The variable geometry orifice (VGO) is an active flow control technology designed to improve the turbocharger compressor stability margin at the low-end operating region of a turbocharged engine. As far as the orifice design, previous studies reported optimized design parameters such as the insertion angle and radial height of the orifice, but the gap between the orifice and the impeller inlet remains insufficiently explored. This study experimentally and numerically investigates a centrifugal compressor equipped with an orifice positioned at varying axial locations in front of the impeller. Both experimental and numerical results consistently demonstrate that translating the orifice plate axially toward the impeller inlet significantly enhances the compressor’s stability margin. Specifically, at 60,000 rpm, the surge-boundary mass flow can be increased by up to 14.75%, corresponding to an approximately 2.8% improvement in isentropic efficiency. Furthermore, Dynamic Mode Decomposition (DMD) analysis indicates that when the orifice is positioned close to the impeller, it effectively impedes backflow in the inlet duct and suppresses low-frequency, high-energy unsteady modes in the vaneless diffuser, thereby enhancing flow stability. These findings suggest that during designing a turbocharger compressor with VGO configuration, the orifice plate should be designed close to the impeller inlet.
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