Abstract
An extensive set of LII signals measured in a Diesel spray flame has been simulated using a refined LII model built upon a comprehensive version of soot heat- and mass-balance equations. This latter includes terms standing for saturation of linear, single- and multi-photon absorption processes, cooling by sublimation, conduction, radiation and thermionic emission in addition to mechanisms depicting soot oxidation and annealing, non-thermal photodesorption of carbon clusters as well as corrective factors allowing considering shielding effect and multiple scattering (MS) within aggregates. A complete parameterization of the so-proposed model has been achieved by means of an advanced optimization procedure coupling design of experiments with a genetic algorithm-based solver. Doing so, the values of different factors involved in absorption and sublimation terms have been assessed for a 1064-nm laser excitation wavelength including the multi-photon absorption cross section for C2 photodesorption and the saturation coefficients for linear- and multi-photon absorption, among others. This parameterized model has then been demonstrated to effectively reproduce signals measured in different combustion media including a CH4/O2/N2 premixed flat flame and a diffusion ethylene flame. As a result of the data derived from the analysis of the Diesel flame, a thermal accommodation coefficient value of 0.49 has been assessed against 0.34 when neglecting the shielding effect. In addition, values of the soot absorption function (Eleft( m right)) comprised between 0.18 and 0.31 have been inferred depending on the particle maturation stage. On the other hand, Eleft( m right) 24% higher on average have been estimated when neglecting MS thus illustrating the importance of aggregate characteristics on soot properties derived through LII modeling. Eventually, the Eleft( m right) evolution observed herein has been compared with results issued from studies conducted with varied hydrocarbons which led to highlight the crucial role played by the soot maturity level over the nature of the burnt fuel as far as optical properties are concerned.
Highlights
Due to the increasing concern regarding the impact of soot emissions on human health and on the environment, continuous efforts have been devoted to the development of advanced diagnostics allowing the formation1 3 Vol.:(0123456789) 138 Page 2 of 19R
The most significant contributions to laser-induced incandescence (LII) model refinement probably remain those proposed by Michelsen who integrated mechanisms accounting for soot melting and annealing, non-thermal photodesorption of carbon clusters from the particle surface, saturation of the linear, singleand multi-photon absorption leading to the photodesorption of C2 clusters at high fluences in addition to oxidation and thermionic emission [17, 21]
The absence of explicit contribution of the multi-photon absorption within the expression standing for the absorption flux together with the selection of a two-photon mechanism to account for the C2 photodesorption at 1064 nm may possibly explain the so-observed gaps while illustrating the difficulties related to the selection and parameterization of adapted LII model formulations
Summary
Due to the increasing concern regarding the impact of soot emissions on human health and on the environment, continuous efforts have been devoted to the development of advanced diagnostics allowing the formation. An extended LII model built upon a comprehensive version of soot heat- and mass-balance equations has been implemented It includes an absorption term accounting for saturation of the linear, single- and multi-photon absorption, expressions depicting cooling processes by sublimation, conduction, radiation and thermionic emission in addition to mechanisms standing for soot oxidation and annealing as well as non-thermal photodesorption of carbon clusters Additional model predictions have been compared with signals measured at different HAB in the Diesel flame so as to infer and discuss the values taken by some parameters of interest in LII studies such as the thermal accommodation coefficient driving the conductive cooling process and the maturity-dependent absorption function of soot
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