Yoder Gregory D, Diwakar Prasoon K, Hahn David W
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611-6300, USA.
Appl Opt. 2005 Jul 10;44(20):4211-9. doi: 10.1364/ao.44.004211.
Although laser-induced incandescence (LII) has been successfully used for soot volume fraction and particle size measurements, uncertainties remain regarding issues of soot vaporization leading to mass loss and morphological changes occurring in soot due to intense heating. Prompt LII detection schemes are often based on the assumption that the associated time scale is shorter than the time scale of soot vaporization or sublimation. The validity of such assumptions is the focus of the current study. Time-resolved light-scattering measurements were made in combination with LII measurements to quantify soot particle vaporization effects resulting from the LII laser pulse. The light-scattering measurements revealed a sharp decrease in total soot particle mass during the time course of the 25 ns full-width LII laser pulse for fluences in the range of 0.5 J/cm2. Light-scattering theory was used to invert the scattering data, revealing approximately 80%-90% reductions in the soot particle volume for LII fluences of 0.47 and 0.61 J/cm2. In addition, the time-resolved scattering measurements show that the time scale of soot vaporization is completely confined to the LII laser pulse itself. Light scattering revealed no soot vaporization only for fluences of approximately 0.1 J/cm2, which is consistent with recent work on low-fluence LII. Possible mechanisms for soot vaporization are discussed, notably for near-threshold fluences.
尽管激光诱导白炽（LII）已成功用于测量烟灰体积分数和颗粒尺寸，但由于烟灰汽化导致质量损失以及强烈加热导致烟灰形态变化等问题，仍存在不确定性。快速LII检测方案通常基于这样的假设，即相关时间尺度短于烟灰汽化或升华的时间尺度。此类假设的有效性是当前研究的重点。结合LII测量进行了时间分辨光散射测量，以量化由LII激光脉冲导致的烟灰颗粒汽化效应。光散射测量显示，对于0.5 J/cm²范围内的能量密度，在25 ns全宽LII激光脉冲的时间过程中，总烟灰颗粒质量急剧下降。利用光散射理论对散射数据进行反演，结果表明，对于能量密度为0.47和0.61 J/cm²的LII，烟灰颗粒体积减少了约80%-90%。此外，时间分辨散射测量表明，烟灰汽化的时间尺度完全局限于LII激光脉冲本身。光散射显示，仅在能量密度约为0.1 J/cm²时没有烟灰汽化，这与近期关于低能量密度LII的研究一致。文中讨论了烟灰汽化的可能机制，特别是对于接近阈值的能量密度。
