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Numerical computations of pulsatile flow in a turbo-charger
KTH, School of Engineering Sciences (SCI), Centres, Centre for Internal Cumbustion Engine Research Opus, CICERO (closed 20101231). KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Centres, Centre for Internal Cumbustion Engine Research Opus, CICERO (closed 20101231). KTH, School of Engineering Sciences (SCI), Mechanics.
2008 (English)In: 46th AIAA Aerospace Sciences Meeting and Exhibit, 2008Conference paper, Published paper (Refereed)
Abstract [en]

The non-pulsatile and pulsatile three-dimensional flow in the turbine part of a radial turbo-charger have been computed with different modeling approaches for the turbulence; using the Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES). The performance of the turbine for the non-pulsatile computations have been compared with measured performance for the same geometry and the computations slightly over predict the pressure ratio and the shaft power for a given mass flow. The discrepancy between the measured and computed turbine performance can be attributed, among others, to uncertainties in the walls boundary conditions (i.e. using smooth and adiabatic), and in the inflow conditions in addition to the uncertainty in the bearing losses which are included in the shaft power in the measured data. To asses the effect of inlet condition three different cases with different frequencies and mass flow pulses have been considered. A comparison of the computed shaft power with results from a one-dimensional engine simulation code shows fairly good agreement. The computations also shows that the mass flow and pressure is out of phase, and the phase shift is not constant during the engine cycle, which also affects the calculated isentropic efficiency. The flow field in turbine has been further studied and the vortex cores are visualized to give a better insight into the unsteady flow field and the effects of the inflow pulsations.

Place, publisher, year, edition, pages
2008.
Series
AIAA, 2008-735
Keyword [en]
Bearing loss, Different frequency, Engine simulation, Inflow conditions, Inlet conditions, Isentropic efficiency, Mass flow, Measured data, Modeling approach, Numerical computations, Out of phase, Pressure ratio, Reynolds-Averaged Navier-Stokes, Shaft power, Three-dimensional flow, Turbine parts, Turbine performance, Turbo charger, Vortex cores
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-8074Scopus ID: 2-s2.0-78249258744ISBN: 978-156347937-3 (print)OAI: oai:DiVA.org:kth-8074DiVA: diva2:13297
Conference
46th AIAA Aerospace Sciences Meeting and Exhibit; Reno, NV; United States; 7 January 2008 through 10 January 2008
Note

QC 20101111

Available from: 2008-03-06 Created: 2008-03-06 Last updated: 2014-10-10Bibliographically approved
In thesis
1. 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
2. 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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