Experimental Investigations of the Aerodynamics of an Annular Compressor Cascade at Reversed Flow Conditions

Compressor unsafe operating regimes yield unsteady high speed turbulent flows in which complex aeroelastic phenomena occur. If the blade flutter and forced response behavior (i.e. aeroelastic stability) can be predicted reliably for normal flow conditions, its assessment at severe-off design conditions remains a critical task for compressor development programs. Due to complex flow fields and highly transient phenomena, reversed flow conditions are still difficult to predict. Within this frame, the understanding of the physical mechanisms of surge onset and events is essential to improve surge occurrence prediction and control. This paper presents steady-state results of an experimental investigation performed on an annular compressor cascade subjected to constant subsonic backflow inlet conditions. The investigations were carried out at EPF Lausanne, in the annular test facility for non-rotating cascades. With an upstream swirled flow corresponding to real axial turbomachine conditions, an axisymmetric flow can be achieved in the test section. This paper constitutes an introduction and a basis to an associated forthcoming experimental study, dedicated to aeroelastic investigations. The steady-state flow conditions are measured upstream and downstream of the test section, with 5-hole aerodynamic probes. The compressor cascade consists of 20 blades, mounted on 20 independent elastic springs and masses to enable the excitation of different IBPA during the aeroelastic measurements. A pair of blades was instrumented with respectively pressure and suction side pressure taps along the airfoil chord length in order to measure the surface steady-state pressure. Static pressure taps were also inserted in the casing of the test section to assess flow characteristics in the blade tip area. To provide a comparison tool to the measurements, associated CFD calculations at offdesign conditions were performed. The comparison between CFD and measurements shows a high level of confidence. Since not many experiments exist at severe off-design conditions, these experimental results are a precious data source for CFD validation.