Experimental and numerical investigation of heat enhancement using a hybrid nanofluid of copper oxide/alumina nanoparticles in water

Abstract

The following work experimentally and numerically investigated the thermal performance of a hybrid nanofluid, prepared by decorating a nanostructured aluminum oxide support with copper oxide nanostructures, in a flow system of porous open-cell foam metals. The porous medium was comprised of 6061-T6 aluminum with a porosity of 0.91 and a permeability of 9.54788 × 10⁻⁷ m². Experiments were performed under variable heat flux, using a hybrid nanofluid consisting of a 0.1 mass% aqueous solution of CuO@Al₂O₃ nanocomposite particles 28 ± 11 nm in size. Thermal performance was evaluated with respect to the Nusselt number and the index of performance with pressure. Remarkably, the implementation of a copper oxide/alumina nanocomposite with the use of porously filled channels resulted in significant thermal enhancement (6–11%) relative to commercial alumina nanofluid, despite a total copper concentration of only 0.0001 mass% in the hybrid nanofluid. Increased performance is attributed to a combination of ultralow copper content and the approach to hybrid nanofluid design. Specifically, a small amount of copper significantly increased the local Nusselt number, indicative of superior heat extraction. At the same time, a numerical model of the system was also developed and agreed with experimental measurements within an error of 5%. Numerical results predicted a slightly higher pressure drop for the hybrid nanofluid, but also showed higher absolute pressures for the hybrid fluid all along the channel in the three-channel configuration. Simulation also produced an interesting discrepancy between the performances of the hybrid nanofluid as a function of heat flux, possibly related to different channel pressures inherent to the two heat sink models under investigation. This could point to a heightened pressure sensitivity of the thermal properties of hybrid nanofluids as well as a greater need to consider experimental design in the comparison of heat enhancement across nanofluidic systems. In terms of material design, decorating alumina nanoparticles with copper nanoparticles rather than mixing two individual nanostructured components appears to have been a beneficial strategy. The photochemical methodology used to prepare the nanocomposite material may also have improved thermal performance by yielding smaller (< 5 nm) copper oxide nanoparticles and provided access to the synergistic properties of a true nanocomposite material. This study demonstrates that heat enhancement by nanofluids can be achieved using a much smaller amount of copper than previously described in the literature and further highlights that synthetic methodology and material characterization can have a dramatic impact upon the performance of applied nanocomposite materials. Additionally, this work delivers a practical example of how progressive nanofunctionalization of materials can enhance thermal functionality of nanofluids.