408
6 Applications: Two-Region Pebble Beds
pebbles has been presented. Third, the equilibrium conditions of the establishment
of the two regions have been discussed. Fourth, through investigating the flow field
in the experimental vessel with different heights, it has been determined that the
heightened part of the pebble packing can improve the uniformity of the flowing
in the lower. Finally, through the validated DEM program, the strategy to eliminate the stagnant zone has been verified. The entire validation of the re-circulating
two-region-designed pebble-bed reactor has been completed.
6.2 Size Effects
6.2.1 Numerical Setup
In the present simulation, the structure of the two-region pebble bed is based on the
experiment facility [3]. The vessel is a flat rectangular container with a slope in the
lower half (Fig. 6.8), and filled with glass particles. The vessel walls have the same
properties as those of the particles. The geometrical and physical parameters for the
present simulation are listed in Table 6.1. The vessel is 1.0 m in height, 0.8 m in
width, and 60 mm in thickness. The width of the bottom is 0.12 m, and the slope
angle of the base is 45
◦ C. The particles generated randomly are inserted through
the top region (Region A, B, and C, 36 mm in height) and discharged at a given
rate from the bottom. The same type of particles of 12 mm in diameter (type 1) is
inserted between regions A and C. A different type of particles with larger or smaller
diameters (type 2) is inserted through region B. There are two guide plates to ensure
no mixing taking place between different types of particles at the initial state.
Region A is as wide as Region C. The half-width of Region B is defined as L 1 . L 2
is the half-width of the total vessel. For investigating the effect of particle diameter
ratio and the width of the inserting region, two non-dimensional parameters, i.e.,
particle size ratio R D and region width ratio R L are considered. Here D 1 and D 2
are the particle diameters of type 1 (in Region A and C) and type 2 (in Region B),
respectively. In this analysis, 15 cases of different combinations of R D and R L are
simulated, and the detailed arrangements are shown in Table 6.2. In the initial state,
large quantities of spheres settle down inside the bed by gravity and fill the vessel.
Then, the guiding plates are removed. The rate of particle discharge is 100 particles
per second for R D = 1.0, 1.5, and 2.0. As the particle number for R D = 0.5 is much
larger than other cases. The discharge rate is 200 particles per second until 150 s and
then increases to 1,000 per second. The void in the top region will increase when the
particles are continuously removed from the granular system, and particles will be
reinserted into the pebble bed. f is the number percentage of type 2 particles in the
pebble bed, varying from 0.042 for Case 11 to 0.83 for Case 4.
The particles are not discharged from the bed freely. As seen in Fig. 6.8, a transport
unit is installed at the bottom of real reactors to let only one pebble pass through the
6 Applications: Two-Region Pebble Beds
pebbles has been presented. Third, the equilibrium conditions of the establishment
of the two regions have been discussed. Fourth, through investigating the flow field
in the experimental vessel with different heights, it has been determined that the
heightened part of the pebble packing can improve the uniformity of the flowing
in the lower. Finally, through the validated DEM program, the strategy to eliminate the stagnant zone has been verified. The entire validation of the re-circulating
two-region-designed pebble-bed reactor has been completed.
6.2 Size Effects
6.2.1 Numerical Setup
In the present simulation, the structure of the two-region pebble bed is based on the
experiment facility [3]. The vessel is a flat rectangular container with a slope in the
lower half (Fig. 6.8), and filled with glass particles. The vessel walls have the same
properties as those of the particles. The geometrical and physical parameters for the
present simulation are listed in Table 6.1. The vessel is 1.0 m in height, 0.8 m in
width, and 60 mm in thickness. The width of the bottom is 0.12 m, and the slope
angle of the base is 45
◦ C. The particles generated randomly are inserted through
the top region (Region A, B, and C, 36 mm in height) and discharged at a given
rate from the bottom. The same type of particles of 12 mm in diameter (type 1) is
inserted between regions A and C. A different type of particles with larger or smaller
diameters (type 2) is inserted through region B. There are two guide plates to ensure
no mixing taking place between different types of particles at the initial state.
Region A is as wide as Region C. The half-width of Region B is defined as L 1 . L 2
is the half-width of the total vessel. For investigating the effect of particle diameter
ratio and the width of the inserting region, two non-dimensional parameters, i.e.,
particle size ratio R D and region width ratio R L are considered. Here D 1 and D 2
are the particle diameters of type 1 (in Region A and C) and type 2 (in Region B),
respectively. In this analysis, 15 cases of different combinations of R D and R L are
simulated, and the detailed arrangements are shown in Table 6.2. In the initial state,
large quantities of spheres settle down inside the bed by gravity and fill the vessel.
Then, the guiding plates are removed. The rate of particle discharge is 100 particles
per second for R D = 1.0, 1.5, and 2.0. As the particle number for R D = 0.5 is much
larger than other cases. The discharge rate is 200 particles per second until 150 s and
then increases to 1,000 per second. The void in the top region will increase when the
particles are continuously removed from the granular system, and particles will be
reinserted into the pebble bed. f is the number percentage of type 2 particles in the
pebble bed, varying from 0.042 for Case 11 to 0.83 for Case 4.
The particles are not discharged from the bed freely. As seen in Fig. 6.8, a transport
unit is installed at the bottom of real reactors to let only one pebble pass through the