Simultaneous imaging of H and OH in flames using a single broadband femtosecond laser source

Abstract

Atomic hydrogen (H) and hydroxyl radical (OH) are two key intermediate species in the combustion of hydrocarbon fuels. For example, the spatial and temporal distribution of H is useful to better understand the ignition/extinction processes, whereas OH distribution profiles can yield flame structure and reaction zone in different flames conditions. In the present study, simultaneous laser-induced fluorescence (LIF) measurements of H and OH are obtained using a single ultrashort femtosecond-duration laser pulse near λ=307.7 nm for 3pLIF of H. Measurements are performed in CH4-air and C2H4-air flames stabilized over the Hencken burner. Recently developed three-photon LIF (3pLIF) technique is used for H, whereas the same excitation laser pulse can simultaneously excite OH A2Ʃ+←X2Π (0,0) transition. Red-shifting the excitation wavelength to 307.7 nm as opposed to commonly used two-photon excitation using 205 nm deep-ultra-violet (DUV) laser pulses enables the reduction of absorption losses of excitation laser pulse energy when transmitting through thick optical windows of high-pressure vessels, for example. Furthermore, the efficiency of the fs wavelength generation scheme of commercial optical parametric devices is approximately a factor of two higher near 307.7 nm as compared to 292 nm for 3pLIF of H. In this study, H and OH number densities have been reported for a range of flame equivalence ratios in CH4/air and C2H4/air flames established over the Hencken burner. Simultaneous imaging of H and OH in the premixed Bunsen flame is presented. Prospective improvements for increasing the signal-to-noise and hence potential kHz-rate simultaneous single-shot imaging are also discussed.