Investigating Swirl Brake Designs for Improving Rotordynamic Stability in Oil-Free Centrifugal Compressors Using Refrigerants

Abstract With the rising demand for energy-efficient cooling systems, particularly in high-demand applications like data centers, oil-free centrifugal compressors have become integral to reduce energy consumption and environmental impact. A critical component in these compressors is the swirl brake, which reduces the inlet swirl ratio at seals, directly influencing seal force coefficients and rotor stability. This study investigates and optimizes swirl brake designs for tooth-on-stator labyrinth seals, focusing on three key parameters: vane number, vane length, and vane shape. Computational Fluid Dynamics (CFD) simulations were conducted using R134a and R1234ze refrigerants to evaluate the impact of these designs on the inlet swirl ratio. A bulk flow model was employed to estimate the corresponding rotordynamic force coefficients, providing insights into the relationship between swirl ratio and rotor stability. The results reveal that increasing the number and length of swirl brake vanes reduces the inlet swirl ratio, with optimal configurations achieving negative swirl ratios that stabilize the rotor by reversing flow directions. While straight vanes demonstrated superior performance, inclined and C-shaped vanes were less effective, challenging conventional assumptions about vane geometry. The analysis confirms that reducing the cross-coupled stiffness through effective swirl brake design significantly mitigates destabilizing forces and enhances rotor stability. This research underscores the importance of tailored swirl brake designs to optimize seal performance and improve the overall efficiency and reliability of oil-free centrifugal compressors. The findings provide valuable insights for advancing compressor technology in refrigeration and cooling applications.