(541f) Effect of Pebble Sizes On Axial Gas Dispersion In a Pebble Bed

Despite the recent interest in the pebble bed reactors (PBRs) for the 4^th generation nuclear energy that motivates various types of studies; there is still a lack of fundamental understanding of the complex gas flow structure and mixing pattern. In this work, the non-ideal flow behavior of the gas phase which is used for cooling has been investigated experimentally in a 0.3 m diameter cold flow pebble bed. The extent of mixing and dispersion of the gas phase has been qualified in terms of axial dispersion coefficient. The effect of pebble size on the axial dispersion coefficient of gas phase has been investigated with three different sizes of pebbles (1.25 cm, 2.5 cm and 5 cm). The superficial gas velocities were ranging from 5 to 60 cm/s which cover both laminar and turbulent flow regimes. An advanced gaseous tracer technique was developed to measure the gas residence time distributions (RTDs) in the bed. The technique accounts properly for the extra dispersion encountered in different parts of the experimental set-up such as plenum, distributor, sampling lines, etc. Experiments results indicate that the increase of pebble size causes an increase in the axial dispersion coefficient. This can be attributed to an increase in the packing porosity of the bed which yields less pressure drops across the bed. Also, it has been found that the axial dispersion coefficient increases as superficial gas velocity increased.