Modeling laser-induced incandescence of soot: a new approach based on the use of inverse techniques

Two LII models derived from the literature have been tested to simulate signals provided in a recently published extensive set of experimental data collected in a non-smoking laminar diffusion flame of ethylene. The first model classically accounts for particle heating by absorption and cooling by radiation, sublimation and conduction. The second one also integrates an alternative absorption term that accounts for saturation of the linear, single-photon and multi-photon absorption leading to C-2-photodesorption at high fluence, a heating flux attributable to oxidation and a cooling process based on thermionic emission. Obtained results illustrate that both models fail to reproduce the LII signals experimentally monitored on a wide range of fluences (up to similar to 1 J cm(-2)) regardless of the value implemented for the main parameters involved in the energy- and mass-balance equations. We therefore originally proposed a new modeling approach based on the use of inverse techniques to gain information about the specific terms that should be integrated into the calculation. The inverse procedure allows inferring the temporal evolution of the soot diameter as well as the temporal and fluence dependence of additional energy rates that have to be considered to fulfill the particle energy and mass balances while providing a complete fit with experimental data. Conclusions issued from the present work indicate that modeling soot LII using only absorption, radiation, conduction and sublimation rates (as these fluxes are generally expressed and computed in the literature) is inadequate to correctly simulate the soot heating and cooling mechanisms over a wide range of fluences. The inverse modeling procedure also gave insights concerning the relevance of integrating photolytic mechanisms such as multi-photon absorption and carbon cluster photodesorption as previously proposed by Michelsen. Additional calculations performed using a new model formulation integrating such processes finally led to predictions merging on a single curve with experimental data. Additional works should be undertaken, however, to complete this first-approach analysis especially to address the large uncertainties existing in the input parameters and equations accounting for photolytic processes that are likely to significantly impact soot LII.

从文献中得出的两种激光诱导白炽光（LII）模型已被用于模拟在乙烯的非吸烟层流扩散火焰中收集的一组近期广泛发表的实验数据所提供的信号。第一种模型经典地考虑了粒子通过吸收热量以及通过辐射、升华和传导冷却的情况。第二种模型还集成了一个替代吸收项，该项考虑了线性、单光子和多光子吸收的饱和，这种饱和在高能量密度下会导致C - 2光解吸，还考虑了可归因于氧化的热通量以及基于热电子发射的冷却过程。所得结果表明，无论在能量和质量平衡方程中涉及的主要参数所采用的值如何，这两种模型都无法在很宽的能量密度范围（高达约1 J/cm²）内重现实验监测到的LII信号。因此，我们最初提出了一种基于反演技术的新建模方法，以获取有关应纳入计算的特定项的信息。反演过程可以推断出 soot直径的时间演化，以及为满足粒子能量和质量平衡而必须考虑的额外能量速率的时间和能量密度依赖性，同时与实验数据完全吻合。本研究得出的结论表明，仅使用吸收、辐射、传导和升华速率（正如文献中通常表达和计算这些通量的方式）对soot的LII进行建模，不足以在很宽的能量密度范围内正确模拟soot的加热和冷却机制。反演建模过程还提供了关于整合光解机制（如多光子吸收和碳簇光解吸，如Michelsen先前提出的）的相关性的见解。使用集成了这些过程的新模型公式进行的额外计算最终使得预测结果与实验数据合并在一条曲线上。然而，还应该开展更多的工作来完善这种初步方法的分析，特别是要解决输入参数和考虑光解过程的方程中存在的大量不确定性，这些不确定性可能会对soot的LII产生重大影响。