What are nanofluids and which are their applications?

Today more than ever, cooling is one of the most pressing needs of many industrial technologies because of their ever-increasing heat generation rates at both micro-level (such as computer chips) and macro-level (such as car engines). However, conventional heat transfer fluids such as air, water, ethylene glycol, and
oil shows very low thermal conductivity compared to solids.

Studies made for the enhancement of the poor thermal conductivity of liquids by adding solid particles to them began more than a century ago, when the great scientist James Clerk Maxwell developed a theoretical model of the electrical conductivity of heterogeneous solid particles. Since then, the classical Maxwell model has been applied while investigatigating the thermal conductivity of mixtures of solid particles and liquids. However, all these studies have been conducted with millimeter or micrometer-sized particles. The major problem with the use of microparticles is that they settle very rapidly in liquids. They also cause abrasion, clogging, and additional pressure drops. Furthermore, high particle concentrations are required to obtain appreciable improvements in the thermal conductivities of these suspensions.

These problems severely limit the use of conventional solid-liquid suspensions as practical heat transfer fluids. Despite tremendous efforts, the technical barriers mentioned above still remain after more than 100 years. Modern nanotechnology has enabled the production of nanoparticles with average particle sizes below 100 nm. Nanoparticles generally have superior mechanical, optical, electrical, magnetic, and thermal properties.

Nanofluids are a new class of nanotechnology-based heat transfer fluids, obtained by dispersing and stably suspending nanoparticles with typical dimensions on the order of 10 nm.

Nanofluids (Nanoparticle fluid suspensions) is the term coined by Choi (1995) to describe this new class of nanotechnology-based heat transfer fluids with augmented thermal properties, both superior to the properties of their own hosting fluids and the conventional particle fluid suspensions.

The goal of nanofluids is to achieve the highest possible thermal properties at the smallest possible concentrations (preferably<1% by volume) by uniform dispersion and stable suspension of nanoparticles (preferably<10 nm) in host fluids. To achieve this goal it is vital to understand how nanoparticles enhance
energy transport in liquids.

Since Choi conceived the new concept of nanofluids in the spring of 1993, many talented scientists in the rapidly growing nanofluids community have made important scientific discoveries not only in discovering unexpected thermal properties of nanofluids, but also in proposing new mechanisms behind enhanced
thermal properties of nanofluids, developing new mathematical models for the nanofluids, identifying unusual opportunities to develop next-generation coolants such as smart coolants for computers and safe coolants for nuclear reactors.

As a result, the research regarding nanofluids is now receiving increasing interest worldwide, as shown by the exponentially increasing number of publications, which from 1993 to 2012 have exceeded the number of 2400.

Combinations of nanoparticles and base fluids can produce many heterogeneous nanofluids. Nanoparticle materials may include:

• Oxide ceramic – Al2O3, CuO
• Metal carbides – SiC
• Nitrides – AlN, SiN
• Metals – Al, Cu
• Nonmetals – Graphite, carbon nanotubes
• Layered – Al + Al2O3, Cu + C
• PCM – S/S
• Functionalized nanoparticles

whereas base fluids may include:

• Water
• Ethylene- or tri-ethylene-glycols and other coolants
• Oil and other lubricants
• Bio-fluids
• Polymer solutions
• Other common fluids

There are two main procedures for preparing nanofluids, called two-step and one-step method, respectively. Two-step method is the most widely used: nanoparticles, nanofibers, nanotubes, or other nanomaterials are first produced as dry powders with chemical or physical processes. Then, the nanosized
powder is be dispersed into the host fluid as second step with one of following process: intensive magnetic force agitation, ultrasonic agitation, high-shear mixing, homogenizing, and ball milling. This method is the most economic one to produce nanofluids in large scale, because nanopowder synthesis techniques have already been scaled up to industrial production levels. Due to the high surface area and surface activity, nanoparticles have the tendency to aggregate. Due to the difficulty of preparing stable nanofluids with the process, several advanced techniques have been developed to produce nanofluids, including the one-step
method.

The one-step process consists of simultaneously making and dispersing the particles in the fluid. With this method, the processes of drying, storage, transportation, and dispersion of nanoparticles are avoided, so the agglomeration of nanoparticles is minimized, and the stability of fluids is increased. The one-step processes can prepare uniformly dispersed nanoparticles, and the particles can be stably suspended in the base fluid at the same time.

Unfortunately, the one-step physical method cannot synthesize nanofluids in large scale, and the cost is also very high, so alternative one-step chemical processes are developing rapidly. An example of a nanofluid prepared using the one step method is reported in Figure 1.

Figure 1: Bright-field TEM micrograph of Cu nanoparticles produced by direct evaporation into ethylene glycol

Figure 2: Carbon Nano-Tube (CNT) nanofluids with and without dispersant.

One of the most crucial issues related to nanofluids is the stability of the nanoparticles suspension. The agglomeration of nanoparticles results in the settlement and clogging of microchannels, and also the decreasing of thermal conductivity of nanofluids. An economical possible solution to enhance the stability of a nanofluids is represented by the use of surfactant, also called s dispersants. Dispersants consists of a hydrophobic tail portion, usually a long-chain hydrocarbon, and a hydrophilic polar head group. Dispersants are employed to increase the contact of two materials, sometimes known as wettability. Of course the selection of the proper dispersant for a given nanofluids is a key issue. In Figure 2, you can see the effect of the use of the proper surfactant on the nanoparticles settlement.

Even if in the last decade, the nanofluids research has seen a large amount of experimental works, it is still in initial phase and there are several open issues, like the lack of agreement regarding the results obtained by different researchers, and the lack of theoretical understanding of the mechanisms responsible for
changes in the properties. In fact, there are many important variables and issues related to the production and the usage of nanofluids, which may cause significant discrepancy in the acquired experimental data. The type of nanoparticle, its size, its shape and distribution are important properties that cannot be easily measured nor well-defined or properly reported in the publications. The type of base fluids used, the method for the nanofluid production, use of surfactants and stabilization additives, including pH adjusters, etc. are other important factors.

Two nanofluid samples with all of the parameters being the same but different type and amount of surfactants and/or pH adjusters used, may result in quite different thermo-physical properties and thermofluodynamic properties. These and other unknown factors may explain anomalous and controversial results
obtained by different researches.

Since the origination of the nanofluid concept about a decade ago, the potentials of nanofluids in heat transfer applications have attracted more and more attention. Nanofluids with Al2O3 and Cu nanoparticles, CNTs, etc., shows to able to increase the thermal conductivity of the host fluids making them promising
solutions in thermal management applications. Looking at the possible uses of nanofluids, a variety of different possibilities opens: transportation (Engine cooling/vehicle thermal management), electronics cooling, defense and space, nuclear systems cooling, heat exchangers, biomedicine and other biomedical
applications, heat pipes, fuel cell, solar water heating, chillers, domestic refrigerator, diesel combustion, drilling, lubrications, thermal storage, and many others.

Just to give an overview among the mentioned possible applications: nanofluids have great potentials in transportation, to improve automotive and heavy-duty engine cooling rates by increasing the efficiency, lowering the weight and reducing the complexity of thermal management systems. According to different
researches, the application of nanofluids in industrial cooling will result in great energy savings and emissions reductions. For the US industry, the replacement of cooling and heating water with nanofluids has the potential to conserve 1 trillion Btu of energy. Since the nanofluids shows interesting two phase heat transfer capabilities during vaporization, they can be used in nuclear plants. The Massachusetts Institute of Technology has established an interdisciplinary center for nanofluid technology for the nuclear energy industry; specifically, the use of nanofluids with at least 32% higher critical heat flux (CHF) could enable a 20% power density uprate in current plants without changing the fuel assembly design and without reducing the margin of CHF.

Moreover, magnetic fluids are kinds of special nanofluids wich take advantage of the magnetic properties of the nanoparticles inside. For example, magnetic liquid rotary seals operating with no maintenance and extremely low leakage in a very wide range of applications. Finally, some special kinds of nanoparticles
shows antibacterial properties or drug-delivery properties, so the nanofluids containing these nanoparticles may shows relevant bio-medical properties.

We can conclude this brief overview of the nanofluids considering that these kind of fluids shows enormously exciting potential applications but at the same time many issues must be solved before they can be introduced in commercial applications.

References

• Das S.K., Choi S.U.S., Yu W., Pradeep T., 2008, Nanofluids – science and technology, Ed. John Wiley and Sons, Hoboken, New
  Jersey, USA. 
• Choi S.U.S., 2008, Nanofluids: A New Field of Scientific Research and Innovative Applications, Heat Transfer Engineering,
  29(5):429–431. 

Correlated topics

• Phase Change Materials for heat transfer applications