Boron nitride nanotubes-based nanofluids with enhanced thermal properties for use as heat transfer fluids in solar thermal applications

Abstract

Nanofluids are considered as promising alternative in heat exchange processes to the classical fluids, which usually present poor thermal properties. One interesting application for nanofluids is as heat transfer fluid in solar thermal applications plants. Boron nitride nanotubes present interesting thermophysical properties for use in nanofluids. Therefore, nanofluids based on boron nitride nanotubes were prepared by a two-step method, dispersing this nanomaterial in a heat transfer fluid typically used. Stability, rheological and thermal properties of the nanofluids were analysed. To check the stability, ultraviolet–visible spectroscopy and particle size and ζ-potential measurements were performed for a month, obtaining that the nanofluids were stable. Furthermore, surface tension was measured and no significant differences were observed with regard to the base fluid. In a variable range of temperature, nanofluids show Newtonian behaviour with a slight increase in viscosity. Besides, the boron nitride nanotubes caused an increase in thermal conductivity of up to 33% with regard to the base fluid. The use of these nanofluids also led to an improvement in the heat transfer coefficient under turbulent flow conditions of up to 18%. Finally, the analysis of the outlet temperature in solar thermal applications shows that these nanofluids are a promising alternative in this application.