The so-called "choking" phenomenon in fluidized systems lacks understanding from micro-scale hydrodynamics and hence always causes disputes. To solve this problem, a newly established multi-scale CFD (computational fluid dynamics) approach, which is based on drag correction from the EMMS (energy-minimization multi-scale) model, is used to simulate our experiments about choking and relevant flow-regime transitions. The computed flow-regime diagram is described with the functional relation between the imposed pressure drop and the solids flux at given gas flow rates. A bell-shaped area with an abrupt plateau, within which the solids flux equals the saturation carrying capacity and the dense upflow coexists with the dilute pneumatic transport, marks the choking transition in this diagram. The non-choking transition, however, is characterized by a smooth shift from the dilute flow to the dense flow, without an area for coexistence of flow regimes. The critical point, the summit of the bell-shaped area, demarcates between the choking and the non-choking transitions, near which the fluctuation of solids volume fraction reaches a maximum. In general, this CFD simulation of flow regime transitions suggests a good perspective of the EMMS based multi-scale approach. (c) 2006 Elsevier Ltd. All rights reserved.