Model investigations on fluidised beds : a contribution to the coating of nuclear fuel particles

The objectives of the present paper were: 1. Simulation of the processes of movement in the high temperature coating bed at room temperature. 2. Optimisation of the fluidisation conditions in order to improve the deposition conditions with regard to polyhedralityof the coated particles. In the f irst part of the paper we report on the theoretical fundamentals required to determine conditions in the coating bed and for producing physical similarity in the model bed. We derive similarity parameters in order to describe the states in the coating and model bed, we describe numerical methods for determining material characteristics, especially at high temperature, and give formulae for calculating the gas and particle velocities in the fluid bed. Variational calculations showed that the same parameters feature in a model with petunia seeds (petunia hybridia) and a nitrogen/hydrogen mixture as in the coating bed. In the model test the same types of movement (spouting, bubbling, slugging) were observed as in the coating bed. In order to gain a knowledge of the processes occurring on coating in a conical fluid bed, the effect of gas flow and gas distribution, of particle loading and of bed and jetgeometry on particle movement was investigated in the model experiment. We report on these matters in the second part of the paper. We found the following results: 1. Due to the different gas distribution between central and annular jet the three types of movement appear. 2. With increasing gas velocity the effect of particle loading on the type of movement diminishes. 3. Tests with variable jet geometry showed that the type of movement is determined by the jet outlet velocities. At a large central jet area spouting disappears. 4. The most favourable conditions were shown by tests involving different cone aperture angles to occur for = 3o deg.(stable bubble movement, good circulation of particles). 5. Tests involving different bed diameters showed that in the 1 inch bed relatively good fluidised beds are produced while for the larger bed geometries very inhomogeneous conditions and so unfavourable coating conditions occur due to low particle concentration in the central coating zone. 6. A cyclone bed was developed with a view to impoving the flow conditions. By producing rotational flow by means of auxilliary jets in the cone and a spiral annular gas inlet the shearlayers may be fluidised and the deposition conditions homogenised 7. In coating tests involving various types of movement the best results with regard to polyhedrality of the coated particles were established for bubbling, and the worst for slugging. 8. Coating in the cyclone bed gave small polyhedrality values even for !arge thicknesses of layer