Extracting blade condition information from the pressure field around a turbine blade

Abstract

Turbine stages are exposed to a variety of excitation sources in the power industry. The resulting forced vibration excitation of the blades may occur near a blade’s natural frequency. Blade vibration is an inevitable, inherent characteristic of turbines as the rotor blades travel through the trailing wakes of the upstream stator blades. Blade vibration can be worsened by other mechanisms such as pitting, corrosion fatigue and stress corrosion cracking commonly experienced in the power industry. Measuring turbine blade vibration allows for condition monitoring of the blades for damage. This is often coupled with finite element models of the blades or with computational fluid dynamic models of the flow field around the blades. These numerical methods, although well-established, lack the complexity of the true multiphysics phenomena within a turbine. As the blade vibration measurement techniques essentially capture blade vibration that is the result of fluid-structure interaction (FSI), blade vibration should be modelled as a coupled problem, but this is usually computationally expensive. A rudimentary yet fundamentally correct numerical model of a turbine stage is thus required to model the fluid-structure interaction while minimising computational costs and retaining accuracy. If this can be achieved and blade health information can be detected in the flow field within the model, further analyses can then be put forth to predict blade health over time. The main objective of this study is to investigate the extent to which blade condition information can be extracted from a transient three-dimensional two-way FSI model of a blade passage containing a single rotor and stator blade. An experimental single-stage test turbine with five stator and five rotor blades is used to gather experimental data. The experimental data is used to validate the FSI model in the time and frequency domains. Two rotor blade assemblies were tested with the first configuration consisting of five healthy blades, and the second configuration consisting of four healthy blades and one damaged blade. All simulations are performed at constant rotational speeds for one single revolution of the rotor. Structural damping of the rotor blades is not considered. All numerical simulations are carried out using the commercial multiphysics software package of Ansys R2 2019 and the explicit use of CFX for the CFD simulations. The results of the FSI model compare well to the experimental results when considering the simplifying assumptions made for the development of the numerical model. The first natural frequency and blade passing frequencies of the healthy and damaged blades can be extracted from the pressure field of the FSI model at critical speeds. Similar findings were observed in the fluid mesh deformation time profiles around the blade tips. Blade excitation is strongly coupled to engine-ordered vibration frequencies, specifically the blade passing frequencies and its first harmonic. Challenges are realised when modelling a single damaged blade that is part of a larger, healthy assembly of rotor blades. The compromise of reducing computational effort is highlighted here. However, very promising results pertaining to blade condition information extraction from the two-way FSI model pressure field are obtained. These results have established a foundation on which a more complex FSI model can be built and coupled with a fatigue or remaining useful life study. It is suggested that future work should include structural damping of the rotor blades, a larger computational domain, and investigation of longer simulation times.