On trade-off for dispersion stability and thermal transport of Cu-Al₂O₃ hybrid nanofluid for various mixing ratios

The dispersion stability and thermophysical properties of metal–metal oxide hybrid nanofluid for various mixing ratios are investigated in this paper. Cu nanoparticles having high thermal conductivity and poor dispersion stability are dispersed in water with Al₂O₃ nanoparticles that have a high dispersion stability and low thermal conductivity, in mixing ratios 0.3:0.7 (MR-1), 0.5:0.5 (MR-2) and 0.7:0.3 (MR-3) to achieve a Cu-Al₂O₃ hybrid nanofluid with improved hydrothermal properties. Dispersion stability and thermophysical properties of the hybrid nanofluid were studied for 240 h using various experimental techniques such as zeta/particle size analyser, UV–Vis spectroscopy, transmission electron microscope, sedimentation, thermal analyser and viscometer. The results show that the hybrid nanofluid transforms into low, medium and high concentration stratified zones over time. Also, stability in Cu/Al₂O₃ single particle nanofluids and MR-3 hybrid nanofluid is related to both sedimentation velocity and zeta potential while sedimentation velocity has the dominating effect on stability of MR-1 and MR-2 hybrid nanofluids. Mixing ratio above MR-2 is identified for rapid settling due to high sedimentation velocity. Also, MR-2 is determined as an optimum mixing ratio to achieve enhanced overall hydrothermal properties for the hybrid nanofluid due to its improved thermal conductivity and relatively better stability.