FIELD OF INVENTION
The present invention relates to a method and an apparatus to induce instability and
increase in localized heat transfer rate by convection of nanoparticle suspension. More
particularly, the invention relates to a system for implementing a method of achieving
an improved heat transfer performance in a heat exchanger by the mechanism of
instability of nanofluid convection.
The field of application is broadly a very general one, starting from the case of indirect
heat exchanger to the case of the passive core decay heat removal system for
advanced heavy water reactor, specifically nuclear reactor.
BACKGROUND OF THE INVENTION
Ever growing pursuit of research community to find the availability of a superior heat
conducting fluid has led the exploration of the development of new fluid in the area of
heat transfer and fluid mechanics. Recent technological progresses have made the use
of nanofluid where a metallic or non-metallic oxide nanoparticle in a small volume is
mixed in water or other solvent. Initial findings on higher level of thermal conductivity
value of nanofluids have prompted the researchers to advance the further use of
nanofluids 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. Most of the studies published in the literature regarding heat transfer
of nanofluid concerns with the enhancement of the thermal conductivity of nanofluid,
and the related model development for the same. Four possible mechanisms for
increase in the thermal conductivity have been proposed by Keblinski in the published
literature titled as "mechanism of heat flow in suspensions of nano-sized partides
(nanofluids)". These are: (i) Brownian motion (ii) Molecular level layering of the liquid at
the liquid/particle interface (iii) Clustering effect from the nanoparticles and (iv) the
nature of heat transport in the nanoparticles. Metal oxide nanofluids are shown to have
higher thermal conductivity than that of base fluid by Lee et al. in their studies titled
"measuring thermal conductivity of fluids containing oxide nanoparticles". Studies on
water Cu nanofluids by Xuan and Li titled as "investigation on convective heat transfer
and flow features of nanofluids" with a concentration approximately of 2% by volume
confirmed a heat transfer co-efficient of 60% higher than the heat transfer co-efficient
of water.
Following the review of the effect of several parameters, J.Buongiomo in published
literature titles as "convective transport in nanofluids", postulated that Brownian
diffusion and thermophoresis are the important mechanisms in nanofluids for
convective transport. Hwang et al. in their study titled as "buoyancy driven heat
transfer of water based AI2O3 in a rectangular cavity" published in international Journal
of Heat and mass transfer investigated natural convective instability and heat transfer
characteristics of water-based AI2O3 nanofluids when heated from underneath ?.
rectangular cavity to conclude that the natural convection of water-based AI2O3
nanofluids was more stable than the base fluid in a rectangular cavity when heated
from the underneath. When the volume fraction of nanoparticles increases, the size of
nanoparticles deaeases, or the average temperature of nanofluids inaeases. The ratio
of heat transfer co-efficient 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.
Enhanced heat transfer performance of nanofluids may be attributed to the
characteristics of nanoparticles for (i) increased surface area and heat capacity of the
fluid (ii) higher number of collisions and interactions amongst the fluid, particles and the
flow passage surface which enhances the heat transfer performance of nanofluids (iii)
higher turbulence and mixing fluctuation of the fluid. Apart from the same, convection
effect due to temperature gradient and concentration gradient have drawn the
researchers towards the study of Soret effect and Dufour effect in nanofluid. A strong
destabilizing effect due to combined behavior of Brownian motion and thermophoresis
of nanoparticles has been found in the analytical study for the instability of natural
convection of nanofluids between two plates, heated from below referred in the
instability studies made by Tzou and titled as "Thermal instability of nanofluids in
natural convection". Tzou and Kim et al. in their study titled as "analysis of convective
instability and heat transfer characterise of nanofluids" have mostly dealt with the
theoretical analysis and the scale analysis for nanofluid instability during natural
convection. Kim et al. studied the effect of nanoparticles on the convective instability
and convective heat transfer characteristics of nanofluids, under one-fluid model, by
introducing a new factor, which measures the ratio of the Rayleigh number of a
nanofluid to that of a base fluid. In a binary component system, an applied temperature
gradient results in a mass flow by means of Soret effect. Using the linear stability
theory under one-fluid model, theoretical investigation has been made by Kim on
convective instabilities in binary nanofluids where in Dufour effect, an effect of heat
transfer due to concentration gradient, has been analyzed in addition to Soret effect.
The same analysis confirmed that the Soret and the Dufour effects made the nanofluids
unstable. Also the initial denser concentration of nanoparbcles enables the nanofluids to
be more unstable. Numerical simulation has been used to study the effects of thermal
diffusion in a fluid suspension of alumina nanoparticles enclosed between two
differentially heated horizontal, relatively closely spaced plates (Benard configuration).
Different modes of convective instabilities have been noted to be present in the system,
which are associated with the gravity force and the density differences induced by
concentration gradients.
Patent in connection with the use of nanofluid can be traced to the early work by Choi
and Eastman in their studies titled as "extended heat transfer using nanofluids" have
devised a method of and apparatus for enhancing heat transfer in fluids such as
deionized water, ethylene glycol by dispersing Nanocrystalline particles of substances
such as copper, copper oxide, aluminum oxide or the like in the fluid. Also found is
another use of metallic nanoparticles combined with caboxylates to provide excellent
corrosion protection and improved thermal conductivity properties in functional fluids
such as lubricants and grease in the European patent of Maes et al. titled as "Heat
transfer fluid containing nanoparticles and carboxylates". While most of the above
works and the patents are more focused towards the study of the enhancement of
thermal conductivity of the nanofluids, this concept focuses on the use of convective
heat transfer of nanofluids. The scope of the further research to investigate the
consequent nature of instability formed inside nanofluid due to natural convection
induced by side wall heating lies in the background of the present invention.
References;
[1] S.Choi, enhancing thermal conductivity of fluids with nanoparticles, FED 231(1995)
99-103.
[2] Choi, S.Z. Zhang, W.Yu, F. Lockwood, and E.GruIke, Anomalously thermal
conductivity enhancement in nanotube suspensions. Appl.Phys.Lett 79(14)(2001), 2252-
2254.
[3] Eastman, J,A, Choi, U, S, Li, S, Yu, W & Thompson, L, J, 2001, Anomalously
increased effective thermal conductivities of ethylene glycol-based nanofluids containing
copper nanoparticles, AppI, Phys. Lett, vol.76(6)(2001)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 Magnefsm, second ed. Oxford
University press, Cambridge, pp.435-441.
[6] Hamilton R. & 0. Grosser, 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. Int. J. 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, S., and Eastman, J.A., Measuring Thermal ConductJ9vity of
fluids containing Oxide Nanoparticles, J. Heat Transfer, vol.l21,pp.280-289, 1999.
[10] Xuan, Y. M., Li, Q., (2003). Investigation on convective heat transfer and flow
features of nanofluids, ASME J. Heat Transfer, 125, 151-155.
[11] J. Buongiomo, Convective Transport in Nanofluids, ASME J. Heat Transfer,
128(2006) 240-250.
[12] K.S. Hwang, J.H.Lee, S.P. Jang, Buoyancy-driven heat transfer of water-based
. AI2O3 nanofluids in a rectangular cavity. International Journal of Heat and Mass Transfer
50(2007) 4003-4010.
(13] D.Y. Tzou, Thermal instability of nanofluids in natural convection. International
Journal of Heat and Mass Transfer 51 (2008) 2967-2979.
[14] J.Kim, Y.T. Kang, C.K. Choi, Analysis of convective instability and heat transfer
characteristics of nanofluids, Phys, Fluids, 16(2004) 2395-2401.
[15] J.Kim, Y.T. Kang, C.K. Choi, Soret and Dufour effects on convective instabilities in
binary nanofluids for absorption application. Int. J. Refrigeration 30(2007) 323-328.
[16] Savino, R., Paterna, D., Thermodiffusion in nanofluids under different gravity
conditions. Phys. Fuids, vol.20(2008) 017101.
[17] S.U.S. Choi and J.A. Eastman, Extended heat transfer using Nanofuids, patent No.
US 6221275 Bl, April 24, 2001.
[18] Jean-Pierre Maes, C.Libot and P. Roose and S.Lievens, Heat Transfer Fluid
containing nanoparticle and caboxylates, European patent No. EP 1 167 486 Bl, 2006.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an apparatus and a method to
induce instability and increase in localized heat transfer rate by convection of
nanoparticle suspension which is capable of enhancing the heat transfer in localized
zone by instability mechanism of natural convection of nanofluid.
Another object of the invention is to propose an apparatus and a method to induce
instability and increase in localized heat transfer rate by convection of nanoparticle
suspension which is able to obtain a stable nanofluid form the dispersion of
nanoparticles in a fluid.
A further object of the invention is to propose an apparatus and a method to induce
instability and increase in localized heat transfer rate by convection of nanoparticle
suspension which develops the instability of natural convection of the nanofluid inside
an enclosure of square cavity cross section by indirect heat exchange from side wall
heating.
A still further object of the invention is to propose an apparatus and a method to induce
instability and increase in localized heat transfer rate by convection of nanopartide
suspension which is capable of developing the heat transfer enhancement from the
natural convection of the nanofluid inside the enclosure.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - shows the SEM picture of AI2O3 nanoparticles in as received condition.
Fig.2 - shows the apparatus according to the invention (an experimental setup with
Cavity)
DETAILED DESCRIPHON OF A PREFERRED EMBODIMENT OF THE
INVENTION
The present invention discloses an apparatus devised for the
improvement/enhancement of heat transfer performance by the instability mechanism
of natural convection of nanofluid. Nanofluid contained in an enclosure of square aoss
section is proposed to be heated from one vertical side. The instability due to the
nanofluid convection leads to heat transfer enhancement in different locations of the
flow in the enclosure.
The apparatus (A) comprises of a heat exchanger unit (H), containing two chambers
(1,2), one containing heating mechanism (1) and the other chamber (2) where the
instability of flow due to natural convection of having nanofluid has the potential to
cause the enhancement of heat transfer. The steps involved are:
• Forming a nanofluid
• Sidewall heating of the chamber containing nanofluid by indirect method.
Commercially available aluminum oxide nanopartides have been used for the
preparation of AI2O3 water suspension or nanofluid. The Scanning Electron Microscope
(SEM) image of the AI2O3 nanopartides in as received condition has been depicted in
Rg.l. Different concentrations of alumina-water nanofluid without any addition of
surfactant were prepared. For the operational purpose, the concentration in different
amount has been used.
ng.2 illustrates the experimental set up of a heat exchanger, consisting of two
chambers (1,2) made up of plexiglass where two chambers have been provided. One
chamber (1) equipped with heater (6), has been termed as a heating chamber (1) and
is used for the purpose of heating the fluid or water. The outer chamber (2) adjacent to
the heating chamber, is separated by a stainless steel plate (3) for exchanging the heci;
using a nanofluid. The heater has been placed at a distance away from the adjoining
surfaces of the chamber such that the heater (6) does not touch the surface of the
Plexiglass surface (7) of the system. One Resistance Temperature Detector (RTD)
passing through the top cover of the heating chamber touches the free top surface of
the liquid/water. Another RTD is inserted through the bottom surface of the heating
chamber.
10
The square cavity chamber (2) containing nanofluid has been equipped with four
numbers of Resistance Temperature Detectors (RTDs). Total numbers of RTDs are six
(RTDl to RTD6) for two chambers (1,2) with the capability of measuring temperature
up to 150 degree Celsius. Two RTDs (RTDl and RTD3) pass through the top cover of
the nanofluid chamber (2) to dip near the free surface of the chamber. There are two
RTDs (RTD2 and RTD4) placed at the bottom surface of the nanofluid chamber (2) that
protrude through the bottom surface. A data logger records the temperature of all RTDs
apart from six digital displays (not shown) with the provision of display of the
temperature of all the RTDs. A controller with a relay for the cut off of the heater
beyond a set value of 50 degree Celsius has been connected along with the provision
for the heater (6) to operate between 50.5 and 49.5 degree C. This implies that the
temperature of water in the hot chamber (1) during the heating is allowed to fluctuate
within the tolerance limit of +/-0.5°C. The heater (6) trips once the water temperature
reaches 50.5 degree, following which the water temperature starts drooping and again
it restarts at a temperature of 49.5 degree Celsius. The relay for the cut-off value
operates based on the temperature recorded by the RTD (RTD6) located at the top
cover surface of the hot chamber, as shown in ng.2. Two motor (8,9) operated
mechanical stirrers (4,5) with regulators (10,11) have been installed through the centre
of the top covers of each chamber for stirring the fluid inside the chamber as and when
required for better mixing and the stirring. Any fluid can be poured inside through a
hole provided with cover located at the top cover of each chamber. A drain with a valve
at the bottom of each chambers allows the easy draining of any liquid inside the
11
cnamDer. Proper insulation around an open outer surface of each chamber ensures the
minimization of the heat loss.
The experiment has been planned to operate in the following sequence. A volume of 2
liter of water/fluid in heating chamber (1) is scheduled to be heated to a temperature of
50 degree following which the heater trips. A nanofluid suspension of 0.5 to 2 vol% of
aluminum oxide will be prepared and added to the cooling chamber (2). The volume c*
nanofluid added is the same as that of the water in the heating chamber. The
temperature of nanofluid, added at room temperature, starts rising following the heat
transfer from the heating chamber (1). The heater trips and again starts due to the
relay operation when the temperature of water in hot chamber is kept in an average
temperature of 50 degree Celsius. The continuous increase of nanofluid temperature is
___ r
manifested by the recordings of the RTDs. The instability is caused by the inaease of
the temperature of binary mixture of nanofluid. The temperature gradient leads to the
Soret effect as well as Dufour effect in this case for nanofluid. This instability in the
nanofluid is prone to the localized heat accumulation zone and the directional increase
of temperature.
WE CLAIM
1. A method to induce instability and inaease in localized heat transfer rate by
convection of nanoparticie suspension comprising:
providing scheduled volume of water/fluid in heating chamber (1);
providing nanofluid suspension of aluminium oxide in the nanofluid chamber (2);
characterised in that,
heating of w/ater/fluid in heating chamber (1) is carried out till the heater (6)
trips at a preset temperature w/hen the temperature of the nanofluid starts rising
following the heat transfer from the heating chamber (1) and the increase of
nanofluid temperature is recorded by the RTDs.
2. A method as claimed in claim 1, wherein the tripped heater starts again due to
relay operation to keep the temperature of the water/fluid in the hot chamber to
an average temperature of 50 degree Celsius.
3. A method as claimed in claim 1 and 2, wherein the nanofluid suspension added
to the nanofluid chamber (2) is of 0.05 vol% to 0.2 vol% of aluminum oxide.
4. A method as claimed in claim 1 to 3, wherein the volume of nanofluid added in
chamber (2) is the same as that of the water/fluid in the heating chamber (1).
5. A method as claimed in claim 1 to 4, wherein the nanofluid is added in chamber
(2) at room temperature.
6. An apparatus (A) to induce instability and increase in localized heat transfer rate
by convection of nanopartide suspension comprising:
a heat exchanger unit (H), the said heat exchanger consists of two chambers
(1,2);
hot chamber (1) carrying the heater for heating the fluid;
nanofluid chamber (2) for carrying the nanofluid suspension;
two motors (8,9) operated mechanical stirrers (4,5) with regulators (10,11);
a plurality of resistance temperature detectors (RTDl to RTD6) disposed in both
the chambers (1,2) for recording the temperature;
six digital displays for displaying the temperature of all the RTDs;
characterised in that,
the apparatus (A) is equipped with a controller and a relay for cutting off the
heater (6) beyond a set value allowing the temperature of the fluid in the hot
chamber (1) to fluctuate within the tolerance limit of ±0.5°C, wherein both
chambers (1,2) are properly insulated for minimising the heat loss.
7. An apparatus as claimed in daim 6, wherein both the chambers (1,2) are made
of plexiglass (7) and separated by stainless steel plate (3).
8. An apparatus as claimed in daim 6 and 7, wherein four RTDs (RTDl to RTD4) for
chamber (2) and two RTE)s (RTDS and RTD6) for chamber (1) are disposed with.
9. An apparatus as claimed in claim 6 to 8, wherein two RTDs (RTDl and RTD3)
pass through the top cover of the nanofluid chamber (2) to dip near the free
surface of the chamber (2) when two other RTDs (RTD2 and RTD4) are placed
at the bottom surface of the nanofluid chamber (2) that protrude through the
bottom surface.
10. An apparatus as claimed in claim 6 to 9, wherein two RTDs (RTDS and RTD6)
are disposed with hot chamber (1) for recording the temperature.
11. An apparatus as claimed in claim 6 to 10, wherein a data logger records the
temperature of all RTDs.

The present invention relates to a method to induce instability and increase in localized
heat transfer rate by convection of nanoparticle suspension. The method comprises of
providing a scheduled volume of water/fluid in a heating chamber (1), providing a
nanofluid suspensions of aluminium oxide and water/fluid in the nanofluid chamber (2)
and then heating the water/fluid in heating chamber (1) being carried out till the heater
(6) trips when the temperature of the nanofluid starts rising following the heat transfer
from the heating chamber (1) and the increase of nanofluid temperature is recorded by
the RTDs.