1. Introduction
Heat dissipation is very important because it is directly related to the life and safety of electronic devices. Accordingly, various studies for managing heat in electronic devices are being conducted [
1]. Among them, this study aimed to improve the heat transfer performance using ferrofluid. Ferrofluid is a colloidal suspension, containing magnetic nanoparticles with a diameter of 3 to 15 nm [
2]. There are many kinds of particles, such as
,
, and
, which have ferromagnetic properties. Due to the magnetism of the particles, this fluid has the characteristic of reacting with a magnetic field. When a magnetic field is applied, the fluid has kinetic energy and forms a specific shape [
3]. That is, there are the advantages that a flow can be created only by a magnetic force without direct contact with the fluid, and that control is possible with a relatively simple configuration. In addition, it was confirmed that nanofluids have superior thermal performance compared to general fluid, due to the properties of the particles [
4,
5].
Yamaguchi et al. [
6] observed heat transfer by natural convection when a magnetic fluid is contained in a cavity. AR (aspect ratio) values, such as 1, 1.5, and 2 of the cavity, were set as variables.
(magnetic Rayleigh number), Ra (Rayleigh number),
(critical Rayleigh number) corresponding to each variable were observed. As AR increased, the
tended to decrease, and it was confirmed that many vortex fields appeared. In addition, when a magnetic field was applied, the improvement in heat transfer was confirmed. It is due to the occurrence of various vortex shapes and strong circulation.
Selimefendigil et al. [
7] observed the flow phenomenon in a partially heated cavity. In their study, Ra, the location of the heat source and the strength and the location of the magnetic field were set as variables. Circulation was related to the strength of a magnet, and was caused by the partial change in susceptibility, according to the temperature gradient. It was concluded that the change in the circulation shape increases when the value of Ra also increases.
Ghorbani et al. [
8] analyzed the heat transfer performance, according to the strength, number, and position of magnets when high-temperature ferrofluid was injected. Five different magnetic field strengths were specified, and magnets were arranged in six places. When the magnetic field was stronger than a certain standard, the cooling performance was proportionally increased. The small circulation was combined to create a big vortex, which circulated the low-temperature ferrofluid to the relatively high-temperature wall. Finally, the position of magnets showed that the heat transfer performance was improved when all magnets were placed near the cold wall and closer to the inlet area.
Bahiraei et al. [
9] placed a permanent magnet in the toroidal loop. Thermomagnetic convection characteristics in which circulation occurs depending on the heat source were used. The magnitude of the heat flux from the heat source, the temperature of the heat sink, the position of the magnet, and the strength of the magnetic field were set as variables. It was shown that the ferrofluid in the loop can be cycled continuously without additional energy consumption. In addition, as the temperature difference between the heat source and the heat sink increased, the circulation speed in the loop increased, indicating that the heat transfer performance was improved.
Bahiraei et al. [
10] studied the heat transfer performance when two phases (water and Fe
3O
4) exist in a square channel. The density, particle size, and magnetic field strength of the nanofluid were set as variables. As a result, the heat transfer performance improved as the density and the particle size increased. In addition, it was concluded that the circulation of the fluid increases as the magnetic field strength increases.
Zheng et al. [
11] studied the plate heat exchanger under a magnetic field, while working fluids, hot water, and cold ferrofluid were used. Positions of magnets were set into six cases, and the position showing the best heat transfer performance was selected. The case where resistance loss and an improvement in heat transfer performance can be achieved at the same time is when the magnet is set vertically. It was confirmed that the pressure drop decreased when the magnets were arranged without overlapping. Finally, the heat transfer performance and pressure drop were considered at the same time. It was concluded that the optimal effect can be obtained when the magnets are placed vertically in a non-overlapping state.
Bezaatpour et al. [
12] compared the cooling performance, according to the magnetic flux density in a fin-tube heat exchanger. Five different densities were set as variables. The cooling performance of the tube farthest from the inlet was the lowest among the three tubes. It was also concluded that the change in the flow was greater when the magnetic flux density was strong.
The purpose of this study was to improve cooling performance, by using ferrofluid in the fin-tube heat exchanger when there is a permanent magnet outside. The numerical analysis was carried out in the absence of magnetic fields, and this result was set as a reference model. Based on this, three conditions were set. The first is a case in which a magnetic field was applied to the reference model. Nanofluids responded to the magnetic field and began to create flows in the heat exchanger. The second involved a case where permanent magnets were added. The last case involved installation of VGs (vortex generators). The heat transfer coefficient was used as an output parameter to compare the cooling performance of these cases. It is believed that these research results can be used as a means to improve the heat dissipation performance of the system in a gravity-free environment, where external forces cannot be applied.
2. Model and Process Description
The reference model used in this study is shown in
Figure 1.
Figure 1a is a 3D fin-tube heat exchanger model, and
Figure 1b is an arbitrary cross-section of a 3D model. The permanent magnet was a neodymium magnet. The space set as the inner chamber was for the ferrofluid. Three tubes with a diameter of 0.6 cm were filled with water and arranged inside the chamber. The distance between them was 1.4 cm (
Figure 1h). The distance from the center of the tube to the
Figure 1a side was set to 1.5 cm (
Figure 1i). The type of ferrofluid used in this study was EFH-1. This liquid uses oil as the base fluid and is composed of Fe
3O
4 particles. The properties for EFH-1 are shown in
Table 1. Nanoparticles in EFH-1 generate flow by an external magnetic field. By controlling the flow using a magnetic field, it is possible to move the fluid in various directions. In addition, there is the advantage that a vortex can be generated by changing the strength of the magnet.