| 1. |
- Drikakis, D., et al.
(författare)
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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
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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)
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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)
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LES of supersonic combustion in a scramjet engine model
- 2007
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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
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Ingår i: Journal of Pharmaceutical Sciences. - 0022-3549.
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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. |
- 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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| 9. |
- 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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| 10. |
- 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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