| 1. |
- Drikakis, D., et al.
(författare)
-
Numerics for iles : Limiting algorithms
- 2007
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Bok (övrigt vetenskapligt/konstnärligt)abstract
- Large eddy simulation (LES) has emerged as the next-generation simulation tool for handling complex engineering, geophysical, astrophysical, and chemically reactive flows. As LES moves from being an academic tool to being a practical simulation strategy, the robustness of the LES solvers becomes a key issue to be concerned with, in conjunction with the classical and well-known issue of accuracy. For LES to be attractive for complex flows, the computational codes must be readily capable of handling complex geometries. Today, most LES codes use hexahedral elements; the grid-generation process is therefore cumbersome and time consuming. In the future, the use of unstructured grids, as used in Reynolds-averaged NavierâStokes (RANS) approaches, will also be necessary for LES. This will particularly challenge the development of high-order unstructured LES solvers. Because it does not require explicit filtering, Implicit LES (ILES) has some advantages over conventional LES; however, numerical requirements and issues are otherwise virtually the same for LES and ILES. In this chapterwe discuss an unstructured finite-volume methodology for both conventional LES and ILES, that is particularly suited for ILES. We believe that the next generation of practical computational fluid dynamics (CFD) models will involve structured and unstructured LES, using high-order flux-reconstruction algorithms and taking advantage of their built-in subgrid-scale (SGS) models. ILES based on functional reconstruction of the convective fluxes by use of high-resolution hybrid methods is the subject of this chapter. We use modified equation analysis (MEA) to show that the leading-order truncation error terms introduced by such methods provide implicit SGS models similar in form to those of conventional mixed SGS models.
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| 2. |
- Karlsson, G., et al.
(författare)
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Numerical study of heat transfer, flow fields, turbulent length scales, and anisotropy in corrugated heat exchanger channels
- 2022
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:5
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we report on large eddy simulation (LES) of convectively dominated heat transfer in a corrugated heat exchanger channel using the computational fluid dynamics toolbox, OpenFOAM. A chevron pattern domain with 63 contact points is used to represent the conditions in a real plate heat exchanger (PHE). The unsteady nature of the flow is elucidated using visualization techniques based on volume rendering of temperature and iso surfaces of vorticity defined using the λ2-criterion and contours of wall shear stress and wall heat flux to illustrate the heat transfer process. Global surface averaged temperature and pressure drop are extracted from the LES on successively finer grids, approaching direct numerical simulation resolution, to increase the understanding of grid resolution requirements for LES in PHEs. Industry standard Reynolds-averaged Navier-Stokes simulations are compared to the LES results along selected profiles to demonstrate similarities and differences between the two techniques. The differences detected are further investigated using anisotropy invariant mapping, energy spectra, and turbulence length scale distributions. Significant differences between the model classes are detected and detailed. Moreover, the LES resolution requirements for the flow and the heat transfer processes are found to be different with the latter being more severe.
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| 3. |
- Zettervall, N., et al.
(författare)
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LES of Combustion Dynamics in an Ethylene-Hydrogen-Air Ramjet
- 2022
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Ingår i: 33rd Congress of the International Council of the Aeronautical Sciences, ICAS 2022. - 9781713871163 ; 7, s. 4903-4919
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Konferensbidrag (refereegranskat)abstract
- A combustion Large Eddy Simulation (LES) is used to examine the flow, mixing, fuel-injection and combustion dynamics of a ramjet combustor with a cavity flame holder. The combustor is a running 50/50, in mole, ethylene/hydrogen fuel mixture. A direct-connect facility dual-mode ramjet/scramjet combustor presents the target case, with in the literature available experimental data is used in the present study for validation of the current LES results. Experimental data for time-averaged chemiluminescence, represented by the CH* signal, and CH-PLIF and OH-PLIF, are used to validate the LES. The LES, using a compact 66-step reaction mechanism for the ethylene/hydrogen/air combustion, predicts a highly dynamic combustion behavior, where the flame oscillates between longer sequences in a cavity stabilized state and shorter ones with a jet-wake stabilized state. A volume averaging in cross-section slabs along the combustor length, plotted over time, is used to further examine and visualize the dynamic combustion and the effects of the dynamics on the temperature, pressure, heat release and axial velocity. Such cross-section slabs, and constant volume simulations, is used to further investigate the predictive effect of the accumulation of H2O2 on the combustion dynamics and the sudden increases in flame size associated with the dynamic flame behavior.
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| 4. |
- Bai, X. S., et al.
(författare)
-
Closing Remarks
- 2022
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Ingår i: Advanced Turbulent Combustion Physics and Applications. - : Cambridge University Press. ; , s. 460-463
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 5. |
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| 6. |
- Berglund, Magnus, et al.
(författare)
-
LES of supersonic combustion in a scramjet engine model
- 2007
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 31, s. 2497-2504
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Tidskriftsartikel (refereegranskat)abstract
- In this study, Large Eddy Simulation (LES) has been used to examine supersonic flow and combustion in a model scramjet combustor. The LES model is based on an unstructured finite volume discretization, using total variational diminishing flux reconstruction, of the filtered continuity, momentum, enthalpy, and passive/reactive scalar equations, used to describe the combustion process. The configuration used is similar to the laboratory scrarmjet at the Institute for Chemical Propulsion of the German Aerospace Center (DLR) and consists of a one-sided divergent channel with a wedge-shaped flameholder at the base of which hydrogen is injected. Here, we investigate supersonic flow with hydrogen injection and supersonic flow with hydrogen injection and combustion. For the purpose of validation, the LES results are compared with experimental data for velocity and temperature at different cross-sections. In addition, qualitative comparisons are also made between predicted and measured shadowgraph images. The LES computations are capable of predicting both the non-reacting and reacting flowfields reasonably well-in particular we notice that the LES model identifies and differentiates between peculiarities of the flowfields found in the experiments. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 7. |
- Cappelletto, Elia, et al.
(författare)
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Impact of Post Manufacturing Handling of Protein-Based Biologic Drugs on Product Quality and User Centricity
- 2024
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Ingår i: Journal of Pharmaceutical Sciences. - : Elsevier B.V.. - 0022-3549 .- 1520-6017.
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Forskningsöversikt (refereegranskat)abstract
- This article evaluates the current gaps around the impact of post-manufacturing processes on the product qualities of protein-based biologics, with a focus on user centricity. It includes the evaluation of the regulatory guidance available, describes a collection of scientific literature and case studies to showcase the impact of post-manufacturing stresses on product and dosing solution quality. It also outlines the complexity of clinical handling and the need for communication, and alignment between drug providers, healthcare professionals, users, and patients. Regulatory agencies provide clear expectations for drug manufacturing processes, however, guidance supporting post-product manufacturing handling is less defined and often misaligned. This is problematic as the pharmaceutical products experience numerous stresses and processes which can potentially impact drug quality, safety and efficacy. This article aims to stimulate discussion amongst pharmaceutical developers, health care providers, device manufacturers, and public researchers to improve these processes. Patients and caregivers' awareness can be achieved by providing relevant educational material on pharmaceutical product handling.
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| 8. |
- Cappelletto, Elia, et al.
(författare)
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Impact of Post Manufacturing Handling of Protein-Based Biologic Drugs on Product Quality and User Centricity
- 2024
-
Ingår i: Journal of Pharmaceutical Sciences. - : Elsevier. - 0022-3549 .- 1520-6017.
-
Tidskriftsartikel (refereegranskat)abstract
- This article evaluates the current gaps around the impact of post-manufacturing processes on the product qualities of protein-based biologics, with a focus on user centricity. It includes the evaluation of the regulatory guidance available, describes a collection of scientific literature and case studies to showcase the impact of post-manufacturing stresses on product and dosing solution quality. It also outlines the complexity of clinical handling and the need for communication, and alignment between drug providers, healthcare professionals, users, and patients. Regulatory agencies provide clear expectations for drug manufacturing processes, however, guidance supporting post-product manufacturing handling is less defined and often misaligned. This is problematic as the pharmaceutical products experience numerous stresses and processes which can potentially impact drug quality, safety and efficacy. This article aims to stimulate discussion amongst pharmaceutical developers, health care providers, device manufacturers, and public researchers to improve these processes. Patients and caregivers' awareness can be achieved by providing relevant educational material on pharmaceutical product handling.
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| 9. |
- Duwig, Christophe, et al.
(författare)
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Large eddy simulation of unsteady lean stratified premixed combustion
- 2007
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 151:1-2, s. 85-103
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Tidskriftsartikel (refereegranskat)abstract
- Premixed turbulent flame-based technologies are rapidly growing in importance, with applications to modern clean combustion devices for both power generation and aeropropulsion. However, the gain in decreasing harmful emissions might be canceled by rising combustion instabilities. Unwanted unsteady flame phenomena that might even destroy the whole device have been widely reported and are subject to intensive studies. In the present paper, we use unsteady numerical tools for simulating an unsteady and well-documented flame. Computations were performed for nonreacting, perfectly premixed and stratified premixed cases using two different numerical codes and different large-eddy-simulation-based flamelet models. Nonreacting simulations are shown to agree well with experimental data, with the LES results capturing the mean features (symmetry breaking) as well as the fluctuation level of the turbulent flow. For reacting cases, the uncertainty induced by the time-averaging technique limited the comparisons. Given an estimate of the uncertainty, the numerical results were found to reproduce well the experimental data in terms both of mean flow field and of fluctuation levels. In addition, it was found that despite relying on different assumptions/simplifications, both numerical tools lead to similar predictions, giving confidence in the results. Moreover, we studied the flame dynamics and particularly the response to a periodic pulsation. We found that above a certain excitation level, the flame dynamic changes and becomes rather insensitive to the excitation/instability amplitude. Conclusions regarding the self-growth of thermoacoustic waves were drawn. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 10. |
- Ehn, A., et al.
(författare)
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Investigations of microwave stimulation of a turbulent low-swirl flame
- 2017
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 36:3, s. 4121-4128
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Tidskriftsartikel (refereegranskat)abstract
- Irradiating a flame by microwave radiation is one of several plasma-assisted combustion (PAC) technologies that can be used to modify the combustion chemical kinetics in order to improve flame-stability and to delay lean blow-out. One practical implication is that engines may be able to operate with leaner fuel mixtures and have an improved fuel flexibility capability including biofuels. In addition, this technology may assist in reducing thermoacoustic instabilities that may severely damage the engine and increase emission production. To examine microwave-assisted combustion a combined experimental and computational study of microwave-assisted combustion is performed for a lean, turbulent, swirl-stabilized, stratified flame at atmospheric conditions. The objectives are to demonstrate that the technology increases both the laminar and turbulent flame speeds, and modifies the chemical kinetics, enhancing the flame-stability at lean mixtures. The study combines experimental investigations using hydroxyl (OH) and formaldehyde (CH2O) Planar Laser-Induced Fluorescence (PLIF) and numerical simulations using finite rate chemistry Large Eddy Simulations (LES). The reaction mechanism is based on a methane (CH4)-air skeletal mechanism expanded with sub-mechanisms for ozone, singlet oxygen, chemionization, electron impact dissociation, ionization and attachment. The experimental and computational results show similar trends, and are used to demonstrate and explain some significant aspects of microwave-enhanced combustion. Both simulation and experimental studies are performed close to lean blow off conditions. In the simulations, the flame is gradually subjected to increasing reduced electric field strengths, resulting in a wider flame that stabilizes nearer to the burner nozzle. Experiments are performed at two equivalence ratios, where the leaner case absorbs up to more than 5% of the total flame power. Data from experiments reveal trends similar to simulated results with increased microwave absorption.
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| 11. |
- Ehn, Andreas, et al.
(författare)
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Plasma assisted combustion: Effects of O3 on large scale turbulent combustion studied with laser diagnostics and Large Eddy Simulations
- 2015
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 35:3, s. 3487-3495
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Abstract In plasma-assisted combustion, electric energy is added to the flame where the electric energy will be transferred to kinetic energy of the free electrons that, in turn, will modify the combustion chemical kinetics. In order to increase the understanding of this complex process, the influence of one of the products of the altered chemical kinetics, ozone (O3), has been isolated and studied. This paper reports on studies using a low-swirl methane (CH4) air flame at lean conditions with different concentrations of O3 enrichment. The experimental flame diagnostics include Planar Laser Induced Fluorescence (PLIF) imaging of hydroxyl (OH) and formaldehyde (CH2O). The experiments are also modeled using Large Eddy Simulations (LES) with a reaction model based on a skeletal CH4-air reaction mechanism combined with an O3 sub-mechanism to include the presence of O3 in the flame. This reaction mechanism is based on fundamental considerations including reactions between O3 and all other species involved. The experiments reveal an increase in CH2O in the low-swirl flame as small amounts of O3 is supplied to the CH4-air stream upstream of the flame. This increase is well predicted by the LES computations and the relative radical concentration shift is in good agreement with experimental data. Simulations also reveal that the O3 enrichment increase the laminar flame speed, su, with ∼10% and the extinction strain-rate, Ïext, with ∼20%, for 0.57% (by volume) O3. The increase in Ïext enables the O3 seeded flame to burn under more turbulent conditions than would be possible without O3 enrichment. Sensitivity analysis indicates that the increase in Ïext due to O3 enrichment is primarily due to the accelerated chain-branching reactions H 2 + O â OH + H , H 2 O + O â OH + OH and H + O 2 â OH + O . Furthermore, the increase in CH2O observed in both experiments and simulations suggest a significant acceleration of the chain-propagation reaction CH 3 + O â CH 2 O + H .
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| 12. |
- Feymark, Andreas, 1984, et al.
(författare)
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LES of an Oscillating Cylinder in a Steady Flow
- 2010
-
Ingår i: 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition. - Reston, Virigina : American Institute of Aeronautics and Astronautics. - 9781600867392
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Konferensbidrag (refereegranskat)
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| 13. |
- Fureby, C., et al.
(författare)
-
Incompressible wall-bounded flows
- 2007
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Bok (övrigt vetenskapligt/konstnärligt)abstract
- Almost all flows of practical interest are turbulent, and thus the simulation of turbulent flow and its diversity of flow characteristics remains one of the most challenging areas in the field of classical physics. In many situations the fluid can be considered incompressible; that is, its density is virtually constant in the frame of reference, moving locally with the fluid, but density gradients may be passively convected with the flow. Examples of such flows of engineering importance are as follows: external flows, such as those around cars, ships, buildings, chimneys, masts, and suspension bridges; and internal flows, such as those in intake manifolds, cooling and ventilation systems, combustion engines, and applications from the areas of biomedicine, the process industry, the food industry, and so on. In contrast to free flows (ideally considered as homogeneous and isotropic), wall-bounded flows are characterized by much less universal properties than free flows and are thus even more challenging to study. The main reason for this is that, as the Reynolds number increases, and the thickness of the viscous sublayer decreases, the number of grid points required to resolve the near-wall flow increases. The two basic ways of computing turbulent flows have traditionally been direct numerical simulation (DNS) and Reynolds-averaged NavierâStokes (RANS) modeling. In the former the time-dependent NavierâStokes equations (NSE) are solved numerically, essentially without approximations. In the latter, only time scales longer than those of the turbulent motion are computed, and the effect of the turbulent velocity fluctuations is modeled with a turbulence model.
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| 14. |
- Fureby, C., et al.
(författare)
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Investigations of microwave stimulation of turbulent flames with implications to gas turbine combustors
- 2017
-
Ingår i: AIAA SciTech Forum - 55th AIAA Aerospace Sciences Meeting. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624104473
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Konferensbidrag (refereegranskat)abstract
- Efficient and clean production of electrical energy and mechanical (shaft) energy for use in industrial and domestic applications, surface- and ground transportation and aero-propulsion is currently of significant general concern. Fossil fuels are mainly used for transportation and aero-propulsion, but also for power generation. Combustion of fossil fuels typically give rise to undesired emissions such as unburned hydrocarbons, carbon dioxide, carbon monoxide, soot and nitrogen oxides. The most widespread approach to minimize these is to apply various lean-burn technologies, and sometimes also dilute the fuel with hydrogen. Although efficient in reducing emissions, lean-burn often results in combustion instabilities and igniteon issues, and thus become challenging itself. Another desired aspect of today’s engines is to increase the fuel flexibility. One possible technique that may be useful for circumventing these issues is plasma-assisted combustion, i.e. to supply a small amount of electric energy to the flame to stimulate the chemical kinetics. Although not new, this approach has not yet been fully explored, partly because of it’s complexity, and partly because of apparent sufficiency. Recently, however, several research studies of this area have emerged. This paper attempts to provide a brief summary of microwave-assisted combustion, in which microwaves are utilized to supply the electrical energy to the flame, and to demonstrate that this method is useful to enhance flame stabilization, delay lean blow-off, and to increase combustion efficiency. The main effect of microwaves (or electrical energy) is to enhance the chemical kinetics, resulting in increased reactivity and laminar and turbulent flame speeds. Here we will demonstrate that this will improve the performance of gas turbine combustors.
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| 15. |
- Fureby, C., et al.
(författare)
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Large-Eddy Simulation of Supersonic Combustion in a Mach 2 Cavity-Based Model Scramjet Combustor
- 2024
-
Ingår i: AIAA SciTech Forum and Exposition, 2024. - 9781624107115
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Konferensbidrag (refereegranskat)abstract
- In this study we report on reactive Large Eddy Simulations (LES) of flow and combustion in the US Air Force Research Laboratory Research Cell 19 cavity-based scramjet combustor. This case involves combustion of ethylene that is injected into a cavity. It has previously been studied experimentally with multiple techniques as well as numerically using hybrid RANS/LES. Here, we build on the existing knowledge and provide an in-depth investigation of the flow and combustion by use of a pure LES approach. A range of fuel injection rates are studied as well as the non-reacting case. This way we can disseminate how different amounts of combustion, and the associated volumetric expansion, in the cavity affects the shear-layer above the cavity, the recirculation zone, and the downstream shock-train. For example, an additional shock-train emerges from the start of the cavity when combustion is present, and it grows in intensity with increasing fueling rate from 50 to 110 slpm. For the cases studied, good agreement is found between simulations, LES, and experiments, Particle Image Velocimetry (PIV), Hyperspectral Imagining (HsI) and Laser Induced Breakdown Spectroscopy (LIBS). More specifically, the primary flow features, such as the shock train, primary and secondary cavity recirculation regions, and shear-layer lift due to exothermicity, are found to evolve similarly with increasing fueling rate in the experiments and the LES which enables using the more complete LES result to further investigate the flow and combustion physics.
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| 16. |
- Fureby, C.
(författare)
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Subgrid models, reaction mechanisms, and combustion models in large-eddy simulation of supersonic combustion
- 2021
-
Ingår i: AIAA Journal. - 0001-1452. ; 59:1, s. 215-227
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Tidskriftsartikel (refereegranskat)abstract
- Here, we report on large-eddy simulation (LES) of supersonic flow, mixing, self-ignition, and combustion in a model scramjet combustor. The combustor has been experimentally investigated at DLR, German Aerospace Center Lampoldshausen and consists of a one-sided divergent channel with a wedge-shaped flame-holder: at the base of which, hydrogen is injected. This combustor has been extensively studied by many research groups and constitutes a good validation case for model development and physics elucidation. The LES model used is based on an unstructured finite volume discretization of the filtered mass, momentum, species, and energy equations, as well as an explicit flow solver. Three subgrid flow models, three reaction mechanisms, and three LES combustion models are employed to examine the sensitivity to these modeling parameters. The LES predictions are compared with experimental data for velocity, temperature, wall pressure at different cross sections, as well as schlieren and OH chemiluminesence images, showing good agreement for both first-and second-order statistics, as well as the instantaneous flow structures. Furthermore, these LES results are used to illustrate and explain the intrinsic flow, mixing, and combustion features of this combustor. The intrinsic relationship between the different modeling parameters is also discussed.
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| 17. |
- Larsson, A., et al.
(författare)
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Skeletal Methane-Air Reaction Mechanism for Large Eddy Simulation of Turbulent Microwave-Assisted Combustion
- 2017
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Ingår i: Energy and Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 31:2, s. 1904-1926
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Tidskriftsartikel (refereegranskat)abstract
- Irradiating a flame via microwave radiation is a plasma-assisted combustion (PAC) technology that can be used to modify the combustion chemical kinetics in order to improve flame stability and to delay lean blow-out. One practical implication is that combustion engines may be able to operate with leaner fuel mixtures and have an improved fuel flexibility capability including biofuels. Furthermore, this technology may assist in reducing thermoacoustic instabilities, which is a phenomenon that may severely damage the engine and increase NOX production. To further understand microwave-assisted combustion, a skeletal kinetic reaction mechanism for methane-air combustion is developed and presented. The mechanism is detailed enough to take into account relevant features, but sufficiently small to be implemented in large eddy simulations (LES) of turbulent combustion. The mechanism consists of a proposed skeletal methane-air reaction mechanism accompanied by subsets for ozone, singlet oxygen, chemionization, and electron impact reactions. The baseline skeletal methane-air mechanism contains 17 species and 42 reactions, and it predicts the ignition delay time, flame temperature, flame speed, major species, and most minor species well, in addition to the extinction strain, compared to the detailed GRI 3.0 reaction mechanism. The amended skeletal reaction mechanism consists of 27 species and 80 reactions and is developed for a reduced electric field E/N below the critical field strength (of ∼125 Td) for the formation of a microwave breakdown plasma. Both laminar and turbulent flame simulation studies are carried out with the proposed skeletal reaction mechanism. The turbulent flame studies consist of propagating planar flames in homogeneous isotropic turbulence in the reaction sheets and the flamelets in eddies regimes, and a turbulent low-swirl flame. A comparison with experimental data is performed for a turbulent low-swirl flame. The results suggest that we can influence both laminar and turbulent flames by nonthermal plasmas, based on microwave irradiation. The laminar flame speed increases more than the turbulent flame speed, but the radical pool created by the microwave irradiation significantly increases the lean blow-out limits of the turbulent flame, thus making it less vulnerable to thermoacoustic combustion oscillations. Apart from the experimental results from low-swirl flame presented here, experimental data for validation of the simulated trends are scarce, and conclusions build largely on simulation results. Analysis of chemical kinetics from simulations of laminar flames and LES on turbulent flames reveal that singlet oxygen molecule is of key importance for the increased reactivity, accompanied by production of radicals such as O and OH.
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| 18. |
- Liefvendahl, Mattias, et al.
(författare)
-
Grid requirements for LES of ship hydrodynamics in model and full scale
- 2017
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Ingår i: Ocean Engineering. - : Elsevier Ltd. - 0029-8018 .- 1873-5258. ; 143, s. 259-268
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Tidskriftsartikel (refereegranskat)abstract
- A review is presented of estimates for the grid resolution requirements for wall-resolved and wall-modeled large-eddy simulation, of flows with an important influence of turbulent boundary layers. The estimates are described within a classification scheme which is formulated based on the grid resolution relative to the length scales of the turbulent boundary layer. The grid resolution estimates are then applied to discuss the computational cost of ship hull hydrodynamics simulations, both in model and in full scale. The hulls of one submarine and one bulk carrier are included in this discussion. Two simulation cases are included in the paper to demonstrate a complete simulation methodology, to illustrate the implications of the grid resolution estimates, and to investigate the resulting predictive accuracy. The first simulation case consists of fully developed turbulent channel flow, for which a comparison is made with direct numerical simulation results. The second simulation case consists of the flow around an axisymmetric body, which is based on a bare-hull version of a generic submarine model, and for which wind tunnel measurement data are available for validation.
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| 19. |
- Liefvendahl, Mattias, et al.
(författare)
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LES and DES of high reynolds number wall bounded flows
- 2006
-
Konferensbidrag (refereegranskat)abstract
- High Reynolds number wall bounded flow is here investigated using Large Eddy Simulation (LES), Detached Eddy Simulation (DES) and Reynolds Averaged Navier Stokes (RANS). The first case considered is the fully developed turbulent channel flow at Re, « 395,590,1800 and 10,000. This flow clearly indicates the development of the undisturbed boundary layer and related events, such as streaky structures, hairpin vortices and ejection events. The second case is the flow over an axisymmetric hill in a channel, here the flow contains complex structures such as a turbulent boundary layer with several unsteady separations and reattachments. It is three-dimensional due to both streamwise and spanwise pressure gradients on the lee-side of the hill. The shallowness of the separation region makes the flow a demanding test case for any computational fluid dynamics model. The third case is the flow past an axisymmetric submarine hull with an elliptic forebody and a smoothly tapered stern - the DARPA Suboff model AFF-1. This flow case is highly demanding due to the long midship section, on which the boundary layer is developed, in combination with the elliptic forebody and the tapered stern. Both LES and DES performs well in all cases considered, while RANS has slightly lower accuracy in the channel flow and the axisymmetric hull, and fails to predict some flow features for the axisymmetric hill. Also DES has some problems with the axisymmetric hill case, related to the inlet condition of the modified eddy viscosity.
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| 20. |
- Nilsson, E. J.K., et al.
(författare)
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A setup for studies of laminar flame under microwave irradiation
- 2019
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 90:11
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Tidskriftsartikel (refereegranskat)abstract
- Plasma assisted combustion is a very active research field due to the potential of using the technology to improve combustion efficiency and decrease pollutant emission by stabilizing lean burning flames. It has been shown in a number of studies that a small amount of electrical energy can be deposited in the flame by applying microwaves, resulting in enhanced flame propagation and thus improved flame stabilization and delayed lean blow-out. However, the effects have not yet been properly quantified since there are significant experimental challenges related to the determination of both the laminar burning velocity and the electric field strength. In the present work, a novel setup is described, where a well-defined burner system is coupled to a microwave cavity. The burner is of heat flux type, where a flat laminar flame is stabilized on a perforated burner head. The advantage of this burner for the current use is that the method and related uncertainties are well studied and quantified, and the geometry is suitable for coupling with the microwave cavity. The setup, experimental procedure, and data analysis are described in detail in this article. Laminar burning velocity for a methane-Air flame at φ = 0.7 is determined to certify that the burner works properly in the microwave cavity. The flame is then exposed to pulsed microwaves at 1 kHz with a pulse duration of 50 μs. The laminar burning velocity at these conditions is determined to be 18.4 cm/s, which is an increase by about 12% compared to the laminar burning velocity that is measured without microwave exposure. The setup shows potential for further investigations of lean flames subjected to various microwave pulse sequences. The data are of high quality with well-defined uncertainties and are therefore suitable to use for validation of chemical kinetics models.
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| 21. |
- Nilsson, E. J.K., et al.
(författare)
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Chemical Kinetics
- 2022
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Ingår i: Advanced Turbulent Combustion Physics and Applications. - : Cambridge University Press. - 9781108671422 - 9781108497961 ; , s. 200-239
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Bokkapitel (refereegranskat)
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| 22. |
- Nilsson, T., et al.
(författare)
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Large Eddy Simulation of Ramjet to Scramjet Transition
- 2022
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Ingår i: 33rd Congress of the International Council of the Aeronautical Sciences, ICAS 2022. - 9781713871163 ; 6, s. 4513-4527
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Konferensbidrag (refereegranskat)abstract
- The dual-mode ramjet engine is one of the most promising propulsion systems for hypersonic flight because it can be operated in a wide range of flight Mach numbers. This engine type will be quite complicated to use in practice since it requires Ma<2 and therefore needs to be complemented by a turbojet engine for take-off, landing and low speed maneuvering. The dual mode ramjet engine will operate in a thermally choked ramjet mode for moderate speeds with 3
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| 23. |
- Nilsson, T., et al.
(författare)
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LES of H2-air jet combustion in high enthalpy supersonic crossflow
- 2021
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:3
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Tidskriftsartikel (refereegranskat)abstract
- Here, we report on large eddy simulation (LES) of supersonic flow, mixing, self-ignition, and combustion in a supersonic hydrogen jet in a crossflow configuration. The configuration has been experimentally investigated at Stanford and consists of a rectilinear channel with a ramp inlet in which a hydrogen jet discharges at a 90° angle to the high enthalpy supersonic crossflow. This configuration has been extensively studied by several research groups and constitutes a good validation case for model development and physics elucidation. The LES model used is based on an unstructured finite volume discretization of the filtered mass, momentum, species, and energy equations and an explicit flow solver. In this study, we investigate the effects of the jet-to-crossflow momentum ratio, the chemical reaction mechanism, and the combustion subgrid model by comparing predictions and by comparing with experimental data including OH* chemiluminescence images and jet penetration data. In general, good agreement is found but with some departures for the smallest reaction mechanisms and some of the LES combustion models. The LES results are also used to elucidate the flow, mixing, and combustion features of this configuration.
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| 24. |
- Nogenmyr, Karl-Johan, et al.
(författare)
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Large eddy simulation and laser diagnostic studies on a low swirl stratified premixed flame
- 2009
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 156:1, s. 25-36
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents numerical simulations and laser diagnostic experiments of a swirling lean premixed methane/air flame with an aim to compare different Large Eddy Simulations (LES) models for reactive flows. An atmospheric-pressure laboratory swirl burner has been developed wherein lean premixed methane/air is injected in an unconfined low-speed flow of air. The flame is stabilized above the burner rim in a moderate swirl flow, triggering weak vortex breakdown in the downstream direction. Both stereoscopic (3-component) PIV and 2-component PIV are used to investigate the flow. Filtered Rayleigh scattering is used to examine the temperature field in the leading flame front. Acetone-Planar Laser Induced Fluorescence (PLIF) is applied to examine the fuel distribution. The experimental data are used to assess two different LES models: one based on level-set G-equation and flamelet chemistry, and the other based on finite rate chemistry with reduced kinetics. The two LES models treat the chemistry differently, which results in different predictions of the flame dynamic behavior and statistics. Yet, great similarity of flame structures was predicted by both models. The LES and experimental data reveal several intrinsic features of the low swirl flame such as the W-shape at the leading front, the highly wrinkled fronts in the shear layers, and the existence of extinction holes in the trailing edge of the flame. The effect of combustion models, the numerical solvers and boundary conditions on the flame and flow predictions was systematically examined. (c) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 25. |
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| 26. |
- Nogenmyr, Karl-Johan, et al.
(författare)
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Structure and stabilization mechanism of a stratified premixed low swirl flame
- 2011
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 1567-1574
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents numerical and experimental investigations of the structure and stabilization of a low swirl turbulent stratified lean premixed methane/air flame. Large-eddy simulations are performed using a two-scalar flamelet model based on mixture fraction for predicting the stratification in the fuel/air mixture and a level-set G-function for tracking the flame. Measurements are carried out with LDV for velocity field and simultaneous PLIF of OH radicals and fuel tracer acetone to identify the structures of the flame. The leading edge flame front is observed to exhibit large-scale flame front wrinkling with a particular W-shaped front frequently occurring. This structure is formed due to the interaction of flame front with the large-scale flow motion in the inner low speed zone and the outer high-speed shear-layer of the burner. The W-structures are formed and destroyed periodically at 15 Hz. The flame stabilization is shown to be dictated by the large-scale vortex rings in the shear-layer. This flame stabilization mechanism is rather different from that found in typical bluff-body stabilized flames and high-swirl flames. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 27. |
- Persson, T., et al.
(författare)
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Numerical investigation of the flow over an axisymmetric hill using LES, DES, and RANS
- 2006
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Ingår i: Journal of Turbulence. - : Informa UK Limited. - 1468-5248. ; 7, s. 1-17
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Tidskriftsartikel (refereegranskat)abstract
- The flow around an axisymmetric hill, mounted in a channel with a fully developed approach flow, is investigated. The flow contains complex structures such as a turbulent boundary layer with several unsteady separations and reattachments. It is highly three-dimensional due to both streamwise and spanwise pressure gradients on the leeside of the hill. The shallowness of the separation region makes the flow a very demanding test case for any computational fluid dynamics model. Three different strategies are used in this study: Reynolds-averaged Navier-Stokes (RANS), large eddy simulation (LES), and detached eddy simulation (DES). The computed flow, in terms of velocity and pressure profiles, compared with measurement data and the results show that LES and DES are indeed capable of handling this complicated flow in a correct way whereas RANS clearly fails to predict several important flow features. Furthermore, the influence of the size of the computational domain, the grid resolution and the inflow boundary conditions is also studied. It is found that the pressure field is sensitive to the location of the inlet and the DES model is very sensitive to the inlet boundary condition on the eddy viscosity. To significantly improve the predictions, it is believed that the near-wall resolution must be increased substantially, in particular in the spanwise direction, or a better wall handling has to be incorporated.
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| 28. |
- Rezaeiravesh, Saleh, et al.
(författare)
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On grid resolution requirements for les of wall-bounded flows
- 2016
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Ingår i: ECCOMAS Congress 2016 - Proceedings of the 7th European Congress on Computational Methods in Applied Sciences and Engineering. - Athens : National Technical University of Athens. ; 4, s. 7454-7465, s. 7454-7465
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Tidskriftsartikel (refereegranskat)abstract
- Grid requirements for LES of wall-bounded flows are considered. The setting is a zero pressure gradient turbulent boundary layer on a flat plate, but the results are intended to be of use generally for the simulation of flows with an important influence of turbulent boundary layers. The basis for the grid estimates are expressions for the thickness and the viscous length scale of a turbulent boundary layer. The literature is reviewed, and a new power law is proposed, the coefficients of which have been determined using recent high-Re experimental data. An estimation for the number of grid points required for NWM-LES is derived, which is more general than previously published such estimates. A complete simulation methodology, including a numerical tripping device for transition to turbulence in the boundary layer, is demonstrated for NWM-LES of a flat plate turbulent boundary layer. The predictive accuracy is assessed by comparison with DNS data.
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| 29. |
- Yu, R., et al.
(författare)
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Evolution equations for the decomposed components of displacement speed in a reactive scalar field
- 2021
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 911
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Tidskriftsartikel (refereegranskat)abstract
- The study of a turbulent premixed flame often involves analysing quantities conditioned to different iso-surfaces of a reactive scalar field. Under the influence of turbulence, such a surface is deformed and translated. To track the surface motion, the displacement speed (Sd) of the scalar field respective to the local flow velocity is widely used and this quantity is currently receiving growing attention. Inspired by the apparent benefits from a simple decomposition of Sd into contributions due to (i) curvature, (ii) normal diffusion and (iii) chemical reaction, this work aims at deriving and exploring new evolution equations for these three contributions averaged over the reaction surface. Together with a previously obtained Sd-evolution equation, the three new equations are presented in a form that emphasizes the decomposition of Sd into three terms. This set of equations is also supplemented with a curvature-evolution equation, hence providing a new perspective to link the flame topology and its propagation characteristics. Using two direct numerical simulation databases obtained from constant-density and variable-density reaction waves, all the derived equations and the term-wise decomposition relations are demonstrated to hold numerically. Comparison of the simulated results indicates that the thermal expansion weakly affects the key terms in the considered evolution equations. Thermal expansion can cause variations in the averaged Sd and its decomposed parts through multiple routes more than introducing a dilatation term. The flow plays a major role to influence the key terms in all equations except the curvature one, due to a cancellation between negatively and positively curved surface elements.
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| 30. |
- Zettervall, N., et al.
(författare)
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A reduced chemical kinetic reaction mechanism for kerosene-air combustion
- 2020
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 269
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Tidskriftsartikel (refereegranskat)abstract
- Development of a new reduced chemical kinetic reaction mechanism for kerosene-air combustion is presented. The new mechanism uses a modular based development technique and is a further development on previously presented kerosene-air mechanisms. The new mechanism consists of 30 species and 77 irreversible reactions and is developed to accurate reproduce key flame parameters yet being small enough to be used in finite rate Large Eddy Simulations (LES), Direct Numerical Simulations (DNS) and in Reynolds Average Navier-Stokes (RANS) simulations. The well-proven development technique uses a refined fuel breakdown oxidation sub-mechanism, a simplified C2 intermediate species sub-mechanism and a more detailed set of reactions for the H/C1/O chemistry. The mechanism has been modified to be able to predict ignition delay times for a wide range of temperatures, including in the negative temperature regime. The mechanism has been evaluated for combustion parameters related to flame propagation and ignition over a wide range of equivalence ratios, initial gas temperatures and pressures. Agreements to experimental data and a set of detailed and skeletal mechanisms are good for all target parameters. The proposed mechanism shows good agreement at a computational cost far below all tested reference mechanisms, making it highly suitable for use in combustion computational fluid dynamic (CFD) simulations.
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| 31. |
- Zettervall, N., et al.
(författare)
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Combustion LES of a multi-burner annular aero-engine combustor using a skeletal reaction mechanism for jet- a air mixtures
- 2015
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Ingår i: 51st AIAA/SAE/ASEE Joint Propulsion Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103216
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Konferensbidrag (refereegranskat)abstract
- In this study we describe combustion simulations of a single sector and a fully annular generic multi-burner aero-engine combustor. The objectives are to facilitate the understanding of the flow, mixing and combustion processes to help improve the combustor design and the design process, as well as to show that it is now feasible to perform high-fidelity reacting flow simulations of full annular gas turbine combustors with realistic combustion chemistry. For this purpose we use a carefully validated finite rate chemistry Large Eddy Simulation (LES) model together with a range of reaction mechanisms for kerosene-air combustion. The influence of the chemical reaction mechanism on the predictive capability of the LES model, and on the resulting understanding of the combustion dynamics has recently been proved very important and here we extend this for kerosene-air combustion. As part of this work a separate study of different kerosene-air reaction mechanism is comprised, and based on this evaluation the most appropriate reaction mechanisms are used in the subsequent LES computations. A generic small aircraft or helicopter aero-engine combustor is used, and modeled both as a conventional single sector configuration and more appropriately as a fully annular multi-burner configuration. The single-sector and fully annular multi-burner LES predictions are similar but with the fully annular multi-burner configuration showing different combustion dynamics and mean temperature and velocity profiles. For the fully annular multi-burner combustor azimuthal pressure fluctuations are clearly observed, resulting in successive reattachment-detachment of the flames in the azimuthal direction.
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| 32. |
- Zettervall, N., et al.
(författare)
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Large Eddy Simulation of a premixed bluff body stabilized flame using global and skeletal reaction mechanisms
- 2017
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 179, s. 1-22
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Tidskriftsartikel (refereegranskat)abstract
- The increasing computational capacity in recent years has spurred the growing use of combustion Large Eddy Simulation (LES) for engineering applications. The modeling of the subgrid stress and flux terms is well-established in LES, whereas the modeling of the filtered reaction rate terms is under intense development. The significance of the reaction mechanism is well documented, but only a few computational studies have so far been conducted with the aim of studying the influence of the reaction mechanism on the predicted flow and flame. Such an investigation requires the availability of well documented, thoroughly tested, and accurate reaction mechanisms suitable for use in practical engineering simulations. Global and detailed reaction mechanisms are available for many fuel mixtures, whereas skeletal reaction mechanisms suitable for LES are in rather short supply. This research attempts to close this gap by using combustion LES to examine a well-known bluff-body stabilized premixed propane–air flame using two well-known global reaction mechanisms and a novel skeletal reaction mechanism, developed as part of this study. These reaction mechanisms are studied for laminar flames, and comparison with experimental data and detailed reaction mechanisms demonstrates that the skeletal mechanism shows improved agreement with respect to all parameters studied, in particular the laminar flame speed and the extinction strain rate. The LES results reveal that the choice of the reaction mechanism does not significantly influence the instantaneous or time-averaged velocity, whereas the instantaneous and time-averaged species and temperature are influenced. The agreement with the experimental data increases with increased fidelity of the reaction mechanism, and the skeletal reaction mechanism provides a more realistic basis for e.g. emission predictions.
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| 33. |
- Zettervall, N., et al.
(författare)
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Small Skeletal Kinetic Mechanism for Kerosene Combustion
- 2016
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Ingår i: Energy and Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 30:11, s. 9801-9813
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Tidskriftsartikel (refereegranskat)abstract
- The development and validation of a new skeletal mechanism for kerosene combustion, suitable for reacting direct-, large-eddy, and Reynolds averaged Navier-Stokes Simulations, are presented. The mechanism consists of 65 irreversible reactions between 22 species and is built on a global fuel breakdown approach to produce a subset of C2 intermediates. A more detailed set of reactions for H/O/C1 chemistry largely determines the combustion characteristics. The mechanism is validated for combustion characteristics related to ignition, flame propagation, and flame extinction over a wide range of pressure, temperature, and equivalence ratios. Agreement with experiments and a more complex reference mechanism are excellent for laminar burning velocities and extinction strain rate, while ignition delays are overpredicted at stoichiometric and rich conditions. Concentration profiles for major stable products are in agreement with reference mechanism, and also a range of intermediate species and radicals shows sufficient agreement. The skeletal mechanism shows an overall good performance in combination with a numerical stability and short computation time, making it highly suitable for combustion Large Eddy Simulation (LES).
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