Axial Transonic Rotor and Stage Behavior Near the Stability Limit

Transient casing pressure data from a transonic rotor and rotor-stator stage measured using high-speed pressure probes embedded in the casewall over the rotor tips are analyzed. Using long data sets sampled at a high frequency, low-frequency (less than once-per-revolution) nonaxisymmetric flow phenomena were detected while operating at steady-state conditions near stall. Both the rotor and stage cases are investigated, and the difference in behavior of a rotor with and without a stator blade row is investigated. Data for both cases over the speed range 70–100% of design and from choke to near the stability limit (stall or surge) are presented. The root mean square power of the low-frequency signal as well as its fraction of the total pressure signal is presented. It was thought that the behavior of these signals as stall was approached could lead to some method of detecting the proximity of stall. For the rotor-only configuration, the strength of these nonaxisymmetric phenomena increased as stall was approached for all speed-lines. However, for the stage configuration, more representative of an operational machine, these were of a lower magnitude and did not exhibit a clearly increasing trend as stall was approached. This would seem to indicate that the stator suppressed these signals somewhat. It is also shown that these nonaxisymmetric phenomena led to a significant variation of the mean relative inlet flow angle into the rotor blade. During stable operation near to stall at 100% speed for the rotor-only case, a 1.9 deg variation of this angle was measured. This compared with a 5.6 deg variation over the entire speed-line. Further, it was observed that while the rotor and stage cases had different stability limits, their peak relative inlet flow angles near stall were similar for both along most speed-lines.