Test of a new model for the temperature and pressure dependence of the thermal conductivity of a packed pebble bed in gas: lithium zirconate in helium

This paper tests a recently developed theory for the thermal conductivity of a packed pebble bed in the presence of a gas, at substantially higher temperatures than previously used to test the theory, by fitting previous measurements on lithium zirconate in helium. The ultimate goal of the theory is the prediction of the thermal conductivity of a packed bed using only parameters that can be measured directly from the particles. The main innovations in the theory are its description of the conduction across the narrow gaps between irregularly shaped particles having several points of contact, in terms of the average gap width and area, and the introduction of a new expression for bridging the change in gas conductivity between the low and high pressure limits. In the case of the lithium zirconate beds, the theoretical fits require a third fitting parameter to take account of the oblateness of these pebbles. The fitted curves then describe very well both the temperature and pressure dependence of the experimental data, and provide reasonable values for all the fitting parameters. The model shows that conduction through the points of contact can be ignored, at least for the small test beds modeled.