Réduction des contraintes secondaires en tension dans les pales en céramique de rotors de turbine en configuration renversée

Depuis quelque temps, un groupe de chercheurs du Groupe d'innovation Createk de l'Université de Sherbrooke travaille sur une nouvelle configuration de rotor de turbine qui utilise un carénage rotatif structurel pour tenir des pales en céramique sous compression, sous l’action du champ centrifuge. Cette configuration de rotor, baptisée Inside-Out Ceramic Turbine (ICT) vise à augmenter la température d'entrée de la turbine (TIT) pour les petites turbines, pour lesquelles le refroidissement et les techniques de fabrication complexes sont d'un coût prohibitif. Plusieurs prototypes et itérations du rotor ICT ont été testés au cours des dernières années, augmentant le temps de fonctionnement et la température d’opération atteignables. Au fur et à mesure que la confiance de l'équipe dans le rotor ICT s'améliore, les problèmes auxquels il est confronté deviennent importants à identifier et à résoudre. Cette thèse vise à étudier et à améliorer la fiabilité des pales en céramique dans des conditions normales de fonctionnement. Ceci est essentiel à l'adoption de la technologie ICT dans un grand nombre de moteurs. La thèse est découpée en trois parties : (1) isoler la cause la plus probable de défaillance des pales grâce à l'observation des résultats expérimentaux et numériques passés, (2) quantifier l'influence relative des principaux paramètres de conception sur la fiabilité des pales à l’aide d’un modèle numérique, et (3) tester des solutions potentielles qui ciblent les variables de conception les plus critiques, pour obtenir une meilleure fiabilité des pales ICT. Les pales étant en céramique technique monolithique, actuellement en nitrure de silicium, elles supportent mal les contraintes en tension. Le niveau de contrainte le plus élevé dans les pales s’avère être à l'interface avec les composants métalliques de support, car les pales sont maintenues en place par friction sous une force normale importante, et des contraintes élevées en tension se manifestent à l'interface. Cela demande de réduire d'abord le nombre d'interfaces au minimum, c'est-à-dire uniquement au niveau du bout de pale en appui contre le carénage structurel tournant. Deuxièmement, à cette interface subsistante, les efforts pour réduire le coefficient de frottement, ainsi que pour réduire la différence de dilatation thermique et de déformation tangentielle entre la pale et le carénage, sont essentiels pour obtenir de faibles contraintes en tension à l'extrémité de la pale. La validation expérimentale des revêtements a été menée avec succès : le revêtement de barrière thermique (TBC) appliqué entre l'extrémité de la pale et le carénage pourrait augmenter la température de l'extrémité de la pale et diminuer la température du carénage, conduisant ainsi à une meilleure correspondance de la dilatation thermique ; le nitrure de bore hexagonal (hBN) est un lubrifiant solide à haute température qui réduit considérablement le coefficient de frottement, même sous une charge extrême, et pourrait conduire à au moins doubler la tolérance à la différence de déformation. Ainsi, un double revêtement de TBC et de hBN devrait réduire la charge en bout de pale. Les niveaux de contrainte réels dépendent de la géométrie de la pale et de la conception globale de la turbine, mais un outil numérique simple a été développé qui permet au concepteur de déterminer quel niveau de réduction de frottement et d'ajustement de dilatation thermique est nécessaire pour obtenir une fiabilité adéquate dans la pale.Abstract : For some time now, a group of researchers in the Createk Innovation Group at Université de Sherbrooke have been working on a novel turbine rotor configuration which uses a structural, rotating shroud to compress ceramic blades under centrifugal load. This rotor configuration, dubbed Inside-Out Ceramic Turbine (ICT) aims at increasing turbine inlet temperature (TIT) for small turbines, for which intricate cooling and complex manufacturing techniques are prohibitively costly. Several prototypes and iterations of the ICT rotor have run over the last years, achieving consistently better run times and firing temperatures. As the team’s confidence in the ICT rotor improves, the issues facing it become important to identify and address. This thesis aims at investigating and improving reliability of the ceramic blades under normal operating conditions. This is central to the adoption of ICT technology in mainstream engines. The thesis is cut into three parts: (1) isolating the most probable cause of failure in the blades through observation of past experimental and numerical results, (2) quantifying the relative influence of the main design parameters on blade reliability through a numerical model, and (3) testing potential solutions which target the most critical design variables, to achieve a better reliability in the ICT blades. As the blades are made of monolithic technical ceramic, currently silicon nitride, they do not tolerate tensile stress fields well. The highest stress level in the blades was found to be at the interface with supporting metallic components, as the blades are maintained in place through friction under large normal force, and a strain mismatch is present at the interface. This leads to first reduce the number of interfaces to a minimum, i.e., only at the blade tip pressing up against the rotating structural shroud. Second, at this remaining interface, efforts to reduce coefficient of friction especially, as well as reduce thermal expansion mismatch and hoop strain between the blade tip and the shroud, are key to achieving low tensile stresses at the blade tip. Experimental validation of coatings was successfully conducted: thermal barrier coating (TBC) applied between the blade tip and the shroud, could increase blade tip temperature and decrease shroud temperature, thus leading to a closer match in thermal expansion; hexagonal boron nitride (hBN) is a high temperature solid lubricant which significantly reduces coefficient of friction, even under extreme load, and could lead to at least double the amount of strain mismatch tolerance. Thus, a double coating of TBC and hBN is expected to reduce the load at the blade tip. Actual stress levels depend on blade geometry and overall turbine design, but a simple numerical tool was developed which could allow the designer to determine what level of friction reduction and thermal expansion fit is required to achieve an adequate reliability in the blade

ANALYTIC APPROXIMATE SOLUTION FOR NONLINEAR DYNAMICMODELING OF THE ROTATING ELASTIC 2D BEAM WITH A SINGLE CRACK

In this paper, the 2D lateral vibration analysis of a rotating cracked beam as a rotary structure is investigated through the Homotopy perturbation analysis and compared with the numerical Newmark-beta (Nβ) algorithm. The structure and crack are modeled as the Euler-Bernoulli (EB) theory and simple torsional spring, respectively. The nonlinear equations of motion are derived using Galerkin and the Assumed Mode Method (AMM). The system’s stability is analyzed through phase plane and time response for different angular velocities of the base, initial values, external disturbances, crack stiffness, and locations. A comparative study presents simulation results for free (first nonlinear frequency) and forced vibration. It is shown that the proposed semi-analytical approach is beneficial as it provides a benchmark for a more precise analysis and further investigation of cracked rotary structures

Scalable Domain Decomposition for Parallel Solution of 3D Finite Element Multibody Rotorcraft Aeromechanics

A specialized mesh partitioner is developed for large-scale multibody three-dimensional finite element models. This partitioner enables modern domain decomposition algorithms to be leveraged for the parallel solution of complex, multibody, three-dimensional finite element-based rotor structural dynamics problems. The partitioner works with any domain decomposition algorithm, but contains special features for FETI-DP, a state-of-the-art iterative substructuring algorithm. The algorithm was implemented into an aeroelastic rotor solver X3D, with several modifications to improve performance. The parallel solver was applied to two practical test cases: the NASA Tiltrotor Aeroacoustic Model (TRAM) and the NASA Rotor Optimization for the Advancement of Mars eXploration (ROAMX) rotor blade. The mesh partitioner was developed from two sets of requirements: one standard to any domain decomposition algorithm and one specific to the FETI-DP method. The main feature of the partitioner is the ability to robustly partition any multibody structure, but with several special features for rotary-wing structures. The NASA TRAM, a 1/4 scale V-22 model, was specially released by NASA as a challenge test case. This model contained four flexible parts, six joints, nearly twenty composite material decks, a fluid-structure interface, and trim control inputs. The solver performance was studied for three test problems of increasing complexity: 1) an elementary beam, 2) the isolated TRAM blade, and 3) the TRAM blade and hub assembly. A key conclusion is that the use of a skyline solver for the coarse problem eliminates the coarse problem scalability barrier. Overall, the principle barrier of computational time that prevented the use of high-fidelity three-dimensional structures in rotorcraft is thus resolved. The two selected cases provided a template for how 3D structures should be used in the future. A detailed aeromechanical analysis of the NASA TRAM rotor was conducted. The solver was validated against experimental results in hover. The stresses in the blade and hub components were examined, illustrating the unique benefit of 3D structures. The NASA ROAMX blade was the first rotor blade to our knowledge designed exclusively with 3D structures. The torsional stability, blade loads, blade deformations, and 3D stresses/strains were evaluated for multiple blade designs before the final selection. The aeroelastic behavior of this blade was studied in steady and unsteady hover. Inertial effects were found to dominate over aerodynamics on Mars. The rotor blade was found to have sufficient factor of safety and damping for all test conditions. Over 20 thousand cases were executed with detailed stresses/strains as means of downselection, demonstrating the efficiency and utility of the parallel solver, and providing a roadmap for its use in future designs

Failure Analysis and Mechanisms of Failure of Fibrous Composite Structures

The state of the art of failure analysis and current design practices, especially as applied to the use of fibrous composite materials in aircraft structures is discussed. Deficiencies in these technologies are identified, as are directions for future research

Materials for Wind Turbine Blades: An Overview

A short overview of composite materials for wind turbine applications is presented here. Requirements toward the wind turbine materials, loads, as well as available materials are reviewed. Apart from the traditional composites for wind turbine blades (glass fibers/epoxy matrix composites), natural composites, hybrid and nanoengineered composites are discussed. Manufacturing technologies for wind turbine composites, as well their testing and modelling approaches are reviewed