M.S.

Dans ce travail, un prototype de turbine activée par une décharge d’air comprimé a été produit et testé pour trouver sa géométrie optimale, et ce, sur la base d'un modèle thermodynamique. Un dispositif expérimental, a été spécialement conçu pour tester la turbine. Il comprend un système de piston et de valves servant à charger un réservoir d’air comprimé, ainsi qu'une valve de fin de course servant à déclencher la décharge du réservoir à travers la turbine. L'appareil expérimental permet la mesure directe de la pression et de la température dans le réservoir et utilise un système d'acquisition de données basé sur LabVIEW. Pour assurer une mesure précise de ces paramètres, des thermocouples à réponse rapide et des capteurs de pression sont utilisés. Le modèle thermodynamique de charge et de décharge d'un réservoir est déterminé par un processus polytropique afin de prévoir la pression et la température de l’air à l'intérieur du réservoir. De plus, l’influence des paramètres de la turbine, tels que la vitesse angulaire initiale de l’arbre de la turbine et les bords avant ou arrière des aubes du rotor et du stator, a été étudiée en fonction de ses performances. Les résultats montrent que la turbine avec une vitesse angulaire initiale peut fournir un rendement supérieur à chaque impulsion. De plus, la plus grande vitesse angulaire initiale de la turbine contribue à une puissance plus élevée. En outre, les angles appropriés peuvent limiter la probabilité de séparation sur la trajectoire du mouvement du fluide.

Ce mémoire a comme objectif de prédire les performances d’une turbine modèle de type bulbe à l’aide de simulations numériques RANS k-ε. Tout d’abord, par des simulations numériques, on valide les éléments de la méthodologie les plus susceptibles d’influencer la qualité des résultats. Par la suite, on obtient la colline de rendement numérique et on analyse l’effet du jeu de bout d’aube ainsi que des jeux au moyeu sur la prédiction de performance. Les résultats indiquent que la colline de rendement numérique se compare bien à la littérature portant sur les turbines bulbes. On montre, de plus, que tenir compte du jeu de bout d’aube dans la simulation numérique diminue de manière significative le rendement prédit. L’impact des jeux au moyeu est cependant négligeable sur la prédiction de performance de la turbine. Finalement, une modification de l’aspirateur est proposée pour la suite du projet entrepris par le consortium de recherche.

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2013-2014. / Ce mémoire présente l'étude expérimentale de l'écoulement dans le canal inter-aube d’une roue de turbine de type bulbe. Pour ce faire, deux campagnes de mesures ont été réalisées. La première a fait usage de l’anémométrie laser à effet Doppler (LDV). La seconde campagne a fait appel à l’anémométrie par image de particules (PIV) et à un montage stéréoscopique endoscopique conçu sur mesure pour atteindre la région ciblée. Les données recueillies permettent de caractériser plusieurs phénomènes. L’importance du sillage des directrices entre les aubes de la roue est mise en évidence par les deux techniques de mesure. La campagne de mesure par LDV permet plus spécifiquement de cibler les tourbillons de jeu de bout d’aube et d’identifier des débalancements fixe et rotatif. En complément, les mesures par PIV révèlent la présence d’un tourbillon qui provient du bord d’attaque près du moyeu lorsque la turbine opère à charge partielle. / This work presents the experimental study of the flow in the inter-blade channel of the runner of a bulb turbine. To do so, two measurement campaigns were carried out. The first used laser Doppler velocimetry (LDV). The second campaign used particle image velocimetry (PIV) and a custom designed stereoscopic endoscopic setup allowed reaching this otherwise difficult to access measurement plane. A comparison of the two sets of data collected indicates a good match over the entire area on which they overlap. The gathered data allows characterising many phenomena. The importance of the guide vanes wake on the runner flow is highlighted by both measurement techniques. The LDV measurement campaign allows characterizing the blade tip vortices and identifying fixed and rotary flow imbalances. In addition, the PIV measurements reveal the presence of a vortex that originates from the leading edge near the hub when the turbine operates at partial load.

The persistent thrust for a cleaner, greener environment has prompted air pollution regulations to be enforced with increased stringency by environmental protection bodies all over the world. This has prompted gas turbine manufacturers to move from non-premixed combustion to lean, premixed combustion. These lean premixed combustors operate quite fuel-lean compared to the stochiometric, in order to minimize CO and NOx productions, and are very susceptible to oscillations in any of the upstream flow variables. These oscillations cause the heat release rate of the flame to oscillate, which can engage one or more acoustic modes of the combustor or gas turbine components, and under certain conditions, lead to limit cycle oscillations. This phenomenon, called thermoacoustic instabilities, is characterized by very high pressure oscillations and increased heat fluxes at system walls, and can cause significant problems in the routine operability of these combustors, not to mention the occasional hardware damages that could occur, all of which cumulatively cost several millions of dollars. In a bid towards understanding this flow-flame interaction, this research works studies the heat release response of premixed flames to oscillations in reactant equivalence ratio, reactant velocity and pressure, under conditions where the flame preheat zone is convectively compact to these disturbances, using the G-equation. The heat release response is quantified by means of the flame transfer function and together with combustor acoustics, forms a critical component of the analytical models that can predict combustor dynamics. To this end, low excitation amplitude (linear) and high excitation amplitude (nonlinear) responses of the flame are studied in this work. The linear heat release response of lean, premixed flames are seen to be dominated by responses to velocity and equivalence ratio fluctuations at low frequencies, and to pressure fluctuations at high frequencies which are in the vicinity of typical screech frequencies in gas turbine combustors. The nonlinear response problem is exclusively studied in the case of equivalence ratio coupling. Various nonlinearity mechanisms are identified, amongst which the crossover mechanisms, viz., stoichiometric and flammability crossovers, are seen to be responsible in causing saturation in the overall heat release magnitude of the flame. The response physics remain the same across various preheat temperatures and reactant pressures. Finally, comparisons between the chemiluminescence transfer function obtained experimentally and the heat release transfer functions obtained from the reduced order model (ROM) are performed for lean, CH4/Air swirl-stabilized, axisymmetric V-flames. While the comparison between the phases of the experimental and theoretical transfer functions are encouraging, their magnitudes show disagreement at lower Strouhal number gains show disagreement.

Gas turbine reheat thrust augmenters known as afterburners are used to provide additional thrust during emergencies, take off, combat, and in supersonic flight of high-performance aircrafts. During the course of reheat development, the most persistent trouble has been the onset of high frequency combustion instability, also known as screech, invariably followed by rapid mechanical failure. The coupling of acoustic pressure upstream of the flame stabilizer with in-phase heat-release downstream, results in combustion instability by which the amplitude at various resonant modes — longitudinal (buzz — low frequency), tangential or radial (screech — high frequency) – amplifies leading to deterioration of the afterburner components. Various researchers in early 1950s have performed extensive testing on straight jet afterburners, to identify screech frequencies. Theoretical and experimental work at test rig level has been reported in the case of buzz to validate the heat release combustion models. In this work, focus is given to study the high frequency tangential combustion instability by vibro-acoustic software and the tests are conducted on the scaled bypass flow afterburner for confirmation of predicted screech frequencies. The wave equation for the afterburner is solved taking the appropriate geometry of the afterburner and taking into account the factors affecting the stability. Nozzle of the afterburner is taken into account by using the nozzle admittance condition derived for a choked nozzle. Screech liner admittance boundary condition is imposed and the effect on acoustic attenuation is studied. A new combustion model has been proposed for obtaining the heat release rate response function to acoustic oscillations. Acoustic wave – flame interactions involve unsteady kinetic, fluid mechanic and acoustic processes over a large range of time scales. Three types of flow disturbances exist such as : vortical, entropy, and acoustic. In a homogeneous, uniform flow, these three disturbance modes propagate independently in the linear approximation. Unsteady heat release also generates entropy and vorticity disturbances. Since flow is not accelerated in the region of uniform area duct, vortical and entropy disturbances are treated as in significant, as these disturbances are convected out into atmosphere like an open-ended tube, but these are considered in deriving the nozzle admittance condition. Heat release fluctuations that arise due to fluctuating pressure and temperature are taken into consideration. The aim is to provide results on how flames respond to pressure disturbances of different amplitudes and characterised by different length scales. The development of the theory is based on large activation energy asymptotics. One-dimensional conservation equations are used for obtaining the response function for the heat release rate assuming the laminar flamelet model to be valid. The estimates are compared with the published data and deviations are discussed. The normalized acoustic pressure variation in the afterburner is predicted using the models discussed earlier to provide an indication of the resonant modes of the pressure oscillations and the amplification and attenuation of oscillations caused by the various processes. Similar frequency spectrum is also obtained experimentally using a test rig for a range of inlet mean pressures and temperatures with combustion and core and bypass flows simulated, for confirmation of predicted results. Without the heat source only longitudinal acoustic modes are found to be excited in the afterburner test section. With heat release, three additional tangential modes are excited. By the use of eight probes in the circumferential cross section of afterburner it was possible to identify the tangential modes by their respective phase shift in the experiments. Comparison of normalized acoustic pressure and phase with and without the incorporation of perforate liner is made to study the effectiveness of the screech liner in attenuating the amplitude of screech modes. By the analysis, conclusion is drawn about modes that get effectively attenuated with the presence of perforate liner. Parametric study of screech liner porosity factor of 1.5 % has not shown appreciable attenuation. Whereas with 2.5 % porosity significant attenuation is noticed, but with 4 % porosity, the gain is very minimal. Hence, the perforate screech liner with the porosity of 2.5 % is finalized. From the rig runs, first pure screech tangential mode and second screech coupled tangential modes are captured. The theoretical frequencies for first and second tangential modes with their phases are comparable with experimental results. Though third tangential mode is predicted, it was not excited in the experiments. There was certain level of deviation in the prediction of these frequencies, when compared to the experimentally obtained values. For this test section of length to diameter ratio of 5, no radial modes are encountered both in the analysis and experiments in the frequency range of interest. In summary, an acoustic model has been developed for the afterburner combustor, taking into account the combustion response, the screech liner and the nozzle to study the acoustic instability of the afterburner. The model has been validated experimentally for screech frequencies using a model test rig and the results have given sufficient confidence to apply the model for full scale afterburners as a predictive design tool.

A novel combustor design, referred to as the Stagnation Point Reverse-Flow (SPRF) combustor, was recently developed that is able to operate stably at very lean fuel-air mixtures and with low NOx emissions even when the fuel and air are not premixed before entering the combustor. The primary objective of this work is to elucidate the underlying physics behind the excellent stability and emissions performance of the SPRF combustor. The approach is to experimentally characterize velocities, species mixing, heat release and flame structure in an atmospheric pressure SPRF combustor with the help of various optical diagnostic techniques: OH PLIF, chemiluminescence imaging, PIV and Spontaneous Raman Scattering. Results indicate that the combustor is primarily stabilized in a region downstream of the injector that is characterized by low average velocities and high turbulence levels; this is also the region where most of the heat release occurs. High turbulence levels in the shear layer lead to increased product entrainment levels, elevating the reaction rates and thereby enhancing the combustor stability. The effect of product entrainment on chemical timescales and the flame structure is illustrated with simple reactor models. Although reactants are found to burn in a highly preheated (1300 K) and turbulent environment due to mixing with hot product gases, the residence times are sufficiently long compared to the ignition timescales such that the reactants do not autoignite. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zones regime throughout the combustor, and it tends to become more flamelet like with increasing distance from the injector. Fuel-air mixing measurements in case of non-premixed operation indicate that the fuel is shielded from hot products until it is fully mixed with air, providing nearly premixed performance without the safety issues associated with premixing. The reduction in NOx emissions in the SPRF combustor are primarily due to its ability to stably operate under ultra lean (and nearly premixed) condition within the combustor. Further, to extend the usefulness of this combustor configuration to various applications, combustor geometry scaling rules were developed with the help of simplified coaxial and opposed jet models.

Call number: LD2668 .T4 1984 L58 / Master of Science

Rural electrification has long been the most important topic on the development agenda of many countries. The needs for power supplies to rural areas increased significantly in the past decades. Extending electricity grids to rural areas is of a very high initial cost and is not viable economically. Micro hydroelectric power plants provide a good economical solution, which is also environmentally very friendly. The current study concentrates on selecting and optimizing a suitable cross-flow micro-turbine to be used in micro hydroelectric power plants. Cross-flow turbines are in general of simple structure, low cost, easy to fabricate and of modest efficiency. The main purpose of the present work is to optimize the performance of a selected turbine by establishing the optimal turbine’s design parameters. A complete analysis of the internal flow, which is of turbulent, two-phase and three dimensional in nature, was undertaken by simulating it using various CFD simulation codes. This study reports on the flow simulation using ANSYS CFX with a two-phase flow model, water-air free surface model and shear stress transport (SST) turbulence model. Prediction velocity and pressure fields of inside the turbine are, subsequently, used to characterize the turbine performance for different geometric parameters including the number of runner blades, the angle of attack, the ratio of inner to outer diameter, the nozzle profile, the blade profile, the nozzle throat width, the nozzle to runner blades width and the runner blades width to outer runner diameter. The results revealed the highly complex nature of the flow and provided a very good insight to the flow structure and performance optimization parameters.

Στην διπλωματική εργασία που ακολουθεί, παρουσιάζεται η προσομοίωση και μελέτη ενός υβριδικού συστήματος διανεμημένης παραγωγής. Το σύστημα που εξετάζεται είναι ένα υβριδικό σύστημα ηλεκτροπαραγωγής που αποτελείται από μία ανεμογεννήτρια, μια φωτοβολταϊκή συστοιχία και μία γεννήτρια diesel. Για τη προσομοίωση του συστήματος χρησιμοποιείται το πρόγραμμα σχεδίασης και προσομοίωσης ηλεκτρικών συστημάτων PSCAD. Η εργασία επικεντρώνεται στη δομή και λειτουργία του συστήματος τόσο στην μόνιμη κατάσταση λειτουργίας όσο και στη συμπεριφορά του συστήματος σε διάφορα μεταβατικά φαινόμενα. Τα μεταβατικά φαινόμενα που εξετάζονται είναι τα εξής: • Μείωση προσπίπτουσας ακτινοβολίας σε φωτοβολταϊκη συστοιχία • Αποσύνδεση Ανεμογεννήτριας από το δίκτυο • Τριφασικό βραχυκύκλωμα ως προς την γή στον ζυγό της γεννήτριας diesel • Απότομη αύξηση του φορτίου Στο 1ο Κεφάλαιο, γίνεται μια αναφορά στα υβριδικά συστήματα παραγωγής ηλεκτρικής ενέργειας, στα συστήματα κατανεμημένης παραγωγής, στις δομές τους και στις ανανεώσιμες πηγές ενέργειας. Επίσης αναφέρονται μερικές εφαρμογές τους. Στο 2ο Κεφάλαιο, περιγράφονται ξεχωριστά τα στοιχεία παραγωγής ηλεκτρικής ενέργειας καθώς και τα υπόλοιπα στοιχεία του υβριδικού συστήματος διανεμημένης παραγωγής που απαρτίζουν την εργασία. Ταυτόχρονα παρουσιάζονται και αναλύονται οι μαθηματικές σχέσεις σύμφωνα με τις οποίες λειτουργεί το σύστημα. Στο 3ο Κεφάλαιο, εισερχόμαστε στο περιβάλλον του προγράμματος μοντελοποίησης PSCAD, παρουσιάζεται και περιγράφεται η δομή και τα στοιχεία που αποτελούν το υβριδικό δίκτυο ηλεκτροπαραγωγής, ενώ γίνεται και αναφορά στις προδιαγραφές που πρέπει να τηρούνται από το σύστημα διανεμημένης παραγωγής. Στο 4ο Κεφάλαιο, παρατηρείται η συμπεριφορά ολόκληρου του συστήματος στην μόνιμη κατάσταση λειτουργίας καθώς και των επιμέρους υποσυστημάτων. Στο 5ο Κεφάλαιο, εξετάζεται η συμπεριφορά ολόκληρου του συστήματος καθώς και των επιμέρους υποσυστημάτων στις διαταραχές που έχουμε περιγράψει παραπάνω. Στο 6ο Κεφάλαιο και τελευταίο, καταλήγουμε σε συμπεράσματα τα οποία προέκυψαν από την μελέτη του υβριδικού συστήματος που παρουσιάστηκε. / The thesis that follows, presents the simulation and design of a hybrid distributed system of electrical power generation. The system presented, consists of a wind turbine, a photovoltaic array and a diesel generator. For the system simulation we use the drawing and simulation of electrical systems program, PSCAD. The project focuses on the structure and operation of the system both at steady -permanent state and in system behavior in various transient situations. The transients considered are the following: • Reduction in incident solar radiation into array • Wind Turbine disconnection from the network • Three-phase short circuit as for ground under the rule of the diesel generator • Sharp load increase In 1st chapter, there is a reference to hybrid power generation systems, distributed generation systems, their structures and the renewable sources of energy. Also some applications are mentioned. In 2nd chapter, there is a description of the elements that produce electrical energy and the other elements of the hybrid system of distributed production of electrical energy which are used in the system. At the same time there are presented and analyzed the mathematical relations under which the system operates. In 3rd chapter, we enter the interface of the PSCAD modeling program, present and describe the structure and components of the hybrid power grid, while we also make a reference to the specifications that have to be met by the system of distributed production. In 4th chapter, is examined the behavior of the whole system as well as of the individual subsystems in steady-permanent state. In 5th chapter, is examined the behavior of the entire system as well as of the individual subsections during the disturbances which we have described above. In 6th chapter and last, we come to conclusions that emerged from the study of the hybrid system presented.