Gas turbine blade natural frequency measurement using external casing vibrations

Abstract

Currently tip timing methods are the pseudo industry standard for measuring Gas Turbine blade vibration. The physical placement of tip timing probes is however reasonably prohibitive due to the high pressure/temperature environment of a turbine. A simpler method of sensor setup would be through the use of externally mounted accelerometers to measure the casing vibration response and relate this back to the internal blade vibration.The vibration of a gas turbine casing is driven by the strong rotating pressure which develops around the rotating blade stages. The oscillating motion of the rotor blades phase modulates this pressure signal. An analytical formulation of the internal pressure signal is developed in this paper. The effects of blade motion on this internal pressure signal is then investigated as the speed of the engine is increased/decreased such that the driving frequency traverses a rotor blade natural frequency. Experimental measurements on a simplified test turbine are presented comparing the results of the analytical internal pressure signal along with results of the measured casing vibration during engine run up/down. It is shown that the spectrum of the internal pressure and casing vibration signal contains information which can be used to estimate the rotor blade natural frequencies.