Gas turbine efficiency can be improved with tighter turbine tip clearances. An approach being developed by engine manufacturers deploys active tip clearance monitoring where the turbine casing diameter is actively controlled in-service either mechanically or thermally. Typically current engines operate at about 1% clearance of blade span. With active control this could potentially be reduced significantly.

Ideally active tip clearance control requires closed loop feedback measurements to maintain very small clearances without the risk of blade tip contact with the casing liner. Therefore reliable and robust sensors systems are required that can operate at the elevated temperatures found in modern gas turbines. Currently there are limited sensor systems available that can operate at these temperatures and survive typical sensor life requirements of many thousands of hours.

This study details development of a high temperature eddy current sensor system for hot section applications. The investigation encompasses development and validation of an integrated sensor design to provide tip clearance measurements. The sensor is designed to withstand temperatures of order 1500 to 1600K. Test facilities used to validate the system include a RB168 Mk 101 Spey engine and a Rolls-Royce VIPER engine. The turbine casings of both engines were modified to fit sensors directly above the rotor. The accuracy of the system was validated in a high speed rotor test facility with engine representative blading. Accuracy of the eddy current sensor was compared and validated against a dynamic laser micrometer system.

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