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

¹
and
2.2 Generation of appropriate tracer-particles
2
100
Frequency distribution
1
Cumulative percentage
50
d=0.5, p=0.5, n=4
®d=1.0, p=0.5, n=4
®d=2.0, p=0.5, n=4
®d=1.0, p=0.5, n=12
®®d=1.0, p=0.5, n=4, Imp
0
0
2 4
8
« 6¬ ª 0©
­
10
Particle size [ µm] ¨
2
100
10
Particle size [ µm] ¨ 1§
Frequency distribution
1
Cumulative percentage
50
d=0.5, p=1.0, n=4
®d=1.0, p=1.0, n=4
®d=2.0, p=1.0, n=4
®d=1.0, p=1.0, n=12
®®d=1.0, p=1.0, n=4, Imp
0
0
2 4
8
« 6¬ ª 0©
­
10
Particle size [ µm] ¨
10
Particle size [ µm] ¨ 1§
†¯±°.²`ƒ&²´³mm), FIGURE 2.5: Volumetric particle size distribution for various hole diameters (
pressure
number of holes per nozzle 12) and impactor.
( (µ ‚
andƒbar),
the effect of an impactor behind the output of the seeding generator. The upper left
plot of figure 2.5 implies that shortly above the minimum pressure required for the production
of particles, only the impactor is well suited to keep the particle size distribution sufficiently
†¯±°
narrow and the mean particle diameter sufficiently small for accurate flow investigations, but
‚
it
could be seen by visual inspection, that the particle concentration produced is quite low when
only a single four-hole nozzle is used. The effect of the impactor can be clearly seen by comparing
the dot-dashed line with the dotted graph. It should be mentioned that additional use of
more impactors (cascade arrangement) has no further effect on the particle size distribution.
By increasing the pressure from 0.5 to*bar, the exit velocity of the air jet reaches the speed
of sound and
£€*C#)
the performance of the two high mass flux nozzles with —) and
becomes comparable with the desired impactor distribution but the number of
particles produced is several orders of magnitude higher as could be qualitatively seen by visual
inspection. The other nozzles, in contrast, need much higher pressure before the shoulder
disappears and the mean diameter becomes about*‡m, see figure 2.6. This implies that a contaminated
nozzle hole, due to polluted air or encrusted oil, will tend to create larger particles
(rº£»#^_¹ £
as the effective diameter decreases. This was validated in several experiments. Unfortunately,
(r¢£€*_¹
the high pressure investigation could not be performed by using the high flow rate nozzles,
‚¸· and
19