Heat Transfer in Pebble Beds for Fusion Blankets

The design of a fusion reactor blanket based on a bed of lithium-containing ceramic pebbles or a mixture of ceramic and metallic pebbles requires knowledge of the effective thermal conductivity of such beds. Binary mixtures of spheres with the same diameter but different conductivities as well as beds formed by one type of sphere are investigated. The pebbles are made of Al₂O₃ (diameter = 1, 2, and 4 mm), Li₄SiO₄ (diameter = 0.5 mm), aluminum (diameter = 2 mm), and steel (diameter = 2 and 4 mm). The experimental apparatus consists of a stainless steel cylinder with a heating rod along the symmetry axis. The pebble bed is contained in the annular space between the two concentric cylinders. Experiments with stagnant and flowing gas are performed. The experimental values of the effective thermal conductivity and the wall heat transfer coefficient are compared with those predicted by correlations available from the literature. On the basis of the present experimental results, modifications of the existing models are suggested.