Abstract

Extension of laser diagnostic methods to high pressure is critically important because most practical combustion systems operate at elevated pressures. However, increased collisional quenching, fluorescence trapping, and other spectroscopic complications make high-pressure laser diagnostics extremely challenging. As the pressure increases, collisional effects broaden and shift excitation spectral lines, reducing the excitation quantum efficiency of the laser-induced fluorescence (LIF) technique, in particular when using conventional narrowband, nanosecond (ns)-duration laser pulses. In this work, spectroscopic investigation of broadband, femtosecond two-photon LIF (fs-TPLIF) of carbon monoxide (CO) was performed in a high-pressure static gas cell, up to total pressures of 20 bar. The fluorescence emission spectrum broadened marginally at the highest pressure investigated and hence can be neglected in most cases. The subquadratic dependence of the CO fs-TPLIF signal on the laser fluence increased as the pressure is increased. Moreover, the CO fs-TPLIF signal decays more slowly with increasing pressure compared to the previously reported ns-TPLIF data. The signal drop when the total pressure is increased from 1 to 13 bar is approximately 30% in the fs excitation, as compared to approximately a 60% drop in the ns excitation in $\mathrm{CO}/{\mathrm{N}}_2/{\mathrm{O}}_2$ mixtures. The pressure effects on the fluorescence signal were observed to be similar in the Ångström and third positive bands, suggesting that the third positive band could also be used for CO measurements with broadband fs laser pulses. Furthermore, experimentally investigated are the effect of pressure on the CO fluorescence signal in different quenching gases using fixed CO mole fractions, as well as number densities. Overall, the fs-TPLIF scheme is shown to be a promising diagnostics tool for CO detection in practical combustion systems at elevated pressures.

© 2019 Optical Society of America

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