THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS - 345 E. 47th St, New York, N.Y. 10017 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published 95-GT-33 in an ASME Journal. Authorization to photocopy material for internal or personal use under circumstance not falling within the fair use provisions of the Copyright Act is granted by ASME to libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of $0.30 per page is paid directly to the CCC, 27 Congress Street, Salem MA 01970. Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright © 1995 by ASME All Rights Reserved Printed in U.S.A. 3D UNSTEADY FLOW AND FORCES IN CENTRIFUGAL IMPELLERS WITH CIRCUMFERENTIAL DISTORTION OF THE OUTLET STATIC PRESSURE Antonios Fatsis Stephane Pierret Rene Van den Braembussche Turbomachinery Department von Karman Institute For Fluid Dynamics Rhode-St-Genese Belgium ABSTRACT This paper describes the numerical investigation of the centrifugal impeller response to downstream static pressure distortions imposed by volutes at off-design operations. An unsteady 3D Euler solver with non-reflecting upstream and downstream boundary conditions and phase-lagged period- icity conditions is used for this purpose. The mechanisms governing the unsteady flow field are analysed. A parametric study shows the influence of the acoustic Strouhal number on the amplitude of the flow per- turbations. Radial forces calculated on backward leaned and radial ending centrifugal impellers, show non-negligible influence of the impeller geometry. NOMENCLATURE F Force L Length of blade channel M Mach number NbI Number of the impeller blades NCI RC Number of angular positions of impeller P Static pressure R Radius S Surface Sr Acoustic Strouhal number V Absolute velocity W Relative velocity c Speed of sound f Frequency of unsteadiness n Number of positions per blade pitch t Time At Time step Sl Reduced frequency Absolute flow angle Relative flow angle p Density w Non-dimensional rotational impeller speed Subscripts m Meridional r Radial 9 Circumferential z Axial 0 Total conditions 2 Impeller trailing edge Superscripts 1 Impeller inlet INTRODUCTION Centrifugal compressor impellers are exposed to down- stream circumferential static pressure distortions when fol- lowed by non-axisymmetric volutes operating at off-design conditions. The static pressure distortion propagates up- stream and creates a circumferential variation of the blade loading and forces in the impeller. A first consequence is a non-zero radial force on the shaft and extra load on the bearings. It also means unsteady flow in the rotating blade passages resulting in blade vibrations and additional noise. This increases losses and decreases the operating range of the compressor. Experimental studies on the flow in centrifugal compres- sor impellers with downstream static pressure distortion (Pfau (1967), Uchida et al. (1987), Sideris (1988)) indicate a non-linear variation of the outlet flow with large variations in the volute tongue region. Non-negligible circumferential flow variations are also detected upstream from the impeller, resulting in a variation of incidence. Presented at the International Gas Turbine and Aeroengine Congress & Exposition Houston, Texas - June 5-8, 1995 This paper has been accepted for publication in the Transactions of the ASME Discussion of it will be accepted at ASME Headquarters until September 30, 1995 Downloaded from https://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78781/V001T01A004/2405669/v001t01a004-95-gt-033.pdf by guest on 28 June 2020