Gas turbine health monitoring is an important area of research. As the performance of aircraft and power plants increase, they will require better sensors for health monitoring systems to prevent failures. Health monitoring systems help in preventive maintenance reducing unnecessary downtime and maintenance costs.
Gas turbine blades are subjected to dynamic loads caused by rotor imbalances, distortions in the intake flows etc. These loads cause low or high cycle fatigues and the blades can fail over time. Tip-timing and tip-clearance systems makes it possible to assess turbomachinery blade vibration by using non-contact measurement systems such as optical, eddy current, hall effect, capacitve etc. The most widely used systems in industry are optical, however, these systems are still largely prone to contamination problems from dust, dirt, oil, water etc. Further development of these systems for in-service use is problematic because of the difficulty in eliminating contamination of the optics. Other systems, although immune to contamination, may not be able to measure both tip-clearance and tip-timing at the same time due to their operating principle. Another limitation is that they cannot be used in high temperature applications such as in a high pressure turbine where the temperatures can reach 1400°C. Eddy current sensors are found to be quite robust and can measure both tip-timing and tip-clearance. They are currently being used for gas turbine health monitoring applications at low temperatures such as in the compressor stage and last stage of a steam turbine.
A new high temperature eddy current sensor has been developed in-house at the University of Oxford for application in gas turbine tip-timing and tip clearance measurements to assess blade vibrations. The current sensor is a modified version of the existing eddy current sensor that is able to operate at high temperatures of about 1400°C. The paper presents the development of the sensor and experimental results of tip clearance measurements in the high pressure turbine stage of a jet engine. In the engine tests, two blades were reduced in height to increase the tip-clearance and the measurements were taken at both idle and max operating speeds. The sensor was found to work in these harsh environments and was sufficiently sensitive to accurately determine the tip clearance at these elevated temperatures. Tests were carried out mainly to demonstrate the technique of obtaining good tip clearance measurements and the survivability of the sensors in the high temperature and pressure environment.