The ingestion of hot mainstream gas into the wheel-space between the rotor and staler discs is one of the most important internal cooling problems for gas turbine designers. To solve this problem, engineers design a rim seal at the periphery of wheel-space and direct a sealing flow from the internal cooling system to prevent ingress. The main aim of this thesis is to build a simple computational model to predict the scaling effectiveness of externally-induced ingress for engine designers. The axisymmetric model represents a gas turbine wheel-space and provides useful information related to the fluid dynamics and heat transfer in the wheel-space. At the same time, this model saves much computation time and cost for engine designers who currently use complex and time-consuming 3D models. The- computational model in this -thesis is called the prescribed ingestion model. Steady simulations are carried out using the commercial CFD code, ANSYS CFX with meshes built using ICEM CFD. Boundary conditions are applied at the ingress inlet of the model using experimental measurements and a mass-based averaging procedure. Computational parameters such as rotational Reynolds number, non-dimensional sealing flow rate and thermal conditions on the rotor are selected to investigate the fluid dynamics and heat transfer at typical experimental rig operating conditions. Different rim seal geometries arc investigated and results are compared with experimental data. In addition to the prescribed ingestion model, two typical axisymmetric rotor-stator system models without ingress arc established. The aim of these rotor-stator models is to investigate the fluid dynamics and heat transfer of the wheel-space in the situation without ingress. The effects of geometry and turbulence model also arc studied in these simulations. Most results from these simulations are in good agreement with experimental data from the literature, which enhances confidence in the prescribed Ingestion model.

An experimental apparatus was designed and built to study the film cooling effectiveness from a single row of holes at various angles and hole spacings, using a foreign gas technique. Mixtures of Freon 12 and air were injected into an air mainstream to give a range of density ratios encompassing the values found in a gas turbine. The density ratio was found to be of importance and none of the commonly used parameters - e.g. blowing parameters - can be used to scale results, unless the density ratio is correctly modelled. The boundary layer thickness was varied independently of other parameters, and an increase in thickness was found to decrease the effectiveness, for normal and angled injection geometries for 20 hole diameters downstream. Favourable and adverse pressure gradients over the injection holes were tested and found to have little effect on the film cooling effectiveness. Changing the hole spacing produced considerable variations, with the smallest hole spacing giving the best performance in all respects. A hole spacing of greater than 3.75 diameters was found to be the maximum to give overall coverage above 0.10 effectiveness. The injection angle was also investigated and for low blowing rates the shallow angles gave the best results; but at high blowing rates, i.e. greater than 1.4, normal injection gave the best performance as the shallow angles rapidly became detached from the surface with increasing velocity ratio. The normal injection was also superior in terms of lateral distribution of coolant at all values of blowing rate. A correlation was proposed that included the density and velocity ratios and hole spacing for normal injection and, in a modified form, for angled injection at 3 diameter spacings. This was found to work well for the experimental results obtained here and by other researchers.

Αρκετές φορές υπήρξαν επιστημονικές ανακαλύψεις που περίμεναν την δημιουργία νέων ή την βελτίωση γνωστών υλικών, προτού βρούν εφαρμογή. Σε άλλες περιπτώσεις πάλι, καινοτομίες στην τεχνολογία των υλικών υπήρξαν το κίνητρο για την ανάπτυξη νέων εφαρμογών και εφευρέσεων. Είναι πράγματι εντυπωσιακό το πόσα από τα σύγχρονα τεχνολογικά επιτεύγματα οφείλουν την ύπαρξή τους στην, τυχαία πολλές φορές, ανακάλυψη ενός πρωτοποριακού υλικού, μιας νέας μεθόδου επεξεργασίας, ή άγνωστων ιδιοτήτων σε γνώριμα υλικά. Σήμερα ο τομέας των υλικών αποτελεί ανεξάρτητο πεδίο επιστημονικής έρευνας με έντονη δραστηριότητα. Παράλληλα επιτελείται σημαντικό έργο με στόχο την αξιοποίηση από την τεχνολογία, των ερευνητικών αποτελεσμάτων της επιστήμης των υλικών. Μία άποψη της δραστηριότητας αυτής είναι η εισαγωγή σε πειραματικό επίπεδο, νέων υλικών σε γνωστές εφαρμογές. Αν και αυτό αποτελεί ένα σημαντικό βήμα, ωστόσο η υιοθέτηση ενός υλικού από την βιομηχανία με την είσοδό του στην παραγωγή, εξαρτάται από πολλούς παράγοντες. Ένας από τους πιο καθοριστικούς, ειδικά όσο αφορά εφαρμογές υψηλής τεχνολογίας, είναι η δυνατότητα προσαρμογής των διαδικασιών σχεδιασμού στις συνθήκες που διαμορφώνονται από τα ιδιαίτερα χαρακτηριστικά της συμπεριφοράς του υλικού, σε συνδυασμό με τις απαιτήσεις της εφαρμογής. Σε αυτό το γενικό πλαίσιο εντάσεται η παρούσα εργασία, η οποία έχει ως αντικείμενο την ανάπτυξη μεθοδολογίας σχεδιασμού, κατάλληλης για εφαρμογή σε στοιχεία από Σύνθετα Κεραμικά Υλικά Συνεχών Ινών (Continuous Fiber Ceramic Composites – CFCC’s) καταπονούμενα από θερμομηχανικά φορτία. Τα ΣΚΥ Συνεχών Ινών αποτελούν εξελιγμένη μορφή των Προηγμένων Κεραμικών Υλικών, τα οποία είναι ίσως τα πλέον ανθεκτικά υλικά σε περιβάλλοντα υψηλών θερμοκρασιών που έχουν αναπτυχθεί έως σήμερα. Τα ΣΚΥ είναι αντικείμενο συνεχούς έρευνας και ανάπτυξης και αναμένεται να επιφέρουν σημαντική πρόοδο σε πολλούς τομείς. Μία από τις πολλές εφαρμογές για τις οποίες θεωρούνται ελκυστικά, είναι οι στροβιλοκινητήρες αεροχημάτων και οι στροβιλομηχανές παραγωγής ενέργειας. Η αντικατάσταση των μεταλλικών κραμμάτων από ΣΚΥ στα «θερμά» μέρη των στροβιλομηχανών εκτιμάται ότι είναι δυνατό να επιφέρει αύξηση του βαθμού απόδοσης έως και 20% με ταυτόχρονη μείωση των εκπεμπόμενων ρύπων. Στην περιοχή αυτή εστιάζεται η παρουσίαση της προτεινόμενης μεθοδολογίας και συγκεκριμένα στον σχεδιασμό και την ανάλυση ενός πρότυπου θαλάμου καύσης στροβιλομηχανής κατασκευασμένου από ΣΚΥ. Η μεθοδολογία αυτή επιχειρεί να καλύψει τις ιδιαίτερες απαιτήσεις της μοντελοποίησης των ΣΚΥ και της ανάλυσης στοιχείων κατασκευών από ΣΚΥ, που προκύπτουν από τα χαρακτηριστικά της συμπεριφοράς τους στις συνθήκες λειτουργίας των στροβιλομηχανών. Ως βασικό στοιχείο της μεθοδολογίας σχεδιασμού συμπεριλαμβάνεται στην παρούσα εργασία και η λεπτομερής περιγραφή του πειραματικού χαρακτηρισμού ενός τύπου ΣΚΥ με μήτρα και ενίσχυση οξειδίων. Στην διαδικασία αυτή εισάγεται και μια πρωτότυπη μέθοδος ποσοτικού χαρακτηρισμού που στηρίζεται σε μη-καταστροφικές δοκιμές με χρήση υπερήχων. Η διαδικασία εφαρμογής της μεθόδου, όσο αφορά το πειραματικό και το υπολογιστικό μέρος της αναπτύχθηκε εξ’ολοκλήρου στα πλαίσια της παρούσας εργασίας και επιτρέπει την ανίχνευση και τον ποσοτικό προσδιορισμό ανισότροπης βλάβης σε ΣΚΥ. Η επεξεργασία των αποτελεσμάτων της διαδικασίας χαρακτηρισμού, οδηγεί στην διαμόρφωση μαθηματικού μοντέλου που περιγράφει την μηχανική συμπεριφορά του υλικού κάτω από δεδομένες συνθήκες λειτουργίας. Συγκεκριμένα, όσο αφορά τα ΣΚΥ με μήτρα και ενίσχυση από οξείδιο του αλουμινίου στα οποία εστιάζεται η εργασία, διαπιστώθηκε υποβάθμιση των ελαστικών ιδιοτήτων και της αντοχής λόγω παραμονής του υλικού σε περιβάλλον υψηλής θερμοκρασίας. Η υποβάθμιση αυτή προσδιορίστηκε ποσοτικά σαν συνάρτηση της θερμοκρασίας και του χρόνου έκθεσης. Η μεθοδολογία σχεδιασμού στηρίζεται σε μεγάλο βαθμό στην ανάλυση και προσομοίωση της λειτουργίας στοιχείων κατασκευών με την μέθοδο των Πεπερασμένων Στοιχείων. Σημαντικό μέρος της παρούσας εργασίας αφορά την ανάπτυξη υπολογιστικού κώδικα ο οποίος επιτυγχάνει την προσαρμογή του πρωτότυπου μοντέλου υλικού σε εμπορικό λογισμικό ανάλυσης με ΠΣ. Επιπρόσθετα ο υπολογιστικός κώδικας εφαρμόζει μοντελοποίηση προοδευτικής αστοχίας που περιλαμβάνει τρείς ξεχωριστούς μηχανισμούς δομικής αστοχίας. Τέλος, ο κώδικας αυτός χρησιμοποιείται στην προσομοίωση της λειτουργίας ενός τυπικού θαλάμου καύσης στροβιλομηχανής παραγωγής ενέργειας σε συνθήκες σταθερής κατάστασης (steady state), κατασκευασμένου από στρωματοποιημένο ΣΚΥ συνεχών ινών και παρουσιάζονται τα αποτελέσματα της ανάλυσης. / -

This work presents a model of the fouling mechanism and the evaluation of compressor washing on line. The results of this research were obtained from experimental and computational models. The experimental model analyzed the localization of the particle deposition on the blade surface and the change of the surface roughness condition. The design of the test rig was based on the cascade blade arrangement and blade aerodynamics. The results of the experiment demonstrated that fouling occurred on both surfaces of the blade. This mechanism mainly affected the leading edge region of the blade. The increment of the surface roughness on this region was 1.0 μm. This result was used to create the CFD model (FLUENT). According to the results of the CFD, fouling reduced the thickness of the boundary layer region and increased the drag force of the blade. The model of fouling was created based on the experiment and CFD results and was used to calculate the engine performance in the simulation code (TURBOMATCH). The engine performance results demonstrated that in five days fouling can affect the overall efficiency by 3.5%. The evaluation of the compressor washing on line was based on the experimental tests and simulation of the engine performance. This system demonstrated that it could recover 99% of the original blade surface. In addition, this system was evaluated in a study case of a Power Plant, where it proved itself to be a techno-economic way to recover the power of the engine due to fouling. The model of the fouling mechanism presented in this work was validated by experimental tests, CFD models and information from real engines. However, for further applications of the model, it would be necessary to consider the specific conditions of fouling in each new environment.

Combustion instabilities (thermo-acoustic pressure oscillations) have been recognised for some time as a problem limiting the development of low emissions (e.g., lean burn) gas turbine combustion systems, particularly for aviation propulsion applications. Recently, significant research efforts have been focused on acoustic damping for suppression of combustion instability. Most of this work has either been experimental or based on linear acoustic theory. The last 3-5 years has seen application of density based CFD methods to this problem, but no attempts to use pressure-based CFD methods which are much more commonly used in combustion predictions. The goal of the present work is therefore to develop a pressure-based CFD algorithm in order to predict accurately acoustic propagation and acoustic damping processes, as relevant to gas turbine combustors. The developed computational algorithm described in this thesis is based on the classical pressure-correction approach, which was modified to allow fluid density variation as a function of pressure in order to simulate acoustic phenomena, which are fundamentally compressible in nature. The fact that the overall flow Mach number of relevance was likely to be low ( mildly compressible flow) also influenced the chosen methodology. For accurate capture of acoustic wave propagation at minimum grid resolution and avoiding excessive numerical smearing/dispersion, a fifth order accurate Weighted Essentially Non-Oscillatory scheme (WENO) was introduced. Characteristic-based boundary conditions were incorporated to enable accurate representation of acoustic excitation (e.g. via a loudspeaker or siren) as well as enable precise evaluation of acoustic reflection and transmission coefficients. The new methodology was first validated against simple (1D and 2D) but well proven test cases for wave propagation and demonstrated low numerical diffusion/dispersion. The proper incorporation of Characteristic-based boundary conditions was validated by comparison against classical linear acoustic analysis of acoustic and entropy waves in quasi-1D variable area duct flows. The developed method was then applied to the prediction of experimental measurements of the acoustic absorption coefficient for a single round orifice flow. Excellent agreement with experimental data was obtained in both linear and non-linear regimes. Analysis of predicted flow fields both with and without bias flow showed that non-linear acoustic behavior occurred when flow reversal begins inside the orifice. Finally, the method was applied to study acoustic excitation of combustor external aerodynamics using a pre-diffuser/dump diffuser geometry previously studied experimentally at Loughborough University and showed the significance of boundary conditions and shear layer instability to produce a sustained pressure fluctuation in the external aerodynamics.

Ο σκοπός της διατριβής αυτής είναι η γενίκευση και η μελέτη των προβλημάτων ελέγχου του δυαδικού οριακού στρώματος σε ένα γενικότερο πρόβλημα στο οποίο το μαγνητικό πεδίο και η έγχυση ξένου ρευστού συνυπάρχουν και αλληλεπιδρούν, ενώ ταυτόχρονα η ροή χαρακτηρίζεται ως ροή ολίσθησης, λόγω της αραίωσης του μέσου. / --

Current and future legislation demands ever decreasing levels of pollution from gas turbine engines, and with combustor performance playing a critical role in resultant emissions, a need exists to develop a greater appreciation of the fundamental causes of unsteadiness. Particle Image Velocimetry (PIV) provides a platform to enable such investigations. This thesis presents the development of PIV measurement methodologies for highly turbulent flows. An appraisal of these techniques applied to gas turbine combustors is then given, finally allowing a description of the increased understanding of the underlying fluid dynamic processes within combustors to be provided. Through the development of best practice optimisation procedures and correction techniques for the effects of sub-grid filtering, high quality PN data has been obtained. Time average statistical data at high spatial resolution has been collected and presented for generic and actual combustor geometry providing detailed validation of the turbulence correction methods developed, validation data for computational studies, and increased understanding of flow mechanisms. These data include information not previously available such as turbulent length scales. Methodologies developed for the analysis of instantaneous PIV data have also allowed the identification of transient flow structures not seen previously because they are invisible in the time average. Application of a new `PDF conditioning' technique has aided the explanation of calculated correlation functions: for example, bimodal primary zone recirculation behaviour and jet misalignments were explained using these techniques. Decomposition of the velocity fields has also identified structures present such as jet shear layer vortices, and through-port swirling motion. All of these phenomena are potentially degrading to combustor performance and may result in flame instability, incomplete combustion, increased noise and increased emissions.

In modern, low emission, gas turbine combustion systems the amount of air available for cooling of the flame tube liner is limited. This has led to the development of more complex cooling systems such as cooling tiles i.e. a double skin system, as opposed to the use of more conventional cooling slots i.e. a single skin system. An isothennal experimental facility has been constructed which can incorporate 10 times full size single and double skin (cooling tile) test specimens. The specimens can be tested with or without effusion cooling and measurements have been made to characterise the flow through each cooling system along with the velocity field and cooling effectiveness distributions that subsequently develop along the length of each test section. The velocity field of the coolant film has been defined using pneumatic probes, hot-wire anemometry and PIV instrumentation, whilst gas tracing technique is used to indicate (i) the adiabatic film cooling effectiveness and (ii) mixing of the coolant film with the mainstream flow. Tests have been undertaken both with a datum low turbulence mainstream flow passing over the test section, along with various configurations in which large magnitudes and scales of turbulence were present in the mainstream flow. These high turbulence test cases simulate some of the flow conditions found within a gas turbine combustor. Results are presented relating to a variety of operating conditions for both types of cooling system. The nominal operating condition for the double skin system was at a coolant to mainstream blowing ratio of approximately 1.0. At this condition, mixing of the mainstream and coolant film was relatively small with low mainstream turbulence. However, at high mainstream turbulence levels there was rapid penetration of the mainstream flow into the coolant film. This break up of the coolant film leads to a significant reduction in the cooling effectiveness. In addition to the time-averaged characteristics, the time dependent behaviour of the .:coolantfilm was. also investigated. In particular, unsteadiness associated with large scale structures in the mainstream flow was observed within the coolant film and adjacent to the tile surface. Relative to a double skin system the single skin geometry requires a higher coolant flow rate that, along with other geometrical changes, results in typically higher coolant to mainstream velocity ratios. At low mainstream turbulence levels this difference in velocity between the coolant and mainstream promotes the generation of turbulence and mixing between the streams so leading to some reduction in cooling effectiveness. However, this higher momentum coolant fluid is more resistant to high mainstream turbulence levels and scales so that the coolant film break up is not as significant under these conditions as that observed for the double skin system. For all the configurations tested the use of effusion cooling helped restore the coolant film along the rear of the test section. For the same total coolant flow, the minimum value of cooling effectiveness observed along the test section was increased relative to the no effusion case. In addition the effectiveness of the effusion patch depends on the amount of coolant injected and the axial location of the patch. The overall experimental data suggested the importance of the initial cooling film conditions together with better understanding of the possible mechanisms that results in the rapid cooling film break-up, such as high turbulence mainstream flow and scales, and this will lead to a more effective cooling system design. This experimental data is also thought to be ideal for the validation of numerical predictions.