Unsteady Investigation of Side Loading Effect on Single-Stage Centrifugal Compressors

Abstract Single-stage centrifugal compressors have historically found usage in several industrial and civil applications thanks to their high efficiency, robustness, and wide operating range. However, despite the simplicity in the architecture, the overhung configuration represents a challenging arrangement for the design of these types of machines. In particular, the presence of a discharge volute with variable cross-sectional area around circumference further exacerbates such task due to its intrinsic asymmetric shape. Indeed, focusing on the volute-impeller interaction, static pressure distortions could introduce non-negligible forces that act on journal bearings. Against this backdrop, a good prediction of static pressure fields is pivotal. In this perspective, a methodological framework leveraging unsteady and steady CFD analyses is proposed and validated using on-site measurements of a real working machine by exploiting the built-in instrumentation of the active magnetic bearing system. The main goal of the present work is to provide and validate a numerical methodology useful for the prediction of the magnitude and the angular position of the induced side force on single-stage centrifugal compressors. Specifically, the theoretical contribution of the research is to investigate the limits of steady computations against unsteady modelling for the evaluation of volute-impeller interactions in single-stage centrifugal compressors with vaneless diffuser. On a practical level, a criticality analysis on the importance of several modelling aspects on the calculations of side load prediction is provided. The proposed approach is verified on a centrifugal compressor stage for expander-compressor applications. Results show good agreement between CFD results and field data. The present research lays the foundations for future studies aiming to predict volute-impeller interaction in single-stage centrifugal compressors.