Numerical prediction of packing behavior and thermal conductivity of pebble beds according to pebble size distributions and friction coefficients

Abstract

In the design process of a solid-type ceramic breeding blanket, it is important to identify the characteristics of functional materials in the form of pebble beds. In this study, the mechanical and thermal behaviors of lithium ceramic pebbles are numerically simulated using an in-house code developed for the discrete element analysis. First, the packing configurations of pebble beds under cyclic loadings are found and discussed in terms of the packing factor, average stress, and average coordination number. In particular, we investigate the effects of pebble size distributions and friction coefficients on the pebble packing. Subsequently, based on the resulting packing configurations, we analyze the heat conduction through the lithium ceramic pebble beds considering the elastic contact between the spherical pebbles. We then qualitatively evaluate the effective thermal conductivities of pebble beds, which also depend upon the pebble size distributions and friction coefficients. The results of the discrete element simulations reveal that a large size difference and low friction coefficient of pebbles lead to close packing and high thermal conductivity of the pebble beds.