Nanofluid boiling: The effect of surface wettability

Abstract

Nanofluid boiling has shown potential to increase boiling heat transfer, but the mechanisms responsible are poorly understood. One likely mechanism for nanofluid enhancement is an improvement in surface wettability. This study is targeted towards investigating whether or not nanofluids improve the critical heat flux (CHF) by altering the surface energy as has been observed for pure fluids. The surface of a heater was systematically altered by oxidizing its surface to varying degrees or by depositing metal onto the surface, and the surface energy was characterized by measuring the advancing three-phase contact angle. Boiling curves on these surfaces were measured for pure fluids and for water and ethanol based nanofluids with aluminum oxide nanoparticle concentrations from 0.001 g/L to 10 g/L. Dilute suspensions of nanoparticles were found to degrade or have no effect on the boiling performance. Greater concentrations (⩾0.5 g/L) led to modest (up to ∼37%) increase in the CHF. Poorly wetting systems (e.g. water on polished copper) could be enhanced by the addition of nanoparticles, whereas better wetting systems (e.g. ethanol on glass) showed no improvement or a larger degradation. Furthermore, the addition of nanoparticles to water was found to improve wetting, but only when the surface was fouled by the particles. Interestingly, similar CHF enhancement was achieved without nanofluids using an oxidized surface, which was easily wetted with pure fluids. In fact, surface treatment alone resulted in similar CHF enhancement but at ∼20 °C lower wall superheat than when using nanofluids.