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

¿
8 Summary
The first part of this thesis examines different aspects of the Particle Image Velocimetry which
are of fundamental importance for accurate measurements but which were not considered in
detail in the literature. Starting point in chapter 2 is the general problem associated with the
reproducible generation of sufficiently monodisperse particles with an appropriate size, shape
and density that they follow the macroscopic flow motion faithfully without disturbing the
flow or fluid properties. It could be shown quantitatively that high concentrations of particles
with a narrow band size distribution and a mean diameter below m can be generated best
by means of multi-hole nozzles under over-critical pressure conditions ¿:9 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 was explained
by assuming that the nozzle has basically three functions. First, 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, because
the fluid mechanical parameters are 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 it may act as an active impactor
as the shearing of the bubbles which carry the particles to the liquid surface is enhanced. In
order to examine the generation, transport and delivery of the particles inside the atomiser in
detail, qualitative visualisation experiments were performed. It could be shown that bubbles
arise from the liquid-feed holes of the Laskin nozzle. This implies that no particles can be
generated at the ring side of the jet, in contrast to previous explanations in the literature.
Beside the fundamental problems associated with the generation of appropriate tracer particles,
the registration, the storage and the read-out of the particle images were investigated in
detail. This is another key element in PIV as the accuracy of the technique strongly depends on
the precision with which the image displacement can be related to particle locations and their
respective particle displacements. The discussion of the physical aspects of commonly used
CCD sensors and electronics implies that the measurement noise in PIV is strongly affected
by the geometrical properties of the picture elements but also by nonlinearities in the pixel
response, the read-out of the images, gain and offset errors during the AD-conversion, amplification
and transportation via long cable connections. It can be concluded from the analysis
that the measurement error introduced by the CCD sensor increases with decreasing dimensions
of the interrogation window. Thus, for accurate displacement estimation the size of the
interrogation window should be selected in such a way that the errors introduced by the CCD
are averaged out. In this case the remaining measurement error is mainly determined by the
performance of the peak fitting routine for sub-pixel accuracy. This error was also investigated
in the same chapter. It could be shown that under ideal experimental conditions this remaining
error is approximately 0.1 pixel for all displacements when a two-dimensional Gaussian peak
163