Critical Heat Flux Enhancement via Surface Modification Using Colloidal Dispersions of Nanoparticles (Nanofluids)

Abstract

Nanofluids are engineered colloidal dispersions of nanoparticles (1-100nm) in common fluids (water, refrigerants, or ethanol…). Materials used for nanoparticles include chemically stable metals (e.g., gold, silver, copper), metal oxides (e.g., alumina, zirconia, silica, titania) and carbon in various forms (e.g., diamond, graphite, carbon nanotubes). The attractive properties of nanofluids include higher thermal conductivity, heat transfer coefficients (HTC) and boiling critical heat flux (CHF) than that of the respective base fluid. Nanofluids have been found to exhibit a very significant enhancement up to 200% of the boiling CHF at low nanoparticle concentrations. In this study, nanofluids were investigated as an agent to modify a heater surface to enhance Critical Heat Flux (CHF). First, the CHF of diamond, Zinc Oxide and Alumina water-based nanofluids at low volume concentration (<1 vol%) were measured to determine if nanofluid enhances CHF as seen in literature. Subsequently, the heaters are coated with nanoparticles via nucleate boiling of nanofluids. The CHF of water was measured using these nanoparticle precoated heaters to determine the magnitude of the CHF enhancement. Characterization of the heaters after CHF experiments using SEM, confocal, and contact angle were conducted to explain possible mechanisms for the observed enhancement. The coating thickness of the nanoparticle deposition on a wire heater as a function of boiling time was also investigated. Finally, theoretical analyses of the maximum CHF and HTC enhancement in term of wettability were performed and compared with the experimental data. The CHF of nanofluids was as much as 85% higher than that of water, while the nanoparticle pre-coated surfaces yielded up to 35% CHF enhancement compared to bare heaters. Surface characterization of the heaters after CHF experiments showed a change in morphology due to the nanoparticles deposition. The coating thickness of nanoparticle was found to deposit rather quickly on the wire surface. Within five minutes of boiling, the coating thickness of more than 1 μm was achieved. Existing CHF correlations overestimated the experimental data.