02-07-2014, 12:52 PM
An overview of Nanofluids: A new media towards green environment
An overview of Nanofluids A new media towards green environment.pdf (Size: 190.51 KB / Downloads: 140)
ABSTRACT
Recent advancements in nanotechnology have originated the new emerging heat transfer
fluids called nanofluids. Nanofluids are prepared by dispersing and stably suspending
nanometer sized solid particles in conventional heat transfer fluids. Past researches have
shown that a very small amount of suspending nanoparticles have the potential to enhance the
thermo physical, transport and radiative properties of the base fluid. Due to improved
properties, better heat transfer performance is obtained in many energy and heat transfer
devices as compared to traditional fluids which open the door for a new field of scientific
research and innovative applications. The aim of this paper is to present the broad range of
nanofluid based current and future applications. Some barriers and challenges are also
focused for implementing these new class of working fluids. At last future opportunities in
nanofluid research are identified and directions are given so that the vision of nanofluid can
be completed.
Introduction
Nanotechnology provides new area of research to process and produce materials with average
crystallite sizes below 100 nm called nanomaterials. The term “nanomaterials” encompasses
a wide range of materials including nanocrystalline materials, nanocomposites, carbon
nanotubes and quantum dots. Xuan and Li (2000) explained that due to its nanostructural
features, nanomaterials exhibit enhanced properties (mechanical, thermal, physical, chemical),
phenomenon and processes than conventional materials. In general, there are four types of
nanomaterials: Carbon based nanomaterials (eg: Carbon nanotubes), Metal based
nanomaterials (metal oxides such as aluminium oxides), Dendrimers (nanosized polymers)
and Composites (nanosized clays
Preparation of nanofluids
To prepare nanofluids by suspending nanoparticles into base fluids, some special
requirements are necessary such as even suspension, durable and stable suspension, low
agglomeration of particles and no chemical change of fluid. There are three general methods
used for preparation of stable nanofluid: (1) Addition of acid or base to Change the pH value
of suspension (2) Adding surface active agents and/or dispersants to disperse particles into
Applications of Nanofluids
Nanofluids can be used in broad range of engineering applications due to their improved heat
transfer and energy efficiency in a variety of thermal systems. The following section gives a
brief idea of different areas of nanofluid applications based on available literatures
Applications in automotive
In automobile arena, nanofluids have potential application as engine coolant, automatic
transmission fluid, brake fluid, gear lubrication, transmission fluid, engine oil and greases.
The first application in cooling automatic power transmission system done by Senthilraja et
al (2010) show that CuO nanofluids have the lowest temperature distribution and accordingly
the best heat transfer performance
Industrial Cooling Applications
Routbort et al. (2010) employed nanofluids for industrial cooling and showed great energy
savings and resulting emission reductions. They showed that replacement of cooling and
heating water with nanofluids has the potential to conserve about 300 million kWh of energy
for industries. For the electric power industry using nanofluids could save about 3000-9000
million kWh of energy per year which is equivalent to the annual energy consumption of
about 50,000-150,000 households. The associated emission reductions would be
approximately 5600 million kg of carbon dioxide, 8.6 million kg of nitrogen oxides and 21
million kg of sulfur dioxide.
In the Defence Advanced Projects demonstrated cooling enhancement by ~ 8-30% using
nanofluids in compact heat exchangers. The nanofluids were found to precipitate nanofins on
the heater surface and there augment the heat flux. The nanoparticles used in this study were
ex-foliated graphite and multi-walled carbon nanotubes (MWCNT). It was observed that the
nanofluids specific heat capacity was enhanced by 50%. Hence, it was concluded that
nanofluids have better efficacy in thermal energy storage applications compared to cooling
applications.
Solar Devices
Direct absorption solar collectors have been proposed for a variety of applications such as
water heating; however the efficiency of these collectors is limited by the absorption
properties of the working fluid. Otanicar et al. (2010) demonstrated efficiency improvements
of up to 5% in solar thermal collectors by utilizing nanofluids as the absorption mechanism.
The experimental and numerical results demonstrate an initial rapid increase in efficiency
with volume fraction, followed by a leveling off in efficiency as volume fraction continues to
increase.
For domestic hot water system, Golden and Otanicar (2009) resulted that the nanofluid based
solar collector has a slightly longer payback period but at the end of its useful life has the
same economic savings as a conventional solar collector. The nanofluid based solar collector
has a lower embodied energy ~9% and approximately 3% higher levels of pollution offsets
than a conventional collector.
Poor long term stability of suspension
Long term physical and chemical stability of nanofluids is an important practical issue
because of aggregation of nanoparticles due to very strong vander walls interactions so the
suspension is not homogeneous. Physical or chemical methods have been applied to get
stable nanofluids such as (i) an addition of surfactant; (ii) surface modification of the
suspended particles; (iii) applying strong force on the clusters of the suspended particles. Lee
and Choi (1996) found that Al2O3 nanofluids kept after 30 days exhibit some settlement
compared to fresh nanofluids. Particles settling must be examined carefully since it may lead
to clogging of coolant passages.
Increased pressure drop and pumping power
Pressure drop development and required pumping power during the flow of coolant
determines the efficiency of nanofluid application. It is known that higher density and
viscosity leads to higher pressure drop and pumping power. There are many studies showing
significant increase of nanofluids pressure drop compared to base fluid. One of the
experimental study by Choi (2009) calculated 40% increase of pumping power compared to
water for a given flow rate.
Future Research
As far as for better understanding of nanofluids, further research is needed. An important
focus for future research should be determining the key energy transport mechanism in
nanofluids. Mostly heat transfer depends on Thermal conductivity of nanofluid. The thermal
conductivity of nanofluids can be a function of parameters such as particle shape, particle
agglomeration etc. therefore future research should be focused on finding out the main
parameters affecting the thermal conductivity of nanofluids. Theoretical predictions should
be evaluated in terms of agreement with experiments regarding concentration, particle size
and temperature dependence. Currently, the available nanoparticles are limited and their
specifications are not accurate. The challenging point is to obtain the desirable nanoparticle
product. The development of the nanoparticle production technique will be very helpful for
the nanofluid research. We have to face public concern about their safety both in production
and in use. Nanofluids engineers would be prudent to pursue green designs by choosing
nontoxic or biodegradable nanoparticles. So in last we can say that low cost, high volume
Conclusion
This paper presents overview about nanofluid, an exiciting new class of heat transfer fluid, in
terms of application, barriers and further research. It is concluded that nanofluids are
important because they can be considered as a potential candidate for numerous applications
involving heat transfer and their use will continue to grow. It was also found that the use of
nanofluids appears promising, but the development of the field faces several challenges.
Nanofluid stability and its production cost are major factors in using nanofluids. The
problems of nanoparticle aggregation, settling, and erosion all need to be examined in detail
in the applications. We can say that once the science and engineering of nanofluids are fully
understood and their full potential researched, they can be reproduced on a large scale and
used in many applications. It is also suggested that further research still has to be done on the
synthesis and applications of nanofluids so that they may be applied as more efficient and
compact heat transfer systems, maintaining cleaner and healthier environment and unique
applications