Experimental Studies on Pool Boiling Heat Transfer Using Alumina and Graphene Oxide Nanofluids

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 03 Issue: 01 | Jan-2016 www.irjet.net p-ISSN: 2395-0072
© 2016, IRJET | Impact Factor value: 4.45 | ISO 9001:2008 Certified Journal | Page 674
EXPERIMENTAL STUDIES ON POOL BOILING HEAT TRANSFER USING
ALUMINA AND GRAPHENE OXIDE NANOFLUIDS
Sunil L J1, Dr. S. Kumarappa2, Dr. R. K. Hegde3
1 M. Tech. Thermal Power Engineering, BIET, Davanagere , Karnataka, India
2 Professor and HOD, Department of Mechanical Engineering, BIET, Davanagere, Karnataka, India
3 Professor and HOD, Department of Automobile/Aeronautical Engineering, SIT- Valachil, Mangaluru, Karnataka,
India
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Abstract - Energy crisis around world has triggered
more attention to energy saving and efficient energy
conversion devices. One of the effective methods for
transferring thermal energy with large heat flux is
liquid-vapour phase change process is called boiling.
The aim is development of special surface geometry or
texture for enhancing nucleate pool boiling heat
transfer is to decrease the boiling incipience and to
delay the critical heat flux (CHF). During pool boiling
and subsequent inspection, formation of a porous layer
of nanoparticles on the heating surface occurred during
nucleate boiling. This layer significantly changes
surface texture of the heater wire surfaces which could
be the region for the improved in the CHF value.
Common fluids with particles of the order of the
nanometers in size are termed as a “nanofluid” which
has created considerable interest in recent times for
their improved heat transfer capabilities. With very
small volume fraction of such particles the thermal
conductivity and convective heat transfer capability of
these suspension are significantly enhanced without
the problems encountered in common slurries such as
clogging, erosion, sedimentation and increase in
pressure drop. In this work, experiments on Alumina
and Graphene oxide nanofluids was used to study the
effect on pool boiling heat transfer and its effect on heat
transfer enhancement was studied on the horizontal
test wire immersed in different concentration of
nanofluid viz. 0.01g, 0.1g/l and 1g/l. The effect on heat
transfer enhancement was investigated by flow
visualization and by surface inspection of the heater
surface by SEM images.
1. INTRODUCTION
Nanofluids are dilute liquid suspended of nanoparticles
with at least one critical dimension smaller than 100 nm.
Pool Boiling Heat Transfer using nanofluids (which are
suspended of nanoparticles in a base fluid) has been a
subject of investigations and incoherent results. In the
past, experiments were conducted in nucleate pool boiling
with varying parameters such as particle size,
concentration, surface roughness etc. Critical Heat Flux
(CHF) is the heat flux at which a boiling crisis occurs
accompanied by a sudden increase of the heat transfer
surface or decrease of the heat transfer rate. The CHF
imposes a limit in designing and operating boiling heat
transfer equipments in power generations industries such
as nuclear power plants and fossil fuel power plants. Thus,
an increase in the critical heat flux can enhance the safety
margins and allows for more economical design and
operation at higher heat fluxes. As the capacity of nuclear
power plants or fossil boilers increases, a higher heat load
per unit heat transfer area is required. In fusion reactors,
the maximum heat flux may be higher than 10 MW/m2 for
this special heat transfer measures are necessary. Among
all the regimes of pool boiling the most effective heat
transfer mechanism is nucleate boiling, the upper
boundary of which is dictated by the CHF. Therefore,
efforts have been exerted to expand the regime of nucleate
boiling.
1.1 Boiling Heat Transfer (BHT)
Heat transfer is the process of transferring the heat from
high temperature region to low temperature region at a
certain rate without aid of any external agency. Hence it
obeys the Second Law of Thermodynamics. There are
three modes of heat transfer viz. conduction, convection
and radiation.
Boiling is a convection process involving a change of phase
from liquid to vapour. Boiling may occur when a liquid is
in contact with a surface maintained at a temperature
higher than the saturation temperature of the liquid. The
transfer of heat takes place in the boiling according to the
Newton’s law of cooling, It states that the rate of heat
transfer in convection process is directly proportional to
the product of temperature difference and convective heat
transfer coefficient (It is constant for the given fluid) per
unit area.
i.e.
Where, is called Excess temperature,
And h = Convective or Boiling Heat Transfer Coefficient. If
the heat is provided to a liquid from a submerged solid
surface, the boiling process is called pool boiling. In this
process the vapour produced may form bubbles, which

grow and subsequently detach themselves from the
surface rising to the free surface due to buoyancy effect.
BHT coefficient is the rate of heat transfer per unit area
due to the change in the thermal forces ( ). Finally the
BHT coefficient is a measure of the heat transfer due to
phase change of a liquid during boiling.
2. OBJECTIVES
This project focuses on the fundamental mechanisms in
pool boiling responsible for heat transfer enhancement
techniques using alumina and Graphene oxide nanofluid.
The primary objectives of this research work are as
follows:
 To study the pool boiling phenomenon till the
Burnout point or Critical Heat Flux (CHF) using a
horizontal test surface immersed in different
concentrations of alumina and Graphene oxide
nanofluid at atmospheric pressure.
 To investigate the reasons for increasing or
decreasing in Critical Heat Flux by subjecting the
test surface to different concentrations by using
SEM Images.
 To study the flow visualization in pool boiling.
3. MATERIALS AND METHODOLOGY
3.1 Apparatus used: Glass beaker, electric heater,
electrodes, condenser, thermocouple, voltmeter, ammeter,
dimmer stat, test wire, distilled water and nanofluids.
3.2 Specifications:
 Capacity of heating coil - 250 W
 Distance between electrodes- 85 mm
 Diameter of the test wire-0.193 mm (SWG 36)
 Capacity of dimmer:
Single phase current Input - 240 V
Maximum load -15 A
Output - 0 to 270 V
Frequency - 50 HZ
 Nichrome wire:
Nickel 80%, Chromium 20% alloy
Density ≈ 8200 Kg/m3
Resistance = 34 Ω/m
Melting temperature = 14000 C
Maximum continuous service
temperature = 12000 C
Non magnetic
 Glass beaker:
Inner diameter = 125 mm
Height = 180 mm
 Electrode:
Copper alloy (Brass)
Diameter = 8 mm
Length = 100 mm
3.3 Formulae:
Heat supplied (Q):
Q = VI (Watts)
Where,
V = Voltage in Volts (V)
I = Current in Amperes (A)
Critical heat flux (q):
q = Q/A (W/m2)
Where,
A = Surface area of wire (m2)
= πdl (m2)
d = Diameter of wire (m)
l = Length of wire (m)
3.4 Properties of nanoparticles used
3.4.1 Graphene oxide
Graphene oxide is electrically insulated due to disruptive
sp2 bonding network but it can be converted conductive
by restoring network.
3.4.2 Aluminum oxide
Parameters Approximate Values
Average nanoparticle size 40 - 60 nm
Purity 99%
Number of Layers Average number of
layers 1 - 2
Surface Area 200 m2/g
Bulk Density 3.7 g/cm3
Nanoparticle morphology Spherical
Crystallographic structure Cubic
Wires
Cooling water out
Cooling water in
Temparature indicator
Electrodes
Heater
Thermocouple
Input Output
+ +__
Dimmer
Nichrome wire
Fig -3.1: Line diagram of Experimental setup
Parameters Approximate Values
Thickness 0.8 - 1.6 nm
Purity 98%
Number of Layers Average number of
layers 1 - 2
Surface Area 350m2/g
Bulk Density 0.121g/cm3
Electrical Conductivity Insulator [Tuneable to
conductor]
Length < 10 - 20 micron

3.5 Experimental Methodology
 Fill the beaker with 2 liters of distilled water.
 Hold the test wire between the two electrodes.
 Place the lid over the vessel such that heater,
electrodes and thermocouple are immersed in the
distilled water.
 Place the condenser firmly in the hole made in the
lid and supply the cooling water through the
condenser inlet.
 Switch on the experimental setup. Initially the
distilled water in the beaker is heated by setting a
temperature value (Saturation Temperature of
distilled water) in the temperature controller.
 The temperature controller cuts off the power
supply to the heating coil when the fluid reaches
the preset temperature (saturation temperature
of fluid).
 Then the power supplied through the electrodes
by the dimmer creates a resistance in the
nichrome wire which is held between electrodes
causing to get heat supply.
 Meanwhile the voltmeter and ammeter readings
are observed.
 When the supply of heat is increased gradually
the formation of minute bubbles appears on the
surface of the test wire indicating nucleate boiling.
 When the heat supply is further increased the
rapid formation of bubbles occurs, indicating
departure from nucleate boiling.
 When the heat supply is increased furthermore
the film boiling regime attains and in the end the
wire gets burnt out at the centre.
 The ammeter and voltmeter readings at the burn
out point are noted.
 If the temperature of the fluid decreases less than
the saturation temperature in the middle of the
experimentation then stop supplying the heat.
Wait for a while to attain the saturation
temperature level and continue the process.
 The procedure is repeated for 8 to 10 times by
mounting the fresh test wire each time.
 After that the distilled water is replaced by the
nanofluid.
 The required nanofluid concentration say 0.1g/l is
obtained by mixing 0.2 gram of Alumina
nanoparticles in 2 liters of distilled water in a
stirrer for 1 hour.
 The experiment is carried out at the various
concentrations of Alumina and Graphene-0xide
nanoparticles.
 After each experimentation for distilled water as
well as for all the concentration of Alumina and
Graphene-oxide nanofluid. Collect the burnout
test wire carefully without touching the surface in
the air tight test-tube.
 The wire collected in the test tube is seen by the
Scanning Electron Microscope (SEM) for the
images captured at different magnification to
investigate the reasons for why the critical heat
flux increases or decreases.
3.6 Methodology for Flow visualization
 The experiments are conducted by the above
procedure (Content 3.7) for 0.01g/l of aluminum
oxide and 0.01g/l and 0.1g/l of graphene oxide.
During the experimentation photographs of
bobbles formation on the surface of the wire are
captured for different Voltage variations (2V,
4V…up to burnout point).
 The photographs are taken for all the mentioned
nanofluids at the same Voltage and Current
variation.
 These photographs are analyzed and found out
the number of bubbles and diameter of each
bubble in the selected section (The section has
constant dimensions for all the photographs) by
manual zooming and interpolation method.
 The rate of retention of the nanofluid can be
calculated by taking the photographs of the fluid
sample in the test tube at the intervals of 0 hour, 1
hour and 2 hour after preparing the nanofluid.
4. RESULTS AND DISSCUSION
4.1 Comparison of CHF with distilled water,
Aluminum oxide Graphene oxide for various
concentrations
Fig -4.1: Variation of Voltage with Current for various
concentrations of Alumina nanofluid with Distilled water
The above Fig -4.1 shows the Voltage v/s Current
variations for the alumina nanofluid. It shows that the
nanofluid at concentration of 0.01g/l, 0.1g/l and 1g/l are
enhanced 17.3%, 32.6% and 56.27% of CHF compared to
distilled water.

Fig -4.2: Variation of Voltage with Current for various
concentration of Graphene oxide nanofluid with Distilled
water
The above Fig. 4.2 shows the Voltage v/s Current
variations for the Graphene oxide nanofluid. It shows that
the nanofluid at concentration of 0.01g/l, 0.1g/l and 1g/l
are enhanced 13.4%, 29.5% and 51% of CHF compared to
distilled water.
Fig -4.3: Variation of CHF with various concentrations of
Alumina, Graphene-oxide, and Distilled water
The above Fig. 4.3 shows that the comparison of CHF v/s
various concentration of alumina, Graphene oxide and CHF
of distilled water. It clearly shown that at 0.01g/l
concentration the alumina enhances the 17.3% whereas
Graphene oxide enhances the 13.4%, at 0.1g/l
Graphene oxide enhances the 29.5% and at 1g/l
Graphene oxide enhances the 51% CHF.
4.2 SEM Images
4.2.1 Aluminum oxide 1g/l:
Fig -4.4: Al2O3 (1g/l) nichrome wire at 1000X
magnification
magnification
magnification
The burnout wire is taken at a concentration of aluminum
oxide 1g/l and it is observed in the SEM shown in the
figure at different magnification. At this concentration the
CHF is obtained 1.769 MW/m2. The CHF of the Distilled is
1.132 MW/m2 under atmospheric pressure. 56.27% of
CHF is enhanced at this concentration.

4.2.2 Graphene oxide 1g/l:
Fig -4.7: GO (1g/l) nichrome wire at 1000X magnification
Fig -4.8: GO (1g/l) nichrome wire at 25000X
magnification
Fig -4.9: GO (1g/l) nichrome wire at 100000X
magnification
At the Graphene oxide nanofluid concentration of 1g/l
SEM Images are shown in figure at different magnification.
At this concentration CHF is obtained 1.71 MW/m2 and
51% of CHF is increased compared to the distilled water.
4.3 Rate of retention
By comparing the photographs (photographs were taken
at 0hr, 1hr, 2hr after preparing the nanoflid) the rate of
retention is found more in the Graphene oxide nanofluids
than the alumina nanofluids due to the inhomogeneous
solution.
The rate of retention is found less in case of high
concentration range and it is noticed high in case of low
concentration range. The nanoparticles have the high
intermolecular forces (cohesive forces) so it is difficult to
break the particles at low concentration range of solution.
But in case of high concentration range, more numbers of
particles are involved in the mixing, the rate of collision of
the particle is more hence the mixing is well and rate of
retention is less in high concentration range due to proper
mixing. This is more in case of graphene oxide compared
to the alumina.
4.4 Flow visualization
The wetted surface is increases by increasing the
concentration of nanofluid and also increased creating the
porous layer over the surface of heating wire.
5. CONCLUSIONS
Pool boiling CHF characteristics in nanofluids are
investigated with different concentrations of Alumina and
graphene oxide nanofluid ranging from 0.01g/l, 0.1g/l and
1g/l and the effect of nanoparticles in pool boiling CHF for
each concentration is studied experimentally and the
effect on heat transfer enhancement are investigated by
flow visualization and by surface inspection of the heater
surface.
 It is observed that with low concentration of Al2O3
(0.01g/l) the dispersion is homogeneous while
higher concentration of Al2O3 (1g/l) resulted in
the formation of coating over the heater surface
and fouling of the vessel inner surface. Whereas in
case of the GO the dispersion is not much
homogenous in all the concentrations compared
to Al2O3 but in 1g/l concentration of GO is better
homogeneous compared to 0.01g/l concentration
of GO. In mid range concentrations (0.1g/l)
characteristics of both the fluids place in between
higher and lower concentrations characteristics of
corresponding fluid.
 While Al2O3 shows an increase in CHF by 17.3%,
at 0.01g/l concentration (Fig -4.3) and 56.27%
increase (Fig -4.3) in CHF at 1g/l concentration.
The maximum CHF was recorded at 1.769
MW/m2 for Al2O3 at 1g/l concentrations against
1.132 MW/m2 of DI water.

 In GO the CHF is increase 13.4% at 0.01g/l
concentration (Fig -4.3) and 51.5% increase (Fig -
4.3) in CHF at 1g/l concentration. The CHF values
are obtained slightly less than the Al2O3. It can be
observed by the Graph (Fig -4.3).
 Though GO has more good qualities than Al2O3due
to lack of molecular level mixing (Fig -4.7, 4.8, 4.9
molecules in bulk form) with the distilled water
and high rate of retention the CHF is obtained
slightly less than the Al2O3.
 The nanoparticle coating layer over the heating
surface is improved surface wettability and
enhanced the CHF is also supported by the SEM
and EDOX observations and flow visualisation.
 Nanofluids can be considered as a potential
candidate for heat transfer enhancement and are
highly suitable for applications in practical heat
transfer processes.
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