Performance Testing of a Microwave Tip Clearance Sensor

Abstract

*The largest barrier to effective operation of tip clearance sensors in aero engines has been survivability and sensor accuracy within difficult thermal environments, especially in the turbine area. Thermal cycling, high thermal gradients and total time at temperature often contribute to premature sensor degradation and failure. This paper describes the results of testing a microwave tip clearance sensor designed to operate within either the compressor or turbine. The testing included resolution and linearity measurements along with thermal cycling of the cable in a high temperature furnace. I. Introduction HE measurement of tip clearance within gas turbine engines is normally performed within the context of providing information back to engine designers. It is desired that tip clearances be designed as close as possible, yet still not rub the case as the engine goes through its operating envelope. It has been documented that tip clearance pinch points occur on takeoff and landing as differences in thermal expansions between the case and rotor occur during the full output power condition at takeoff as well as the thermal soak back that occurs immediately after landing. 1 It is difficult to obtain actual tip clearance data in the field, due to the difficulty in getting tip clearances sensors to provide accurate and reliable data for an extended period of time. Therefore, design tip clearances can often be conservative, since it normally desirable to operate at a larger clearance then risk significant mechanical contact between the blades and case. Large tip clearances can have a substantial impact on efficiencies. It has been reported in the literature that a 0.001” the tip clearance can be closed can contribute as much as an additional 0.1% in efficiency improvement. 2 Normal operation of the engine typically causes clearances to increase over time as the blades and case coatings gradually erode due to the gas path or mechanical contact between the blades and case. If an active clearance control system could be designed able to adjust the tip clearances dynamically for the current engine condition, then the clearances could be optimized for the current location within the operating envelope as well as assist in maintaining constant engine performance in-between overhauls. A variety of active tip clearance control methods have been proposed including thermal techniques that control the clearance using compressor bleed air as well as active mechanical techniques that shift the rotor axially or use some type of segmented case to squeeze the case around the blades. New large commercial engine designs use compressor bleed air to actively control the tip clearance. Due to the inability to directly measure clearances in the engine, the tip clearance is scheduled using available data off of the engine combined with various models and algorithms. This technique also has some conservatism within it, which would benefit from actual clearance data to close the loop and thus optimize the control system. To date, the main types of sensors that are used for measuring tip clearance include capacitive, eddy current, optical, and microwave. Each of these different sensing principals has positives and negatives associated with them. The largest barrier to date in implementing a tip clearance sensor on a production engine has been sensor reliability, especially in the hottest areas of the engine. Sensor reliability in the turbine will typically be tens to hundreds of hours without some type of external active cooling, such as nitrogen or circulated water. Therefore, the ability to implement a sensor with long term reliability remains the most significant barrier to improved active clearance control performance. The microwave sensor described in this paper contains some unique characteristics, which make it highly suitable for use in the turbine environment. This paper will describe those characteristics as well as provide the results of performance testing showing the applicability of the sensor for use within an active clearance control system.