Numerical investigation of flutter vibrations in industrial low pressure steam turbines

Numerical techniques have been widely used to investigate fluid dynamics problems. Several models have been proposed during the years, showing diferent levels of accuracy and computational costs. In the last decades, the large improvements achieved in computing technology led to a massive increase in the use of numerical tools for turbomachinery applications. In an industrial framework, the current state of the art is represented by URANS techniques which grant a good accuracy in the results of the simulations with an afordable computational cost. The development and validation of new numerical methods is of high interest since it provides a sound basis for the design of future technologies. Thanks to CFD, it is possible to obtain detailed information on the flow field and to study complex unsteady phenomena such as flutter instability, as addressed by the present thesis which focuses on the aeromechanical investigation of low pressure steam turbines. This kind of machines is relevant in the green economy scenario since it is possible to feed a steam power plant with renewables or nuclear energy. The constant efort to maximize the efficiency and to cover a wider operative range enhanced the aeromechanical risk, thus requiring a dedicated study of these phenomena. Considering the load conditions, the stability of a low pressure rotor was numerically investigated in a wide range of both low and high load configurations, highlighting the energy exchange mechanisms that affect flutter vibrations. Then, a study of the impact of part span connectors used to avoid vibrations was developed to verify the capabilities of the used URANS strategy in dealing with the presence of this kind of elements. Finally, the results obtained were used to move towards a coupled flutter approach accounting for both fluid and structural solvers to match closer the real behavior of a blade.