The present invention generally relates to a method for improvement
of convective heat transfer performance including overall heat
transfer characteristics of a heat exchanger with the use of a heat
conducting fluid. More particularly the invention relates to a system
for convective heat transfer in a heat exchanger by adapting a high
performing heat-carrier fluid.
BACKGROUND OF INVENTION
The limitation in the availability of high performance heat conducting
fluid has led the researchers to explore the possibility of producing
fluids having better heat conducting properties. Recent work in
several other technical field with the use of nanofluid, have prompted
the researchan adaptability of nanoparticles to form heat conducting
fluid which could enhance the cooling effectiveness. Efforts are
further being made to use the nanofluid in several spheres of life
including industries for example, steel industries, electronic
industries, nuclear reactor and any other applications requiring a high
rate of heat dissipation.
3
Any particle less than the size of 100 nanometers (nm) come under
the field of nanotechnology. Instead of cooling water,nanofluid has
been perceived as a cooling agent. Traditionally nanofluids are heat
transfer fluid. It is a suspension of solid particles in the size of
nanometers. The dispersion of the solid particles in the base medium
increases the thermal conductivity. With the addition of 4% volume
of cupric oxide nanoparticles, the ethylene glycol exhibits an increase
of thermal conductivity by 40%. Similarly a substantially large volume
in the increase of thermal conductivity is noticed by the addition of
aluminum oxide nanoparticles dispersed in water.
Compared to the normal cooling water, the thermal conductivity of
the nanofluid being higher, it offers an exclusive and undisputed
possibility to act as a high heat conducting fluid.
The use of the nanofluid for effective cooling has been explored for
more than a decade time when the researchers have found several
interesting characteristics of nanofluid. Most of the studies published
in the literature however focus on the higher thermal conductivity of
nanofluid that is produced, and model for development of the same.
The state of art of using nanofluid known to the inventors can be
summarized as under :
4
[1] S. Choi, 1995, enhancing thermal conductivity of fluids with
nanoparticles FED 231, 99-103.
[2] Choi, S. Z. Zhang, W. Yu, F Lockwood and E. Grulke, 2001
Anomalously thermal conductivity enhancement in nanotube
suspensions. Appl. Phys. Letter 79(14), 2252-2254.
[3] Eastman, J, A, Choi, U, 5, Li, 5, Yu, W & Thompson, L, J, 2001,
'Anomalously increased effective thermal conductivities of ethylene
glycol-based nanofluids containing copper nanoparticles', Appl, Phys,
Lett, vol. 76, no. 6, pp. 718-720.
[4] Patel H., S. Das, T. Sundararajan, A. Sreekumaran, B. George &
T. Pradeep, 2003. Thermal conductivities of naked and monolayer
protection metal nanoparticle based nanofluids: manifestation of
anomalous enhancement and chemical effects. Appl. Phys. Lett. 83
(14), 2931-2933.
[5] Maxell, J. c., 1904. A Treatise on Electricity and Magnestism,
second ed. Oxford University press, Cambridge, pp. 435-441.
5
[6] Hamilton R. & O. Crosser, 1962, Thermal conductivity of
heterogeneous two component systems, I & EC Fundamentals,
125(3), 187-191.
[7] Xuan Y. & Q. Li, 2000, Heat transfer enhancement of nanofluids.
IntJ. Heat Fluid Flow 21, 58-64.
[8] Keblinski, P., Phillpot, S.R., Choi, S.U.-S., Eastman, J.A. 2002.
Mechanisms of heat flow in suspensions of nano-sized particles
(nanofluids). Int. J. Heat Mass Transfer 45, 855-863.
[9] Lee,S., Choi, S.U.S., Li, 5., and Eastman, J. A., Measuring
Thermal Conductivity of fluids Containing Oxide Nanoparticles, J.
Heat Transfer, vol. 121 pp. 280-289, 1999.
[10] Jang, S. P., and Choi, S.U.S., Role of Brownian Motion in the
Enhanced Thermal Conductivity of Nanofluids, Applied Physics
Letters, vol. 84, no. 21, pp. 4316-4318, 2004.
[11] Xue, Q.Z., Model for Thermal Conductivity of Nanofluids, Physics
Letters, A. Vol. 307 (2003), pp. 313-317.
6
[12] D. H. Kumar, H. E. Patel, V. R. R. Kumar, T. Sundararajan, T.
Pradeep and S. K. Das, Model for Heat Conduction in Nanofluids,
Physical Review Letters, Vol. 93(2004), pp. 144301-1-4.
[13] Xue, Q. and W - M., A Model of Thermal Conductivity of
Nanofluids with Interfacial Shells, Materials Chemistry and Physics,
Vol. 90(2005), p.298.
[14] Xuan Y. and Li, Q., Heat Transfer Enhancement of Nanofluids,
International Journal of Heat and Fluid Flow, Vol.21 (2000), pp. 58-
64.
[15] Xuan, Y. M., Li, Q., (2003). Investigation on convective heat
transfer and flow features of nanofluids, ASME J. Heat Transfer, 125,
151-155.
[16] J. Buongiorno, Convective Transport in Nanofluids, J. Heat
Transfer - March 2006 - volume 128, Issue 3, 240, 001:
10.1115/1.2150834.
[17] Kyo Sik Hwang, Ji-Hwan Lee, Seok Pil Jang, Buoyancy-driven
heat transfer of water-based AI203 nanofluids in a rectangular cavity,
7
International Journal of Heat and Mass Transfer 50 (2007) 4003-
4010.
[18] Y. Xuan, Q. Li, Investigation on convective heat transfer and
flow features of nanofluids, ASMETrans. J. Heat Transfer 125 (2003)
151-155.
[19] D. Wen, Y. Ding, experimental investigation into convective heat
transfer of nanofluid at the entrance region under laminar flow
conditions, Int. J. Heat MassTransfer 47(2004) 5181-5188.
Choi [1] has experimentally shown that the nanoparticles indeed
exhibit highly conductive properties. Nanofluids basically consist of
nanoparticles of very low volume fractions distributed in quiescent
fluids ('nanofluids'). Choi et al. [2] Eastmanet al. [3], Patel et al. [4]
disclose that around 30 nm diameter particles in volume
concentrations between 0.001 to 6 percent, are capable to generate
a thermal conductivity of the nanofluid solution higher than at least
30% of the base fluid. In the study by Choi et al [2] and Eastmanet
al. [3] the anomalous behaviour in the increase of thermal
conductivity of nanofluid has been reported where the study was
related to carbon nanotube and copper nanoparticles.
8
Traditional mathematical models and correlations for thermal
conductivity have been found in literature from the work of Maxwell
[5], and Hamilton and Crosser [6]. Recent attempt to evaluate and
establish a new correlation for the thermal conductivity has been
proposed by Xuan and Li [7]. Keblinski et al.[8] have proposed four
possible mechanisms to provide a physical explanation for an
increase in the thermal conductivity. The four mechanisms can be
summarized as : (1) Brownian motion of the nanoparticles, (2)
Molecular level layering of the liquid at the liquid/particle interface
(3). Clustering effect from the nanoparticles, and (4) the nature of
heat transport in the nanoparticles.
The thermal conductivity of metal oxide nanofluids are shown to be
higher than the base fluid in the work of Lee et al. [9]. Several novel
models have also been explored for expressing the effectiveness and
correlation for conductivity in the recent times.
A new model based on Brownian motion of the suspended particles
has been developed to explain the effect of the size and temperature
on thermal conductivity by Jang and Choi [10]. However, only a
limited quantum of data obtained from the experimental results have
been used to prove the validity of the most of the known correlation
models.
9
Basedon Maxwell'stheory and average polarization theory, Xue [11]
has formulated a thermal conductivity model, which has been
compared with the experimental results. Kumar et al. [12] have
shown by means of a stationary particle model that thermal
conductivity is dependent on the particle size and volumetric
concentration. By considering the particle size and the interfacial
properties, a further correlation has been developed for calculation of
the thermal conductivity of nanofluids by Xue et al. [13]. Xuan et al.
[14] analysed the enhanced heat transfer of nanofluid with the help
of a dispersion model. Studies conducted using water-Cu nanofluids
by Xuan and Li [15] with a concentration approximately of 2% by
volume was shown to have a heat transfer coefficient of 60% higher
than the heat transfer co-efficient when pure water was used J.
Buongiorno [16] have reviewed the effect of several parameters and
observed that Brownian diffusion and thermophoresis are only
important slip mechanisms in nanofluids for convective transport.
Hwang et al. [17] investigated natural convective instability and heat
transfer characteristics of water-based Ab03 nanofluids when heated
from underneath a rectangular cavity. Hwang et al concluded that
the natural convection of water basedAI203 nanofluids is more stable
than the base fluid in a rectangular cavity when heated from the
underneath by noting the phenomenon that with the increase in the
volume fraction of nanoparticles,the size of nanoparticles decreases,
10
or average temperature of nanofluids increases. The ratio of heat
transfer coefficient of nanofluids to that of the base fluid is decreased
with the increase in the size of nanoparticles or with the decrease in
the average temperature of nanofluids, Wen and Ding [18] have
experimentally investigated the convective heat transfer of
nanofluids, made of gamma- AI203 nanoparticles and de-ionized
water, flowing through a copper tube in the laminar flow regime. This
establishes a considerable enhancement of convective heat transfer
using the nanofluids.
In general it is believed that the advantages of nanofluids are the
reducedsize and reduced cost. The reason of enhanced heat transfer
performance of nanofluids may be attributed to the characteristics of
nanoparticlesfor (1) increasedsurface area and heat capacity of the
fluid (2) higher number of collisions and interactions amongst the
fluid particles and the flow passagesurface which enhances the heat
transfer performance of nanofluids (3), higher turbulence and mixing
fluctuation of the fluid.
Thus, the above-mentioned prior art suggest that the thermal
conductiVity of nanofluids are function of the particle size and
interfacial properties. Accordingly, the prior art findings have
stimulated a concept that the nanofluids are indeed adaptable for
11
better heat transfer mechanism. Application of nanofluids in the area
of medicineand biomedical engineering is already known.
In most of the earlier research work, the focus was concentrated on
the study of thermal conductivity or the development of a stable
nanofluid.
The known convection studies in the field of nanofluid relate to heat
convection results observed in tubes carrying nanofluid and most of
them deal with the prediction of correction of Nusselt number for
different Reynolds number. In the area of convective heat transfer,
the experimental studies are related to forced convection or mostly
centred around numerical analysis. Very few studies on experimental
work dealing with natural convection can be found in laid open
publications.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a system for
convective heat transfer in a heat exchanger by adapting a high
performing heat-carrier fluid.
12
Another object of the invention is to propose a system for convective
heat transfer in a heat exchanger by adapting a high performing
heat-carrier fluid, which achieves a faster cooling rate of a hot
medium in a heat exchanger via indirect cooling.
A further object of the invention is to propose a system for
convective heat transfer in a heat exchanger by adapting a high
performing heat-carrier fluid, in which the hot-medium and the high
performing heat-carrier fluid are disallowed to come into direct
contact with each other.
SUMMARY OF THE INVENTION
The heat conductivity of water is generally not very high. An addition
of nanoparticles with a lower volume concentration in water/base
fluid or the suspension of nanoparticles as noted through
experimentation makes the solution an efficient heat conducting
liquid. Thus, a nanofluid increases the thermal conductiVity by an
amount 40%. According to the invention, a nanofluid suspension is
prepared for cooling the hot strip. A cooling chamber is filled with the
nanofluid, and a hot chamber is filled with hot water where water is
heated by at least one heater. The hot water in the heating chamber
is cooled by an indirect heat exchangefrom the nanofluid kept in the
13
cooling chamber. Thus in turn, the nanofluid picks up the heat and
the hot water is cooled down.
According to the invention, the system comprises a heat exchanger
unit, contain~ng two chambers one each for heating and cooling, the
nanofluid acting as a convective heat transfer medium. The nanofluid
is a suspension of nanoparticles of particular concentration and
stable.
The steps in the cooling method involved are:
• Forming a nanofluid;
• Determining the cooling phenomena of a hot medium while
allowing an indirect heat-transfer by the nanofluid contained in
a cooling chamber;
• Optimizing the cooling rate; and
• Commencing the heat - transfer process in the system.
14
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Block diagram of a strip cooling system in ROT of HSM
Figure 1A - Shows a sectional view of the top-cover of the heating
chamber.
FIGURE 1B - Shows a sectional view of the bottom plate of the
heating chamber
FIGURE 1C - Shows a sectional view of the bottom plate of the
cooling chamber.
FIGURE 1D - Shows a sectional view of the top cover of the cooling
chamber.
DETAIL DESCRIPTION OF THE INVENTION
Preparation of nanofluid :
A nanofluid is prepared from Alumina nanoparticles. Commercially
available nanoparticles are used for preparation of the nanofluid.
Different concentration of alumina is first prepared. A dispersant of
Sodium Hexa-Metaphosphate is added in the nanofluid which
maintains the stability. For the operational purpose, the
concentration of 0.50/0 and 1% is used.
15
The present invention will be better understood from the following
brief description of the system reference to the accompanying
drawings.
Figure 1 illustrates a heat exchangersystem (1) where two chambers
have been provided. A first chamber, constitutes a heating chamber
(2), is built for heating the fluid or water, and a second chamber (3)
is for exchanging the heat using the nanofluid. The two chambers
(2,3) are separated by disposing a stainless steel plate (19) in the
system (1).
In the first heating chamber (2), at least one heater (4), is provided
for heating water in the hot chamber (2). The heater (4) is placed at
a distance away from the adjoining surfaces of the chamber (2) such
that the heater does not touch the surface of the chambers (2, 3).
Two RTDsare provided on the bottom side of coling chamber (3) and
two RTDsat the top of the heating chamber (2). One RTD at the
bottom of the heating chamber (2), close to the separating plate (19)
is provided and another RTD is placed at the top of the hearing
chamber (2).
16
The heating chamber (2) is provided with one RTD (6) at the top
through a cover (TCHC) of the chamber (2), while the top cover of
the cooling chamber (TCC) is provided with two RTDs, (7,8) as
shown in figure 1. Total numbers of RTDsare six for two chambers
(2,3) for measuring temperature upto 150 degree Celsius.The RTDs
(6,7,8) at the top should only dip inside the water, while the bottom
RTDs(5,9,10) should not protrude upward exceeding 10-20 mm from
the bottom surface (SPHC,SPCC)of each chamber (2,3).
Six digital display (not shown) with the provision of display of the
temperature of all the RTDs(5,6,7,8,9,10) are provided in the display
board for recording of RTD data from the digital display to a
datalogger (not shown).
A controller with as relay for the cut off of the heater (4) after a set
value of temperature for example, 60 degree Celsius has been
attained, is provided. Temperature of water in the hot chamber (2)
during the heating should not fluctuate +/-0.5°C. i.e. it should
operate between 50.5 and 49.5 degree C. The relay for the cut-off
value operates based on the temperature recorded by the RTD (9)
located at the top cover surface (TCHC) of the chamber (2), as
shown in Figure 1.
17
An energy meter is provided for recording the energy input to the
heater (2) during heating for a definite time. For better mixing and
stirring of the nanoparticles in the chambers (2,3), two mechanical
stirrers (11,11) are placed vertically which passthrough the top cover
(TCHC,TCCC)of each of the chambers (2,3). Each stirrer (11,11) is
driven by a motor (12) and having a shaft rod (13) which is not
allowed to touch the top covers (TCHC, TCCC) of each chamber
(2,3). Means is provided to separate the stirrer rod (13) from the
motor (12) so that the stirrer (11) can be removed at any stage
during the operation from the chambers (2,3). A regulator (14) for
each motor (12) controls the rotation or speed of the motor (2) of
each stirrer (11,11).
As shown in figures lA & lD, a hole (15) each provided at the top
cover (TCHC,TCCC)of each chamber (2,3) ensures pouring of either
water or any liquid during the operation. Each cover for each
chamber (2,3) allows passageof the RTDs,stirrers, and heater.
As shown in figures lA & lC, a drain (16) each allows draining of any
liquid inside the chambers (2,3) through the bottom (SPCC,SPHC)of
each chamber (2,3). The drain (16) at the base of each chamber
(2,3) is installed with a valve so that the chambers (2,3) can be
emptied as necessary.
18
The base of the heat exchanger (1) is located at a higher level such
that drainage of liquid from each chamber (2, 3) is facilitated. A
separate mounting/framework holds the motor-stirrer assembly (11,
12). The separate mounting/framework for the stirrer and motor
assembly (11,12) facilitates easy removal of the chamber assembly
(2,3).
Proper insulation around an open outer surface of each chamber
(2,3) ensures minimization of the heat loss. Plug points and switches
along with fuse in a switch board are provided for six RTDs, one
heater and two strirrers. An electric cable is provided for connecting
the switch board to the plug point.
As shown in figure - lA, a hole passage (17) for the stirrer (11)
including a hole with lid (15) is provided on the top cover of the
heating chamber (TCHC).As shown in Figure ID, a hole passage(18)
for the stirrer (11) is provided on the top cover of the cooling
chamber (TCCC).

WE CLAIM:
1. A System for convective heat transfer in a heat exchanger
by adapting a high performing heat-carrying fluid, the
system comprising:
a heat exchanger having at least two chambers (2,3),
a first chamber (2) accommodating water for heating
by a heater (4) disposed therein, a second chamber
(3) filled with nanofluids for cooling the hot water in
the first chamber (2) through indirect heat exchange,
wherein the first chamber (2) and the second chamber
(3) is separated by a stainless plate (19).
2. The system as claimed in claim 1, wherein the first chamber
(2) and the second chamber (3) each has a top cover
(TCHC, TCCC) and a bottom cover (BPHC, BPCC)wherein
the top cover of the first chamber (2) has a single RTD (6),
the bottom cover of the first chamber (2) has two RTDs(9,
10), and wherein the top cover of the second chamber (3)
has two RTDs (7,8), the bottom cover of the second
chamber (3) has a single RTD(5).
20
3. The system as claimed in claim 1 or 2, wherein each RTDis
provided with a digital display means for displaying
temperature data, and wherein the digital display means are
connected to a datalogger.
4. The system as claimed in claims 1 to 3, wherein a controller
with a cut-off relay is provided to switch off the heater (4)
when the display means relating to the RTD (9) exhibits a
temperature exceeding a threshold value.
5. The system as claimed in claim 1, wherein the first and
second chamber (2, 3) each comprises a stirrer (11, 11)
provided with a motor (12) including a shaft rod (13) for
stirring and mixing the contents in the chambers (2,3), and
wherein each motor (12) is equipped with a regulator (14).
6. The system as claimed in claim 1 or 5, wherein a hole (15) is
provided on the top cover (TCHC)of the first chamber (2),
and wherein a hole passage (18) is provided on the top
cover (TCCC)of the secondchamber (3).
21
7. The system as claimed in claim 1, wherein the heatconductive
fluid is prepared from alumina nanoparticles
added with sodium hexa-metaphosphate.
8. A System for convective heat transfer in a heat exchanger
by adapting a high performing heat-carrying fluid as
substantially described and illustrated herein with reference
to the accompanying drawings.