62
2 Experiments in Pebble Flows
right above the orifice move out of the bed and leave the voids they previously
occupied; then the next above pebbles move into these voids under gravity and leave
the next voids they previously occupied. There will be no pebble motion without
the voids remaining below. The motion of the pebbles propagates from the orifice
upward to the top, and then spreads sideward to the surrounding, during which the
stacking state of pebbles is changed to form a distribution of the porosity determined
by the propagation of pebble motion. However, pebbles do not flow into the voids
immediately but under certain conditions when the void grows large enough, which
is usually enclosed by several pebbles with the size from less than one diameter
to several diameters of the pebble. Because the size of the pebbles is much less
than that of the vessel, it is impossible to accumulate very large cavities (except for
pebbles arched above the orifice). When pebbles flow into the voids, they do not
move until new conditions are re-met, and then fresh flows are driven. As a result,
unlike the flow of fluid, the flow of pebbles is intermittent and noncontinuous and
presents discretization. Consequently, an interpretation of what is the quasi-static
gravity-driven dense pebble flow is given below.
Greatly different from the steady flow of the pebbles in the drainage pebble mode,
firstly, the quasi-static pebble flow experiences a process from rest to motion and back
to rest. The steady pebble flow has no obvious intervals of resting time, but always in
motion. Secondly, the disturbance produced by the steady flow in the drainage pebble
manner is significantly stronger than that in the quasi-static flow. The interpretation
of the quasi-static flow is as follows: it is only driven by gravity without extra loads,
and experiences a process from rest to motion and back to rest under the small
disturbance, during which the relaxation time is much less than the processing time.
This means the falling pebbles inside the pebble packing quickly re-achieve their
equilibrium state and almost have no collision effects.
The flow of the pebbles in the real pebble-bed reactor is very slow and pebbles are
in a static equilibrium most of the time. Therefore the resting time can be reduced
while still keeping the characteristics of the quasi-static flow to quicken the rate of
circulation in practical experiments significantly.
2.4.2 Distribution of Contact Force
For example, the left and right figures in Fig. 2.13 show the DEM simulation on
the distribution of the contact force in the pebble packing under friction coefficient
μ= 0.2, conical angle of bed A bed = 30
◦ C at t = 0 and t = 22 h, respectively. It is
shown that the stacking of the pebble packing is different between the initial state
and the final state. The deeper color represents the more significant contact force,
less porosity, or more compact arrangement. It is found that the contact force in the
lower section is more significant than that in the upper section at the initial state,
while at the final state just the opposite is exact; furthermore, two areas with the
more considerable contact force emerge in the corners of the vessel, in which the
stagnant zones exist. It is noted that an area with a somewhat larger contact force
2 Experiments in Pebble Flows
right above the orifice move out of the bed and leave the voids they previously
occupied; then the next above pebbles move into these voids under gravity and leave
the next voids they previously occupied. There will be no pebble motion without
the voids remaining below. The motion of the pebbles propagates from the orifice
upward to the top, and then spreads sideward to the surrounding, during which the
stacking state of pebbles is changed to form a distribution of the porosity determined
by the propagation of pebble motion. However, pebbles do not flow into the voids
immediately but under certain conditions when the void grows large enough, which
is usually enclosed by several pebbles with the size from less than one diameter
to several diameters of the pebble. Because the size of the pebbles is much less
than that of the vessel, it is impossible to accumulate very large cavities (except for
pebbles arched above the orifice). When pebbles flow into the voids, they do not
move until new conditions are re-met, and then fresh flows are driven. As a result,
unlike the flow of fluid, the flow of pebbles is intermittent and noncontinuous and
presents discretization. Consequently, an interpretation of what is the quasi-static
gravity-driven dense pebble flow is given below.
Greatly different from the steady flow of the pebbles in the drainage pebble mode,
firstly, the quasi-static pebble flow experiences a process from rest to motion and back
to rest. The steady pebble flow has no obvious intervals of resting time, but always in
motion. Secondly, the disturbance produced by the steady flow in the drainage pebble
manner is significantly stronger than that in the quasi-static flow. The interpretation
of the quasi-static flow is as follows: it is only driven by gravity without extra loads,
and experiences a process from rest to motion and back to rest under the small
disturbance, during which the relaxation time is much less than the processing time.
This means the falling pebbles inside the pebble packing quickly re-achieve their
equilibrium state and almost have no collision effects.
The flow of the pebbles in the real pebble-bed reactor is very slow and pebbles are
in a static equilibrium most of the time. Therefore the resting time can be reduced
while still keeping the characteristics of the quasi-static flow to quicken the rate of
circulation in practical experiments significantly.
2.4.2 Distribution of Contact Force
For example, the left and right figures in Fig. 2.13 show the DEM simulation on
the distribution of the contact force in the pebble packing under friction coefficient
μ= 0.2, conical angle of bed A bed = 30
◦ C at t = 0 and t = 22 h, respectively. It is
shown that the stacking of the pebble packing is different between the initial state
and the final state. The deeper color represents the more significant contact force,
less porosity, or more compact arrangement. It is found that the contact force in the
lower section is more significant than that in the upper section at the initial state,
while at the final state just the opposite is exact; furthermore, two areas with the
more considerable contact force emerge in the corners of the vessel, in which the
stagnant zones exist. It is noted that an area with a somewhat larger contact force