Modeling laser-induced incandescence of Diesel soot-Implementation of an advanced parameterization procedure applied to a refined LII model accounting for shielding effect and multiple scattering within aggregates for αT and E(m) assessment

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 C-2 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/O-2/N-2 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 (E(m)) comprised between 0.18 and 0.31 have been inferred depending on the particle maturation stage. On the other hand, E(m) 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 E(m) 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.

使用基于 soot热质平衡方程综合版本建立的精细激光诱导白炽（LII）模型，对柴油喷雾火焰中测量到的大量LII信号进行了模拟。后者包括代表线性、单光子和多光子吸收过程饱和的项，除了描述碳烟氧化和退火、碳簇的非热光解吸以及考虑团聚体内屏蔽效应和多次散射（MS）的修正因子外，还包括升华冷却、传导、辐射和热电子发射机制。通过将实验设计与基于遗传算法的求解器相结合的先进优化程序，实现了所提出模型的完整参数化。通过这样做，针对1064nm激光激发波长，评估了吸收和升华项中涉及的不同因子的值，包括C - 2光解吸的多光子吸收截面以及线性和多光子吸收的饱和系数等。然后，该参数化模型被证明能有效地重现不同燃烧介质中测量到的信号，包括CH4/O - 2/N - 2预混平面火焰和扩散乙烯火焰。对柴油火焰分析得出的数据结果表明，考虑屏蔽效应时热适应系数值为0.49，而忽略屏蔽效应时为0.34。此外，根据颗粒成熟阶段推断出碳烟吸收函数（E(m)）的值在0.18到0.31之间。另一方面，忽略多次散射时，E(m)平均估计高出24%，这说明了团聚体特性对通过LII建模得出的碳烟特性的重要性。最后，将此处观察到的E(m)演变与对不同碳氢化合物进行研究得出的结果进行了比较，结果强调了就光学性质而言，碳烟成熟度相对于燃烧燃料的性质所起的关键作用。