Capacitance tip timing techniques in gas turbines (2003)

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