Acoustic Modes Measurement in the Duct of High Speed Compressor

Abstract

The dominant spinning modes in the ducted sound field of the high speed compressor are related to the blade number and the unstable phenomena in principle, its mode order normally is high, it is difficult to get the high-order acoustic spinning modes information by microphones flush mounted on the case wall if the microphones number is less than the twice of the order number of the desired spinning mode based on the Shannon's Sampling Theorem. In this paper a approach to analyze the spinning modes is presented, with this approach the high-order mode components could be assessed from the low-order mode components which could be measured and processed by the data measured from less microphones and may be mixed with the high-order modes components due to the aliasing. An experiment to measure the acoustic spinning modes of the duct in a transonic compressor rig was conducted, the spinning modes of the acoustic pressure components similar to the rotation instability components which had been measured and investigated in low speed compressor by the previous researchers as well as the spinning modes of rotor blade passing frequency components which are the significant parts in transmitting fan/compressor noise were measured and studied by present approach, from the measured modes components some high-order modes mixed in it are assessed, the acoustic model analysis were discussed, some interesting results concerning instability components and its acoustic modes and some information about the sound modes transmitting in the duct of high speed compressor were obtained. 1、 Introduction It is important to get the information of the acoustic modes in the duct of fan/compressor in the study on noise generation, propagation and attenuation for the purpose of noise suppression and the diagnosis of rotating instability. The measurement of acoustic spinning modes in a circular duct of fan/compressor normally is carried out by the microphones flush mounted on the case wall and evenly spaced circumferentially or by the microphones movable in the circumferential direction by which it is easier to obtain the higher order spinning modes than by the first one, but it is difficult to use in the high speed compressor. The paper introduces an approach to measure the spinning modes in high speed compressor according to the first method. Based on Shannon's Sampling Theorem the mode components resulted from circumferentially mounted microphones inevitably contain the higher order modes folded back, so that it is possible to abstract some information about higher order spinning modes from the measurement results of low order modes combining the knowledge of the noise sources, the acoustic modes propagation and attenuation in duct. The paper presents an experiment to measure the acoustic spinning modes in the duct of compressor by the new approach, conducted in the transonic compressor rig in Jet Propulsion School of Beihang University (previous name is Beijing University of Aeronautics and Astronautics). The analysis and discussion of the experiment results illustrate that the approach is valuable and some interesting phenomena have been observed. 9th AIAA/CEAS Aeroacoustics Conference and Exhibit 12-14 May 2003, Hilton Head, South Carolina AIAA 2003-3143 Copyright © 2003 by Tongqing Wang and Huaiyu Wu. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. 2、 The experiment facility The experiment is conducted in the transonic compressor rig of Beihang University, its flux range in 0~25kg/s,rotating speed in 8000~22000 rpm. The tested compressor is a single stage compressor with 17 rotor blades, 29 exit guide vanes, and 3 support struts in the entry, the diameter of the rotor is 365mm. A model consisting of one section of linings, or two section of linings or a hard wall duct was amounted in the upstream of the compressor in order to study the acoustic performance of linings, two sets of microphones with 16 pieces were flush mounted and circumferentially event spaced on the duct wall of the both sides of the linings model. The experiment facility is shown in figure 1. 3、 The aliasing of spinning modes and the identification of dominant mode propagating in the duct of compressor From Shannon's Sampling Theorem the spinning modes obtained based on the sound pressure measured by the microphone of N pieces will be ranged in (-N/2+1~+N/2) into which the modes with higher order will fold back. The coefficient of mode with the order α could be calculated from the complex sound pressure n P measured by the microphone on the casing by,