This paper deals with the issue of modeling isothermal mono- and polydisperse bubbly flows in the framework of Eulerian approach, based on the Diffusion Inertia Model (DIM) coupled with method of delta-function approximation. The model takes into account the interphase momentum transfer due to drag and non-drag forces, bubble break-up and coalescence processes as well as dependency of the relative velocity, drag and lift coefficients on the bubble diameter. In order to take into account polydispersity effect, a new model of bubble break-up and coalescence is presented in comparison to a well-known model by Yao and Morel (2004). The considered models are based on the mechanism of exchanging energy between turbulent eddies and interfacial area of a bubble. The study starts with validation of the expressions for interfacial forces based on simulation of monodisperse bubbly flows. This has allowed separating polydispersity effect and carefully analysing the role of interfacial forces, acting on bubbles. The presented model for the polydisperse bubbly flow has been validated by means of comparison to experiments MT-Loop (where the tube diameter is 50.8 mm) and TOPFLOW (with the tube diameter of 195 mm), providing an adequate agreement with experimental data. All numerical calculations were performed in an open-source CFD package OpenFOAM. (C) 2014 Elsevier Ltd. All rights reserved.