High-speed planar laser-induced fluorescence of the CH radical using the C²Σ⁺ - X²Π(0, 0) band

The potential for kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the Methylidyne (CH) radical using its C²Σ ⁺ - X²Π(v = 0, v = 0) band was investigated. We show that due to its strength and the excitation wavelength (~314 nm), which is conveniently generated by a dye laser operating with a red dye, the C-X(0,0) band is a good choice for CH LIF studies wherein suppression of background scattering is not required. While interference from polycyclic aromatic hydrocarbons is small, that caused by hydroxyl (OH) can be significant. Nonetheless, the OH lines can be avoided, and we observe good CH image fidelity. Most importantly, we show that due to the favorable properties of the CH C-X band (i.e., good signal strength and convenient wavelength), it can be used for PLIF at kHz acquisition rates using a continuously pulsing laser system. This is demonstrated in laminar and turbulent CH₄-air flames with a laser system operating at 10 kHz and delivering ~0.2 mJ/pulse at 314 nm.