| 1. |
- Li, Zhongshan, et al.
(författare)
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Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame
- 2010
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 157:6, s. 1087-1096
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Tidskriftsartikel (refereegranskat)abstract
- High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH2O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 mu m and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH2O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH2O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH2O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH2O from the reaction zones to the preheat zone. The CH2O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH2O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O-2 and CO2 in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH2O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 2. |
- Yan, Beibei, et al.
(författare)
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Structures and stabilization of low calorific value gas turbulent partially premixed flames in a conical burner
- 2010
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Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 34:3, s. 412-419
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Konferensbidrag (refereegranskat)abstract
- Experiments are carried out on partially premixed turbulent flames stabilized in a conical burner. The investigated gaseous fuels are methane, methane diluted with nitrogen, and Mixtures of CH4, CO, CO2, H-2 and N-2, Simulating typical products from gasification of biomass, and co-firing of gasification gas with methane. The fuel and air are partially premixed in concentric tubes. Flame stabilization behavior is investigated and significantly different stabilization characteristics are observed in flames with and without the cone. Planar laser induced fluorescence (LIF) imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are carried out to gain further understanding of the flame/flow interaction in the cone. The data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities. (C) 2009 Elsevier Inc. All rights reserved.
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