SiO multi-line laser-induced fluorescence for quantitative temperature imaging in flame-synthesis of nanoparticles
Silicon monoxide (SiO) is an intermediate in the gas-phase synthesis of SiO2 nanoparticles and coatings. We demonstrate a method for in situ imaging the gas-phase temperature via multi-line laser-induced fluorescence (LIF) using excitation in the A 1Π- X 1Σ+(0,0) band near 235 nm. A low-pressure lean (3 kPa, φ = 0.39) premixed hydrogen/oxygen flame was seeded with 210 ppm hexamethyldisiloxane (HMDSO) to produce SiO2 nanoparticles. Spectral regions with no interference from other species in the flame were located, and the excitation-spectral range that provides the best temperature sensitivity was determined from numerical experiments. Quenching rates of the selected transitions were also determined from fluorescence lifetime measurements, and found to be independent of the excited rotational state. Upon laser light-sheet excitation, images of fluorescence were recorded for a sequence of excitation wavelengths and pixel-wise multi-line fitting of the spectra yields temperature images. The results were compared against multi-line NO-LIF temperature imaging measurements using the A 2Σ+- X 2Π(0,0) band near 225 nm from 500 ppm NO added to the gas flow as a thermometry target. Both methods show good qualitative agreement with each other and demonstrate that temperature can be evaluated from the zone in the reactor where SiO is naturally present without adding tracers. SiO LIF exhibited high signal-to-noise ratios of the order of ten times that of NO LIF.