Matthieu Gancedo

Numerical Flow Analysis of a Centrifugal Compressor with Ported and without Ported Shroud

Turbochargers are commonly used in automotive engines to increase the internal combustion engine performance during off design operation conditions. When used, a most wide operation range for the turbocharger is desired, which is limited on the compressor side by the choke condition and the surge phenomenon. The ported shroud technology is used to extend the operable working range of the compressor, which permits flow disturbances that block the blade passage to escape and stream back through the shroud cavity to the compressor inlet. The impact of this technology on a speed-line at near optimal operation condition and near surge operation condition is investigated. A numerical study investigating the flow-field in a centrifugal compressor of an automotive turbocharger has been performed using Large Eddy Simulation. The wheel rotation is handled by the numerically expensive sliding mesh technique. In this analysis, the full compressor geometry (360 deg) is considered. Numerical solutions with and without ported shroud for a near optimal operation condition and near-surge operation condition. The flow-field of the different cases is analyzed to elucidate the functionality of the ported shroud. In agreement with previous observations, it was found that the ported shroud reduces the flow disturbances in the blade passage for all operating conditions. However, the compressor efficiency for the off-design operation condition was found to be higher without the ported shroud, supporting the findings reported recently by an experimental investigation. The computational results are validated with experimental measurements in terms of the performance parameters and available Particle Image Velocimetry data. Copyright

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.

Optimal Pressure based Detection of Compressor Instabilities using the Hurst Exponent

The compressor surge line of automotive turbochargers can limit the low-end torque of an engine. In order to determine how close the compressor operates to its surge limit, the Hurst exponent of th ...

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.

Bleed Slot Benefits on Turbocharger Centrifugal Compressor Stability

Bleed slots located in the inducer region of centrifugal compressors have been demonstrated to extend the surge margin with minimal negative impact on performance. This paper describes the investigation of the effect of a bleed slot on map width enhancement of a turbocharger centrifugal compressor used for heavy duty diesel engine application. The goal is to evaluate the overall pressure instabilities on the compressor map and to study the dynamic phenomena occurring at low mass flow rates to better understand the benefits of the bleed slot on the compressor stability and surge line. The obstruction of the bleed slot permitted to compare the compressor behavior with the recirculation feature and without it. The pressure instability levels were measured along the accessible compressor map for the two cases using a high speed response pressure transducer at the compressor outlet. In addition, static and dynamic pressure measurements were conducted within the diffuser using respectively pressure taps and high speed response pressure transducers. The compressor with open bleed slot proved to have lower instability levels at low mass flow rates when not experiencing deep surge. Frequency responses at low mass flow rates showed that the implementation of the bleed slot suppresses broad band frequencies below the rpm frequency, which improves the overall stability. These frequencies are associated with rotating instabilities (RIs) with changing propagation speed depending on the rotational speed. At lower speeds, RIs are propagated with the wheel rotation whereas at higher speeds, they tend to propagate with the speed associated with their characteristic frequency.Copyright

Experimental Investigation of Flow Instability in a Turbocharger Ported Shroud Compressor

Turbocharger centrifugal compressors are equipped with a “ported shroud” to reduce flow instabilities at low mass flow rate. This passive stability control device using flow recirculation has been demonstrated to extend the surge margin of a compressor, without substantially sacrificing performance. However, the actual working mechanisms of the system remain not well understood.In this paper, the relationship between inlet flow recirculation and instability control is studied using stereoscopic particle image velocimetry (PIV) in conjunction with dynamic pressure transducers at the inlet of a turbocharger compressor with and without a ported shroud. Both stable and unstable operational points are analyzed with use of phase-locked PIV measurements in surge.Detailed description of unstable flow in a centrifugal compressor is presented with the reconstruction of the complex flow structure evolution at the compressor inlet during surge. Rather than one-dimensional, the surge flow is described by a three-dimensional axisymmetric structure of combined entering and exiting swirling flows, alternating in magnitude during the self-excited pressure cycle. The correlation between pressure and velocity measurements shows that the development of compressor unsteadiness is accompanied with swirling reversed flow at the impeller tip. The influence of the ported shroud on the inlet velocity flowfield is seen with the presence of localized flow recirculation. Stability improvement with a ported shroud is thus explained by removing swirling backflow at the impeller inducer tip and recirculating it to the impeller inlet to increase the near shroud inlet blade loading and incidence angle.Copyright

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.