Last Stage, Analysis in Detail

Regarding the last stage of the compressor, the downstream stability of rotor and stator wakes along the machine axis can be illustrated. The shape of the rotor tip clearance ft>w depends strongly on varying operating conditions. Last but not least, a flow separation in the last stator, resulting in a hub corner stall, occurs already at design conditions and is influenced by the wakes of the rotor upstream. Further throttling leads to a significantly growing separation, and it is assumed that the last stator triggers surge at the 100% speedline.

Last Blade. The ft>w field downstream the last blade (Fig. 6) is dominated by the wakes of the second vane, which are tilted in the direction of the cir­cumferential velocity. Basically, two differences exist between the operating points OP1 and OP3. In the wake area of the second vane close to the hub, high RMS values occur at OP3. At this position, the decreased meridional velocity in the wake of the vane causes an increasing incidence on the pressure side in the inlet plane of the last blade. Due to the incidence, the stochastic flictua – tions increase periodically and a fluctuating separation on the suction side is likely to occur in the hub region of the blade. At OP1, the region infhenced by the tip clearance vortex can be identified by maximum gradients of the RMS values. Though similar regions can be detected at OP3 as well, the gradients are smaller, and there is an additional region above 83% span with high RMS values over the entire pitch. The mechanism of the tip clearance flow and its variation due to different aerodynamic loading can be analyzed in more detail using the dynamic pressure field at the casing above the rotor tip (Fig. 7). Ex­cept for the shock wave, the flow field corresponds widely to the one of the first rotor. The increased aerodynamic loading at OP3 results in a pressure minimum moving upstream from 47% chord at OP1 to 35% chord at OP3. At

Trajectory of tip clearance flow

OP r Pressure minimum ‘ Indexing by S2

Figure 7. Snapshot of the dynamic wall pressure distribution, last rotor, RMS, 100% speed­line both operating points, it is apart from the suction side of the blade. At OP1, the trajectory of the tip clearance ft>w is detected which originates close to the leading edge and leaves the blade duct close to the pressure side of the adjacent blade. At OP3, a trajectory of this shape is not detected. A corresponding area extends over the entire pitch from the leading edge to 71% chord. Furukawa et al. (2000) report on similar effects investigating an one-stage axial compressor with a NACA 65 blading. At design conditions, the leakage ft>w originates at the leading edge up to 30% chord downstream. In the region of the pressure minimum, the fbw changes into a coil-shaped structure. Downstream of the pressure minimum, the resulting vortex grows and moves to the pressure side of the adjacent blade. Furukawa et al. (2000) called this structure a spiral-type vortex. Approaching the surge margin, the leakage ft>w is coiled very close to the tip clearance. Downstream of the pressure minimum, a breakdown of the resulting vortex occurs, and it drifts to the pressure side of the adjacent blade. This effect is accompanied by a deceleration and the appearance of reversed ft>w regions covering the entire pitch. Consequently the leakage ft>w can not be detected anymore at the casing in the vicinity of the trailing edge. Never­theless, the ft>w appears downstream the blade row in the upper 20% of span (see Fig. 6). It can be concluded, that the vortex dives into mid-duct direction. Furukawa et al. (2000) call this structure a bubble-type vortex. Besides that, a remarkable spillage upstream the leading edge is obtained. As Suder and Celestina (1994) already mentioned, this spillage is caused by a significant positive incidence and reversed flow due to the axial pressure gradient at this particular spanwise position. Unlike the front stage, the ft>w interacts periodi­cally with the wake of the vane upstream. Using animated temporal plots, this indexing can clearly be seen in the fluctuating maxima of the RMS distribution

Figure 8. Snapshot of the dynamic total pressure distribution downstream the last stator, ensemble average, 100% speedline

both in intensity and axial position, and is more distinctive at OP3. The region of the maximum RMS values in time can be assigned to the wake of the second vane upstream. The boundary layer of the vane upstream grows at the surge line (OP3) resulting in a wider wake. This effect causes increasing flictua – tions of the incidence angle upstream of the last blade, and a time dependent variation of its ft>w field, especially of the tip leakage fbw.

Last Vane. By design intent, the third stator exhibits the highest aerody­namic loading of all airfoils in this particular compressor resulting in a hub corner stall which is already present at design conditions. Further information about the mechanism of the hub corner stall in general can be found in Hah and Loellbach (1997). The extent of the stall in the circumferential as well as radial direction becomes very clear in the ensemble averaged distribution of the total pressure downstream the last vane (Fig. 8). Besides that, the separation grows at OP1 when the wake of the last rotor is passing the last vane. The fluctuating incidence due to the transient wake is forcing a periodic fbw separation. At OP3, the ft>w is separated all the time, damping the periodic flictuations. The differences concerning the tip clearance flow which were obtained by the dy­namic wall pressure distribution can still be found at the exit of the last vane. Whereas at OP1 the infhence of the leakage ft>w is still visible, at OP3 no corresponding phenomenon is detected. With the lower aerodynamic loading at OP1, the mechanisms of mixing and vortex breakdown are less intensive and take up an increased axial distance. At the exit of the last stator, the wakes of the second stator are still visible. The wakes and the hub corner stall of the last stator strongly inflrence the ft>w in the outlet diffuser as well. Measurements 144% chord downstream of the last vane show a remarkable inhomogeneous ft>w field in radial and circumferential direction. The downstream stability of viscous fbw phenomena is confirmed, which has already been seen in the front stage.

2. Summary

This paper presents measurements with high resolution both in space and time in an industry-like three-stage axial compressor with inlet guide vanes. Besides a brief description of the experimental facility and its overall behav­ior, the detailed analysis of the ft>w field is focused on the front stage and the last stage of the compressor. The front stage operates at the overall highest Mach number level which results in transonic ft>w conditions at the tip of the first rotor. Due to the fact that upstream of the first rotor, no other periodic disturbances of the flow field occur, the potential upstream influence can be isolated. Regarding the first rotor, the structure and the intensity of the tip clearance ft>w change due to different throttlings of the compressor. At design conditions, the tip leakage ft>w can be identified as a spiral-type vortex. With the approach towards the surge line, a bubble-type vortex occurs. Though the wakes of the blade rows become wider with higher aerodynamic loading, they are less stable along the machine axis. Concerning the aerodynamic loading, the differences between the investigated operating points are more significant in the last stage of the compressor. The effect on the development of sec­ondary ft>w phenomena becomes more clear. Firstly, the downstream stability of stator wakes along the machine axis is confirmed. The wake of the sec­ond stator causes slight periodic ft>w separations in the adjacent rotor blade and is still visible downstream of the last stator. As well as the first one, the last rotor shows a varying shape of its tip clearance flow dependent on aero­dynamic loading. Because of a high aerodynamic loading of the last vane, a flow separation occurs already at design conditions resulting in a corner stall. It is influenced by the wakes of the rotor upstream. Further throttling leads to a significantly growing separation and it is assumed that the last stator triggers surge at the 100% speedline.

Acknowledgments

This work was supported by the Forschungsvereinigung Verbrennungskraft – maschinen e. V. (FVV) and the Arbeitsgemeinschaft Industrieller Forschungs – vereinigungen e. V. (AIF), which is gratefully acknowledged.

References

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