Influence of tip clearance on flow behavior and noise generation of centrifugal compressors in near-surge conditions

• Centrifugal compressor aeroacoustics sensitivity to tip clearance is investigated.
• 3 different clearance ratios are set in accordance to expected operating values.
• Pressure spectra do not depend on tip clearance ratio in near-surge conditions.
• DES performs better than URANS in predicting compressor acoustic signature.
• Flow field observation reveals that tip clearance is immersed in rotating backflow.

CFD has become an essential tool for researchers to analyze centrifugal compressors. Tip leakage flow is usually considered one of the main mechanisms that dictate compressor flow field and stability. However, it is a common practice to rely on CAD tip clearance, even though the gap between blades and shroud changes when compressor is running. In this paper, sensitivity of centrifugal compressor flow field and noise prediction to tip clearance ratio is investigated. 3D CFD simulations are performed with three different tip clearance ratios in accordance to expected operating values, extracted from shaft motion measurements and FEM predictions of temperature and rotational deformation. Near-surge operating conditions are simulated with URANS and DES. DES shows superior performance for acoustic predictions. Cases with reduced tip clearance present higher pressure ratio and isentropic efficiency, but no significant changes in compressor acoustic signature are found when varying clearance. In this working point, tip clearance is immersed in a region of strongly swirling backflow. Therefore, tip leakage cannot establish any coherent noise source mechanism.