Effects of particle size on bubble dynamic behaviors in a quasi-two-dimensional gas-solid fluidized bed

For thermal energy storage reactors with gas-solid bubbling fluidized bed, the bubbles have important effects on the heat and mass transfer capabilities as well as temperature homogeneity. Therefore, it is necessary to investigate the bubble dynamic behaviors, which is affected importantly by the particle size, for the design, and operation of bubbling fluidized bed reactors. In the present work, four particles with narrow size were sieved as the bed materials to study the effects of particle size on bubble dynamic behaviors at superficial gas velocity of 0.56 m·s−1 in a quasi-two-dimensional (quasi-2D) fluidized bed. The bubble dynamic behaviors, including bubble equivalent diameter, bubble size distribution, average bubble density, bubble aspect ratio, bubble hold-up, bed expansion ratio, bubble velocity, and bubble rising angle, at full bubbling fluidization were derived by digital image analysis (DIA) post-processing technique. The results shown that the bubble equivalent diameter increased with the increasing of particle size. For four particles, the bubble size distributions shown the similar distribution features, which indicated that the particle size had rare effects on it. Near the air distributor and freeboard, with increasing particle size, the average bubble density increased. The modes of bubble aspect ratio were all lower than 1, indicating that most of bubbles were vertically oblong ellipse. Additionally, the mode of the bubble aspect ratio tended to increase with the increasing of the particle size. Both time-averaged bed expansion ratio and bubble hold-up increased with increasing particle size. Bed fluctuation of smaller particles was more intense, but it was converse for the bubble hold-up. Inside the fluidized bed, two uniform "core-annular" flows were formed, and there was no evident difference in the radial distribution among four particles. Additionally, the particle size had little effects on the bubble rising velocity, which increased with increasing bubble equivalent diameter only, and proposed correlation with velocity coefficients of 1.09 was in line with the correlation coefficients in the literatures. Depending on the bubble rising angle, the bubble movement from the biased random movement to the main upward movement was dominant. Mastering the effects of particle size on bubble dynamic behaviors comprehensively is beneficial for the design, operation, and optimization of thermal energy storage reactors with gas-solid bubbling fluidized bed.