An Optical Fiber Bundle Sensor for Tip Clearance and Tip Timing Measurements in a Turbine Rig

Abstract

When it comes to measuring blade-tip clearance or blade-tip timing in turbines, reflective intensity-modulated optical fiber sensors overcome several traditional limitations of capacitive, inductive or discharging probe sensors. This paper presents the signals and results corresponding to the third stage of a multistage turbine rig, obtained from a transonic wind-tunnel test. The probe is based on a trifurcated bundle of optical fibers that is mounted on the turbine casing. To eliminate the influence of light source intensity variations and blade surface reflectivity, the sensing principle is based on the quotient of the voltages obtained from the two receiving bundle legs. A discrepancy lower than 3% with respect to a commercial sensor was observed in tip clearance measurements. Regarding tip timing measurements, the travel wave spectrum was obtained, which provides the average vibration amplitude for all blades at a particular nodal diameter. With this approach, both blade-tip timing and tip clearance measurements can be carried out simultaneously. The results obtained on the test turbine rig demonstrate the suitability and reliability of the type of sensor used, and suggest the possibility of performing these measurements in real turbines under real working conditions.

Tip-Clearance Measurement in the First Stage of the Compressor of an Aircraft Engine

“…In previous works, we developed a reflective intensity-modulated optical fiber sensor to carry out TC measurements in turbines [20] and rotating components of aircraft engines [28]. The essential component of this sensor was a bundle of optical fibers.…”

“…Each ring of receiving fibers was gathered into a leg on the other end of the bundle, which was connected to a photodetector in order to convert the optical signals into an electrical one. Finally, the quotient of these two voltage signals (V 1 and V 2 ) was related to the distance of the target by a linearized calibration curve as the one shown in Figure 1 (for a more detailed explanation of the sensor operation see [20,29]).…”

“…Regarding TC measurement, optical sensors yield the best resolution. On the contrary, they are seriously affected by contamination and debris, thus being suitable only for the clean parts of the engine such as the fan and the compressor, or for the testing of rigs during the engine-development phase [20]. …”

Blade tip clearance and time of arrival immediate measurement method using an optic probe

“…An optic fiber sensor based on a trifurcated bundle of optical fibers has been developed [17]. It has a central fiber to transmit the light from the laser to the probe end.…”

Blade resonance parameter identification based on tip-timing method without the once-per revolution sensor

“…Compared to the traditional strain gauges based vibration measurement, the blade tip-timing method is a non-contact method and it can measure vibration parameters of all blades dynamically [4,5]. This method has become an industry-standard procedure for blade vibration measurement of turbomachinery and it is applied for damage prognosis of the turbine blades [6,7].…”