86
2 Experiments in Pebble Flows
Fig. 2.26 The experimental images of particles (left inset), the locations of the particles marked
by the red crosses (middle inset), and the trajectories of individual particles (right inset)
The images are preprocessed to reduce the background and the noise, which
includes convolving the image with a Gaussian filter and then an average filter. The
circular-shaped filter has been added to the algorithm to improve the particle tracking
as for the circular shape of the glass particles.
In PTV, the Relaxation Method (RM) is used [11]. In this algorithm, the location
of the pebble is identified with the Dynamic Threshold Binarization (DTB), where
the threshold level is detected particle by particle. The DTB is better than the SingleThreshold Binarization (STB) using a uniform threshold level to detect particles
[15].
2.5.3.12 Snapshots of Velocity Profiles
As aforementioned, the main focus of this section is to explore the flow field characteristics under the six types of bed configurations.
The transverse velocity is indicative of the character of horizontal dispersion in
the pebble flow (Fig. 2.27). The horizontal velocities in the central and near-wall
regions are low, while they are higher in the middle of the two regions at the same
heights. This is based on the fact that particles tend to drift horizontally toward
a zone with faster downward flows because they are likely to get more space to
move in the transverse direction. A largely uniform transverse velocity profile with
a smaller magnitude (less than 0.01 d/s) is established across the radius of the silo
above h = 30d. Below this height, the velocity profile is not uniform and becomes
increasingly concentrated toward the outlet region at lower heights.
The highest vertical velocity component v 0 (approximately 0.0158 d/s) is found
along the centerline on the level of h = 0d under all types of bed configurations.
In this experiment, for visualization, seven different zones have been identified in
the pebble bed during recirculation depending on the ratio α of local actual vertical
velocity component v to V 0 . A central rapidly flowing pipe in the lower part with a
velocity higher than 0.0048 (α=30%) was surrounded by a slowly flowing zone and
a nearly stagnant zone near the silo wall. The stagnant zone herein is defined by the
zones with velocities less than 0.0008 d /s (α=5%) located in the lower corner of the
2 Experiments in Pebble Flows
Fig. 2.26 The experimental images of particles (left inset), the locations of the particles marked
by the red crosses (middle inset), and the trajectories of individual particles (right inset)
The images are preprocessed to reduce the background and the noise, which
includes convolving the image with a Gaussian filter and then an average filter. The
circular-shaped filter has been added to the algorithm to improve the particle tracking
as for the circular shape of the glass particles.
In PTV, the Relaxation Method (RM) is used [11]. In this algorithm, the location
of the pebble is identified with the Dynamic Threshold Binarization (DTB), where
the threshold level is detected particle by particle. The DTB is better than the SingleThreshold Binarization (STB) using a uniform threshold level to detect particles
[15].
2.5.3.12 Snapshots of Velocity Profiles
As aforementioned, the main focus of this section is to explore the flow field characteristics under the six types of bed configurations.
The transverse velocity is indicative of the character of horizontal dispersion in
the pebble flow (Fig. 2.27). The horizontal velocities in the central and near-wall
regions are low, while they are higher in the middle of the two regions at the same
heights. This is based on the fact that particles tend to drift horizontally toward
a zone with faster downward flows because they are likely to get more space to
move in the transverse direction. A largely uniform transverse velocity profile with
a smaller magnitude (less than 0.01 d/s) is established across the radius of the silo
above h = 30d. Below this height, the velocity profile is not uniform and becomes
increasingly concentrated toward the outlet region at lower heights.
The highest vertical velocity component v 0 (approximately 0.0158 d/s) is found
along the centerline on the level of h = 0d under all types of bed configurations.
In this experiment, for visualization, seven different zones have been identified in
the pebble bed during recirculation depending on the ratio α of local actual vertical
velocity component v to V 0 . A central rapidly flowing pipe in the lower part with a
velocity higher than 0.0048 (α=30%) was surrounded by a slowly flowing zone and
a nearly stagnant zone near the silo wall. The stagnant zone herein is defined by the
zones with velocities less than 0.0008 d /s (α=5%) located in the lower corner of the