Blade Tip Clearance Sensors for Use in Engine Health Monitoring Applications

Blade tip clearance is a key design parameter for gas turbine designers. This parameter is often measured during engine testing and development phases as part of design validation but has yet to be utilized during normal engine fleet operation. Although blade tip clearance measurements are often mentioned for fleet operation in the context of active clearance control, the use of blade tip clearance measurements can provide an additional benefit for engine health monitoring. This paper explores the use of blade tip clearance sensors for engine condition monitoring of hot section blades. Blade tip clearance, especially in the first stage turbine, has an impact on exhaust gas temperature. The use of tip clearance measurements can provide supplementary information to traditional EGT measurements by providing a direct measurement of wear on the blade tips. In addition, blade creep and cracking can be measured and tracked if the sensors are able to provide clearance values of individual blades. Broken or missing blades can be identified allowing corrective action be taken in advance of normal inspection intervals. In order to enable health monitoring applications, blade tip clearance sensors must be survivable, repeatable, and provide high fidelity data. This paper discusses a microwave blade tip clearance sensor and discusses some of the key sensor attributes needed for successful health monitoring.