1. INTRODUCTION A wide variety of industrial processes involve the transfer of heat energy. Throughout any industrial facility, heat must be added, removed, or moved from one process stream to another and it has become a major task for industrial necessity. These processes provide a source for energy recovery and process fluid heating/cooling. The enhancement of heating or cooling in an industrial process may create a saving in energy, reduce process time, raise thermal rating and len gthen the working life of equipment. Some processes are even affected qualitatively qualitatively by the action of enhanced heat transfer. The development of high performance thermal systems for heat transfer enhancement has become popular nowadays. A number of work has been performed to gain an understanding of the heat transfer performance for their practical application to heat transfer enhancement. Thus the advent of high heat flow processes has created significant demand for new technologies to enhance heat transfer. There are several methods to improve the heat transfer efficiency. Some methods are utilization of extended surfaces, application of vibration to the heat transfer surfaces, and usage o f micro channels. Heat transfer efficiency can also be improved by increasing the thermal conductivity of the working fluid. Commonly used heat transfer fluids such as water, ethylene glycol, and engine oil have relatively low thermal conductivities, when compared to the thermal conductivity of solids. High thermal conductivity of solids can be used to increase the thermal conductivity of a fluid by adding small
solid particles to that fluid.
The feasibility of the usage of such suspensions of solid particles with sizes on the order of 2 millimeters or micrometers was previously investigated by several researchers and the following significant drawbacks were observed. 1.The particles settle rapidly, forming a layer on the surface and reducing the heat transfer capacity of the fluid. 2.If the circulation rate of the fluid is increased, sedimentation is reduced, but the erosion of the heat transfer devices, pipelines etc.,increases rapidly. 3.The large size of the particles tends to clog the flow channels, particularly if the cooling channels are narrow. 1
4. The pressure drop in the fluid increases considerably. 5.Finally, conductivity enhancement based on particle concentration is achieved (i.e., the greater the particle volume fraction is, the greater the enhancement — — and and greater the problems, as indicated above). Thus, the route of suspending particles in liquid was a well-known but rejected option for heat transfer applications.
1.1 Emergence of Nanofluids
In recent years, nanofluids have attracted more and more attention. The main driving force for nanofluids research lies in a wide range of applications. Nanofluids, the fluid suspensions of nanomaterials, have shown many interesting properties, and the distinctive features offer unprecedented potential for many applications. Nanofluids are a new class of fluids engineered by dispersing nanometer-sized materials (nanoparticles, nanofibers, nanotubes, nanowires, nanorods, nanosheet, or droplets) in base fluids. In other words, nanofluids are nanoscale colloidal suspensions containing condensed nanomaterials. They are two-phase systems with one phase (solid phase) in another (liquid phase). Nanofluids contains nanoparticles (1 – 100 100 nm) which are uniformly and stably distributed in a base fluid. These distributed nanoparticles, generally a metal or metal ox ide greatly enhance the thermal conductivity of the nanofluid, increases conduction and convection coefficients, allowing for more heat transfer . These distributed nanoparticles have been found to enhance thermo physical properties such as thermal conductivity, thermal diffusivity, viscosity, and convective heat transfer coefficients of base fluids like oil or water. Compared to conventional solid – liquid liquid suspensions for heat transfer intensifications, nanofluids having properly dispersed nanoparticles possess the following advanta ges: ❖
High specific surface area and therefore more heat transfer surface between particles and fluids. High dispersion dispersion sstability with predominant Brownian motion of particles.
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Reduced pumping power as compared to pure liquid to achieve equivalent heat transfer intensification. 2
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Reduced particle clogging as compared to conventional slurries, thus promoting system miniaturization.
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Adjustable properties, including thermal conductivity and surface wettability, by varying particle cconcentrations to suit different applications.
1.2 Hybrid Nanofluids
Recently, a new class of nanofluid, “hybrid nanofluid” is being used to further enhance the heat transfer rate. Hybrid nanofluids are very new kind of nanofluids, which can be prepared by suspending (i) different types (two or more than two) of nanoparticles in base fluid, and (ii) hybrid (composite) nanoparticles in base fluid. A hybrid material is a substance which combines physical and chemical properties of different materials simultaneously and provides these in a homogeneous phase. Synthetic hybrid nanomaterials exhibit remarkable physicochemical properties that do not exist in the individual components. A significant amount of research has been done regarding the properties of these composites and hybrid materials consisting of carbon nanotubes (CNTs) have been used in electrochemical-sensors, bio-sensors, nanocatalysts, etc. but the use of these hybrid nanomaterials in nanofluids has not developed as such. Work on hybrid nanofluids is very limited and a lot of experimental study is still being done. The main objective of synthesizing hybrid nanofluids is to obtain the properties of its constituent materials. A single material does not possess all the favorable characteristics required for a particular purpose; it may either have good thermal properties or rheological properties. But in many practical applications, it is required to trade-off between several properties and that is where the use of hybrid nanofluid comes. Furthermore, the hybrid nanofluid is expected to yield better thermal conductivity compared to individual nanofluids due to synergistic effect. Carbon nanotubes have a multitude of unique properties like its physical strength, chemical stability, mechanical resistance, very high electrical and thermal conductivity, etc. These characteristics have attracted the researchers towards carbon nanotubes as well as in development of a new category of hybrid nanomaterials consisting of a composite of carbon nanotubes with metallic, semi-conductive or non-conductive nanoparticles.
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1.3 Nanoparticle material
The nanoparticle materials include chemically stable me tals(e.g. gold, copper), metal oxides (e.g. alumina, silica, zirconia, titania), oxide ceramics (e.g. Al2O3, CuO), metal carbides (e.g. SiC), metal nitrides (e.g. AIN, SiN), carbon in various forms (e.g. diamond, graphite, carbon nanotubes, fullerene) and functionalized nanoparticles . Most of the research works were focused mainly on water and ethylene glycol based nanofluids, very few reports of the synthesis of oil- based nanofluids have been found. Xuan and Li have found that oil based nanofluids exhibited better enhancement of heat transfer characteristics compared to water based nanofluids, and that the viscosity of the oil could be crucial for the dispersion and stability of nanofluids . Hwang et al.have also showed similar conclusion that a higher thermal conductivity enhancement can be obtained if a base fluid of lower thermal conductivity is used. Therefore, oil-based nanofluids containing carbon nanotubes, TiO2, CuO, Al2O3, AlN and SiO2, for industrial and engineering applications, have attracted some more attention in recent. Hence the project aims on preparation of hybrid nanoparticle of TiO2 & CuO in which CuO adds the property of high thermal conductivity and TiO2 adds stability of nanofluid.
1.4. Thermophysical properties of Nanofluids:
Thermo physical properties of the nanofluids are qu ite essential to predict their heat transfer behavior. It is extremely important in the control for the industrial and energy saving perspectives. There is great industrial interest in nanofluids. Nanoparticles have great potential to improve the thermal transport properties compared to conventional particles fluids suspension, millimetre and micrometer sized particles. In the last decade, nanofluids have gained significant attention due to its enhanced thermal properties. Experimental studies show that thermal conductivity of nanofluids depends on many factors such as particle volume fraction, particle material, particle size, particle shape, base fluid material, and temperature. Amount and types of additives and the acidity of the nanofluid were also shown to be effective in the thermal conductivity enhancement. Hence the project also pays attention to study all these factors.
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2. LITERATURE SURVEY 2.1 Synthesis of Alumina-Copper/water hybrid nanofluids using two step method and its thermo physical properties [1] S.Suresh, K.P.Venkitaraj, P.Selvakumar, M.Chandrasekar
OBJECTIVE
To synthesis Al2O3-Cu hybrid nanoparticle by hydrogen reduction technique and to study thermal as well as physical properties of different nanofluid with water as base fluid at different concentration of hybrid nanoparticle. PROCEDURE
Al2O3-Cu hybrid nanoparticles were synthesized by hydrogen reduction technique from the powder mixture of Al2O3 and CuO in 90:10 weight proportions obtained from a chemical route synthesis. The steps consisted of following stages: spray-drying, oxidation of precursor powder, reduction by hydrogen and homogenisation. The SEM and XRD were then done on the obtained hybrid nanopowder. Then Al2O3/water hybrid nanofluids with volume fractions from 0.1 to 2% were prepared using two step method by dispersing it in deionized water. Thermal conductivity was then measured with the help of a thermal property analyser. The viscosity of the nanofluid was measured using Brookfield cone and plate viscometer. The stability of the nanofluids were measured by using pH and Zeta potential measurements. RESULT AND DISCUSSION
It was found that the nanofluids were least stable at the isoelectric point. At isoelectric point, the repulsive forces between particles are too weak, allowing the particles to approach each other and eventually agglomerate affecting the stability of the suspensions. It was observed that pH values of the hybrid nanofluid increased with increase in concentration indicating stability of the nanofluids are dependent on the volume concentration of nanoparticles. From the values obtained from the thermal property analyzer it was found that thermal conductivity decreased with time ultimately reaching a constant value. Also thermal conductivity increased with increase in nanoparticle concentration in nanofluid. The viscosity measurements of the nanofluids showed that viscosity was independent of shear rate. The viscosity of the hybrid nanofluid increased with volume concentrations. 5
CONCLUSION
The experimental results show that there was a significant enhancement in effective thermal conductivity of the prepared hybrid nanofluids compared with deionised water. It was also observed that the thermal conductivity of nanofluids increased remarkably with increase in volume fraction of nanoparticles. Viscosity measurements indicate that Al2O3/water nanofluids behave as Newtonian fluids. The viscosity values for hybrid fluids are higher when compared to the viscosity of alumina/water nanofluids.
2.2 Experimental study on heat transfer and rheological characteristics of hybrid nanofluids for cooling applications [2] D. Madhesh &S. Kalaiselvam OBJECTIVE
Effects of Nusselt number, Peclet number, Reynolds number, heat transfer coefficient and pressure drop were investigated for various volume concentrations of copper-titania hybrid nanocomposite (HyNC). PROCEDURE
The HyNF was prepared by the following procedure: Initially 5 g of Titania was dispersed in aqueous solution. Subsequently 0.5 g of Copper Acetate was mixed in aqueous solution by intense stirring. Ascorbic acid and Sodium borohydride which act as reducing agents were then added to the blend of Titania and Copper Acetate solution. The prepared solution was allowed to stand for 2 hours at 45 °C temperature and ambient pressure to obtain HyNC colloids. The HyNC colloids were then washed and filtered, followed by vacuum drying. To prepare the HyNF, the as prepared HyNC powder was re-dispersed in the base fluid, in volume concentrations ranging from 0.1% to 1.0% using the ultrasonic vibrator.Various characterization techniques such as XRD and EDAX were performed in order to confirm the material homogeneity and particle size are within limits. RESULTS AND DISCUSSION
The fine dispersion of nanoparticles in the base fluid showed rise in thermal conductivity. Thermal conductivity depended not only on particle size and temperature but also on chemical 6
parameters such as Hamaker constant, Zeta pot ential, PH value and ion concentration. Formation of fine crystalline and heat conductive nature of the Copper n anoparticles on the surface of Titania particles was different and an important reason to dissipate the thermal transport to the surrounding fluids.
CONCLUSION
Without surfactant, the surface functionalized and highly crystalline nature of HyNC has contributed to creating effective thermal interfaces with the fluid medium, thereby achieving improved thermal conductivity and heat transfer potential of nanofluids.For the volume concentration up to 0.7%, convective heat transfer coefficient of HyNF was enhanced by 59.3%.
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3. SCOPE AND OBJECTIVES 3.1 PROJECT VISION
The preliminary study suggests that nano fluids have begun to replace normal coolants used in heat transfer equipments used in process industry. Even though most of the nano fluids possess several inherent advantages such as , high thermal conductivity and improved heat transfer characteristics resulting in a higher economy in industries , proper choice of nano fluids with optimal characteristics is still a barrier due to their low thermal and oxidative stability and poor temperature characteristics. With the addition of certain synthetic and natural additives, the drawbacks of such Nano fluids can be overcome. In such scenario we are trying to incorporate higher heat transfer character of copper which has a decreased stability and TiO2 which has a higher stability but poor thermal conductivity in a single base fluid. This study aims at achieving the following objectives:
Preparation of hybrid nanoparticles of copper and TiO2
Characterisation of prepared samples using 1. X-ray diffraction 2. FTIR 3. SEM 4. EDAX
To ensure the size of prepared samples are within limits
Preparation of Nano fluids by dispersing the prepared sample in base fluids( water and ethylene glycol)
Vary the concentrations of fluids from 0.01% to 0.1% (wt %)
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Visual observation of stability or determination of settling time of samples
Application of prioritised samples in miniature heat exchanger to find the enhancement ratio.
Find the optimal fluid in terms of stability ,cost and heat transfer characteristics
Find the optimal temperature range of nanofluids
4. MATERIALS AND METHOD Table 4.1: Materials Used Name
Type
Copper acetate
Ascorbic acid Titanium dioxide
Deionized water
Ethylene glycol
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Table 4.2: Apparatus used Apparatus
Lab
Hot air oven
ct lab
Sonicator
mechanical nano technology research lab
Centrifuge dryer
mechanical nano technology research lab
FTIR
Chemical department ftir lab
XRD
chemistry xrd
idax
mechanical nano technology research lab
Weighing balance Beakers Vials Cultural tubes
Table 4.3: Sample Details Sample
Composition
Composition
(base fluid water)
(base fluid ethylene glycol)
Wt %
Wt %
1
0.01
0.01
2
0.03
0.03
3
0.05
0.05
4
0.07
0.07
5
0.09
0.09
6
0.1
0.1
10
4.1 METHODS USED
In order to study the properties and characteristics of various samples, different tests were performed on the samples prepared .They are listed below. 1. FTIR 2. XRD 3. EDAX 4.4.1 FTIR Fourier transform infrared spectroscopy (FTIR) is a technique which is used to obtain an inf rared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high spectral resolution data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer which measures intensity over a narrow range of wavelengths at a time.
4.4.2 XRD
X-ray diffraction (XRD) is a powerful nondestructive technique for characterizing crystalline materials. It provides information on crystal structure, phase, preferred crystal orientation (texture), and other structural parameters, such as average grain size, crystallinity, strain, and crystal defects. X-ray diffraction peaks are produced by constructive interference of a monochromatic beam of X-rays diffracted at specific angles from each set of lattice planes in a sample. The peak intensities are determined by the distribution of atoms within the lattice. Consequently, the X-ray diffraction pattern is the fingerprint of the periodic atomic arrangements in a given material. A search of the ICDD (International Centre for Diffraction Data) database of X-ray diffraction patterns enables the phase identification of a large variety of cr ystalline samples. 4.4.3 DLS
Dynamic Light Scattering (DLS) measures the translational diffusion coefficients Dt of nanoparticles and colloids in solution by quantifying dynamic fluctuations in scattered light. Sizes and size distributions, in turn, are calculated from the diffusion coefficients in terms of hydrodynamic radius r h or hydrodynamic diameter dh.DLS is suitable for ensemble measurements
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ranging from r h values of 0.2 nm up to 5,000 nm. Wyatt offers high-throughput, automated DLS as well as conventional, cuvette-based formats.
5. PROCEDURE 5.1 PREPARATION OF HYBRID NANO PARTICLE
About 0.5g of copper acetate , 5g of ascorbic acid and titanium dioxide was mixed in aqueous medium The mixture was well agitated and set in a hot air oven at 45 degree Celsius for two hours The resulting solution was washed several times and dried using centrifugal drying technique Following centrifugal drying the obtained dry powd er was finely ground using mortar. The sample was preserved in vacuum desiccator.
5.2 PREPARATION OF NANO FLUIDS
For the preparation of nanofluids base fluids selected were water and ethylene glycol Two sets of samples were prepared each set with different base fluid Each of the set consisted of 6 samples of with concentration varying from 0.01% to 0.1%(mass percentage) of hybrid nanoparticles .(concentration range was arbitrarily chosen preferring lower concentrations.) For fine dispersing the powder in base fluid all samples are kept in sonication bath for a minimum time period of 12 hours
5.3 PREPARATION OF SAMPLE FOR DLS
To 15 ml of DI water about 5 mg of a hybrid nanopowder is added. The mixture is then kept in sonication bath for 10 to 15 minutes. After sonication a uniform dispersed solution was formed which was taken for DLS analysis. 5.4 XRD AND FTIR
For XRD and FTIR dry powder form of hybrid is used. In FTIR the powder is initially pelletized before performing analysis.
5.5 STABILITY ANALYSIS
For stability analysis visual observation of the synthesised nanofluids is preferred. The prepared samples of different concentrations are kept under surveillance till they settle. The time from synthesising to settling is considered as the stable period for storage of the n anofluids. 12
RESULTS AND DISCUSSION
6.1 FTIR Analysis
Figure 6.1: FT-IR Spectrogram
6.1.1 Discussions
Strong band absorptions were observed in the region of 3500 – 3200 −1and 1636.3 −1 caused by corresponding to O – H stretching vibrations. The stretching vibrations of Cu-O can be seen at frequencies of 670.9 −1. The stretching vibrations of Ti-O bond can be seen at 500-600
−1
indicating the presence of TiO2.
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6.2 XRD Analysis
Figure 6.2: XRD analysis
Fig 6.2 represents the XRD of the material Rigaku Miniflex 600 Cu - K alpha radiation source (= 1.504) 10 to 90 degree with an increment of 0.12 angle was used to determine the diffraction of the material. The synthesized hybrid nano powder was taken for XRD. The peaks found were satisfactory confirming the following results on comparison with a standard range of theta values of TiO2 and Cu: ● The major peak in the range of 20 - 30 degrees of theta are indicative of the presence of 2 and
a minor peak is also found in the neighbourhood of 55 degree for TiO2.
● The presence of CuO is confirmed from the peaks appearing near to 40 degrees and at 62 degree of theta. 14
6.3 DLS Analysis
Fig 6.3: DLS Analysis
On conducting DLS the following results were obtained. ● The observed major peak was found to be at the size of 389.1nm. ● The size of the sample can be averaged as 341.9 nm which is quite satisfactory to confirm the particle size in the nano region. ● The PDI (particle distribution index) is an indication of uniformity of the mixture. A higher PDI indicates less uniformity in the sample. ●
The PDI observed is 0.216 which implies that the sample is uniform in size and has a lower settling tendency.
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● The shifted peaks towards the right indicate the presence of large sized particles in the sample with a lower intensity. The large sized particles are responsible for settling tendency of nanofluids. ● From the results of DLS analysis the resultant quality of the sample is obtained as good. It resembles the prepared sample is having a uniform size distribution. 6.5 Stability Analysis
The stability of the prepared sample were primarily analyzed by visual observation and the time period for each sample to settle down was noted. The results are tabulated below: Table 6.1
SL NO
BASE FLUID
CONCENTRATION
SETTLING PERIOD
( Wt%)
(days)
1
Water
0.01
-
2
Water
0.03
-
3
Water
0.05
3
4
Water
0.07
2
5
Water
0.09
4
6
Water
0.1
1
7
Ethylene Glycol
0.01
7
8
Ethylene Glycol
0.03
3
9
Ethylene Glycol
0.05
2
10
Ethylene Glycol
0.07
1
11
Ethylene Glycol
0.09
1
12
Ethylene Glycol
0.1
1
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(a)
(b)
(c)
(d)
Fig: 6.5.a to 6.5.d - represents nanofluids with base fluid water for a given time period. (a) Day 1 (b) Day 2 (c) Day 3 (d) Day 4
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(e)
(f)
(g)
(h)
Fig: 6.5.e to 6.5.h - represents nanofluids with base fluid ethylene glycol for a given time period. (a) Day 1 (b) Day 2 (c) Day 3 (d) Day 7
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6. CONCLUSION In any chemical industries heating and cooling utilities have a high significance,since throughout any industrial facility, heat must be added, removed, or moved from one process stream to another and it has become a major task for industrial necessity. They are a major factor of cost expenditure. The incorporation of these nanofluids as a coolant produces a large reduction in the cost expenditure by reducing the utilities requirement since nanofluids are having a high heat transfer enhancement than the common process fluids or base fluids. By means of combing through relevant papers we have shortlisted that a hybrid nanofluids have better thermal conductivity than the common fluids like water and other synthesised nanofluids. We realised that two step method is the most appropriate and stable method than the one step method for the hybrid nanofluids through investigation o f different journals. The hybrid nanoparticle Cu-TiO2 is selected on the basis of following reasons: ● High conductivity of Cu ● High stability of TiO2 Since the high thermal conductivity of Cu particle enhances the thermal of conductivity of the nanofluid while it destabilizes also. In Order to encounter the stability problem TiO 2 is used for high stabilization in the nanofluid. Nanofluids have different effects and variation with the physical parameters: ● ● ● ● ● ●
Temperature Type of base fluids Concentration Acidity Particle material Viscosity of base fluid The following experimental conclusions were obtained from the synthesis and
characterization of hybrid Cu-TiO2 nanofluid: 1. We observed from the stability test that the lower concentration nanoparticle in any base fluid is having the highest stability. 2. The base fluid water is showing more stability for Cu-TiO 2 nanoparticle than ethylene glycol as settlement of the particle is high in eth ylene glycol.
3. The characterisation is an important factor to check the stability of nanoparticle and from the DLS results, we obtained a size range of 300 nm which resembles the good stability of nanofluid and also obtained a good PID value. It was observed that more the brownian motion in the nanofluid, the better will be the stability and we are able to observe that agglomeration is very small in the low concentration samples which showed the desired results in the stability test. The nanofluid will become successful only when it meets the stability requirements and it's important to check it after the synthesis which will eventually produce better results in the heat enhancement process. Therefore characterisation and stability test are the most relevant step to the production of a better nanofluid. Nanofluids ability to enhance the heat transfer and optimisation of utility make it a better source of utility to any chemical industry and most of the refineries are developing nanofluids in order to replace the conventional fluids nowadays. It can expected that in the coming years the nanofluid technology will be implemented in most of the industries and make it more reliable.
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