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representatiovef thevelocity profile across each individual blade passage (all the passages were assumed to be identical). All of the data presented in this report are based on passage- averaged velocity distributions. All of the laser anemometer measurements were acquired in coincidence mode with a coincidence window width of 8 tasec, which ensured that velocity data were recorded only if a veloc- ity measurement occurred in both the blue and green fringe systems within 8 lusec of one another. Since the time interval between each of the 200 pitchwise measurement points was also 8 _sec, we are confident that all measured velocity com- ponents occurred in the same circumferential measurement interval. An inhibiting circuit driven by the shaft angle encoders was used to interrupt the laser anemometer signal processor while an impeller blade was in the measurement volume. This was done so that light scattered from the blade surface would not trigger a laser anemometer measurement. Without the inhibit- ing circuit, approximately 80 percent of the acquired data would have been triggered by light scattered from the blade surface. In most regions of the flow field, 50 000 velocity measure- ments were acquired at each of the laser anemometer locations shown in figure 6. This would yield an average of 250 meas- urements at each of the 200 points in the ensemble-averaged velocity distribution if the measurements were evenly distrib- uted across the blade pitch. In reality, since the rate at which the seed particles arrive at the laser anemometer volume meas- urement location is directly tied to the streamwise momentum, the number of measurements acquired per unit time can vary considerably across the pitch. In order to acquire at least 100 to 200 measurements at each of the 200 pitchwise points in the ensemble-averaged velocity distributions that cut across low- momentum regions, we had to acquire as many as 200 000 measurements for a given spanwise and streamwise measure- ment location. At each measurement point in the flow field, the laser an- emometer optical axis was directed into the compressor at an azimuth angle _ (see fig. 11). The angle _ ranged from 90 ° at the impeller inlet to 0° at the impeller exit. Although the blades were designed as straight-line elements from hub to tip, there was considerable blade-lean away from the meridional plane in some regions of the impeller. In order to obtain velocity meas- urements near the blade surfaces at any azimuth angle _, the optical axis could be deflected out of the meridional plane by a declination angle rl (see fig. 12). The probe volume x,y,z- location and the optical axis orientation angles _ and rl could all be varied independently. For a given (_,q) orientation of the laser anemometer optical axis, two velocity components were measured in a plane per- pendicular to the optical axis by the blue and green fringe sys- tems. In order to determine all three components of the total velocity at a point in the flow field, measurements were ac- quired at two different _ orientations of the optical axis for each measurement point. At each 4, the r I was oriented such that the optical axis was tangent to the blade surface, thus minimizing optical blockage. In order to maintain measure- ment accuracy, the two optical axis orientations used at each point were generally separated by 20 ° to 30 °. As a result, four measurements were available (blue and green optical axes at each of two (_,q) orientations) for use in calculating the three orthogonal components of the total velocity vector. However, since only three measurements were needed, the calculated to- tal vector was over-specified. We chose to use the information from all four measurements and applied a least-squares fit to all four velocity component measurements to calculate the three-components of the total velocity vector (see Calculation Procedures). The aforementioned measurement technique was developed in order to measure the spanwise velocity at any point in the flow field, since the spanwise velocity had not been measured by most previous laser anemometer investigations in centrifu- gal compressors. The accuracy of this technique in measuring the spanwise velocity component was verified at the upstream aerodynamic survey station (station 23 in fig. 6) by comparing axisymmetric-averaged pitch angles derived from the laser anemometer measurements to those derived from five-hole probe measurements. The pitch angle is defined as o_ = tan -i (VflVz), where Vr and Vz are the radial and axial velocity com- ponents respectively. Note that the spanwise velocity compo- nent must be accurately measured in order to accurately meas- ure the pitch angle, but it cannot be measured when the laser anemometer optical axis is directed along the span. The results of this exercise are shown in figure 13. The five-hole probe measurements of pitch angle vary smoothly in the spanwise di- rection from 0 ° at the shroud to 21 ° at the hub, which is the pitch angle of the rotor spinner surface. The laser measure- ments tend to depart from the five-hole probe measurements at lower spans; this is probably due to the curved window altering the laser beam paths and thereby distorting the laser measure- ment volume. However, the agreement between the laser and the five-hole-probe measurements of the pitch angle is better than 2° over the outer 70 percent of the span (20 cm immer- sion). These results indicate that the laser measurement tech- nique is capable of accurately measuring the relatively small pitch angles near the shroud, and they give us confidence that with this technique we can accurately measure the spanwise velocity component. Flow Visualization Flow direction on the blade surfaces was visualized by the ammonia-Ozalid technique described by Joslyn and Dring (1987). Ammonia gas was leaked into the flow stream through existing static pressure taps on the blade surfaces via a pneu- matic slip ring. A remotely actuated pressure regulator con- trolled the rate of ammonia leakage into the flow stream, and a strobe light and camera monitored the process through the la- ser windows. Wherever the ammonia contacted a 25.4-ram- wide strip of 0.001-ram-thick Ozalid paper taped immediatelyPDF Image | Laser anemometer measurements of the three-dimensional rotor flow
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