The effects of nanoparticles in distilled water are investigated in mixed convective flows in a tube with different inclination angles. Heat transfer and hydrodynamic features of the flow itself can be roughly estimated by any single-phase or homogenous models in the literature. However, the precise values, and especially the particle tracking, would be impossible with such models. Therefore, discrete modelling of the particles is employed in this study to understand the nature of migration and concentration of nanoparticles in complex mixed convection flow. An attempt is made to clarify the significant phenomena in particle-fluid and particle-wall interactions. A proper model of heat transfer between nanoparticles and fluid is considered and implemented in ANSYS-Fluent 17.0 through user-defined functions (UDFs). The results are validated by comparing them to similar experimental measurements for nanofluids in the literature, and good agreement is found. The results of the tracking and concentration of nanoparticles are naturally found in complete contractions compared to other models such as two-phase multiphase models. The deposition of particles on the walls due to low flow velocity is predicted in this paper. (C) 2018 Elsevier B.V. All rights reserved.