Large eddy simulations of corner separation in varying dihedral angle diffusers

The dihedral angle, an inherent geometric characteristic between the suction surface of the compressor blade and the endwall, significantly influences corner separation. To deeply investigate the impact of the size and variation of the dihedral angle on corner separation, this paper designs physical models of varying dihedral angle diffusers equivalent to compressors. These models leverage the deceleration and pressurisation effects of diffusers on airflow. Using these models, a study is conducted employing large eddy simulation (LES) to investigate the impact of varying dihedral angles on corner separation under adverse pressure gradients. The research results indicate that in varying dihedral angle diffusers, where the dihedral angle decreases axially, the following three strategies can be employed to enhance aerodynamic performance: (1) Keeping the inlet dihedral angle constant while increasing the outlet dihedral angle. (2) Keeping the difference in dihedral angles between the inlet and outlet constant while increasing the inlet dihedral angle. (3) Keeping the outlet dihedral angle constant while increasing the inlet dihedral angle. Among these strategies, the first and second can delay corner separation and promote flow reattachment. While the third strategy can also delay corner separation, it increases the axial variation rate of the dihedral angle, exacerbating corner separation at the end of the expansion section. Additionally, secondary flow in the diffuser develops from the center toward the periphery, forming oppositely rotating vortex pairs near the corner and wall center, with increasing intensity downstream. The highest Reynolds stresses are concentrated in the corner region, where the flow exhibits strong anisotropy. Overall, these findings improve understanding of corner separation flow under varying dihedral angle conditions and provide new insights and data to support the design of modern compressors.