1 EXPERIMENTAL INVESTIGATION OF ROTATING STALL IN A HIGH SPEED MULTI-STAGE AXIAL COMPRESSOR N. Courtiade 1 - X. Ottavy 1 - N. Gourdain 2 1: Laboratoire de Mécanique des Fluides et d’Acoustique, Ecole Centrale de Lyon, 69130 - Ecully, France, nicolas.courtiade@ec-lyon.fr , xavier.ottavy@ec-lyon.fr 2: CERFACS, CFD Team, 31057 - Toulouse, France, nicolas.gourdain@cerfacs.fr ABSTRACT Pressure measurements using high frequency response sensors have been carried out in the 3½-stage high speed compressor CREATE (rotation speed: 11543 RPM, Rotor 1 tip speed: 313 m/s). The pressure signals obtained at several axial positions during the onset of instabilities are first presented. Before the surge, 12 rotating stall cells are characterized and are localized in the region of the interactions between the tip clearance flow and the stator wakes. Their rotation velocity (85% of the rotation speed of the compressor shaft) has been measured using 6 sensors equally spaced over the circumference. This paper shows how the rotating stall cells influence the flow field in the whole compressor and finally collapse into one large and unique cell, leading to the surge. NOMENCLATURE BPF blade passing frequency CFD computational fluid dynamic DFT discrete Fourier transform NS near stall PE peak efficiency URANS Unsteady Reynolds averaged Navier-Stokes RSI rotor/stator interaction(s) Ω shaft rotation speed of the compressor shaft Ω cells rotation speed of the stall cells Θ 16 circumferential period of the compressor based on the blades number (2π/16) T 16 temporal period of the compressor based on the blades number (2π/16/Ω shaft ) F cell rotating stall cells induced frequency F iN reduced frequency : i occurrences over T 16 INTRODUCTION As the compressors tend to become smaller, strong rotor/stator interactions (RSI) occur, leading to complex flow phenomena which impact the performance. When increasing the loading of a compressor, the intensity of the RSI also increases, and rotating instabilities might occur in the tip flow region, especially for machines with significant tip clearance März et al. (2002). Indeed, at low mass flow rate, the aerodynamics instabilities are characterized by the presence of rotating stall and/or surge phenomena. According to Tan et al. (2010), the surge is preceded by rotating stall in most of the cases. Camp & Day (1998) described the two patterns of rotating stall inception: modal or spike types. The modal wave pattern, already studied by Garnier et al. (1991), is characterized by a long-length scale wavelike disturbance rotating slower than the compressor shaft, and it can occur hundreds of revolutions prior to stall onset. The spike type pattern is initiated by a small cell, localized at one blade passage, rotating at nearly the speed of the rotor. The stall cell reduces its speed when growing, and rotating stall occurs within a few rotor revolutions.