Rotating Stall Event at 60% Speed

The complete rotating stall event at 60% speed lasts for 3.6 seconds from initiation to fan recovery. The time histories from the ring of Kulites positioned half an axial chord upstream of the fan against circumferential position show

a distinct change in the fhctuations caused by the rotating stall transitioning from one stall cell to two at approximately 1.1 seconds after initiation (Fig. 2). Figure 3 shows the Fourier analysis for one of the upstream Kulites with a distinct transition in frequency from one to two cells, the figure also shows the harmonics of the cells. This transition is unusual with previous fans typically only showing a one cell pattern rotating at 0.6EO: the reason for this dividing of the stall cell is unknown. However, data reported by Pampreen (Ref. 3) show single and double stall cells in a single stage system rotating at approximately 60% of the rotational speed. Figure 4 shows a zoom of the time histories with a ‘spike’ initiation of the stall event being picked up first at 305 degrees. The stall cell development is a rapid process occurring over two revs with the cell rotating at 0.60 EO after 10 revs. After the transition to two cells the rotational cell speed increases slightly to 0.63EO. Kulites positioned at the same circumferential position ahead of and behind the fan and downstream of the OGV, see the same phase, suggesting the stall cell is axial aligned and not spiraling; this is analogous to a cut-off pressure wave as defined by Tyler & Sofrin (Ref. 4).

The over tip high frequency data were acquired after the transition, approx­imately 0.6 seconds prior to the recovery point. The data show the stall cell passing at both the leading edge and mid chord as an increase in pressure and a fall in pressure at the trailing edge. This is consistent with a fbw reduction at inlet in the stall cell and an increase in blockage at the fan trailing edge, which effectively increases the exit Mach number.

To resolve the frequency components of the over-tip data a power spectral density analysis has been performed, both in rotating stall and post stall. The results for the leading edge Kulites are shown in Fig. 5. For post stall only, the blade passing frequency and first harmonic are clearly defined. During rotating stall the stall frequency and the first two harmonics are clearly visible. Additional frequencies around the blade passing frequency can be seen. These are offset from blade passing by the rotating stall frequency and its harmonics, showing the strong interaction between the stall cell and the blading. These ’sum and difference’ tones are expected and are explained further in Refs. 5 and 6. A similar pattern is seen around the first harmonic of blade passing. Although not presented, the power spectra for all gauges were inspected and it was found that similar trends could be seen throughout the passage.

The overtip data suggest that the size and structure of both stall cells were very similar, with the cells occupying approximately four blade passages. The data presented in animation format shows that, even during the ’stable’ portion of the fbw between stall cells, there are significant blade to blade flictuations in the rotor ft>w field.