Effects of different concentrations of Al2O3 nanoparticles and base fluid types on pool boiling heat transfer in copper foam with bottom condensed reflux

Abstract A closed-loop type experimental system of pool boiling heat transfer with condensed reflux in the bottom was developed to study the boiling heat transfer of copper foams in different working fluids, including deionized water, water-based Al2O3 nanofluids, ethylene glycol/deionized water mixed fluid and ethylene glycol/deionized water-based Al2O3 nanofluids. The pore densities of copper foams are 5, 30 and 60 pores per inch (PPI) and the foam thickness are 2, 4, 6 and 8 mm, while the porosity remains a fixed value of 0.9. Al2O3 nanoparticles with an average diameter of 50 nm are used in this study and their mass concentrations are 0.01%, 0.05% and 0.1%. The results show that the concentrations and types of nanofluids and the structural parameters of copper foams have significant effects on the boiling heat transfer performance. Introducing the condensed reflux back to the bottom of boiling chamber can enhance the boiling heat transfer performance compared with the conventional pool boiling experimental condition. Under the current experimental settings, the gravity-capillary driven condensed reflux can offer extra liquid replenishment to the copper foam surface, improving the rewetting ability and therefore enhancing the boiling heat transfer. In addition, the scouring effect of condensed reflux can promote the bubble departure and decrease the bubble release resistance, and inhibit the deposition of nanoparticles on the heating surface.