6.2 Size Effects
423
system, one type-1 or type-2 particle colored either by red or green is inserted into
the pebble bed correspondingly at the same rate of discharge.
By comparing Fig. 6.18a–c, type-2 particles are more likely to flow downside for
R D = 0.5, and both type-1 and type 2 particles flow in bulks for R D = 2.0. Because it is
much easier for the type-1 particles in the side regions to be driven by the large-sized
particles (with large inertia) in the central region to flow downside than by smallsized particles (with small inertia) in the central region. Thus, Fig. 6.18d–f show
that the stagnant region for R D = 2.0 (large inertia particle in the central region and
small inertia particle on the two sides) is much smaller than that for R D = 0.5 (small
inertia particle in the central region and large inertia particle on the two sides). In
other words, arranging the large-sized particles in the central region and small-sized
particles in the side regions will be beneficial to reduce the stagnant region near the
two sidewalls.
In addition, the effects of multi-size on the pebble flow system can be considered
by analogical prediction. For example, for the system composed of m-species-sized
particles, an averaged particle diameter ratio can be defined by R D =
D 2
D 1
+
D 3
D 1
+···+
Dn
D 1
(n−1)
,
and an averaged region ratio can be defined by R L =
L 2
L 1
+
L 3
L 1
+···+
Ln
L 1
(n−1)
. Analogical to
the binary mixtures, the conclusion of multi-sized particles may be predicted, for
example, as follows: 1). R D has larger effects on the mixing and dispersion of pebble
than R L ; 2). The mixing process will be enhanced when the averaged particle diameter
ratio increases at R D > 1; 3). When more large-sized particles are inserted into the
region near the center at the top, the stagnant zone will decrease significantly.
6.2.6 Short Summary
The two-region pebble bed is an advanced design aiming at improving the economical
efficiency of HTGR. In this section, the simulations are performed by DEM. For
quantifying the mixing process of the two-region pebble bed, the Particle-Scale
Mixing Index (PSMI) and mixing entropy are used. The influence of particle size
and region width is investigated in different cases at a given discharge rate.
A steady two-region structure is formed when different types of particles in the
pebble bed reach a balance dynamically. Then, a similarity of mixing under different
discharge rates is found in the steady two-region distribution structure. The mixing
region is more complicated in shape for the cases with larger particle size differences.
The fractal dimension increases quantitatively from 1.316 for R D = 1.0 to 1.393 for
R D = 2.0. Both the total particle volume and mixing entropy of the mixing interface
in the relatively steady state increase with the rise of size ratio for R D >1.0, which
is hardly influenced by region ratio R L .
The effect of region ratio on PSMI of two-region pebble bed is also remarkable.
There is an increasing tendency of PSMI when the region ratio or size ratio rises.
When the size ratio varies from 0.5 to 2.0, the diffusion of small particles into the
423
system, one type-1 or type-2 particle colored either by red or green is inserted into
the pebble bed correspondingly at the same rate of discharge.
By comparing Fig. 6.18a–c, type-2 particles are more likely to flow downside for
R D = 0.5, and both type-1 and type 2 particles flow in bulks for R D = 2.0. Because it is
much easier for the type-1 particles in the side regions to be driven by the large-sized
particles (with large inertia) in the central region to flow downside than by smallsized particles (with small inertia) in the central region. Thus, Fig. 6.18d–f show
that the stagnant region for R D = 2.0 (large inertia particle in the central region and
small inertia particle on the two sides) is much smaller than that for R D = 0.5 (small
inertia particle in the central region and large inertia particle on the two sides). In
other words, arranging the large-sized particles in the central region and small-sized
particles in the side regions will be beneficial to reduce the stagnant region near the
two sidewalls.
In addition, the effects of multi-size on the pebble flow system can be considered
by analogical prediction. For example, for the system composed of m-species-sized
particles, an averaged particle diameter ratio can be defined by R D =
D 2
D 1
+
D 3
D 1
+···+
Dn
D 1
(n−1)
,
and an averaged region ratio can be defined by R L =
L 2
L 1
+
L 3
L 1
+···+
Ln
L 1
(n−1)
. Analogical to
the binary mixtures, the conclusion of multi-sized particles may be predicted, for
example, as follows: 1). R D has larger effects on the mixing and dispersion of pebble
than R L ; 2). The mixing process will be enhanced when the averaged particle diameter
ratio increases at R D > 1; 3). When more large-sized particles are inserted into the
region near the center at the top, the stagnant zone will decrease significantly.
6.2.6 Short Summary
The two-region pebble bed is an advanced design aiming at improving the economical
efficiency of HTGR. In this section, the simulations are performed by DEM. For
quantifying the mixing process of the two-region pebble bed, the Particle-Scale
Mixing Index (PSMI) and mixing entropy are used. The influence of particle size
and region width is investigated in different cases at a given discharge rate.
A steady two-region structure is formed when different types of particles in the
pebble bed reach a balance dynamically. Then, a similarity of mixing under different
discharge rates is found in the steady two-region distribution structure. The mixing
region is more complicated in shape for the cases with larger particle size differences.
The fractal dimension increases quantitatively from 1.316 for R D = 1.0 to 1.393 for
R D = 2.0. Both the total particle volume and mixing entropy of the mixing interface
in the relatively steady state increase with the rise of size ratio for R D >1.0, which
is hardly influenced by region ratio R L .
The effect of region ratio on PSMI of two-region pebble bed is also remarkable.
There is an increasing tendency of PSMI when the region ratio or size ratio rises.
When the size ratio varies from 0.5 to 2.0, the diffusion of small particles into the