Measurement Techniques

The results shown in this paper include a comparison of the thermal-resistive techniques used, and shows the inflience of mass ft>w and rotating speed on the boundary layer transition position on the compressor blades. These tech­niques also give detailed information of the unsteady boundary layer develop­ment on rotor and stator blades at m=6.4 and 5.85 kg/s, as well as at near stall conditions.

Surface Hot Wire Application

Figures 6 and 7 compare the skewness distribution on stator vanes at the compressor design point (m=6.82 kg/s) using hot film and hot wire techniques. The figures are plotted as Campbell-diagrams (t-s-diagram) over a time span of t=2 ms, corresponding to four wake periods. The results show a good concor­dance of both transition lines, which indicates the point of ^3=0 at streamwise positions between ж/с=30-40%. The transition lines are clearly modulated by the periodically incoming wakes generated by the up-stream rotor. The results from individual measurements at two stator vanes show slight changes in posi­tion, as well as expansion of the lines. These results imply a good performance of the novel hot wire technique used for unsteady measurements on turbo ma­chine components.

Boundary Layer Transition shifting

A general overview of the mass ft>w and speed of rotation infhence on the axial-compressor rows are given in the Figures 8 and 9. The results show the streamwise transition development at 3 different rotating speed frequencies over a range of mass fbws (m). The results indicate clear up-stream shifting of the transition due to a throttling of the machine and a corresponding mass flow decrease. Additionally, Figure 8 shows a good correlation of the data of both thermal-resistive sensors used for unsteady measurements on the stator vanes.

Data Acquisition. For measurements of periodically unsteady conditions in the cascade, a phase-locked ensemble average technique was used. This technique was proposed by Lakshminarayana et al. [4]. Data acquisition was carried out using a shaft encoder coupled with the rotor shaft to get a trigger-impulse. In the case of the present measurements sensor-signals were recorded 200 times using sample rates up to n=1024 and a scanning frequency of f=20 kHz. The ensemble-averaged sensor signals were analyzed using the AC-output voltages representing quasi wall-shear stress fhctuations. Statisti­cal values of root-mean-square (rms) and skewness (^3), as well as analysis functions such as power spectra and cross-correlation, were used to analyze the data.

Surface Hot Wire. As an alternative to a surface hot film, a new flish mounted hot wire with a better dynamic response was used for measurements on a stator vane for the experimental project part. The surface hot wire (SHW) is a thermal-resistive sensor, which is directly welded over a tiny cavity, flush – mounted to the surface [5]. The device offers a promising technique for skin friction measurements on compressor blades. The sensor design considerably reduces the heat conduction into the substrate and, therefore, yields an im­proved signal-to-noise ratio and a higher frequency response compared to a common surface hot film. Figure 3 shows a sketch of the sensor. A platinum – coated, 5 ^m diameter tungsten wire is used as sensor element while a 30 ^m copper-layer serves as substrate. The bonded sensor element has a resistance of approximately R=5.5 ^ and is positioned several wire diameters above the wall. The array is very flexible and allows for easy attachment onto curved surfaces.

Blade Instrumentation. The measurements on rotor and stator were con­ducted at blade mid-span using Senfhx hot film arrays. 16 and 25 single sen­sors were applied along the streamwise direction of a rotor and stator blade, respectively. In addition, a stator vane was equipped with a 16 single sensor surface hot wire array during the first experiment (see Figure 4). All sensors were operated in the constant-temperature mode with typical overheat ratios. The sensors on the rotor blade were adjusted using miniature anemometry – circuits. 16 electric circuits were designed on a ring-shaped printed circuit board (PCB) and implemented on the rotor disk (Figure 5). The integrated telemetry-system was used for a cable-less data transfer as well as a power supply for the 16-channel miniature constant-temperature anemometry ring.

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