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Table 1. THERMAL CONDUCTVITY (Whn-K) OF VARIOUS
MATERIALS AT 30[ K UNLESS
OTHWlS[NIED
Material Thermal
Conductivity
Metahic Solids
Silver 429
Copper 401
Aluminum 237
Nonmetallic Solids
Silicon 148
Metallic Liquids
Sodium 9 644 K 72,3
NonmetallIc Liquids
Water 0.613
Engine oil 0.145
heat transfer fluids. In fact, numerous theoretical and
experimental studies of the effective thermal conductivity of
dispersions that contain solid particles have been condticted
since Maxwell's theoretical work was published more than 100
years ago (Maxwell, 188I- However, all of the studies on
thermal conductivity of suspensions have been confined to
millimeter- or micrometer-sized particles, Maxwell's model
shows that the effective thermal conductivity of suspensions
that contain spherical particles increases with the volume
fraction of the solid particles. It is also known that the
thermal conductidvity of suspensions increases with the ratio
of the surface area to volume of the particle.
It is proposed that ranomete-sized metallic particles can be
suspended in industrial heat transfer fRids such as water,
ethylene glycol, or engine oil to poduce a new class of
engineered fluids with high henna conductiv4i The author
has coined the terra nanofluids (NFs) for this new class of
engineered heat transfer fluids, which conrein metallic
particles with average particle sizes of about 10 nanometers
and can be produced by curnt nanophase technology.
Narnofloids are expected to exhibit nipalor properties when
compared with conventionat heat transfer fluids and fluids that
contain micromtor-sized metallic particles, Because heat
transfer takes place at the surface of the particle, it is desirable
to use a particle with a lorge surface area. Nanopraniles have
extremely large surface areas and therefore have a great
potential for application in heat transfer. The much larger
rolativ soifao prcas a nanophase powders, whcn compared
with conventional micrometer-sized powders. should markedly
improve the heat transfer capabilities and stability of the
suspensions,
Researchers at Argonne National Laboratoy (ANL) have
been developing advanced fluids for industrial applications,
including district heating and cooling systems (Cho and Tram,
1991; Choi et al.. 1992a and 1992b). One of the problems
identified in this R&D program was that micrometer-sized
particles cannot be used in practical best transfer equipment
because of severe clogging problems- However, nanophasemetals are believed in be ideally suited for applications in
which fluids flow through small passages. because the metallic
nanoparticles are small enough that they are expected to
behave like molecules of liquid. Therfore, nanomzter-sized
particles will not clog flow passages, but will improve the
thermal conductivity of the ftuids. This will open up the
possibility of using nanoparticles even in microchannels for
many envisioned high-heat-Ioad applications. More receritly,
a project was begun at Alt to demorsutie the feasibility of
the concept of nwneflds Suaxasful employs of
nanofluids will meult in sigdificamt energy and cost savings
and will support the current ind'trial trend towars
component IuniEaturizathia by enabling the design of smaller
and lighter heat exchanger systems.
The purpose of the paper is to demonstrate theoretically the
feasibility of the concept of amofluids. Afer briefly
describing the technology for producing nanoparddtes wnd
suspensions, e shall estimate the thermal conductivity of
naiofiuids with copper nanophase materials and the
subsequem heat transfer eshancement as a function of thermal
cotnductivity. We will aLso explore the potential benefits of
nanofluids in the expectation that the ultra-high-perforanve
nanoluds may have major implications for many industries.
TECHNOLOGY FOR PRODLUTiON OF
NANOPARTICLE AND SUSPENSIONS
Moden fabrication technology provides great
opportunities to actively process materials on micro- and
nnometer scales. Materials with novel properties can be
produced on nanometer scales. Nanstorumcred or nanophase
materials are nanometer-sized solid substances engineered on
the atomic or molecular scale to produce cithcr new o-r
enhanced physical properties not exhibited by conventional
bulk solids. All physical mechanisms have a critical length
scale, below which the physical properties of rnateials are
changed. Therefore, paticles < 100 UM in diamner exhibit
properties different from dose of conventional solids. The
noble properties of nanophase materials come frn the
itlatively high surface-area-to-volume ratio that is due to the
high proportion of constituent atoms that reside at the grain
boundaries, ' thermal, mechamcal, optical. magnetic, and
electrical properties of namophase materials are superior to
itwac of contoParl pnatrials with coarse grain stmctures.
Consequently, th1 exploration in research and development of
nanophase m-ates has drawn considerable attention from
material scientists acd engineers alike (Doncan and Rouvray.
1989: Siegel, 199 1).
Much progress has been made in the production of
nanophase maerials, said currac nanophase technology can
produce large quantities of powders with average particle sizes
in the 10-nm range. Several 'mdern" Fiaophase ateruls
have been prepared by physical gas-phase condensation or
chemical synthesis techniques (Siegel, 191). The gas-phase
condensation process involves the evaporation of a source
material and the rapid condensation of vapor idf nanorherer-
sized crystallites or loosely agglomerated clusters in a cool,
inert, reduced-pressure aimosphere A chermniry-based
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Choi, S. U. S. & Eastman, J. A. Enhancing thermal conductivity of fluids with nanoparticles, article, October 1, 1995; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc671104/m1/3/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.