Blade Tip Clearance and Blade Vibration Measurements Using a Magnetoresistive Sensor

A Blade Tip Timing (BTT) system enables the measurement of turbomachinery blade vibrations by means of contactless sensors mounted in the casing. In addition, the so-called Blade Tip Clearance (BTC) measurement system ensures the monitoring of the existing running gaps between the blade tip and the casing. This paper focuses on the results achieved so far using a novel magnetoresistive sensor for simultaneous BTT and BTC measurements. Several prototypes of the probe have been realized and tested. Measurements of clearance are possible with an uncertainty of U = ± 22 μm (20:1) within a gap range [1 ÷ 1.5] mm, in line with the commonly used probes. In terms of measurement of vibration, a dedicated calibration bench for any type of BTT and BTC probe was realized. Experimental investigations are presented. The magnetoresistive sensor has low manufacturing costs and the metrological characteristics fulfill the requirements of industrial instrumentation standards. NOMENCLATURE BTC Blade Tip Clearance BTT Blade Tip Timing CT Calibration Tower DAS Data Acquisition System I Current M Magnetic Field OPR Once Per Revolution signal SNR Signal to Noise Ratio θ Theta: angle between the Magnetic Field and Current vectors U95 Expanded Uncertainty V Voltage INTRODUCTION The blade tip clearance and the vibration of the blades play a fundamental role in the design and development of any turbomachine. For instance, the efficiency of the rotor can be improved by minimizing the distance between the blade tip and the casing in order to reduce leakage flows. However, during operation the tip clearance changes due to mechanical loads and varying temperature and pressure conditions inside the turbomachine. To prevent a fatal damage, an accurate online monitoring system is therefore mandatory for optimized and safe operations. The so-called blade tip clearance (BTC) measurement systems are based on non-contact probes sensitive to the distance between the passing blade tip and the casing. As reported by Lattime and Bruce (2004), a typical sensor for such applications requires an accuracy of about 25 μm. In addition, all turbomachinery blades and components experience vibrations during their operation. This is due to several sources like: mistuning, pressure distortions, flutter, stall and surge; as explained more in Proceedings of 11 European Conference on Turbomachinery Fluid dynamics & Thermodynamics ETC11, March 23-27, 2015, Madrid, Spain