| 1. |
- Brackmann, Christian, et al.
(författare)
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Picosecond excitation for reduction of photolytic effects in two-photon laser-induced fluorescence of CO
- 2013
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 3541-3548
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Tidskriftsartikel (refereegranskat)abstract
- Two-photon laser-induced fluorescence for detection of carbon monoxide (CO) frequently shows interferences by emission from photolytically produced C-2 radicals encountered under fuel-rich combustion conditions. Reduced C-2 interference for excitation with laser pulses in the picosecond regime is here demonstrated by comparison with excitation using nanosecond pulses for measurements in laminar premixed ethene-air flames. Compared with nanosecond pulses of 8 ns duration and 4 mJ pulse energy, picosecond pulses of 80 ps duration and around 0.5 mJ pulse energy gave similar to 10 times higher peak power, which allowed for efficient CO excitation and resulted in stronger signal with lower C-2 interference. CO fluorescence with picosecond excitation showed a linear to quadratic power dependence, indicating photoionization, whereas a more quadratic dependence was found for the C-2 interference. A sub-nanosecond effective lifetime of CO resulted in a rapid fluorescence decay compared with C-2 and allowed for efficient reduction in C-2 interference by minimizing the detection gate. In addition, interference compensation using time-resolved detection could be demonstrated. Altogether, picosecond pulses provide efficient two-photon excitation of CO in terms of signal strength as well as reduced C-2 interference. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 2. |
- Rosell, Joakim, et al.
(författare)
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Comparison of Three Schemes of Two-Photon Laser-Induced Fluorescence for CO Detection in Flames.
- 2013
-
Ingår i: Applied Spectroscopy. - : SAGE Publications. - 1943-3530 .- 0003-7028. ; 67:3, s. 314-320
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Tidskriftsartikel (refereegranskat)abstract
- Two-photon excitation laser-induced fluorescence of carbon monoxide suffers from interference from mainly C and strong pressure quenching. This paper presents an investigation of three excitation/detection schemes for two-photon excitation laser-induced fluorescence on carbon monoxide. The schemes are evaluated for pressure and quenching partner dependencies and C interference. Three different emission bands lie in the Hopefield-Birge system: The Ångström BΣ→AΠ band, with two-photon excitation through BΣ←XΠ around 230 nm; the Herzberg band CΣ→AΠ, with two-photon excitation through, CΣ←XΠ, around 217 nm; and the third positive group bΣ→aΠ, also with excitation of BΣ←XΠ around 230 nm. The measurements are performed in laminar premixed flames with various equivalence ratios as well as in a high-pressure cell, where pressure and species concentrations are varied in order to investigate the fluorescence quenching dependence.
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| 3. |
- Rosell, Joakim, et al.
(författare)
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Multi-species PLIF study of the structures of turbulent premixed methane/air jet flames in the flamelet and thin-reaction zones regimes
- 2017
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 182, s. 324-338
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Tidskriftsartikel (refereegranskat)abstract
- Simultaneously planar laser-induced fluorescence (PLIF) measurements of OH, CH, CH2O and toluene are carried out to investigate the structures of turbulent premixed methane/air jet flames in the flamelet regime and the thin-reaction zones regime. A premixed flame jet burner of an inner diameter of 1.5 mm is employed. Stoichiometric methane/air mixtures introduced as a jet are ignited and stabilized in a hot co-flow generated by a coaxial porous plug pilot flame surrounding the jet. The Reynolds number for the studied jet ranges from 960 to 11,500 with the characteristic Karlovitz number ranging from 1 to 60. The focus of this study is on the characterization of the structures and turbulent burning velocity of premixed flames in the flamelet and the thin-reaction zones regimes. The preheat zone is analyzed using the CH2O and toluene PLIF fields, whereas the reaction zone is analyzed using the CH and OH PLIF fields. Laser Doppler Anemometer (LDA) measurements are performed to characterize the turbulence field and it is noted that when the Reynolds/Karlovitz number increases a successive thickening of the preheat zone is observed, whereas the reaction zone, characterized by the CH layer maintains nearly the same thickness. The heat release zone, characterized by the combination of the OH and CH2O PLIF fields, is shown to nearly maintain the same thickness under the present experimental conditions. The flame surface wrinkle ratio is shown to be Reynolds number and Karlovitz number independent when the Reynolds number is high enough such that the smallest wrinkle scales reach to the length scales of the thin reaction layers. The global fuel consumption speed of the jet flame is analyzed using the toluene PLIF field and the OH PLIF field. A discrepancy in the two consumption velocities is found as the Karlovitz number increases. This is found to be a result of the broadening of the oxidation zone. These findings provide experimental support to the flamelet and thin-reaction zone regime hypotheses of turbulent premixed combustion.
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| 4. |
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| 5. |
- Sjöholm, Johan, et al.
(författare)
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Simultaneous visualization of OH, CH, CH2O and toluene PLIF in a methane jet flame with varying degrees of turbulence
- 2013
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 1475-1482
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents simultaneous, single shot planar laser-induced fluorescence (PLIF) imaging of four species: OH, CH, CH2O (formaldehyde) and toluene (C6H5CH3) in methane/air jet flames. The jet flames were stabilized by a flat pilot flame generated using a McKenna type burner. Both flames were operated with a stoichiometric premixed methane/air mixture at room temperature and atmospheric pressure. Several flames, with varying jet flow speeds, were investigated, spanning from laminar (10 m/s jet exit velocity) up to highly turbulent flame conditions, with high Karlovitz numbers, (150 m/s jet exit velocity). Measuring the four species presented above provides detailed information on jet flame structures including the transition from fuel (indicated by the fuel tracer toluene) via the preheat zone (indicated by CH2O) and the inner layer of the flame front (indicated by CH) to the oxidation layer and the postflame zone (both indicated by OH). Furthermore, the simultaneously recorded PLIF images enable the study of correlations between these key species. Especially, overlapping regions between the species in the flames is of interest. The result indicates that turbulence in the present jet flames affects primarily the mixing in the preheat zone and the wrinkling of the reaction layers. It does however not significantly affect the inner flame front structures, represented by the CH radicals, as the thickness of the CH layer remains fairly constant under the investigated flame conditions. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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