3 Dynamics of Spray Granulation in Continuously …
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3.1.2 Materials
All experiments were conducted with porous γ -alumina particles (Sasol Germany
GmbH). The material, a model substance in drying and layering experiments, was
chosen because of its thermal and mechanical robustness, i.e., there is only limited
breakage and abrasion of particles even after repeated particle-particle or particlewall collisions. This simplified the tracking of particles. Two different size distributions were used: One with a mean diameter of 1.8 mm and a standard deviation
of 0.1 mm, the other with a mean diameter of 3 mm and a standard deviation of
0.1 mm. The particles belong to group D in the Geldart classification with minimum
fluidization velocities of 0.5 m/s (1.8 mm) and 0.81 m/s (3 mm), respectively.
3.2 Method Development: Particle Tracking Velocimetry
(PTV)
Particle tracking velocimetry (see [23–27] for details and successful applications)
involves the identification of individual particles in images, constructing individual particle trajectories and determining individual (Lagrangian) particle velocities
(Fig. 5). The main advantage of PTV is that particles are tracked directly and individually, allowing also the detection of small numbers of particles moving opposite to a
dominating particle flow, making PTV very suitable for the investigation of exchange
rates at weirs.
3.2.1 Image Acquisition
Particle movement in the two chambers was recorded with a high-speed camera
system: It consists of a 1024 × 1024 pixel Photron camera with a CMOS chip,
mounted on a solid frame. Two halogen bulbs with 400 W each were positioned to
provide uniform lighting of the field of view (FOV). The camera was operated at full
resolution at 1000 Hz, with an exposure time of 1/31000 s and a dynamic range of 10
bits. Operating the camera at these settings allows recording of 5000 images (5 s of
process time), a constraint imposed by the built-in memory chip. An objective lens
(60 mm, f = 4) was used to give the desired depth of field and light exposure. The
field of view (95 mm × 95 mm) was placed as sketched in Fig. 4. This position was
chosen in pre-trials and allows capturing the particle movement without losing spatial
resolution by the observation of empty zones or of regions where particle movement
is not of interest for weir passage (far left, far right of the weir). The overall setup was
controlled by the DaVis image acquisition software (LaVision GmbH, Germany).
75
3.1.2 Materials
All experiments were conducted with porous γ -alumina particles (Sasol Germany
GmbH). The material, a model substance in drying and layering experiments, was
chosen because of its thermal and mechanical robustness, i.e., there is only limited
breakage and abrasion of particles even after repeated particle-particle or particlewall collisions. This simplified the tracking of particles. Two different size distributions were used: One with a mean diameter of 1.8 mm and a standard deviation
of 0.1 mm, the other with a mean diameter of 3 mm and a standard deviation of
0.1 mm. The particles belong to group D in the Geldart classification with minimum
fluidization velocities of 0.5 m/s (1.8 mm) and 0.81 m/s (3 mm), respectively.
3.2 Method Development: Particle Tracking Velocimetry
(PTV)
Particle tracking velocimetry (see [23–27] for details and successful applications)
involves the identification of individual particles in images, constructing individual particle trajectories and determining individual (Lagrangian) particle velocities
(Fig. 5). The main advantage of PTV is that particles are tracked directly and individually, allowing also the detection of small numbers of particles moving opposite to a
dominating particle flow, making PTV very suitable for the investigation of exchange
rates at weirs.
3.2.1 Image Acquisition
Particle movement in the two chambers was recorded with a high-speed camera
system: It consists of a 1024 × 1024 pixel Photron camera with a CMOS chip,
mounted on a solid frame. Two halogen bulbs with 400 W each were positioned to
provide uniform lighting of the field of view (FOV). The camera was operated at full
resolution at 1000 Hz, with an exposure time of 1/31000 s and a dynamic range of 10
bits. Operating the camera at these settings allows recording of 5000 images (5 s of
process time), a constraint imposed by the built-in memory chip. An objective lens
(60 mm, f = 4) was used to give the desired depth of field and light exposure. The
field of view (95 mm × 95 mm) was placed as sketched in Fig. 4. This position was
chosen in pre-trials and allows capturing the particle movement without losing spatial
resolution by the observation of empty zones or of regions where particle movement
is not of interest for weir passage (far left, far right of the weir). The overall setup was
controlled by the DaVis image acquisition software (LaVision GmbH, Germany).