Hydrodynamics and flow structure, gas and solids mixing behavior, and choking phenomena in gas-solid fluidization.

摘要

In this study, the dynamic flow behavior and gas and solids mixing behavior in the turbulent fluidized beds are investigated by using Electrical Capacitance Tomography (ECT) technique and tracer technique.; The ECT study reveals that the time-averaged solids holdup distribution exhibits radial symmetry in the turbulent regime, which is not the case for the bubbling regime. The standard deviations of the fluctuations of the cross-sectional averaged bubble/void phase fraction, and the cross-sectional averaged solids holdups in the bubble/void phase and in the emulsion phase, all peak at the transition velocity from the bubbling to the turbulent regimes, U c. The addition of 10% fine particles decreases the solids concentration in the emulsion phase due to the breakup of the large bubbles. The bubble/void phase fraction does not vary with bed temperature up to 400°C; the emulsion phase voidage is observed to increase as the bed temperature increases. More than one spiral motion of bubble swarms is observed in the bubbling regime for the 0.3 m ID fluidized bed. The transition velocity, Uc, decreases with increasing bed diameter. The gas and solids mixing behavior varies significantly with the flow regimes. The temperature and pressure are found to have little effect on the gas and solids mixing behavior. A small quantity of fine particles is noted to drastically affect the gas and solids mixing behavior in the turbulent fluidized bed.; The term "choking" commonly refers to a gas-solid fluidization phenomenon in which a small change in gas or solids flow rate prompts a large change in the hydrodynamic behavior such as the pressure drop or solids holdup during the gas-solid flow. In this study, the flow structure variations during the regime transition and the choking phenomenon in a 0.05 m ID pneumatic conveying system and a 0.1 m ID circulating fluidized bed for both Group A and Group B particles are examined by utilizing the Electric Capacitance Tomography (ECT). The criterion for the occurrence of choking transition in a circulating fluidized bed for both Group A and Group B particles is given.; In summary, a systematic study has been conducted to investigate the dynamic flow behavior and gas and solids mixing behavior in the turbulent fluidized beds under different operating conditions. The choking phenomenon and the regime transition in the circulating fluidized beds are discerned from the viewpoints of flow structure variations obtained by the ECT technique. It establishes a comprehensive database and provides profound understanding on the dynamic flow behavior of the complex gas-solid two-phase flow systems. (Abstract shortened by UMI.)