454
7 Applications: Pebble Flow Optimizations
are forced to move toward the bulk region rather than flow along the wall. This
mechanism is simple but effective. Wall structures with large radial dimension, like
“triangle_12_10”, “triangle_12_20”, and “arc_12_20”, can push near-wall pebbles
farther away from the wall than other ones. One might notice that “triangle_12_20”
experiences a decrease in the last 20 s. That is because some colored pebbles flow
into the hopper part of the bed and are excluded from the calculation, reducing the
mean value. The dispersion effect is complicated, and many factors like friction and
random crystallization are also responsible for its mechanism.
7.3 Flow-Corrective Insert Optimization
A flow-corrective insert is a potentially useful device that can be adopted in a pebblebed High-Temperature Gas-cooled Reactor (HTGR) if necessary to improve the flow
performance of the slow discharge-controlled pebble flow. In this section, DEM
simulation is utilized to verify that an adequately designed insert can help change
the unsatisfactory flow field to the preferred flow pattern in a pebble bed significantly.
Three characteristic values aimed at evaluating stagnant zone, radial uniformity, and
flow sequence of the pebble flow are used to study simulation results, and the pebble
bed equipped with the insert works better than a normal bed from all these three
aspects. Based on repeating experiments, several universal tips for an insert design
in a pebble-bed reactor have been concluded.
7.3.1 Numerical Setup
In some industrial areas, an alternative method to promote mass flow without changing the geometry of a silo is the application of a flow-corrective insert (e.g., Fig.
7.10) [5–7]. These inserts have various forms, such as flat, column, taper, spindle,
and hopper. Although various types of inserts are used in industries, the understanding of the mechanism with a fixed insert placed in a silo is not known with certainty.
The usual procedure for finding the proper form, size, and placement is by trial and
error. The left side of Fig. 7.10 shows various inserts utilized in industry or experiments, and they are double cone, cone-in-cone, double cone-in-cone, and inverted
cone [5]. The right side displays the one adopted in the simulation, which belongs to
the cone-in-cone type. What is more, all of these applications are designed for free
outflow.
This subsection aims to verify the applicability of the insert effect in the specific
very slow discharge-controlled pebble flow applied in pebble-bed HTGRs through
3D simulations. Some strategies to help design inserts for the pebble-bed reactors
are proposed. The discrete element method is employed to carry out the simulations
of a pebble flow.
The longitudinal section of the three-dimensional cylindrical pebble bed with a
contraction bottom is depicted in Fig. 7.11a. The bed with 1 m total height and 0.8 m
7 Applications: Pebble Flow Optimizations
are forced to move toward the bulk region rather than flow along the wall. This
mechanism is simple but effective. Wall structures with large radial dimension, like
“triangle_12_10”, “triangle_12_20”, and “arc_12_20”, can push near-wall pebbles
farther away from the wall than other ones. One might notice that “triangle_12_20”
experiences a decrease in the last 20 s. That is because some colored pebbles flow
into the hopper part of the bed and are excluded from the calculation, reducing the
mean value. The dispersion effect is complicated, and many factors like friction and
random crystallization are also responsible for its mechanism.
7.3 Flow-Corrective Insert Optimization
A flow-corrective insert is a potentially useful device that can be adopted in a pebblebed High-Temperature Gas-cooled Reactor (HTGR) if necessary to improve the flow
performance of the slow discharge-controlled pebble flow. In this section, DEM
simulation is utilized to verify that an adequately designed insert can help change
the unsatisfactory flow field to the preferred flow pattern in a pebble bed significantly.
Three characteristic values aimed at evaluating stagnant zone, radial uniformity, and
flow sequence of the pebble flow are used to study simulation results, and the pebble
bed equipped with the insert works better than a normal bed from all these three
aspects. Based on repeating experiments, several universal tips for an insert design
in a pebble-bed reactor have been concluded.
7.3.1 Numerical Setup
In some industrial areas, an alternative method to promote mass flow without changing the geometry of a silo is the application of a flow-corrective insert (e.g., Fig.
7.10) [5–7]. These inserts have various forms, such as flat, column, taper, spindle,
and hopper. Although various types of inserts are used in industries, the understanding of the mechanism with a fixed insert placed in a silo is not known with certainty.
The usual procedure for finding the proper form, size, and placement is by trial and
error. The left side of Fig. 7.10 shows various inserts utilized in industry or experiments, and they are double cone, cone-in-cone, double cone-in-cone, and inverted
cone [5]. The right side displays the one adopted in the simulation, which belongs to
the cone-in-cone type. What is more, all of these applications are designed for free
outflow.
This subsection aims to verify the applicability of the insert effect in the specific
very slow discharge-controlled pebble flow applied in pebble-bed HTGRs through
3D simulations. Some strategies to help design inserts for the pebble-bed reactors
are proposed. The discrete element method is employed to carry out the simulations
of a pebble flow.
The longitudinal section of the three-dimensional cylindrical pebble bed with a
contraction bottom is depicted in Fig. 7.11a. The bed with 1 m total height and 0.8 m