ReviewAdvanced applications of tunable ferrofluids in energy systems and energy harvesters: A critical review
Introduction
Currently, cooling is one of the most important scientific challenges in production related industries, such as transportation, manufacturing and microelectronics. Technological advancements have led to increased thermal loads and thus, improvements to cooling systems have become a necessity. Maximising the surface area of heat exchanger systems is the conventional approach to enhance heat dissipation. However, this method needs an unwanted rise in the size of thermal management systems, thus, there is an urgency for novel coolants with enhanced performance [1]. One such coolant is the innovative concept of ‘nanofluids’, which are a mixture of metallic/nonmetallic nanoparticles in a base fluid. The term nanofluid was originally introduced by Choi at Argonne National Laboratory [2]. A substantial improvement in liquid thermal conductivity, specific heat and viscosity are the unique features of nanofluids. The relatively large overall surface area of nanoparticles not only improves heat transfer capabilities but also increases the stability of the suspension by alleviating particle settling phenomenon. There are also several potential benefits from nano-suspension testing, specifically: better long-standing stability compared to the millimetre or even micrometre sized particle suspensions and lower erosion and pressure drop, especially in micro-channels [3]. Nanofluids are very potential fluids for heat transfer application [4], [5], [6], [7].
A unique class of nano-suspensions named magnetic nanofluids demonstrate both fluid and magnetic properties. Magnetic nanofluids have drawn considerable attention due to the possibility of tuning their heat transfer and flow properties through the application of an external magnetic field [8], [9]. The nanoparticles can be either ferromagnetic materials, such as cobalt or iron, or ferrimagnetic materials such as magnetite. Magnetite nanoparticles are not susceptible to oxidation and therefore are a far better alternative to iron or cobalt nanoparticles which tend to lose their magnetic characteristics over time because of oxidation [10]. Magnetite-based nanofluids with particle sizes less than 10 nm, known as ferrofluids, were initially introduced by Stephen Pappell in the 1960s (at NASA) as an advanced method for controlling fluid in space [11]. He concluded that magnetite nanofluids have a wide range of applications from lubricating rotary shafts to biomedicine. These types of nanofluids have improved thermal properties (such as heat capacity, thermal conductivity, and viscosity), as well as magnetic properties, both of those tunable characteristics help to control the heat transfer and the movement of the particle by applying the magnetic fields. As a result, they are believed to be one of the promising fluids in different engineering fields such as bioengineering, thermal engineering, electronics, and energy harvests [12], [13].
However, when subjected to an external magnetic field, the thermal conductivity of magnetite nanofluid can be raised to levels much higher than any other nanofluid. In one such example, Philip et al. [14] have shown remarkable enhancements in thermal conductivity of up to 300% for a magnetite based nanofluid. This high rise in thermal conductivity is associated with the effective heat conduction through the chain-like structures induced in the magnetite nanofluid. The advantage of using a magnetically polarisable nanofluid, such as magnetite nanofluid, is that the size, shape and form of aggregates can be precisely controlled by the external magnetic field. More importantly, unlike other nanofluids, the aggregation observed in magnetite nanofluids is perfectly reversible due to the super-paramagnetic nature of particles [15]. This tunable nature offers great opportunities for resolving the inherent problems associated with conventional nanofluids, such as lower heat transfer capacity, clogging and blockage of the flow passage. The ferrofluid with magnetic fields in the application of heat transfer is considered as the compound heat transfer method [16]. The magnetic nanofluids offer the following advantages compared to the nonmagnetic nanofluids [17];
- (a)
the temperature gradient and non-uniform magnetic field are induced by using a magnetic field, which may initiate a flow in the fluid. Such phenomenon is called thermomagnetic convection [18] and it is readily handled;
- (b)
the thermomagnetic convection is higher compare to the gravitational convection;
- (c)
the possibility of changing thermal properties of ferrofluids by applying external magnets/solenoids [19], [20]
Furthermore, a ferrofluid is a suspension of solid-liquid, made-up from nano-sized permanent magnetic dipoles [21]. The magnetic nanofluids are potential fluids for vibrational energy harvesting application due to their fluidity and magnetic properties in which they act as a soft ferromagnetic substance [22], [23]. Energy harvesting is an alteration of the environmental energy to the electrical energy at a small scale and was originally introduced in 1966 [24]. The energy sources for energy harvesters are freely available in the environment. Examples of energy sources include vibration, wind energy, wave energy, and thermal temperature gradients. Nowadays energy harvesting has become a hot topic for research because of its exceptional advantages. The cost of the battery supply, in particular, the maintenance cost to replace the discharged batteries, make the energy harvesters a very attractive option. The conventional energy harvesting techniques only capable to generate a few micro-watts, whereas vibration energy harvesters demonstrate high performance in less vibration by replacing the solid magnets with ferrofluid.
Recently magnetite nanofluids have come to the attention of the research community after showing high enhancement in heat transfer by applying external magnetic field as well as their usage in the electromagnetic energy harvesters to generate more power with a small vibration with compare to the traditional harvesters. The objective of this paper is to concentrate on the recent investigations of magnetic nanofluids in the application of heat transfer and energy harvesting. It is anticipated that this review paper will help to provide a clear idea about the current status of ferrofluid, and also specify the recommendations for the future study.
Section snippets
Preparation and characterisation of magnetic nanofluids
The magnetic nanofluids are prepared by dispersing of super-paramagnetic nanoparticles into a non-magnetic base fluid such as water, hydrocarbon oil and so on [25]. Generally, the preparation of ferrofluids consist of two steps: (a) the magnetic nanoparticles preparation, (b) the dispersion of the synthesised magnetic nanoparticles in different polar/non-polar carrier liquids. In the first step, the nano-sized magnetic particles can be produced by various processes such as ball milling [26],
Stability of magnetic nanofluids
A nanofluids’ stability relies on a number of aspects [53]: (a) nanofluids are multiphase dispersion systems with high surface energies and hence, are thermodynamically unstable, (b) nanoparticles dispersed in the nanofluids have strong Brownian motions. The nanoparticles’ movement can offset their sedimentation due to gravity, (c) the dispersed nanoparticles in the fluids may settle out with time because of nanoparticle aggregation, which is initiated by van der Waals forces, (d) no chemical
Mechanism of heat transfer enhancement using magnetic nanofluids
Numerous methods have been identified on the mechanism of the heat transfer enhancement in the different studies. The thermal properties especially the thermal conductivity of magnetic nanofluids was the main focus of some investigations, but the mechanisms to justify the experimental data in the presence and absence of external magnetic field are still desirable. The Brownian motion [66], [67], [68], [69], liquid layering on the particle-liquid interface, and the effects of nanoparticles
An overview of thermal properties of magnetic nanofluids
Investigations on magnetic nanofluids without the application of external magnet exhibit that thermophysical properties of nanofluids are affected by different factors such as nanoparticle size, types and intensities of magnetic fields, nanoparticle volume concentration, base fluid properties, temperature, the chemical composition of the nanoparticle, the coating around the nanoparticles etc. The properties of magnetic nanofluids and their effects on the volume fraction of nanoparticle and
Convective heat transfer and pressure drop characteristics of magnetic nanofluids
Nanofluids are dilute suspensions of functionalised nanoparticles and their objective is to enhance the heat transfer performance of coolants/fluids, and nowadays has evolved into a promising nanotechnological area [122], [123]. Although thermal conductivity of magnetic nanofluids in the absence and presence of magnetic field has been the subject of many past studies, relatively little effort has been focused on the convective heat transfer of magnetic nanofluids. It has been proven that the
Ferrofluid based energy harvester
Over the current years, interest has raised in the development of micro-electromechanical systems (MEMS) and miniaturised system. In this approach, there has been great effort to decrease the energy consumption of these devices from being in the order mW to the order μW [137]. Applications such as sensors in buildings, medical implants, wireless sensors networks, sensors used in military applications, monitoring of the environment, sensors that may provide different information about the
Conclusion and recommendations for future work
This paper involved the recent development on magnetic nanofluids for energy systems and energy harvesters. Recent studies have discussed the thermomagnetic convection and thermomagnetic effects. Few investigations have been done on the other features of heat transfer such as the improvement and control of thermal properties of ferrofluid under the external magnetic field. The use of magnetic nanofluids in heat transfer, as well as electromagnetic energy harvester applications, appear
Acknowledgments
The authors gratefully acknowledge the financial support provided by the University of Newcastle (Australia), Granite Power Pty Ltd and the Australian Research Council through the ARC-Linkage grant LP100200871, for the present study.
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