Abstract
Research on nanofluids has increased markedly in the last two decades. Initial attention has focused on conventional or mono nanofluids, dispersions of one type of solid nano-sized particles in a base fluid. Despite various challenges such as dispersion stability or increased pumping power, nanofluids have become improved working fluids for various energy applications. Among them, convective heat transfer has been the main research topic since the very beginning. Hybrid nanofluids, dispersions of two or more different nanoadditives in mixture or composite form, have received attention more recently. Research on hybrid nanofluids aims to further enhance the individual benefits of each single dispersion through potential synergistic effects between nanomaterials. Multiple experimental studies have been conducted independently analysing the convective heat transfer performance of mono or hybrid nanofluids for single-phase and two-phase convective heat transfer applications. However, there are still no general conclusions about which nanofluids, mono or hybrid, present better prospects. This review summarizes the experimental studies that jointly analyse both hybrid and mono nanofluids for these applications and the results are classified according to the heat transfer device used. Based on this criterion, three large groups of devices were noticed for single-phase convective heat transfer (tubular heat exchangers, plate heat exchangers and minichannel heat exchangers/heat sinks), while one group was identified for two-phase convective heat transfer (heat pipes). The main outcomes of these studies are summarized and critically analysed to draw general conclusions from an application point of view.
Highlights
As a society, we are encouraged to comply with the 2 K global warming limit of the Paris Agreement [1] and thereby we should replace conventional primary energy generation methods with low-emission replacements [2]
The interest in the research of hybrid nanofluids has significantly grown in the last years [7,8] on the assumption or excuse that an enigmatic improvement in relation to mono nanofluids occurs in the heat transfer process [9,10]
The results showed a maximum h improvement for the Al2O3:multi-walled carbon nanotubes (MWCNT)/water hybrid nanofluid, 42.2%
Summary
We are encouraged to comply with the 2 K global warming limit of the Paris Agreement [1] and thereby we should replace conventional primary energy generation methods (for instance burning fossil fuels) with low-emission replacements [2]. An analysis of the literature shows that both passive (for instance, heat transfer enhancing surfaces or nanofluids) and active techniques has been applicable to heat exchangers [6] It is well rec ognised that these procedures entail a remarkable potential to improve the heat exchanger thermal–hydraulic performance. It is worth investigating the optimization of the efficiency of existing energy systems involving heat exchangers, including those available in renew able energies, from the perspective of fostering the development and practical implementation of nanofluids In this regard, the interest in the research of hybrid nanofluids has significantly grown in the last years (see Fig. 2a) [7,8] on the assumption or excuse that an enigmatic improvement in relation to mono nanofluids occurs in the heat transfer process [9,10]. The common characteristics and results between studies will contribute to develop conclusions for the scientific debate on whether to select “hybrid or mono nanofluids for convective heat transfer applications”
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