Fluidization experiment was carried out in a 368 mm×368 mm square fluidized bed with FCC particles to investigate the regime transition from bubbling to turbulent fluidization. The differential pressures at different axial locations were measured and the s standard deviation of pressure gradient fluctuations was used to determine the transition velocity, Uc, defined as the superficial gas velocity at which the standard deviation reaches its maximum. It was shown that, during the transition, the average behaviors of gas-solids fluidization in square fluidized bed are symmetric about the bed cross-section and non-uniformly distributed along the bed height, always with a dense particle-phase at the bed bottom. The Uc was found to be different at different axial locations. From the upper section of bed downwards to the lower, Uc increases linearly, indicating that the regime transition takes place first at the upper section of bed and then spreads downwards with increasing superficial gas velocity. Also, the static bed height has a noticeable effect on Uc and, corresponding to an increased static bed height, Uc becomes increased at all axial locations, simultaneously. The experimental results showed that the most predictive correlations reported in literatures obviously need to be improved.
