Interference-free two-photon LIF imaging of atomic hydrogen in flames using picosecond excitation

Abstract

Interference-free, two-photon-excited planar laser-induced fluorescence (PLIF) imaging of atomic hydrogen is demonstrated in steady premixed methane flames. PLIF measurements of atomic hydrogen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the laser-induced fluorescence (LIF) signal on laser intensity. Previous two-photon LIF measurements of atomic hydrogen in hydrocarbon flames using nanosecond laser excitation have identified complications from photolytic production of hydrogen atoms by the excitation laser. Recent results from line-imaging studies in our laboratory indicate a significant advantage to using picosecond excitation for imaging atomic hydrogen with negligible photolytic interference. In the current study, we extend our capabilities for interference-free PLIF imaging of atomic hydrogen in steady premixed CH4/O2/N2 flames. Peak single-shot signal-to-noise ratios of approximately 6–8 are achieved, and the estimated single-shot detection limit is on the order of 1016cm-3. Avoidance of interference and stimulated emission and the effects of fluorescence quenching are discussed. Averaged composite PLIF images are generated by combining images from multiple axial locations in the flame. The images show enhanced number densities of atomic hydrogen near the flame tip, in accordance with numerical predictions of diffusional focusing of H-atoms resulting from the sharp curvature of the flame front.