Experimental Investigation of Fluid Structure Interaction of Impeller Like Disks in Super Critical Carbon Dioxide

Abstract

In radial compressors working with high density gases, such as super critical carbon dioxide, strong interactions between the fluid and the impeller can cause large dynamic loads that can lead to damage or even failure. Acoustic resonances in the compressor cavities couple strongly with the vibration modes of the impeller. The coupled acoustic-structural modes can be predicted by means of complex models that need to be validated. To this end a test rig has been developed in which impeller like structures can be tested under high density gas conditions. In this paper the experimental setup is described with which the required data for the validation of the models is generated. The focus of this paper is not on data analysis and comparisons with models since this is part of two separate PhD projects. To demonstrate the relevance of the results a short preliminary comparison with finite element modeling is made. The present test rig has been operated with (super critical) carbon dioxide in a pressure range from atmospheric pressure to 200 bar and temperatures between 40 and 60 degrees C. Two types of simplified impellers have been tested: a single disk and a double disk with ribs. The latter mimics the shape of an impeller. The diameter of the impeller disk is comparable to a real impeller. The disks are mounted in a housing that represents the compressor stator including a diffuser. The housing is placed inside a specially designed pressure vessel. Several parameters can be varied, such as the width of the cavities, the clearance between the impeller and the diffuser, etc. In the test set up multiple pressure transducers are installed to measure the acoustic modes. To identify the modal deformation of the impeller, glass fiber sensors (so-called FBG’s) have been installed at 38 of points on the impeller. The transfer functions from fluid to structure are measured by means of acoustic excitation. Vice versa the transfer function from structure to fluid is measured by means of impulse excitation of the disk. In this paper the test rig is described and preliminary results are presented.