Saw-Tooth Ramps for the Suppression of Flow-Acoustic Coupling in a High-Frequency Silencer for Turbocharger Compressors

The ported shroud casing treatment for turbocharger compressors is desirable for mitigating broadband/whoosh noise and enhancing boost pressures at low to mid flow rates. Yet, it is accompanied by elevated narrowband noise at the blade-pass frequency (BPF). Compressor BPF noise occurs at high frequencies where wave propagation is often multi-dimensional, rendering traditional planar wave silencers invalid. An earlier work introduced a novel reflective high-frequency silencer (baseline) targeting BPF noise in the 8-12 kHz range using an “acoustic straightener” that promoted planar wave propagation along arrays of quarter-wave resonators (QWRs). The design, however, faced challenges with high-amplitude tonal noise generation at specific flow conditions due to flow-acoustic coupling at the opening of the QWRs, thereby compromising the noise attenuation. The current study explores two QWR interface geometries that weaken the coupling, including linear and saw-tooth ramps on the upstream edge of each QWR. Computational fluid dynamics is utilized to study the mechanism of flow-acoustic coupling and assess the effectiveness of these modified interface geometries. Prototypes of the improved designs fabricated by additive manufacturing are then experimentally evaluated on a flow bench against the baseline silencer. The saw-tooth ramps prove particularly effective, achieving superior suppression of flow-acoustic coupling with lower flow restriction relative to the linear ramps. Next, the insertion loss of the baseline silencer and that with the saw-tooth ramps are determined on a turbocharger gas stand equipped with a rotating inlet duct. This unique setup is utilized to calculate the transmitted sound power upstream of the configurations with and without the silencer and therefore its insertion loss. The sharp valleys of negative insertion loss indicating tonal noise due to flow-acoustic coupling in the baseline silencer are eliminated by the saw-tooth ramps, therefore enhancing the overall noise attenuation.