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Nowadays, the use of nanofluids as alternative to commonly-used industrial heat transfer fluids is a topic of increasing interest. Analysing the improved efficiency of heat transfer processes according to advanced nanomaterials and obtaining stable nanofluids is one of the most interesting challenges. This paper presents a study of nanofluids based on boron nitride nanotubes and using an aqueous solution of Triton X-100 (which acts as a surfactant) as the base fluid. UV-vis spectroscopy, particle size measurements (size between 150-170 nm) and  potential (at about -25 mV) showed that stable nanofluids were obtained. Surface tension measurements were also performed. The surface tension of water was weakly affected by the presence of any amount of nanoparticles and was mainly governed by the presence of surfactant. The rheological properties of the fluids were also analysed, as were their isobaric specific heat and thermal conductivity values. A Newtonian behaviour was observed for the base fluid and the nanofluids, with no significant increase in viscosity. The isobaric specific heat increased by 8% and thermal conductivity by 10 % compared with the base fluid.Thus, the results obtained are interesting because while thermal properties improved with nanoparticle content, rheological behaviour did not change. Consequently, the nanofluids studied in the current paper do not raise the pressure drop and pumping power significantly and may therefore be a good option for thermal system applications. HighlightsStable nanofluids based on boron nitride nanotubes were prepared and analysed Isobaric specific heat increased by 8% and thermal conductivity by 10 % Newtonian behaviour was found for nanofluids with no significant increase in viscosity Surface tension was mainly governed by the presence of Triton X-100 as surfactant Rheological behaviour is not changed with nanoparticle content

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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 -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.

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