Natural Frequencies of Centrifugal Compressor Impellers for High Density Gas Applications

Abstract

Characteristics of natural frequencies of an impeller and an equivalent disc were investigated in high-density gas to develop a method for predicting natural frequencies of centrifugal compressor impellers for high-density gas applications. The equivalent disc had outer and inner diameters equal to those of the impeller. We expected that natural frequencies would decrease with increasing the gas density because of the added-mass effect. However, we found experimentally that some natural frequencies of the impeller and the disc in high-density gas decreased but others increased. Moreover, we observed, under high-density condition, some resonance frequencies that we did not observe under low-density condition. These experimental results cannot be explained by only the added-mass effect. For simplicity, we focused on the disc to understand the mechanism of the behavior of natural frequencies. We developed a theoretical analysis of fluid-structure interaction considering not only the mass but also stiffness of gas. The analysis gave a qualitative explanation of the experimental results. In addition, we carried out a fluid-structure interaction analysis using the finite element method. The behavior of natural frequencies of the disc in high-density gas was predicted with errors less than 6%.

Identification of Coupled Natural Frequencies in a Rotor-Stator Test-Rig for Different Gas Properties

“…For increased pressure levels, the resonance frequencies diverge from each other. This behavior for coupled modes has been identified by Magara et al (2008), too. They named the respective branches in-phase (decreasing resonance frequency for increasing pressure) and out-ofphase (increasing resonance frequency for increasing pressure) mode.…”

“…The experimental results are compared with the numerical modal analysis and the analytical results of Magara et al (2008). In Figure 14 the relative deviations of the respectively determined resonance frequencies to the resonance frequency determined with the strain gage data is displayed.…”

“…The experimental results are compared with a numerical modal analysis and a qualitative analytical approach presented by Magara et al (2008).…”

“…Calculations have been performed with fluid properties set to the experimental ones as listed in Table 2. Magara et al (2008) propose a qualitative analytical approach to estimate the natural frequency shift of a similar coupled system with a disk enclosed in a gas cavity. They couple the equations of motions of the gas and the structure and assume an inviscid and compressible gas.…”

“…These are the coupled natural frequencies of the system. Magara et al (2008) have introduced the terms "disc origin" and "fluid origin mode" for the respective branches of the parabola, depending on the uncoupled natural frequencies. The terms are renamed to "structure dominant" and "acoustic dominant" within this paper.…”