Hydroxyl radical planar imaging in flames using femtosecond laser pulses

Abstract

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 (kHz–MHz) 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 A2Ʃ+ ← 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 310–325 nm 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.