| 1. |
- Guillou, E., et al.
(författare)
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Surge characteristics in a ported shroud compressor using PIV measurements and large eddy simulation
- 2010
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Ingår i: 9th International Conference on Turbochargers and Turbocharging - Institution of Mechanical Engineers, Combustion Engines and Fuels Group. ; , s. 161-170
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 2. |
- Hellström, F., et al.
(författare)
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Stall development in a ported shroud compressor using PIV measurements and Large Eddy Simulation
- 2010
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Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- Surge is a phenomenon that limits the operational range of the compressor at low mass flow rates. The objective of this research is to study effective operational range for a ported shroud compressor. The size of the compressor is typical for a turbocharger used on diesel engines. To be able to extend the operational range, the surge characteristics have to be assessed. This is done by performing measurement of the flow at the inlet to the compressor wheel and pressure fluctuations at the inlet and outlet of the compressor housing. Detailed numerical computations of the flow in the entire compressor section under similar operating conditions have also been carried out. The experimental work includes Particle Imaging Velocimetry (PIV) measurements of the instantaneous and mean velocity field at the inlet. At surge, low frequency pulsations are detected that seem to result from back flow already observed in stall. The numerical computations include details of the flow by having good spatial and temporal resolution and using Large Eddy Simulations (LES) to account for the turbulence. LES is most suitable for the surge flow since it resolves the large scale structures, such as flow separation and reversed flow, which characterize surge.
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| 3. |
- Prahl Wittberg, Lisa, et al.
(författare)
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Effects of aortic irregularities on blood flow
- 2016
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Ingår i: Biomechanics and Modeling in Mechanobiology. - : Springer. - 1617-7959 .- 1617-7940. ; 15:2
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Tidskriftsartikel (refereegranskat)abstract
- Anatomic aortic anomalies are seen in many medical conditions and are known to cause disturbances in blood flow. Turner syndrome (TS) is a genetic disorder occurring only in females where cardiovascular anomalies, particularly of the aorta, are frequently encountered. In this study, numerical simulations are applied to investigate the flow characteristics in four TS patient- related aortic arches (a normal geometry, dilatation, coarctation and elongation of the transverse aorta). The Quemada viscosity model was applied to account for the non-Newtonian behavior of blood. The blood is treated as a mixture consisting of water and red blood cells (RBC) where the RBCs are modeled as a convected scalar. The results show clear geometry effects where the flow structures and RBC distribution are significantly different between the aortas. Transitional flow is observed as a jet is formed due to a constriction in the descending aorta for the coarctation case. RBC dilution is found to vary between the aortas, influencing the WSS. Moreover, the local variations in RBC volume fraction may induce large viscosity variations, stressing the importance of accounting for the non-Newtonian effects.
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| 4. |
- Ahn, Myeonghwan, et al.
(författare)
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Flow and Near-field Pressure Fluctuations of Twin Square Jets
- 2021
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Ingår i: AIAA Propulsion and Energy Forum, 2021. - Reston, Virginia : American Institute of Aeronautics and Astronautics Inc, AIAA.
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Konferensbidrag (refereegranskat)abstract
- We aim to investigate the aerodynamic and acoustics characteristics of a twin square jet using an implicit Large Eddy Simulation (ILES). A screeching cold jet condition, a nozzle pressure ratio (NPR) of 3.0, is considered to simulate a coupled twin-jet. A second-order central scheme with a modified version of Jameson’s artificial dissipation is adopted to damp numerical oscillations and to mimic the effect of small-scale turbulence without an explicit subgrid-scale (SGS) model. Numerical results show that the overall trends of time-averaged streamwise velocity profiles are similar to the experimental data, with the largest differences observed at locations associated with the presence of the shock-cell structures. A detailed investigation of the flow fluctuations in jet shear layers is performed. The amplitude of the velocity fluctuations is highly dependent on the location of the shear layers with respect to the twin-jet configuration (upper, lateral, or inner). The coupling mode of twin jets associated with the screech tone is determined as a symmetrical flapping mode be a two-points spacetime cross-correlation analysis. The overall trends of near-field pressure fluctuation spectra by LES agree well with the experimental results in both upstream and downstream regions. Near-field pressure fluctuation spectra by ILES agree well with the experimentally obtained spectra at different locations in the nozzle exit plane as well as at several downstream locations in the near-field acoustic region. The highest screech tone is observed at the inter-nozzle region where superposition of in-phase waves and standing waves are found. Fourier phase and amplitude fields at the fundamental frequency also confirm the symmetrical flapping mode of the twin jets by showing in-phase relations of hydrodynamic/acoustic waves and noise directivities.
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| 5. |
- Duwig, Christophe, et al.
(författare)
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Experimental and numerical study of flameless combustion in a model gas turbine combustor
- 2008
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:2, s. 279-295
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Tidskriftsartikel (refereegranskat)abstract
- Flameless combustion is an attractive solution to address existing problems of emissions and stability when operating gas turbine combustors. Theoretical, numerical and experimental approaches were used to study the flameless gas turbine combustor. The emissions and combustion stability were measured and the limits of the flameless regime are discussed. Using experimental techniques and Large Eddy Simulation (LES), detailed knowledge of the flow field and the oxidation dynamics was obtained. In particular the relation between the turbulent coherent structures dynamics and the flameless oxidation was highlighted. A model for flameless combustion simulations including detailed chemistry was derived. The theoretical analysis of the flameless combustion provides 2 non-dimensional numbers that define the range of the flameless mode. It was determined that the mixture that is ignited and burnt is composed of similar to 50% of fresh gases and similar to 50% vitiated gases.
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| 6. |
- Ghulam, M. M., et al.
(författare)
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Characterization of Non-reacting Swirling Flow in a Gas Turbine Fuel Injector
- 2021
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Ingår i: AIAA Scitech 2021 Forum. - Reston, Virginia : American Institute of Aeronautics and Astronautics (AIAA). ; , s. 1-22
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Konferensbidrag (refereegranskat)abstract
- The current work investigates the swirling flow of a gas fuel injector utilized in the Lean Direct Injection (LDI) combustion system. Planer particle image velocimetry (PIV) measurements and large eddy simulation (LES) numerical analysis are conducted to have a profound understanding of the swirling flow characteristics. Specifically, the impacts of the level of confinement with a rectangular cross-section and different Reynolds number are examined. Increasing the Reynolds number increases the strength of swirling jets and reverse flow region. More significant changes occurred on the mean flowfield due to the confinement effect such as increasing the width of the reverse flow region and increasing/decreasing the size of the recirculation zones which in turn effects the inlet jet penetration. The inlet jet spreads at a larger angle as the size of the outer recirculation zone (ORZ) increases with the confinement ratio. The shape of the inner recirculation zone (IRZ) vortex structure on the unconfined flow is characterized to be a thin and short vortex and located on top of the nozzle exit, and it becomes thicker and longer vortex located further downstream from the nozzle exit upon confinement. The increased size of the IRZ vortex structure in confined cases is an indication of the increased thickness of the inner shear layer (ISL) that increases linearly as the confinement ratio increases. LES results reveled there is a connection channel between the reverse flow region and the ORZ of the swirling flow emanating from the multiple-jet LDI nozzle. Higher level of turbulence is associated with the location of the IRZ vortex structure. Proper orthogonal decomposition (POD) analysis is preformed to extract coherent fluctuating flow features. The swirling flow of the LDI nozzle exhibits the single-helical and double-helical precessing vortex core (PVC) modes, with the first one being the most energetic mode. The general flow structure of the coherent single-helix PVC mode on the unconfined flow consists of four vortices: two corner vortices rotating in opposite of each other, and a tiny vortex on top of the nozzle exit followed by a huge central vortex rotating in a different direction. Upon confinement the outer vortices attached to the wall of the combustor and the central vortex becomes about twice bigger. The preexistence of the outer vortices on the unconfined flow suggests that the formation of the ORZ is not caused by the confinement, but rather it is a part of the natural behavior of a highly turbulent swirling flow which magnified in the case of confined environment. The single-helix PVC mode gains higher energy value and becomes less-sensitive to the increase of the Reynolds number as the confinement ratio decreases. This is linked to the asymmetry mode shapes, and energy content linearity between the axial and radial components associated with the single-helix PVC mode.
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| 7. |
- Gojon, Romain, et al.
(författare)
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On the response of a rectangular supersonic jet to a near-field located parallel flat plate
- 2017
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Ingår i: 23rd AIAA/CEAS Aeroacoustics Conference, 2017. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- In this paper, the flow and acoustic fields of a rectangular over-expanded supersonic jet interacting with a parallel plate are investigated using compressible LES. The jet exits from a converging diverging rectangular nozzle of aspect ratio 2 and of design Mach number 1.5. Four simulations with four different distances between the lower inner lip of the minor axis of the rectangular jet and the plate ranging from 0 to 3 equivalent diameters are performed. The geometry of the nozzle, the positions of the plate, and the exit conditions are chosen in order to match those in an experimental study conducted at the University of Cincinnati. Snapshots and mean velocity fields are first presented. A good agreement with the PIV experimental measurements is found. The Overall Sound Pressure Levels are then plotted along the minor and major axis. In a previous paper, the corresponding free jet has been found to undergo a strong flapping motion along the minor axis due to the screech feedback mechanism. In the present study, it is seen that the intensity of the screech feedback mechanism increases for some distances and decreases for some others compared to the one in the corresponding free jet. A study of the jets shear-layers is then proposed first by looking at two points space-time cross correlation of the axial velocity. The convection of the turbulent is thus studied. Then, two points space-time cross correlation of the pressure along the jets shear-layers are proposed and an amplification of the aeroacoustic feedback mechanism leading to screech noise is observed in the lower jet shear-layers for two cases. It is also observed that the screech feedback mechanism establishes mainly between the nozzle lips and the end of the tenth shock cell. The acoustic loading on the plate is finally studied. As pointed out in a previous study, the flapping motion of the jet at the screech frequency seems to yield to an asymmetric organization of the Mach wave radiation also at the screech frequency. Those organized Mach waves impinge in the plate and propagate back to the jet, exciting the shear-layer at the screech frequency. This will amplify the screech mechanism in the lower jet shear-layer. However, this amplification happens only for some nozzle-to-plate distances. Indeed, the screech mechanism leads to the formation of a standing wave pattern in terms of pressure loading at the screech frequency on the plate. There are regions with high amplitude, meaning the acoustic loading is organized mainly at the screech frequency, and regions with low amplitude, which means the acoustic loading is not organized mainly at the screech frequency. The amplification then depends on the location of the standing wave compared to the overall acoustic loading on the plate. If a region of high amplitude of the standing wave pattern coincide with the region of maximal acoustic loading, there is amplification of the screech mechanism.
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| 8. |
- Hafsteinsson, Haukur, 1984, et al.
(författare)
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Exploration of temperature effects on the far-field acoustic radiation from a supersonic jet
- 2014
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Ingår i: 20th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102851
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Konferensbidrag (refereegranskat)abstract
- Jet engines designed for high-speed aircraft commonly include C-D nozzles to obtain supersonic speeds. The radiated noise from the jet exhaust reaches acoustic levels which may cause hearing damage to the pilot and the air-field personnel even though state of the art noise protection such as noise-canceling ear muffs are employed. It is therefore extremely important to keep the noise levels as low as possible. Understanding the noise generation mechanism is of great importance in order to reduce strength of the noise sources. Typical far-field noise spectral characteristics from the supersonic jet exhaust consist of turbulent mixing noise and shock-associated noise. Another noise component named'crackle' is radiated from the jet under certain circumstances. Although it does not appear in the noise spectra due to its characteristics, its rasping character is perceived as a dominant annoyance factor by the human ear. Since it does not appear in the spectrum other measures are needed to identify the existence of'crackle'. Statistical tools like Skewness and Kurtosis applied to the far- and near-field pressure signals and the time derivate of the pressure signal have been shown in literature to be useful for identification of'crackle' events. In this paper the near-field and far-field acoustic radiation from a supersonic jet is analyzed using LES with a code developed at Chalmers University of Technology. The code has previously shown to accurately capture far-field noise spectra of supersonic jets under a variety of moderately cool jet conditions. In the present study we move towards more realistic high-speed aircraft conditions with higher jet exhaust temperatures. The nozzle is operated at slightly underexpanded conditions (NPR = 4.0) and three different stagnation temperature ratios NTR = 1.0, NTR = 2.0 and NTR = 3.0. The LES results are compared with experiments conducted at the Gas Dynamics and Propulsion Laboratory at the University of Cincinnati.
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| 9. |
- Iudiciani, Piero, et al.
(författare)
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LES of the interaction between a premixed flame and complex turbulent swirling flow
- 2011
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 318, s. 092007-092007
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Konferensbidrag (refereegranskat)abstract
- In this paper the Triple Annular Research Swirler, a fuel injector characterized by complex design with three concentric air passages, has been studied numerically. A swirl-stabilized lean premixed flame has been simulated by means of Large Eddy Simulation. The computations characterize successfully the dynamics of the flame and their interactions with the complex swirling flow. The flame is stabilized upstream the fuel injector exit, and the dynamics are led by a Precessing Vortex Core which seems to originate in the inner air passage. The results obtained by Proper Orthogonal Decomposition analysis are in agreement with previous findings in the context of swirling flows/flames.
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| 10. |
- Szasz, R., et al.
(författare)
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Hysteretic Dynamics of Flashback in a Low-Swirl Stabilized Combustor
- 2017
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Ingår i: Combustion Science and Technology. - : Taylor & Francis. - 0010-2202 .- 1563-521X. ; 189:2, s. 266-289
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Tidskriftsartikel (refereegranskat)abstract
- The hysteretic behavior of flashback (FB) and flash forward (FF) in methane and natural gas flames, stabilized by a low swirl fuel injector, is investigated using high speed OH* chemiluminescence and particle image velocimetry. Due to the lack of vortex breakdown, the two mechanisms discussed are boundary layer and turbulence induced FB. Two hysteresis cycles were identified, one when FB is induced by increasing the equivalence ratio starting from lean conditions, and the other by decreasing the equivalence ratio starting from rich conditions. Impact of relevant parameters including Reynolds number (Re), equivalence ratio, fuel type, combustion chamber geometry, preheating, and mixing tube protrusions are investigated. As Re is increased, the equivalence ratio at which both rich and lean flashbacks occur approaches stoichiometric conditions. However, the range of the hysteresis cycle between FB and FF is independent on Re. The transition processes during FB and FF are quite variable and their duration is independent on Re. The mean duration of FB transition initiated from lean conditions is nearly twice longer than the rich branch and also longer than both the lean and rich FF. The geometry of the combustion chamber affected neither FB nor FF. However, preheating increased the equivalence ratio at which FB occurred but did not affect FF. Also, FB had significant effect on the mean flow field.
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| 11. |
- Szász, Robert-Zoltán, et al.
(författare)
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Algorithm for automatic quantification of flashback and flash forward events from high-speed chemiluminescence recordings
- 2016
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Ingår i: Springer Proceedings in Physics. - Cham : Springer Science+Business Media B.V.. - 9783319306001 - 9783319306025 ; , s. 519-526
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Konferensbidrag (refereegranskat)abstract
- Three methods are employed to identify and quantify flashback and flash forward events based on chemiluminescence recordings of swirling flames. The approaches differ in the procedure to determine the instantaneous flame position. The results revealed that the most robust method is to determine a threshold relative to the instantaneous maximum intensity. Analysis of the complete dataset indicated that flashback events are significantly slower than flash forward events.
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| 12. |
- Yellugari, K., et al.
(författare)
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Effect of nozzle spacing on flow behavior in a lean direct injection combustor
- 2021
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Ingår i: AIAA Scitech 2021 Forum. - Reston, Virginia : American Institute of Aeronautics and Astronautics (AIAA). ; , s. 1-13
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Konferensbidrag (refereegranskat)abstract
- A numerical study has been performed on a linear array of five nozzles in a lean direct injection (LDI) combustor for three mass flow rates and four different inter-nozzle spacings, using two equation realizable k − ɛ turbulence model. It is observed that the mass flow rates do not affect the flow patterns in this five nozzle configuration. The two smaller inter-nozzle spacings, s = 1.25d and 1.5d, developed asymmetric flow patterns. Especially at 1.5d, the asymmetry is quite dominant in the core flows of nozzles N2 and N4, due to the non-merging of jets in the shear layers. But, at higher nozzle spacings, s = 1.75d and 2d, the jets merge in the shear layers and move downstream as a single jet. Due to the slower expansion of the flow in the radial direction, strong and compact central toroidal recirculation zones (CTRZ) are formed at smaller inter-nozzle spacings, 1.25d and 1.5d. These compact CTRZs contribute to higher turbulence kinetic energy (TKE) in regions between the nozzles and closer to the dome-plate. These regions correspond to higher velocity and higher shear stress in the flow. As the inter-nozzle spacing is increased, the intensity of TKE decreases between the nozzles.
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