In this paper, the improved incompressible smoothed particle hydrodynamics (ISPH) method is used to conduct a numerical simulation for the buoyancy driven flow inside an enclosure including a cross shape that is filled with moving/fixed solid particles. In this study, a Cu-water nanofluid is assumed as a working fluid. The sidewalls of the cavity are cooled, the horizontal cavity walls are adiabatic and the cross shape with the inner solid particles were heated. Different cases are taken into account based on the temperature conditions on the inner solid particles, namely, fixed and cold solid particles, moving and cold solid particles, fixed and hot solid particles and moving and hot solid particles. The controlling parameters in this study are dimensions of the cross shape L-cross, depth of the solid particles L-solid, the Rayleigh number Ra and the nanoparticles volume fraction phi. The main results revealed that the decrease in the cross shape lengths by 0.6 increases values of the stream function by 27.8% and the isotherms lines are enhanced. In addition, the case of the cold and moving solid particles gives the higher rate of the heat transfer while the lowest rate of the heat transfer is observed in case of fixed and cold solid particles. (C) 2019 Elsevier Ltd. All rights reserved.