The complex multiphase flow physics of spray-swirl interaction in both reacting and non-reacting environment is of fundamental and applied significance for a wide variety of applications ranging from gas turbine combustors to pharmaceutical drug nebulizers. Understanding the intricate dynamics between this two phase flow field is pivotal for enhancing mixing characteristics, reducing pollutant emissions and increasing the combustion efficiency in next generation combustors. The present work experimentally investigates the near and far-field break-up, dispersion and coalescence characteristics of a hollow cone spray in an unconfined, co¬annular isothermal swirling air jet environment. The experiments were conducted using an axial-flow hollow cone spray nozzle having a 0.5 mm orifice. Nozzle injection pressure (PN = 1 bar) corresponding to a Reynolds number at nozzle exit ReN = 7900 used as the test setting. At this setting, the operating Reynolds number of the co-annular swirling air stream number (Res) was varied in four distinct steps, i.e. Res = 1600, 3200, 4800 and 5600. Swirl was imparted to the co¬axial flow using a guided vane swirler with blade angle of Ф=45° (corresponding geometric swirl number SG = 0.8). Two types of laser diagnostic techniques were utilized: Particle / Droplet imaging analysis (PDIA) and shadowgraph to study the underlying physical mechanisms involved in the primary breakup, dispersion and coalescence dynamics of the spray. Measurements were made in the spray in both axial and radial directions and they indicate that Sauter Mean Diameter (SMD) in radial direction is highly reliant on the intensity of swirl imparted to the spray. The spray is subdivided into two zones as function of swirl in axial and radial direction: (1) near field of the nozzle (ligament regime) where variation in SMD arises predominantly due to primary breakup of liquid films (2) far-field of the nozzle where dispersion and collision induced coalescence of droplets is dominant. Each regime has been analyzed meticulously, by computing probability of primary break-up, probability of coalescence and spatio-temporal distribution of droplets which gives probabilistic estimate of aforementioned governing phenomena. In addition to this, spray global length scale parameters such as spray cone angle, break-up length, wavelength of liquid film has been characterized by varying Res while maintaining constant ReN.

Les procédés existant pour la fabrication de sous-couches et de systèmes barrière thermique pour les aubes mobiles des turbomachines sont complexes, onéreux, et, de leur reproductibilité dépend la durée de vie de ces systèmes. Cette étude montre la faisabilité d'obtention de sous-couches γ-Ni + γ'-Ni3Al enrichies en platine et de systèmes barrière thermique complets, par l'utilisation du procédé de Spark Plasma Sintering (SPS). Les paramètres du procédé SPS (pression, durée, température et nombre de paliers) ont été ajustés afin de fabriquer les différents types de systèmes. Des améliorations ont été apportées à l'outillage, en vue notamment de mieux appréhender la température de la pièce lors du cycle thermique et, d'empêcher la formation de carbures. Les propriétés microstructurales des revêtements obtenus sont caractérisées et mises en relation avec les paramètres d'élaboration. De plus, les propriétés d'usage (cinétique d'oxydation, résistance au cyclage thermique et à la corrosion par les aluminosilicates fondus, CMAS, conductivité thermique) ont été évaluées. Un large domaine de composition de sous-couches a pu être exploré, notamment par l'addition, par pulvérisation cathodique, d'éléments réactifs (Hf, Y, Si) et d'autres éléments (Ag, Au, Cu) ayant un effet sur le domaine de stabilité de la phase γ'. La possibilité de réaliser des systèmes barrière thermique mono et bi-couches céramiques en une seule étape par le procédé SPS est également démontrée. Un mode d'endommagement spécifique des systèmes barrière thermique élaborés par SPS a été mis en évidence lors d'essais de cyclage thermique et interprété avec l'aide de simulations numériques par éléments finis. Les nombreuses compositions et architectures réalisées au cours de cette étude, sont un encouragement à poursuivre les améliorations apportées au procédé pour l'obtention de systèmes plus complexes et plus fiables. / Fabrication of bond coatings and thermal barrier coating systems for aircraft engine turbine blades and vanes, rely on complex and costly processes, and the lifetime of the systems highly depends on their reproducibility. This work demonstrates the feasibility of platinum rich γ-Ni + γ'-Ni3Al bond coatings and complete thermal barrier coating systems using the Spark Plasma Sintering (SPS) process. Processing parameters (pressure, number, temperature and duration of dwells) have been finely tuned to fabricate the different types of systems. Some necessary adjustments to the equipments have been made to prevent the specimens from pollution, and to better control the samples temperature, and are discussed. The microstructure characteristics and their relation with process parameters have been investigated. Properties such as oxidation kinetics, thermal cycling resistance and CMAS (molten aluminosilicate) corrosion are evaluated. Physical vapor deposition have been used for bond coat doping with reactive elements (Hf, Y, Si) and elements that extend the γ' phase stability domain (Au, Ag, Cu). The possibility to fabricate complete thermal barrier coating systems with a mono or a bi-ceramic top coat layer in a single step is demonstrated. SPS thermal barrier coating systems exhibit a singular spalling behavior during thermal cycling. A finite-element numerical model has been developed and allows its understanding. The versatility of the SPS process has allowed the fabrication of a large number of bond coat compositions and thermal barrier coatings architectures. There is a strong incentive in developing this process for fabricating more reliable and competitive systems.

Au sein des parties statiques d’une turbine à gaz, l’étanchéité du système d’air derefroidissement est couramment assurée par un montage de segments. De par sa rigiditéélevée, ce type de joint s’adapte mal aux pièces avec lesquelles il entre en contact. Uneméthodologie, associant métrologie et calcul éléments finis, a effectivement montré quel’ouverture, à l’interface entre un segment et sa piste, est très supérieure à celle qui caractériseun contact simplement rugueux. Une analyse en ordres de grandeur a révélé que lesmécanismes d’écoulement du gaz, sur un champ d’ouverture typique de l’interface entre la pisteet le segment, sont conditionnés par deux nombres adimensionnels. Trois régimes peuvent êtredistingués en fonction des valeurs prises par ces nombres. L’un d’eux, le régime laminairecompressible inertiel, est particulièrement intéressant car il est présent dans les zones de laturbine où l’étanchéité est cruciale. Un modèle quasi analytique a été développé pour décrire cerégime. En l’associant à un modèle de déformation, on aboutit à un outil prédictif du débit degaz induit par un segment. Des essais sur banc partiel ont permis de le valider et de démontrersa précision. L’outil développé fournit le moyen de quantifier l’impact du champ de température,du champ de pression et des écarts géométriques, propres à un stator de turbine, sur lesperformances de l’étanchéité. Ce travail a débouché sur des règles de dimensionnement et deconception qui portent notamment sur l’obturation du jeu inter-extrémités, sur le choix dunombre de segments et du sens de serrage. / The sealing of gas turbine static parts is usually performed using piston rings.Because of its high stiffness, this type of gasket does not adapt itself to the parts with which it isdue to be in contact. A method involving metrology and a FEM actually revealed that the ringboreor ring-piston contact aperture is far larger than the one within a rough contact. Using ascale analysis, we showed that gas flow mechanisms, through the aperture field of the contactbetween the ring and the inner or outer part (i.e. the piston or the bore), depend on twodimensionless numbers. Three regimes can be distinguished depending on the values of thesenumbers. One of them, the compressible inertial laminar regime, is of particular interestbecause of its relevance in turbine areas where sealing is strongly critical. A quasi analyticalmodel was developed to describe this regime. By a coupling with a strain model, it wasupgraded to a predictive tool of leakage through a piston-ring or bore-ring contact. Acomparison with measurements carried out on a partial test rig allowed to prove its accuracy.This tool made it possible to quantify the influence of temperature field, pressure field andgeometrical defects, which are typical of a turbine stator, on sealing performance. This worklead us to propose design rules concerning, in particular, the ring gap type, number of rings tobe mounted and choice of tightening direction.

A lot of research effort is spent worldwide in order to reduce the environmental impact of the transportation and power generation sector. To minimize the environmental pollution the role of hydrogen fuelled gas turbines is intensively discussed in several research scenarios, like the IGCC-technology or the application of hydrogen as large scale storage for renewable energy sources. The adaptation of the applied gas turbine combustion chamber technology and control technology is mandatory for a stable and secure low NOx operation of a hydrogen fuelled gas turbine.<p>The micromix combustion principle was invented at Aachen University of Applied Sciences and achieves a significant reduction of the NOx-emissions by the application of multi miniaturized diffusion-type flamelets. Based on the research experiences, gained during the two European hydrogen research programs EQHHPP and Cryoplane at Aachen University of Applied Sciences, the intention of this thesis was to continue the scientific research work on low NOx hydrogen fuelled gas turbines. This included the experimental characterization of the micromix combustion principle, the design of an improved combustion chamber, based on the micromix combustion principle, for industrial gas turbine applications and the improvement of the gas turbine’s control and metering technology.<p>The experimental characterization of the micromix combustion principle investigated the impact of several key parameters, which influence the formation of the NOx-emissions, and allows therefore the definition of boundary conditions and design laws, in which a low NOx operation of the micromix combustion principle is practicable. In addition the ability of the micromix combustion principle to operate at elevated energy densities up to 15 MW/(m2bar) was successfully demonstrated. The improved combustion chamber design concept includes the experiences gained during the experimental characterization and covers the industrial needs regarding scalability and manufacturability.<p>The optimization and testing is done with an Auxiliary Power Unit GTCP 36-300. The original kerosene fuelled gas turbine was modified for the hydrogen application. Therefore several hardware and software modifications were realized. The improved gas turbine’s control and metering technology enables stable and comparable operational characteristics as in kerosene reference. An improved hydrogen metering unit, which is controlled by the industrial Versatile Engine Control Box, was successfully implemented. <p>The combination of the micromix combustion technology and of the optimized control and metering technology allows a stable, secure and low NOx hydrogen fuelled gas turbine operation.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

The growing need of mobile power sources with high energy density and the robustness to operate also in the harshest environmental surroundings lead to the idea of downscaling gas turbines to ì-scale. Classified as PowerMEMS devices, a couple of design attempts have emerged in the last decade. One of these attempts was the Belgian “PowerMEMS” design started back in 2003 and aiming towards a ì-scale gas turbine rated at 1 kW of electrical power output.<p>This PhD thesis presents the scientific evaluation and development history of different combustion chamber designs based upon the “PowerMEMS” design parameters. With hydrogen as chosen fuel, the non-premixed diffusive “micromix” concept was selected as combustion principle. Originally designed for full scale gas turbine applications in two different variants, consequently the microcombustor development had to start with the downscaling of these two principles towards ì-scale. Both principles have the advantage to be inherently safe against flashback, due to the non-premixed concept, which is an important issue even in this small scale application when burning hydrogen. By means of water analogy and CFD simulations the hydrogen injection system and the chamber geometry could be validated and optimized. Besides the specific design topics that emerged during the downscaling process of the chosen combustion concepts, the general difficulties of microcombustor design like e.g. high power density, low Reynolds numbers, short residence time, and manufacturing restrictions had to be tackled as well.<p>As full scale experimental test campaigns are still mandatory in the field of combustion research, extensive experimental testing of the different prototypes was performed. All test campaigns were conducted with a newly designed test rig in a combustion lab modified for microcombustion investigations, allowing testing of miniaturized combustors according to full engine requirements with regard to mass flow, inlet temperature, and chamber pressure. The main results regarding efficiency, equivalence ratio, and combustion temperature were obtained by evaluating the measured exhaust gas composition. Together with the performed ignition and extinction trials, the evaluation and analysis of the obtained test results leads to a full characterization of each tested prototype and delivered vital information about the possible operating regime in a later UMGT application. In addition to the stability and efficiency characteristics, another critical parameter in combustor research, the NOx emissions, was investigated and analyzed for the different combustor prototypes.<p>As an advancement of the initial downscaled micromix prototypes, the following microcombustor prototype was not only a combustion demonstrator any more, but already aimed for easy module integration into the real UMGT. With a further optimized combustion efficiency, it also featured an innovative recuperative cooling of the chamber walls and thus allowing an cost effective all stainless steel design.<p>Finally, a statement about the pros and cons of the different micromix combustion concepts and their correspondent combustor designs towards a possible ì-scale application could be given. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

M.Sc. / The increased demand for electricity in South Africa is expected to exceed supply between 2004 and 2007. Electricity supply options in the country would be further complicated by the fact that older power stations would reach the end of their design life beyond the year 2025. In light of this and considering the long lead times required for the commissioning of new plants, new power supply options need to be proactively investigated. The environmental impacts associated with coal-fired generation of electricity have resulted in increased global concern over the past decade. To reduce these impacts, new technologies have been identified to help provide electricity from fossil fuels. The alternatives considered are gas-fired generation technologies and the Integrated Gasification Combined Cycle (IGCC). This study attempts to document and understand the environmental aspects related to gas-fired and IGCC electricity generation and evaluate their advantages in comparison to conventional pulverised coal fired power generation. The options that could be utilised to make fossil fuel electricity generation more environmentally friendly, whilst remaining economically feasible, were also evaluated. Gas-fired electricity generation is extremely successful as electricity generation systems in the world due to inherently low levels of emissions, high efficiencies, fuel flexibility and reduced demand on finite resources. Associated benefits of a Combined Cycle Gas Turbine (CCGT) are lower operating costs due to the reduced water consumption, smaller equipment size and a reduction in the wastewater that has to be treated before being returned to the environment. A CCGT plant requires less cooling water and can be located on a smaller area than a conventional Pulverised Fuel (PF) power station of the same capacity. All these factors reduce the burden on the environment. A CCGT also employs processes that utilises the energy of the fuel more efficiently, with the current efficiencies approaching 60%. Instead of simply being discharged into the atmosphere, the gas turbines’ exhaust gas heat is used to produce additional output in combination with a Heat Recovery Steam Generator (HRSG) and a steam turbine. Furthermore, as finite resources become increasingly scarce and energy has to be used as wisely as possible, generating electricity economically and in an ecologically sound manner is of the utmost importance. The clean, reliable operation of gas-fired generation systems with significantly reduced noise levels and their compact design makes their operation feasible in heavily populated areas, where electricity is needed most. At the same time, energy can be consumed in whatever form needed, i.e. as electricity, heat or steam. The dependence of the South African economy on cheap coal ensures that it will remain a vital component of future electricity generation options in the country. This dominance of coal-fired generation in the country is responsible for South Africa’s title as the largest generator of carbon dioxide (CO2) emissions on the continent and the country could possibly be requested to reduce its CO2 emissions at the next international meeting of signatories to the Kyoto Protocol. Carbon dioxide emissions can be reduced by utilising gas-fired generation technologies. However, the uncertainty and costs associated with natural gas in South Africa hampers the implementation of this technology. There are currently a number of initiatives surrounding the development of natural gas in the country, viz. the Pande and Temane projects in Mozambique and the Kudu project in Namibia, and this is likely to positively influence the choice of fuel utilised for electricity generation in the future. The economic viability of these projects would be further enhanced through the obtaining of Clean Development Mechanism (CDM) credits for greenhouse gases (GHG) emissions reduction. Alternatively, more efficient methods of generating electricity from coal must be developed and implemented. IGCC is capable of achieving this because of the high efficiencies associated with the combined cycle component of the technology. These higher efficiencies result in reduced emissions to the atmosphere for an equivalent unit of electricity generated from a PF station. An IGCC system can be successful in South Africa in that it combines the benefits of utilising gas-fired electricity generation systems whilst utilising economically feasible fuel, i.e. coal. IGCC systems can economically meet strict air pollution emission standards, produce water effluent within environmental limits, produce an environmentally benign slag, with good potential as a saleable by-product, and recover a valuable sulphur commodity by-product. Life-cycle analyses performed on IGCC power plants have identified CO2 release and natural resource depletion as their most significant positive lifecycle impacts, which testifies to the IGCC’s low pollutant releases and benign by-products. Recent studies have also shown that these plants can be built to efficiently accommodate future CO2 capture technology that could further reduce environmental impacts. The outstanding environmental performance of IGCC makes it an excellent technology for the clean production of electricity. IGCC systems also provide flexibility in the production of a wide range of products including electricity, fuels, chemicals, hydrogen, and steam, while utilizing low-cost, widely available feedstocks. Coal-based gasification systems provide an energy production alternative that is more efficient and environmentally friendly than competing coalfuelled technologies. The obstacle to the large-scale implementation of this technology in the country is the high costs associated with the technology. CDM credits and by-products sales could possible enhance the viability of implementing these technologies in South Africa.

Finding limit cycles and their stability is one of the central problems of nonlinear thermoacoustics. However, a limit cycle is not the only type of self-excited oscillation in a nonlinear system. Nonlinear systems can have quasi-periodic and chaotic oscillations. This thesis examines the different types of oscillation in a numerical model of a ducted premixed flame, the bifurcations that lead to these oscillations and the influence of external forcing on these oscillations. Criteria for the existence and stability of limit cycles in single mode thermoacoustic systems are derived analytically. These criteria, along with the flame describing function, are used to find the types of bifurcation and minimum triggering amplitudes. The choice of model for the velocity perturbation field around the flame is shown to have a strong influence on the types of bifurcation in the system. Therefore, a reduced order model of the velocity perturbation field in a forced laminar premixed flame is obtained from Direct Numerical Simulation. It is shown that the model currently used in the literature precludes subcritical bifurcations and multi-stability. The self-excited thermoacoustic system is simulated in the time domain with many modes in the acoustics and analysed using methods from nonlinear dynamical systems theory. The transitions to the periodic, quasiperiodic and chaotic oscillations are via sub/supercritical Hopf, Neimark-Sacker and period-doubling bifurcations. Routes to chaos are established in this system. It is shown that the single mode system, which gives the same results as a describing function approach, fails to capture the period-$2$, period-$k$, quasi-periodic and chaotic oscillations or the bifurcations and multi-stability seen in the multi-modal case, and underpredicts the amplitude. Instantaneous flame images reveal that the wrinkles on the flame surface and pinch off of flame pockets are regular for periodic oscillations, while they are irregular and have multiple time and length scales for quasi-periodic and chaotic oscillations. Cusp formation, their destruction by flame propagation normal to itself, and pinch-off and rapid burning of pockets of reactants are shown to be responsible for generating a heat release rate that is a highly nonlinear function of the velocity perturbations. It is also shown that for a given acoustic model of the duct, many discretization modes are required to capture the rich dynamics and nonlinear feedback between heat release and acoustics seen in experiments. The influence of external harmonic forcing on self-excited periodic, quasi-periodic and chaotic oscillations are examined. The transition to lock-in, the forcing amplitude required for lock-in and the system response at lock-in are characterized. At certain frequencies, even low-amplitude forcing is sufficient to suppress period-$1$ oscillations to amplitudes that are 90$\%$ lower than that of the unforced state. Therefore, open-loop forcing can be an effective strategy for the suppression of thermoacoustic oscillations. This thesis shows that a ducted premixed flame behaves similarly to low-dimensional chaotic systems and that methods from nonlinear dynamical systems theory are superior to the describing function approach in the frequency domain and time domain analysis currently used in nonlinear thermoacoustics.

This work is concerned with the fluid dynamic processes and the associated loss of acoustic energy produced by circular apertures within noise absorbing perforated walls. Although applicable to a wide range of engineering applications particular emphasis in this work is placed on the use of such features within a gas turbine combustion system. The primary aim for noise absorbers in gas turbine combustion systems is the elimination of thermo-acoustic instabilities, which are characterised by rapidly rising pressure amplitudes which are potentially damaging to the combustion system components. By increasing the amount of acoustic energy being absorbed the occurrence of thermo-acoustic instabilities can be avoided. The fundamental acoustic characteristics relating to linear acoustic absorption are presented. It is shown that changes in orifice geometry, in terms of gas turbine combustion system representative length-to-diameter ratios, result in changes in the measured Rayleigh Conductivity. Furthermore in the linear regime the maximum possible acoustic energy absorption for a given cooling mass flow budget of a conventional combustor wall will be identified. An investigation into current Rayleigh Conductivity and aperture impedance (1D) modelling techniques are assessed and the ranges of validity for these modelling techniques will be identified. Moreover possible improvements to the modelling techniques are discussed. Within a gas turbine system absorption can also occur in the non-linear operating regime. Hence the influence of the orifice geometry upon the optimum non-linear acoustic absorption is also investigated. Furthermore the performance of non-linear acoustic absorption modelling techniques is evaluated against the conducted measurements. As the amplitudes within the combustion system increase the acoustic absorption will transition from the linear to the non-linear regime. This is important for the design of absorbers or cooling geometries for gas turbine combustion systems as the propensity for hot gas ingestion increases. Hence the relevant parameters and phenomena are investigated during the transition process from linear to non-linear acoustic absorption. The unsteady velocity field during linear and non-linear acoustic absorption is captured using particle image velocimetry. A novel analysis technique is developed which enables the identification of the unsteady flow field associated with the acoustic absorption. In this way an investigation into the relevant mechanisms within the unsteady flow fields to describe the acoustic absorption behaviour of the investigated orifice plates is conducted. This methodology will also help in the development and optimisation of future damping systems and provide validation for more sophisticated 3D numerical modelling methods. Finally a set of design tools developed during this work will be discussed which enable a comprehensive preliminary design of non-resonant and resonant acoustic absorbers with multiple perforated liners within a gas turbine combustion system. The tool set is applied to assess the impact of the gas turbine combustion system space envelope, complex swirling flow fields and the propensity to hot gas ingestion in the preliminary design stages.

No description available.

Driven by intense competition, increasing operational cost and strict environmental regulations, the modern process and energy industry needs to find the best possible way to adapt to maintain profitability. Optimization of control and operation of the industrial systems is essential to satisfy the contradicting objectives of improving product quality and process efficiency while reducing production cost and plant downtime. Use of optimization not only improves the control and monitoring of assets but also offers better coordination among different assets. Thus, it can lead to considerable savings in energy and resource consumption, and consequently offer a reduction in operational costs, by offering better control, diagnostics and decision support. This is one of the main driving forces behind developing new methods, tools and frameworks that can be integrated with the existing industrial automation platforms to benefit from optimal control and operation. The main focus of this dissertation is the use of different process models, soft sensors and optimization techniques to improve the control, diagnostics and decision support for the process and energy industry. A generic architecture for an optimal control, diagnostics and decision support system, referred to here as a learning system, is proposed. The research is centred around an investigation of different components of the proposed learning system. Two very different case studies within the energy-intensive pulp and paper industry and the promising micro-combined heat and power (CHP) industry are selected to demonstrate the learning system. One of the main challenges in this research arises from the marked differences between the case studies in terms of size, functions, quantity and structure of the existing automation systems. Typically, only a few pulp digesters are found in a Kraft pulping mill, but there may be hundreds of units in a micro-CHP fleet. The main argument behind the selection of these two case studies is that if the proposed learning system architecture can be adapted for these significantly different cases, it can be adapted for many other energy and process industrial cases. Within the scope of this thesis, mathematical modelling, model adaptation, model predictive control and diagnostics methods are studied for continuous pulp digesters, whereas mathematical modelling, model adaptation and diagnostics techniques are explored for the micro-CHP fleet. / FUDIPO – FUture DIrections for Process industry Optimization