Discussion: “Rotordynamic Force Prediction of Centrifugal Compressor Impellers Using Computational Fluid Mechanics” by J. J. Moore, D. L. Ransom, and F. Viana

Abstract

The authors are to be commended for an interesting demonstraion of computational fluid dynamics CFD methods in the analyis of rotordynamic forces for impellers in centrifugal compresors. CFD approaches clearly represent the future for this mportant calculation. As they state, data are vitally needed to nchor predictions for impeller coefficients for compressors. In act, better data would also be helpful for pump impellers. The ata of Bolleter et al. 4 yield whirl frequency ratios that range pward from 0.75 to 2.2. To the extent that the destabilizing orces arise from fluid rotation, WFR cannot exceed the average ircumferential velocity ratio, and more reasonable values would e on the order of 0.5 as predicted by the authors . The authors provide an illuminating discussion of the Wachel odel and its current variations with comparisons to their CFD pproach. They mentioned Gupta’s MS thesis 6 , but apparently verlooked the more accessible 2006 work by Gupta and Childs 1 who use a bulk-flow model to predict the forces for the front nd back compressor shroud faces and compared their predictions o Wachel’s formula. Gupta and Childs showed reasonable agreeent with measured results for pump impellers from Bolleter et l. and presented calculated results for the forces developed by the hroud faces of an industrial compressor. In reviewing CFD literature related to calculation of rotordyamic coefficients for impellers, the authors overlooked the first uch calculation by Baskharone et al. 2 . In 1994, he used a nite-element CFD model and produced reasonable comparisons o measured results by Bolleter et al. In Ref. 8 , Moore showed omparable comparisons for Bolleter’s measured pump data using i Moore’s CFD developments, ii Childs’ bulk-flow predictions 5 , and iii predictions of Baskharone et al.. In their Summary and Conclusions section, the authors state, Based on this result, it can be concluded that the majority of the estabilizing force of a centrifugal impeller arises from the shroud assage, not the impeller-to-diffuser interaction, since the instabilty can be predicted by the shroud force alone.” Are the authors uggesting that this is a new and unexpected outcome? In regard