This paper aims at describing the complex unsteady flow which develops in a radial vaned diffuser of a centrifugal compressor stage designed and built by Safran Helicopter Engines. This research compressor is composed of four rows: inlet guide vanes, a splittered unshrouded impeller, a splittered radial vaned diffuser and axial outlet guides vanes. It is mounted on a 1MW test rig at the Laboratoire de Mécanique des Fluides et d'Acoustique, École Centrale de Lyon, France. The experimental analysis is based on the signals given by unsteady pressure sensors distributed over the entire circumference in the diffuser at hub and shroud.

Depending on the rotation speed of the compressor, two distinct modes of the flow in the radial diffuser are observed at stabilized operating points near the surge, an asymmetric and a symmetric mode.

At medium rotation speed, an asymmetric mode consisting of a two-channel pattern in the radial diffuser develops. One passage over two is stalled and its pressure is high at the inlet; the adjacent passage is free and is characterized by a jet flow near the suction side in the front part of the vane inducing a very low pressure level at the vane entry. This alternate pattern interacts with mild surge (characterized by the propagation of a low frequency pressure wave in the entire compressor) and pulses without rotating in the circumferential direction at a frequency of roughly 12 Hz which is close to the Helmholtz frequency of the test rig.

By lowering the rotation speed, the two-channel pattern fades away and gives way to a periodical behavior of the radial diffuser passages called symmetric mode. The flow in each channel is identical presenting a stalled behavior pulsating in phase at a higher frequency of roughly 42 Hz.

The origin of these different modes are then discussed through various approaches. It has been shown that some geometrical parameters foster the installation of an alternate pattern in the radial diffuser. Moreover, an experimental characterization of the test rig acoustics shows a correlation of the test rig modes with the aforementioned modes. Those correlations may indicate a lock-in of the frequencies of the instabilities with the acoustic modes of the test rig.