Abstract
The effective thermal conductivity (ETC) is a key parameter of the heat transfer models in high temperature pebble-bed reactors. The ETC in a simple cubic packed bed of graphite pebbles dominant by the passive heat transfer coupled of the conduction, radiation and natural convection is studied numerically, with the emphasis on the effects of the natural convection. It is found that the natural convection has little influence on the solid pebble temperatures but it would affect the fluid temperatures. The ETCs in the bulk region are consistently greater than ETCs in the near-wall region. The natural convection plays a positive role in ETCs in the bulk region while it has a negative role in ETCs in the near-wall region. When the temperature increases, the natural convection is less important to the ETC. The natural convection effect on the ETC could be ignored in the high temperature conditions (T > 600 K).
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