Dissertations / Theses on the topic 'Turbomachinery measurements'

Thesis (M.S. in Aeronautical Engineering) Naval Postgraduate School, June 1997.
Thesis advisor, Raymond P. Shreeve. AD-A333 428. Includes bibliographical references (p. 49-50). Also available online.

Accurate unsteady measurements are required for studying the flows in high speed turbomachines, which rely on the interaction between rotating and stationary components. Using statistics of phase locked ensembles simplifies the problem, but accurate frequency response in the 10-100 kHz range significantly limits the applicable techniques. This research advances the state of the art for phase resolved measurement techniques using for high speed turbomachinery flows focusing on the following areas: development, validation, and uncertainty quantification. Four methods were developed and implemented: an unsteady total pressure probe, the multiple overheat hot-wire method, the slanted hot-wire method, and the phase peak yaw hot-wire method. These methods allow for the entire phase locked average flow field to be measured (temperature, pressure, and velocity components, swirl angle, etc.). No trusted reference measurement or representative canonical flow exists for comparison of the phase resolved quantities, making validation challenging. Five different validation exercises were performed to increase the confidence and explore the range of applicability. These exercises relied on checking for consistency with expected flow features, comparing independent measurements, and cross validation with CFD. The combined uncertainties for the measurements were quantified using uncertainty estimates from investigations into the elemental error sources. The frequency response uncertainty of constant temperature hot-wire system was investigated using a novel method of illuminating the wire with a laser pulse. The uncertainty analysis provided estimates for the uncertainty in the measurements as well as showing the sensitivity to various sources of error.

Thesis (Degree of Aeronautical and Astronautical Engineer) Naval Postgraduate School, December 1997.
"December 1997." Thesis advisor(s): Raymond P. Shreeve. Includes bibliographical references (p. 83-84). Also available online.

A new pressure-measurement technique which employs the tools of molecular spectroscopy has recently received considerable attention in the fluid mechanics community. Measurements are made via oxygen-sensitive molecules attached to the surface of interest as a coating, or paint. The pressure-sensitive-paint (PSP) technique is now commonly used in stationary wind-tunnel tests; this thesis presents the extension of the technique to advanced turbomachinery applications. New pressure- and temperature-sensitive paints (TSPs) have been developed for application to a state-of-the-art transonic compressor where pressures up to 2 atm and surface temperatures up to 140° C are expected for the first-stage rotor. PSP and TSP data has been acquired from the suction surface of the first-stage rotor of a transonic compressor operating at its peak-efficiency condition. The shock structure is clearly visible in the pressure image, and visual comparison to the corresponding computational fluid dynamics (CFD) prediction shows qualitative agreement to the PSP data.
Master of Science

This research developed two innovative test methods that were used to experimentally evaluate the performance of a novel blade tip timing (BTT) system from Prime Photonics, LC. The research focused on creating known blade tip offsets and tip vibrations so that the results from a BTT system can be validated. The topic of validation is important to the BTT field as the results between many commercial systems still are not consistent. While the system that was tested is still in development and final validation is not complete, the blade tip offset and vibration frequency validation results show that this BTT system will be a valuable addition to turbomachinery research and development programs once completed. For the first test method custom rotors were created with specified blade tip offsets. For the blade tip offset alternate measurement, the rotors were optically scanned and analyzed in CAD software with a tip location uncertainty of 0.1 mm. The BTT system agreed with the scanned results to within 0.13 mm. Tests were also conducted to ensure that the BTT system identified and indexed the blades properly. The second developed test method used an instrumented piezoelectric blade to create known dynamic deflections. The active vibration rotor was able to create measureable deflection over a range of frequencies centered on the first bending mode of the blade. The results for the 110 Hz, 150 Hz, 180 Hz first bending resonance, 200 Hz, and 1036 Hz second bending resonance cases are presented. A strain gage and piezoelectric sensor were attached to the active blade during the dynamic deflection tests to provide an alternate method for determining blade vibration frequency. The BTT system correctly identified the active blade excitation frequencies as well as a 120 Hz frequency from the drive motor. This thesis also explored applying BTT methods and testing to more realistic blade geometry and vibration. Blade vibrations are usually classified by their frequency relative to the rotation speed. Synchronous vibrations are integer multiples of the rotational speed and are often excited by struts or vanes fixed to the engine case. For this reason, special probe placement algorithms were explored that use sine curve fitting to optimize the probe placement. Knowing how the blade will vibrate at operation before testing is critical as well. In preparation for future research, ANSYS Mechanical was used to predict the first three modes of a PT6A-28 first stage rotor blade at 1,966, 5,539, and 7,144 Hz. These frequencies were validated to within 4% using scanning laser vibrometry. The simulation was repeated at speed to produce a Campbell Diagram to highlight synchronous excitation crossings.
Master of Science

Improvements in the efficiency of power generation via turbomachinery are essential in order to reduce greenhouse gas emissions throughout the world. Advancements in measurement techniques are therefore crucial to understanding the main areas of energy loss in turbines and compressors. This thesis presents a novel system which allows that loss to be characterised using fast response, 3-D measurements of the pressure field within industrial scale rigs. Turbulent energy dissipation rates give an insight into where useable energy is lost from. To gain an insight into such rates, measurement techniques must be able to take data in all three dimensions simultaneously at high sampling rates, usually over 50 kHz. Traditional methods of flow characterisation such as optical techniques and pneumatic pressure probes are unable to capture the rapid fluctuations in pressure and velocity which lead to energy loss from the turbomachine. A new system was therefore designed and implemented into a 6-stage compressor rig to take fast response measurements at sampling frequencies up to 100 kHz behind the last stage stator. A fast-response 5-sensor pressure head, acquired from Kulite Semiconductor Products Inc, has been embedded into a bespoke stem to allow turbulence measurements in a range of turbomachinery applications. The five-sensor (5S) probe was calibrated for pressure sensitivity as well as aerodynamically to give total and static pressure along with velocity magnitude and direction. Individual sensors were calibrated and characterised at temperatures within a range of 200C and 500C, which corresponds to the conditions found within the final application. The probe was also used in a vortex shedding experiment where alternative eddies were detected from the 5S probe measurements in both the time and frequency domain. The aerodynamic calibration of the 5S probe consists of exposing the probe sensors to a range of flow angles in order to map their response between ±200 in both the yaw and pitch directions. This results in four non-dimensional coefficients, two to represent pressure and two to signify the flow angles. A linear interpolation method was written and implemented to deduce pressure and flow angles from experimental query points and the calibration data. The linear interpolation was used as an alternative to the standard surface fit method, where the calibration data is expressed as system of polynomial equations. It was found that the linear method was applicable to the interpolation of flow angles and gave a reduction in computation time of the order of 104. The total and static pressure values do however require the more tried and tested polynomial interpolation method due to the need for higher order interaction terms in the surface fit equation describing the terms. The fully calibrated 5S probe was then implemented into a 6-stage industrial scale rig where it acquired fast response pressure data from the flow field at the exit of the last stage vane. The data was processed to give time resolved, 3-D measurements of total and static pressure, flow angle and velocity. Due to the simultaneous capture of data from all 5 sensors, the resulting velocity vectors can be decomposed into their mean and periodic components to obtain values of energy loss from the turbomachine. The acquisition of such data from an industrial rig marks a novel advancement in the area of turbomachinery flow characterisation and the use of the 5S probe in a range of applications will begin to fulfil the need for a database of fast response data from chaotic and turbulent flow fields.

The application of laser Doppler vibrometry to high speed rotating structures has been hampered by technical limitations. Whereas full-field three-dimensional velocity measurements can be made on stationary structures, the capability on rotating structures is limited to low speed, one-dimensional, steady state operation. This work describes the implementation of a self-tracking laser vibrometry system which overcomes many of the limitations of current techniques for vibration measurements on rotating structures. A model of the self-tracker is developed and used to predict the effects of static misalignments on the position and velocity errors. These predictions are supported by experimental results and simplified models of the self-tracker. NOTE: (02/2011) An updated copy of this ETD was added after there were patron reports of problems with the file.
Ph. D.

The noise (unwanted sound) from fans of all sizes, operating in close proximity to people, can be a design constraint due to annoyance or, in the worse cases, health damage. Of the total noise, aeroacoustic noise - produced by unsteadiness in the air - often represents a significant source and is intrinsically linked to the aerodynamic features of the flow field. In this work, the aeroacoustics of low-speed fans are investigated using a compact mixed-flow fan as a test case. The low-speed regime is less developed compared to large-scale, high-speed machines and is increasingly relevant to applications such as micro air vehicles, small wind turbines, and other environmental comfort technologies found in buildings or vehicles. The test case fan Reynolds number is of the order of 104 which is a couple of orders lower than those generally found in gas turbines. Its main sources are therefore best identified experimentally in the absence of proven alternative methods. In order to do this, a way of quantifying fan noise is developed in tandem with control of the aerodynamic operating point. Following a study of sources of the significant broadband and tonal noise, a low-order noise prediction scheme is developed and applied to predict tonal noise with reference to Reynolds number effects. The new, duct-based rig and method has several advantages over the existing sound power measurement rig built to the ISO 5136 standard at Dyson. The approach, which makes no assumptions about the relative power of different modes, has resulted in a rig that is much shorter. Unlike the ISO rig, it is capable of accurate narrow-band tone measurements with sources which excite strong non-plane-wave duct modes (as the modal structure of the sound is determined) for the frequencies of interest. Tests have been carried out at different operating points with a range of geometry modifications produced with 3D printing techniques. In terms of tonal sources which particularly impact sound quality, the mixed-flow impeller alone produces tones due to very high sensitivity to inflow distortion of the mean flow (giving unsteady blade loading). This means that the product inlet must be designed very carefully to optimally condition the flow. Periodicity in the impeller outlet flow produces rotor-stator interaction tones even with a number of guide vanes chosen to satisfy the Tyler-Sofrin theory cut-off criteria. This is thought to be due to abrupt radius change after the guide vanes in the rig (while the theory assumes constant radius). In the product, abrupt radius change also occurs. The sensitivity of the broadband level to inflow turbulence was confirmed to be low in the rig, although the in-product inflow appears much less ideal. The main broadband noise source in rig tests is suggested to be impeller self-noise as only small reductions in rotor-stator interaction noise are achieved with far fewer vanes. The low-order modelling scheme to understand the fundamental unsteady loading noise mechanism compares well to experiments for sample rotor-stator interaction tones. The velocity fluctuations which induce this noise, measured experimentally with a 2D hotwire, are shown to increase in intensity as Reynolds number is reduced towards 104. This is due to a higher importance of viscosity which can give boundary layers that are thicker and liable to laminar separation. Surface treatments such as boundary layer trips could be used to prevent such separation and potentially reduce noise. Based on the thesis findings, further tests, simulations and possible design modifications are suggested to understand and reduce the important noise sources.

L’objectif de cette ´étude est de d´écrire en détail l´écoulement de jeu afin d’en ´évaluer la sensibilité´e auxparamètres de conception ainsi que la r´réceptivité´e `a des dispositifs de contrôle par injection d’air au carter. Pourcela, on considère une géométrie simplifiée qui est constituée d’une aube fixe isolée, placée perpendiculairement `aune plaque plane représentant le carter. L’analyse de cette configuration simplifiée s’appuie sur la complémentarité´edes mesures en soufflerie et simulations numériques. La taille du jeu est le paramètre principal qui affectel’écoulement de jeu. L’épaisseur de la couche limite incidente au carter et le chargement de l’aube ont ´égalementun effet visible sur la position latérale du tourbillon de jeu. Le calcul du taux de création d’entropie local a permisd’identifier plusieurs zones de pertes dans l’écoulement. Ensuite, les pertes de pression totale ont été décomposéesen la somme d’un terme li´e au tourbillon et d’un autre li´e au déficit de vitesse longitudinale. Ce terme li´e autourbillon est responsable de l’augmentation des pertes de pression totale avec la taille du jeu. Enfin, un modèleempirique a été développé pour estimer la circulation du tourbillon de jeu ainsi que les pertes de pression totaleen fonction de la taille du jeu. Un système d’injection continue d’air dans le jeu a été évalué, les jets étant orientésperpendiculairement au carter. D’une part, cette stratégie de contrôle permet de rapprocher le tourbillon de jeude l’aube, ce qui pourrait permettre d’augmenter le domaine de fonctionnement stable d’un compresseur. D’autrepart, le champ de vorticité axiale dans le tourbillon de jeu devient plus homogène, ce qui serait intéressant pourlimiter les interactions rotor-stator. Cependant cette approche tend `a augmenter les pertes de pression totale etperd en efficacité avec un élargissement du jeu
This study aims at providing a detailed description of the tip-leakage flow, in order to analyzeits sensitivity to design parameters and to control devices based on air injection from the casing. The setup iscomposed of a single blade, set orthogonal to a flat plate that plays the role of the casing wall. The analysis isbased on experiments conducted in a low-speed wind tunnel that are complemented by CFD calculations. Thetip-leakage flow is primarily driven by the gap height. The incoming boundary layer thickness and the bladeloading also have a notable effect on the lateral position of the tip-leakage vortex. The distribution of local entropycreation rate has been used to identify areas of losses in the flow. Moreover, the total pressure losses have beendecomposed in two terms identified as a vortex loss and a wake loss. This vortex loss drives the increase of totalpressure losses with the gap height. An empirical model has been developped to predict the evolution of thetip-leakage vortex circulation and of the total pressure losses with respect to the gap height. A steady injection ofair from the casing has been evaluated, using normal jets in the gap. With this control strategy, the tip-leakagevortex tends to be closer to the blade, which could lead to an extension of the range of stable operation for axialcompressors. In addition to that, the control device smoothes out the axial vorticity field in the tip-leakage vortex,which could be interesting to reduce rotor-stator interactions. However, this control strategy leads to higher totalpressure losses and is less effective with larger gaps

Le constant besoin d’amélioration de l’efficacité et d’allègement des turbomachines demande aux constructeurs des efforts permanents pour ouvrir les domaines de conception. En particulier, les jeux fonctionnels entre les parties fixes et tournantes des turbomachines modernes sont de plus en plus réduits, permettant des diminutions substantielles des pertes de rendement. Toutefois, l’allègement des composants entraîne leur assouplissement, et donc une importance croissante des phénomènes dynamiques dans le cycle de vie des structures fixes comme tournantes. L’effet des couplages multi-physiques se retrouve lui aussi exacerbé, que ce soit celui du couplage thermomécanique ou aéroélastique. Il est donc, dans ce contexte, nécessaire d’améliorer les outils de dimensionnement de façon à prévoir plus précisément le cycle de vie des composants des moteurs, afin de réduire les coûts de développement, tout en augmentant leur sûreté de fonctionnement et donc la sécurité des passagers. Cette thèse s’est inscrite dans la continuité de travaux précédents sur le sujet du contact aubes–stator. À savoir, introduire plus de physique dans les modèles numériques en tenant compte de phénomènes et de géométries de plus en plus complexes. L’objectif principal a été d’introduire la modélisation de phénomènes thermiques dus aux évènements de contact, intervenant à l’interface entre les pièces fixes et tournantes, en se basant sur des géométries industrielles installées sur un banc d’essais. Cependant, l’introduction de cette nouvelle physique dans les modélisations devait être faite en tenant compte des contraintes de simulation sur des systèmes complexes, c’est-à-dire en trouvant un compromis entre vitesse et précision des calculs. Enfin, une phase de corrélation entre ces simulations et des essais sur banc était à effectuer, pour s’assurer de la pertinence des outils de dimensionnement mis en oeuvre. Durant cette thèse, une adaptation et une ré-instrumentation du banc d’essais CASTOR (Contact Aubes StaTOR) ont d’abord été effectuées. Plusieurs essais de contact sur banc ont ensuite été réalisés en mesurant les comportements vibratoires et thermiques des parties fixes et tournantes. Puis, des travaux menés en parallèle ont porté sur la réduction de modèles éléments finis décrivant le comportement thermomécanique d’éléments de compresseurs centrifuges de turbomachines aéronautiques. Par ailleurs, plusieurs méthodologies d’intégration temporelle des problèmes de contact, conventionnellement utilisées dans un cadre purement mécanique, ont été évaluées dans un cadre de simulation thermomécanique pour s’assurer de leur capacité à fonctionner pour ce type d’études. Certaines difficultés ont été levées en exploitant des méthodes numériques issues de la communauté scientifique traitant de la dynamique non-régulière. Enfin, des simulations ont été effectuées avec divers paramétrages pour montrer à la fois les capacités de l’outil développé, et confronter les résultats numériques aux observations expérimentales
The constant need for efficiency and lightweightness of aeroengines demands OEM continued efforts to open design domain. In particular, operations clearances between static and rotating parts of modern engines become narrower, leading to better efficiencies. However, lighter components generally have lower stiffnesses, causing a growth in dynamic phenomena participation in the engines life-cycle. Mutli-physics coupling effects are aggravated in the same manner, whether they are of thermomechanical or aeroelastic nature. In this context, it is therefore crucial to improve design tools so as to predict more accurately the operational conditions of the engine components, with a general objective to cut down development and operational costs, while ensuring engine reliability and passenger safety. This thesis closely follows previous work on blade–casing contacts, all aimed at modelling more accurately the underlying more and more complex phenomena and structures. The main objective of this work has been to introduce a model for the thermal phenomena occurring during contacts at the interface between rotating and static parts, based on industrial geometries of components, which are set in a test rig. Due to the sophistication of the parts, the addition of these phenomena in the model had to be performed while paying attention to high simulation constraints. In other words, a trade-off had to be found between speed and precision of the computations. Finally a correlation phase was to be performed between simulations and experimental trials was to be performed to assess the relevance of the proposed numerical tools. During this thesis, a modification and a new instrumentation of the CASTOR test rig were performed. Multiple contact trials were carried out, during which vibratory and thermal behavior of the components were measured. In parallel to these experimental operations, multiple numerical developments were tackled. Among them, a model reduction methodology of thermoelastic models of turbo-engine centrifugal compressors was developed. Also, multiple time-stepping procedures, originally dedicated to solve contact problems in a purely mechanical context, we extended to perform thermomechanical computations. Several complications were removed taking advantage of advanced methods stemming from the non-smooth dynamics community. Eventually, simulations were performed with diverse setups to both show the capabilities of the numerical tool as well as confront numerical results and experimental observations

The present work is a result of collaboration between the LMFA (Laboratoire de Mécanique des Fluides et d'Acoustique, Ecole Centrale de Lyon - France), Snecma and the Cerfacs. It aims at studying the flow in the 3.5-stages high-speed axial compressor CREATE (Compresseur de Recherche pour l'Etude des effets Aérodynamique et TEchnologique - rotation speed: 11543 RPM, Rotor 1 tip speed: 313 m/s), designed and built by Snecma and investigated at LMFA on a 2-MW test rig. Steady measurements, as well as laser velocimetry, fast-response wall static and total pressure measurements have been used to experimentally investigate the flow. The analysis focuses on two main aspects: the study of the flow at stable operating points, with a special interest on the rotor-stator interactions, and the study of the instabilities arising in the machine at low mass flow rates.The description of the unsteady flow field at stable operating points is done through measurements of wall-static pressure, total pressure and velocity, but also total temperature, entropy and angle of the fluid. It is shown that the complexity and unsteadiness of the flow in a multistage compressor strongly increases in the rear part of the machine, because of the interactions between steady and rotating rows. Therefore, a modal analysis method developed at LMFA and based on the decomposition of Tyler and Sofrin is presented to analyze these interactions. It is first applied to the pressure measurements, in order to extract the contributions of each row. It shows that all the complex pressure interactions in CREATE can be reduced to three main types of interactions. The decomposition method is then applied to the entropy field extracted from URANS CFD calculations performed by the Cerfacs, in order to evaluate the impact of the interactions on the performance of the machine in term of production of losses.The last part of this work is devoted to the analysis of the instabilities arising in CREATE at low mass flows. It shows that rotating pressure waves appear at stable operating points, and increase in amplitude when going towards the surge line, until reaching a critical size provoking the onset a full span stall cell bringing the machine to surge within a few rotor revolutions. The study of these pressure waves, and the understanding of their true nature is achieved through the experimental results and the use of some analytical models. A precise description of the surge transient through wall-static pressure measurements above the rotors is also provided, as well as a description of a complete surge cycle. An anti-surge control system based on the detection of the amplitude of the pressure waves is finally proposed.

Le rendement des turboréacteurs peut être amélioré en minimisant le jeu aube-carter, réduisant ainsi les pertes aérodynamiques. Ces jeux réduits occasionnent des risques de contact entre les aubes en rotation à grande vitesse et le carter moteur. Des matériaux sacrificiels, appelés matériaux abradables, sont alors déposés sur le carter pour limiter les endommagements induits par ces contact. Ces interactions font intervenir un grand nombre de mécanismes d’endommagement bénéfiques ou néfastes au bon fonctionnement du joint abradable et à la fiabilité du moteur. L’objectif de cette thèse est alors de comprendre, prédire et quantifier les différents endommagements et les efforts d’interaction associés pour des matériaux abradables obtenus à l’aide de paramètres procédés différents. Un dynamomètre triaxial a été développé afin de reproduire l’interaction locale entre l’extrémité de l’aube et le matériau abradable à très grande vitesse (50 – 300 m/s) lors de phases transitoires. La mesure d’efforts d’interaction lors de contacts de très courte durée (300 µs – 1 ms) nécessite une bande passante importante. Une méthode de correction basée sur l’analyse modale expérimentale a été mise en œuvre afin d’étendre la bande passante naturelle du dynamomètre et d’atténuer les couplages entre les différentes voies de mesure. Les mécanismes d’endommagements des abradables ont été étudiés à partir d’analyses post-mortem et corrélés aux efforts et vitesse d’interactions
The turbofan efficiency can be improved by minimizing the blade-casing gap, thus reducing the aerodynamic loss. The reduced gap conduces to contact risk between the high-speed rotating blades and the engine case. Sacrificial materials, called abradable materials, are deposited on the casing to limit the damage caused by these contacts. These interactions involve a lot of damage mechanisms, which can be adverse or beneficial to the proper performance of the abradable seal and to the reliability of the engine. The aim of this thesis is to understand, predict and quantify the different damages and the interaction forces associated for abradable materials obtained with different process parameters. A triaxial dynamometer was developed to reproduce the local high-speed interactions (50 – 300 m/s) between the blade tip and the abradable material during transitional phases. The interaction forces measurement during short-lived contacts (300 µs – 1 ms) requires a large bandwidth. A correction method based on experimental modal analysis was implemented to extend the natural bandwidth of the device and attenuate the crosstalk between the different measurement channels. The damage mechanisms of abradable materials were studied by post-mortem analysis and correlated to the interaction forces and velocity

Understanding the dynamic characteristics of blades is important in the online condition monitoring of turbomachinery. Conventionally contact methods are used for this purpose. However improvements in technology now allow for the use of non-contact methods. Contact measurement techniques for turbomachinery blade vibration analysis typically involve the use of strain gauges and accelerometers. These present some complications when analyzing rotating machinery. Being contact in nature, mass loading can affect the integrity of measurements captured. Turbomachines typically operate under the adverse conditions of high temperatures, high flow rates and sometimes wet environments. This significantly reduces the life of contact transducers installed to capture the blade dynamics. Installation of telemetry systems for signal transmission is also necessary. In addition to being invasive and expensive, telemetry systems can introduce electrical noise. These factors make it desirable to explore the applicability of various optical non-contact methods for analyzing turbomachine blade vibrations, such as Laser Doppler Vibrometry (LDV) and photogrammetry. Both techniques have been successfully used to analyze vibrations of structures. Photogrammetry is a full-field measurement technique which allows for non-intrusive simultaneous measurement of vibrations at different locations on a blade. This is particularly important for the updating of numerically developed models of structures, investigation of structural global dynamics, and more effective localization of damage. Accelerometers have been used to validate a variation of photogrammetry, three dimensional point tracking (3DPT), for rotational applications and discrepancies attributed to the contact nature of accelerometers were observed. To build confidence in the use of 3DPT as a non-contact method for analyzing rotating machines, it is necessary to investigate how well it correlates with various non-contact methods. Through such an investigation aspects that need to be addressed when using 3DPT to analyse turbomachines can be identified. If reliable measurements can be obtained using this technique, further investigations such as online damage detection and characterization in rotating structures can be conducted. In this study Tracking Laser Doppler Vibrometry (TLDV) and 3DPT are used as non-contact methods to investigate the online vibrations of a turbomachine test rotor. To employ TLDV on the test rotor, the dynamics of the scanning mirrors of a Polytec Scanning Vibrometer (PSV) are characterized using a frequency response approach. Look-up tables are constructed to provide the necessary phase angle compensation for the two signals supplied to the mirrors, to obtain a circular scanning path. Photogrammetric 3DPT is then validated by tracking the TLDV laser spot focused on one of the test rotor blade using high speed cameras, and comparing the 3DPT measurements to TLDV blade out-of-plane vibration measurements. The correlation between the two non-contact measurement techniques is presented. This establishes the validity of the employed scanning system, and also serves to show how well the two non-contact methods correlate with each other, when investigating dynamics of turbomachinery blades. 3DPT is then used to analyze the responses of the test rotor blades under excitation. Various Operational Deflection Shapes (ODSs) of the blades are identified and the results obtained are presented. The use of ODSs obtained from 3DPT to investigate irregularities along turbomachinery blades is also presented. This information is used to show that ODSs captured using 3DPT can be used to online detect and localize blade damage in turbomachines.
Dissertation (MEng)--University of Pretoria, 2015.
tm2015
Mechanical and Aeronautical Engineering
MEng
Unrestricted

碩士
國立中興大學
機械工程學系
91
Abstract This study sets up a non-contact measurement system with light probes for revealing vibration characteristics of rotor blades by using blade tip clearance. The two-parameter method in conjunction with measurements on a small fan is studied. The study finds an analytical relationship for the two-parameter method based on the linear vibration equation. This analytical result describes the relation between axis ratio of the blade dynamic ellipse and phase difference of the probes. The speed of fan is in the range from 800 to 2500 rpm for the measurements of the tip clearance for dynamic analysis of the blade vibrations. Experiments include the one-probe method and two-probe method. The one-probe method measures vibration of an arbitrary blade with one probe located at a fixed position on the casing. Its purpose is to identify the resonant frequency and resonant amplitude of blade vibration. The two-probe method employs two probes located with an appropriate spacing on the casing. Blade displacements measured by two probes under different rotor speed are found. The application of two-parameter method is discussed by comparing with the results of experiment and simulation. Finally, the blade vibration involving rotor speed drifting is studied by simulation. In this simulation, the amplitude and the frequency of disturbance on rotor speed for blade vibration are discussed. Key words: non-contact measurement, light probes, blade tip clearance, blade dynamic mode.

The on-line condition monitoring of turbomachinery blades is of utmost importance to ensure the long term health and availability of such machines and as such has been an area of study since the late 1960s. As a result a number of on-line blade vibration measurement techniques are available, each with its own associated advantages and shortcomings. In general, on-blade sensor measurement techniques suffer from sensor lifespan, whereas non-contact techniques usually have measurement bandwidth limitations. One non-contact measurement technique that yields improvements in the area of measurement bandwidth is laser Doppler vibrometry. This thesis presents results and findings from utilizing laser Doppler vibrometry in an Eulerian fashion (i.e. a fixed reference frame) to measure on-line blade vibrations in axial-flow turbomachinery. With this measurement approach, the laser beam is focussed at a fixed point in space and measurements are available for the periods during which each blade sweeps through the beam. The characteristics of the measurement technique are studied analytically with an Euler-Bernoulli cantilever beam and experimental verification is performed. An approach for the numerical simulation of the measurement technique is then presented. Associated with the presented measurement technique are the short periods during which each blade is exposed to the laser beam. This characteristic yields traditional frequency domain signal processing techniques unsuitable for providing useful blade health indicators. To obtain frequency domain information from such short signals, it is necessary to employ non-standard signal processing techniques such as non-harmonic Fourier analysis. Results from experimental testing on a single-blade test rotor at a single rotor speed are presented in the form of phase angle trends obtained with non-harmonic Fourier analysis. Considering the maximum of absolute unwrapped phase angle trends around various reference frequencies, good indicators of blade health deterioration were obtained. These indicators were verified numerically. To extend the application of this condition monitoring approach, measurements were repeated on a five-blade test rotor at four different rotor speeds. Various damage cases were considered as well as different ELDV measurement positions. Using statistical parameters of the abovementioned indicators as well as time domain parameters, it is shown that with this condition monitoring approach, blade damage can successfully be identified and quantified with the aid of artificial neural networks.
Thesis (PhD)--University of Pretoria, 2010.
Mechanical and Aeronautical Engineering
unrestricted

Metal mesh foil bearings (MMFBs) are a promising low cost gas bearing technology for support of high speed oil-free microturbomachinery. Elimination of complex oil lubrication and sealing system by installing MMFBs in oil free rotating machinery offer distinctive advantages such as reduced system overall weight, enhanced reliability at high rotational speeds and extreme temperatures, and extended maintenance intervals compared to conventional turbo machines. MMFBs for oil-free turbomachinery must demonstrate adequate load capacity, reliable rotordynamic performance, and low frictional losses in a high temperature environment. The thesis presents the measurements of MMFB break-away torque, rotor lift off and touchdown speeds, temperature at increasing static load conditions, and identified stiffness and equivalent viscous damping coefficients. The experiments, conducted in a test rig driven by an automotive turbocharger turbine, demonstrate the airborne operation (hydrodynamic gas film) of the floating test MMFB with little frictional loses at increasing loads. The measured drag torque peaks when the rotor starts and stops, and drops significantly once the bearing is airborne. The estimated rotor speed for lift-off increases linearly with increasing applied loads. During continuous operation, the MMFB temperature measured at one end of the back surface of the top foil increases both with rotor speed and static load. Nonetheless, the temperature rise is only nominal ensuring reliable bearing performance. Application of a sacrificial layer of solid lubricant on the top foil surface aids to reduce the rotor break-away torque. The measurements give confidence on this simple bearing technology for ready application into oil-free turbomachinery. Impact loads delivered (with a soft tip) to the test bearing, while resting on the (stationary) drive shaft, evidence a system with large damping and a structural stiffness that increases with frequency (max. 200 Hz). The system equivalent viscous damping ratio decreases from ~ 0.7 to 0.2 as the frequency increases. In general, the viscous damping in a metal mesh structure is of structural type and inversely proportional to the frequency and amplitude of bearing motion relative to the shaft. Impact load tests, conducted while the shaft rotates at 50 krpm, show that the bearing direct stiffness is lower (~25% at 200 Hz) than the bearing structural stiffness identified from impact load tests without shaft rotation. However, the identified equivalent viscous damping coefficients from tests with and without shaft rotation are nearly identical. The orbits of bearing motion relative to the rotating shaft show subsynchronous motion amplitudes and also backward synchronous whirl. The subsynchronous vibration amplitudes are locked at a frequency, nearly identical to a rotor natural frequency. A backward synchronous whirl occurs while the rotor speed is between any two natural frequencies, arising due to bearing stiffness asymmetry.