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Numerical computations of the unsteady flow in turbochargers
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
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 [en]
Turbochargers, turbine, compressor, unsteady pulsatile flow, perturbations, Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-12742ISBN: 978-91-7415-632-4 (print)OAI: oai:DiVA.org:kth-12742DiVA: diva2:318494
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
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
4. Numerical computations of the pulsatile flow in a turbocharger with realistic inflow conditions from an exhaust manifold
Open this publication in new window or tab >>Numerical computations of the pulsatile flow in a turbocharger with realistic inflow conditions from an exhaust manifold
2009 (English)In: ASME Turbo Expo 2009: Power for Land, Sea and Air, 2009Conference paper, Published paper (Refereed)
Abstract [en]

The combined effect of different secondary perturbations at the turbine inlet and the pulsatile flow on the turbine performance was assessed and quantified by using Large Eddy Simulation. The geometrical configuration consists of a 4-1 exhaust manifold and a radial turbine. At the inlet to each port of the manifold, engine realistic pulsatile mass flow and temperature fields are specified. The turbine used in this numerical study is a vaneless radial turbine with 9 blades, with a size that is typical for a turbocharger mounted on a 2.0 liters IC engine of passenger cars. The flow field is investigated and the generated vortices are visualized to enable a better insight into the unsteady flow field. Correlations between the turbine inflow conditions, such as massflow rate, strength of secondary flow components, and the turbine performance have also been studied. The results show that the flow field entering the turbine is heavily disturbed with strong secondary flow components and disturbed axial velocity profile. Between the inlet to the turbine and the wheel, the strength of the secondary flow and the level of the disturbances of the axial flow decrease which gives large losses in this region. Even though the magnitude of the disturbances decrease, the flow entering the wheel will still be disturbed, resulting in a perturb inlet flow to the wheel which affects the shaft power output from the turbine.

Series
GT2009-59619
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-12737 (URN)
Conference
ASME Turbo Expo 2009, Power for Land, Sea and Air
Note
QC 20100617Available from: 2010-05-07 Created: 2010-05-07 Last updated: 2010-06-22Bibliographically approved
5. Heat transfer effects on the performance of a radial turbine working under pulsatile flow conditions
Open this publication in new window or tab >>Heat transfer effects on the performance of a radial turbine working under pulsatile flow conditions
2010 (English)In: 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010, 2010-0903- p.Conference paper, Published paper (Refereed)
Abstract [en]

Turbo charging of internal combustion in general and spark ignited engine in particular has become more common the last decay since it reduces fuel consumption and emissions. The turbine works under highly unsteady flow conditions, since the exhaust flow entering the turbine is pulsatile and turbulent with a varying strength of the axial and secondary flow components. Heat transfer from the fluid to the turbine housing is different for a pulsatile flow compared to a non-pulsatile flow. A numerical study with Large Eddy Simulation is applied to study the effects of heat transfer at the walls on the turbine performance working under pulsatile flow conditions. Two cases are considered, one with adiabatic walls and another with heat transfer at the walls. The flow field is assessed with focus on the differences between the two cases. The results show that difference in the shaft power is small, although the temperature distribution in the turbine is different for the two cases. The mechanisms for these differences in the flow field are assessed and discussed.

Keyword
Adiabatic wall, Exhaust flow, Flow condition, Heat transfer effects, Internal combustion, Numerical studies, Radial turbines, Shaft power, Spark-ignited engines, Turbine housings, Turbine performance
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-12738 (URN)2-s2.0-78649843392 (Scopus ID)978-160086739-2 (ISBN)
Conference
48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition; Orlando, FL; United States; 4 January 2010 through 7 January 2010
Note

QC 20140829

Available from: 2010-05-07 Created: 2010-05-07 Last updated: 2014-08-29Bibliographically approved
6. Assessment of Heat Transfer Effects on the Performance of a Radial Turbine using Large Eddy Simulation
Open this publication in new window or tab >>Assessment of Heat Transfer Effects on the Performance of a Radial Turbine using Large Eddy Simulation
2010 (English)In: 9th International Conference on Turbochargers and Turbocharging - Institution of Mechanical Engineers, Combustion Engines and Fuels Group, 2010, 281-291 p.Conference paper, Published paper (Refereed)
Abstract [en]

One way to reduce fuel consumption and emissions is to downsize the engine in combination with turbo-charging. The turbine works under highly unsteady flow conditions, since the exhaust flow is pulsatile, turbulent and with a varying strength of the axial and secondary flow components. The heat transfer from the fluid to the turbine housing will be different for a pulsatile flow compared to a non-pulsatile flow. Therefore, the effects of heat transfer at the walls on the turbine performance working under pulsatile flow conditions are assessed and quantified by performing a numerical study with Large Eddy Simulation. Two cases are considered, one case with adiabatic walls and one case with heat transfer at the walls. The results show that the difference in the obtained shaft power is small. Even the differences in the time mean efficiency is small, it only differs with 2 percent units, even though the heat transferred to surroundings is as large as approximately 60 percent of the delivered shaft power.

Keyword
Adiabatic wall, Exhaust flow, Flow condition, Heat transfer effects, Numerical studies, Radial turbines, Shaft power, Turbine housings, Turbine performance
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-12739 (URN)2-s2.0-79959600664 (Scopus ID)
Conference
9th International Conference on Turbochargers and Turbocharging; Westminster, London; United Kingdom; 19 May 2010 through 20 May 2010
Note

QC20100622

Available from: 2010-05-07 Created: 2010-05-07 Last updated: 2017-04-28Bibliographically approved
7. ON NUMERICAL COMPUTATIONS OF THE UNSTEADY FLOW FIELD IN A RADIALTURBINE: ASSESSMENT AND VALIDATION OF DIFFERENT MODELINGSTRATEGIES
Open this publication in new window or tab >>ON NUMERICAL COMPUTATIONS OF THE UNSTEADY FLOW FIELD IN A RADIALTURBINE: ASSESSMENT AND VALIDATION OF DIFFERENT MODELINGSTRATEGIES
(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:nbn:se:kth:diva-12740 (URN)
Note
QC20100622Available from: 2010-05-07 Created: 2010-05-07 Last updated: 2010-06-22Bibliographically approved
8. Large Eddy Simulation of the Unsteady Flow in a Radial Compressor Operating Near Surge
Open this publication in new window or tab >>Large Eddy Simulation of the Unsteady Flow in a Radial Compressor Operating Near Surge
2012 (English)In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 134, no 5, 051006- p.Article in journal (Refereed) Published
Abstract [en]

The flow in a centrifugal compressor has been computed using large eddy simulation (LES). The investigated geometry is that of a ported shroud compressor with a 10 blade impeller with an exducer diameter of 88 mm. The computational data compares favorably with measured data for the same compressor and operational point. For the considered operational point near surge, the flow field in the entire compressor stage is unsteady. Back-flow occurs in the diffuser, wheel, and the ported shroud channels resulting in back-flow at the walls in the inlet region of the compressor. In the diffuser and volute, the flow is highly unsteady with perturbations that are convected around the volute, affecting the flow field in most of the entire compressor. The mechanism driving this unsteadiness is assessed by flow visualizations, frequency analysis, and correlations of pressure and velocity data in order to gain a more comprehensive understanding of the mechanism leading to stall and surge.

Keyword
Blade impeller, Compressor stage, Computational data, Frequency Analysis, Measured data, Radial compressors
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-12741 (URN)10.1115/1.4003816 (DOI)000308404500006 ()2-s2.0-84860681469 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20120621. Updated from manuscript to article in journal.

Available from: 2010-05-07 Created: 2010-05-07 Last updated: 2017-12-12Bibliographically approved

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Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
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