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

Spectrally-narrow- (~0.003 cm⁻¹) and broadband (>1 cm⁻¹) fluorescence excitation of the \(\tilde{A}^{1} A_{2} - \tilde{X}^{1} A_{1} ,4_{0}^{1}\) 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.