Quasi-simultaneous detection of CH2O and CH by cavity ring-down absorption and laser-induced fluorescence in a methane/air low-pressure flame

The combination of two-dimensional, planar laser-induced fluorescence (PLIF) and cavity ring-down (CRD) absorption spectroscopy is applied to map quantitatively the spatial distributions of CH2O and CH in a methane/air flame at 25 Torr. Both species are detected in the same spectral region using the overlapping CH2O A (1)A(2) - X (1)A(1) 4(1)(0) and CHB - X(1, 0) bands. The combination of diagnostic techniques exploits the spatial resolution of LIF and the quantitative CRD absorption measure of column density. The spatially resolved PLIF provides the distribution of absorbers and line-of-sight CRD absorption the absolute number density needed for quantitative concentration ima es. The peak CH2O concentration is (3.5 +/- 1.4) x 10(14) cm(-3), or 1450 +/- 550 ppm at 1000 K. The lack of precise absorption cross-section data produces these large error limits. Although a flame model predicts lower amounts, these large uncertainties limit this measurement's usefulness as a test of the flame chemistry.