Purpose – The purpose of this paper is to investigate the combined effects of buoyancy force and variable viscosity on unsteady flow and heat transfer of water-based nanofluid containing copper and alumina as nanoparticles through a porous pipe. Design/methodology/approach – Using the Boussinesq and boundary-layer approximations with Buongiorno nanofluid model. The governing nonlinear partial differential equations for the continuity, momentum and energy balance are formulated. The equations obtained are solved numerically using a semi-discretization finite difference method (know) as method of line coupled with Runge-Kutta-Fehlberg integration scheme. Findings – Numerical results for the skin-friction, heat transfer and for the velocity and temperature profiles are obtained. The results show that with suction, Cu-water produces higher skin friction and heat transfer rate than Al2O3-water. Both nanofluids velocity and temperature increase with a decrease in viscosity and an increase in buoyancy force intensity. Practical implications – Buoyancy-driven flow and heat transfer in porous geometries has many significant applications in industrial and engineering such as, electrical and microelectronic equipments, solar-collectors, geothermal engineering, petroleum reservoirs, thermal buildings insulation. This work provides very important information for researchers on this subject. Originality/value – This paper illustrates the effects of buoyancy force and temperature dependent on heat transfer and fluid flow problem using Cu-water and Al2O3-water nanofluids in a porous pipe.

Heat transfer characteristics of a Berman flow of water based nanofluids containing copper (Cu) and alumina (Al2O3) as nanoparticles in a porous channel with Navier slip, viscous dissipation, and convective cooling are investigated. It is assumed that the exchange of heat with the ambient surrounding takes place at the channel walls following Newton’s law of cooling. The governing partial differential equations and boundary conditions are converted into a set of nonlinear ordinary differential equations using appropriate similarity transformations. These equations are solved analytically by regular perturbation methods with series improvement technique and numerically using an efficient Runge-Kutta Fehlberg integration technique coupled with shooting scheme. The effects of the governing parameters on the dimensionless velocity, temperature, skin friction, pressure drop, and Nusselt numbers are presented graphically and discussed quantitatively.

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In this paper, the combined effects of variable viscosity, Brownian motion, thermophoresis and convective cooling on unsteady flow of nanofluids in a pipe with permeable wall are investigated. It is assumed that the pipe surface exchange heat with the ambient following the Newton's law of cooling. Using a semi discretization finite difference method coupled with Runge-Kutta Fehlberg integration scheme, the nonlinear governing equations of momentum and energy balance, and the equation for nanoparticles concentration are tackled numerically. Useful results for the velocity, temperature, nanoparticles concentration profiles, skin friction and Nusselt number are obtained graphically and discussed quantitatively.