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