Simulations of ACLII signals considering soot maturity and size

International audienceIt is well established that soot particles are harmful to the environment and health of living beings, mainly due to their small sizes and chemical composition. In this sense, development of advanced experimental diagnostics for detailed characterization of soot properties is fundamental. Unfortunately, accurate characterization of soot properties requires knowledge of its optical properties, which are difficult to obtain and are subject to ongoing debate and investigation in the community [1]. The soot optical properties can be related to its composition and internal nanostructure in terms of the amount of organic and graphitic compounds, which allows to define a level of maturity [2]. Indeed, soot maturity should be considered in the analysis of optical signals obtained experimentally. Particularly, the auto-compensating laser induced incandescence (ACLII) technique offers a great opportunity to obtain soot properties; however, large errors can be produced when converting these signals to soot volume fraction, temperature and sizes without considering soot maturity.The maturity of soot particles can be quantified as a power-law spectral dependence of the ratio of absorption function Em to the ratio of avelengths λ, which defines a maturity level β. This is closely related to the Angstrom exponent ξ that uses the ratio of absorption coefficients κabs or absorption cross section instead of ratio of Em [1]. The determination of ξ relies on a power-law fit of the data retrieved for the ratios of κabs at different pairs of wavelengths, which may vary according to the spectral range considered, especially if κabs is determined from extinction measurements and not corrected for scattering. Moreover, the multiple-scattering effects within soot aggregates can also play a role and should be accounted for. Nevertheless, these effects can be quantified using a numerical framework, where a reference can be established.In this study, the impact of composition, morphology, multiple-scattering and excitation wavelength on the determination of soot temperature, volume fraction and diameter of primary spheres through the ACLII technique is quantified. For this purpose, the numerical method considers the following. First, simulation of a canonical Gülder type flame is performed, using the CoFlame code, providing reference 2D fields of soot volume fraction, temperature, diameter of primary particles, number density of aggregates and primary particles. Next, these properties are used as input for solving the conservation equations of energy and mass of laser-heated soot aggregates. Last, the modelled temperature-time curve is used to simulate a pair of timeresolved LII signals at different detection wavelengths, which are then used to recover the input parameters, in particular, soot volume fraction, temperature and particle diameter. The effect of soot maturity is considered by imposing a β coefficient field based on recent results [2]. The scattered values of ξ found after a literature review are used as reference for the sensitivity analysis. Finally, multiple-scattering effects are taken into account through the corrections proposed by [3], and the effect of size distribution is also assessed. This framework allows to quantify the uncertainties associated with the hypotheses employed in laser-based techniques