Improvements in the design of an optical sensor for tip-clearance measurements in turbines

For the last few years, we have been carrying out tip-clearance (TC) measurements in turbine rigs using optical sensors in collaboration with the Aeronautical Technologies Center. Several turbines with completely different blade profiles have been tested with satisfying results. The reflective intensity-modulated sensor used in these tests is based on a trifurcated bundle of optical fibers. This sensor is the ideal candidate for TC measurements because of its low cost, simplicity, robustness and the capability of performing tip-timing measurements (TT) employing the same probe. In the case of TC measurements, the main requirement is a precision of at least 30 μm. In order to get this precision, the latest improvements of the sensor have been focused on reducing the modal noise at the endface of the transmitting fiber of the bundle. For this purpose, different approaches were developed using mode-scramblers, plastic optical fibers and a single-mode illuminating fiber. The results obtained in the tests demonstrate that it is possible to achieve the demanded precision. Hence, in next test campaign, three sensors will be used to determine clearance at three different points of a rotating disk that belongs to a real aircraft engine.