Journal of Thermal Science Vol.19, No.1 (2010) 34−41 Received: November 2009 Xavier OTTAVY: Dr. Permanent Researcher www.springerlink.com DOI: 10.1007/s11630-010-0034-4 Article ID: 1003-2169(2010)01-0034-08 Unsteady Pressure Measurements in a High-Speed Centrifugal Compressor N. Bulot, X. Ottavy, and I. Trebinjac Laboratoire de Mécanique des Fluides et d’Acoustique (LMFA), UMR CNRS 5509 Ecole Centrale de Lyon, UCBLyon I, INSA 36 av. Guy de Collongue, 69134 Ecully cedex, France Tel : 33-4-72-18-61-83, Fax : 33-4-78-64-71-45, E-mail : Xavier.Ottavy@ec-lyon.fr © Science Press and Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg 2010 This paper presents the unsteady data acquisition system used to measure the pressure field in high speed compressors. Details and electronic sketches are given for the conditioners developed in-house that have been used to amplify and to filter the pressure signal with the aim of acquiring data up to 150 kHz. A discussion of the experimental results carried out in a centrifugal compressor is proposed. Through different processing of the pressure signals and a comparison with URANS simulations, the excitation of the pressure transducers by the pressure waves generated by shock waves that occur between the impeller and the diffuser is highlighted. The levels of pressure fluctuations measured when entering into surge are also presented and reveal very repetitive behaviour of the flow instabilities. Keywords: unsteady pressure measurements, conditioner, high frequency, high speed, centrifugal compressor. Introduction Next generation engine design tends towards compact, highly efficient and large operability configurations. The operating range of high speed compressors is limited by choking at high mass flow rates and by the onset of in- stabilities at low mass flow rates. A safety margin, known as the ‘surge margin’, prevents the compressor from op- erating close to the stability limit, precisely where the pressure ratio reaches its highest level. Therefore, in or- der to increase the operating range, there is great interest in predicting the condition at which instability will occur in a compressor. That requires reaching a comprehensive understanding of the physical phenomena which trigger rotating stall and/or surge. Over the last few years the development of numerical tools such as Computational Fluid Dynamics and high performance parallel computers has led to significant progress, enhancing the understanding of flows in tur- bomachines. Numerical methods and various models have also been proposed to reduce the cost and to permit unsteady calculations. But in most of the cases, strong assumptions concerning the spatial and time scales limit the validity domain around design conditions, and the results obtained at non-stable operating points have to be validated by experimental results. Experimental valida- tion is all the more necessary as the case under investiga- tion is a realistic high-speed machine with high complex- ity induced by technological effects. The Turbomachinery Team of the Fluid Mechanics Acoustic Laboratory (LMFA) at Ecole Centrale de Lyon (ECL) has two high-speed compressor test rigs dedicated