The performance of gas turbines relies heavily on the blade design geometry. To raise the thermal efficiency of the turbine, a high inlet temperature is required, which leads to the formation of hot-spots on the blade surfaces and thereby reduces its useful life cycle. Additionally, the blades are also subjected to pressure loads from the fluid flow, which causes structural deformation and may lead to the development of bends and cracks. The blade geometry as such is a vital component in turbines to decide the overall performance.
Evaluating the blade design for structural integrity and fatigue life cycle is often carried out using destructive testing methods, which are often costly and time consuming. Although they are of great importance from design perspectives, the tests do not reveal comprehensive information on temperature distribution, stress concentration and flow characteristics. Non-destructive inspections on the other hand are time consuming as the entire blade geometry is inspected gradually to identify deformation and cracks.
However, to reduce the inspection time in non-destructive testing, computational techniques can serve as an assistive tool. With FEM and CFD analyses, engineers can better understand temperature distribution and pressure loadings over the blade surface and identify critical regions, which can later be inspected using non-destructive testing; rather then evaluating entire geometry.
Thermal loading and the resulting stress and deformation values can be obtained using finite element method. The stress concentration along the blade geometry can then be identified requiring critical attention. Although finite element analysis is one of the widely used techniques to evaluate mechanical structures, their reliability is majorly dependent on the solver capabilities and design assumptions being made in the FE model. It is quite possible to expect deviations from the experimental results, if boundary conditions are not suitably assigned. As such, it is essential to ensure that the FE model represents real physical conditions as much as possible.
A more comprehensive evaluation of the blade can be achieved by also simulating the blades using computational fluid dynamics. As the blades are also subjected to gas pressure, considering the effects of thermal loading alone is practically of no use. This requires developing a solid blade and a fluid domain surrounding the blade to capture the flow physics. Applying proper turbulence model and boundary conditions will allow engineers to visualize the effect of pressure on the blade surface and identify critical regions that are expected to deform and develop cracks.
The information obtained through FEM and CFD analyses can then be utilized by service technicians to inspect the blades more comprehensively, ensuring that the critical regions simulated are not missed for crack inspection. Having pre-conceived information about the inspection significantly reduces the testing time and ultimately leads to the reduction in maintenance costs. CAE techniques thus serves as an assistive tool for non-destructive inspection techniques used to evaluate turbine blades more effectively.
Image Credit: laborelec.be/ENG/news_cat/non-destructive-testing-2/