Unsteady mixed convection boundary layer flow and heat transfer of nanofluids due to stretching sheet

The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. A numerical analysis has been presented to investigate the unsteady mixed convection boundary layer flow and heat transfer due to uncertainties of thermal conductivity and dynamic viscosity of nanofluid over a stretching vertical surface. The unsteadiness in the flow and temperature fields is caused by the time-dependent of the stretching velocity and the surface temperature. The governing partial differential equations with the auxiliary conditions are converted to ordinary differential equations with the appropriate corresponding conditions via scaling transformations. Different water-based nanofluids containing Cu, Ag, CuO, Al2O3, and TiO2 are taken into consideration. The effects of pertinent parameters such as the solid volume fraction of nanoparticles, Prandtl number, mixed convection parameter, and the unsteadiness parameter have been discussed. Furthermore, different models of nanofluid based on different formulas for thermal conductivity and dynamic viscosity on the flow and heat transfer characteristics is discussed. Comparison with known results for steady state flow is presented and it found to be in excellent agreement.