By Josh Perry, Editor
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In an article from Science Trends, researchers reviewed recent studies of ionanofluids (INF), new heat transfer fluids that consist of nanoparticles suspended in ionic liquids (IL) that can be tailored towards specific applications.

Ionic fluids are being explored for their heat transfer capabilities. (Wikimedia Commons)

As the article explained, “Ionic liquids are also an innovative class of fluids, which consist entirely of ions and have the melting point lower than 100°C…As these liquids are not combustible or volatile at ambient conditions and also are recyclable, they are considered as environmentally-friendly fluids. ILs also have an extremely low vapor pressure, high thermal stability, and high heat capacity, and the combination of these features makes these fluids as better heat transfer fluids at low or very low pressures or even under vacuum conditions.”

Ionic liquids have been studied since the turn of the century as possible heat transfer fluids in heat exchangers and other liquid cooling applications. Nanofluids are a growing trend as well, so there is a natural connection between enhanced, engineered fluids and incorporating ionic fluids, which have greater thermal characteristics than typical fluids.

The first inclusion of nanofluids with IL was in 2007 when researchers added carbon nanotubes at room temperature to form buck gels. Further experimentation showed that INF can be used in solar applications because they efficiently absorb solar radiation.

According to the article, the most important conclusions were:

• In almost all considered cases, the thermal conductivity of the ionanofluids remained almost constant while temperature increases and increased with increasing the nanoparticles concentration.
• The experimental results for specific heat indicated a slight increase with temperature and a minor decrease while adding the nanoparticles to the base ionic liquid. It was revealed that very limited efforts have been devoted to this key property of INF and more systematic studies are to be performed.
• Only a handful of studies are reported on the density of INFs and their results showed that densities of both ILs and their INFs decrease with increasing temperature. However, densities of INFs were found to be slightly lower than those of their base ILs. More studies are needed to better identify the effects of nanoparticles loading and temperature on this property of INF.
• The data for viscosities of ILs and their INFs are scattered and contradictory as some researchers found a decrease with increasing temperature while others observed the contrary. Effect of nanoparticles concentration on the viscosity of INFs is also inconsistent and contradictory as well. Although most of the researchers found an increase in viscosity of INF while adding nanoparticles to the base ionic liquid, some researchers reported opposite.
• The numerical analysis revealed that in general, with increasing flow, the heat transfer coefficient increases considerably. Moreover, it seems that the thermal conductivity plays a bigger role in laminar convection, while viscosity is less significant.

The full review was published in Renewable and Sustainable Energy Reviews. Read the abstract and full article at https://sciencedirect.com/science/article/pii/S1364032118302314.