The goal of the presented research is to study the effective operational range for a centrifugal vaneless diffuser turbocharger compressor with ported shroud typically used in diesel engines. A turbocharger bench facility was designed and tested in order to define the performances of the compressor and to better understand the occurrence of instabilities in the housing. Specific emphasis was given to the low mass flow rate region of the compressor performance characteristics where instabilities occur with fluctuations that can be significantly large in the case of surge. Static pressures and dynamic pressure fluctuations were measured at the inlet, the outlet, as well as at different positions around the volute and diffuser sections of the compressor in order to assess the development and propagation of flow instabilities.
The dynamic signature of the flow was measured along with the elaboration of the compressor mapping. Hence, data covering the entire compressor map were collected and then analyzed with specific emphasis given at low mass flow rates where unstable phenomena, including stall and surge, occur. In this study, three regimes of operation were identified from the combination of the dynamic and performance analysis that are: the stable regime, the stall (mild surge) regime and the deep surge regime. Signal features from the different sensor locations are discussed for a single speedline. Use of cross-correlations between measurements allows revealing the propagation and susceptible origin of the instabilities. In particular, pressure oscillations in surge seem to originate under the volute tongue to then propagate in the rest of the housing. Additionally, the total set of data is compared with a similar compressor without bleeding slot in order to objectively evaluate the impact of the ported shroud on the development and occurrence of instabilities.