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Thermal Conductivity of Ionic Liquids and IoNanofluids and Their Feasibility as Heat Transfer Fluids

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journal contribution
posted on 16.04.2018, 00:00 by João M. P. França, Maria José V. Lourenço, S. M. Sohel Murshed, Agílio A. H. Pádua, Carlos A. Nieto de Castro
Ionic liquids and ionanofluids were studied in recent years as possible alternatives to current engineering fluids, namely in the area of heat transfer. Excellent thermal properties, like high heat capacity per unit volume and thermal conductivity, allied to the dispersion of nanoparticles in them, have created great expectations, as the enhancement of their thermophysical properties liquids can contribute to better efficiency in heat transfer. The thermal conductivity of [P66614]­[N­(CN)2], [P66614]­[Br], [C2mim]­[SCN], [C4mim]­[SCN], [C2mim]­[C­(CN)3], and [C4mim]­[C­(CN)3] in the temperature range of 293–343 K at 0.1 MPa and their ionanofluids with multiwalled carbon nanotubes are reported in the present work. While we could not obtain stable suspensions with phosphonium based ionic liquids, thermal conductivity enhancement of cyano-based ionic liquids was compared with our previous work using dicyanamide ionic liquids. The thermal conductivity of C2mim+ ionic liquids and ionanofluids is generally higher than the corresponding C4mim+ fluids. Temperature dependence of thermal conductivity enhancement hinders the conception of a unified thermal conductivity enhancement predictive model of the presented ionanofluids, current theories under-predicting its value for the dispersions studied. Finally, we selected a specific heat transfer process and calculated the heat transfer area necessary using currently commercialized heat transfer fluids, ionic liquids, and ionanofluids. While the addition of nanomaterial to the ionic liquids leads to an increase in the heat transfer available area, the enhancement of the thermophysical properties leads to a smaller variation of the area with temperature. Depending on the ionic liquid, some of the ionanofluids studied are head-to-head with a significant number of currently used heat transfer fluids concerning the heat transfer area necessary to transfer the same amount of heat.