Optimal approaches to formaldehyde planar laser-induced fluorescence

Abstract

We assessed approaches to i) detecting formaldehyde (CH2O) via planar laser-induced fluorescence (PLIF) in atmospheric-pressure premixed flames, including those with pre-vaporized jet fuels, and ii) improving signal-to-noise ratio (SNR) of CH2O PLIF more generally. We compared excitation wavelengths and schemes – employing the third-harmonic output of a Nd:YAG laser vs. the frequency-doubled output of a dye laser. Specifically, we identified the optimal excitation line accessible via the frequency-doubled dye laser at 353.07 nm, which was ∼4× more efficient than tripled-Nd:YAG excitation, but considering that the tripled-Nd:YAG laser output is generally ∼10× more energetic, it will produce more CH2O-LIF signal. We then determined the optimal detection setup to include a 100-mm focal length (f/1.4) objective lens coupled to a standard achromatic lens, which provided ∼4× greater signal relative to our baseline lens configuration, and subsequently we demonstrated that a Gen III GaAsP intensifier produced remarkably high PLIF SNR (>15). Measurements in premixed turbulent flames using pre-vaporized kerosene fuels showed that interference from fuel-based LIF was largely absent in all cases and that high-quality CH2O PLIF was efficacious.