Influence of tip clearance on the flow field and aerodynamic performance of the centrifugal impeller

This paper studies numerically the influence of the tip clearance on the three-dimensional viscous flowfield and performance of the NASA Low-Speed Centrifugal Compressor (LSCC) impeller with a vaneless diffuser. A three-dimensional viscous code developed by the authors is applied with several acceleration methods: local time-step, multigrid and residual smoothing. The computations were performed under several operating conditions with four different tip clearance sizes (0.0%, 50%, 100% and 200% design tip-clearance). The numerical results show that: (1) the throughflow wake is considerably influenced by the tip clearance. The low-kinetic-energy fluid accumulated in the shroud/pressure side of the unshrouded impeller, but in the shroud/suction region for 0.0 tip clearance; (2) there possibly exists an optimum size of the tip-clearance which is not the zero tip clearance to make the flow loss minimized.