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7 Applications: Pebble Flow Optimizations
and has to be selected carefully. An optimum distance can resolve crystallization
without increasing much resistance to a pebble flow. In the investigated conditions
in this analysis, 20 cm (about 16.7 times of the pebble diameter) shows the best
performance.
For the impact of a flow-corrective insert on the flow field in a pebble-bed reactor,
it has been verified that a proper application of an insert can improve the flow performance of a pebble bed by optimizing uniformity and decreasing stagnant zone. The
insert helps to turn typical funnel flow to the desirable mass flow for HTGR engineering, and this is achieved without modifying bed configuration and particle–wall
friction.
For the sake of studying three crucial issues of a pebble flow in a pebble-bed
HTGRs, the stagnant zone, the radial uniformity, and flow sequence, three corresponding characteristic values (stagnant rate—R stag , vertical standard deviation—
D v , and mean sequence value—S M ) are defined to evaluate the flow performance
of different pebble beds. The pebble bed equipped with an insert gets significantly
better results than a normal bed from these three aspects.
Several universal tips for the insert design in a pebble-bed HTGR have been
concluded after repeating simulations. The key to the insert design is to balance
flows inside and outside the insert, and a proper insert can restrict dominated central
flow and promote disadvantaged side flow to create a relatively uniform flow field.
The outlet diameter of an insert is the critical parameter that determines the balancing
mechanism, and there exists an optimized value for each case. The reasonable insert
design should exactly cover the region where unbalanced flow happens, and minimize
the side effect on the upper bin part by adjusting the insert height and location.
Specifically, the height of the insert is recommended not to be larger than the hopper
part. The insert should be located close to the discharge hole, and its inlet should not
exceed the hopper–bin junction surface too much to avoid an adverse effect on the
upper flow field.
For the bed-configuration effects on the pebble flow uniformity and stagnation
characteristics in a gravity drained pebble bed, the results demonstrated that the pebble flows under a brachistochrone-shaped contraction configuration of the bed are the
most uniform, far better than any other cases using arc-shaped bed configurations
{R n }. It has almost no stagnated pebbles inside the bed even after 2Np recirculated pebbles. It also has the most concentrated distribution of recycling rates. The
general sequence of the flow uniformity level is “Brachistochrone-curve > R 2 >
R 3 or R ∞ > R 1 ”.
Furthermore, the radial distribution of the velocity and residence time shows the
quantified evaluation of the bed configuration effects. The distinguished uniform
distributions of the velocity and residence time under the brachistochrone-shaped
bed configuration are illustrated. It is strongly suggested to use a brachistochroneshaped configuration for the pebble-bed reactor core. If not, the alternative choice
would be the R 2 configuration which is quite near the brachistochrone configuration.
Concerning the friction effects, it is validated that reducing particle–wall friction
and particle–particle friction can help to form the preferred flow pattern—mass flow
in a pebble-bed HTGR. Compared with particle–particle friction, particle–wall fric-
7 Applications: Pebble Flow Optimizations
and has to be selected carefully. An optimum distance can resolve crystallization
without increasing much resistance to a pebble flow. In the investigated conditions
in this analysis, 20 cm (about 16.7 times of the pebble diameter) shows the best
performance.
For the impact of a flow-corrective insert on the flow field in a pebble-bed reactor,
it has been verified that a proper application of an insert can improve the flow performance of a pebble bed by optimizing uniformity and decreasing stagnant zone. The
insert helps to turn typical funnel flow to the desirable mass flow for HTGR engineering, and this is achieved without modifying bed configuration and particle–wall
friction.
For the sake of studying three crucial issues of a pebble flow in a pebble-bed
HTGRs, the stagnant zone, the radial uniformity, and flow sequence, three corresponding characteristic values (stagnant rate—R stag , vertical standard deviation—
D v , and mean sequence value—S M ) are defined to evaluate the flow performance
of different pebble beds. The pebble bed equipped with an insert gets significantly
better results than a normal bed from these three aspects.
Several universal tips for the insert design in a pebble-bed HTGR have been
concluded after repeating simulations. The key to the insert design is to balance
flows inside and outside the insert, and a proper insert can restrict dominated central
flow and promote disadvantaged side flow to create a relatively uniform flow field.
The outlet diameter of an insert is the critical parameter that determines the balancing
mechanism, and there exists an optimized value for each case. The reasonable insert
design should exactly cover the region where unbalanced flow happens, and minimize
the side effect on the upper bin part by adjusting the insert height and location.
Specifically, the height of the insert is recommended not to be larger than the hopper
part. The insert should be located close to the discharge hole, and its inlet should not
exceed the hopper–bin junction surface too much to avoid an adverse effect on the
upper flow field.
For the bed-configuration effects on the pebble flow uniformity and stagnation
characteristics in a gravity drained pebble bed, the results demonstrated that the pebble flows under a brachistochrone-shaped contraction configuration of the bed are the
most uniform, far better than any other cases using arc-shaped bed configurations
{R n }. It has almost no stagnated pebbles inside the bed even after 2Np recirculated pebbles. It also has the most concentrated distribution of recycling rates. The
general sequence of the flow uniformity level is “Brachistochrone-curve > R 2 >
R 3 or R ∞ > R 1 ”.
Furthermore, the radial distribution of the velocity and residence time shows the
quantified evaluation of the bed configuration effects. The distinguished uniform
distributions of the velocity and residence time under the brachistochrone-shaped
bed configuration are illustrated. It is strongly suggested to use a brachistochroneshaped configuration for the pebble-bed reactor core. If not, the alternative choice
would be the R 2 configuration which is quite near the brachistochrone configuration.
Concerning the friction effects, it is validated that reducing particle–wall friction
and particle–particle friction can help to form the preferred flow pattern—mass flow
in a pebble-bed HTGR. Compared with particle–particle friction, particle–wall fric-