Natural Frequency Shift in a Centrifugal Compressor Impeller for High-Density Gas Applications

Abstract

In designing an impeller for centrifugal compressors, it is important to predict the natural frequencies accurately in order to avoid resonance caused by pressure fluctuations due to rotor-stator interaction. However, the natural frequencies of an impeller change under high-density fluid conditions. The natural frequencies of pump impellers are lower in water than in air because of the added mass effect of water, and in high-pressure compressors the mass density of the discharge gas can be about one-third that of water. So to predict the natural frequencies of centrifugal compressor impellers, the influence of the gas must be considered. We previously found in the nonrotating case that some natural frequencies of an impeller decreased under high-density gas conditions but others increased and that the increase of natural frequencies is caused by fluid-structure interaction, not only the added mass effect but also effect of the stiffness of the gas. In order to develop a method for predicting natural frequencies of centrifugal compressor impellers for high-density gas applications, this paper presents experimental results obtained using a variable-speed centrifugal compressor with vaned diffusers. The maximum mass density of its discharge gas is approximately 300 kg/m3. The vibration stress on an impeller when the compressor was speeding up or slowing down was measured by strain gauges, and the natural frequencies were determined by resonance frequencies. The results indicate that for high-density centrifugal compressors, some natural frequencies of an impeller increased in high-density gas. To predict this behavior, we developed a calculation method based on the theoretical analysis of a rotating disk. Its predictions are in good agreement with experimental results.

Study of variation in natural frequencies of bio-composites due to structural damage

“…Horta-Rangel et al (2008) analysed the shift in natural frequencies of structures and proposed the best method to repair damaged bridges. Magara et al (2013) proposed a theoretical method to find the natural frequency for centrifugal pumps and implemented experimental methods to verify results. Their results indicated that for high-density centrifugal compressors, some natural frequencies of an impeller increased with a high-density gas.…”

Experimental technique to measure mass under high pressure conditions using oscillatory motions of a spring-mass system

“…Because changes in the frequency of oscillation due to the surrounding fluid are usually generated entirely by the added mass [24,25], observing the oscillatory behavior of a known mass has allowed the behavior of this added mass occurrence to be explored in many studies [26][27][28][29]. Hamilton presented a vibrational technique for determining the added mass of submerged objects during deployment of subsurface moorings [26].…”

“…Wu and Hsieh measured added mass and added mass moment of inertia of several floating bodies under heave and pitch motions as functions of frequency [27]. Magara et al characterized the added mass of an impeller rotating in dense gas by analyzing the shift in its natural frequency [28]. Zhao et al developed a method to measure fluid density using a vibrating micro-cantilever [29].…”