Flow mechanisms of two-regime flow instability caused by centrifugal compressor volute

Except for the progressive and the sudden flow instabilities, a novel two-regime flow instability is observed and studied in current centrifugal compressor experiments. To explore its formation mechanisms, a fully three-dimensional numerical model of the compression system is established. Based on this numerical model, the evolution of the two-regime flow instability is predicted successfully, and the matching relationships among the impeller, the diffuser, and the volute are studied. The results show that, in the two-regime flow instability evolution, the reverse flow at the impeller inlet unexpectedly enhances the pressure rise capacity of the impeller, but the reverse flow near the volute outlet caused by the large area expansion of the volute adaptor leads to significant flow fluctuations and results in the mild surge. Decreasing the mass flow rate further from the mild surge, an unstable equilibrium point (PUE) that is the most representative characteristic of the two-regime flow instability is reached. At PUE, a steady reverse-flow channel is formed temporarily in the volute, and the reverse flow originates near the volute outlet, bypasses the volute tongue, and re-enters the volute spiral. These findings reveal the flow mechanisms underlying the two-regime flow instability and enrich the theory of centrifugal compressor flow instability.