Takao Yokoyama

Investigation of Unsteady Flow Field in a Vaned Diffuser of a Transonic Centrifugal Compressor

Transonic centrifugal compressors are used with high-load turbochargers and turboshaft engines. These compressors usually have a vaned diffuser to increase the efficiency and the pressure ratio. To improve the performance of such a centrifugal compressor, it is required to optimize not only the impeller but also the diffuser. However the flow field of the diffuser is quite complex and unsteady because of the impeller located upstream. Although some research on vaned diffusers has been published, the diffuser flow is strongly dependent on the particular impeller exit flow, and some of the flow physics remain to be elucidated. In the research reported here, detailed flow measurements within a vaned diffuser were conducted using a particle image velocimetery (PIV). The vaned diffuser was designed with high subsonic inlet conditions marked by an inlet Mach number of 0.95 for the transonic compressor. As a result, a complex three-dimensional flow with distortion between the shroud and the hub was observed. Also, unsteady flow accompanying the inflow of the impeller wake was confirmed. Steady computational flow analysis was performed and compared with the experimental results.

Influence of Volute Cross-Sectional Shape of a Nozzleless Turbocharger Turbine Under Pulsating Flow Conditions

This paper presents an experimental and computational investigation of the influence of volute cross-sectional shape on the performance of a radial turbocharger turbine under pulsating conditions. Two volute configurations (denoted volute A and B) with the same area-to-radius ratio (A/R) distribution but different aspect ratios are rapid prototyped and tested with a same radial rotor. Experimental results show that the turbine with smaller aspect ratio volute (volute A), which has a squarish shape, shows consistently better cycle averaged efficiency at different loadings and frequencies, and the magnitude of improvement is influenced by the operational conditions. In consequent to experiments, a reduced order unsteady computational fluid dynamics (CFD) method was employed to investigate the mechanism of the performance discrepancies between volute A and B. Computational results show a stronger flow angle distortion in both circumferential and spanwise direction for volute B. Furthermore, compared with the volute A, the flow distortion near the shroud at the rotor inlet is evidently amplified by the volute B under pulsating conditions compared with the corresponding steady condition. Results in this paper, in general, demonstrate a direction for desired volute cross-sectional shape to be used in a turbocharger radial turbine.© 2014 ASME

Investigation of Unsteady Flow Field in Vaned Diffuser of a Transonic Centrifugal Compressor

Transonic centrifugal compressors are used with high-load turbochargers and turboshaft engines. These compressors usually have a vaned diffuser to increase the efficiency and the pressure ratio. To improve the performance of such a centrifugal compressor, it is required to optimize not only the impeller but also the diffuser. However the flow field of the diffuser is quite complex and unsteady because of the impeller located upstream. Although some research on vaned diffusers has been published, the diffuser flow is strongly dependent on the particular impeller exit flow, and some of the flow physics remain to be elucidated. In the research reported here, detailed flow measurements within a vaned diffuser were conducted using a particle image velocimetery (PIV). The vaned diffuser was designed with high subsonic inlet conditions marked by an inlet Mach number of 0.95 for the transonic compressor. As a result, a complex three-dimensional flow with distortion between the shroud and the hub was observed. Also, unsteady flow accompanying the inflow of the impeller wake was confirmed. Steady computational flow analysis was performed and compared with the experimental results.Copyright

Unsteady behaviours of a volute in turbocharger turbine under pulsating conditions

Turbochargers are currently in their prime utilization period, which pushes for performance enhancement from conventional turbochargers and more often than not revisiting its design methodology. A turbocharger turbine is subjected to pulsating flow, and how this feeds a steady flow design volute is a topic of interest for performance enhancement. This article investigates unsteady effects on flow characteristics in the volute of a turbine under pulsating flow conditions by numerical method validated by experimental measurement. A single pulse with sinusoidal shape is imposed at the turbine inlet for the investigation on unsteady behaviours. First, pulse propagation of different flow parameters along the volute passage, including pressure, temperature and mass flow rate, is studied by the validated numerical method. Next, the unsteady effect of the pulsating flow on the flow angle upstream the rotor inlet is confirmed by simulation results. The mechanism of this unsteady effect is then studied by an analytical model, and two factors for flow angle distributions are clearly demonstrated: the configuration of the volute A/Rc and the unsteady effect that resulted from mass imbalance. This article demonstrates unsteady behaviours of the turbine volute under pulsating conditions, and the mechanism is discussed in details, which can lead to the improvement of volute design methodology tailoring for pulsating flow conditions.

Development of twin-entry scroll radial turbine for automotive turbochargers using unsteady numerical simulation

In recent years, responding to growing interest in environmental issues, in particular many automotive technologies for fuel efficient vehicles and countermeasure to emission control have been developed actively. Among such technologies, turbochargers have been increasingly applied for downsizing of engines. In addition, Fuel consumption at actual engine operating conditions such as low engine speed are focused. Twin-entry scroll turbine gains increased attention because of high efficiency at low engine speed by utilizing engine exhaust gas pulsation. For further enhancement of fuel efficiency it is necessary to improve efficiency of turbochargers. On the other hand, a radial turbine used for a turbocharger operates under exhaust gas pulsation. Because large pulsation increases flow unsteadiness, it was difficult to understand flow phenomena and improve turbine efficiency.