Recent measurement of solid concentration in gas-solid riser flows by Electric Capacitance Tomography (ECT) reveals a strong heterogeneous structure, typically represented by a core-annulus-wall zone pattern. In this paper we present a mechanistic model in which the formation of the heterogeneous structure is due to the radial migration of solid flow from the wall toward center as well as due to the non-uniform acceleration of solids across the cross-section near the bottom of the riser. Firstly we present the general governing equations and discuss problem closure; then a simplified model with one-way flow coupling between the wall region and the core-annulus region is proposed to simulate the formation and development process of heterogeneous flow structures in the riser. Typical results of the three-zone flow structure along the riser are illustrated, which include the axial distributions of solids concentration and phase velocities in each zone, in addition to the pressure distributions. The model is also validated against the ECT measurements.
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