Thermal conductivity of a beryllium gas packed bed

An unsintered packed bed has been suggested as a material form for the solid breeder and multiplier in fusion reactor blankets. Study of the effective bed thermal conductivity can provide tools for analysis of the blanket performance under different operating conditions, and for analysis of how to control actively the thermal behavior of the blanket. Issues of particular interest are the ability to predict and to control the thermal conductivity. The UCLA 2-D model is used to study the effects of the particle diameter, solid-to-gas conductivity ratio, bed porosity, contact area and surface roughness characteristics on the bed thermal conductivity. The study shows that all the parameters except the bed porosity play important roles in determining the bed thermal controllability. The effect of the bed porosity is minimal. Four models (the UCLA 2-D model, the modified Hall-Martin model, the SZB model, and the Kunii-Smith model) were compared with the recent UCLA single-size beryllium packed bed experimental data. The sensitivity of each model to uncertainties in the input parameters, such as the surface roughness characteristics and particle-to-particle contact area, are examined. The UCLA 2-D model gives the most reliable prediction of the Be-He packed bed effective thermal conductivity, using reasonable parameters. The modified Hall-Martin model predictions agree well with the experimental data, using a larger empirical particle-to-particle contact area. The SZB model works well for BeN2 or Be-air particle beds. The Kunii-Smith model is not suitable for a packed bed with a high solid-to-gas conductivity ratio.