This paper presents a novel fibre optic laser Doppler position sensor for single blade tip clearance and vibration measurements at turbo machines, which offers high temporal resolution and high position resolution simultaneously. The sensor principle is based on the generation of a measurement volume consisting of two superposed fan-like interference fringe systems with contrary fringe spacing gradients using wavelength division multiplexing. A flexible and robust measurement system with an all-passive fibre coupled measurement head has been realized employing diffractive and refractive optics. Measurements of tip clearance and rotor vibrations at a transonic centrifugal compressor performed during operation at up to 50 000 rpm (833 Hz) corresponding to 21.7 kHz blade frequency and 586 m s−1 blade tip velocity are presented. The results are in excellent agreement with those of capacitive probes. The mean uncertainty of the position measurement was around 20 µm and, thus, considerably better than for conventional tip clearance probes. Consequently, this sensor is capable of fulfilling the requirements for future active clearance control systems and has great potential for in situ and online tip clearance and vibration measurements at metallic and non-metallic turbine blades with high precision.
No abstract
Concerning the further development of gas turbine engines, advances of the aero-thermodynamic design can be achieved most efficiently by cooperative efforts aimed at the improvement of both the numerical simulation methods and the experimental test and measurement techniques. Rapid development of numerical capability is accompanied by increasing demands on experimental data. In this context significant instrumentation research efforts are being conducted to develop the needed measurement technologies. Because of the need for reduced experimental costs planar measurement techniques have undergone a rapid pace of development. Three newly developed quantitative light sheet techniques utilizing the scattered light of tracer particles are described in this paper. First a Doppler global velocimetry (DGV) system optimized for time-averaged three component velocity measurements is presented. The system, which uses a single viewing direction in conjunction with three different illumination directions enables very accurate velocity measurements. Second a quantitative light sheet (QLS) technique for quantitative mass fraction measurements in mixing processes is treated. To apply the technique the inflow of the mixing experiment must consist at least of two separate flows, one of which can be seeded while the other remains unseeded. DGV and QLS results obtained from experimental investigation in a model combuster are presented. Third a method named tracer-based shock visualization (TSV) is described which is capable of determining the shape and structure of shock waves in transonic flows by analysing the sudden increase of flow density across a shock. Results taken in a transonic compressor are presented.
Similar to the Doppler technique the principle of this method is also based on light scattered by small particles as they are normally contained in every fluid. Two light beams (laser beams are not necessary) are focused in two very small light spots in the measuring volume. Particles passing both the spots emit two light pulses which can be detected by a photomultiplier. From the time interval between these two pulses and from the known spots distance the flow velocity can be calculated. A statistical analysis of many signals, taken at the same treasuring point, allows calculation of the mean velocity, the mean flow angle and the turbulence degree of the velocity components in a plane normal to the beam axis. Due to the light intensity of the light spots (102 to 103 times higher than the intensity in a Doppler probe volume under similar conditions) a sufficient signal-to-noise ratio is achieved so that velocity measurements even in unseeded air flows are possible up to 500 m/s in backscattering by using a 5 mW laser. Tests were carried out in a supersonic wind tunnel up to a Mach number of 2.2. For measurements in the rotor blade channels of turbomachines a specific trigger optic has been installed.
Numerous results—most of them are published—have proved the applicability of the L2F-velocimeter to experimental studies of the complex compressor flow: e.g. the blade wakes and the shock-wave system within the blade passages. Furthermore, measurements close to the compressor surfaces provide information about boundary layers and flow separation. However, the measuring procedure is rather time-consuming. It takes about one hour to accumulate 10 to 15 vector measurements within a rotor blade channel. This paper presents a technique whereby the measuring time is reduced by a factor of ten. Mathematical considerations of the L2F-signal statistics lead to a modified calculation model and a new measuring procedure. Now, about 100 to 150 flow points can be acquired in one hour which includes the magnitude and direction of the mean flow vector as well as its turbulent components. This paper describes the innovations in optics, operational procedures, and electronics that have resulted in an enhanced state-of-the-art in flow measurement. Experimental results are also submitted and discussed.
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