144
3 Experiments in Pebble Bed Heat Transfer
Fig. 3.14 Effective thermal diffusivities of C1–C5 in two vacuum tests (T1–T6)
Fig. 3.15 Effective thermal diffusivities of C1–C5 in two helium tests (T1–T6)
decreases due to packing structure variation. This wall effect was also found by the
HTTU facility in South Africa [10, 11]. Other conclusions are similar to those of
Sect. (3.3.3.1).
Furthermore, comparing the results of vacuum and helium tests, it is found that
there is an observable growth in the low-temperature region of helium results and
the difference of them is insignificant in the high-temperature region. This can be
explained by the variation of the heat transfer effect with temperature. At lowtemperatures, the effective thermal diffusivity and conductivity are attributed mainly
by solid heat conduction and helium-gas heat convection, and radiation effect is
small compared with them. However, at high-temperatures, the radiation effect is
prominent in the total heat transfer. Figure (3.11b) shows the thermal diffusivity and
conductivity of graphite solid descend with the increase of temperature. Therefore,
the similar results of the vacuum and helium tests at high-temperatures reveal the
helium-gas heat convection does not rise prominently with the temperature increase
under constant-pressure conditions.
3 Experiments in Pebble Bed Heat Transfer
Fig. 3.14 Effective thermal diffusivities of C1–C5 in two vacuum tests (T1–T6)
Fig. 3.15 Effective thermal diffusivities of C1–C5 in two helium tests (T1–T6)
decreases due to packing structure variation. This wall effect was also found by the
HTTU facility in South Africa [10, 11]. Other conclusions are similar to those of
Sect. (3.3.3.1).
Furthermore, comparing the results of vacuum and helium tests, it is found that
there is an observable growth in the low-temperature region of helium results and
the difference of them is insignificant in the high-temperature region. This can be
explained by the variation of the heat transfer effect with temperature. At lowtemperatures, the effective thermal diffusivity and conductivity are attributed mainly
by solid heat conduction and helium-gas heat convection, and radiation effect is
small compared with them. However, at high-temperatures, the radiation effect is
prominent in the total heat transfer. Figure (3.11b) shows the thermal diffusivity and
conductivity of graphite solid descend with the increase of temperature. Therefore,
the similar results of the vacuum and helium tests at high-temperatures reveal the
helium-gas heat convection does not rise prominently with the temperature increase
under constant-pressure conditions.