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  • 1. Guillou, E.
    et al.
    Gancedo, M.
    DiMicco, R. G.
    Gutmark, E.
    Hellström, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. SAAB Automobile Powertrain, Sweden .
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Mohamed, A.
    Surge characteristics in a ported shroud compressor using PIV measurements and large eddy simulation2010In: 9th International Conference on Turbochargers and Turbocharging - Institution of Mechanical Engineers, Combustion Engines and Fuels Group, 2010, p. 161-170Conference paper (Refereed)
    Abstract [en]

    Compressors operating range is limited at low mass flow by the development of surge. The objective of this research is to study effective operational range for a turbocharger ported shroud compressor typically used in diesel engines. Surge characteristics are assessed by planar flow measurements in the vicinity of the compressor inlet along with numerical computations in the entire compressor geometry. In this paper, satisfying characterization of the compressor instabilities was achieved. Experimental measurements yielded a better understanding of the flow interactions occurring at the compressor entrance and the validation of the computational results in stable regime for this specific model.

  • 2.
    Hellström, Fredirk
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Renberg, Ulrica
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Westin, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Centre for Internal Cumbustion Engine Research Opus, CICERO (closed 20101231).
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Centre for Internal Cumbustion Engine Research Opus, CICERO (closed 20101231).
    Predictions of the Performance of a Radial Turbine with Different Modeling Approaches: Comparison of the Results from 1-D and 3-D CFD2010In: SAE technical paper series, ISSN 0148-7191, no 01-1223Article in journal (Refereed)
    Abstract [en]

    In this paper, the performance of a radial turbine working under pulsatile flow conditions is computed with two different modeling approaches, time resolved 1-dimensional (1-D) and 3-dimensional (3-D) CFD. The 1-D modeling approach is based on measured turbine maps which are used to compute the mass flow rate and work output from the turbine for a given expansion ratio and temperature at the inlet. The map is measured under non-pulsatile flow conditions, and in the 1-D method the turbine is treated as being a quasi-stationary flow device. In the 3-D CFD approach, a Large Eddy Simulation (LES) turbulence approach is used. The objective of LES is to explicitly compute the large scales of the turbulence while modeling the effects of the unresolved scales.

    Three different cases are considered, where the simplest case only consist of the turbine and the most complex case consist of an exhaust manifold and the turbine. Both time resolved data, such as pressure ratio, temperature and shaft torque and time mean data from the two different modeling approaches are compared. The results show that the computed time mean shaft power differs between the two different modeling approaches with as much as 100%. Since the considered operation point for the engine in this study is 1500 rpm with wide open throttle, the turbine operates in an area where the turbine map is extrapolated. Only by using a few operation points from CFD to extend the map, an improvement is achieved for the 1-D results, but still the deviation is large. Also, the pressure ratio and temperature drop over the turbine differs for the used modeling approaches. The causes for the deviations are assessed and discussed to get a better understanding of eventually limitations of the 1-D modeling approach.

  • 3.
    Hellström, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical computations of the unsteady flow in a radial turbine2008Licentiate 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.

  • 4.
    Hellström, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Numerical computations of the unsteady flow in turbochargers2010Doctoral 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.

  • 5.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. SAAB Automobile Powertrain, Sweden .
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Assessment of Heat Transfer Effects on the Performance of a Radial Turbine using Large Eddy Simulation2010In: 9th International Conference on Turbochargers and Turbocharging - Institution of Mechanical Engineers, Combustion Engines and Fuels Group, 2010, p. 281-291Conference 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.

  • 6.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Centres, Centre for Internal Cumbustion Engine Research Opus, CICERO. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Effects of inlet conditions on the turbine performance of a radial turbine2008In: PROCEEDINGS OF THE ASME TURBO EXPO 2008: Power for Land, Sea and Air, 2008, Berlin, New York: AMER SOC MECHANICAL ENGINEERS , 2008, p. 1985-2001Conference 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.

  • 7.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Heat transfer effects on the performance of a radial turbine working under pulsatile flow conditions2010In: 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010, p. 2010-0903-Conference 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.

  • 8.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical computation of the pulsatile flow in a turbocharger with realistic inflow conditions from an exhaust manifold2009In: Proceedings of ASME Turbo Expo 2009, 2009, no PART B, p. 1317-1329Conference 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 mis 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 mass flow 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 me wheel which affects the shaft power output from the turbine.

  • 9.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Centre for Internal Cumbustion Engine Research Opus, CICERO (closed 20101231). KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Centres, Centre for Internal Cumbustion Engine Research Opus, CICERO (closed 20101231). KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical computations of pulsatile flow in a turbo-charger2008In: 46th AIAA Aerospace Sciences Meeting and Exhibit, 2008Conference 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.

  • 10.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Numerical computations of steady and unsteady flow in bended pipes2007In: Collection of Technical Papers - 37th AIAA Fluid Dynamics Conference: 37th Fluid Dynamics Conference, Miami Fl, 2007, p. 1850-1859Conference 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.

  • 11.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Numerical computations of the pulsatile flow in a turbocharger with realistic inflow conditions from an exhaust manifold2009In: ASME Turbo Expo 2009: Power for Land, Sea and Air, 2009Conference 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.

  • 12.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    ON NUMERICAL COMPUTATIONS OF THE UNSTEADY FLOW FIELD IN A RADIALTURBINE: ASSESSMENT AND VALIDATION OF DIFFERENT MODELINGSTRATEGIESManuscript (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.

  • 13.
    Hellström, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Gutmark, Ephraim
    Department of Aerospace, Engineering and Engineering Mechanics, University of Cincinnati.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Large Eddy Simulation of the Unsteady Flow in a Radial Compressor Operating Near Surge2012In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 134, no 5, p. 051006-Article in journal (Refereed)
    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.

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