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Effects of inlet conditions on the turbine performance of a radial turbine
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
KTH, School of Engineering Sciences (SCI), Centres, Centre for Internal Cumbustion Engine Research Opus, CICERO. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
2008 (English)In: PROCEEDINGS OF THE ASME TURBO EXPO 2008: Power for Land, Sea and Air, 2008, Berlin, New York: AMER SOC MECHANICAL ENGINEERS , 2008, 1985-2001 p.Conference paper, Published paper (Refereed)
Abstract [en]

For a turbocharger working under internal combustion engine operating conditions, the flow will be highly pulsatile and the efficiency of the radial turbine will vary during the engine cycle. In addition to effects of the inflow unsteadiness, there is also always a substantial unsteady secondary flow component at the inlet to the turbine depending on the geometry upstream. These secondary motions may consist of swirl, Dean vortices and other cross-sectional velocity components formed in the exhaust manifold. The strength and the direction of the vortices vary in time depending on the unsteady flow in the engine exhaust manifold, the engine speed and the geometry of the manifold itself. The turbulence intensity may also vary during the engine cycle leading to a partially developed turbulent flow field. The effect of the different perturbations on the performance of a radial nozzle-less turbine is assessed and quantified by using Large Eddy Simulations. The turbine wheel is handled using a sliding mesh technique, whereby the turbine wheel, with its grid is rotating, while the turbine house and its grid are kept stationary. The turbine performance has been compared for several inflow conditions. The results show that an inflow-condition without any perturbations gives the highest shaft power output, while a turbulent flow with a strongly swirling motion at the inlet results in the lowest power output. An unexpected result is that a turbulent inflow yields a lower shaft power than a turbulent inflow with a secondary flow formed by a pair of Dean vortices. The flow field for the different cases is investigated to give a better insight into the unsteady flow field and the effects from the different inlet conditions.

Place, publisher, year, edition, pages
New York: AMER SOC MECHANICAL ENGINEERS , 2008. 1985-2001 p.
Series
GT2008-51008
Keyword [en]
Turbocharger, radial turbine, performance, inlet perturbations, Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-12734DOI: 10.1115/GT2008-51088ISI: 000262646901018Scopus ID: 2-s2.0-69949153516ISBN: 978-0-7918-4316-1 (print)OAI: oai:DiVA.org:kth-12734DiVA: diva2:318435
Conference
53rd ASME Turbo Expo 2008 Berlin, GERMANY, JUN 09-13, 2008
Note
QC 20100622Available from: 2010-05-07 Created: 2010-05-07 Last updated: 2011-09-07Bibliographically 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
2. Numerical computations of the unsteady flow in a radial turbine
Open this publication in new window or tab >>Numerical computations of the unsteady flow in a radial turbine
2008 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Non-pulsatile and pulsatile flow in bent pipes and radial turbine has been assessed with numerical simulations. The flow field in a single bent pipe has been computed with different turbulence modelling approaches. A comparison with measured data shows that Implicit Large Eddy Simulation (ILES) gives the best agreement in terms of mean flow quantities. All computations with the different turbulence models qualitatively capture the so called Dean vortices. The Dean vortices are a pair of counter-rotating vortices that are created in the bend, due to inertial effects in combination with a radial pressure gradient. The pulsatile flow in a double bent pipe has also been considered. In the first bend, the Dean vortices are formed and in the second bend a swirling motion is created, which will together with the Dean vortices create a complex flow field downstream of the second bend. The strength of these structures will vary with the amplitude of the axial flow. For pulsatile flow, a phase shift between the velocity and the pressure occurs and the phase shift is not constant during the pulse depending on the balance between the different terms in the Navier- Stokes equations.

The performance of a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has also been investigated by using ILES. To assess the effect of pulsatile inflow conditions on the turbine performance, three different cases have been considered with different frequencies and amplitude of the mass flow pulse and different rotational speeds of the turbine wheel. The results show that the turbine cannot be treated as being quasi-stationary; for example, the shaft power varies with varying frequency of the pulses for the same amplitude of mass flow. The pulsatile flow also implies that the incidence angle of the flow into the turbine wheel varies during the pulse. For the worst case, the relative incidence angle varies from approximately −80° to +60°. A phase shift between the pressure and the mass flow at the inlet and the shaft torque also occurs. This phase shift increases with increasing frequency, which affects the accuracy of the results from 1-D models based on turbine maps measured under non-pulsatile conditions.

For a turbocharger working under internal combustion engine conditions, the flow into the turbine is pulsatile and there are also unsteady secondary flow components, depending on the geometry of the exhaust manifold situated upstream of the turbine. Therefore, the effects of different perturbations at the inflow conditions on the turbine performance have been assessed. For the different cases both turbulent fluctuations and different secondary flow structures are added to the inlet velocity. The results show that a non-disturbed inlet flow gives the best performance, while an inflow condition with a certain large scale eddy in combination with turbulence has the largest negative effect on the shaft power output.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. viii, 67 p.
Series
Trita-MEK, ISSN 0348-467X ; 2008:02
Keyword
Pulsatile flow, radial turbines, pipe flow, effects of inlet conditions
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-4660 (URN)978-91-7178-906-8 (ISBN)
Presentation
2008-03-28, E3, Osquarsbacke 14,, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20101111Available from: 2008-03-06 Created: 2008-03-06 Last updated: 2010-11-11Bibliographically approved

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