<p dir="ltr">This research presents a culmination of work into uncovering the underlying fluid dynamic behaviors of supercritical CO2 as it relates to high pressure turbine design using a combined fundamental and practical numerical and experimental analysis. The fundamental analysis of the thermo-fluid dynamic properties of supercritical CO2 boundary layers and separation is analyzed against the air counterparts for non-dimensional quantities of interest – pressure ratio, Mach number, Reynolds number – and combinations of these quantities. The coupling of density derivatives with pressure and temperature are investigated within the operating conditions of the first stage turbine of a supercritical CO2 oxyfuel power cycle. Armed with the information garnered from this analysis, a 3D optimization is run using computational fluid dynamics to investigated nearly 3000 unique blade shapes, focusing on increasing the isothermal corrected efficiency and decreasing the heat load to the blade. Three different families of blade shapes are identified from the analysis and their aerodynamic qualities discussed. A single advanced blade design is chosen for in depth analysis and experimental testing against the baseline blade from which the optimization was started. Mechanical design for the experimental campaign in the Big Rig for Aerothermal Stationary Turbine Analysis (BRASTA) is presented for a novel sector-based off-axis design and the results of the aerothermal measurements discussed. In tandem with blade design and analysis, the Tip Gap Experimental Research Article for Large Scale Injection Layouts (Tiger Lily), a canonical model for the large-scale investigation of tip flows in high Reynolds number flows, is developed and the mechanical and aerodynamic design discussed. Aerothermal analysis for different tip coolant injection configurations is performed using Improved Delayed Detached Eddy Simulation (IDDES) computational fluid dynamics analysis to resolve turbulent structures resulting from coolant injection and over tip flow interaction. Experimental investigation of Tiger Lily is presented, validating the structures and features seen in the numerical analysis. The conclusion of these investigations results in the increased understanding of the underlying fluid dynamic behaviors of supercritical CO2 in high pressure turbines.</p>

In modern gas turbines, with the increase of inlet gas temperature to raise thework output, the importance of accurate aero-thermal analysis has become of vitalimportance. These analysis are required for temperature prediction throughoutthe turbine and to predict the thermal stresses and to estimate the cooling requiredfor each component.In the past 20 years, computational fluid dynamics (CFD) methods have becomea powerfool tool aero-thermal analysis. Due to reasons including numericallimitation, flow complications caused by blade row interactions and the effect offilm cooling, using simple steady state CFD methods may result in inaccuratepredictions. Even though employing transient simulations can improve the accuracyof the simulations, it will also greatly increase the simulation time and cost.Therefore, new methods are constantly being developed to increase the accuracywhile keeping the computational costs relatively low. Investigating some of thesedeveloped methods is one of the main purposes of this study.A simplification that has long been applied in gas turbine simulations hasbeen the absence of cooling cavities. Another part of this thesis will focus onthe effect of cooling cavities and the importance of including them in the domain.Therefore, all transient and steady state simulations have been examined for twocases; a simplified case and a detailed case. The results are then compared tothe experimental measurements to evaluate the importance of their presence inthe model. The software used to perform all simulations is the commercial codeANSYS CFX 15.The findings suggest that even though including cooling cavities would improvethe results, the simulations should be run in transient. One important finding wasthat when performing transient simulations, especially the Time Transformationmethod, not only is the pitch ratio between every subsequent blade row important,but also the pitch ratio between the stators is highly influential on the accuracyof the results.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The use of gas turbines in central receiver solar power plant cycles has become an increasingly popular research topic. This has led to the need to investigate and analyse the effect of the solar receiver on the gas turbine cycle. The aim of this thesis is to construct an experimental gas turbine setup to accommodate further research on utilizing solar energy to power gas turbines. The gas turbine under consideration is the Rover Gas Turbines 1S/60. The focus of this investigation is the centrifugal compressor of the gas turbine. An increase in static pressure is required for the gas turbine to cope with anticipated pressure drops in the central receiver that will be part of the gas turbine cycle. The standard compressor design is analysed by means of 3-D (CFD) analysis using CFX® and experimental data. The new centrifugal compressor is designed by means of 1-D and 3-D (CFD) analysis using CompAero and CFX®. The aim is to design a compressor that maximizes the total-to-static pressure ratio. The size of the compressor is highly constrained by the geometry parameters of the gas turbine. Since the turbine rotor will remain unchanged, the power input, mass flow rate and rotational speed must stay the same. The experimental setup was build and the numerical results of the standard compressor were validated against the experimental results. A new centrifugal compressor was designed. The total-to-static pressure ratio was increased from 2.50 to 3.30 at an operating speed of 46 krpm. The efficiency of the compressor was improved from 63.8% to 85.6%. The input power of the new compressor design deviated 1.6% from the set benchmark, and 1.3% from the numerical data of the standard compressor. / AFRIKAANSE OPSOMMING: Die gebruik van gasturbines in sonkragstasiesiklusse met ’n sentrale ontvanger het gegroei tot ’n gewilde navorsingsonderwerp. Dit het gelei tot die behoefte om die effek van die sonontvanger op die gasturbinesiklus te ondersoek en te analiseer. Die doel van hierdie tesis is om ’n eksperimentele gasturbine opstelling te bou vir verdere navorsing oor die benutting van sonenergie om ’n gasturbine aan te dryf. Die gasturbine in oorweging is die Rover Gas Turbines 1S/60. Die fokus van hierdie ondersoek is die sentrifugale kompressor van die gasturbine. ’n Toename in statiese druk word benodig vir die gasturbine om die verwagte drukverlies in die sentrale ontvanger, wat deel uit maak van die gasturbinesiklus, te hanteer. Die standaard kompressor ontwerp is geanaliseer deur middel van 3-D Berekenings Vloeimeganika (BVM) analises met behulp van CFX® en eksperimentele data. Die nuwe sentrifugale kompressor is ontwerp deur middel van 1-D en 3-D BVM analises met behulp van CompAero en CFX®. Die doel is om ’n kompressor te ontwerp wat die totale-tot-statiese drukverhouding maksimeer. Die grootte van die kompressor is beperk deur die geometrie van die gasturbine omhulsel. Aangesien die turbinerotor onveranderd sal bly, moet die insetdrywing, massa-vloeitempo en rotasiespoed dieselfde bly. Die eksperimentele opstelling is gebou en die numeriese resultate van die standaard kompressor is teenoor die eksperimentele resultate gevalideer. ’n Nuwe sentrifugale kompressor is ontwerp. Die totale-tot-statiese drukverhouding is verhoog van 2.50 tot 3.30 teen ’n rotasiespoed van 46 000 omwentelings per minuut. Die doeltreffendheid van die kompressor is verbeter van 63.8% tot 85.6%. Die insetdrywing van die nuwe kompressor ontwerp het met 1.6% afgewyk van die vasgestelde maatstaf, en met 1.3% van die numeriese data van die standaard kompressor.

<p>This research investigates the forced response of mistuned rotor blades that can lead to excessive vibration, noise, and high cycle fatigue failure in a turbomachine. In particular, an embedded rotor in the Purdue Three-Stage Axial Compressor Research Facility is considered. The prediction of the rotor forced response contains three key elements: the prediction of forcing function, damping, and the effect of frequency mistuning. These computational results are compared with experimental aerodynamic and vibratory response measurements to understand the accuracy of each prediction.</p><p>A state-of-the-art time-marching computational fluid dynamic (CFD) code is used to predict the rotor forcing function. A highly-efficient nonlinear frequency-domain Harmonic Balance CFD code is employed for the prediction of aerodynamic damping. These allow the compressor aerodynamics to be depicted and the tuned rotor response amplitude to be predicted. Frequency mistuning is considered by using two reduced-order models of different levels of fidelity, namely the Fundamental Mistuning Model (FMM) and the Component Mode Mistuning (CMM) methods. This allows a cost-effective method to be identified for mistuning analysis, especially for probabilistic mistuning analysis.</p><p>The first topic of this work concerns the prediction of the forcing function of the embedded rotor due to the periodic passing of the neighboring stators that have the same vane counts. Superposition and decomposition methods are introduced under a linearity assumption, which states that the rotor forcing function comprises of two components that are induced by each neighboring stator, and that these components stay unchanged with only a phase shift with respect to a change in the stator-stator clocking position. It is found that this assumption captures the first-order linear relation, but neglects the secondary nonlinear effect which alters each stator-induced forcing functions with respect to a change in the clocking position.</p><p>The second part of this work presents a comprehensive mistuned forced response prediction of the embedded rotor at a high-frequency (higher-order) mode. Three steady loading conditions are considered. The predicted aerodynamics are in good agreement with experimental measurements in terms of the compressor performance, rotor tip leakage flow, and circumferential distributions of the stator wake and potential fields. Mistuning analyses using FMM and CMM models show that the extremely low-cost FMM model produces very similar predictions to those of CMM. The predicted response is in good agreement with the measured response, especially after taking the uncertainty in the experimentally-determined frequency mistuning into consideration. Experimentally, the characteristics of the mistuned response change considerably with respect to loading. This is not very well predicted, and is attributed to un-identified and un-modeled effects. A significant amplification factor over 1.5 is observed both experimentally and computationally for this higher-order mode.</p> / Dissertation

Film cooling is vital for the cooling of the blades and vanes in the high temperature environment of a jet engine high pressure turbine stage. Previous research into film cooling has typically concentrated on its time-mean performance. However, results from other studies upon more simplified geometries, suggest that coherent unsteadiness is likely to also be present in film cooling flows. The research presented in this thesis, therefore, aims to characterise what coherent unsteadiness, if any, is present within film cooling flows. Cylindrical and shaped cooling holes, located upon the pressure surface of a turbine blade within a large scale linear cascade, have been investigated. A blowing ratio range of 0.5 to 2.0 has been investigated, with either a plenum or perpendicular crossflow at the cooling hole inlet. Particle Image Velocimetry, high speed photography and Hot Wire Anemometry have been used to investigate the jet downstream of both cooling holes. The impact of crossflow at the hole inlet upon the flowfield inside both cooling holes has been investigated using Hot Wire Anemometry and a further numerical model solved by applying TBLOCK. The results presented in the current thesis, show the existence of two coherent unsteady structures in the jet downstream of both the cylindrical and the shaped holes. These structures are called shear layer vortices and hairpin vortices, and their formation is dependent on the velocity difference across the jet shear layer. Inside the cooling hole coherent hairpin vortices also appear to occur, with their formation dependent on the direction and magnitude of the crossflow at the hole inlet. The coherent unsteadiness presented here is shown for the first time for film cooling flows, and recommendations to build on the current study, in what is potentially an interesting research area, are made at the end of this thesis.

Le fretting est un endommagement induit par le glissement cyclique à très faible amplitude de deux corps en contact. Il se caractérise par un amorçage d’une fissure en surface, qui peut ensuite propager, menant ainsi à la rupture. Le fretting est présent dans de nombreux secteurs industriels où il constitue un critère de résistance à la fatigue plus ou moins sévère. Ce mémoire s’intéresse aux fissures de fretting qui apparaissent dans une liaison entre pale et disque d’une turbomachine. La sollicitation cyclique de contact est dans ce cas le fruit de la combinaison d’un chargement statique à un chargement vibratoire à très haute fréquence (quelques milliers de Hertz). Pouvoir estimer la durée de vie de la liaison sous un tel chargement est indispensable pour la sécurité des vols. La méconnaissance de certains paramètres d’entrée, la non-proportionnalité du chargement ainsi que les fortes concentrations de contraintes mises en jeu, sont autant de verrous techniques à la modélisation. Ce mémoire de thèse propose une méthode numérique permettant le calcul d’une criticité à amorçage de fretting sous sollicitation vibratoire. Le modèle se décompose en une première phase de calcul des contraintes et déformations cycliques par éléments finis, suivie d’une seconde qui consiste à post-traiter les résultats avec le critère de Dang Van. Le modèle est développé grâce au support d’un banc d’essai innovant qui permet de reproduire les chargements subis par un contact d’une liaison pale/disque. Une utilisation intensive du processus de calcul mis au point permet de tirer des conclusions et de mieux comprendre les phénomènes mis en jeu dans ce type d’endommagement. Une confrontation des différentes études numériques réalisées permet de comparer la représentativité des moyens expérimentaux par rapport aux configurations moteurs réelles. / Fretting is a damage induced by small cyclic slip of two bodies in contact. It is characterized by surface crack initiation, which can then propagate, thus leading to failure. Fretting is present in many industrial environments where it is a more or less severe resistance criterion. This work focuses on the fretting cracks that appear in blade/disk roots of turboshaft engines. In this case, the cyclic contact loading is the result of the combination of a static loading and a high frequency vibratory loading (some thousands of Hertz). Being able to estimate the lifetime of the root under such a solicitation is essential for flight safety. The lack of knowledge of certain input parameters, the non-proportionality of the solicitation as well as the high stress gradient involved, make this phenomenon difficult to predict. This work proposes a numerical method allowing the computation of a fretting crack initiation criterion. First, stresses and deformations fields are computed with finite element method. Then, the post-processing of the fields is done woth Dang Van criterion. The model is developed with the support of an innovative test bench which makes it possible to reproduce the loadings sustained by a a blade/disk root. An intensive use of the computation process developed makes it possible to draw conclusions and provides better understanding of the phenomenon involved in this type of damage. The different numerical studies carried out make it possible to compare the representativeness of the experimental means with respect to the actual engine configurations.

Les turbomachines aéronautiques sont composées de plusieurs roues aubagées dont la fonction estde transférer l’énergie de l’air au rotor. Les roues aubagées des modules compresseur et turbine sontdes pièces particulièrement sensibles car elles doivent répondre à des impératifs de performanceaérodynamique, de tenue mécanique, de tenue thermique et de performance acoustique. L’optimisation aéro-méca-acoustique ou aéro-thermo-mécanique des aubages consiste à chercher, pourun ensemble de formes aérodynamiques paramétrées (par plusieurs dizaines de variables), celleassurant le meilleur compromis entre la performance aérodynamique du moteur et la satisfactionde plusieurs dizaines de contraintes souvent contradictoires. Cette thèse introduit une méthode d’optimisation basée sur les métamodèles et adaptée à la grande dimension pour répondre à la problématique industrielle des aubages. Les contributions de cettethèse portent sur deux aspects : le développement de modèles de krigeage, et l’adaptation d’unestratégie d’optimisation pour la gestion du grand nombre de variables et de contraintes.La première partie de ce travail traite des modèles de krigeage. Nous avons proposé une nouvelleformulation du noyau de covariance permettant de réduire le nombre de paramètres du modèleafin d’accélérer sa construction. Une des limitations connues du modèle de krigeage concernel’estimation de ses paramètres. Cette estimation devient de plus en plus difficile lorsque nousaugmentons la dimension du phénomène à approcher. En particulier, la base de données nécessitedavantage de points et par conséquent la matrice de covariance du modèle du krigeage est de plusen plus coûteuse à inverser. Notre approche consiste à réduire le nombre de paramètres à estimer en utilisant la méthode de régression des moindres carrés partiels (PLS pour Partial Least Squares). Cette méthode de réduction dimensionnelle fournit des informations sur la relation linéaire entre les variables d’entrée et la variable de sortie. Ces informations ont été intégrées dans les noyaux du modèle de krigeage tout en conservant les propriétés de symétrie et de positivité des noyaux. Grâce à cette approche, la construction de ces nouveaux modèles appelés KPLS est très rapide étant donné le faible nombre de paramètres nécessaires à estimer. La validation de ces modèles KPLS sur des cas test académiques ou industriels a démontré leur qualité de prédiction équivalente voire même meilleure que celle des modèles de krigeage classiques. Dans le cas de noyaux de covariance de type exponentiel, laméthode KPLS peut être utilisée pour initialiser les paramètres du krigeage classique, afin d’accélérerla convergence de l’estimation des paramètres du modèle. La méthode résultante, notée KPLS+K, a permis d’améliorer la qualité des modèles dans le cas de fonctions fortement multimodales. La deuxième contribution de la thèse a consisté à développer une stratégie d’optimisation globale sous contraintes pour la grande dimension, en s’appuyant sur les modèles KPLS ou les modèlesKPLS+K. En effet, nous avons étendu la méthode d’optimisation auto-adaptative connue dans lalittérature sous le nom "Efficient Global Optimisation, EGO" pour gérer les problèmes d’optimisationsous contraintes en grande dimension. Différents critères d’enrichissement adaptatifs ont pu êtreexplorés. Cette stratégie a permis de retrouver l’optimum global sur des problèmes académiquesjusqu’à la dimension 50. La méthode proposée a été confrontée à deux types de problèmes industriels, le cas test MOPTA issu de l’industrie automobile (124 variables d’entrée et 68 fonctions contraintes) et le cas test Snecma des aubes de turbomachines (50 variables d’entrée et 31 fonctions contraintes). Les résultats ont permis de montrer la validité de la démarche ainsi que les limites de la méthode pour une application dans un cadre industriel. / Aerospace turbomachinery consists of a plurality of blades. Their main function is to transfer energybetween the air and the rotor. The bladed disks of the compressor are particularly important becausethey must satisfy both the requirements of aerodynamic performance and mechanical resistance.Mechanical and aerodynamic optimization of blades consists in searching for a set of parameterizedaerodynamic shape that ensures the best compromise solution between a set of constraints.This PhD introduces a surrogate-based optimization method well adapted to high-dimensionalproblems. This kind of high-dimensional problem is very similar to the Snecma’s problems. Ourmain contributions can be divided into two parts : Kriging models development and enhancementof an existing optimization method to handle high-dimensional problems under a large number ofconstraints. Concerning Kriging models, we propose a new formulation of covariance kernel which is able toreduce the number of hyper-parameters in order to accelerate the construction of the metamodel.One of the known limitations of Kriging models is about the estimation of its hyper-parameters.This estimation becomes more and more difficult when the number of dimension increases. Inparticular, the initial design of experiments (for surrogate modelling construction) requires animportant number of points and therefore the inversion of the covariance matrix becomes timeconsuming. Our approach consists in reducing the number of parameters to estimate using the Partial LeastSquares regression method (PLS). This method provides information about the linear relationshipbetween input and output variables. This information is integrated into the Kriging model kernelwhile maintaining the symmetry and the positivity properties of the kernels. Thanks to this approach,the construction of these new models called KPLS is very fast because of the low number of newparameters to estimate. When the covariance kernel used is of an exponential type, the KPLS methodcan be used to initialize parameters of classical Kriging models, to accelerate the convergence of theestimation of parameters. The final method, called KPLS+K, allows to improve the accuracy of themodel for multimodal functions. The second main contribution of this PhD is to develop a global optimization method to tacklehigh-dimensional problems under a large number of constraint functions thanks to KPLS or KPLS+Kmethod. Indeed, we extended the self adaptive optimization method called "Efficient Global Optimization,EGO" for high-dimensional problems under constraints. Several enriching criteria have been tested. This method allows to estimate known global optima on academic problems up to 50 inputvariables. The proposed method is tested on two industrial cases, the first one, "MOPTA", from the automotiveindustry (with 124 input variables and 68 constraint functions) and the second one is a turbineblade from Snecma company (with 50 input variables and 31 constraint functions). The results showthe effectiveness of the method to handle industrial problems.We also highlight some importantlimitations.

<div>This dissertation presents a strategy that incorporates nature and bio-inspired shapes to redesign turbine airfoils and stator-rotor rim seal cavity.</div><div><br></div><div>The first objective consists of the development of tools to optimize the turbine velocity triangles and then the 3D shape using 75 parameters. Design trends that minimize loss in the stator and rotor were discussed. The second objective expands on the first by incorporating wavy structures at the leading and trailing edges as well as the suction side mimicking design features of seal whiskers and tubercles of a whale. The airfoils were optimized to maximize the efficiency of a highly loaded high-pressure turbine at positive incidence.</div><div><br></div><div>The last objective addressed the design of the cavity to reduce cooling massflow and protect the turbine platform. A novel strategy was proposed to assess and optimize the shape of the cavity. In an attempt to simply the problem and identify the main physical phenomena, a slice of the flow was examined by considering a purely a 2D case in the relative frame of reference. This simplification enabled the cavity to be optimized in 2D using a geometry inspired by the meandering of rivers. The optimization produced designs that reduce the heat flux in the rear rotor platform and are less sensitive towards variations in gap and cavity total pressure. The methodology was demonstrated in 3D rotating cavity and later in a full turbine stage configuration. The strategy and design tools developed in this dissertation seek to provide understanding of the effects of bio-inspired shapes on turbine blades and lay the foundation for future experimental research into cavity flows.<br></div>

Torsional vibration is an oscillatory angular twisting motion in the rotating members of a system. It can be deemed quite dangerous in that it cannot be detected as easily as other forms of vibration, and hence, subsequent failures that it leads to are often abrupt and may cause direct breakage of the shafts of the drive train. The need for sufficient analysis during the design stage of a rotating machine is, thus, well justified in order to avoid expensive modifications during later stages of the manufacturing process. In 1998, a project was initiated by the Turbomachinery Research Consortium (TRC) at Texas A&M University, College Station, TX, to develop a suite of computer codes to model torsional vibration of large drive trains. The author had the privilege of developing some modules in Visual Basic for Applications (VBA-Excel) for this suite of torsional vibration analysis codes, now collectively called XLTRC-Torsion. This treatise parleys the theory behind torsional vibration analysis using both the Transfer Matrix approach and the Finite Element approach, and in particular, validates the results generated by XLTRC-Torsion based on those approaches using experimental data available from tests on a 66,000 HP Air Compressor.

<p>This research presents the detailed investigation of coupled-mode flutter and non-synchronous vibration in turbomachinery. Coupled-mode flutter and non-synchronous vibration are two aeromechanical challenges in designing turbomachinery that, when present, can cause engine blade failure. Regarding flutter, current industry design practices calculate the aerodynamic loads on a blade due to a single mode. In response to these design standards, a quasi three-dimensional, reduced-order modeling tool was developed for identifying the aeroelastic conditions that cause multi-mode flutter. This tool predicts the onset of coupled-mode flutter reasonable well for four different configurations, though certain parameters were tuned to agree with experimentation. Additionally, the results of this research indicate that mass ratio, frequency separation, and solidity have an effect on critical rotor speed for flutter. Higher mass-ratio blades require larger rotational velocities before they experience coupled-mode flutter. Similarly, increasing the frequency separation between modes and raising the solidity increases the critical rotor speed. Finally, and most importantly, design guidelines were generated for defining when a multi-mode flutter analysis is required in practical turbomachinery design. </p><p>Previous work has shown that industry computational fluid dynamics can approximately predict non-synchronous vibration (NSV), but no real understanding of frequency lock-in and blade limit-cycle amplitude exists. Therefore, to understand the causes of NSV, two different reduced-order modeling approaches were used. The first approach uses a van der Pol oscillator to model a non-linear fluid instability. The van der Pol model is then coupled to a structural degree of freedom. This coupled system exhibits the two chief properties seen in experimental and computational non-synchronous vibration. Under various conditions, the fluid instability and the natural structural frequency will lock-in, causing structural limit-cycle oscillations. This research shows that with proper model-coefficient choices, the frequency range of lock-in can be predicted and the conditions for the worst-case, limit-cycle-oscillation amplitude can be determined. This high-amplitude limit-cycle oscillation is found at an off-resonant condition, i.e., the ratio of the fluid-shedding frequency and the natural-structural frequency is not unity. In practice, low amplitude limit-cycle oscillations are acceptable; this research gives insight into when high-amplitude oscillations may occur and suggests that altering a blade's natural frequency to avoid this resonance can potentially make the response worse.</p><p>The second reduced-order model uses proper orthogonal decomposition (POD) methods to first reconstruct, and ultimately predict, computational fluid dynamics (CFD) simulations of non-synchronous vibration. Overall, this method was successfully developed and implemented, requiring between two and six POD modes to accurately predict CFD solutions that are experiencing non-synchronous vibration. This POD method was first developed and demonstrated for a transversely-moving, two-dimensional cylinder in cross-flow. Later, the method was used for the prediction of CFD solutions for a two-dimensional compressor blade, and the reconstruction of solutions for a three-dimensional first-stage compressor blade. </p><p>This research is the first to offer a van der Pol or proper orthogonal decomposition approach to the reduced-order modeling of non-synchronous vibration in turbomachinery. Modeling non-synchronous vibration is especially challenging because NSV is caused by complicated, unsteady flow dynamics; this initial study helps researchers understand the causes of NSV, and aids in the future development of predictive tools for aeromechanical design engineers.</p> / Dissertation