Critical investigation of heat transfer enhancement using nanofluids in microchannels with slip and non-slip flow regimes

Abstract

The Reynolds number in nanofluid studies depends on the inlet velocity and the kinematic viscosity of nanofluid. The nanofluid kinematic viscosity increases with an increase in nanoparticles volume fraction while the inlet velocity should be increased to keep the Reynolds number constant. Therefore, it is not clear that either increasing the nanoparticles volume fraction or increasing the inlet velocity has a major role on heat transfer enhancement in nanofluids flow studies which are done at constant Reynolds numbers. In this study, forced convection Al 2 O 3 -water nanofluid flows in two-dimensional rectangular microchannels have been investigated to study heat transfer enhancement due to addition of the nanoparticles to the base fluid especially in microchannels at low Reynolds number. Three different cases are examined to evaluate proportion impact of increasing nanoparticles volume fraction ( ϕ ) and the inlet velocity ( u in ) on heat transfer enhancement. Two-dimensional Navier–Stokes and energy equations accompany with the slip velocity and the jump temperature boundary conditions expressions have been discretized using the Finite Volume Method (FVM). The Brownian motions of nanoparticles have been considered to determine the thermal conductivity of nanofluids. The calculated results show good agreement with the previous numerical and analytical data. It is found that at a given Reynolds number, the major enhancement in the Nusselt number is not due to increasing the nanoparticle concentrations but it is due to the increasing the inlet velocity to reach a constant Re . Constant Reynolds number studies of nanofluids are not sufficient approach to evaluate the heat transfer and the skin friction factor due to the nanofluids usage. ► At a specific Reynolds number, the major enhancement on the Nusselt number is due to increasing the inlet velocity to reach a constant Re. ► In the nanofluids studies, fixing the Reynolds number is not a sufficient comparison to evaluate the heat transfer and the skin friction factor. ► A logical comparison will be achieved when the pumping power is assumed constant.