Narrowband versus broadband excitation for CH2O PLIF imaging in flames using a frequency-tripled Nd:YAG laser

Narrowband versus broadband excitation for CH2O PLIF imaging in flames using a frequency-tripled... Spectrally-narrow- (~0.003 cm−1) and broadband (>1 cm−1) fluorescence excitation of the $$\tilde{A}^{1} A_{2} - \tilde{X}^{1} A_{1} ,4_{0}^{1}$$ A ~ 1 A 2 - X ~ 1 A 1 , 4 0 1 electronic transition of formaldehyde (CH2O) in laminar premixed and non-premixed flames is investigated using the third-harmonic output from a tunable, injection-seeded Nd:YAG laser. Spectrally-resolved, CH2O fluorescence excitation spectra are examined over a broad range of conditions including room-temperature vapor cells and lean-to-rich premixed methane/air and dimethyl ether/air flames in order to understand the origin of the fluorescence using both narrowband and broadband excitation strategies. The measured CH2O excitation spectra are nearly identical in all conditions considered which cover a broad range of composition and temperature conditions. These results imply that the predominant emission signature is CH2O and suggest the potential for quantitative in-flame CH2O LIF measurements using room-temperature calibration and existing fluorescence models. A specific emphasis of this study is on CH2O isolation and potential fluorescence interference in the context of single-shot planar laser-induced fluorescence (PLIF) imaging in flames. The PLIF results indicate that for the premixed flames investigated, both narrowband (Nd:YAG laser operating in single mode) and broadband (no injection seeding) excitation yield a reliable marker of the CH2O distribution, with no indication of major interference from additional species. However, frequency-tuned narrowband excitation resulted in a collected fluorescence emission signal that increased by a factor of two as compared to broadband excitation. In the methane-based non-premixed flames, evidence of the excitation of additional species (such as PAH) was noted; however, the impact of this interference is reduced when using narrowband excitation. Similar to the premixed flames, the CH2O fluorescence emission signal increased by approximately a factor of two when using spectrally tuned, narrowband excitation from the third-harmonic output of an injection-seeded Nd:YAG laser. The current results indicate that narrowband excitation of CH2O near 355 nm using the third-harmonic output of an injection-seeded Nd:YAG laser results in increased fluorescence emission signal and hence a reduced effect of interference from additional flame-generated species as compared to conventional broadband excitation using a frequency-tripled Nd:YAG laser. Experiments in Fluids Springer Journals

Narrowband versus broadband excitation for CH2O PLIF imaging in flames using a frequency-tripled Nd:YAG laser

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Springer Berlin Heidelberg
Copyright © 2014 by Springer-Verlag Berlin Heidelberg
Engineering; Engineering Fluid Dynamics; Fluid- and Aerodynamics; Engineering Thermodynamics, Heat and Mass Transfer
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