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Enhancing thermal conductivity of fluids with nanoparticles


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Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.
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1 1 19S5
Stephen U. S. Choi* and J. A. Eastman^
1 Energy Technology Division and 2Materials Science Division
Argonne National
Argonne, IL 60439
October 1995
The submitted manuscript has bean authored
by a contractor of the U.S. Government
under contract No. W-31-109-ENG-38.
Accordingly, the U.S. Government retains a
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contribution, or allow others to do so, for
This report was prepared as an account of work sponsored by an agency of the United States
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United States Government or any agency
Abstract to be submitted to ASME International Mechanical Engineering Congress & Exposition,
November 12-17,1995, San Francisco, CA.
by the U.S.
Department of
Basic Energy
Sciences-Materials Sciences, under contract
Stephen U. S. Choi 1and Jeffrey A. Eastman 2
1Energy Technology Division and 2Materials Science Division
Argonne National Laboratory
Argonne, Illinois
Low thermal conductivity is a primary limitation in the
development of energy-efficient heat transfer fluids that are
required in many industrial applications. In this paper we
propose that an innovative new class of heat transfer fluids
can be engineered by suspending metallic nanoparticles in
conventional heat transfer fluids. The resulting "nanofluids"
are expected to exhibit high thermal conductivities compared
to those of currently used heat transfer fluids, and they
represent the best hope for enhancement of heat transfer. The
results of a theoretical study of the thermal conductivity of
nanofluids with copper nanophase materials are presented, the
potential benefits of the fluids are estimated, and it is shown
that one of the benefits of nanofluids will be dramatic
reductions in heat exchanger pumping power.
d Pipe diameter
f Fanning friction factor
h Heat transfer coefficient
k Thermal conductivity
L Length
n Shape factor
Nu Nusselt number
P Pumping power
Pr Prandtl number
Re Reynolds number
V Velocity
a Particle volume fraction
Sp Pressure drop
P Density
¥ Sphericity
Sub scripts
eff Effective
m Metallic particle
nf Nanofluid
o Reference fluid without nanoparticles
Fluids are often used as heat carriers in heat transfer
equipment. Examples of important uses of heat transfer
fluids include vehicular and avionics cooling systems in the
transportation industry, hydronic heating and cooling
systems in buildings, and industrial process heating and
cooling systems in petrochemical, textile, pulp and paper,
chemical, food, and other processing plants. In all of these
applications, the thermal conductivity of heat transfer fluids
plays a vital role in the development of energy-efficient heat
transfer equipment. With an increasing global competition,
industries have a strong need to develop advanced heat transfer
fluids with significantly higher thermal conductivities than
are presently available.
Despite considerable previous research and development
efforts on heat transfer enhancement, major improvements in
cooling capabilities have been constrained because of the low
thermal conductivity of conventional heat transfer fluids.
However, it is well known that at room temperature, metals in
solid form have orders-of-magnitude larger thermal
conductivities than fluids. For example, the thermal
conductivity of copper at room temperature is =700 times
greater than that of water and =3000 times greater than that of
engine oil, as shown in Table 1. The thermal conductivity of
metallic liquids is much greater than that of nonmetallic
liquids. Therefore, the thermal conductivities of fluids that
contain suspended solid metallic particles are expected to be
significantly enhanced when compared with conventional
* To be presented at ASME International Mechanical Engineering Congress & Exposition, November 12-17, 1995, San Francisco,
Material Thermal
Metallic Solids
Silver 429
Copper 401
Aluminum 237
Nonmetallic Solids
Silicon 148
Metallic Liquids
Sodium® 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 conducted
since Maxwell's theoretical work was published more than 100
years ago (Maxwell, 1881). 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 conductivity of suspensions increases with the ratio
of the surface area to volume of the particle.
It is proposed that nanometer-sized metallic particles can be
suspended in industrial heat transfer fluids such as water,
ethylene glycol, or engine oil to produce a new class of
engineered fluids with high thermal conductivity. The author
has coined the term nanofluids (NFs) for this new class of
engineered heat transfer fluids, which contain metallic
particles with average particle sizes of about 10 nanometers
and can be produced by current nanophase technology.
Nanofluids are expected to exhibit superior properties when
compared with conventional heat transfer fluids and fluids that
contain micrometer-sized metallic particles. Because heat
transfer takes place at the surface of the particle, it is desirable
to use a particle with a large surface area. Nanoparticles have
extremely large surface areas and therefore have a great
potential for application in heat transfer. The much larger
relative surface areas of nanophase powders, when compared
with conventional micrometer-sized powders, should markedly
improve the heat transfer capabilities and stability of the
Researchers at Argonne National Laboratory (ANL) have
been developing advanced fluids for industrial applications,
including district heating and cooling systems (Choi and Tran,
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 heat transfer equipment
because of severe clogging problems. However, nanophase
metals are believed to 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. Therefore, nanometer-sized
particles will not clog flow passages, but will improve the
thermal conductivity of the fluids. This will open up the
possibility of using nanoparticies even in microchannels for
many envisioned high-heat-load applications. More recently,
a project was begun at ANL to demonstrate the feasibility of
the concept of nanofluids. Successful employment of
nanofluids will result in significant energy and cost savings
and will support the current industrial trend towards
component miniaturization 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 nanofluids. After briefly
describing the technology for producing nanoparticles and
suspensions, we shall estimate the thermal conductivity of
nanofluids with copper nanophase materials and the
subsequent heat transfer enhancement as a function of thermal
conductivity. We will also explore the potential benefits of
nanofluids in the expectation that the ultra-high-performance
nanofluids may have major implications for many industries.
Modern fabrication technology provides great
opportunities to actively process materials on micro- and
nanometer scales. Materials with novel properties can be
produced on nanometer scales. Nanostructured or nanophase
materials are nanometer-sized solid substances engineered on
the atomic or molecular scale to produce either new or
enhanced physical properties not exhibited by conventional
bulk solids. All physical mechanisms have a critical length
scale, below which the physical properties of materials are
changed. Therefore, particles < 100 nm in diameter exhibit
properties different from those of conventional solids. The
noble properties of nanophase materials come from the
relatively high surface-area-to-volume ratio that is due to the
high proportion of constituent atoms that reside at the grain
boundaries. The thermal, mechanical, optical, magnetic, and
electrical properties of nanophase materials are superior to
those of conventional materials with coarse grain structures.
Consequently, the exploration in research and development of
nanophase materials has drawn considerable attention from
material scientists and engineers alike (Duncan and Rouvray,
Siegel, 1991).
Much progress has been made in the production of
nanophase materials, and current nanophase technology can
produce large quantities of powders with average particle sizes
in the 10-nm range. Several "modern" nanophase materials
have been prepared by physical gas-phase condensation or
chemical synthesis techniques (Siegel, 1991). The gas-phase
condensation process involves the evaporation of a source
material and the rapid condensation of vapor into nanometer-
sized crystallites or loosely agglomerated clusters in a cool,
inert, reduced-pressure atmosphere. A chemistry-based
solution-spray conversion process starts with water-soluble
salts of source materials. The solution is then turned into an
aerosol and dried by a spray-drying system. Rapid
vaporization of the solvent and rapid precipitation of the
solute keeps the composition identical to that of the starting
solution. The precursor powder is then placed in a fluidized-
bed reactor to evenly pyrolyze the mixture, drive off volatile
constituents, and yield porous powders with a uniform
homogeneous fine structure (Ashly, 1994). A third technique
is to generate nanophase materials by condensation of metal
vapors during rapid expansion in a supersonic nozzle (Hill, et
1963; Andres, et al., 1981; Brown, et al., 1992).
If powders are produced by one of these processes, some
agglomeration of individual particles may occur. It is well
known, however, that these agglomerates, which are typically
1 micrometer or so in size, require little energy to fracture into
smaller constituents, and thus it is possible they will not
present a problem in this application. If, however,
agglomeration is a problem, it would prevent realization of
the full potential of high surface areas of nanoparticles in
nanofluids. Under such conditions, these conventional
technologies for production of nanophase materials are not
suitable for nanofluids.
Another promising
technique for producing
nonagglomerating nanoparticles involves condensing
nanophase powders from the vapor phase directly into a
flowing low vapor pressure fluid. This technique was
developed in Japan more than 10 years ago by Akoh et al.
but has been essentially ignored by the
nanocrystalline-materials community because of difficulties in
subsequently separating the particles that are produced from
the fluids to make dry powders or bulk materials by sintering
individual nanometer-sized particles.
Because of the absence of a theory for the thermal
conductivity of nanofluids, two existing models that were
developed for conventional solid-liquid systems with fine
particles are used in this study to estimate the effective thermal
conductivity of nanofluids. Batchelor and O'Brien (1977)
have developed an expression for the effective thermal
which is applicable to two-phase systems
that contain metal powders with particle diameters on the order
of micrometers, i. e.,
keff/ko = 41n(km/k0)-ll. (1)
where km is the thermal conductivity of the metallic particle
and k0 is the thermal conductivity of the reference fluid.
However, it should be noted that the theory of Batchelor and
O'Brien (1977) was originally developed for a point-contact
porous medium. When there is no contact between the
particles, the effective thermal conductivity is independent of
the conductivity ratio. Thus, for values of the conductivity
ratio ranging from 100 to 10,000, the effective thermal
conductivity of noncontacting systems is estimated from the
keff/ko = 4 (2)
If it is assumed that this expression is applicable to
nanofluids, nanoparticles are expected to increase the thermal
conductivity of the base fluids by a factor of 4. However, this
expression seems unfeasible for nanofluids because it does not
involve the particle volume fraction or particle shape.
Hamilton and Crasser (1962) have developed a more
elaborate model for the effective thermal conductivity of two-
component mixtures as a function of the conductivity of the
pure materials, the composition of the mixture, and the shape
of the dispersed particles. For mixtures in which the ratio of
conductivities of two phases is > 100, the effective thermal
conductivity of two-component mixtures can be calculated as
keff/ko = [km+ (n-1) ko - (n-1) a (ko - km)] / [km+ (n-1) ko
+ a(ko -km)] (3)
where a is the particle volume fraction and n is the empirical
shape factor given by
n =
/ y, (4)
where \|/ is the sphericity, defined as the ratio of the surface
area of a sphere with a volume equal to that of the particle to
the surface area of the particle. This model shows that
nonspherical shapes (all other circumstances being the same)
will increase the conductivity above that of spheres.
Applying the Hamilton and Crasser model to copper
nanoparticles in water, the effective thermal conductivity of
the copper-water system has been estimated for three values
for y. The effects of particle volume fraction and sphericity
on the thermal-conductivity ratio for a copper-water system
are plotted in Fig. 1. The results clearly show that the thermal
conductivity of the fluid-particle system depends on both the
particle volume fraction and the shape. Assuming that the
sphericity of copper nanoparticles is 0.3, the thermal
conductivity of water can be enhanced by a factor of 1.5 at the
low volume fraction of 5% and by a factor of almost 3.5 at the
high volume fraction of 20%. This finding demonstrates,
theoretically, the feasibility of the concept of nanofluids, i.e.,
metallic nanoparticles are capable of significantly increasing
the thermal conductivity of conventional heat transfer fluids.
Furthermore, Masuda et al. (1993) have shown experimentally
that y-Al203 particles at a volume fraction of
can increase
the effective thermal conductivity of water by =30%. The
agreement between the estimated and measured conductivities
is satisfactory.
For turbulent convection transfer of heat in smooth pipes,
the heat transfer coefficient can be calculated from the Dittus-
Boelter correlation,
Nu = 0.023 Re08 Pr1/3. (5)
If it is assumed that only the thermal conductivity of the
nanofluid system varies and other properties, such as the
specific heat, density, and dynamic viscosity, are the same as
for the reference fluid, then we obtain from Eq. 5,
which shows that the heat transfer coefficient h may be
increased by increasing the velocity v or the thermal
conductivity of the fluid k.
In heat exchangers that use conventional fluids, the heat
transfer coefficient may only be increased by significantly
increasing the velocity of the fluid in the heat transfer
equipment. However, the pumping power significantly
increases with increasing velocity. The frictional pressure
drop for fully developed turbulent flows in a pipe is given as
8p = 2fpLv2/d, ' (7)
where p is the density of the fluid, L the length of the pipe, d
the pipe diameter, and f the Fanning friction factor given by
f = 0.079 Re"025. (8)
It can be shown that the frictional pressure drop is given by
the relationship
op ~ v175. (9)
Because pumping power P is proportional to the product of the
pressure drop and the flow rate, it can be expressed by the
P - v2-75. (10)
From Eqs. 6 and 10, enhancement of heat transfer due to
increased pumping power can be estimated from the following
h/h0 = (P/P0)0'29. (11)
For a nanofluid flowing in the same heat transfer equipment
at a fixed velocity, enhancement of heat transfer due to
increased thermal conductivity can be estimated from the
hnf/h0 = (kn/k0)2/3. (12)
The effects of thermal conductivity and pumping power on
heat transfer are plotted in Fig. 2. In heat exchangers that use
conventional fluids, heat transfer can only be improved by
significantly increasing flow rates. For example, to improve
the heat transfer by a factor of 2, the pumping power should be
increased by a factor of =10. However, if a nanoparticle-based
fluid with a thermal conductivity of =3 times that of a
conventional fluid were used in the same heat transfer
equipment, the rate of heat transfer would be doubled.
Liu et al. (1988) have studied the influence of particle
loading and size on the pressure drop of slurry. Their data
show that solids suspensions in the 20% volume fraction
range incur little or no penalty in pressure drop as compared
with single-phase fluids of comparable flow rate. Therefore, it
is reasonable to assume that the nanofluid pressure drop
behaves like that of a single-phase fluid at volume fractions up
to 20%. Then, the potential savings in pumping power is
particularly significant as the heat transfer enhancement ratio
is increased, as shown in Fig. 3. This could lead to a major
technological breakthrough in the development of energy-
efficient industrial heat transfer fluids. Therefore, the potential
benefits of nanofluids could provide tremendous performance,
size/weight, and cost advantages.
The research effort to produce and characterize the heat
transfer behavior of nanofluids will consist of five main tasks.
Nanophase metal powders will be produced in existing
state-of-the-art gas-condensation preparation systems at ANL.
The particle size and agglomeration behavior of nanophase
powders in liquids will be studied.
A new technique for producing nonagglomerating
nanoparticles for nanofluids by directly condensing
nanophase powders into a flowing fluid will be developed,
based on the system designed by Akoh et al. (1978). The
properties of nanofluids produced by this technique will be
compared with those produced by inert-gas condensation.
Technology for production of nanoparticle
suspensions will be developed and the stability, dispersion,
and rheological/transport properties of these nanofluids will
be investigated.
The flow characteristics of dilute and concentrated
suspensions of nanoparticles will be studied. Heat transfer
tests with nanoparticles in a range of up to 10 volume fraction
will be conducted to demonstrate the expected dramatic
improvement in energy efficiency from nanofluids.
Practical applications of nanofluids will be
The concept of nanofluids is an innovative idea. The
feasibility of the concept of high-thermal-conductivity
nanofluids has been demonstrated by applying the Hamilton
and Crosser (1962) model to copper nanoparticles in water,
together with some experimental results of Masuda, et al.
(1993) for y-Al203 particles in water. The potential benefits of
nanofluids with copper nanophase materials have been
estimated. One of the benefits of nanofluids will be dramatic
reductions in heat exchanger pumping power. For example, to
improve the heat transfer by a factor of 2, the pumping power
with conventional fluids should be increased by a factor of
However, if a nanoparticle-based fluid with a thermal
conductivity of =3 times that of a conventional fluid were used
in the same heat transfer equipment, the nanoparticle-based
fluid would double the rate of heat transfer without an increase
in pumping power. The invention of nanofluids presents new
challenges and opportunities for thermal scientists and
This work was supported by the U.S. Department of Energy,
under Contract W-31-109-ENG-38. The author would like to
express special thanks to Argonne's Coordinating Council for
Science and Technology for their interest and support of this
work. Thanks are also given to M. W. Wambsganss for
valuable discussions.
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I i i i | i i i i | i i i i | i i i i 1 i i ! 1
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-a o _
i i i T i p i i I i i i i 1 i i 1 1 1 1 1 1 1
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... The significant enhancement in thermal conductivity of the base fluid by suspending nanometer-sized solid particles in it was first reported by Choi [1] and the resulting fluid was termed 'Nanofluid'. The phenomenon of enhancement in thermal conductivity suggests the possibility of using nanofluids in many science and engineering applications. ...
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The leading investigation focuses on the current scenario of peristaltic action with gyrotactic micro-organisms and Williamson nano fluid through the arterial wall under the highlighted impacts of solar mimetic appliance and magneto-hydrodynamics effect. The animation of chemical reaction takes place with the collaboration of activation energy and solar system as encountered through Rosseland’s estimations. The Buongiorno model is applied for further analysis of Brownian diffusion and thermophoresis effects on thermal radiation, chemical reactions, and the dynamics of gyrotactic micro-organisms. In such peristaltic action, viscosity as well as conductivity is presumed to vary with temperature. The controlled DE’s (differential equations) are interpreted by taking the assumption of the lowest Reynolds number and highest wavelength, and numerical findings for such a non-dimensional set of equations were launched by employing the BVP4c technique. Through the use of pictorial interpretations, the implications of diverse physical parameters upon the flow stream, the dynamics of gyrotactic micro-organisms, thermal radiations and concentration distribution were calculated. Major findings about velocity depict that viscosity and the Williamson variable decline the velocity profile due to the resistive behavior of these forces. The magneto-hydrodynamics effect produces a Lorentz retarded force, which slows blood motion during surgical scenarios. The heating phenomenon is accelerated by the Brownian motion variable and the thermophoresis parameter. The activation energy parameter results in a low concentration distribution, whereas the Brownian motion parameter causes a higher density of motile microorganisms. The bio-convection constant and peclet number diminish the motile micro-organism density. The conductivity parameter increases the temperature profile in the pumping section while the Prandtl number slows down the heating phenomenon.
The present work explores the hybrid nanofluid flow of water conveying aluminium oxide (Al2O3) and copper (Cu) on a convectively heated stretching sheet in presence of a magnetic field. The radiative flow considered here is under slip velocity at the edge and heat transference mechanism due to thermal convection. The problem is developed mathematically in terms of partial differential equations (PDEs). The governing PDEs are transformed into a system of ODEs by using an apposite similarity transformation and then are explained numerically. Graphical illustrations for the dimensionless velocity and temperature are presented and discussed in detail from the physical point of view. The behavior of the rate of heat transmission and the skin friction number are also deliberated numerically for various flow parameters. The result shows that thermal outlines have a linear relationship with a thermal number and slip parameters but velocity outlines disclose a reverse relationship with the magnetic field. When compared to nanofluid flow, shear stress at the wall as well as heat transmission rate is changed for hybrid nanofluid flow due to the existence of high-conducting nanoparticles Al2O3 and Cu in the base fluid water.
Heat transfer is a branch of physics that describes how and how quickly thermal energy is transmitted. In daily life, we encounter applications of heat transmission; for instance, the human body regularly emits heat, and people use clothing to regulate their body temperature to the surrounding environment. For cooking, refrigeration, and drying, heat transfer is employed to control temperature in our buildings. Additionally, it is used directly in radiators of automobile and to regulate the temperature of electronic gadgets. Many of these components needed instant heat dissipation in order to function properly and achieve maximum system efficiency. Devices have gotten smaller as technology has advanced, necessitating better thermal management. In general, the need for efficient cooling technology increases as the size decreases. As a result, thermal engineering's focus on improving heat transport is crucial. Nanofluids are attracting increasing interest because of improved thermophysical characteristics and their capacity to be assimilated into a variety of thermal applications, including improving the efficiency of heat exchangers utilized in various industries and solar energy scavenging for the generation of sustainable energy. In this paper a brief and updated review is provided to utilize nanofluids in variety of heat transfer devices such as electronic cooling, automobile radiators, refrigeration system etc.
This research addresses buoyancy-driven stretching flow of non-Newtonian (third-grade) rheological liquid confined by a vertically stretchable surface. The nanoliquid considered for modeling encompasses Brownian movement and thermophoresis aspects. Heat-mass transportation characteristics are scrutinized under modern approaches (i.e., Cattaneo-Christov heat-mass fluxes consideration). Such consideration overwhelms the paradoxes of heat conduction and mass diffusion via heat-mass flux relation times. Steady-state and chemically reactive magnetohydrodynamic boundary-layer flow satisfying incompressibility condition is modeled. The governing nonlinear boundary-layer expressions are coupled and highly nonlinear due to mixed convection consideration. The homotopy scheme yielding convergent solutions is implemented. Numerical data along with plots is presented to ensure convergence. The achieved outcomes are exhibited graphically and elaborated thoroughly. Keywords Buoyancy-driven stretching flow · Third-grade nanoliquid · Cattaneo-Christov theories of heat-mass transfer · Brownian movement · Magnetohydrodynamic boundary-layer flow · Thermophoresis list of symbols ν = μ ρ f Kinematic viscosity σ Electrical conductivity x, y Cartesian coordinates
Micropolar fluids have microstructures and have applications in lubricating theory and porous media. This study inculpates the Magnetohydrodynamic (MHD) flow of micropolar nanofluid past the stretching sheet enclosed in a permeable medium. The fluid is flowing through convective boundary and is thought to be constant and laminar. Velocity slips through the surface is considered in conjunction with temperature effected viscosity and thermal radiation. Through a similarity transformation, governing model's pde's are converted to a set of nonlinear odes which are then attempted numerically by RK-5 technique along with shooting skills. Different non-dimensional parameters are examined graphically for velocity, micro-rotation, temperature and concentration profiles. The magnetic field parameter shows a decrease in the velocity profile, while opposite results are shown for temperature and concentration profile. The graphical outcomes also show that the momentum boundary layer thickness increases with the values of stretching ratio parameter. The temperature is growing due to enhancing the Eckert number and radiation parameter. The thermal boundary layer and concentration upsurge for rising values of thermophoresis parameter while for Brownian temperature profile rises whereas reduction in concentration is shown. Computed values of physical quantities like wall shear stress component, Nusselt number and Sherwood number are given through bar charts. Results include the increasing value of Biot number causes an increment in the magnitude of temperature gradient. Moreover, comparison of results with previous work gives the compatibility of adopted techniques for solution.
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The material under investigation consists of particles of relatively large conductivity embedded or immersed in a matrix, the volume fraction of the particles being so high that they are in, or nearly in, contact. The particles are arranged randomly, and the material is statistically homogeneous. A general formula gives the effective conductivity of the material in terms of the average thermal (or electrical) dipole strength of a particle. The thermal flux across the surface of a particle is concentrated in areas near points of contact with another particle, and the dipole strength is approximately equal to a weighted sum of the fluxes across the areas near contact points. It is thus necessary to calculate the flux between two adjoining particles at different temperatures, and we do this by solving numerically an integral equation for the distribution of temperature over the (locally spherical) surface of one of the particles near the contact point. The flux between the two particles is found to be proportional to loge a/h when α 2h/a >> 1 and to loge α when α 2h/a << 1, where h is the minimum gap between the particle surfaces, a-1 the mean of their local curvatures, and α the ratio of the conductivities of the particles and the matrix. In the case of two particles pressed together to form a circular flat spot of radius ρ , the flux occurs almost wholly in the particle material, and is proportional to ρ when α ρ /a >> 1. Explicit approximate results are obtained for the effective conductivity of the granular material in the case of uniform spherical particles. For a close-packed bed of particles making point contact the effective conductivity is found to be 4.0k loge α where k is the matrix conductivity. This asymptotic relation (applicable when α >> 1 is seen to be consistent with the available measurements of the conductivity of packed beds of spheres. Values of the effective conductivity for packed beds of particles of different shape are not expected to be greatly different.
Nanostructured or nanophase materials are solid substances engineered almost atom by atom to produce either new or enhanced mechanical, optical, magnetic, or other physical properties. Comprising several different physical and chemical processing routes, nanostructure manufacturing techniques can produce unique compositions that would not naturally occur. Scientists are now exploring a variety of compositions and structural configurations to find systems that mechanical engineers and designers can exploit their strength, hardness, toughness, thermal expansion, or conductivity.
How much the thermal conductivity of a liquid can be altered by dispersing a small amount of ultra-fine particles into it has been studied. Fine powders of Al2O3, SiO2 and TiO2 were used as the ultra-fine particles, and water was selected as the base liquid. Three dispersed systems were made by applying the technique of electrostatic repulsion. For the systems of water-Al2O3 and water-TiO2, effective thermal conductivities were seen to increase much more as the particle concentration was increased, but that of water-SiO2 system almost never increased. Viscosities of their dispersed systems were also measured, and the characteristics were made clear.
A study has been made of the condensation of metal vapors in nozzles, using the liquid drop nucleation theory, to predict the incidence of condensation and its effect on the properties of the fluid stream. The theory, which has been verified by accurate experimental data on steam, predicts that in the absence of charged particles initially saturated mercury vapor, for example, is very reluctant to condense in a nozzle. On the other hand, the vapors of sodium, potassium, and rubidium should condense very readily. In each case the typical size and growth rate of the condensed drops is so small that significant slip between vapor and liquid phases is very unlikely to develop. The nucleation process is rapid so that equilibrium is approached quickly. In reasonably long nozzles it may be quite in order to idealize the transition between supersaturated and equilibrium states as a step change.
Ultrafine particles of ferromagnetic metals (Fe, Co and Ni) were prepared by the vacuum evaporation onto a running oil substrate. Particles thus obtained were suspended in the oil and their average diameter was about 25 Å. An electron diffraction analysis indicated that the particles were oxidized and the main component was Fe3O4, CoO and NiO for Fe, Co and Ni fine particles, respectively. From magnetic measurements in the temperature range between 4.2 and 300 K, the main magnetic behavior of the fine particles can be explained in terms of the superparamagnetism or superantiferromagnetism. Furthermore, it was concluded that the Fe3O4 particles may be covered with several atomic layers of α-Fe2O3. For CoO and NiO particles, existence of an imperfect oxide structure was required besides the main components in order to explain the magnetic behavior.
Argonne National Laboratory (ANL), under sponsorhip of the US Department of Energy, Office of Buildings and Community Systems, has been conducting a comprehensive, long-range program to develop high- performance advanced energy transmission fluids for use in district heating and cooling (DHC) systems. The current study focuses on the development of phase-change slurries as advanced energy transmission fluids. The objectives are (1) to establish proof-of-concept of enhanced heat transfer by a slurry, with and without phase change, relative to heat transfer in a pure carrier liquid; (2) to investigate the effect of particle volumetric loading, size, and flow rate on the slurry pressure drip and heat transfer behavior with and without friction-reducing additives; and (3) to generate pressure drop and heat transfer data needed for the development and design of improved DHC systems. Two types of phase-change materials were used in the experiments: ice slush for cooling, and cross-linked, high- density polyethylene (X-HDPE) particles with diameters of 1/8 and 1/20 in. (3.2 and 1.3 mm) for heating. The friction-reducing additive used in the tests was Separan AP-272 at 65 wppm. This report describes the test facility, discusses the experimental procedures, and presents significant experimental results on flow and heat transfer characteristics of the non-melting slurry flows. 51 refs., 36 figs., 6 tabs.
Divide and subdivide a solid and the traits of its solidity fade away one by one, like the features of the Cheshire cat, to be replaced by characteristics that are not those of liquids or gases. They belong instead to a new phase of matter, the microcluster. Microcluster consist of tiny aggregates comprising from two to several hundred atoms. They pose questions that lie at the heart of solid-state physics and chemistry and the related field of materials science: How small must an aggregate of particles become before the character of the substance they once formed is lost How might the atoms reconfigure themselves if freed from the influence of the matter that surrounds them If the substance is a metal, how small must its cluster of atoms be to avoid the characteristic sharing of free electrons that underlies conductivity Do growing clusters proceed gradually from one stable structure to another, largely through the simple addition of atoms, or do they undergo radical transformation as they grow Many cluster properties are determined by the fact that a cluster is mostly surface. A closely packed cluster of 20 atoms has only one atom in its interior; a cluster made up of 100 atoms may have only 20. Other properties stem from clusters unfilled electronic bonding capability, which leaves them naked and hence extremely reactive. This reactivity makes them effective tools for the study of the solid state and, potentially, for such industrial processes as the growing of crystals, selective chemical catalysis and the creation of entirely new materials with made-to-order electronic, magnetic and optical properties. Such materials, in turn, could enhance the performance of products as diverse as lasers, photographic film, electrosensitive phosphors, magnetic disks and supercomputers.
Conference Paper
Three alternate fluids, ice-water slurry, friction reduction additive and the combination of them, have been compared for use in District Cooling Systems (DCS). The effect of the fluids on cost and cooling capacities were considered for the two cases of new and existing DCS separately. Two criteria were used in comparisons among fluids in each case: constant pumping power which allows for the most benefit, and constant velocity which is more practical consideration. An economic assessment for a 500 ton system shows a potential cost difference in the total pipe cost for a new system of 70% when a 30% ice slurry is used in place of chilled water. The pipe diameter is reduced to 40% using the slurry. These results apply to the constant comparison and are independent of the use of additive. Friction reduction additives serve to reduce pumping power and pressure drop. The ice-water slurry also has a significant impact on existing district cooling systems. It can potentially expand the cooling capacity by 500% without new piping being installed while maintaining the same pumping power, velocity and pressure-drop as the chilled water system. Again, friction reduction additives serve to reduce pumping power and pressure-drop. They do not influence cooling capacity. The cost for expanding the piping to increase the cooling capacity by the same amount by the use of conventional district cooling technology has been shown to be extremely high compared to the ice-water slurry system.