Hydroxyl radical planar imaging in flames using femtosecond laser pulses
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2019, Springer-Verlag GmbH Germany, part of Springer Nature. Hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) imaging is one of the most widely used laser diagnostic techniques to investigate reacting flows such as flames and plasmas. In conventional PLIF experiments, 10-Hz, commercial Nd:YAG/dye-laser-based systems are often used for OH excitation. In recent years, significant developments are also reported in using diode-pumped solid-state and pulse-burst laser systems for high-repetition-rate (kHzMHz) measurements. In general, all these laser sources generate nanosecond-duration, narrowband laser pulses which are to be tuned to a specific ro-vibrational excitation transition line. In the present work, we investigate the use of broadband, ultrashort femtosecond-duration (fs) laser pulses for OH-PLIF imaging in flames. The fs excitation of the OH A2+ X2 (1, 0) transition is followed by fluorescence detection from the (0, 0) and (1, 1) vibrational bands. Because of the broad bandwidth, the excitation laser is coupled to a large number of OH ro-vibrational transitions at the same time; hence, species selectivity is obtained by detecting fluorescence emission in the 310325nm spectral window. This scheme is shown to be free from fluorescence from other flame species as confirmed by high-resolution fluorescence spectra recorded under a variety of flame conditions. Measured OH number density profiles in CH4, C2H4 and H2 calibration flames are in good agreement with model predictions. Two-dimensional imaging of OH at 1-kHz repetition rate is also demonstrated in a turbulent diffusion flame. The present fs OH-PLIF scheme can find novel applications in fundamental chemical physics research, as well as in practical engine combustion and flame diagnostics.
