Stefan Dankers

Soot Temperature Measurements and Implications for Time-Resolved Laser-Induced Incandescence (TIRE-LII)

Emission spectroscopy has been used to determine soot particle temperatures in an ethene diffusion flame both under normal combustion conditions and also after irradiation with an intense laser pulse. On the basis of these measurements, a check on the models and an improvement of parameters underlying time-resolved laser-induced incandescence (TIRE-LII) was performed. With this technique a two-dimensionally resolved measurement of soot primary particle sizes is feasible in a combustion process from the ratio of emission signals obtained at two delay times after a laser pulse, as the cooling behavior is characteristic of particle size. For accurate measurements, local gas temperatures must be known, which can be derived from the temperatures of the soot particles themselves. These have been measured by fitting full Planck curves to line-of-sight emission spectra after an inversion algorithm. The temperature and heat of vaporization of soot, which govern the energy and mass loss at high temperatures, were obtained by measurements of maximum particle temperature for various laser irradiances and a fit procedure to the theoretical dependence. Finally, the temperature decay of laser-heated soot was measured with high temporal resolution. Comparisons with model predictions show that soot temperatures are roughly 300 K higher than expected after the onset of vaporization, which indicates deficiencies in the present models of vaporization. It is demonstrated that the TIRE-LII performance is essentially unaffected by these shortcomings if LII signals are detected in a period where conductive heat transfer dominates and an appropriate correction is performed.

Determination of primary particle size distributions from time-resolved laser-induced incandescence measurements

For a polydisperse nanoparticle ensemble the evaluation of time-resolved laser-induced incandescence (LII) measurements yields a weighted average value for the primary nanoparticle size. Although this value is sufficient for narrow size distributions, a comprehensive characterization of a particle-evolution process requires the reconstruction of the size distribution. An easy-to-use online approach is presented to evaluate the LII signal regarding higher moments of the distribution. One advantage of this approach is that the size distribution results in a deceleration of the LII signal decay with time after the laser pulse. Therefore LII signal-decay curves are evaluated in two different time intervals after the laser pulse, providing information about the desired distribution parameters that has been tested successfully with experimental curves taken in different soot-formation processes.

Application of laser-induced incandescence for the determination of primary particle sizes of nanoparticles demonstrated using carbon blacks

Time-resolved laser-induced incandescence (TIRE-LII), which relies on the heating of particles by a laser pulse and subsequent detection of the thermal radiation, has been successfully tested for the particle sizing of nanoscale carbon blacks. For this purpose, different types of commercially available carbon black powders are dispersed in a measurement chamber by means of a dry ultrasonic dispersion. After the sedimentation of big clusters out of the measurement volume reproducible LII-measurements can be performed. A good correlation between primary particle sizes measured by LII and specified product properties, which are provided by transmission electron microscopy (TEM)-analysis, is found. Furthermore, it turns out that the LII results are not affected by the aggregate structure.