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

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2.2 Generation of appropriate tracer-particles
Air
Liquid
Air
Liquid
Air
Liquid
Feedholering
Impactorring
FIGURE 2.3: Principal side and top view of three utilised atomiser nozzles with different mechanisms
for particle generation. From left to right: reference nozzle, Laskin nozzle and suction nozzle with
impactor ring.
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13320-1, see [34]. This is possible for spherical particles with known optical properties, when
the particle concentration within the measurement domain is sufficiently low such that multiple
scattering and optical interference between the scattered radiation from different particles can
be neglected. Otherwise the measurements would be biased, as the diffraction pattern of
a particle ensemble is no longer identical to the superposition of the individual scattering
patterns of all particles present.
2.2.2 Particle size analysis
To ensure controlled boundary conditions before starting the experiment, each nozzle was
cleaned and the symmetry of the emerging jets was analysed, as any asymmetry would indicate
damages at the nozzle exit due to deposition of old oil or burr, for example. In addition,
all particles produced during the adjustment of the desired pressure level were completely
removed from the generator and the tube leading to the optical system. The reproducibility
of the generated particle distribution was finally confirmed by performing four independent
measurements for each set of parameters. To exclude
£–¥"!0¥G#./kg/(ms)
the influence of physical properties of
the liquid to be atomised, only with‡ vegetable kg/m˜ oil and
£¥"!0¥G#.-kg/(ms) with‡ DEHS used for all experiments, as both
liquids behave very similarly.
Figure 2.4 reveals the volumetric particle size distribution for the described nozzles for two
typical pressure states (0.5 and*bar) and, in addition, the distribution generated by means of a
smoke generator has been added for comparison. First, it can be seen that the performance
andcp£–45(—
of
the smoke generator is quite good with respect to the other atomisers, especially if the shape,
bandwidth and upper limit of the particle size distribution are considered. Unfortunately,
andcp£›š^*C#kg/m˜ were
17