A double pulse LII experiment on carbon nanoparticles: insight into optical properties

In this work cooled carbon nanoparticles are investigated with the aim of gaining knowledge on their properties. To this purpose, a double pulse experiment is employed consisting essentially of a modified Laser-Induced Incandescence (LII) approach. Before the conventional LII measurements, nanoparticles are additionally irradiated applying different laser fluences. The investigation is performed on carbon nanoparticles sampled from a rich premixed ethylene/air flame at two heights in order to compare the irradiation effects on young and mature particles. Two-color LII measurements are carried out on pristine and irradiated nanoparticles varying the LII laser fluence. In particular, the effects on the incandescence signal, temperature and concentration are investigated. Two phenomena are isolated, namely (1) a significant increase of the apparent particle volume fraction with the applied laser fluence; and (2) a noticeable increase of the LII signal depending on the laser irradiation fluence applied prior to LII. The effects are found to be stronger for young carbon nanoparticles compared to mature ones. These effects are discussed with the aim of understanding the phenomena occurring under laser irradiation and to suggest a possible role of the electrical properties of the particles under analysis.