Front Stage, Analysis in Detail

This section deals with the analysis of the unsteady ft>w field of the IGV, as well as the first rotor blades. Due to the fact, that upstream of the first rotor no other periodic disturbances of the flow field occur, its potential upstream infhence can be isolated. With the dynamic pressure measurements above the first rotor, a change in structure and intensity of the tip clearance flow is detected due to the different operating conditions at OP1 and OP3.

Inlet Guide Vane. At the outlet of the IGV, a remarkable potential up­stream inflience of the first rotor is obtained. Figure 3 shows snapshots of field traverses with total pressure probes downstream the IGV. The results are ensemble averaged and related to the total pressure averaged in space and time over the entire measuring field. In addition to the experimental results, the theoretical position of the trailing edge of the IGV is depicted. The upstream influence of the first rotor is indicated by low values of the ensemble averaged total pressure. Neglecting the losses in the stagnation area of the leading edge, the total pressure is assumed to be constant over the entire pitch. Besides that, the stochastic fluctuations increase in the stagnation area, and lower values of the ensemble averaged total pressure occur. Throttling the compressor from OP1 to OP3 the aerodynamic load of the first rotor changes significantly. Re­garding the higher load at OP3 the affected area expands in circumferential and midspan direction. The potential field of the first rotor significantly influences the boundary layer development on the IGV, as has been illustrated by Niehuis etal. (2003).

First Blade. Regarding the ft>w field downstream the first rotor (Fig. 4), at

OP1, the tip clearance vortex, indicated by transient maximum RMS values, in­teracts with the convected wake of the IGV. Passing the wake, the RMS values, as well as the spatial extent of the affected area, decrease. This effect is due to the lower meridional velocity, and therefore less intensive secondary flow in

PtRMS^Pl

0.0500 0.0471 0.0443 0.0414 0.0386 0.0357 0.0329 0.0300 0.0271 0.0243 0.0214 0.0186 0.0157 0.0129 0.0100

OP3

Figure 4. Snapshot of the dynamic total pressure distribution downstream R1, RMS, 100% speedline the wake area. At OP3, a similar phenomenon is detected, but the tip clearance fbw leaves the blade duct closer to the casing, and comparatively high RMS values are obtained over the entire pitch. In contrast to OP1, a sharply defined wake of the rotor is only detected on the left and the right side of the measur­ing field where no disturbance of the rotor inflow by the wake of the IGV is present. Similar results are presented by Suder and Celestina (1994) investi­gating a comparable rig. A further analysis of the tip clearance fbw is enabled considering the measurements with flush mounted dynamic pressure transduc­ers at the casing above the blades. Figure 5 shows a snapshot of the ensemble

averaged data as well as the position of the moving rotor tip. While at OP1, the stagnation point can be found close to the leading edge, it moves to the pressure side with the increased incidence at OP3. Besides that, the axial position of the separated pressure minimum moves upstream from 48% chord (OP1) to 38% chord (OP3). Since the inlet Mach number at the tip section is almost identical at both operating points, a normal shock wave is generated downstream of the pressure minimum in both cases. The time resolved ensemble averaged pres­sure distribution captures the trajectory of the tip clearance vortex. At OP3 the trajectory close to the suction side of the blade is more inclined in the direc­tion of the circumferential velocity. A similar result was found by Mailach et al. (2001) in a low-speed compressor without any shock wave present. They explain this effect with the different momentum of tip-clearance ft>w and core flow. With a higher aerodynamic loading, the momentum of the tip clearance flow increases due to the enlarged pressure gradient between the pressure and suction side. Simultaneously, the momentum of the core flow decreases due to the reduced mass flow. The resulting force on the tip leakage fluid turns in the direction of the circumferential velocity. Interacting with the perpendicular shock at OP3, the trajectory bends in meridional direction. The same effect was again detected by Suder and Celestina (1994). The shock causes a loss of momentum of the leakage fhid, and the resulting force on the fhid turns. Con­trary to the measurements downstream of the rotor (see Fig. 4), an indexing of the tip clearance ft>w by the wake of the IGV can not be detected at the casing above the rotor tip. A further analysis of the leakage flow would be enabled regarding the RMS distribution. As the ft>w field is widely similar to the one of the last rotor, the phenomena will be discussed below in more detail.