Capacitance tip timing techniques in gas turbines (2003)

The vibration of turbomachinery blades is an important phenomenon to understand, observe and predict&#13;\nand is the reason for developing a tip timing measurement system. Vibration leads to High Cycle Fatigue&#13;\n(HCF), which limits blade durability and life. HCF can result in blade failure, having expensive&#13;\nconsequences for the engine involved. The traditional method for monitoring blade vibration under test&#13;\nconditions is to use blade mounted strain gauges. However, strain gauges are costly and time consuming&#13;\nto install. They have a limited operating life as they are subjected to the harsh on-engine conditions. Only&#13;\na limited number of blades can be monitored with strain gauges as the number that can be used is limited&#13;\nby the number of channels in the slip ring or telemetry. They can also interfere with the assembly&#13;\naerodynamics. Consequently non-intrusive alternative techniques such as tip timing are sought.&#13;\nCapacitance probe based clearance measurement systems see widespread use in turbomachinery&#13;\napplications to establish rotor blade tip clearance. This thesis reports investigations into an alternative&#13;\nand additional use in aero-engine rotor blade tip timing measurement for these commercially available&#13;\nsystems. Tip clearance is of great importance in the gas turbine industry; this is clear from the fact that&#13;\ngas turbine efficiency has an inverse relationship with tip clearance. Large tip clearance leads to large&#13;\nleakage flows, hence low efficiency, thus the common use of the capacitance probe clearance&#13;\nmeasurement technique in monitoring turbomachinery.&#13;\nOptical systems have been successfully used to measure rotor blade tip timing on test rigs with several&#13;\noptical probes mounted equally spaced around the turbomachine casing. However, there are practical&#13;\nproblems associated with mounting such monitoring systems on in-service jet engines. Optical probes&#13;\nrequire high maintenance to keep the lenses clean, probably incorporating a purge air system to keep the&#13;\nlenses from fouling. Such impracticalities and added weight make it unlikely that an optical probe based&#13;\ntip timing system will be fitted on an in-service engine in the foreseeable future.&#13;\nIn this thesis the scope for a dual use sensor to measure both turbomachinery tip clearance and tip timing&#13;\nis investigated. Since it is impractical to measure blade tip clearance with an optical probe, then the&#13;\nobvious choice for such a sensor is a capacitance probe. Therefore, a commercially available FM&#13;\ncapacitance probe based blade tip clearance measurement system is used in a series of tip timing practical&#13;\ninvestigations. The equipment and instrumentation designed, assembled and produced to facilitate this&#13;\ninvestigation is documented. These include the development of an optical once per revolution sensor and&#13;\nthe design of an independent vibration measurement system based on blade mounted strain gauges.&#13;\nThrough an extensive body of experimental work the practicalities in this alternate use of the tip&#13;\nclearance measurement equipment have been assessed. System responses pertaining to tip timing&#13;\nmeasurement have been investigated, characterised and quantified. The accuracy by which tip timing can&#13;\nbe measured using the system has been reported through the findings of an experimental programme&#13;\ncarried out on a full-sized, low-speed compressor.&#13;\nSpecifically, dual capacitance probe tip timing derived vibration amplitudes have been compared to&#13;\nthose derived from blade mounted strain gauge signals. Sources of error have been identified and&#13;\nquantified. Amplitudes were found to agree within the calculated error bands. Instantaneous resonant&#13;\nblade vibrations measured through single capacitance probe tip timing have been correlated with strain&#13;\ngauge derived vibration levels. This has also been done as the rotor traverses blade resonant speed. In&#13;\nthis case the vibration phase change across resonance expected from theory was successfully detected&#13;\nthrough tip timing. Also, the accuracy by which blade time of arrival can be determined by using&#13;\ncapacitance probe tip timing has been assessed using a precision OPR sensor and a non-vibrating&#13;\ncompressor rotor blade. The characteristics of a DC capacitance probe based clearance measurement&#13;\nsystem's response to movement in 3D space in proximity to a blade tip have been mapped. Detection of&#13;\nsmall vibrations have also been investigated in a series of static impulse tests.