Enhancement of Boiling Surfaces Using Nanofluid Particle Deposition.

Abstract

To meet increasing power demands across several industries, advanced thermal management systems based on boiling heat transfer have been proposed. Furthermore, nanofluids, a relatively new class of coolants created by suspending 1-100 nm sized particles in a base fluid, have been shown to improve a fluid’s thermal properties. This research focuses on two methods using nanofluids to deposit nanoparticles for the creation of enhanced surfaces for boiling heat transfer. Since many of these thermal management systems require a non-conductive fluid, the electrical conductivity of nanofluids is also studied. Pool boiling studies of nanofluids have demonstrated either enhanced or diminished boiling heat transfer, yet have been unable to distinguish the contributions of increased surface roughness and suppression of bubble transport by suspended particles. This uncertainty is resolved by studying the boiling performance of a surface exposed to a series of boiling tests that alternate between water and a water-based nanofluid. The boiling performance of the coated surfaces increases significantly with each cycle. The measured surface roughness of the intervening nanoparticle layers is used with a model to explain the measured increase in performance. The results demonstrate that the effect of increased surface roughness due to nanoparticle layering can enhance boiling for the base fluid. A novel method to create enhanced boiling surfaces is electrophoretic deposition of nanoparticles from a nanofluid. A surface was coated using electrophoretic deposition from a ZnO-propylene glycol based nanofluid. With adequate coating time, such a surface modification method can increase the boiling heat transfer coefficient by about 200%, which was correlated to an increase in the nucleation site density. In addition, on chip cooling techniques require low conductivity coolants. However, the electrical conductivity of nanofluids has not been widely studied. The particle size and concentration effects on nanofluid electrical conductivity were experimentally investigated and compared to a model based on colloidal suspensions in a salt-free medium. The results showed the electrical conductivity increased with increasing volume fraction and decreasing particle size. At higher volume fractions, the increase of electrical conductivity begins to level off, which is attributed to ion condensation effects in the high surface charge regime.