Abstract
A highly sensitive fibre bundle-based reflective optical sensor has been designed and fabricated for Tip Clearance measurements in a turbine rig. The sensor offers high spatial and temporal resolution. The sensor probe consists of a single-mode transmitting fibre and two concentric rings of receiving multimode fibres that collect reflected light in a differential detection gain configuration, yielding a highly linear calibration curve for distance measurements. The clearance measurement range is approximately 2 mm around the central point fixed at 3.2 mm from the probe tip, and the sensitivity of the probe is 61.73 mm−1. The fibre bundle has been designed to ensure that the distance security specifications required for the experimental program of the turbine are met. The optical sensor has operated under demanding conditions set by the blade and casing design. The experimental results obtained so far are promising and lead us to think that the optical sensor has great potential for online clearance measurements with high precision.
Highlights
In aeronautics, Tip Clearance (TC) refers to the gap existing between the blade tip and its surrounding case
We report on the design and fabrication of a highly sensitive fibre bundle-based reflective optical sensor that has been tested in an aircraft turbine rig
To avoid modal noise at the output [28], a central single-mode fibre is used as transmitting fibre of red laser light, which after exiting the fibre bundle and being reflected by the target object located at a distance d from the fibre bundle tip, is partially gathered by two concentric rings of multimode optical fibres arranged around the central transmitting fibre; the inner ring consists of 5 optical fibres (fibre bunch 1; core diameters of 200 μm and Numerical Aperture (NA) of 0.2), whereas the outer ring consists of 17 optical fibres
Summary
Tip Clearance (TC) refers to the gap existing between the blade tip and its surrounding case. Whereas high TC values allow an amount of air to flow without generating useful work, a lack of clearance accelerates blade tip and shroud wear over time due mainly to rubs, and can put engine integrity at risk [1]. The clearance varies with the operation point of the mission profile (take-off, cruise and landing) as well as with the engine aging [2,3]. TC changes are caused by two types of loads, namely engine and flight loads. Some of the most relevant benefits of reducing the TC include efficiency increase as well as increased payload and mission range capabilities [1,5].
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