| 1. |
- Ehn, A., et al.
(author)
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Investigations of microwave stimulation of a turbulent low-swirl flame
- 2017
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 36:3, s. 4121-4128
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Journal article (peer-reviewed)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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| 2. |
- Ehn, Andreas, et al.
(author)
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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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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 35:3, s. 3487-3495
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Journal article (other academic/artistic)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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| 3. |
- Fureby, C., et al.
(author)
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Investigations of microwave stimulation of turbulent flames with implications to gas turbine combustors
- 2017
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In: AIAA SciTech Forum - 55th AIAA Aerospace Sciences Meeting. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624104473
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Conference paper (peer-reviewed)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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| 4. |
- Larsson, A., et al.
(author)
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Skeletal Methane-Air Reaction Mechanism for Large Eddy Simulation of Turbulent Microwave-Assisted Combustion
- 2017
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In: Energy and Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 31:2, s. 1904-1926
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Journal article (peer-reviewed)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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| 5. |
- Liefvendahl, Mattias, et al.
(author)
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Grid requirements for LES of ship hydrodynamics in model and full scale
- 2017
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In: Ocean Engineering. - : Elsevier Ltd. - 0029-8018 .- 1873-5258. ; 143, s. 259-268
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Journal article (peer-reviewed)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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| 6. |
- Nilsson, E. J.K., et al.
(author)
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A setup for studies of laminar flame under microwave irradiation
- 2019
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 90:11
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Journal article (peer-reviewed)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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| 7. |
- Rezaeiravesh, Saleh, et al.
(author)
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On grid resolution requirements for les of wall-bounded flows
- 2016
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In: 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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Journal article (peer-reviewed)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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| 8. |
- Zettervall, N., et al.
(author)
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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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In: 51st AIAA/SAE/ASEE Joint Propulsion Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103216
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Conference paper (peer-reviewed)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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| 9. |
- Zettervall, N., et al.
(author)
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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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In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 179, s. 1-22
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Journal article (peer-reviewed)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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| 10. |
- Zettervall, N., et al.
(author)
-
Small Skeletal Kinetic Mechanism for Kerosene Combustion
- 2016
-
In: Energy and Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 30:11, s. 9801-9813
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Journal article (peer-reviewed)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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