançant els enfocaments de turbulència U-RANS i LES en un marc Eulerià-Lagrangià. La resolució numèrica d'aquest problema multiescala s'aborda mitjançant la resolució completa del flux al llarg de tots els elements que constitueixen la maqueta experimental, incloent el seu pas pel swirler i l'entrada a la càmera de combustió. Açò es duu a terme a través de dos codis CFD que involucren estratègies de mallat diferents: una basada en la generación automàtica de la malla i en l'algoritme de refinament adaptatiu (AMR) amb CONVERGE i l'altra que es basa en una tècnica de mallat estàtic més tradicional amb OpenFOAM. D'una banda, s'ha definit una metodologia per tal d'obtindre una estrategia de mallat òptima mitjançant l'ús de l'AMR i s'han explotat els seus beneficis front als enfocaments tradicionals de malla estàtica. D'aquesta forma, s'ha demostrat que l'aplicabilitat de les ferramente de control de malla disponibles en CONVERGE com el refinament fixe (fixed embedding) i l'AMR són una opció molt interessant per tal d'afrontar aquest tipus de problemes multiescala. Els resultats destaquen una optimització de l'ús dels recursos computacionals i una major precisió en les simulacions realitzades amb la metodologia presentada. D'altra banda, l'ús d'eines CFD s'ha combinat amb l'aplicació de tècniques de descomposició modal avançades (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificació numèrica dels principals modes acústics a la càmera de combustió ha demostrat el potencial d'aquestes ferramentes al permetre caracteritzar les estructures de flux coherents generades com a conseqüència del trencament dels vòrtex (VBB) i dels raigs fortament arremolinats presents al cremador LDI. A més, la implantació d'estos procediments matemàtics ha permès recuperar informació sobre les característiques de la dinàmica del flux i proporcionar un enfocament sistemàtic per tal d'identificar els principals mecanismes que sustenten les inestabilitats a la càmera de combustió. Finalment, la metodologia validada ha sigut explotada a traves d'un Diseny d'Experiments (DoE) per tal de quantificar la influència dels factors crítics de disseny en el flux no reactiu. D'aquesta manera, s'ha avaluat la contribución individual d'alguns paràmetres funcionals (el nombre de pales del swirler, l'angle de les pales, l'amplada de la càmera de combustió i la posició axial de l'orifici de l'injector) en els patrons del camp fluid, la distribució de la mida de gotes del combustible líquid i l'aparició d'inestabilitats en la càmera de combustió mitjançant una matriu ortogonal L9 de Taguchi. Aquest estudi estadístic és un bon punt de partida per a futurs estudis de injecció, atomització i combustió en cremadors LDI. / [EN] Aeronautical gas turbine engines present the main challenge of increasing the efficiency of the cycle while keeping the pollutant emissions below stringent restrictions. This has led to the design of new injection-combustion strategies working on more risky and problematic operating points such as those close to the lean extinction limit. In this context, the Lean Direct Injection (LDI) concept has emerged as a promising technology to reduce oxides of nitrogen (NOx) for next-generation aircraft power plants In this context, this thesis aims at contributing to the knowledge of the governing physical mechanisms within an LDI burner and to provide analysis tools for a deep characterisation of such complex flows. In order to do so, a numerical CFD methodology capable of reliably modelling the 2-phase nonreacting flow in an academic LDI burner has been developed in an Eulerian-Lagrangian framework, using the U-RANS and LES turbulence approaches. The LDI combustor taken as a reference to carry out the investigation is the laboratory-scale swirled-stabilised CORIA Spray Burner. The multi-scale problem is addressed by solving the complete inlet flow path through the swirl vanes and the combustor through two different CFD codes involving two different meshing strategies: an automatic mesh generation with adaptive mesh refinement (AMR) algorithm through CONVERGE and a more traditional static meshing technique in OpenFOAM. On the one hand, a methodology to obtain an optimal mesh strategy using AMR has been defined, and its benefits against traditional fixed mesh approaches have been exploited. In this way, the applicability of grid control tools available in CONVERGE such as fixed embedding and AMR has been demonstrated to be an interesting option to face this type of multi-scale problem. The results highlight an optimisation of the use of the computational resources and better accuracy in the simulations carried out with the presented methodology. On the other hand, the use of CFD tools has been combined with the application of systematic advanced modal decomposition techniques (i.e., Proper Orthogonal Decomposition and Dynamic Mode Decomposition). The numerical identification of the main acoustic modes in the chamber have proved their potential when studying the characteristics of the most powerful coherent flow structures of strongly swirled jets in a LDI burner undergoing vortex breakdown (VBB). Besides, the implementation of these mathematical procedures has allowed both retrieving information about the flow dynamics features and providing a systematic approach to identify the main mechanisms that sustain instabilities in the combustor. Last, this analysis has also allowed identifying some key features of swirl spray systems such as the complex pulsating, intermittent and cyclical spatial patterns related to the Precessing Vortex Core (PVC). Finally, the validated methodology is exploited through a Design of Experiments (DoE) to quantify the influence of critical design factors on the non-reacting flow. In this way, the individual contribution of some functional parameters (namely the number of swirler vanes, the swirler vane angle, the combustion chamber width and the axial position of the nozzle tip) into both the flow field pattern, the spray size distribution and the occurrence of instabilities in the combustion chamber are evaluated throughout a Taguchi's orthogonal array L9. Such a statistical study has supposed a good starting point for subsequent studies of injection, atomisation and combustion on LDI burners. / Belmar Gil, M. (2020). Computational study on the non-reacting flow in Lean Direct Injection gas turbine combustors through Eulerian-Lagrangian Large-Eddy Simulations [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/159882