The document discusses heat transfer enhancement through the use of nanofluids, which are suspensions of nanoparticles in conventional fluids, aiming to improve thermal conductivity and convective heat transfer performance. It outlines various synthesis methods for nanoparticles, including physical, chemical, and biological approaches, and describes the advantages and disadvantages of different manufacturing techniques for nanofluids. Additionally, the impact of surfactants on dispersion stability and the effects of nanoparticles on thermal conductivity and viscosity in convective heat transfer applications are examined.

1. Heat transfer enhancement using
nanofluids

2. Introduction
➢ In general, heat transfer is the most fundamental process applied in numerous
industries and engineering fields such as cooling/heating applications and other
industrial systems.
➢ A mechanistic modification of the convective heat transfer can improve the heat
transfer characteristics with the application of an extension or change in the core
elements of the heat exchanger.
➢ Most studies in this area have aimed to promote convective heat transfer
performance and find the possibility of improving the convective heat transfer
through nanotechnology. Therefore, the methods to increase the heat transfer by
using nanofluid, which has nano-sized nanoparticles, and to overcome the
deficiency in the thermal conductivity of conventional working fluids such as water,
propylene glycol, and ethylene glycol have been actively studied in the literature.
➢ A mixture of nano-sized particles and a conventional heat transfer fluid is called
nanofluid.

3. Nanofluids are mixtures of traditional fluids (base fluids such as water, propylene
glycol, and ethylene glycol) to which various types and concentrations of
nanoparticles (nanoscale materials <100nm) with high thermal conductivities have
been added.
These nanoparticles can be:
➢ metals (Cu, Al, Fe, Au, Ag, Zn, Ni, Mn, Ti, etc.)
➢ metal oxides (TiO2, Al2O3, SiO2, CuO, Fe3O4),
➢ carbon nanotubes (SWCNT- and MWCNT), or graphene.

4. When compared to the heat transfer characteristics of conventional liquid or solid–
liquid suspensions, nanofluids possess many advantages.
• First, the nanomaterials have a much higher surface area to volume ratio than that
of more traditionally sized microparticles, which results in more significant surface
interaction between the individual nanoparticles and the surrounding fluid
molecules.
• In addition, the physical properties of the nanoparticles, such as viscosity and
thermal conductivity, can be easily adjusted by many approaches, including surface
modification, applying external stimuli, and using dispersing agents(surfactants) to
achieve desired applications.

5. Fig. 1. Numbers of papers related to nanofluids
Research studies into nanofluids have
intensively increased over the last 20
years.
Currently, many researchers are focusing
on a new type of nanofluid by dispersing
more than two different particles into
conventional heat transfer fluids, which
are called hybrid nanofluids. In general, a
hybrid nanofluid can achieve better
dispersion stability than other nanofluids
because each nanofluid can be used with
lower concentration.

6. Synthesizing methods of particles
A variety of nanoparticle
synthesis methods have
been reported, and that
can be divided into three
main categories, namely
physical, biological, and
chemical, depending on
the process used.
Figure . 2. Nanoparticle synthesizing methods.

7. ➢ Among the physical methods, mechanical ball milling is widely used to
manufacture dry powder nanoparticles.
➢ In this method, a container filled with hardened metal balls such as steel and
tungsten is used. To create dry powder nanoparticles using a ball milling method,
the bulk material is forced against rotating walls in an inert gas atmosphere at high
speed.
➢ The absence of a chemical solvent during the preparation is one of the main
advantages of a physical method when compared to chemical methods.
➢ Compared to a physical method, a chemical method uses simple techniques that
require inexpensive instrumentation. Besides, a chemical synthesis method can be
used at low temperatures (<350 ◦C) compared to the physical method.
➢ A biological method uses microorganisms such as fungi, yeast, bacteria, and
actinomycetes, which are extracted from plants or enzymes

8. Preparation method of nanofluids
Figure .3. Nanofluid manufacturing methods
Nanofluid manufacturing is generally classified as a one and two-step
method.

9. ❖ The two-step method is the most generally used approach for preparing
nanofluids.
➢ Nano-sized materials with favorable thermal and rheological properties are
prepared as dry powders through a chemical or physical process.
➢ Then, the nanoparticles are dispersed into the base fluid with the help of intensive
mixing methods such as a magnetic agitator, ultrasonic probe sonicator, ball
milling, or high shear-stirring device.
The advantages:
o relatively economical friendliness.
o fewer complications compared with
other manufacturing methods.
The disadvantage:
It is the tendency of easy agglomeration
of particles in the nanofluids. To increase
the dispersion stability of a nanofluid,
chemical agents are used as a surfactant
to decrease the surface tension of the
base fluid.

10. ❖ The one step method was established to resolve the dispersion stability issue of a
two-step manufacturing method.
In one-step method:
o Synthesis and dispersion of the nanoparticles are carried out simultaneously in the
base fluid.
o The drying and storing processes of the nanoparticles are eliminated leading to
minimum sedimentation in the nanofluid.
➢ Owing to the high cost, the one-step manufacturing method of nanofluid cannot be
widely used in the industrial market.

11. The effect of surfactant.
Surfactants are used to increase the dispersion stability of nanofluids. They are chemical
agents used to decrease the surface tension of the base fluid thus the nanoparticle
surface can be modified as hydrophilic or hydrophobic by adding a surfactant.
Surfactants can be divided into
▪ Anionic: Sodium dodecyl sulfate (SDS).
▪ Cationic: Cetyl trimethyl ammonium bromide (CTAB).
▪ Non-ionic: Gum arabic (GA) and oleic acid.
▪ Amphoteric types: lecithin.
The type of surfactant and the concentration of surfactant play a crucial role in the
nanofluid’s stability and thermal conductivity improvement.
Using excess surfactants can have an inverse effect on the thermal conductivity of the
nanofluids, as the surfactants are nonconductive, so it may block the heat conduction and
works as a barrier that decreases the thermal conductivity.

12. Thermal conductivity of nanofluids
➢ The thermal conductivity of a fluid is its ability to deliver heat through the
materials.
➢ Nanoparticles with high thermal conductivity are suspended in the base fluid
with low thermal conductivity, which increases the thermal conductivity of
nanofluids.
➢ Several researchers have extensively studied the thermal conductivity of
various nanofluids. Those researchers have mentioned that their proposed
models are more suitable for the prediction of the thermal properties of
nanofluids with a special shape than those of spherical nanoparticles.
➢ A summary of thermal conductivity models for different nanofluids is shown
in Table. 1.

14. ➢ Harandi et al. recently studied the thermal
properties of F-MWCNT-Fe3O4/EG hybrid
nanofluids at various concentrations.
➢ The maximum improvement in the thermal
conductivity of an F-MWCNT-Fe3O4/EG
nanofluid was 30% at a concentration of 2.3
vol% as compared with the base fluid.
➢ They concluded that the thermal conductivity
increases with increasing the temperature and
concentration of the nanofluid.
Figure .4. Thermal conductivity of EG based F-MWCNT-
Fe3O4 hybrid nanofluids with different volume
concentrations.
Thermal conductivity of hybrid
nanofluid

15. ➢ Shahsavar et al. described the effects of a
magnetic field on the thermal conductivity
and viscosity of a water-based CNT-Fe3O4
hybrid nanofluid.
➢ The most substantial improvement of the
thermal conductivity was 151.31% for a
0.9% Fe3O4/1.35% CNT hybrid nanofluid at
a magnetic strength of 470 mT.
➢ Without a magnetic field, the improvement
in the thermal conductivity was 45.4% for
the 0.9% Fe3O4/1.35% CNT hybrid
nanofluid.
➢ They concluded that when the magnetic
field strength is more than 490 mT, the
viscosity and thermal conductivity of the
hybrid nanofluid decrease simultaneously.
Fig. 5. Thermal conductivity over time for Fe3O4-
CNT/water with the effect of magnetic field strength at
25 °C.
Thermal conductivity under
magnetic field

16. Viscosity of nanofluids
➢ Many researchers have developed efficient models, and numerous experiments have been
conducted to compare with other experimental data.
➢ Representative models of the previous study on the viscosity of a nanofluid are summarized
in Table 2.

17. Convective heat transfer of nanofluids
➢ Most convective heat transfer
performance studies have carried out
using various types of heat
exchangers such as straight pipes and
heat sinks.
➢ Among the various experimental
studies conducted, most experiments
on a convective heat transfer need
several types of equipment to control
the desired flow conditions through a
straight pipe, as shown in Fig. 7. Fig. 7. Schematics of convective heat
transfer experimental setup with straight pipe.

18. ➢ Hojjat et al.conducted an investigation about
the turbulent flow region at different
concentrations of gamma-Al2O3, TiO2, and CuO
nanoparticles using newly proposed
correlations for the prediction of the Nu
(Nusselt number) for a nanofluid.
➢ Fig. 8 shows the local convective heat transfer
of three types of nanofluid based on the
functional axial distance from the inlet
region of the tube.
➢ As their results indicate, the convective heat
transfer coefficient and Nu of a nanofluid are
noticeably higher than those of the base fluid.
➢ These improvements in the Nu of a nanofluid
were increased proportionally with the increase
in the particle concentration and Peclet number
of the nanofluid.

19. ➢ Highly thermal conductive CNT group materials have also been studied in different
research areas.
➢ Amrollahi et al. used a functionalized multi-walled carbon nanotube (f-MWCNT) in a
water-based nanofluid, and their results exhibited that the convective heat transfer
coefficient increased by 33% in a laminar flow and 40% in a turbulent flow at a
concentration and temperature of 0.25 wt% and 20 ◦C, respectively, compared with
those of water.
➢ Furthermore, they found that the convective heat transfer coefficient increases with an
increase in the Re. The effect of the nanoparticle concentrations on the convective heat
transfer is shown in Fig. 10. At a concentration of 0.25 wt%, the convective heat transfer
coefficient is higher than that of the base fluid for all temperatures.
➢ the effect of the nanoparticle concentrations on the convective heat transfer is shown
in Fig. 9. At a concentration of 0.25 wt%, the convective heat transfer coefficient is
higher than that of the base fluid for all temperatures.

20. Fig. 9. Investigation into convective heat transfer coefficient
of water-f-MWNT nanofluids under laminar and turbulent

21. Summary

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