46
2 Experiments in Pebble Flows
method, pre-filled stripes method, and pre-filled core method. Before the discussion,
we firstly introduce a specific experiment to illustrate the effect of different operation
modes.
2.2.1 Drainage Pebble Experiment
The drainage pebble experiment includes preloaded central column (Fig. 2.2a) and
preloaded horizontal stripes experiment (Fig. 2.2b). In the beginning, a metal baffle
was installed at the bottom of the drainage hole to prevent pebbles from flowing out.
Then the baffle was removed after pebbles were filled in the vessel. As a consequence,
pebbles started to drain out from the bottom under gravity without refilling freely.
The preloaded central column was designed to display the horizontal component
of pebble motion, and the preloaded horizontal stripes showed the vertical motion
pattern. Drainage pebble experiments illustrate the following:
• The angle of repose is quickly formed on the free surface in the upper section of
the pebble bed, which is equal to that formed by inserting pebbles.
• The horizontal component of the pebble motion in the upper section of the pebble
bed is larger than that in the lower, greatly different from that of the recirculation
mode.
• The drainage pebble mode presents a steady flow other than a quasi-static pebble
flow, different from that in the real pebble-bed reactor.
The drainage pebble experiments indicate that the pebble flow in the experiment
design is not entirely the plug-like flow, and present great non-uniformity, as if almost
all of the pebbles are drained out from a narrow area located in the center of the pebble
packing. These results are significantly different from MIT’s experiments [3, 5–7].
Thus, the operation mode is still a critical factor. In all of the following investigations,
the recirculation mode is adopted, which is the real operation mode of the pebble-bed
reactor.
2.2.2 Central Area Method
The central area method is designed to investigate the establishment of the two-region
arrangement and the mixing zone between regions. In the beginning, the experiment
vessel was pre-filled with about 70,000 colorless glass pebbles forming the initial
state of random pebble packing. Then, black glass pebbles were added in the middle,
whereas colorless pebbles were added on both sides of the vessel. The pebbles were
discharged one by one from the bottom at the same time. The forming process of
the two regions was displayed, and eventually reached a steady state approximately
after 7.5 h. The size of the two regions is determined by the ratio of loading pebbles,
2 Experiments in Pebble Flows
method, pre-filled stripes method, and pre-filled core method. Before the discussion,
we firstly introduce a specific experiment to illustrate the effect of different operation
modes.
2.2.1 Drainage Pebble Experiment
The drainage pebble experiment includes preloaded central column (Fig. 2.2a) and
preloaded horizontal stripes experiment (Fig. 2.2b). In the beginning, a metal baffle
was installed at the bottom of the drainage hole to prevent pebbles from flowing out.
Then the baffle was removed after pebbles were filled in the vessel. As a consequence,
pebbles started to drain out from the bottom under gravity without refilling freely.
The preloaded central column was designed to display the horizontal component
of pebble motion, and the preloaded horizontal stripes showed the vertical motion
pattern. Drainage pebble experiments illustrate the following:
• The angle of repose is quickly formed on the free surface in the upper section of
the pebble bed, which is equal to that formed by inserting pebbles.
• The horizontal component of the pebble motion in the upper section of the pebble
bed is larger than that in the lower, greatly different from that of the recirculation
mode.
• The drainage pebble mode presents a steady flow other than a quasi-static pebble
flow, different from that in the real pebble-bed reactor.
The drainage pebble experiments indicate that the pebble flow in the experiment
design is not entirely the plug-like flow, and present great non-uniformity, as if almost
all of the pebbles are drained out from a narrow area located in the center of the pebble
packing. These results are significantly different from MIT’s experiments [3, 5–7].
Thus, the operation mode is still a critical factor. In all of the following investigations,
the recirculation mode is adopted, which is the real operation mode of the pebble-bed
reactor.
2.2.2 Central Area Method
The central area method is designed to investigate the establishment of the two-region
arrangement and the mixing zone between regions. In the beginning, the experiment
vessel was pre-filled with about 70,000 colorless glass pebbles forming the initial
state of random pebble packing. Then, black glass pebbles were added in the middle,
whereas colorless pebbles were added on both sides of the vessel. The pebbles were
discharged one by one from the bottom at the same time. The forming process of
the two regions was displayed, and eventually reached a steady state approximately
after 7.5 h. The size of the two regions is determined by the ratio of loading pebbles,