Measurement of blade tip clearance using an ultrasonic sensor

Abstract

Higher performance of the gas turbine engine is achieved by keeping the blade tip clearance to the casing as smaller as possible, which may be realized by the on-line measurement and the related closed loop control system. The ultrasonic sensor method described here is considered a promising candidate for the on-line measurement of a blade tip clearance. The sensor used was an air/gas propagation type. The natural frequency was approximately 1.0 MHz and the diameter was 20 mm. An ultrasonic wave, excited from the casing side by a sensor, travels through a clearance space and is echoed at a blade tip, and travels back to the sensor. The rotational blade test rig consisted of 18 blades, with a height of 30 mm and a thickness of 2.3 mm. The rotational speed was approximately 60 m/s or Mach 0.18. The measured clearance by the sensor was compared with another optical measurement and the estimate, calculated by taking into consideration the effect of the centrifugal force. As a result, a good agreement was shown.