1 ROTATING INSTABILITIES IN A RADIAL VANELESS DIFFUSER PART II: COMPARISON OF EXPERIMENTAL AND NUMERICAL RESULTS G. Cavazzini* – A. Dazin** – G. Pavesi* - P. Dupont** - S. Coudert** - G. Ardizzon* – G.Caignaert** – G. Bois** * Department of Mechanical Engineering, University of PADOVA, Italy Ph : +39-49-8276800 Fax : +39-49-8276785 giovanna.cavazzini@unipd.it ** Laboratoire de Mécanique de LILLE (UMR CNRS 8107), Arts et Métiers ParisTech, École Centrale de Lille, France Ph : +33-3-20622168 Fax : +33-3-20622240 antoine.dazin@ensam.lille.fr ABSTRACT The paper refers to the vaneless diffuser flow analysis of a radial flow pump in partial flow operating conditions. Numerical data obtained with the help of the CFX computer code are compared with the experimental results (high speed PIV and unsteady pressure measure- ments) presented in the first part of this paper. Averaged results and Fourier spectra are com- pared, as well as the flow patterns associated with a rotating instability occurring at this flow rate. The numerical data are also used to analyse the flow behaviour at this flow rate. NOMENCLATURE b impeller or diffuser width (m) c absolute velocity (m/s) f frequency (Hz) Q volume flow rate (m 3 /s) r radial position (m) R radius (m) Re Reynolds number u peripheral velocity (u= R) Z number of impeller blades density (kg/m 3 ) angular rotation velocity (rad/s) kinematic viscosity (m 2 /s) pressure coefficient Subscripts 1 impeller inlet 2 impeller outlet 3 diffuser inlet d design r radial component u tangential component z axial component ri relative to the rotating instability b relative to the blade passage imp relative to the impeller t theoretical INTRODUCTION Rotating instabilities can negatively affect the behaviour of a vaneless diffuser in centrifugal pumps or compressors, by causing performance decay, noise and damage. The first part of this paper presented an experimental analysis carried out on the radial diffuser of a centrifugal pump by means of the high speed stereoscopic PIV and of unsteady pressure trans-