A generalized Nusselt number correlation for nanofluids, and look-up diagrams to select a heat transfer fluid for medium temperature solar thermal applications

There are many solar thermal applications in the medium temperature range, 373-573 K. These systems rely heavily on heat transfer fluids. Proper strategy and heat transfer correlation are required for selecting heat transfer fluids. Hence, a consistent, non-dimensional figure of merit is proposed for heat transfer fluids, like synthetic oils, widely-available vegetable oils, and molten salts. To extend this analysis for nanofluids, a correlation for Nusselt number (Nu) is deduced, using the experimental data, for nanoparticles with distinct thermophysical properties, shapes and sizes. A novel separation approach is proposed to filter the effect of nanoparticles on Nu correlation for nanofluids. This leads to a generalized form of Nu correlation for nanofluids, which allows the use of a well-accepted Nu correlation for a wide-range of base-fluid. The generalized form of Nu correlation, for nanofluids, predicts the experiment-based Nu for CuO-water, TiO2-water, Cu-water, Carbon nanotube-water, mostly within +/- 20%, with an exception of Graphene nanoplatelet. One way to adopt this approach, for hybridnanofluids, is discussed. The figure of merit-based qualitative and quantitative look-up diagrams are developed to enable the selection of heat transfer fluids. A comparative assessment shows that the figure of merit, for CuOCanola/TherminolVP1 nano-oil or CuO-Al2O3-Canola/TherminolVP1 hybrid nano-oil increases by about 10-30%, in comparison to Canola or TherminolVP1 oil. Therefore, the use of Canola oil and Canola oil-based nanooil or hybrid-nano-oil will be beneficial for solar thermal systems in the medium temperature range. Moreover, the use of TherminolVP1 based nano-oil or hybrid nano-oil will be beneficial for concentrated solar thermal power generation systems. A preliminary cost-benefit analysis lends additional support to these findings.

Automated summary

This research proposes a consistent figure of merit for heat transfer fluids and derives a correlation for Nusselt number (Nu) for nanoparticles in nanofluids. A novel approach is used to filter out the effect of nanoparticles on Nu correlation, leading to a generalized form for nanofluids. Comparative assessments show that certain nanooils and hybrid-nano-oils can increase the figure of merit by 10-30%.