Sensible heat storage in fluids generates thermal stratification. In order to improve thermodynamic system efficiency, stratification should be promoted much more. To this scope, this article presents a numerical study of transient mixed convection. The study investigates the use of different fluids as a heat storage medium in cylindrical cavities with different aspect ratios. The effect of the fluids is made through the variation of physical properties represented through the Prandtl number. The system consists of a cavity with fluid injection at the top and fluid discharge at the bottom. Transient, two-dimensional, mixed convection flows in a thermal storage tank have been studied using finite volume method. The governing equations are the conservation equations for laminar natural convection flow based on the Boussinesq approximation. Forced convection flow is superimposed through the use of appropriate boundary conditions (inflow and outflow conditions). The study considers three representative fluids i.e. Torada oil, ethylene glycol and water. It considers also cavities with aspect ratios varying from 3 to 1/3. Flow analysis is made through typical transient temperature distributions for the three fluids and for different configurations. The performances of thermal energy storage using these fluids are analyzed through the transient thermal storage efficiency.