A STUDY OF NANOPARTICLE SURFACE MODIFICATION EFFECTS ON POOL BOILING CRITICAL HEAT FLUX

Abstract

Pool boiling critical heat flux (CHF) measurements have been performed on stainless steel and zirconium wires in nanofluids consisting of oxide nanoparticles (7 to 250 am) dispersed in water as well as in high-purity water after coating these wires with a variety of materials and methods. For the nanofluids study, nanoparticles of titania, alumina, zirconia, and yttria-stabilized zirconia (YSZ) were investigated for various sizes and concentrations. Results showed improvements in CHF in the range of 50% to 100%, with titania and zirconia exhibiting the highest and the lowest levels of improvement, respectively. Wires were coated separately with the same oxide nanoparticle materials, as well as pure titanium nanoparticles, using the electrophoretic deposition (EPD) technique and by nanofluid boiling. EPD coatings yielded superior and more consistent improvements in CHF values in clean water, suggesting that this could be a more practical approach than using nanofluids. Coating uniformity plays an important role in dictating the levels of CHF enhancement. In all cases, titania provided for high levels of improvement, while YSZ showed similarly high levels of improvement in some cases. Pure titanium coatings exhibited lower levels of improvement, indicating qualitatively that the lower wettability on metallic substrates (as compared to oxides) may play a role in dictating CHF improvements. Titanium, however, exhibits better adhesion to metallic substrates than do oxides, which is an important property for applications in a reactor environment. Given this, the improvements in CHF achieved by titanium coatings were sfficient to justify further study.