NANOFLUID - AS A HEAT TRANSFER FLUID IN CONCENTRIC TUBE HEAT EXCHANGER
1
MR. Vatsal. S. Patel,
2Dr. Ragesh. G. Kapadia ,
3Dr. Dipak Deore
1
M.E.[Thermal Engg.] Student, Department Of Mechanical Engineering, S.V.M.
Institute Of Technology, Bharuch ,Gujarat.
2
Professor And Principle , Department Of Mechanical Engineering, S.V.M. Institute Of Technology, Bharuch ,Gujarat.
3
Asst.Professor And HOD, Department Chemical Engineering, S.V.M. Institute Of Technology, Bharuch ,Gujarat.
vicky939077@gmail.com
,rag260475@gmail.com, dipakdeore @gmail.com
ABSTRACT :
New energy-efficient heat transfer equipment stands at the point of a miniature increase in heat flux on one hand and an astronomical one on the other. Heat transfer fluids such as water, minerals, oil and ethylene glycol play a vital role in many industry processes, including power generation, chemical processes, heating and cooling processes, transportation, microelectronics and other micro-sized applications.The poor heat transfer properties of these fluids compared with those of most solids are the primary hindrance of high compactness and the effectiveness of the heat exchanger. The key idea is to exploit the very high thermal conductivities of solid particles that can be several hundreds of times greater than all of the conventional fluids combined. Nanofluids clearly exhibit improved thermo-physical properties such as thermal conductivity, thermal diffusivity, viscosity and convective heat transfer coefficient The application of nanofluids or fluids containing suspensions of metallic nanoparticles to confront heat transfer problems in thermal management is one of the technological uses of nanoparticles that hold enormous promise today.In this review paper, present a nano fluid as a effective heat transfer fluid in concentric tube heat exchanger.Also presents a thermophysical properties ,applications, benefits and its making process.
Keywords—Nano fluid, Concentric tube heat exchanger
I. INTRODUCTION
Heat exchangers play an important part in the field of energy conservation, conversion and recovery.
Several studies have focused on direct transfer type heat exchanger, where heat transfer between fluids occurs through a separating wall or into and out of a wall in a transient manner. There are two important phenomena happening in a heat exchanger: fluid flow in channels and heat transfer between fluids and channel walls. Thus, improvements to heat exchangers can be achieved by improving the processes occurring during those phenomena. Firstly the rate of heat transfer depends on the surface area to volume ratio, which means the smaller channel dimensions provide the better heat transfer coefficient. Secondly, improving the properties of the heat transfer fluids (nanofluids) can yield higher heat transfer coefficient in a heat exchanger. In recent years, modem technologies have permitted the manufacturing of particles down to the nanometer scale, which have created a new class of fluids, called nanofluid.
The main advantage for choosing a concentric configuration, as opposed to a plate or shell and tube
heat exchanger, is the simplicity of their design. As such, the insides of both surfaces are easy to clean and maintain, making it ideal for fluids that cause fouling. Additionally, their robust build means that they can withstand high pressure operations.
They also produce turbulent conditions at low flow rates, increasing the heat transfer coefficient, and hence the rate of heat transfer.
II. NANOFLUID
Nanotechnology provides new area of research to process and produce materials with average crystallite sizes below 100 nm called nanomaterials.
The term “nanomaterials” encompasses a wide range of materials including nanocrystalline materials, nanocomposites, carbon nanotubes and quantum dots. Xuan and Li (2000) [5] explained that due to its nanostructural features, nanomaterials exhibit enhanced properties (mechanical, thermal, physical, chemical), phenomenon and processes than conventional materials. [1] In general, there are four types of nanomaterials: Carbon based nanomaterials (eg:
Carbon nanotubes), Metal based nanomaterials (metal oxides such as aluminium oxides),
Dendrimers (nanosized polymers) and Composites (nanosized clays).
Figure - 1: Nano Layer Of Nano Particle With Base Fluid
When these nanoparicles are suspended in conventional fluids (water, oil, ethylene glycol) called “nanofluids”. A study of Kakac and Pramuanjaroenkij (2009) resulted that the nanolayer works as a thermal bridge between the liquid base fluid and the solid nanoparticles and a nanofluid consists of the liquid base fluid, the solid nanoparticles and the nanolayers as seen in figure 1.
Nanofluids clearly exhibit improved thermo-physical properties such as thermal conductivity, thermal diffusivity, viscosity and convective heat transfer coefficient. The property change of nanofluids depends on the volumetric fraction of nanoparticles, shape and size of the nanomaterials as shown by Yang et al. (2005). Increased thermal conductivity of nanofluid in comparison to base fluid by suspending particles is shown in Table 1.
Materials
Thermal conductivity
(W/mk)
Metallic Materials Copper 401
Silver 429
Nonmetallic Materials
Silicon 148
Alumina
(Al2O3) 40
Carbon Carbon
Nano Tubes
(CNT) 2000
Base fluids
Water 0.613
Ethylene
glycol (EG) 0.253
Engine oil
(EO) 0.145
Nanofluids (Nanoparticle concentration %)
Water /
Al2O3 (1.50) 0.629 EG / Al2O3
(3.00) 0.278
EG-Water /
Al2O3 (3.00) 0.382 Water / TiO2
(0.75) 0.682
Water / CuO
(1.00) 0.619
Table 1: Thermal conductivity of some materials, base fluids and nanofluids[1]
III. PREPARATION OF NANOFLUIDS
To prepare nanofluids by suspending nanoparticles into base fluids, some special requirements are necessary such as even suspension, durable and stable suspension, low agglomeration of particles and no chemical change of fluid. There are three general methods used for preparation of stable nanofluid: (1) Addition of acid or base to Change the pH value of suspension (2) Adding surface active agents and/or dispersants to disperse particles into fluid (3) Using ultrasonic vibration.
Methods for producing nanofluids
The delicate preparation of a nanofluid is important because nanofluids need special requirements such as an even suspension, durable suspension, stable suspension, low agglomeration of particles and no chemical change of the fluid. There are two fundamental methods to obtain nanofluids (Mamut 2004).
Two step process:
This technique is also known as Kool-Aid method which is usually used for oxide nanoparticles. In this technique nanoparticles are obtained by different methods (in form of powders) and then are dispersed into the base fluid. The main problem in this technique is the nanoparticle agglomeration due to attractive Van der Waals forces.
Figure - 2: Two step preparation process of nanofluid
Nanoparticle
Base Fluid
Direct mix Dispersant
Nanofluid Ultrasonic
One step process:
In this process the dispersion of nanoparticles is obtained by direct evaporation of the nanoparticle metal and condensation of the nanoparticles in the base liquid and is the best technique for metallic nanofluids such as Cu nanofluids. The main problems in this technique are low production capacity, low concentration of nanoparticles and high costs. While the advantage of this technique is that nanoparticle agglomeration is minimized. The suspensions obtained by either case should be well mixed, uniformly dispersed and stable in time. Also it should be noted that the heat transfer properties of nanofluids could be controlled by the concentration of the nanoparticle and also by the shape of nanoparticles.
These methods can change the surface properties of the suspended particles and can be used to suppress the formation of particle clusters in order to obtain stable suspensions. The use of these techniques depends on the required application of the nanofluid.
Methods for dispersing particles
Due to the high surface energy of nanoparticles they tend to agglomerate to decrease their surface energy. The agglomeration of nanoparticles causes rapid settling which deteriorates the properties of nanofluids. To keep the nanoparticles from agglomeration they are coated with a surfactant (steric dispersion) or charged to repulse each other in a liquid (electrostatic dispersion).Although the addition of the dispersant could influence the thermal conductivity of the base fluid itself, and thus, the real enhancement by using nanoparticles could be overshadowed.
There are other dispersion methods such as using a high-speed disperser or an ultrasonic probe/bath and also changing the pH value of the suspension. The selection of suitable dispersants depends mainly upon the properties of the solutions and particles and the use of these techniques depends on the required application of the nanofluid.
However metallic nanofluids due to their low thermal conductivity have limited interest but metallic nanofluids especially Cu nanofluids and Ag nanofluids due to their high thermal conductivity are the common nanofluids. More specifically we can say that all the metallic nanofluids compared to oxide nanofluids show much more enhancements so that metallic nanofluids and their volume percent is reduced by one order of magnitude at comparable K enhancements. There are a number of factors other than the thermal conductivity of the dispersed phase which should be considered such as the average size of the nanoparticles, the method employed for the
preparation of the nanofluids, the temperature of measurements and the concentration of the dispersed solid phase. [2]
IV. BENEFITS OF NANOFLUIDS
Nanofluids possess the following advantages as compared to conventional fluids which makes them suitable for various applications involving heat exchange. [1]
1. Absorption of solar energy will be maximized with change of the size, shape, material, and volume fraction of the nanoparticles.
2. The suspended nanoparticles increase the surface area and the heat capacity of the fluid due to the very small particle size.
3. The suspended Nanoparticles enhance the thermal conductivity which results improvement in efficiency of heat transfer systems.
4. Heating within the fluid volume, transfers heat to a small area of fluid and allowing the peak temperature to be located away from surfaces losing heat to the environment.
5. The mixing fluctuation and turbulence of the fluid are intensified.
6. The dispersion of nanoparticles flattens the transverse temperature gradient of the fluid.
7. To make suitable for different applications, properties of fluid can be changed by varying concentration of nanoparticles.
V. APPLICATIONS OF NANOFLUIDS
Nanofluids can be used in broad range of engineering applications due to their improved heat transfer and energy efficiency in a variety of thermal systems. The following section gives a brief idea of different areas of nanofluid applications based on available literatures.
The novel and advanced concepts of nanofluids offer fascinating heat transfer characteristics compared to conventional heat transfer fluids. There are considerable researches on the superior heat transfer properties of nanofluids especially on thermal conductivity and convective heat transfer.
Applications of nanofluids in industries such as heat exchanging devices appear promising with these characteristics.
Kostic reported that nanofluids can be used in following specific areas:
Heat-transfer nanofluids.
Tribological nanofluids.
Surfactant and coating nanofluids.
Chemical nanofluids.
Process/extraction nanofluids.
Environmental (pollution cleaning) nanofluids.
Bio- and pharmaceutical-nanofluids.
Medical nanofluids (drug delivery and functional tissue–cell interaction).
Important points about the heat transfer of nanofluids
Finally it should be noted that the most important reason of using nanofluids is to improve the heat transfer of fluids. Some key points for nanofluids are:
1- The thermal conductivity enhancement ratio increases with increasing particle volume fraction.
2- The sensitivity to volume fraction depends on particle material and base fluid (the sensitivity is higher for particle material with higher thermal conductivity and base fluid with lower thermal conductivity).
3- The thermal conductivity of nanofluids shows higher sensitivity to temperature then that of the base fluid, consequently the thermal conductivity enhancement ratio shows also high sensitivity to temperature.
4- The particle shape also affects the thermal conductivity. Elongated particles show higher thermal conductivity enhancement ratio than spherical particles.
5- The additives used to prevent particle agglomeration seem to increase the thermal conductivity enhancement ratio.
6- The thermal conductivity enhancement ratio increases with acidity.
7- For the suspensions containing the same base liquid and nanoparticles, the thermal conductivity enhancements were highly dependent on the specific surface area (SSA) of the nanoparticles.
8- For the suspensions using the same nanoparticles, the enhanced thermal conductivity ratio decreased with increasing thermal conductivity of the base fluid.
VI. THERMOPHYSICAL PROPERTIES:
The physical properties such as the density, viscosity, and specific heat and thermal conductivity of the nanofluid are calculated using the following published correlations. [4]
The density is calculated from Pak and Cho using the following equation:
ρnf =
∅
ρp + (1-∅
) ρwwhere
∅
is the volume fraction of the nanoparticles, ρp is the density of the nanoparticles and ρw is the density of the base fluid.Drew and Passman suggested the well-known Einstein equation for calculating the viscosity, which is applicable to spherical particles in volume fractions of less than 5.0 vol.% and is defined as follows:
μ
nf = (1 + 2.5∅
)μ
wwhere μ nf is the viscosity of the nanofluid and
μ
w is the viscosity of the base fluid.The specific heat is calculated from Xuan and Roetzel as follows:
(ρCp)nf = ∅ (ρCp)p + (1- ∅ )(ρCp)w where (ρCp)nf is the heat capacity of the nanofluid, (ρCp)p is the heat capacity of the nanoparticles and (ρCp)w is the heat capacity of the base fluid.
The thermal conductivity of the nanofluid is calculated from Yu and Choi using the following equation:
Knf =
[ K p +2 K w+2(K p−K w)(1+ β)3 ∅ K p+2 K w−(K p−K w)(1+β )3 ∅ ]
Kw
where knf is the thermal conductivity of the nanofluid, Kp is the thermal conductivity of the nanoparticles, Kw is the thermal conductivity of the base fluid and
β
is the ratio of the nanolayer thickness to the original particle radius. Normally a value of β = 0.1 is used tocalculate the thermal conductivity of the nanofluid . The properties of the nanofluid shown in the above equations are evaluated from water and nanoparticles at average bulk temperature.
Before starting to determine the convective heat transfer coefficient and friction factor of the nanofluid, the reliability and accuracy of the experimental system are estimated by using water as the working fluid. The results of the experimental heat transfer coefficient and friction factors are compared with those obtained from the Gnielinski equation and Colebrook equation which are defined as follows:
The Gnielinski equation is defined as:
Nu = (f /8)(ℜ−1000)P r 1+12.7 (f /8)0.5(Pr 2/3−1) where Nu is the Nusselt number, Re is the Reynolds number, Pris the Prandtl number and f is the friction factor.
The Colebrook equation is defined as:
1
√ f
= -2.0 log( ε/ D 3.7 + ℜ 2.51 √ f )
where ε is the roughness of the test tube.
Moreover, the Pak and Cho and Xuan and Li correlations for predicting the Nusselt number for nanofluid are compared with the results which are defined as follows:
The Pak and Cho correlation is defined as:
Nunf = 0.021
ℜ
nf0.8Pr
nf0.5The Xuan and Li correlation is defined as:
Nunf = 0.0059 (1.0 + 7.6286 ∅ 0.6886
Pe
d0.001 )ℜnf0.9238Prnf0.4
The Reynolds number of the nanofluid is defined as:
Renf =
ρ nf u m D μ nf
The Prandtl number of the nanofluid is defined as:
Prnf =
μ nf Cpnf K nf
The Peclet number of the nanofluid in Eq. is defined as:
Penf =
u m d p α nf
where dp is the diameter of the nanoparticles.
In order to calculate the Peclet number, the thermal diffusivity of the nanofluid ( α nf) is defined as:
α nf =
K nf ρ nf Cp nf
VII. EXPERIMENTS FOR HEAT TRANSFER ENHANCEMENT WITH USE OF NANOFLUID IN CONCENTRIC TUBE HEAT EXCHANGER:
[1] Parham Naderia et al [6] studies to characterize heat transfer in forced convection, the heat transfer coefficient is one of the important parameters to be determined. In this novel work the volume fraction of SiO2 as in base fluid (water) with average of 20 nm was chosen as 0.1% 0.3% and 0.6%. Results
indicated that addition of small amounts of SiO2 to water increased heat transfer remarkably.
In order to verify the accuracy of the experimental system, a series of experiments with pure water were performed and the experimental results were compared excellently well with Dittus-Boelter correlation (Incropera and DeWitt, 2008) as shown in Fig. 3. SiO2/water nanofluid with dilute loadings of 0.1%. 0.3%, and 0.6% were used in this study. The Reynolds number varied between 7000 and 25,000.
Figure - 3 : Comparison of experimental and numerical Nusselt numbers using pure water with 0.12% 0.3% and 0.6%. SiO2
.
[2] Anchupogu Praveen et al [7] investigate in the paper , a theoretical study has been carried out predict heat transfer coefficient of nanofluids Al2O3 / Water The heat transfer coefficient calculated for different temperature ranging from 25°C to 80°C with volume concentrations ranging from 1 to 5%
and heat transfer coefficient is compared with pure water. The results show that the percentage increase in heat transfer coefficient Al2O3/ Water with nanoparticle concentration was determined.
Figure - 4 : Variation of heat transfer coefficient with Reynolds number, Al2O3/Water nanofluid
Based on Xuan and Li correlation the heat transfer coefficient for Al2O3/Water nanofluids are predicted for the turbulent flow. Heat transfer coefficient is calculated at inlet temperature of the nanofluid is varied from 25 oC to 80 oC and the volume fractions ranging from 0.01 to 0.05 and compared with water.
[3] Om Shankar Prajapati et al [8] investigated In this work turbulent flow forced convection heat transfer of Al2O3-water nanofluid inside an annular tube with variable wall temperature was investigated experimentally.The results of the present investigation are summarized as :Heat transfer increases with increase in mass flux at all given range of percent Al2O3-Water nanofluid and heat fluxes.
Heat transfer increases with addition of the Al2O3- nanoparticles in the base fluid. The results of this experimental work show that heat transfer increases with heat flux applied.
Figure - 5: Variation of Nusselt number of Al2O3- water nanofluid with Heat Flux
[4] Weerapun Duangthongsuk et al [4] present this article reports for experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2 vol.% TiO2
nanoparticles. The heat transfer coefficient and friction factor of the TiO2–Water nanofluid flowing in a horizontal double-tube counter flow heat exchangerunder turbulent flow conditions are investigated.
Figure - 6 : Comparison of heat transfer coefficient obtained from water and that from the 0.2 vol.% of TiO2 nanoparticles dispersed in water.
VIII. CONCLUSION
This review paper presents overview about nanofluid, an exiciting new class of heat transfer fluid. It is concluded that nanofluids are important because they can be considered as a potential candidate for numerous applications involving heat transfer and their use will continue to grow. It was also found that the use of nanofluids appears promising.
The possible reason for this enhancement may be associated with the following:
The nanofluid – enhancing the heat transfer process because the suspended ultra-fine particles remarkably increase the thermal conductivity of the nanofluid.
The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature.
The probability of collision between nanoparticles and the heat exchanger wall increases, due to using higher concentration of coolants, the total heat transfer coefficient increases.
The heat transfer coefficient increases with increase in the concentration of nano particle in base fluid.
It is also presents a sucessfully demonstration of a heat exchanger using nano fluid.
REFRENCES:
[1] Gupta H.K, Agrawal G.D, Mathur J, "An overview of Nanofluids: A new media towards green environment ", IJES,Volume 3, No 1, 2012
[2] Sadollah Ebrahimi, Anwar Gavili, Maryamalsadat Lajevardi ," New Class of Coolants: Nanofluids"
[3] P. Sivashanmugam, "Application of Nanofluids in Heat Transfer".
[4] Weerapun Duangthongsuk, Somchai Wongwises,
"Heat transfer enhancement and pressure drop characteristics of TiO2–water nanofluid in a double- tube counter flow heat exchanger", International Journal of Heat and Mass Transfer 52 (2009), 2059–
2067
[5]Yimin Xuan, Qiang Li,."Heat transfer enhancement of nanofuids", International Journal of Heat and Fluid Flow 21 (2000) 58-64
[6] Parham Naderia, A.Moharrerib, H.Goshayshic,
"An Exprimental Study On The Heat Transfer Performance of Sio2-Water Nanofluid in a Double Pipe Heat Exchanger".
[7] Anchupogu.Praveen, Penugonda Suresh Babu, Venkata Ramesh Mamilla, "Analysis On Heat Transfer In Nanofluids For Al2O3 / Water" , International Journal of Advanced Scientific Research And Technology, Volume 2 (April 2012)
[8] Om Shankar Prajapati, "Effect of Al2O3-Water Nanofluids in Convective heat Transfer" , International Journal of Nanoscience Vol. 1, No. 1 (2012) 1-4
