**1. Introduction**

Nanofluids are a mixture of base fluid and small nanoparticles up to 100 nm size. It is proven that nanofluids have better thermal properties than the base fluids [1], therefore, in the last two decades, a substantial amount of research has been conducted related to the nanofluids and their application to the solar systems and other heat exchange devices. A major part of those research studies was focused on the thermophysical properties and heat transfer of nanofluids where the base fluid was water and ethylene-glycol, with nanoparticles with different oxides (Al2O3, TiO2, CuO, etc). Ribeiro et al. [2] introduced ionanofluids, the suspension of nanoparticles in ionic liquids as a new class of nanofluids. One of the first experimental investigations of ionanofluids was done by Altin et al. [3], who analysed the rheological properties of the suspensions of the nanoparticles in the ionic liquids. Following this, Wang et al. [4] and Altamash et al. [5] also experimentally analyzed the rheological properties of ionanofluids. They concluded that ionanofluids show non-Newtonian flow behavior. When it comes to the thermophysical properties of the ionanofluids, Nieto de Castro et al. [6] were among the first who conducted experimental investigations into the thermophysical properties of ionanofluids. They studied the thermal properties of imidazolium and pyrrolidinium and higher wall carbon nanoparticles, and concluded that nanoparticles cause improvement in the thermal conductivity and heat capacity of ionanofluids compared to the base fluids. Ionanofluids are mainly analyzed in

**Citation:** Haseˇci´c, A.; Almutairi, J.H.; Biki´c, S.; Džaferovi´c, E. Numerical Analysis of Heat Transfer Performances of Ionic Liquid and Ionanofluids with Temperature-Dependent Thermophysical Properties. *Energies* **2021**, *14*, 8420. https://doi.org/ 10.3390/en14248420

Academic Editor: Kamel Hooman

Received: 5 October 2021 Accepted: 17 November 2021 Published: 14 December 2021

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respect to their thermophysical properties. The analyses were experimentally conducted by Fox et al. [7], Bridges et al. [8], Titan et al. [9–13] and Bhattacharjee et al. [14], while Minea et al. [15] gave a comparison of thermal conductivity for different ionanofluids. Fox et al. [7] experimentally investigated the influence of alumina nanoparticles on the thermophysical characteristics of ionanofluids, and showed that fibrously shaped Al2O3 nanoparticles show a greater improvement in thermal conductivity. Bridges et al. [8] showed that the increased heat capacity of ionic fluids was improved with alumina particles without a detrimental effect on thermal stability. Titan et al. [9–13] showed that an increased heat transfer coefficient of forced convection and a deterioration in the natural heat transfer of a nanoionic fluid relative to an ionic fluid, as well as showing that there are significant improvements in the thermophysical characteristics of ionanofluids compared to the base ionic fluids. Besides the thermophysical properties, they also analyzed the rheological behavior of the ionanofluids and concluded that ionanofluids show non-Newtonian flow behavior.

As it can be seen from the literature overview, the experimental investigations of ionanofluids are rare in comparison to the published experimental investigations of the nanofluids. When it comes to the analyses of ionanofluids by using computational fluid dynamics, to the authors best knowledge there are only few studies available [16–23], whereas Said [24] analyzed the use of adaptive neuro fussy interface systems to predict the thermal conductivity and viscosity of ionanofluids. Computational fluid dynamic analyses of ionanofluids are mainly focused on the heat transfer performances.

Minea et al. [16] numerically analyzed heat transfer in a square enclosure filled with ionic liquid nanofluid. Although they stated that the thermophysical properties are temperature dependent, it can be concluded that they are only a function of initial temperature. Chereches et al. [17,18] numerically analyzed heat transfer behavior of ionanofluids in laminar flow for different Reynolds numbers and one initial temperature for case without [17] and with [18] insulation over the pipe walls. Titan et al. [19] investigated the natural convection heat transfer of Al2O3 nanoparticle enhanced N-butyl-N-methylpyrrolidinium bis {(trifluoromethyl)sulfonyl} imide ([C4mpyrr][NTf2]) ionic liquid. The heat transfer performance of ionanofluids was also numerically analyzed by Prasad et al. [20] and Rupesh et al. [21]. Prasad et al. [20] analyzed the heat transfer in a 2-D flat plate, whereas Rupesh et al. [21] analyzed the heat transfer performance of ionanofluids around a circular cylinder. The most recent numerical investigations of the heat transfer behavior of particle suspension in ionic liquids were done by Shah et al. [22] and Bouchta et al. [23].

Although it has been shown that ionanofluids flow behavior corresponds to non-Newtonian flow, the assumption of Newtonian fluid was made in many studies [3–5,9–13]. In most numerical research a single-phase assumption was made, whereas the properties of ionanofluids are calculated as the properties of a mixture and a function of the weight percent of nanoparticles and base liquid. Furthermore, the studies all assumed that the properties are constant and related only to the initial and boundary conditions.

In this research, a numerical analysis of steady, laminar forced convection flow of Al2O3 nanoparticles in ([C4mpyrr][NTf2]) ionanofluids in a straight tube with constant heat flux on the tube wall for different Reynolds numbers and different values of initial and inlet temperature is presented. The heat transfer characteristic of the ionic liquid and ionanofluids for different weight percentages were analyzed and compared. The geometry was chosen due to its common application in solar collectors. The main contribution of this research is that this is, to the authors best knowledge, the first research on heat transfer characteristics of ionanofluids in which the thermophysical properties are temperature related and described in corresponding equations and implemented in such form. Moreover, the results are compared with results obtained for constant thermophysical properties for different initial temperature values. Although the term constant thermophysical properties is used, it must be emphasized that the thermophysical properties are a function of the initial/inlet temperature and therefore are constant for the same initial/inlet temperature.
