Recent applications of the WALS-technique
                              Hergen Oltmann1, Stefan Will2
                  1
                   Technische Thermodynamik, Universität Bremen, Germany
      2
      Lehrstuhl für Technische Thermodynamik, Universität Erlangen-Nürnberg, Germany
                            email: stefan.will@ltt.uni-erlangen.de

Nanoparticles produced in combustion processes often exhibit complex fractal
structures. While laser-induced incandescence (LII) is a proven technique for the
determination of primary particle size no information about aggregate sizes can be
obtained. To gather information about aggregate size and fractal dimension elastic
light scattering (ELS) [1] is an often used in situ method.

The wide-angle light scattering (WALS) approach [2] extends classical ELS-concepts
by using a combination of an ellipsoidal mirror and an intensified CCD-camera. The
ellipsoidal mirror redirects the light scattered within a plane onto the CCD-chip (cf.
Fig. 1), which makes it possible to almost instantaneously record a complete
scattering diagram over an angular range of approx. 10° to 170° with an angular
resolution ∆θ of typically 0.6°.

The basic performance of the approach was demonstrated previously by
measurements on soot particles in laminar premixed flames [2]. This contribution
highlights various recent developments and applications of the technique. These
include measurements in a turbulent diffusion flame [3], employing a pulsed laser
and underlining the favourable applicability to unsteady processes. Also
measurements with a particular high resolution of ∆θ = 0.3° were performed which
allow for a detailed investigation of selected angular regions. To simultaneously
measure the vv- and hh-scattering components polarization foils were mounted in
front of the ellipsoidal mirror. Radii of gyration obtained for soot particles in a
premixed ethene flame show good agreement with former results. Furthermore
investigations on silica particles produced in a diffusion flame were carried through
(cf. Fig. 2) for various relative velocities between the precursor flow (nitrogen flow
saturated with hexamethydisiloxane) and the methane/oxygen flow of the supporting
flame. Recorded scattering diagrams indicate a change in the structure of the silica
particles for the different velocities.




Fig. 1: Experimental setup            Fig. 2: Measurement on silica particles in a diffusion flame

[1]   C. M. Sorensen, Aerosol Sci. Technol. 35, 648-687 (2001)
[2]   H. Oltmann, J. Reimann, S. Will, Combust. Flame 157, 516-522 (2010)
[3]   H. Oltmann, J. Reimann, S. Will, Appl. Phys. B 106, 171-183 (2012)

                5th international workshop on Laser-Induced Incandescence
                 May 9-11, 2012, Palais des Congrès, Le Touquet, France