A centrifugal compressor stage designed and built by Safran Helicopter Engines is tested at several IGV (Inlet Guide Vanes) stagger angles. The compressor stage includes 4 blade rows: axial inlet guide vanes, a backswept splittered impeller, a splittered vaned radial diffuser and axial outlet guide vanes. More than a hundred steady pressure and temperature measurements are used for the monitoring and the performance calculation. The methodology for calculating the performance is detailed, including the consideration of humidity in order to minimize errors related in particular to operating atmospheric conditions. Furthermore, 54 unsteady pressure sensors are distributed over the entire stage in order to capture the time-dependent fluctuations of the flow and to investigate instabilities.

The performance map of the compressor has been measured at several rotation speeds from choke to surge for 3 IGV stagger angles: 0°, 20° and 40° (positive pre-swirl). The variation of the choked mass flow rate according to the rotation speed and the stagger angle of the IGV is quantified. At a given rotation speed, the choked mass flow rate is lower as the IGV stagger angle is increased. The effect of the stagger angle on the choked mass flow is not linear: the difference between stagger angles 40° and 20° is higher than between 20° and 0°. The shift of the surge line towards lower mass flow rate as the IGV stagger angle increases highly depends on the rotation speed. The surge line shift is very small at low rotation speeds whereas it significantly increases at high rotation speeds. A first-order stability analysis of the impeller and diffuser sub-components shows that the diffuser (resp. impeller) is responsible for triggering surge at low (resp. high) rotation speeds. This explains why the operating range moves towards lower flow rates only at high rotation speed which is consistent with results extracted from the literature.

At low rotation speeds below a given mass flow rate, rotating instabilities at the impeller inlet were previously detected at zero IGV stagger angle. Their occurrence is conditioned by the incidence flow angle at the impeller blade leading edge. As the IGV stagger angle increases, the mass flow decreases to maintain a given inlet flow angle. Therefore, the onset of the rotating instabilities is delayed towards lower mass flow rates. As the mass flow rate is further reduced mild surge was previously observed (just before deep surge) at zero IGV stagger angle. Its onset is conditioned by the incidence flow angle at the radial diffuser leading edge. Mild surge still exists as the stagger angle is increased but appears at slightly lower mass flow rates.

Overall, the IGV stagger angle modifies the velocity triangles both at the impeller inlet and at the radial diffuser inlet. The main effect is the reduction of the critical mass flow rates below which the compressor experiences instabilities such as rotating instabilities, mild surge and deep surge.


Effects of Inlet Guide Vanes on the Performance and Stability of an Aeronautical Centrifugal Compressor

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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