96
2 Experiments in Pebble Flows
wedged section, δ x (h) is smaller than δ
(h) at each height for the same configuration.
In a sense, the δ x (h) can be regarded as an index to indicate the extended width of the
vessel at each height to decrease the boundary layer effect. Particularly, the extended
width of the vessel at h = 0d (increasing the radius of the outlet tube) is an effective
way to make more pebbles move down with the mass flow pattern throughout the
whole flow field [47, 48].
2.5.4 Pebble Arch Formation
An experimental study of very slow dense pebble flow in a silo bed is conducted
by employing an improved particle tracking method that combines the relaxation
method and the Voronoï diagram. The PTV method is validated by comparison with
DEM simulation and experimental results. Velocity and fluctuation characteristics on
different streamlines are investigated to show the macroscopic kinematics of pebble
flow. Bulk arches on different locations of the pebble bed are identified. Two local
arching characteristics, including size distribution and horizontal span of the arches,
are analyzed. Their effects on the whole flow regime are discussed. It is found that
the arch size distribution follows the second-order polynomial distribution in the
semi-logarithmic scale. The horizontal span of arches provides evidence for the
transition between the ordered and disordered regimes. The probability distribution
of successive angles between the neighbors of arching particles is a good indicator
of arch stability. The relationship between arch breakdown and contact network
change is demonstrated by correlation analysis. Finally, a high correlation between
arching structure and mean velocity in targeted regions is indicated. The characteristic
lifetime of arches and autocorrelation time of fluctuation velocity present a positive
correlation. The bulk arching dynamics are the main reason for the fluctuations in
particle velocities.
2.5.4.1 Experimental Setup
In this section, a 2-D test facility is designed based on a real pebble-bed reactor at
a scale of 1:5 (Fig. 2.25). The experimental setup consists of several main parts.
The particles used here are made up of plexiglass with a density of 1700 kg/m
3
and a friction coefficient of 0.1. Firstly, the vessel is made up of plexiglass with the
dimensions of 800×1000×120mm in width, height, and thickness, respectively. The
base angle of the hopper section is 30
◦ . About 70,000 black glass pebbles with a
diameter of 12mm fill the vessel until the pebble pile is established [1]. Secondly,
three inlet tubes are set on the vessel top through which the pebbles can fall into
the bed. Finally, the discharge hole is at the bottom of the setup, with 120mm in
diameter and 200mm in length. The operation method is as follows [40, 49]: 1).
The experiment begins with a random packing by pre-filling the black pebbles in
the vessel. 2). The pebble bed is fed with 14:122:14 pebbles per minute from the
2 Experiments in Pebble Flows
wedged section, δ x (h) is smaller than δ
(h) at each height for the same configuration.
In a sense, the δ x (h) can be regarded as an index to indicate the extended width of the
vessel at each height to decrease the boundary layer effect. Particularly, the extended
width of the vessel at h = 0d (increasing the radius of the outlet tube) is an effective
way to make more pebbles move down with the mass flow pattern throughout the
whole flow field [47, 48].
2.5.4 Pebble Arch Formation
An experimental study of very slow dense pebble flow in a silo bed is conducted
by employing an improved particle tracking method that combines the relaxation
method and the Voronoï diagram. The PTV method is validated by comparison with
DEM simulation and experimental results. Velocity and fluctuation characteristics on
different streamlines are investigated to show the macroscopic kinematics of pebble
flow. Bulk arches on different locations of the pebble bed are identified. Two local
arching characteristics, including size distribution and horizontal span of the arches,
are analyzed. Their effects on the whole flow regime are discussed. It is found that
the arch size distribution follows the second-order polynomial distribution in the
semi-logarithmic scale. The horizontal span of arches provides evidence for the
transition between the ordered and disordered regimes. The probability distribution
of successive angles between the neighbors of arching particles is a good indicator
of arch stability. The relationship between arch breakdown and contact network
change is demonstrated by correlation analysis. Finally, a high correlation between
arching structure and mean velocity in targeted regions is indicated. The characteristic
lifetime of arches and autocorrelation time of fluctuation velocity present a positive
correlation. The bulk arching dynamics are the main reason for the fluctuations in
particle velocities.
2.5.4.1 Experimental Setup
In this section, a 2-D test facility is designed based on a real pebble-bed reactor at
a scale of 1:5 (Fig. 2.25). The experimental setup consists of several main parts.
The particles used here are made up of plexiglass with a density of 1700 kg/m
3
and a friction coefficient of 0.1. Firstly, the vessel is made up of plexiglass with the
dimensions of 800×1000×120mm in width, height, and thickness, respectively. The
base angle of the hopper section is 30
◦ . About 70,000 black glass pebbles with a
diameter of 12mm fill the vessel until the pebble pile is established [1]. Secondly,
three inlet tubes are set on the vessel top through which the pebbles can fall into
the bed. Finally, the discharge hole is at the bottom of the setup, with 120mm in
diameter and 200mm in length. The operation method is as follows [40, 49]: 1).
The experiment begins with a random packing by pre-filling the black pebbles in
the vessel. 2). The pebble bed is fed with 14:122:14 pebbles per minute from the