[en] A numerical study has been carried out to investigate the solidification of a binary mixture of water and Cu nanoparticles inside a horizontal trapezoidal cavity of different aspect ratio under specific given boundary conditions for temperature and concentration gradients. The vertical side walls of the cavity are insulated while the top wall temperature is kept lower than that of the bottom wall. The effect of parameters such as the ratio of the cavity length to height (aspect ratio), the cold wall temperature (-5 to -30 °C) and the initial temperature of the nanofluid (0 °C to 16 °C) on solidification time is investigated. The moving solid-liquid interface is obtained using Enthalpy-porosity technique in the model. We found that the solidification time decreases with the increase of the aspect ratio (i.e., a longer trapezoidal cavity) and the decrease of cold wall temperature. Aspect ratio is found to give a prominent effect. However, the initial temperature of fluid does not affect the solidification time much. (paper)