FIELD OF THE INVENTION
The invention generally relates to methods to disperse ceramic nanoparticles in a
hydrophobic thermally transfer fluid. More particularly, the invention relates to
determining the effect on thermal conductivity of transformer oil upon dispersion
of ceramic nanoparticles including stability pattern of the formed fluid.
BACKGROUND OF THE INVENTION
Transformer as an electrical device has significant practical application for
transforming an electric current from one voltage to another. The transformer
normally has a primary coil and a secondary coil wound around an iron core to
enhance corresponding magnetic field and flux. A heat transfer liquid such as
transformer oil is used to cool the coils and core and transfer the heat to the
surrounding through a radiator.
In liquid filed transformers, dielectric fluid is used to cool the windings and
provide optimal performance. The fluid flows vertically up from a lowest
temperature level at the bottom portion of the tank to a top portion of the tank
having highest temperature, and finally the fluid exits the main tank and enters a
series of radiators and cooling fans. The fluid thereafter flows downward through
the radiators, where it is cooled, and reenters the main tank at the bottom. In
self-cooled transformers, this cycle is governed naturally by convection. Natural
convection can also be enhanced by a series of fans directing air against the
radiators increasing the rate of heat transfer and subsequent rate of cooling in
the windings. In some large power transformers, it is also possible to have a
level of forced oil circulation where a pump assists in the circulation of the fluid.

This arrangement generally provides a lower top oil temperature and more
uniform temperatures within the windings.
Selection of a heat transfer media for specific applications is dependent on
factors like density, thermal conductivity, specific heat and viscosity. The
maximization of the heat transfer capability in a transformer is important to the
overall energy efficiency, material resource minimization and system cost. The
transformer oil currently available in the market although substantially serves its
purpose, but suffers from overheating which generates hot spots, presumably
due to poor thermal conductivity of the fluids, which interalia leads to cracking of
the oil's molecular composition and jeopardizing insulation properties. Other
factors that affect the performance of transformer oil include environmental
impact, toxicity, flammability, physical state at normal operating temperature and
corrosive nature besides the cost. Therefore, there exists a need to develop an
improved heat transfer fluid composition and methods that are cost effective and
have better performance, which can be used in the transformers.
Nanofluids are colloidal suspensions with dispersed nanoparticles, which are
known to enhance the thermal conductivity of a base fluid. A higher thermal
conductivity would decrease the winding temperature for a given load. On the
other hand, the load of a particular piece of equipment could be increased
without exceeding the specifications, if thermal conductivity of fuel used in the
case can be suitably improved. Either way, the nanofluid when used as the heat
transfer fluid, would result in a tremendous cost recovery by reducing the
maintenance and replacement costs of aghg equipment. However, because the
nature and behaviour of nanofluids is generally not well documented, the side
effects of adding nano-particulate to transformer oil must be examined carefully.

Upon adding nano-particulate to transformer oil, transformer performance may
be governed by a variety of technical effects other than enhanced thermal
conductivity. Furthermore, these effects can be critical to safe, efficient and
reliable operation of the equipment. In general, the issues associated with
nanofluids and oil-immersed transformer performance can be categorized into
electrical, mechanical, chemical and thermal. Electrical effects include dielectric
strength, discharge susceptibility and magnetic interference, for example.
Chemical effects includes stability, suspension, clustering and reaction with
immersed components (including insulating paper). Mechanical effects include
settling, viscosity, infiltration, lubrication and clogging.
Nanofluids based on water as the base solvent using various types of
nanomaterials have been studied extensively worldwide in order to understand
the effect on thermal conductivity as an important parameter. The extensive
research has been carried out in Alumina-water and CuO-water systems besides
few reports in Cu-water, and carbon nanotubes (CNT) based systems. The
nanoparticles used in three main systems, such as AI2O3-based, CuO-based and
Cu-based nanofluids were varied in the range of 13-300, 23-29 and 50-300 nm
respectively. The improvement in the thermal conductivity in such systems was
varied in the range of 1.10-1.29, 1.07-1.54 and 1.002-1.24 respectively. Limited
reports are available in metal oxide based nanofluids in oil and ethylene glycol.
The enhancement of thermal conductivity of base fluid will be a definite
requirement in future to improve the thermal efficiency of different systems.
The concept of introducing additives to cooling media to enhance thermal
conductivity is not new. However, the idea of adding nano particles to cooling
media such as transformer oil was realized by Davidson and Bradshaw disclosed

in their patent US 7,390,428B2. A soya-based oil as the base fluid have
demonstrated an oil based system for improvement in thermal conductivity. The
nanodiamond with average particle size of less than 100 nanometer was
preferred as the additive because it is an inert, dielectric oil compatible material
that is dispersible and readily suspended in transformer oil. With the addition of
this property-enhancing, cost-effective dispersion, the thermal conductivity can
be directly raised resulting in increase in oii life.
Lockwood et al. (US 7,449,432B2) outlined a novel use of nanomaterials as a
viscosity modifier and thermal conductivity improver for gear oil and other
lubricating compositions. The gear oils of the invention had higher viscosity,
higher shear stability and improved thermal conductivity compared to the known
gear oils. The preferred nanoparticles also impart a reduction in the coefficient of
friction, including reduced friction in the boundary lubrication regime. These
properties are obtained by replacing part or all the polymer thickener with
nanomaterials of suitable size, shape and composition.
Zhang et al (US 0287326 Al/2008) described a lubricant oil composition with
nanoparticles with enhanced thermal conductivity for automatic transmission
fluids, power transmission fluids and hydraulic steering applications. Choi et al. in
a Korean patent application (2009-0075989) have described a preparatory
method of lubricating oil having excellent extreme pressure load resistance and
wear resistance properties. Tingler et al. (US patent application 12/6, 93,
569,2010) discloses a lubricating oil composition with nanoparticles for use in a
submersible electric motor.

The first published report on transformer oil based nanofluid was by Xuan and Li
on the dispersion of 18 nm copper nanoparticles in the range of 2-5 vol. % with
22 wt.% Oleic acid as a dispersing agent. Later, Choi et al (Current Appl.
Phys.2007) demonstrated the dispersion of alumina (Al203) and aluminum nitride
(AIN) nanoparticles in transformer oil using oleic acid as a surface active agent.
The authors observed that 0.5% AIN can increase the thermal conductivity of
transformer oil by 8% and overall heat trarsfer coefficient by 20%. The authors
pointed out that excessively thick hydrophobic coating layers formed on the
powder surface has harmful effects on heat transfer viscosity and chemical
stability.
As noted above, there is only one patent available on nanofluid based on
transformer oil. The patent described a costly nano-diamond material in
transformer oil. Limited data is available on other nanomaterials like alumina,
aluminum nitride, and no data with carbon nanotubes, copper oxide and cerium
oxide etc. No research reports are also available on the stability of such
nanofluids. The said patent reports a method of dispersion of carbon nanotubes,
copper oxide and cerium oxide nanoparticles in transformer oil and study the
thermal properties of the base fluid including the stability and effect of
temperature.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process to produce an
improved oil based fluid by dispersing metal oxide nanoparticles in a base fluid
with surface active agents.

Another object of the invention is to propose a process to produce an improved
oil based fluid by dispersing metal oxide nanoparticles in a base fluid with
surface active agents, in which the metal oxides are copper oxide (CuO) and
cerium oxide (CeO2) and multiwalled carbon nanotubes (MWCNT).
A still another object of the present invention is to propose a process to produce
an improved oil based fluid by dispersing metal oxide nanoparticles in a base
fluid with surface active agents, which exhibits an enhancement in thermal
conductivity of the oil based fluids.
Yet another object of the invention is to determine the effect of temperature and
effect of time on the thermal conductivity behavior of said nano fluids.
SUMMARY OF THE INVENTION
According to the invention, a stable dispersion of nanopowders in transformer oil
is done by ultrasonic disruption, and by using surface active agents for different
nanomaterials. The oil based nanofluids produced in this invention, exhibit an
enhancement of thermal conductivity in a range of 5-20% over its base fluid with
addition of nanoparticles in the range of 0.1-2.0 vol %. The enhancement in
thermal conductivity for the oil with the sonication time, is found to be lower
with increase in temperature. Further, the nanofluids were kept for a fixed
duration and thermal conductivity was measured over a time period in order to
determine the effect of nanoparticles for longstanding characteristics of such
fluids.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS :
Figure 1 describes the process for producing an oil based nanofluid including
determination of thermal property.
Figure 2 - depicts a graph illustrating a comparison of the thermal conductivity of
different nanofluids as a function of volume fractions of said nanoparticles.
Figure 3 - graphically concludes the stability of such fluids.
Figure 4 - graphically exhibits the effect of sonication time on the thermal
conductivity of CNT based nanofluid.
Figure 5 - graphically illustrates the effect of temperature on the thermal
conductivity of oil based nanofluids.
Table 1 - shows the properties of different nanoparticles.
DETAIL DESCRIPTION OF THE INVENTION
The present invention provides a process for producing a nanofluid by dispersing
of nanoparticles and surfactant in a base fluid including characterization of the
thermal and electrical properties of the nanofluid. The present invention
therefore provides a process for producing nanofluid and characterization of such
nanofluids. Figure 1 illustrates the steps involved to produce the nanofluid
according to the invention.

The preparation of nanofluid was carried out by ultrasonication process for
dispersion of nanoparticles with surface active agents. A surface active agent or
a surfactant is a substance which lowers the surface tension of the base fluid in
which it is dissolved, and/or the interfacial tension with other phases, and,
accordingly, is positively adsorbed at the liquid/vapour and/or at other interfaces.
Surfactants play an important role in dispersing nanoparticles in a base fluid such
as transformer oil with very low thermal conductivity. For making stable
nanofluids, a controlled amount of surfactant is added. The surface active agents
can be categorized as anionic, cationic and nonionic. The present invention
explored all types of such surface active agents assisting nanoparticles dispersion
in oil. The anionic agents like Sodium lauryl sulphate, Sodium dodecyl sulphate,
Sodium dodecyl benzene sulphonate etc cationic agents like cetyl trimethyl
ammonium bromide and nonionic agents like Triton X-100, Polyvinyl Pyrolidone,
Myristic avid, Castor oil, Oleic acid, Laurie acid, Methacryl oxypropyl
trimethoxysilane (MPS), Polydimethylsiloxane and n-Hexane etc. have been tried
out during development of this inventive process.
The effect of both the process and the agents are complimentary and can not be
used independently to produce stable nanofluids. The additives like MWCNT,
CuO and CeO2 were used in the as received condition after drying in air under an
Infra red lamp. The properties of such materials are listed in table 1. One of the
considerations for using carbon nano tube in the fluid is the high resistance of
nano tube for high surface density, increase thermal conductivity in pivot tube,
lightening of nano tube because of their emptiness and also, more importance of
nano tubes for their high capability in different industry and easy production with
different properties. Copper oxide was another material selected for preferable
application in this invention, for reported capability of such materials in
developing water based nanofluids. However, no study on the oil based
nanofluids using copper

oxide is known in the art. The last material used in this invention is the cerium
oxide which is a rare earth oxide material and is widely used in the energy
applications and to stabilize zirconium oxide resulting in high fracture toughness
to the material. The cerium oxide thus assumes a significant role as an additive
for improving the properties of the base fluid.
The transformer oil conforming to the standard specification of IS 335/1993 was
used in this invention as the base fluid. The oil was kept sealed all the time and
passed through a porous ceramic membrane to filter out any impurities prior to
heating at 70 deg C for 30 min. This was done in order to avoid any moisture
contamination to the oil which could otherwise deteriorate the properties of the
base fluid. A 250 watt probe type ultrasonic disrupter was used to homogenize
the oil and particle mixture and the time required for such operation varies for
different materials and for different volume fractions of such materials.
Sonication is the known process for the preparation of stable nanofluids.
Sonication is the act of applying sound energy to agitate particles in a sample,
for various purposes. Sonication is a high-intensity acoustic energy to change
materials and constitutes a mechanism used in loosening particles adhering to
surfaces. The ultrasonication was carried out at a stretch for about 30 min.
followed by a gap of 10 min, before resuming the next sonication step. In the
case of MWCNT based fluids, breaking of CNT particles due to constant exposure
to ultrasonication was avoided. A hand helc' thermal conductivity meter was used
for thermal conductivity measurement of the base fluid and the nanofluids
following transient heat source method of measurement following EN 500082-1
(1991) standard. With this technique, a 30 second heat pulse is applied to a
needle, and temperature response with time is monitored through the heated
needle or an adjacent needle. The needle is 1.2 mm diameter and 60 mm long.
The nature of a temperature response is a result of the thermal

properties of the material. The single needle algorithm is used which is based on
transient heat. Heat is applied to a single needle for a time, th, and temperature
is monitored in that needle during the pe iod of heating and for an additional
time equal to tn after heating. The temperature during heating is computed from
T =m0 + m2t + m3 In t
Where, m0 is the ambient temperature during heating, m2 is the rate of
background temperature drift, m3 is the slope of a line relating temperature rise
to logarithm of temperature. During cooling the model is T=mi+m2t + m3 In
[t/(t-th)] The thermal conductivity is computed from K=q/4-nm3, where q is the
heat per unit length.
The effect of temperature on the thermal conductivity of the nanofluids was
measured by stabilizing the nanofluids "in a water bath at an applicable
temperature prior to the measurement. The temperature of the fluid in the water
bath was monitored simultaneously with the temperature of the fluid in the cell
containing the nanofluid. The ageing characteristics of such nanofluids were
measured by measuring the variation in thermal conductivity of the samples at a
constant interval of time. The dielectric properties such as breakdown voltage,
dielectric loss and surface resistivity of the base fluid and the nanofluids were
extensively measured for CuO based fluids.
According to the invention, for producing the nanofluids, transformer oil is taken
in a vessel and is heated on a hot plate to 70 deg C for 30 min. to avoid any
moisture contamination. To this oil, surface active agent is added under stirring
condition. The selected nanoparticles is added to this modified oil composition
and ultrasonicated for 2h with pulsed sonication time of 30 min. each. The
ultrasonicated liquid thus results an oil based nanofluid wherein the nanoparticles
are suspended in the base oil. The mixed oil is taken in a tube with stopper

means and a thermal conductivity probe is inserted into it and stabilized for 30
min. prior to taking readings. An average of 10 readings is taken for each fluid in
15 minutes interval in order to estimate the actual enhancement of the thermal
conductivity of such fluids added with nanoparticles. Figure 2, shows the
enhancement of thermal conductivity of oil based nanofluid with the increase in
volume fraction of the nanoparticles. In the case of ageing test, the nanofluid
samples are taken at different intervals of time as shown in figure 3 and the
thermal conductivity is measured after sonication for 10 min. The experiments
enable estimation of the stability of such fluids in terms of affecting the thermal
conductivity with different nanomaterials at different volume fractions. The
resultant thermal conductivity value gets reduced with the passage of time. In
another experiment according to the invention, the effect of sonication time was
studied for CNT based nanofluids. The' data in figure 4 indicate that the
enhancement in thermal conductivity can be achieved with increase in sonication
time. The CNT nanoparticles was dispersed ultrasonically with higher surface
active agent content in the range of 1.5-2.5 %, and the sonication time was
varied. The effect of temperature on the thermal conductivity of nanofluids was
studied in this invention. It is concluded that, the effective thermal conductivity
reduces with increase in temperature. It is known that the thermal conductivity
of most of the liquids decrease with increase in temperature, although water is a
notable exception. The results observed in this invention validate theoretical
prediction. The results depicted in this invention will be however, slightly
different in a dynamic application where the fluid is under circulation. Therefore,
the nanofluids are best thermal efficient fluid mainly for close circuit applications.
The present invention can be better explained with suitable examples.

Example 1: MWCNT based Nanofluid
0.01 - 0.015 g of Polyvinyl Pyrolidone (PVP) surface active agent was added in
approximately 100 ml of pre-heated cooled transformer oil with magnetic stirring
for 10 min. 1-1.5 g MWCNT powder vas added to the modified oil and
ultrasonicated for 2-3 h. The thermal conductivity of such fluid showed an
enhancement of 10-14 % over its base fluid. The nanofluid produced in this
example was found to be stable for four to six weeks without any change in
conductivity value. The temperature variation study carried out on such nanofluid
resulted in reduction in thermal conductivity with rise in temperature.
Example 2 : CuO based Nanofluid
0.25-0.35 g of Triton X-100 with nominal composition (C34H62O11) surface active
agent was added in approximately 100 ml of pre-heated and cooled transformer
oil with magnetic stirring for 10 min. 2.5-3.5 g of CuO was added to the modified
oil and ultrasonicated for 2-3 h. The thermal conductivity of such fluid showed an
enhancement of 5-8 % over its base fluid. The nanofluid produced in this
example was found to be stable for three to six weeks without appreciable
change in conductivity. The temperature variation study carried out on such
nanofluid resulted in reduction in thermal conductivity with rise in temperature.
Example 3 : Cerium oxide based Nanofluid
0.03 - 0.04 g of Oleic acid with nominal composition (C18H34O2) surface active
agent was added in approximately 100 ml of pre-heated and cooled transformer
oil with magnetic stirring for 10 min. 3.2-3.8 g of CeO2 was added to the
modified oil and ultrasonicated for 2-3 h. The thermal conductivity of such fluid

showed an enhancement of 12-16 % over its base fluid. The nanofluid produced
in this example was found to be stable for one to two weeks without appreciable
change in thermal conductivity. The temperature variation study carried out on
such nanofluid resulted in reduction in thermal conductivity with rise in
temperature.
The above examples are not limited to the invention and are exemplary in
nature. Many experiments have been carried out for each materials by varying
the volume fractions and measured the thermal properties. These above
examples along with other experiments have established that the invention is
made on the successful development of oil based nanofluids with various types
of nanomaterials resulting in enhancement of thermal conductivity. The stability
of such nanofluids is varied depending on the characteristics of the nanoparticles
used and the volume fractions of each such materials in the base fluid.
While preferred embodiments have been shown and described, it should be
understood that changes and modifications can be made therein without
departing from the invention in its broader aspects. Various features of the
invention are defined in the following claims :
References Cited:
Patents:
1. Davidson and Bradshaw, US 7,390,428 B2 (20080)
2. Lockwood et al., US 7,449 432 B2 (2J08)
3. Zhang et al., US 0287326 Al, (2008)
4. Choi et al., Korean Patent filed 0075989 (2007)

5. Tingler et al., US 6,93,569 (filed in 2010)
Publications:
1. Xuan and Li, International Journal of Heat and fluid Flow, 21, (2008), 58
2. Choi et al. Current Applied Physics, 8 (2008), 710

WE CLAIM :
1. A method of suspending nanoparticles in transformer oil, comprising the
steps of:
preparing a moisture free transformer oil;
producing surface treated oil from said transformer oil with addition of
surfactants;
adding nanoparticles of differen types into the fluid; and
producing nanofluid by ultrasonic disrupter.
2. The method as claimed in claim 1, wherein the nano particles are selected
from the group consisting of multiwailed carbon nanotubes, copper oxide
and cerium oxide.
3. The method as claimed in claim 2, wherein the volume fraction of the
nanoparticles added to the transformer oil base fluid is in the range of
0.1-2%.
4. The method as claimed in claim 1, wherein the amount of surface active
agents (surfactants) is within a range of 0.5-5 %.
5. The method as claimed in claim 1, wherein the nanofluids so produced
exhibits an enhancement of thermal conductivity in the range of 5-20%
compared to the base fluid transformer oil.
6. The method as claimed in claim 1, wherein, the stability of the suspension
was validated by repeated measurement of the thermal conductivity at
regular interval of time for example -6 weeks.

7. The method as claimed in claim 1, wherein the thermal conductivity of the
nanofluid can be increased with the increase in sonication time upto 3h.
8. The method as claimed in claim 1, wherein the thermal conductivity of the
oil based nanofluid decreases with the rise in temperature.

The present invention relates to compositions of nanofluids containing a
thermally transfer fluid and ceramic nanoparticles. The nanofluid of the present
invention is based on a hydrophobic fluid such as transformer oil and the
nanoparticles suspended are multiwalled carbon nanotubes (MWCNT), copper
oxide (CuO) and cerium oxide (CeO2). The invention further relates to a method
of dispersing the nanoparticles in the base oil and surface active agents to form
a stable suspension. The enhancement O thermal characteristics of these oil
based nanofluids is determined and validated.