Erwann Guillou

Stall development in a ported shroud compressor using PIV measurements and Large Eddy Simulation

Surge is a phenomenon that limits the operational range of the compressor at low mass flow rates. The objective of this research is to study effective operational range for a ported shroud compressor. The size of the compressor is typical for a turbocharger used on diesel engines. To be able to extend the operational range, the surge characteristics have to be assessed. This is done by performing measurement of the flow at the inlet to the compressor wheel and pressure fluctuations at the inlet and outlet of the compressor housing. Detailed numerical computations of the flow in the entire compressor section under similar operating conditions have also been carried out. The experimental work includes Particle Imaging Velocimetry (PIV) measurements of the instantaneous and mean velocity field at the inlet. At surge, low frequency pulsations are detected that seem to result from back flow already observed in stall. The numerical computations include details of the flow by having good spatial and temporal resolution and using Large Eddy Simulations (LES) to account for the turbulence. LES is most suitable for the surge flow since it resolves the large scale structures, such as flow separation and reversed flow, which characterize surge.

PIV Measurements of Surge Incipience in a Ported Shroud Compressor

The range of operability of compressors in turbochargers is limited by surge. At low mass flow rate, surge is characterized by a breakdown of the incoming flow accompanied with large pressure fluctuations and the development of reversal flow. Part of the passive control strategies to increase the performances of compressors includes the implementation of a ported shroud. By releasing the reverse flow and preventing instabilities to occur, it has been demonstrated that larger range of operability could be achieved with minor negative impact on the compressor efficiency. The scope of this research is to study the effective operational range for a turbocharger with port shrouded compressor typically used in diesel engines. In order to better understand and control the surge phenomenon, it is important to identify the aerodynamic changes leading to stall and eventually surge. Particle Image Velocimetry (PIV) was performed at the inlet of the compressor to capture steady state and transient velocity in respectively stable and unstable regimes. Using the dynamic signature of the compressor outlet pressure, it has been possible to phase lock the PIV measurements and reconstruct the evolution of the flow patterns along a full surge cycle. Specific emphasis was given to the understanding of how the ported shroud helps to prevent surge from occurring by comparing different operating conditions. In this paper, both standard 2D-PIV and stereoscopic 3D-PIV measurements are presented and permitted to characterize the flow interactions at the entrance of the compressor.

Dynamic Characterization of a Centrifugal Compressor with Ported Shroud

The scope 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 under study and to better understand the development and occurrence of surge. Pressure fluctuations were measured at the inlet and the outlet of the compressor as well as at different positions around the volute and in the diffuser by use of dynamic pressure transducers. 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 insight 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: the stable regime, the stall regime and the deep surge regime. Signal features from the different measuring instruments and configurations are discussed. Particularly, early detection of stall is made possible from the association of the pressure frequency domain and the statistic temporal analysis.