Experimental study of the E(m, λ)/E(m, 1064) ratio as a function of wavelength, fuel type, height above the burner and temperature

Influence of soot aggregate size and internal multiple scattering on LII signal and the absorption function variation with wavelength determined by the TEW-LII method

“…The authors of this technique [1] observed that at a given position in a flame and for the same detection det , the amplitude, temporal decay, and spectral dependence of the LII signal could be nearly identical for different excitation wavelengths when proper laser pulse energies were selected, as illustrated in Fig. 3.…”

“…Various methods have been proposed and applied to determine the soot refractive index or the soot absorption function variation with wavelength. A fairly simple method to infer the relative spectral variation of E(m) with wavelength is based on low-fluence LII measurements using pulsed lasers of two excitation wavelengths, i.e., TEW-LII [1,14,31]. This method is briefly reviewed in the next section, and the…”

“…It is well known that the soot absorption function is difficult to determine accurately and many methods have been proposed [12,[14][15][16][17][18][19][20][21][22]. Among these methods, the two excitation wavelength LII (TEW-LII) technique seeks to produce identical time-resolved and spectrally resolved LII signals by adjusting the laser pulse energy at two different laser wavelengths and permits to determine the relative spectral dependence of E(m) [1]. The principle of TEW-LII to determine the relative E(m) variation is to ensure that the soot particles under the two excitations are subject to the same amount of energy absorption from the laser pulse.…”

“…This technique has been recently generalized to four pairs of wavelength [1] and shown that the relative spectral dependence of E(m) seems independent of either the fuel type or the position in the flames investigated. As explained by Bejaoui et al [1], these results are not in disagreement with previous studies showing an increase in the absorption function from nascent soot low in rich premixed flat flames to more mature soot (mainly composed of black carbon with an advanced graphitization state) in higher locations from the burner exit, since TEW-LII only provides the relative spectral dependence of E(m), but not the absolute values. Nevertheless, similar to the theoretical expression for the LII signal, the theory of TEW-LII formulated so far has also neglected the multiple scattering effects on aggregate absorption/emission.…”

“…Nevertheless, similar to the theoretical expression for the LII signal, the theory of TEW-LII formulated so far has also neglected the multiple scattering effects on aggregate absorption/emission. Therefore, the second objective of the present paper is to re-analyze the TEW-LII results of Bejaoui et al [1] by taking into account the multiple scattering effects on aggregate absorption cross section using the correction factor of Yon et al [8].…”