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Publications (10 of 24) Show all publications
Sundström, E., Semlitsch, B. & Mihaescu, M. (2018). Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors. Flow Turbulence and Combustion, 100(3), 705-719
Open this publication in new window or tab >>Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors
2018 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 100, no 3, p. 705-719Article in journal (Refereed) Published
Abstract [en]

Flow instabilities such as Rotating Stall and Surge limit the operating range of centrifugal compressors at low mass-flow rates. Employing compressible Large Eddy Simulations (LES), their generation mechanisms are exposed. Toward low mass-flow rate operating conditions, flow reversal over the blade tips (generated by the back pressure) causes an inflection point of the inlet flow profile. There, a shear-layer induces vortical structures circulating at the compressor inlet. Traces of these flow structures are observed until far downstream in the radial diffuser. The tip leakage flow exhibits angular momentum imparted by the impeller, which deteriorates the incidence angles at the blade tips through an over imposed swirling component to the incoming flow. We show that the impeller is incapable to maintain constant efficiency at surge operating conditions due to the extreme alteration of the incidence angle. This induces unsteady flow momentum transfer downstream, which is reflected as compression wave at the compressor outlet traveling toward the impeller. There, the pressure oscillations govern the tip leakage flow and hence, the incidence angles at the impeller. When these individual self-exited processes occurs in-phase, a surge limit-cycle establishes.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Rotating stall, Surge, Centrifugal compressor, Large Eddy Simulations, LES
National Category
Engineering and Technology
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-218399 (URN)10.1007/s10494-017-9877-z (DOI)000426863700006 ()2-s2.0-85034781553 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20171211

Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2018-04-04Bibliographically approved
Nygård, A., Altimira, M., Semlitsch, B., Prahl Wittberg, L. & Fuchs, L. (2016). Analysis of vortical structures in intermittent jets. In: Springer Proceedings in Physics: . Paper presented at 5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015 (pp. 3-10). Springer Science+Business Media B.V.
Open this publication in new window or tab >>Analysis of vortical structures in intermittent jets
Show others...
2016 (English)In: Springer Proceedings in Physics, Springer Science+Business Media B.V., 2016, p. 3-10Conference paper, Published paper (Refereed)
Abstract [en]

The manipulation of jets has since long been subject to research, due to the wide range of industrial applications in which they are used. A vast number of numerical and experimental studies concerning the physics of the breakup process of continuous jets have been published. Improvements in mixing and ambient gas entrainment have been reported experimentally when using intermittent injection, although the responsible mechanisms have not yet been completely revealed. This work presents a systematic analysis of the mechanisms of jet breakup and mixing with the surrounding fluid and its relation to vorticity generation and transport. Comparisons aremade between the redistribution of vorticity and the engulfment of ambient fluid into the core region for different injection strategies. © Springer International Publishing Switzerland 2016.

Place, publisher, year, edition, pages
Springer Science+Business Media B.V., 2016
Keywords
Industrial research, Mixing, Transport properties, Vorticity, Wakes, Ambient fluids, Ambient gas, Continuous jets, Intermittent injection, Numerical and experimental study, Systematic analysis, Vortical structures, Vorticity generation, Fighter aircraft
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-195137 (URN)10.1007/978-3-319-30602-5_1 (DOI)000387431400001 ()2-s2.0-84978998403 (Scopus ID)9783319306001 (ISBN)
Conference
5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015
Note

Correspondence Address: Nygård, A.; Department of Mechanics, KTHSweden; email: alexander@mech.kth.se. QC 20161116

Available from: 2016-11-16 Created: 2016-11-02 Last updated: 2019-01-28Bibliographically approved
Schickhofer, L., Semlitsch, B. & Mihaescu, M. (2016). Numerical Flow Simulations of a Flexible Plate Attached to an Obstacle in Crossflow. In: : . Paper presented at International Conference on Jets, Wakes and Separated Flows ( ICJWSF2015). Springer
Open this publication in new window or tab >>Numerical Flow Simulations of a Flexible Plate Attached to an Obstacle in Crossflow
2016 (English)Conference paper, Published paper (Refereed)
Abstract [en]

For biomedical applications with relevance to the human upper respiratory tract, the knowledge of the tissue behavior when exposed to a particular flow field would be desired. Moreover, there is of importance to quantify how the tissue properties affects the biomechanics of obstruction. Since in-vivo measurements are often not possible or inappropriate, this is assessed computationally and usually using simplified/idealized geometries.

The present work is devoted to analyze a fluid-structure interaction scenario relevant to snoring and Obstructive Sleep Apnea Syndrome (OSAS). The uncertainty of the solution to the most influential parameters will be assessed, with the aim of quantifying the interplay between the most relevant parameters responsible for tissue self-excitation and obstruction dynamics. A statistical description of the behavior shall be developed. The tissue responsible for snoring in sleep apnea patients (the soft palate) is mimicked in this numerical study by a flexible thin plate anchored to an obstacle. The fluid-structure interaction problem is simulated computationally for several configurations in order to quantify the sensitivity of the investigation parameters onto the flow-field development.

Place, publisher, year, edition, pages
Springer, 2016
Series
Springer Proceedings in Physics, ISSN 0930-8989 ; 185
Keywords
Snoring
National Category
Fluid Mechanics and Acoustics
Research subject
Biological Physics
Identifiers
urn:nbn:se:kth:diva-171230 (URN)10.1007/978-3-319-30602-5_25 (DOI)000387431400025 ()2-s2.0-84978933952 (Scopus ID)978-331930600-1 (ISBN)
Conference
International Conference on Jets, Wakes and Separated Flows ( ICJWSF2015)
Note

QC 20150804. QC 20160226

Available from: 2015-07-24 Created: 2015-07-24 Last updated: 2017-01-10Bibliographically approved
Sundström, E., Semlitsch, B. & Mihaescu, M. (2016). Similarities and differences concerning flow characteristics in centrifugal compressors of different size. In: : . Paper presented at International Conference on Jets, Wakes and Separated Flows ( ICJWSF2015). Springer
Open this publication in new window or tab >>Similarities and differences concerning flow characteristics in centrifugal compressors of different size
2016 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The appropriate choice of an automotive turbocharger compressor for an internal combustion engine is based on the compressor performance, which is commonly specified on a compressor map for different operating conditions. A wide operating range for the compressor covering all possible engine working conditions is desired. However, the application range of the compressor is limited. Different compressor designs are used to fit specific engine requirements. Naturally, these will have rather different characteristic compressor maps. The aim of the present investigation is to explain the differences in the compressor maps by analyzing the compressible flow-fields in two compressor designs from the same manufacturer, intended for a light-duty vehicle (passenger car). The flow-fields are assessed by steady-state Reynolds Averaged Navier-Stokes (RANS) simulations for several operating conditions. Similar flow features are observed near optimal efficiency operating conditions when the flow-field parameters are scaled properly. This study exposes the reason for the different measured operating ranges of the two compressors when ran at the same speed lines.

Place, publisher, year, edition, pages
Springer, 2016
Series
Springer Proceedings in Physics, ISSN 0930-8989 ; 185
Keywords
Centrifugal Compressor, Turbocharger, Fuel Efficiency
National Category
Fluid Mechanics and Acoustics
Research subject
Aerospace Engineering; Energy Technology
Identifiers
urn:nbn:se:kth:diva-171231 (URN)10.1007/978-3-319-30602-5_57 (DOI)000387431400057 ()2-s2.0-84979080235 (Scopus ID)978-331930600-1 (ISBN)
Conference
International Conference on Jets, Wakes and Separated Flows ( ICJWSF2015)
Funder
Swedish Energy Agency
Note

QC 20150805

Available from: 2015-07-24 Created: 2015-07-24 Last updated: 2016-12-22Bibliographically approved
Sundström, E., Semlitsch, B. & Mihaescu, M. (2015). Centrifugal Compressor: The Sound of Surge. In: 21st AIAA/CEAS Aeroacoustics Conference: . Paper presented at 21st AIAA/CEAS Aeroacoustics Conference, Meeting Location: Dallas, TX, 22-25 June 2015 (pp. 1-17).
Open this publication in new window or tab >>Centrifugal Compressor: The Sound of Surge
2015 (English)In: 21st AIAA/CEAS Aeroacoustics Conference, 2015, p. 1-17Conference paper, Published paper (Refereed)
Abstract [en]

When the centrifugal compressor operates at low mass flow rates (close to the unstable operating condition called surge), flow instabilities may develop and severe flow reversal may occur in the wheel passage. Under such conditions, noise generation has been reported resulting in a notable discomfort induced to the passengers in the cabin.

The aim with this study is to predict the flow field associated with a centrifugal compressor and characterize the acoustic near-field generation and propagation under stable and off-design (near-surge) operating conditions. The Large Eddy Simulation (LES) approach is employed. The unsteady features in the flow field leading to acoustic noise generation are quantified by means of statistical averaging, Fourier data analysis and flow mode decomposition techniques. The decomposition method is performed inside the rotating impeller region for several stable and off-design (including surge and near-surge) operating condi- tions. The acoustic near-field data are presented in terms of noise directivity maps and sound pressure level spectra.

For the near-surge condition an amplified broadband feature at two times the frequency of the rotating order of the shaft (possible whoosh noise) was captured. However, an amplified feature around 50% of the rotating order was captured as well. These features are present also during the investigated surge operating conditions, but occur at lower amplitudes as compared with the captured low surge frequency of 43 Hz. 

Keywords
Acoustic waves, Aeroacoustics, Centrifugal compressors, Centrifugation, Flow fields, Fourier series, Large eddy simulation, Decomposition methods, Flow instabilities, Low mass flow rates, Noise generation, Operating condition, Rotating impellers, Sound pressure level, Statistical Averaging, Acoustic noise
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-169882 (URN)10.2514/6.2015-2674 (DOI)2-s2.0-84962514109 (Scopus ID)978-1-62410-367-4 (ISBN)
Conference
21st AIAA/CEAS Aeroacoustics Conference, Meeting Location: Dallas, TX, 22-25 June 2015
Note

QC 20160616

Available from: 2015-06-24 Created: 2015-06-24 Last updated: 2017-11-17Bibliographically approved
Semlitsch, B., Wang, Y. & Mihaescu, M. (2015). Flow effects due to valve and piston motion in an internal combustion engine exhaust port. Energy Conversion and Management, 96, 18-30
Open this publication in new window or tab >>Flow effects due to valve and piston motion in an internal combustion engine exhaust port
2015 (English)In: Energy Conversion and Management, ISSN 0196-8904, E-ISSN 1879-2227, Vol. 96, p. 18-30Article in journal (Refereed) Published
Abstract [en]

Performance optimization regarding e.g. exhaust valve strategies in an internal combustion engine is often performed based on one-dimensional simulation investigation. Commonly, a discharge coefficient is used to describe the flow behavior in complex geometries, such as the exhaust port. This discharge coefficient for an exhaust port is obtained by laboratory experiments at fixed valve lifts, room tem- peratures, and low total pressure drops. The present study investigates the consequences of the valve and piston motion onto the energy losses and the discharge coefficient. Therefore, Large Eddy Simulations are performed in a realistic internal combustion geometry using three different modeling strategies, i.e. fixed valve lift and fixed piston, moving piston and fixed valve lift, and moving piston and moving valve, to estimate the energy losses. The differences in the flow field development with the different modeling approaches is delineated and the dynamic effects onto the primary quantities, e.g. discharge coefficient, are quantified. Considering the motion of piston and valves leads to negative total pressure losses during the exhaust cycle, which cannot be observed at fixed valve lifts. Additionally, the induced flow structures develop differently when valve motion is taken into consideration, which leads to a significant disparity of mass flow rates evolving through the two individual valve ports. However, accounting for piston motion and limited valve motion, leads to a minor discharge coefficient alteration of about one to two percent. 

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Internal combustion engines, Fuel economy, Turbocharged engines, Exhaust gas energy, Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-161210 (URN)10.1016/j.enconman.2015.02.058 (DOI)000353729200003 ()2-s2.0-84924262331 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20150311

Available from: 2015-03-10 Created: 2015-03-10 Last updated: 2017-12-04Bibliographically approved
Semlitsch, B., Mihaescu, M. & Fuchs, L. (2015). Large Eddy Simulation of Fluidic Injection into a Supersonic Convergent-Divergent Duct. In: DLES-9: . Paper presented at Direct and Large-Eddy Simulation 9,April 3 - April 5, 2013, Dresden, Germany. Springer
Open this publication in new window or tab >>Large Eddy Simulation of Fluidic Injection into a Supersonic Convergent-Divergent Duct
2015 (English)In: DLES-9, Springer, 2015Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Springer, 2015
Keywords
Compressible Flow, Nozzle Flow, Shocks
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-149566 (URN)10.1007/978-3-319-14448-1_37 (DOI)2-s2.0-84964878169 (Scopus ID)
Conference
Direct and Large-Eddy Simulation 9,April 3 - April 5, 2013, Dresden, Germany
Note

QC 20140825

Available from: 2014-08-24 Created: 2014-08-24 Last updated: 2017-03-13Bibliographically approved
Sundström, E., Semlitsch, B. & Mihaescu, M. (2014). Assessment of the 3D Flow in a Centrifugal compressor using Steady-State and Unsteady Flow Solvers. In: SAE Technical Paper, 2014-01-2856, 2014: . Paper presented at SAE 2014 International Powertrain, Fuels & Lubricants Meeting,October 20-23, 2014,Birmingham, UK. SAE International
Open this publication in new window or tab >>Assessment of the 3D Flow in a Centrifugal compressor using Steady-State and Unsteady Flow Solvers
2014 (English)In: SAE Technical Paper, 2014-01-2856, 2014, SAE International , 2014Conference paper, Published paper (Refereed)
Abstract [en]

Numerical analysis methods are used to investigate the flow in a ported-shroud centrifugal compressor under different operating conditions, i.e. several mass flow rates at two different speed lines. A production turbocharger compressor is considered, which is widely used in the heavy automotive sector. Flow solutions obtained under steady-state and transient flow assumptions are compared with available experimental data.

The steady-state Reynolds Averaged Navier-Stokes method is used to assess the overall time averaged flow and the global performance parameters. Additionally, the Large Eddy Simulation (LES) approach is employed to capture the transient flow features and the developed flow instabilities at low mass flow rates near the surge line.

The aim of this study is to provide new insights on the flow instability phenomena in the compressor flow near surge. Comparison of flow solutions obtained for near-optimal efficiency and near-surge conditions are carried out. The unsteady features of the flow field are quantified by means of Fourier transformation analysis, Proper Orthogonal Decomposition and Dynamic Mode Decomposition. For a near optimal efficiency set-up the frequency spectra are broad- banded with no distinct instabilities. Close to the surge line, the spectra show a distinct surge cycle frequency, which is due to flow pulsation in the compressor.

The modal flow decomposition elucidates a mode occurring at the surge frequency. The mode explains the oscillating pumping effect occurring during surge. The surface spectra contours reveal the shape of the pressure pulsation during surge and support that a pressure gradient occurs with the oscillating modes found with the modal decomposition. 

Place, publisher, year, edition, pages
SAE International, 2014
Series
SAE Technical Paper, ISSN 0148-7191
Keywords
Compressor Flow, Turbocharger, Internal Combustion Engine
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-152344 (URN)10.4271/2014-01-2856 (DOI)
Conference
SAE 2014 International Powertrain, Fuels & Lubricants Meeting,October 20-23, 2014,Birmingham, UK
Funder
Swedish Energy Agency
Note

QC 20141106

Available from: 2014-09-25 Created: 2014-09-25 Last updated: 2017-11-17Bibliographically approved
Semlitsch, B., Wang, Y. & Mihaescu, M. (2014). Flow effects due to pulsation in an internal combustion engine exhaust port. Energy Conversion and Management, 86, 520-536
Open this publication in new window or tab >>Flow effects due to pulsation in an internal combustion engine exhaust port
2014 (English)In: Energy Conversion and Management, ISSN 0196-8904, E-ISSN 1879-2227, Vol. 86, p. 520-536Article in journal (Refereed) Published
Abstract [en]

In an internal combustion engine, the residual energy remaining after combustion in the exhaust gasses can be partially recovered by a downstream arranged device. The exhaust port represents the passage guiding the exhaust gasses from the combustion chamber to the energy recovering device, e.g. a turbocharger. Thus, energy losses in the course of transmission shall be reduced as much as possible. However, in one-dimensional engine models used for engine design, the exhaust port is reduced to its discharge coefficient, which is commonly measured under constant inflow conditions neglecting engine-like flow pulsation. In this present study, the influence of different boundary conditions on the energy losses and flow development during the exhaust stroke are analyzed numerically regarding two cases, i.e. using simple constant and pulsating boundary conditions. The compressible flow in an exhaust port geometry of a truck engine is investigated using three-dimensional Large Eddy Simulations (LES). The results contrast the importance of applying engine-like boundary conditions in order to estimate accurately the flow induced losses and the discharge coefficient of the exhaust port. The instantaneous flow field alters significantly when pulsating boundary conditions are applied. Thus, the induced losses by the unsteady flow motion and the secondary flow motion are increased with inflow pulsations. The discharge coefficient decreased about 2% with flow pulsation. A modal flow decomposition method, i.e. Proper Orthogonal Decomposition (POD), is used to analyze the coherent structures induced with the particular inflow and outflow conditions. The differences in the flow field for different boundary conditions suggest to incorporate a modeling parameter accounting for the quality of the flow at the turbocharger turbine inlet in one-dimensional simulations.

Place, publisher, year, edition, pages
Elsevier, 2014
Keywords
Internal combustion engines, Fuel economy, Turbocharged engines, Exhaust gas energy, Automotive exhaust systems
National Category
Mechanical Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-134843 (URN)10.1016/j.enconman.2014.06.034 (DOI)000340976900051 ()2-s2.0-84903639552 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20140828

Available from: 2013-11-29 Created: 2013-11-29 Last updated: 2017-12-06Bibliographically approved
Wang, Y., Semlitsch, B., Mihaescu, M. & Fuchs, L. (2014). Flow Induced Energy Losses in the Exhaust Port of an Internal Combustion Engine. Journal of Fluids Engineering - Trancactions of The ASME, 137(1), 011105-01-011105-09
Open this publication in new window or tab >>Flow Induced Energy Losses in the Exhaust Port of an Internal Combustion Engine
2014 (English)In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 137, no 1, p. 011105-01-011105-09Article in journal (Refereed) Published
Abstract [en]

A numerical study of the flow in the exhaust port geometry of a Scania heavy-duty diesel engine is performed using the large eddy simulation (LES) and an unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation approach. The calculations are performed at fixed valve positions and stationary boundary conditions to mimic the setup of an air flow bench experiment, which is commonly used to acquire input data for one-dimensional engine simulations. The numerical results are validated against available experimental data. The complex three-dimensional (3D) flow structures generated in the flow field are qualitatively assessed through visualization and analyzed by statistical means. For low valve lifts, the major source of kinetic energy losses occurs in the proximity of the valve. Flow separation occurs immediately downstream of the valve seat. Strong helical flow structures are observed in the exhaust manifold, which are caused due an interaction of the exhaust port streams in the port geometry.

Place, publisher, year, edition, pages
Elsevier, 2014
Keywords
Exhaust port, Internal Combustion Engine, Large Eddy Simulation
National Category
Mechanical Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-134842 (URN)10.1115/1.4027952 (DOI)000348049600005 ()2-s2.0-84907812254 (Scopus ID)
Note

QC 20140930

Available from: 2013-11-29 Created: 2013-11-29 Last updated: 2017-12-06Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7715-863X

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