Generation of entropy during forced convection of heat in nanofluid stagnation-point flows over a cylinder embedded in porous media

Thermodynamics and heat transfer processes during impingement of a nanofluid flow upon a cylinder with constant surface temperature and embedded in porous media are investigated. The surface of the cylinder can feature uniform or non-uniform transpiration and is hotter than the incoming nanofluid flow. Appropriate similarity parameters are employed to reduce the three-dimensional governing equations of nanofluid motion and heat transfer in porous media to simpler equations solvable through using a finite difference scheme. The numerical solutions of these equations reveal the flow velocity and temperature fields as well as the Nusselt number and induced shear stress. These are then used to calculate the rate of entropy generation within the system by viscous and heat transfer irreversibilities. It is demonstrated that changes in the concentration of nanoparticles result in the variation of thermal and hydrodynamic boundary layers and hence can modify the Nusselt number and entropy generation considerably. However, the shear stress on the surface of the cylinder is observed to be less affected by the variations in the concentration of nanoparticles. Further, the Reynolds number of the impinging nanofluid flow and the functional form of transpiration are shown to have significant effects upon the Nusselt number and entropy generation. In particular, it is argued that the influences of Reynolds number on the boundary layer thickness can majorly modify the level of irreversibility and the value of Bejan number.