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Numerical computations of the unsteady flow in a radial turbine
KTH, School of Engineering Sciences (SCI), Mechanics.
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 [en]
Pulsatile flow, radial turbines, pipe flow, effects of inlet conditions
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-4660ISBN: 978-91-7178-906-8 (print)OAI: oai:DiVA.org:kth-4660DiVA: diva2:13299
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
List of papers
1. Numerical computations of steady and unsteady flow in bended pipes
Open this publication in new window or tab >>Numerical computations of steady and unsteady flow in bended pipes
2007 (English)In: Collection of Technical Papers - 37th AIAA Fluid Dynamics Conference: 37th Fluid Dynamics Conference, Miami Fl, 2007, 1850-1859 p.Conference paper, Published paper (Refereed)
Abstract [en]

The steady and pulsative turbulent flows in curved pipes have been computed with two different modeling approaches; the Reynolds Averaged Navier-Stokes (RANS) technique and Large Eddy Simulations (LES). The results from computations of the flow in a single bended pipe have been compared to experimental data. The comparisons show poor agreement for the RANS technique at the exit of the bend, while the LES computations show better agreement with the measured velocity profiles. LES in contrast to RANS, can also provide much more details about the dynamics of the flow. It is also shown that small uncertainties in the inlet boundary conditions can result in significant variations in the flow field. Different types of small amplitude secondary flow at the inlet affect the flow downstream of the bend. The approach enables one to state that for the experiments under consideration the lack of data on the secondary flow prevents a direct validation of the LES results. The pulsatile flow in a double bended pipe has also been investigated and the vortex cores are visualized to enable a better insight into the unsteady flow field and the effects of the inflow pulsations.

Keyword
Pipe flow, Dean vortices, pulsatile flow, Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-12732 (URN)2-s2.0-35649003022 (Scopus ID)978-156347897-0 (ISBN)1563478978 (ISBN)
Conference
37th AIAA Fluid Dynamics Conference; Miami, FL; United States; 25 June 2007 through 28 June 2007
Note

QC20100622

Available from: 2010-05-07 Created: 2010-05-07 Last updated: 2014-11-12Bibliographically approved
2. Numerical computations of pulsatile flow in a turbo-charger
Open this publication in new window or tab >>Numerical computations of pulsatile flow in a turbo-charger
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.

Series
AIAA, 2008-735
Keyword
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:nbn:se:kth:diva-8074 (URN)2-s2.0-78249258744 (Scopus ID)978-156347937-3 (ISBN)
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
3. Effects of inlet conditions on the turbine performance of a radial turbine
Open this publication in new window or tab >>Effects of inlet conditions on the turbine performance of a radial turbine
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
Series
GT2008-51008
Keyword
Turbocharger, radial turbine, performance, inlet perturbations, Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-12734 (URN)10.1115/GT2008-51088 (DOI)000262646901018 ()2-s2.0-69949153516 (Scopus ID)978-0-7918-4316-1 (ISBN)
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

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