Non-contact gas turbine blade vibration monitoring using internal pressure and casing response measurements

Abstract

This thesis addresses the non-contact measurement of rotor blade vibrations in gas turbines. Specifically, use is made of internal casing wall pressure, and external casing vibration measurements. Non-contact measurement of gas turbine blade vibrations has made significant progress over recent years; however, there still exist some limitations in the current techniques available. The current dominant non-contact method uses proximity probes to measure blade arrival time, to be used for blade vibration monitoring. Distinctly with these blade tip timing methods, some of the limitations are: the requirement of a large number of sensors for each engine stage, difficulties in dealing with multiple excitation frequencies, and sensors being located in the gas path. Alternative techniques are examined here, utilising the unsteady casing wall pressure, and external casing vibration measurements which have the potential to rectify some of these limitations. Simulated internal pressure measurements are used in the proposal of a technique for direct rotor blade vibration amplitude estimation. A novel phase demodulation procedure was developed to obtain the blade vibration amplitude estimates from the simulated internal pressure signal. This demodulation technique has the potential to find further application with phase modulated signals, often present in rotating machinery, where the modulating frequency is higher than the carrier frequency. Although the use of internal pressure measurements showed great potential in the direct measurement of rotor blade vibrations, the use of external sensors outside of the flow path has a more discernible advantage. Thus, the development of the response of the external casing of a gas turbine under the internal pressure forces was undertaken, with the unique response of an axi-symmetric structure under moving loads being presented. Once the response of the casing structure is determined, it can then be used in understanding how external casing vibration measurements could be correlated to rotor blade vibrations. It is shown that the stochastic portion of the external casing vibration measurements will contain narrowband peaks located at multiples of shaft speed plus and minus rotor blade natural frequencies. These results, significantly, demonstrated that blade vibration information can be obtained from casing vibration measurements at a single engine running speed.