Numerical Investigation of Unsteady Flow in Tandem Cascades Diffuser

Numerical simulation was employed to investigate the unsteady flow field of tandem cascades diffuser. Results show that inner flow of the centrifugal compressor with tandem cascades diffuser is featured with strongly unsteady, three-dimensional viscous main flow and gap flow. The inner flow in tandem cascades diffuser is significantly unsteady. The propagation of the pressure fluctuation attenuates notably in the passage of front cascade. The dominant frequency of fluctuation pressure in front cascade of diffuser is the blade passing frequency(BPF) of impeller, while it changes to a half BPF of impeller near the leading edge of rear cascade. Near the leading edge of the front cascade, the pressure fluctuation is excited by the impeller; in the gap of tandem cascades, it is dominated by a new excitation which is created by the interaction and superposition of the gap flow, the wake of front cascade and the impeller wake transported in the mainstream. Near the leading edge of front cascade, compared with the root or the tip, the pressure fluctuation near the mid-span is more strongly excited by the wake of both main blades and splitter blades of impeller.