IMAGING OF CO AND CO_2 USING INFRARED PLANAR LASER-INDUCED FLUORESCENCE

摘要

Infrared (IR) planar laser-induced fluorescence (PLIF) techniques for imaging of carbon monoxide and carbon dioxide are reported. These diagnostics employ a tunable IR source to excite overtone and combination band transitions of CO and CO_2, respectively, and one or two InSb focal plane arrays to collect fluorescence emitted via fundamental transitions from excited vibrational states. A brief outline of the theoretical framework for absorption and fluorescence modeling is presented, with most attention paid to the distinct characteristics stemming from the use of vibrational (IR) transitions as compared to more traditional electronic (UV) transitions. Of note are the conclusions that (1) acceptable fluorescence quantum yield (and therefore signal level) can be achieved despite relatively small Einstein A coefficients, as the quenching processes following IR excitation are often slow; and (2) vibration-to-vibration transfer to other IR-active species can enable imaging of more than one species with a single excitation wavelength. Experimentally, the large dynamic range afforded by the IR cameras (14 bit) allows for effective imaging despite the presence of background luminosity, while the use of two cameras enables imaging of multiple species simultaneously and/or common-mode rejection of luminous background in unsteady flows. PLIF imaging of CO and CO_2 is demonstrated for room-temperature mixing processes with signal-to-noise ratio = 1 detection limits near 1000 ppm. Imaging of both CO and CO_2 in a steady laminar coflowing CO/Ar/ H_2 flame is also presented using laser excitation of CO only.