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

2.2 Generation of appropriate tracer-particles
of the jets do not interact with each other (three planes#.¥mm apart from each other, each
with four holes arranged crosswise). As only the flow rate (and thus kinetic energy) entering
the liquid reservoir and available for turbulent mixing differs, it seems likely that the nozzle
has another function besides the generation of the particles, namely the sufficient mixing of
the liquid. This has two effects. First, the fluid mechanical state of the fluid changes into
a two-phase liquid which becomes clearly visible, as the liquid becomes milky when the kinetic
energy entering the liquid reservoir matches the liquid volume. This may promote the
generation of smaller particle size distributions. Secondly, the background turbulence within
the liquid reservoir increases strongly and causes an enhanced shearing of the bubbles which
carry the particles to the liquid surface. This shearing process, caused by the turbulent mixing,
can be seen as an active impactor which may enhance the rejection of large particles. These
assumptions were supported by measurement of the volumetric particle size distribution of a
12-hole nozzle which was dipped into a large liquid reservoir whose fluid mechanical state
and turbulence level could not be altered sufficiently by the kinetic energy entering from the
jet. Further evidence and additional information on this subject can be found in [43].
2.2.3 Flow visualisation
In order to examine the generation, movement, and delivery of the particles above the liquid
surface as a function of the pressure, a reference and Laskin nozzle, each with only one single
*mm drill, were operated in DEHS (vegetable oil is less suited for optical flow analysis due
to the strong absorption). For this visualisation the centreline of the air-jet was illuminated
with a vertically arranged short pulsed laser light-sheet and the scattered light was recorded
by means of a CCD camera. Figure 2.7 shows the global structure of the air jet from the
reference nozzle (left) and Laskin nozzle (right) for various pressure conditions (top to bot-
bar). The increasing horizontal extension of the jets with increasing exit
velocity and the decreasing divergence (top to bottom) are clearly visible. It can also be seen
that the size of the bubble structures changes with increasing free-stream velocity due to the
stronger mixing with the surrounding medium. The white circle in the left column reveals a
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delivered particle cloud which moves upwards in the form of a vortex structure with entrainment,
and at the liquid surface of the lower right images, a bubble is visible just before the
bursting takes place. The white circles and arrows in the right column indicate the rising bubbles
emerging from the liquid feed holes of the Laskin nozzle, as mentioned in the previous
section. This indicates that the holes in the ring are not operating in a way assumed in the
literature. Figure 2.8 shows a selection of three images recorded independently whereby the
white and and black squares in the left image indicate the size and location of the other two
high resolution pictures. The jet axis (see bright centreline of small white square) is approximately—¥mm
below the liquid level (see dark line within black square). The white circles
in the centre image indicate particle filled air bubbles below the liquid level (see white dots)
moving within the liquid, and the right image reveals a particle filled air bubble, which has
penetrated the liquid surface, immediately before bursting. So it is evident that the particles
are generated at the nozzle exit and transported within expanding air bubbles, which are exposed
to the turbulent motion of the surrounding fluid on their way to the liquid surface, where
the particles are delivered.
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