An experimental study on the spectral dependence of light extinction in sooting ethylene counterflow diffusion flames

A light extinction technique is widely-adopted for quantitative measurement of soot volume fractions. The measurement accuracy is dependent on the optical properties of soot, which are expected to vary with the wavelength of incident light and physicochemical environments in which soot is formed. In the present study, a diode laser based light extinction setup, capable of providing light with variable wavelengths ranging from 405 to 1064 nm, was utilized to investigate the in-situ spectral dependence of light absorption for soot formed in counterflow diffusion flames. Soot volume fractions (FV) were inferred from the extinction level of these laser beams for a series of flames parameterized by oxygen/fuel mole fractions, nozzle exit velocities, and fuel types. Special attention was given to distinguish between the soot formation (SF) and soot formation/oxidation (SFO) flames, considering their notable differences in soot evolutions. It was found that the inferred FV as measured with visible light (405−670 nm) was always significantly higher than those measured with near-infrared light (> 780 nm). In addition, the quantitative decrease of FV with the increase in light wavelength (λ) was found to be different for soot particles formed at different flame locations and/or flame conditions, even in the spectral range above 780 nm for which polycyclic aromatic hydrocarbon (PAH) interferences are expected to be minimal. This confirms the wavelength dependence of the soot optical property E(m). In particular, the value of E(m) tends to decrease with increasing wavelength and the rate of decrease is lower for more mature soot particles. Furthermore, by fitting the extinction coefficient with wavelength in the near-infrared range, the quantitative relation of E(m) with λ was derived and compared among various flame conditions. The present study demonstrates that soot formed at different conditions have different optical properties. The results are also expected to provide essential information for uncertainty evaluation in literature FV data in counterflow diffusion flames as measured with light extinction, especially for those performed with visible light sources where PAH interference may not be negligible.