Circumferential Variation of Noise at the Blade-Pass Frequency in a Turbocharger Compressor with Ported Shroud

The ported shroud casing treatment for turbocharger compressors offers a wider operating flow range, elevated boost pressures at low compressor mass flow rates, and reduced broadband whoosh noise in spark-ignition internal combustion engine applications. However, the casing treatment elevates tonal noise at the blade-pass frequency (BPF). Typical rotational speeds of compressors employed in practice push BPF noise to high frequencies, which then promote multi-dimensional acoustic wave propagation within the compressor ducting. As a result, in-duct acoustic measurements become sensitive to the angular location of pressure transducers on the duct wall. The present work utilizes a steady-flow turbocharger gas stand featuring a unique rotating compressor inlet duct to quantify the variation of noise measured around the duct at different angular positions. The acoustic pressure transducers installed on the rotating duct record time-resolved in-duct acoustic pressure at different azimuthal locations while the compressor is held at a steady operating point. Acoustic measurements are performed across the flow range of a ported shroud compressor at three different rotational speeds. A comparison of sound pressure levels measured at different azimuthal locations reveals the significant contribution of high-frequency BPF noise to the variation in the acoustic pressure around the duct.