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 rotorstator interaction. However, the natural frequencies of an impeller change under highdensity 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 highpressure compressors the mass density of the discharge gas can be about onethird 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 highdensity gas conditions but others increased and that the increase of natural frequencies is caused by fluidstructure 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 highdensity gas applications, this paper presents experimental results obtained using a variablespeed 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 highdensity centrifugal compressors, some natural frequencies of an impeller increased in highdensity 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.