Investigating bubble dynamics in a semi-cylindrical gas-solid fluidized bed

Abstract

Bubble dynamics (i.e., bubble shape, bubble size, bubble rise velocity) and bed voidage in a semi-cylindrical gas-solid fluidized bed at various superficial gas velocities and particle sizes were measured by fiber optic probe and image analysis. Measurements in the semi-cylindrical bed were compared with those in the cylindrical bed to investigate the similarity of two fluidized beds and verifying whether a semi-cylindrical bed can be used for measuring hydrodynamic properties of fluidized beds. Experiments were carried out in two Plexiglas semi-cylindrical and cylindrical fluidized beds of 14 cm in diameter at ambient pressure and temperature. Glass beads of various sizes (120, 290 and 450 μm) were used as the bed material. The superficial gas velocity was varied in the range of 0.2–0.8 m/s and both beds were operating in the bubbling fluidization regime. It was shown that increase in the particle size leads to decrease in the emulsion phase fraction while increases the fraction of bubble phase. At low gas velocity, the semi-cylindrical fluidized bed contains normal bubbles while at higher superficial gas velocity, elongated bubbles with normal rising velocity can be found. Increase in the particle size leads to increase in the aspect ratio of bubbles. It was found that the average bubble size and rise velocity increases with increasing particle size and superficial gas velocity. Results indicated that the gas-solid distribution, bubble rise velocity and bubble size in both beds are similar and geometry of the cross-section of column (semi-circular and circular) has no significant effect on these parameters. It was shown that the hydrodynamics of both beds are very similar and the difference is negligible. Since measuring the bubble size and rise velocity in the semi-cylindrical bed can be easily done by visual observation compared to those in a cylindrical bed, using a semi-cylindrical fluidized bed instead of two dimensional fluidized bed in the laboratory scale is the preferred choice which provides bubble dynamics close to that in a three-dimensional bed.