Computational fluid dynamic and thermal stress analysis of coatings for high-temperature corrosion protection of aerospace gas turbine blades

The current investigation presents detailed finite element simulations of coating stress analysis for a 3-dimensional, 3-layered model of a test sample representing a typical gas turbine component. Structural steel, Titanium alloy and Silicon Carbide are selected for main inner, middle and outermost layers respectively. ANSYS is employed to conduct three types of analysis- static structural, thermal stress analysis and also computational fluid dynamic erosion analysis (via ANSYS FLUENT). The specified geometry which corresponds to corrosion test samples exactly is discretized using a body-sizing meshing approach, comprising mainly of tetrahedron cells. Refinements were concentrated at the connection points between the layers to shift the focus towards the static effects dissipated between them. A detailed grid independence study is conducted to confirm the accuracy of the selected mesh densities. The momentum and energy equations were solved, and the viscous heating option was applied to represent improved thermal physics of heat transfer between the layers of the structures. A discrete phase model (DPM) in ANSYS FLUENT was employed which allows for the injection of continuous uniform air particles onto the model, thereby enabling an option for calculating the corrosion factor caused by hot air injection. Extensive visualization of results is provided. The simulations show that ceramic (silicon carbide) when combined with titanium clearly provide good thermal protection; however, the ceramic coating is susceptible to cracking and the titanium coating layer on its own achieves significant thermal resistance. Higher strains are computed for the two-layer model than the single layer model (thermal case). However even with titanium only present as a coating the maximum equivalent elastic strain is still dangerously close to the lower edge. Only with the three-layer combined ceramic and titanium coating model is the maximum equivalent strain pushed deeper towards the core central area. Here the desired effect of restricting high stresses to the strongest region of the gas turbine blade model is achieved, whereas in the other two models, lower strains are produced in the core central zones. Generally, the CFD analysis reveals that maximum erosion rates are confined to a local zone on the upper face of the three-layer system which is in fact the sacrificial layer (ceramic coating). The titanium is not debonded or damaged which is essential for creating a buffer to the actual blade surface and mitigating penetrative corrosive effects. The present analysis may further be generalized to consider three-dimensional blade geometries and corrosive chemical reaction effects encountered in gas turbine aero-engines. Key words: Thermal coating; Silicon Carbide ceramic; ANSYS; Finite element stress analysis; CFD (computational fluid dynamics); mesh density; total deformation; erosion.</i

Les systèmes barrières thermiques pour aubes de turbine

Les systèmes barrières thermiques pour aubes de turbine sont des systèmes multicouches dont la structure et la composition évoluent à haute température. Cette évolution, qui est liée à l'interaction avec l'environnement oxydant, entraîne des modifications des champs de contraintes présents dans ces systèmes et qui peuvent provoquer leur endommagement, voire leur ruine. Ce texte synthétique présente succinctement les systèmes actuellement utilisés et la problématique de l'interaction entre aspects mécaniques et environnement dans la turbine

Microindentation instrumentée de 20 à 900 °C sur matériaux constitutifs de barrières thermiques

Nous décrivons le principe et les possibilités d’appareils de microindentation instrumentée conçus pour fonctionner jusqu’à 1000 °C et 1200 °C. Des exemples de résultats (microdureté, comportement en fluage) sont donnés sur des alliages de type NiAl(Pt), qui sont des matériaux de base pour constituer la couche de liaison de barrières thermiques pour aubes de turbines. La technique de microindentation instrumentée à chaud ouvre la voie à la détermination de lois de comportement mécanique local de revêtements et de phases individuelles dans un matériau multiphasé