Stephan Schraml

Two-dimensional soot-particle sizing by time-resolved laser-induced incandescence

The evaluation of the temporal decay of the laser-induced incandescence (LII) signal from soot particles is introduced as a technique to obtain two-dimensional distributions of particle sizes and is applied to a laminar diffusion flame. This novel approach to soot sizing exhibits several theoretical and technical advantages compared with the established combination of elastic scattering and LII, especially as it yields absolute sizes of primary particles without requiring calibration.

Performance characteristics of soot primary particle size measurements by time-resolved laser-induced incandescence

A detailed analysis of various factors that influence the accuracy of time-resolved laser-induced incandescence for the determination of primary soot particles is given. As the technique relies on the measurement of the signal ratio at two detection times of the enhanced thermal radiation after an intense laser pulse, guidelines are presented for a suitable choice of detection times to minimize statistical uncertainty. An error analysis is presented for the issues of laser energy absorption, vaporization, heat conduction, and signal detection. Results are shown for a laminar ethene diffusion flame that demonstrate that concurring results are obtained for various laser irradiances, detection characteristics, and times of observation.

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.

Comprehensive two-dimensional soot diagnostics based on laser-induced incandescence (LII)

A strategy for a comprehensive description of soot sizes and concentrations is described, which relies on various two-dimensional techniques based on laser-induced incandescence (LII) and elastic scattering. Time-resolved LII (TIRE-LII) is an important tool in this process, it allows a direct measurement of primary particle size dp as a central quantity for the description of soot. The paper discusses the essential features of the method which is based on the acquisition of the LII signals at two delay times after an initial laser pulse, where monomer size can be deduced from the local signal ratio. Delays of about 100 and 800 ns should be chosen as moments of observation in order to obtain precise information for a wide range of primary particle sizes. Local soot volume fractions f, may be directly obtained from the prompt LII signal, where a necessary calibration factor can be deduced from a simple line-of-sight extinction measurement. In combination with elastic scattering, this technique can also be used to give relative values for the volume-equivalent diameter D of soot clusters. Additionally, the local mean number n of monomers within a cluster and number concentrations Np and Na of primary particles and aggregates, respectively. can be determined. Although some uncertainties persist regarding the absolute monomer sizes measured, and some data can be given only on a relative basis, experimental results obtained for the upper region of a laminar ethene diffusion flame are in good agreement with the established models of soot oxidation and aggregation.

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.

Laseroptische Charakterisierung von gasgetragenen Nanoteilchen mit der zeitaufgelösten laserinduzierten Glühtechnik (TIRE-LII) (Laser-Optical Characterization of Air-Borne Nanoparticles by Time-Resolved Laser-Induced Incandescence (TIRE-LII))

Abstract Die Charakterisierung gasgetragener Nanoteilchen ist für eine Vielzahl industrieller und natürlicher Prozesse von entscheidender Bedeutung. Insbesondere die Überwachung von in Verbrennungssystemen erzeugten Rußteilchen und die für die Nanoteilchentechnologie notwendige Produktcharakterisierung und Prozesssteuerung benötigen in zunehmendem Maße geeignete Analysenmesstechniken, die schnelle, empfindliche und zuverlässige Aussagen über eine Vielzahl von Messgrößen zulässt. Mit der laserinduzierten Glühtechnik (laser-induced incandescence, LII) steht ein nicht-invasives laseroptisches Online-Verfahren zur Verfügung, das neben einer hochempfindlichen Konzentrationsbestimmung auch dazu in der Lage ist, die spezifische Oberfläche bzw. die Primärteilchengröße der Partikel zu bestimmen.