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Experimental studies and CFD simulations of conical spouted bed hydrodynamics Wang, Zhiguo

Abstract

Conical spouted beds have been commonly used for drying suspensions, solutions and pasty materials. They can also be utilized in many other processes, such as catalytic partial oxidation of methane to synthesis gas, coating of tablets, coal gasification and liquefaction, pyrolysis of sawdust or mixtures of wood residues. Literature review shows that there is still considerable uncertainty in hydrodynamics as compared to cylindrical spouted beds. No CFD simulation model has been developed to predict static pressure profiles, and there is a lack of experimental data on such characteristics as the evolution of the internal spout, particle velocity distribution, voidage distribution and gas mixing. Moreover, most empirical equations for the minimum spouting velocity and the pressure drop at stable spouting do not agree well with each other. The main objectives of this work include both the experimental research and mathematical modeling of the conical spouted bed hydrodynamics. Pressure transducers and static pressure probes were applied to investigate the evolution of the internal spout and the local static pressure distribution. Optical fibre probes were utilized to measure axial particle velocity profiles and voidage profiles. The step tracer technique using helium as the tracer and thermal conductivity cells as detectors was used to investigate the gas mixing behaviour inside a conical spouted bed. Many factors that might affect the calibration of the effective distance of an optical fibre probe were investigated. A new calibration setup was designed and assembled, and a comprehensive sensitivity analysis was conducted. The analysis included the effect of the glass window, the design of the rotating plate, the distance between the rotating plate (or rotating packed bed) and the probe tip, the particle type, as well as the particle size. A stream-tube model based on the bed structure inside a conical spouted bed was proposed to simulate partial spouting states. The proposed stream-tube model with a single adjustable parameter is capable of predicting the total pressure drop ΔPt under different operating conditions, and estimating the distribution of the axial superficial gas velocity and the gauge pressure, especially for the descending process as well as in the region above the internal spout. A mathematical model based on characteristics of conical spouted beds and the commercial software FLUENT was also developed and evaluated using measured experimental data. The proposed new CFD model can simulate both stable spouting and partial spouting states, with an adjustable solids source term. At stable spouting states, simulation results agree very well with almost all experimental data, such as static pressure profiles, axial particle velocity profiles, voidage profiles etc. A comprehensive sensitivity analysis was also conducted to investigate the effect of all possible factors on simulation results, including the fluid inlet profile, solid bulk viscosity, frictional viscosity, restitution coefficient, exchange coefficient, and solid phase source term. The proposed new CFD model was also used successfully to simulate gas-mixing behaviour inside a conical spouted bed.

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