Nanofluids mixed convection, heat transfer characteristics and effects of volume concentrations in the thermally developing region

• Effects of varying volume concentrations on the Flow Regime range (Laminar, Transition, Turbulent) were experimentally investigated. • The role of Thermal Entrance Effects, Mixed convection and Axial Position on the heat transfer enhancement was experimentally investigated. • The influence of volume concentrations on the Al 2 O 3 – MWCNT Hybrid nanofluids mixed convection heat transfer characteristics was also critically studied and analysed using experimental data. • A method of differentiating Thermal entrance effects from mixed convection effects on Al 2 O 3 – MWCNT hybrid nanofluid heat transfer was devised and validated by experiments. The vast majority of research on nanofluids only focuses on the heat transfer enhancement and pressure drop characteristics of these fluids. But apart from that, there are other intricate and complex phenomena that are very critical to the understanding of these fluid’s characteristics. The effects of free convection, entrance effects, and the influence of volume concentration on boundary conditions (i.e., laminar start and end, transition and turbulence) were often neglected. This limits our knowledge of these fluids. These phenomena were critical to understanding and fully utilising nanofluid potentials. Therefore, this research experimentally investigated the force and mixed convection of Al 2 O 3 − MWCNT/water hybrid nanofluids. Three different concentrations of 0.30, 0.20, and 0.10 were prepared using the two-step method. Results show that an increase in volume concentration significantly affects nanofluid transition boundaries. Because critical Reynolds numbers for all the volume concentrations varied significantly, results show that critical Reynolds numbers of 0.3, 0.2 and 0.1 vol concentrations are Recr . = 2140, 2200, and 2330, respectively. Thermal entrance influences were found to decrease with the increase in volume concentration. Mixed convection effects were increased with a decrease in volume concentration. Results also revealed that the Nusselt number of 0.3 vol concentration at Re = 1000 was enhanced by about 50 % within the thermal entrance length. In contrast, it was enhanced by about 48.84 % due to mixed convection. It was concluded that heat transfer enhancement was much better due to thermal entrance effects than with mixed convection. Also, the fluid viscosity depends on transition region boundaries, thermal entrance effects, and mixed convection heat transfer characteristics. Also, their characteristics often differ at different axial positions.