Two-fluid model (TFM) has been widely used to simulate particle-fluid flows, with two model formulations available: Model A and Model B [1-3]. Previous studies have shown that both models generate comparable results for some flows, but their possible application limitations have not been well addressed. Recently, Zhou et al. [4] discussed this issue in the framework of coupled CFD (computational fluid dynamics) and DEM (discrete element method), indicating that both models are largely applicable to simple flows such as fluidization and pneumatic conveying, but the so-called Model B is not applicable to complicated three-dimensional flows such as that in a hydrocyclone. However, it is not clear such limitations still exist in TFM. In this work, both Model A and Model B are applied to model two typical cases, i.e., gas-solid fluidized bed and hydrocyclone. It is demonstrated that Model B is not applicable to hydrocyclones while both models are applicable to fluidized beds. The results confirm that, Model B, as a simplified model, is not applicable to the flows where the pressure gradient force is significant and its direction is quite different from that of the drag force. To overcome this problem, its original formulations, which are somehow ignored in the literature, should be used. (C) 2018 Elsevier B.V. All rights reserved.