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ON NUMERICAL COMPUTATIONS OF THE UNSTEADY FLOW FIELD IN A RADIALTURBINE: ASSESSMENT AND VALIDATION OF DIFFERENT MODELINGSTRATEGIES
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Today, more advanced turbocharging techniques are used together with downsizing to meet future emission legislations. To be able to keep the development costs on a reasonable level, and to be able to assess complex heat transfer and flow phenomena in the turbocharger, numerical simulations are often used. When using these kinds of tools, it is very important to verify the computed results with measured results. In this study, computed global results are compared with measured data for a radial turbine.The size of the radial turbine is typical for a turbocharger used on a two liter gasoline engine for a passenger car. Different turbulence modeling strategies, the RANS and LES approach, were used. Also, two different modeling approaches for the turbine wheel were used, the sliding mesh technique and the Rotational Reference Frame technique. In order to get the correct inflow conditions to the numerical simulations, PIV measurement of the flow entering the turbine have been performed. The measurements were performed in the new gas-stand at SAAB Automobile, Sweden AB in Trollhttan.The results show that the most advanced technique with sliding mesh and LES gave the best agreement with the measurements.The computed flow field in the turbine is assessed, both with focus on obtaining a deeper knowledge of the transonic flow in the turbine and to assess the differences for the computed flowfield with the different modeling strategies.

National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-12740OAI: oai:DiVA.org:kth-12740DiVA: diva2:318480
Note
QC20100622Available from: 2010-05-07 Created: 2010-05-07 Last updated: 2010-06-22Bibliographically approved
In thesis
1. Numerical computations of the unsteady flow in turbochargers
Open this publication in new window or tab >>Numerical computations of the unsteady flow in turbochargers
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Turbocharging the internal combustion (IC) engine is a common technique to increase the power density. If turbocharging is used with the downsizing technique, the fuel consumption and pollution of green house gases can be decreased. In the turbocharger, the energy of the engine exhaust gas is extracted by expanding it through the turbine which drives the compressor by a shaft. If a turbocharged IC engine is compared with a natural aspirated engine, the turbocharged engine will be smaller, lighter and will also have a better efficiency, due to less pump losses, lower inertia of the system and less friction losses. To be able to further increase the efficiency of the IC engine, the understanding of the highly unsteady flow in turbochargers must be improved, which then can be used to increase the efficiency of the turbine and the compressor. The main objective with this thesis has been to enhance the understanding of the unsteady flow in turbocharger and to assess the sensitivity of inflow conditions on the turbocharger performance.

The performance and the flow field in a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has been assessed by using Large Eddy Simulation (LES). To assess the effects of different operation conditions on the turbine performance, different cases have been considered with different perturbations and unsteadiness of the inflow conditions. Also different rotational speeds of the turbine wheel were considered. The results show that the turbine cannot be treated as being quasi-stationary; for example,the shaft power varies for different frequencies of the pulses for the same amplitude of mass flow. The results also show that perturbations and unsteadiness that are created in the geometry upstream of the turbine have substantial effects on the performance of the turbocharger. All this can be summarized as that perturbations and unsteadiness in the inflow conditions to the turbine affect the performance.

The unsteady flow field in ported shroud compressor has also been assessed by using LES for two different operational points. For an operational point near surge, the flow field in the entire compressor stage is unsteady, where the driving mechanism is an unsteadiness created in the volute. For an operational point far away from surge, the flow field in the compressor is relatively much more steady as compared with the former case. Although the stable operational point exhibits back-flow from the ported shroud channels, which implies that the flow into the compressor wheel is disturbed due to the structures that are created in the shear layer between the bulk flow and the back-flow from the ported shroud channels.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. vi, 100 p.
Series
Trita-MEK, ISSN 0348-467X ; 2010:03
Keyword
Turbochargers, turbine, compressor, unsteady pulsatile flow, perturbations, Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-12742 (URN)978-91-7415-632-4 (ISBN)
Public defence
2010-05-26, F3, Lindsedsv, 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC20100622Available from: 2010-05-10 Created: 2010-05-07 Last updated: 2010-11-11Bibliographically approved

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