###### Abstract  
In this study, experiments, simulations, and optimization were performed to evaluate heat transfer performance of ferrofluids. Ferrofluids are colloidal suspensions containing magnetic-nano particles with a diameter of 5 to 15 nm in a base fluid such as oil or water. Recently, as many devices are miniaturized, the design of heat dissipation systems are being diversified to consider cost and safety, and it is becoming important to separate an ancillary device for cooling from main unit. In ferrofluids, the behavior and vortex of magnetic-nano particles are actively generated by an external magnetic field, and the cooling system can be designed in a simplified manner by using this characteristic. The main design parameter is the arrangement of permanent magnets, and the output variable is the temperature inside the magnetic nanofluid. The permanent magnet can be moved up and down, and the temperature inside the magnetic nanofluid was measured at various locations. A predictive model was created using a design of experiments (DOE) and response surface method (RSM) using selected design and temperature variables. Based on the generated regression model, an optimization was applied to find a permanent magnet arrangement that  
depending on whether magnetic field was applied. When an electromagnet was applied, numerical results were compared and analyzed by increasing Re or changing the frequency. Through this, a difference in the heat transfer performance between the high temperature ferrofluid and the low temperature wall was confirmed. Finally, compared with the case where no electromagnet was applied, the heat transfer was increased by 17.41% in the case of Re = 100 and magnetic flux density (B) = 1 T.  
[PERSON] et al. [7] set up a new cooling system for PV systems using a ferrofluid composed of Fe\({}_{3}\)O\({}_{4}\) based on pure deionized water (DIW) as a cooling medium and a rotating magnetic field (RMF). The experimental setup consisted of four major components. There was a PV system that generated heat, a metal halide lamp, a ferrofluid and permanent magnets. The design variables were the concentration of magnetic nano particles (MNPs = 0.01, 0.02, 0.03, 0.04, 0.05%), the rotational speed of magnet, and the magnetic field induction (B = 0.35, 0.45, 0.57, 0.72, 0.88 T). The system consisted of 72 cells, and 12 temperature measurement points were set. Overall, the cooling performance of the cell surface was improved as the concentration of magnetic nano particles, the strength of the magnetic flux density, and the rotation speed increased. This was due to the performance change by the magnetic torque of particles and the vorticity of the ferrofluid. Finally, a cooling performance improvement of 17.8-30% and a maximum power improvement of 1.67-47.5% was shown. [PERSON] et al. [8] tried to improve the performance of a flat plate solar collector by using four rotary tubes and a magnetic field inducer. As the working fluid inside the collector tube, a ferrofluid composed of Fe\({}_{3}\)O\({}_{4}\) and water was used. The design parameters were set as the tube rotation speed (\(\omega\) = 0-2.5 rpm) and magnetic field intensity (B = 0-8 mT). Heat dissipation occurred through an external magnetic field and tube rotation, and the performance was improved by setting the convective heat transfer coefficient as an output parameter. In conclusion, 27.8% and 10.44% of energy loss was recovered by using a magnetic field inducer and a rotary pipe, respectively. [PERSON] et al. [9] analyzed the heat transfer performance using a low temperature ferrofluid flowing in a rectangular horizontal channel. Design parameters include nanoparticles volume fractions (\(\varphi\) = 0-0.05), Reynolds number (Re = 50-500), and magnetic number (Mn = 0-100). Heat exchange became active by using a magnetic field and the ferrofluid. This is because a recirculation region was created around the magnetic source and the thermal boundary layer was removed.