Transient 3D simulation of the gas-solid flow in large diameter (> 0.3 m) risers is performed using a new density based solution algorithm. Unlike the conventional pressure based algorithm used so far for riser simulation, the density based solution method uses the pre-conditioning of time derivatives, does not have the internal pressure and velocity correction loop and hence provides a much faster convergence speed. The two phase flow is highly oscillatory with many small scale high frequency (10Hz) fluctuations and a few dominating low frequency (0.02 Hz) oscillations. The transient simulation of a case belonging to a fast fluidized regime with Geldart B particles, demonstrated the density inversion phenomena, experimentally observed before. Cluster formation is simulated on a real time scale for a dilute phase riser having solid fraction < 0.0007, although the time averaged flow fields still resemble a core-annular flow structure.

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