Numerical simulations of stall inside a centrifugal compressor

This paper presents a numerical simulation of stall flow phenomenon inside a turbocharger centrifugal compressor with vaneless diffuser. Three-dimensional Reynolds averaged compressible Navier-Stokes equations were solved with k-epsilon turbulence model using computational fluid dynamics software CFX. Entire geometry of the compressor, including the impeller, vaneless diffuser, and volute housing were included in the simulation. The stage performance curve was obtained using steady and unsteady calculations and compared with experimental results. The unsteady flow simulation was performed at reduced mass flowrates (compressor stall region) with a plenum chamber model added at the compressor housing outlet. The results show that there is a distinct stall frequency at the given compressor speed. The amplitude of the static pressure oscillation at this frequency in the diffuser is increased with reduction in compressor mass flow. The stall frequency is only marginally affected by the compressor mass flowrate. Time varying flow fields in the diffuser and in the volute indicate that there is a circumferential standing wave inside the volute. It is proposed that the volute behaves like a tuned pipe resonator and this dictates the stall frequency. The stall is also characterized by a periodically reversing flow at the centre of the volute discharge cross-section.