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Licentiate seminar

Centrifugal compressor flow instabilities at low mass flow rate


Defendant Main Advisor Extra Advisor Date
Elias Sundström Mihai Mihaescu Laszlo Fuchs 2016-04-28

Opponent
Håkan Nilsson, Chalmers Univ. Tech.

Evaluation committee
Anders Dahlkild, KTH, Mekanik

Abstract

Turbochargers play an important role in increasing the energetic efficiency and reducing emissions of modern power-train systems based on downsized recipro- cating internal combustion engines (ICE). The centrifugal compressor in tur- bochargers is limited at off-design operating conditions by the inception of flow instabilities causing rotating stall and surge. They occur at reduced engine speeds (low mass flow rates), i.e. typical operating conditions for a better engine fuel economy, harming ICEs efficiency. Moreover, unwanted unsteady pressure loads within the compressor are induced; thereby lowering the com- pressors operating life-time. Amplified noise and vibration are also generated, resulting in a notable discomfort. The thesis aims for a physics-based understanding of flow instabilities and the surge inception phenomena using numerical methods. Such knowledge may permit developing viable surge control technologies that will allow turbocharg- ers to operate safer and more silent over a broader operating range. Therefore, broadband turbulent enabled compressible Large Eddy Simulation (LES) cal- culations have been performed and several flow-driven instabilities have been captured under unstable conditions. LES produces large amounts of 3D data which has been post-processed using Fourier spectra and Dynamic Mode De- composition (DMD). These techniques are able to quantify modes in the flow field by extracting large coherent flow structures and characterize their relative contribution to the total fluctuation energy at associated. Among the main findings, a dominant mode was found which describes the filling and emptying process during surge. A narrowband feature at half of the rotating order was identified to correspond to co-rotating vortices upstream of the impeller face as well as elevated velocity magnitude regions propagating tangentially in the diffuser and the volute. Dominant mode shapes were also found at the rotating order frequency and its harmonics, which manifest as a spinning mode shape localized at the diffuser inlet. From the compressible LES flow solution one can extract the acoustic infor- mation and the noise affiliated with the compressor. This enable through data correlation quantifying the flow-acoustics coupling phenomena in the compres- sor. Detailed comparison of flow (pressure, velocity) and aeroacoustics (sound pressure levels) predictions in terms of time-averaged, fluctuating quantities, and spectra is carried out against experimental measurements.
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