The elctroosmotic flow and mixing is numerically studied through a Nozzle/Diffuser placed between two large reservoirs connected through a micro channel. The analytical and numerical studies are performed to illustrate the effects of convection and diffusion mechanism due to different aspect ratios of the transformation. The hydrodynamics of the ion and flow components are analyzed through Poisson-Nernst-Plank based Navier Stokes model of Newtonian fluid subjected to specified system of forces. The numerical results for the reservoir effects on velocity, pressure, potential have found a significant variation when the transport mechanism is changing from nozzle to diffuser. Mixing efficiency is also evaluated for different geometric configurations and compared with the plane slit channel when the reservoir is connected. Various effects of Debye-Hückel parameter, conical angles or slopes and reservoir height/ width on mixing efficiency are discussed. A notable findings are obtained for the effective mixing in reservoir connected nozzle channel is very high without implementing any hurdles or heterogeneous zeta potential. The flow solutions observed a low torque when the equilibrium conditions are justified and is quite effective for mixing enhancement which is optimized in terms of aversion of frictional factor and viscous dissipation. Also, the findings manifest the species patterns with high accuracy and versatility, which could possibly help to handle the technical challenges associated with the design of pumpless actuated micro-fluidic devices.