## 1 Introduction
[12] experimented on a plate-fin heat exchanger using Al\({}_{2}\)O\({}_{3}\)-W NFs, and reported an increase in \(N_{Nu}\) ranging from 9% to 15% at a weight concentration of 0.1 _w/w_%. [PERSON] and [PERSON] [13] used a mathematical model to analyze a finned tube heat exchanger using EG-W (60:40 _w/w_%) as the base fluid. The NPs considered were Al\({}_{2}\)O\({}_{3}\) and CuO, which, at 4% volume, provided modeled results with an increase in heat transfer compared to the base fluid of 11.6% and 8.7%, respectively. [PERSON] and [PERSON] [14] chose a microchannel geometry to study the effect of NFs on heat transfer using computer simulation. They found that for CuO-W NFs at 1 and 4 _v/v_% the average enhancement of thermal performance was 15% and 20%, respectively, compared to the base fluid. [PERSON] et al. [15] conducted experiments with Al\({}_{2}\)O\({}_{3}\)-W NFs at 0.3, 0.5, and 1 _v/v_% in a rectangular minichannel, and the results showed up to a 54% enhancement \(N_{Nu}\) for the NF at 1 _v/v_%. Besides the improvement in \(k\), the passive enhancement technique with tube curving has been widely used due to its compactness and high heat transfer coefficient produced by the pattern of secondary flows (i.e., Dean vortices). This secondary heat transport superimposed on the main axial flow dominates the overall process, achieving a higher heat transfer rate per unit of length than in straight tubes [16]. The use of NFs in helical coil heat exchangers has also proven advantageous in achieving heat transfer enhancement [17; 18]. Although heat transport along curved paths exhibits superiority over straight geometries, as the streamlines of Dean vortices are closed curves, their fluid parcels do not mix. Thus, as the fluid parcels near the vortex centers do not approach the tube walls, the temperature fields are heterogeneous in the radial direction [19]. Perturbing the secondary flow in curved geometries by chaotic advection produces chaotic trajectories that enhance the mixing of the particles within the fluid [20]. Chaotic advection in curved ducts is produced by periodic changes in geometry, such as a rotating coil axis over defined segments. The coiled flow inverter (CFI) introduced by [PERSON] and [PERSON] [21] is a particular class of curved geometry that promotes chaotic mixing through a combination of coils and bends; the centrifugal forces acting on the secondary flow change direction at each equally spaced 90\({}^{\circ}\) bend along the length of a straight helical coiled tube. Their pioneering work reported a significant reduction of residence time distribution in the CFI geometry, and it was found that it exhibited up to a 20 times reduction in the dispersion number compared to that in a helical coil geometry. Typically, bent coils are made up by four helical \"arms\" arranged in a square shape. These four-arm CFI units, which are used in different research fields, including heat transfer enhancement, are henceforth referred to as CFI sets. Most of the experimental research in heat transfer characteristics for CFI sets has been performed using water as the working fluid. For instance, [PERSON] et al. [22] evaluated the flow dynamics and thermal performance in a heat exchanger similar to a shell and tube (STHE) with eight CFI sets on the tube side. The values of \(h\) on the tube side were compared to those in a straight coiled geometry predicted by existing correlations. The CFI geometry increased the \(h\) by 25% and 12% in the Reynolds number (\(N_{Re}\)) ranges of 1000 \(<N_{Re}<\) 10,000 and 10,000 \(<N_{Re}<\) 16,000, respectively. Experimental data were fitted to develop correlations accordingly. [PERSON] et al. [23] compared the performance of the CFI heat exchanger used by [PERSON] et al. [22] with that of the conventional PHE and the STHE. The three devices were rated under equivalent heat transfer areas and process conditions.