Gas turbine engine health monitoring systems play an active role to ensure timely maintenance and prevention of failures. Tip-timing and tip clearance measurements form a major part of gas turbine health monitoring systems. They are used to assess turbomachinery blade vibrations using non-contact systems such as optical, capacitive, Hall effect, eddy current etc. Most of these sensors are prone to contamination, non-linearity and cannot measure both tip-timing and tip-clearance together. Eddy current sensors are found to be robust and can measure both tip-timing and tip-clearance simultaneously. They are already being used in gas turbine health monitoring systems to assess compressor and turbine blade vibrations. Apart from assessing blade vibrations, it will be quite beneficial to predict and prevent surge and stall of compressors in an engine. Surge and stall can be disastrous for an aircraft during flight as it can cause severe damage to the engine. Pressure sensors are generally used to study the variations in the inlet flow for surge and stall protection in an engine and play an important role in health monitoring.

A new combined sensor that can measure tip-timing, tip-clearance and dynamic pressure was developed at the University of Oxford for use in gas turbine engine health monitoring. The combined sensor uses a pressure sensor in the centre and is enclosed by an eddy current sensor forming a compact single package. The pressure sensor used here is a fast response optical based sensor that is known to work at high temperatures and is less noisy compared to piezo based pressure sensors. The pressure sensor can also measure the steady state temperature of the casing. The combined sensor is found to be quite robust and is able to operate in harsh environments without any loss in accuracy. Due to the combined package, the space occupied is much less compared to that required by two separate sensors. The sensor has many applications that include measuring vibrations, active flow control, stall/surge of compressors etc.

The paper presents the design and development of this combined sensor along with experimental results on tip timing and unsteady pressures from gas turbine engine fan blades. The engine tests included looking at the effects of squeezing the inlet casing and to study the effect of distorting the inlet flow on blade vibrations by placing varying number of bars in the inlet duct of the engine.

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