Acoustic Measurements From an Automotive Centrifugal Compressor With a Switchable Dual-Port Casing Treatment for Extended Operating Range

Abstract

An effective measure to improve the surge margin of a centrifugal compressor, without sacrificing efficiency, is to implement a recirculating casing treatment inside the compressor cover. However, introduction of an additional sound propagation path directly over the rotating impeller blades exposes the inherently unsteady internal flow-field as an added potential noise source, which is of concern for automotive applications. The present study conducts performance and acoustic measurements of a new compressor which was designed to achieve high isentropic efficiency over a wide flow range, featuring an impeller with splitter blades and a vaneless diffuser. A dual-port active casing treatment (ACT) was also incorporated into the compressor cover to independently extend both the low and high flow rate operating regions of the compressor. The slot of the first (surge) port is positioned between the main and splitter blades of the impeller, similar to passive casing treatments that are already widely adopted. This port extends the low-flow boundary of the compressor operating range by reducing flow separation on the suction surface of the main blades near the shroud. The slot of the second (choke) port is located just behind the splitter blades, and it is studied in both the open and closed positions. This second port allows for increased air flow near choke, due to the slot position just downstream of the aerodynamic throat of the compressor. The current ACT design leaves the surge port open at all times, while the choke port is only opened when the compressor operates near choke conditions. In addition to comparing experimental results from this new compressor in both configurations (choke port open and closed), measurements from a comparable (baseline) compressor without splitter blades and a single-port shroud are utilized to assess the acoustics of the new design. Acoustic measurements were completed over the low to mid-speed operating range, which is a region heavily weighted in customer drive cycles for light and medium duty vehicles. The conscientious design of the impeller and surge slot of the new compressor to minimize flow separation on the suction surface of the inducer blades is shown to not only improve efficiency and extend the low-flow operating range, but (with the choke port closed) broadband noise is significantly reduced in the mid to high flow rate operating region. At low flow rates, the new compressor (with the choke port closed) is slightly louder than the baseline compressor at the inlet duct measurement location, but essentially equal to the baseline compressor at the external microphone location near the compressor inlet duct opening. When the choke port of the new compressor is open, broadband noise increases slightly relative to the closed configuration. More importantly, the peak sound pressure level at (main) blade-pass frequency is reduced by opening the choke port, and the operating region of elevated tonal noise shifts from mid to high flow rates.