| 1. |
- Baudoin, Eric, et al.
(author)
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Comparison of LES Models Applied to a Bluff Body Stabilized Flame
- 2009
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In: Conference Proceeding Series, Digital.
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Conference paper (peer-reviewed)abstract
- Present-day demands on combustion equipment are increasing the need for improved understanding and prediction of turbulent combustion. Large Eddy Simulation (LES), in which the large-scale flow is resolved on the grid, leaving only the small-scale flow to be modeled, provides a natural framework for combustion calculations as the transient nature of the flow is resolved. In most situations, however, the flame is thinner than the LES grid, and subgrid modeling is required to handle the turbulence-chemistry interaction. Here, we examine the predictive capabilities and the theoretical links between LES flamelet models, such as the G-equation model (G-LES), and LES finite rate chemistry models, such as the Thickened Flame Model (TFM-LES), the Partially Stirred Reactor model (PaSR-LES), the Eddy Dissipation Concept (EDC-LES) model, a Presumed Probability Density Function (PPDF-LES) model and the Implicit LES (QL-LES) model. The models are described, and theoretical links between these are discussed in terms of the turbulent flame speed and flame thickness. In addition, the different models are used to study a bluff-body stabilized flame and the resulting predictions are compared with experimental data for two operating conditions.
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| 2. |
- Baudoin, Eric, et al.
(author)
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Effect of partial premixing on stabilization and local extinction of turbulent methane/air flames
- 2013
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In: Flow, Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1573-1987 .- 1386-6184. ; 90:2, s. 269-284
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Journal article (peer-reviewed)abstract
- Abstract in UndeterminedThe stabilization characteristics and local extinction structures of partially premixed methane/air flames were studied using simultaneous OH-PLIF/PIV techniques, and large eddy simulations employing a two-scalar flamelet model. Partial premixing was made in a mixing chamber comprised of two concentric tubes, where the degree of partial premixing of fuel and air was controlled by varying the mixing length of the chamber. At the exit of the mixing chamber a cone was mounted to stabilize the flames at high turbulence intensities. The stability regime of flames was determined for different degree of partial premixing and Reynolds numbers. It was found that in general partially premixed flames at low Reynolds numbers become more stable when the level of partial premixing of air to the fuel stream decreases. At high Reynolds numbers, for the presently studied burner configuration there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime, significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes grow in size leading to eventual flame blowout. Local flame extinction was found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.
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| 3. |
- Baudoin, Eric, et al.
(author)
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Effect of partial premixing on stabilization and local extinction of turbulent methane/air flames
- 2011
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In: Proceedings of the 17th Mediterranean Combustion symposium, MCS 7. - 9788888104126
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Conference paper (peer-reviewed)abstract
- The stabilization characteristics and local extinction structures of partially premixed methane/air flames were studied using simultaneous OH-PLIF/PIV techniques, and large eddy simulations employing a two-scalar flamelet model. Partial premixing was made in a mixing chamber comprised of two concentric tubes, where the degree of partial premixing of fuel and air was controlled by varying the mixing length of the chamber. At the exit of the mixing chamber a cone was mounted to stabilize the flames at high turbulence intensities. The stability regime of flames was determined for different degree of partial premixing and Reynolds numbers. It was found that in general partially premixed flames at low Reynolds numbers become more stable when the level of partial premixing of air to the fuel stream decreases. At high Reynolds numbers, for the presently studied burner configuration there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime,significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes grow in size leading to eventual flame blowout. Local flame extinction was found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.
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| 4. |
- Baudoin, Eric
(author)
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Large Eddy Simulation of Turbulent Premixed and Partially Premixed Combustion
- 2010
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Doctoral thesis (other academic/artistic)abstract
- In this thesis, a computational fluid dynamics (CFD) approach is used to study turbulent premixed and partially premixed combustion. The CFD approach is based on large eddy simulation (LES) in which the large-scale structures of the flow are resolved on a grid, leaving only the small-scale structures (subgrid scales) to be modeled. The combustion modelling is based on the flamelet concept in which the scale separation of the flow and the chemistry is assumed. This thesis work is made up of the following parts. First, focusing on the turbulent premixed combustion, the level-set G-equation flamelet model is applied to study high density ratio flames without explicit filter in order to capture the effect of thin reaction zone embedded in the LES subgrid. Conventional fractional step methods are shown to be numerically instable for high density ratio flames. A highly robust numerical method, which is known as the ghost fluid method (GFM), is implemented. The G-equation based flamelet model and the ghost fluid method are evaluated on a lean propane/air premixed flame stabilized by a bluff body. The methods are shown to be able to capture the density ratio effect on the flame dynamics, including the near flame holder wrinkling due to Kelvin Helmholtz (KH) instabilities, the downstream large scale wrinkling due to the lower frequency Bénard/von-Karman (BVK) instability at low density ratio conditions, and the suppression of BVK instability at high density ratio conditions. In LES, spatial filtering of the reaction zone leads to thickening of the reaction zone. It is shown that thickening of the reaction zone can lead to significant under-prediction of the turbulence intensity at high density ratio conditions. The effects of flame thickening are further studied for the cases of the flame/vortex interaction and hydrodynamic instability. Essentially, with thickening of the reaction zones, the development of flame wrinkling and hydrodynamic instability are suppressed. Second, focusing on the partially premixed combustion, a two-scalar flamelet approach for LES is developed. In this approach, the trailing edge of the flame is assumed to be controlled by diffusion of mass and heat and thereby it is modelled using a steady diffusion flamelet model. The stabilization of the flame is due to the propagation of the leading premixed flame front (triple flame) in turbulent flows. The leading premixed flame is modelled using the level-set G-equation. This model is applied to simulate partially premixed flames of various fuels in a conical burner to understand the structure and the dynamics of the turbulent partially premixed flames operating in the flamelet regime and in certain cases with thicker reaction zones and local flame extinction. It is found that in general partially premixed flames are more stable when the level of partial premixing of air to the fuel stream decreases. However, at high Reynolds number conditions, an optimal level of partial premixing is found where the flame is most stable. There are two possible flammable surfaces in the partially premixed flames in the conical burner, where the mixture is in stoichiometric condition. At low Reynolds number flows, the inner flame is observed experimentally whereas it is not possible to stabilize at high Reynolds number flows. Numerical results based on the two-scalar flamelet model correctly predicted the blowoff of the inner flame at high Reynolds number conditions. It is well known that LES results are sensitive to the inflow conditions. The sensitivity of LES results to inflow turbulence and the mean flow profiles is systematically investigated based on the conical burner. It is shown that in the proximity of the burner the onset of the flow instability is not only dependent on the shape of the mean profiles, but also on the anisotropy of the inflow turbulence and the integral length scale of the inflow turbulence. This calls for special care in validation of LES models and more detailed experimental data for the inflow conditions when preparing for the database for model development and validation.
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| 5. |
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| 6. |
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| 7. |
- Hult, Johan, et al.
(author)
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Optical Characterization of the Combustion Process inside a Large-Bore Dual-Fuel Two-Stroke Marine Engine by Using Multiple High-Speed Cameras
- 2020
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In: WCX SAE World Congress Experience. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191. ; 2020-April
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Conference paper (peer-reviewed)abstract
- Dual-fuel engines for marine propulsion are gaining in importance due to operational and environmental benefits. Here the combustion in a dual-fuel marine engine operating on diesel and natural gas, is studied using a multiple high-speed camera arrangement. By recording the natural flame emission from three different directions the flame position inside the engine cylinder can be spatially mapped and tracked in time. Through space carving a rough estimate of the three-dimensional (3D) flame contour can be obtained. From this contour, properties like flame length and height, as well as ignition locations can be extracted. The multi-camera imaging is applied to a dual-fuel marine two-stroke engine, with a bore diameter of 0.5 m and a stroke of 2.2 m. Both liquid and gaseous fuels are directly injected at high pressure, using separate injection systems. Optical access is obtained using borescope inserts, resulting in a minimum disturbance to the cylinder geometry. In this type of engine, with fuel injection from positions at the rim of the cylinder, the flame morphology becomes asymmetric. The optical spatial mapping and tracking method is demonstrated to be well suited for the study of such an asymmetric combustion system. Spatial mapping and tracking of flame position is applied to both engine operating modes; normal diesel operation and dual-fuel operation with diesel pilot ignition of the gas. Similarities and differences between diesel and gas flame shape and development can thus be visualised directly. The effects of changing charge density, gas injection pressure and injection nozzle geometry on the flame geometry and development are also studied.
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| 8. |
- Hult, Johan, et al.
(author)
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Spatiotemporal flame mapping in a large-bore marine diesel engine using multiple high-speed cameras
- 2020
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In: International Journal of Engine Research. - : SAGE Publications. - 1468-0874 .- 2041-3149. ; 21:4, s. 622-631
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Journal article (peer-reviewed)abstract
- A calibrated multiple high-speed camera arrangement recording the flame emission from three different directions has been demonstrated on an engine. From the multiple views, the flame position inside the engine cylinder can be spatially mapped, allowing quantitative studies of the dynamics of ignition, flame development and propagation. Through space carving, the three-dimensional flame contour can be estimated. From this contour, properties like flame length, flame height, ignition locations and flame directions can be extracted. The technique is demonstrated by measurements on diesel flames inside a marine two-stroke engine with a bore diameter of 500 mm. It is found to be a valuable tool for spatiotemporal flame mapping in this asymmetric industrial combustion system.
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| 9. |
- Li, Bo, et al.
(author)
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Experimental and numerical study of a conical turbulent partially premixed flame
- 2009
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 32, s. 1811-1818
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Journal article (peer-reviewed)abstract
- The structure and dynamics of a turbulent partially premixed methane/air flame in a conical burner were investigated using laser diagnostics and large-eddy simulations (LES). The flame structure inside the cone was charecterized in detail using LES based on a two-scalar flamelet model, with the mixture fraction for the mixing field and level-set G-function for the partially premixed flame front propagation. In addition, planar laser induced florescence (PLIF) of CH and chemiluminiscence imaging with high speed video were performed through a glass cone. CH and CH2O PLIF were also used to examine the flame structures above the cone. It is shown that in the entire flame the CH layer remains very thin, whereas the CH2O layer is rather thick. The flame is stabilized inside the cone a short distance above the nozzle. The stabilization of the flame can be simulated by the triple-flame model but not the flamelet-quenching model. The results show that flame stabilization in the cone is a result of premixed flame front propagation and flow reversal near the wall of the cone which is deemed to be dependent on the cone angle. Flamelet based LES is shown to capture the measured CH structures whereas the predicted CH2O structure is some-what thinner than the experiments.
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