05-09-2016, 12:42 PM
1452856442-Dasetal.Review.pdf (Size: 2.66 MB / Downloads: 13)
Suspended nanoparticles in conventional fluids, called nanofluids, have been the subject of intensive study worldwide
since pioneering researchers recently discovered the anomalous thermal behavior of these fluids. The enhanced thermal
conductivity of these fluids with small-particle concentration was surprising and could not be explained by existing theories.
Micrometer-sized particle-fluid suspensions exhibit no such dramatic enhancement. This difference has led to studies of other
modes of heat transfer and efforts to develop a comprehensive theory. This article presents an exhaustive review of these
studies and suggests a direction for future developments. The review and suggestions could be useful because the literature in
this area is spread over a wide range of disciplines, including heat transfer, material science, physics, chemical engineering
and synthetic chemistry
INTRODUCTION
The last few decades of the twentieth century have seen unprecedented
growth in electronics, communication, and computing
technologies, and it is likely to continue unabated into
the present century. The exponential growth of these technologies
and their devices through miniaturization and an enhanced
rate of operation and storage of data has brought about serious
problems in the thermal management of these devices. Another
important area that has experienced a similar problem in thermal
management is the area of optical devices. Lasers, high-power xrays,
and optical fibers are integral parts of today’s computation,
scientific measurement, material processing, medicine, material
synthesis, and communication devices. The increasing power of
these devices with decreasing size also calls for innovative cooling
technology. Microscale heat transfer is an area of research that has been adequately reviewed by texts such as those by Duncan
and Peterson [1] and Majumdar et al. [2]. However, all of
these texts indicate that the conventional fin-and-microchannel
technology [3] appears to be inadequate for the new generation
of electronic and optical devices. Choi et al. [4] have shown
that power densities of 2000 W/cm2 can be managed by microchannel
heat exchangers that use subcooled liquid nitrogen.
An increasing number of studies on microchannel boiling, such
as those by Kandlikar and Grande [5], Bergles et al. [6], and
Thome et al. [7], also indicates the need for an alternative way
to cool micro-size devices. The advent of nanotechnology and
Micro-Electro-Mechanical Systems (MEMS) has only intensi-
fied this need, asking for a revolution in cooling technology to
keep pace with the new revolution in device technology.
However, it is important to note that miniaturized devices are
not alone in looking for innovative cooling technology. Large
devices (such as transportation trucks) and new energy technology
(such as fuel cells) also require more efficient cooling
systems with greater cooling capacities and decreased sizes.
Thus, big or small, new and enhanced cooling technology is
the need of the hour. This need must be met in two ways: introducing
new designs for cooling devices, such as microchannels
and miniature cryodevices, and enhancing the heat transfer capability of the fluid itself. The present review deals with the
second option. The suspension of nanoparticles in conventional
fluids are usually called nanofluids. Before going into the details
of nanofluids and their potential in cooling technology, it is
worth first examining the rationale behind the idea of nanofluids.
THE RATIONALE BEHIND NANOFLUIDS
It is obvious from a survey of thermal properties that all liquid
coolants used today as heat transfer fluids exhibit extremely
poor thermal conductivity (with the exception of liquid metal,
which cannot be used at most of the pertinent useful temperature
ranges). For example, water is roughly three orders of magnitude
poorer in heat conduction than copper—as is the case with engine
coolants, lubricants, and organic coolants. It goes without
saying that all of the efforts to increase heat transfer by creating
turbulence, increasing area, etc., will be limited by the inherent
restriction of the thermal conductivity of the fluid. Thus, it is
logical that efforts will be made to increase the thermal conduction
behavior of cooling fluids. Using the suspension of solids is
an option that came to mind more than a century ago. Maxwell
[8] was a pioneer in this area who presented a theoretical basis
for calculating the effective thermal conductivity of suspension.
His efforts were followed by numerous theoretical and experimental
studies, such as those by Hamilton-Crosser [9] and Wasp
[10]. These models work very well in predicting the thermal conductivity
of slurries. However, all of these studies were limited
to the suspension of micro- to macro-sized particles, and such
suspensions bear the following major disadvantages.
1. The particles settle rapidly, forming a layer on the surface
and reducing the heat transfer capacity of the fluid.
2. If the circulation rate of the fluid is increased, sedimentation is
reduced, but the erosion of the heat transfer devices, pipelines,
etc., increases rapidly.
3. The large size of the particles tends to clog the flow channels,
particularly if the cooling channels are narrow.
4. The pressure drop in the fluid increases considerably.
5. Finally, conductivity enhancement based on particle concentration
is achieved (i.e., the greater the particle volume fraction
is, the greater the enhancement—and greater the problems,
as indicated in 1–4 above).
Thus, the route of suspending particles in liquid was a wellknown
but rejected option for heat transfer applications. However,
the emergence of nanofluids helped stimulate the reexamination
of this option. Modern materials technology provided
the opportunity to produce nanometer-sized particles which are
quite different from the parent material in mechanical, thermal,
electrical, and optical properties. Thus, nanofluid technology
coupled with new heat-transfer-related studies on microchannel
flow [11] has provided a new option of revisiting suspensions
of nanoparticles. The first proposition in this area was from Argonne National Laboratory (ANL) through the seminal
work of Choi [12], who designated the nanoparticle suspension
a nanofluid. From a purist’s point of view, this designation may
not be acceptable—every fluid is “nano” because of its molecular
chains—but the term has been accepted and become popular
in the scientific community. It must be kept in mind that biologists
have been using the term nanofluid for different types of
particles, such as DNA, RNA, proteins, or fluids contained in
nanopores [13–15].
The attractive features which made nanoparticles probable
candidates for suspension in fluids are a large surface area, less
particle momentum, and high mobility. With respect to conductivity
enhancement, starting from copper, one can go up to
multi-walled carbon nanotubes (MWCNTs), which at room temperature
exhibit 20,000 times greater conductivity than engine
oil [16].
When the particles are properly dispersed, these features of
nanofluids are expected to give the following benefits: