The significance of coherent flow structures for the turbulent mixing ...

2.3 Registration of the particle images
FIGURE 2.8: Left: Laser light-sheet visualisation of an air jet interacting with the liquid inside an
aerosol generator for tracer-particle generation
ƒbar). Centre: detail picture of particle filled air
bubbles immediately behind the nozzle exit. Right: detail picture of particle filled air bubbles above
the liquid surface.
‚ (µ
2.2.4 Conclusion
It could be shown that high concentrations of particles with a narrow band size distribution and
a mean diameter below*‡m can easily be generated by means of multi-hole nozzles under
over-critical pressure conditions bar) provided the kinetic energy, entering the liquid
volume, is sufficient to change the fluid mechanical state of the liquid to a highly turbulent
two-phase fluid. This experimental result can be explained by assuming that the nozzle has
basically three functions. Firstly, it generates the particles at the nozzle exit. Secondly, it
changes the fluid mechanical state of the fluid into a two phase liquid which seems to promote
the generation of smaller particle size distributions, as the fluid mechanical parameters are
(Ä *
different. Thirdly, the remaining kinetic energy which is not consumed for the generation
of the particles or the transition of the fluid mechanical state is transferred into the turbulent
motion of the liquid and may act as an active impactor as the shearing of the bubbles which
carry the particles to the liquid surface is enhanced. Besides this result, it could be qualitatively
shown that bubbles arise from the liquid-fed holes of the Laskin nozzle. This implies that no
particles can be generated at the ring side of the jet, as assumed in the literature, due to the
missing liquid in this region.
All validation experiments, discussed in chapter 2 to 4, were performed with a multihole
nozzle oil atomiser. For the experimental investigations described in chapter 5 to 7 the
particles were generated with a smoke generator because of the large observation distance
and the small aperture of the optical system. Because of the small free-stream velocity, the
velocity lag of the particles is negligible relative to the uncertainties due to the recording and
evaluation methods, which will be described in the following sections.
2.3 Registration of the particle images
Beside the particle dynamics, the registration, storage and read-out of the individual particle
images is another key element in PIV because the accuracy of the technique strongly depends
on the precision with which the image displacement can be related to particle locations
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