Iker García

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

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.

Optical Fiber Sensors for Aircraft Structural Health Monitoring

Aircraft structures require periodic and scheduled inspection and maintenance operations due to their special operating conditions and the principles of design employed to develop them. Therefore, structural health monitoring has a great potential to reduce the costs related to these operations. Optical fiber sensors applied to the monitoring of aircraft structures provide some advantages over traditional sensors. Several practical applications for structures and engines we have been working on are reported in this article. Fiber Bragg gratings have been analyzed in detail, because they have proved to constitute the most promising technology in this field, and two different alternatives for strain measurements are also described. With regard to engine condition evaluation, we present some results obtained with a reflected intensity-modulated optical fiber sensor for tip clearance and tip timing measurements in a turbine assembled in a wind tunnel.

Different Configurations of a Reflective Intensity-Modulated Optical Sensor to Avoid Modal Noise in Tip-Clearance Measurements

Tip clearance is critical to the performance of rotating turbomachinery. The objective of this paper is to develop a noncontact sensor with a precision of 30 μm to measure tip clearance in a turbine rig assembled in a wind tunnel. To carry out the measurements, an optical sensor whose main component is a bundle of optical fibers is employed. We use four different configurations of this sensor, which are tested in two distinct turbines with the aim of minimizing the effect of the noise on the repeatability of the measurements. Each configuration serves to increase the precision until the required performance is achieved for the measurement of the tip clearance. Our results may be helpful to develop applications related to structural health monitoring or active clearance-control systems.

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

In this article, we report the design of a reflective intensity-modulated optical fiber sensor for blade tip-clearance measurement, and the experimental results for the first stage of a compressor of an aircraft engine operating in real conditions. The tests were performed in a ground test cell, where the engine completed four cycles from idling state to takeoff and back to idling state. During these tests, the rotational speed of the compressor ranged between 7000 and 15,600 rpm. The main component of the sensor is a tetrafurcated bundle of optical fibers, with which the resulting precision of the experimental measurements was 12 µm for a measurement range from 2 to 4 mm. To get this precision the effect of temperature on the optoelectronic components of the sensor was compensated by calibrating the sensor in a climate chamber. A custom-designed MATLAB program was employed to simulate the behavior of the sensor prior to its manufacture.

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.

Comparison of three different configurations of an optical sensor for tip-clearance measurements in turbines

The influence of the tip clearance on the performance of rotating turbo machinery is well known. The objective of this work was to measure this parameter using a non-contact sensor with a precision of 30 μm in a real turbine. An optical sensor whose main component is a bundle of optical fibers was selected to carry out the measurements. Three different configurations of the sensor have been tested by taking measurements on two distinct turbines. Tip-clearance measurements are achieved with the desired precision, providing the opportunity to develop applications related to structural health monitoring or active clearance-control systems.

Turbine-blade tip clearance and tip timing measurements using an optical fiber bundle sensor

Traditional limitations of capacitive, inductive or discharging probe sensor for tip timing and tip clearance measurements are overcome by reflective intensity modulated optical fiber sensors. This paper presents the signals and results corresponding to a one stage turbine rig which rotor has 146 blades, obtained from a transonic wind-tunnel test. The probe is based on a trifurcated bundle of optical fibers that is mounted on turbine casing. It is composed of a central illuminating fiber that guides the light from a laser to the turbine blade, and two concentric rings of receiving fibers that collect the reflected light. Two photodetectors turn this reflected light signal from the receiving rings into voltage. The electrical signals are acquired and saved by a high-sample-rate oscilloscope. In tip clearance calculations the ratio of the signals provided by each ring of receiving fibers is evaluated and translated into distance. In the case of tip timing measurements, only one of the signals is considered to get the arrival time of the blade. The differences between the real and theoretical arrival times of the blades are used to obtain the deflections amplitude. The system provides the travelling wave spectrum, which presents the average vibration amplitude of the blades at a certain nodal diameter. The reliability of the results in the turbine rig testing facilities suggests the possibility of performing these measurements in real turbines under real working conditions.

Optical tip clearance measurements for rotating disk characterization

An experimental study about the vibrational behaviour of two prototypes of a rotating disk by means of three optical sensors is presented. Both prototypes were assembled in a wind tunnel in order to reproduce real operation conditions. The optical sensors identified the vibration frequency and the nodal diameter of the first prototype by measuring the clearance of the disk to the casing of the wind tunnel. This method was also employed to check the improvements obtained with an upgraded design of the rotating disk, showing that the measuring system presents a great potential to perform non-contact evaluation of rotating components.

Design, Fabrication and Testing of a High-Sensitive Fibre Sensor for Tip Clearance Measurements

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.

Polymer optical fiber sensors for aircraft structural and engine health monitoring

Aircraft structures and engines require periodic, scheduled inspection and maintenance operations. Such operations are time-consuming, labor intensive and a cost factor to the operations of commercial and defense aircraft fleets. Structural health monitoring (SHM) is a cost-effective approach to meet operational requirements, and to reduce maintenance costs in aircrafts. Polymer fiber optic sensor (PFOS) technology is a mature technology that provides advantages over traditional sensors, and can be used to monitor physical parameters not only of wing surfaces and fuselage sections but also of the engines themselves. Several practical SHM applications for aircraft structures and engines using PFOS will be reviewed in this talk, such as impact detection and location, strain, temperature and deformation measurements on composite structural elements made of carbon fiber-reinforced plastic (CFRP) materials. With regard to engine condition evaluation, we will show some results obtained with a reflected intensity-modulated optical fiber sensor for tip clearance and tip timing measurements in an engine turbine assembled in a wind tunnel and for turbine pressure with a Fabry-Perot fiber interferometer. The reliability of the results in the turbine rig testing facilities suggests the possibility of performing these measurements in real turbines under real working conditions.