Burst-mode OH/CH₂O planar laser-induced fluorescence imaging of the heat release zone in an unsteady flame

The paper presents simultaneous high-speed (7.5 kHz) planar laser-induced fluorescence (PLIF) of formaldehyde (CH₂O) and the hydroxyl-radical (OH) for visualization of the flame structure and heat release zone in a non-premixed unsteady CH₄/O₂/N₂ flame. For this purpose, a dye laser designed for high-speed operation is pumped by the second-harmonic 532 nm output of a Nd:YAG burst-mode laser to produce a tunable, 566 nm beam. After frequency doubling a high-energy kHz-rate narrowband pulse train of approximately 2.2 mJ/pulse at 283 nm is used for excitation of the OH radical. Simultaneously, CH₂O is excited by the frequency-tripled output of the same Nd:YAG laser, providing a high-frequency pulse train over 10 ms in duration at high pulse energies (>100 mJ/pulse). The excitation energies enable signal-to-noise ratios (SNRs) of ~10 and ~60 for CH₂O and OH PLIF, respectively, using a single high-speed intensified CMOS camera equipped with an image doubler. This allows sufficient SNR for investigation of the temporal evolution of the primary heat release zone and the local flame structure at kHz rates from the spatial overlap of the OH- and CH₂O-PLIF signals.