Experimental and numerical investigation of turbomachinery blade vibration

Abstract

Despite constant research on Blade Tip-Timing (BTT) for decades, the major limitation of this turbomachinery blade vibration measurement technique in terms of insufficient data sampling is still not overcome. Complex vibrations cannot reliably be analysed and, to tackle this issue, an innovative data analysis approach to increasing the amount of useful data was developed. Instead of only being used for determining vibration parameters on the basis of times of blade arrival at a sensor, complete waveforms produced by stationary casing-mounted sensors are processed. Such waveforms usually hold multiple data points of the blade vibration trajectory, yet these data points are hidden and not directly accessible when eyeing the sensor output. Their analysis thus provides new opportunities in respect of the following: characterising multi-mode vibrations occurring within the same instant of time; detecting non-linear vibration responses arising from excitation of engine components with non-linear behaviour; ascertaining axial, radial, and tangential blade positions during operation. After an introduction to state-of-the-art BTT, the novel concept of investigating blade vibration in turbomachinery is presented on the principle of Blade Tip-Timing (BTT) and clearance sensor waveform analysis (BLASMA), which was filed for patent protection by Rolls-Royce. It is shown that the modulation of the sensor output by blade vibration can offer additional information compared with under-sampled time-of-arrival (TOA) data from traditional BTT applications. The sensor data can help to not only dispel doubts on the validity of statements on blade vibration but also lessen the dependence on contact-based strain gauges measurements for producing reference data. A numerical and experimental study was conducted to evaluate the merit of sensor waveform analysis for describing multi-harmonic blade vibration. Results are carefully discussed in this work and future strategies for proceeding with BLASMA are proposed.