Ionanofluids, which are an innovative class of new heat transfer fluids, consist of nanoparticles suspended in ionic liquids (ILs), thus, ionanofluids (INFs) are a special type of nanofluids. Since ILs are the only base fluids for ionanofluids, it is important to highlight the main characteristics, advantages, as well as potential applications of ILs.
Ionic liquids are also an innovative class of fluids, which consist entirely of ions and have the melting point lower than 100 ºC. Ionic liquids exhibit several unique features that allow for development and synthesis by tailoring the cation-anion structure for desired physiochemical properties and thus for the target applications. 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.
In fact, due to these features, ILs have been investigated as heat transfer media even from the beginning of the 21-st century. Some studies also suggested that, due to their very low vapor pressure preventing them to be cooled by evaporations, ILs can be used for thermal energy storage in an open system. Thus, ionic liquids are a good medium for thermal storage systems as well as heat transfer fluids in solar power generation applications.
ILs can also be used as heat transfer fluids in heat exchange systems such as conventional heat exchangers. For instance, due to their high heat capacity and thermal conductivity, some researchers studied ILs as possible heat transfer fluids in shell and tube heat exchanger. The heat transfer areas were estimated to be comparable or bigger for ILs as compared to some usual heat transfer fluids. It was later demonstrated that the combination of nanomaterials with ILs shows great potential as heat transfer fluids through the enhancement of the thermal properties.
Apart from the heat transfer and various thermal management systems, ILs can be implemented in an extensive range of techniques and systems such as catalysis, synthesis, solar absorbing panels, lubricants, absorption refrigeration systems, solid liquid separation process, and supercritical fluids. ILs can be also used as the solvent system and as the reactant/catalyst in a reaction process.
In 2007, Fukushima and Aida were the first to mix carbon nanotubes (CNT) in ionic liquids at room temperature to form gels named as “buck gels” that can be applied in many engineering or chemical processes. The “Bucky gels” are actually emulsions of ionic liquids with nanomaterials, so these are the ionanofluids of high concentration of CNT. Further on, numbers of research works were performed in this new area of ionanofluids and a tremendous work has been accomplished by a group (Nieto de Castro and co-workers) from University of Lisbon, Portugal. The research on thermophysical properties of these new heat transfer fluids is still at its very beginning and some results will be carefully reviewed further on.
In terms of real-life applications, few research groups found that the ionanofluids are very well suited for solar energy applications, especially for the efficient absorption of the solar radiation and for its transmission to heating/cooling systems. Anyway, the research is ongoing and a lot of studies are needed in order to clarify the behavior and the potential applications of these new heat transfer fluids.
The idea of this article was to make a comprehensive review on existing research on thermophysical properties of ionic liquid-based nanofluids (ionanofluids) and to implement the results in a computational fluid dynamic code to evaluate their heat transfer behavior flow system under laminar flow condition. The numerical analysis was performed for the ionanofluids that were found well described in the literature in regard to thermophysical properties.
The most important conclusions derived from this review can be summarized as:
- 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.
In conclusion, we strongly affirm that more extensive efforts are needed in order to completely describe these new heat transfer fluids and to give a better framework for their usage in practical applications.
These findings are described in the article entitled A review on development of ionic liquid-based nanofluids and their heat transfer behavior, recently published in the journal Renewable and Sustainable Energy Reviews. This work was conducted by Alina Adriana MINEA from Technical University “Gheorghe Asachi” from Iasi, ROMANIA, and S.M. Sohel MURSHED from the Department of Mechanical Engineering, Instituto Superior Técnico, University of Lisbon, Portugal.