For both diesel and gasoline internal-combustion engines, stringent emissions regulations and fuel economy requirements have significantly intensified the demand on the turbocharger compressor operating flow range with high efficiency levels. To address these needs, the ongoing research and development effort has been focused on advanced turbocharger technologies for a high efficiency and a wide operating range. In this paper, a research effort is reported to enhance further the classic ported shroud or recirculating casing treatment with the addition of two switchable ports along the impeller shroud, which allows an augmented high-speed choke flow range without sacrificing the surge margin and efficiency. Impeller aerodynamic blade design considerations for exploiting this new dual-port casing treatment concept are discussed. Flow bench test results demonstrate that the underlying concepts of the new dual-port casing treatment implemented with a recently developed advanced centrifugal compressor impeller lead to 12% extra flow capacity at the rated speed over the same compressor with a conventional single-port recirculating casing treatment.
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.
Heavy FGR required on diesel engines for future emission regulation compliance has posed a big challenge to conventional turbocharger technology for high efficiency and wide operation range. This study, as part of the U.S. Department of Fnergy sponsored research program, is focused on advanced turbocharger technologies that can improve turbocharger efficiency on customer driving cycles while extending the operation range significantly, compared to a production turbocharger. The production turbocharger for a medium-duty truck application was selected as a donor turbo. Design optimizations were focused on the compressor impeller and turbine wheel. On the compressor side, advanced impeller design with arbitrary surface can improve the efficiency and surge margin at the low end while extending the flow capacity, while a so-called active casing treatment can provide additional operation range extension without compromising compressor efficiency. On the turbine side, mixed flow turbine technology was revisited with renewed interest due to its performance characteristics, i.e., high efficiency at low-speed ratio, relative to the base conventional radial flow turbine, which is relevant to heavy FGR operation for future diesel applications. The engine dynamometer test shows that the advanced turbocharger technology enables over 3% BSFC improvement at part-load as well as full-load condition, in addition to an increase in rated power. The performance improvement demonstrated on an engine dynamometer seems to be more than what would typically be translated from the turbocharger flow bench data, indicating that mixed flow turbine may provide additional performance benefits under pulsed exhaust flow on an internal combustion engine and in the low-speed ratio areas that are typically not covered by steady state flow bench tests. ] high turbine power demand. Also, high back pressure, due to the after treatment system, increases the turbine inlet temperature, thus further reducing UIC. These requirements have driven Journal of Engineering for Gas Turbines and Power
The ultimate goal of an advanced turbocharger development is to have a superior aerodynamic performance while having the turbocharger survive various real world customer applications. Due to the uncertainty of customer usage and driving pattern, the fatigue life prediction is considered one of the most ambiguous analyses in the entire design and analyses processes of the turbocharger. The turbocharger system may have various resonant frequencies, which may be within the range of turbocharger operation for automotive applications. A turbocharger may operate with excessive stresses when running near resonant frequencies. The turbocharger may experience fatigue failures if the accumulative cycles of the turbocharger running across the resonant frequencies exceeds a certain limit. In this study, the authors propose an alternative approach to mitigate this kind of fatigue issues: i.e. engine system approach to improve turbocharger fatigue life via avoiding operating the turbocharger near resonant speeds for extended period of time. A preliminary numerical study was made and presented in this paper to assess the feasibility of such an engine system approach, which is followed by an engine dynamometer test for engine performance sensitivity evaluation when the turbocharger operation condition was adjusted to improve the high cycle fatigue life. The study shows that for a modern diesel engine equipped with electrically controlled variable geometry turbine and EGR for emission control, through the engine calibration and control upgrade, turbocharger operation speed can be altered to stay away from certain critical speeds if necessary. The combined 1D and 3D numerical simulation shows the bandwidth of the turbine “risk zone” near one of the resonant speeds and the potential impact on engine performances if the turbocharger speed has to be shifted out of the “risk zone.”
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