Deep surge is a violent fluid instability that occurs within turbomachinery compression systems and limits the low-flow operating range. It is characterized by large amplitude pressure and flow rate fluctuations, where the cross-sectional averaged flow direction alternates between forward and reverse. When a compressor transitions into deep surge, the time-averaged compressor outlet pressure and temperature decrease and increase, respectively, along with a drastic rise in narrowband, low-frequency noise.

The present study includes both measurements and predictions from a turbocharger centrifugal compressor installed on a gas stand. The compressor breathes air from ambient through an inlet duct with a bellmouth opening. The downstream compression system consists of a compressor outlet duct attached to a plenum with increased cross-sectional area, and an additional duct that connects the plenum outlet to a control valve. A detailed three-dimensional (3D) computational fluid dynamics (CFD) model of this compression system was constructed to carry out unsteady surge predictions. The results included here capture the transition from mild to deep surge, as the flow rate at the outlet boundary (valve) is reduced. During this transition, the amplitude of pressure and flow rate fluctuations greatly increase until they reach a repeating cyclic structure characteristic of deep surge. During the deep surge portion of the prediction, pressure fluctuations are compared with measurements at the corresponding compressor inlet and outlet transducer locations, where the amplitudes and frequency exhibit excellent agreement.

The predicted flow-field throughout the compression system is studied in detail during operation in deep surge, in order to characterize the unsteady and highly 3D structures present within the impeller, diffuser, and compressor inlet duct. Key observations include a core flow region near the center of the inlet duct, where the flow remains in the forward direction throughout the deep surge cycle. The dominant noise generation occurs at the fundamental surge frequency, which is near the Helmholtz resonance of the compression system, along with harmonics at integer multiples of this fundamental frequency.

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