Mechanism of improvement in stability margin and choking flow capacity of a centrifugal compressor with variable geometry casing treatment

Casing treatments in centrifugal compressors conventionally employ a single port positioned above the impeller tip, which effectively increases the surge margin but provides limited enhancement of choke flow capacity due to a mismatch between port location and the shroud surface pressure gradient. In this study, a dual-ported shroud casing treatment with two ports arranged in meridional direction over the impeller tips is proposed to resolve this mismatch. Compressor aerodynamic performance was quantified through complementary experimental tests and three-dimensional numerical simulations. Results show that the dual-ported configuration not only yields a more pronounced improvement in surge margin than the conventional single-port design, but also increases choke flow capacity by up to 14.15% in mass flow rate at the maximum rotational speed. This enhancement arises because the dual-ported shroud provides an auxiliary flow path into the tip region—bypassing the impeller throat and mitigating local flow separation. Finally, an active control strategy is introduced, whereby the shroud ports are dynamically switched to optimize isentropic efficiency and extend the compressor’s stable operating envelope.