LIF diagnostics for selective and quantitative measurement of PAHs in laminar premixed flames

Laser-induced fluorescence (LIF) technique has been widely applied in PAH measurement in sooting flames, but it is still a big challenge to quantitatively identify the specific PAH species due to the complexities of spectral characteristics and calibration method. In this study, we aimed at developing a spectral-resolved LIF analytic model that can obtain PAH mole fractions in flames. A comprehensive LIF spectrum database in the wide temperature and concentration ranges was firstly established by the LIF experiments of gas-phase PAHs in an optical cell. Then, a spectral-resolved LIF analytic model was presented based on linear superposition regulation and an optimization fitting algorithm, and the LIF spectra of PAH components can be decomposed from the LIF spectra of the flame using this model. Finally, the integral fluorescence intensities of each PAH component were calibrated to their mole fractions by the calibration factors at corresponding temperatures. In this way, we can measure the mole fractions of several PAHs. Subsequently, the presented LIF method was applied to investigate the PAH formation characteristics at different flame temperatures. The experimental results showed that the mole fractions of each PAH except naphthalene decreased around 60% to 70% when the temperature increased from 1186 K to 1483 K, while the mole fraction of naphthalene kept nearly constant. The mole fractions simulated by a chemical kinetic model well reproduce the experimental results. According to the chemical kinetic analysis, C2H2 oxidation reaction was promoted and the C2H2-addition reactions were suppressed by increasing temperatures, which suppressed the PAH formation.