Two-dimensional laser-induced incandescence (LII) measurements usually involve the use of a cylindrical lens to illuminate the planar region of interest. This creates a varying laser fluence and sheet width in the imaged flame region which could lead to large uncertainties in the quantification of the 2D LII signals into soot volume fraction distributions. To investigate these effects, 2D LII measurements using a wide range of laser pulse energies were performed on a premixed flat ethylene–air flame while employing a cylindrical lens to focus the laser sheet. Using shorter focal length of the focusing lens resulted in larger variation of the LII signal profiles across the flame. A heat – and – mass – transfer - based LII model was also used to simulate the measurements and good agreement was found. The ratio between focal length (FL) and image length (IL) was introduced as a useful parameter for estimating the bias in estimated soot volume fractions across the flame. The general recommendation is to maximize this FL/IL ratio in an experiment, which in practice means the use of a long focal length lens. Furthermore, the best choices of laser fluence and detection gate width are discussed based on results from these simulations.

中文翻译：

---

用于碳烟体积分数测量的二维激光诱导白炽光：由于激光束聚焦导致的量化问题

二维激光诱导白炽度 (LII) 测量通常涉及使用柱面透镜来照亮感兴趣的平面区域。这会在成像火焰区域产生变化的激光能量密度和片材宽度，这可能导致将 2D LII 信号量化为烟尘体积分数分布的很大不确定性。为了研究这些影响，在使用柱面透镜聚焦激光片的同时，在预混合的扁平乙烯-空气火焰上进行了使用各种激光脉冲能量的 2D LII 测量。使用较短焦距的聚焦透镜会导致火焰中 LII 信号分布的较大变化。还使用了基于传热和传质的 LII 模型来模拟测量结果，并且发现了良好的一致性。焦距 (FL) 和图像长度 (IL) 之间的比率被引入作为一个有用的参数，用于估计整个火焰中估计的烟尘体积分数的偏差。一般建议是在实验中最大化此 FL/IL 比率，这实际上意味着使用长焦距镜头。此外，基于这些模拟的结果讨论了激光能量密度和检测门宽度的最佳选择。