INTRODUCTION:

Nanotechnology has undergone enormous changes and development over the last few decades and emerged as a tool for the future development in the field of science. Many researchers have been carried out and some of them have showed excellent results and new methods are found and preceded. Some examples for nanotechnology are Nanoparticles used for drug delivery system, as anti cancer, nanomedicine etc.

Nanoscience has leaped another step in developing a new field called Nanofluids. It was developed by dispersing nanoparticle in base fluids.

Suspension of metal and metal oxide nanoparticle in base fluid has generated enormous application like enhancement of thermal conductivity, in heat transfer, including microelectronics, fuel cells, pharmaceutical processes, and hybrid-powered engines, engine cooling/vehicle thermal management, domestic refrigerator, chiller, heat exchanger, nuclear reactor coolant, in grinding, machining, in space technology, defense and ships, and in boiler flue gas temperature reduction.

In order to develop thermal conductivity of a base fluid one must consider that thermal conductivity depends upon several factors like: thermal conductivity of the base fluid, volume fraction, surface area, size of the nanoparticle. J.A. Eastman^[5] indicated that increased ratio of surface area to volume percentage with decreasing size and additive acid may stabilize suspended particle in the base fluid. This will result in increase in effective K.

Some of the researchers found with Au and Ag nanoparticle. Thermal conductivity of nanoparticle was Non-linear with temperature and linear with particle size and volume fraction. Researchers demonstrated the mechanism of nanofluid based on some of the theories like Brownian movement, the effects of nanoparticle clustering, nature of heat transport and molecular layering of liquid at liquid/solid interface. At present one of the challenges in industry is transport property. This problem can be overcome by introducing the concept of nanofluid.

PREPARATION OF NANOFLUID:

Preparation of nanofluids is the first key step in experimental studies with nanofluids. Nanofluids are not just dispersion of solid particles in a fluid. The essential requirements that a nanofluid must fulfill are even and stable suspension, adequate durability, negligible agglomeration of particles, no chemical change of the particles or fluid, etc. Nanofluids are produced by dispersing nanometer scale solid particles into base liquids such as water, ethylene glycol, oil, etc. In the synthesis of nanofluids, agglomeration is a major problem.

SINGLE – STEP METOD:

The single-step direct evaporation approach was developed by Akoh et al and is called the VEROS (Vacuum Evaporation onto a Running Oil Substrate) technique. The original idea of this method was to produce nanoparticles, but it is difficult to subsequently separate the particles from the fluids to produce dry nanoparticles. A modified VEROS process was proposed by Wagener et al. They employed high pressure magnetron sputtering for the preparation of suspensions with metal nanoparticles such as Ag and Fe. Eastman developed a modified VEROS technique, in which Cu vapor is directly condensed into nanoparticles by contact with a flowing low-vapor-pressure liquid. Zhu presented a novel one-step chemical method for preparing copper nanofluids by reducing CuSO₄·5H₂O with NaH₂PO₂·H2O in ethylene glycol under microwave irradiation. Results showed that the addition of NaH₂PO₂·H₂O and the adoption of microwave irradiation are two significant factors which affect the reaction rate and the properties of Cu nanofluids.

TWO – STEP METHOD:

In this method, dry nanoparticles are first preparaed, and then they are dispersed in a suitable base fluid, but as nanoparticles have a high surface energy, aggregation and clustering are unavoidable and will appear easily. Then the particles will clog and sediment at the bottom of the container. Thus, making a homogeneous dispersion by two step method remains a challenge. However, there exist some techniques to minimize this problem like high shear and ultrasound. They are different methods of making a stable nanofluid. Nanofluids containing oxide particles and carbon nanotubes can be produced by using this method. Ultrasonic equipments are used for dispersion of nanoparticle on the base fluid. This method works well for oxide nanoparticles and is especially attractive for the industry due to its simple preparing method. But its disadvantage due to quickly agglomerated particles brings about many challenges nowadays. The two-step method is useful for application with particle concentrations greater than 20 vol. % but it is less successful with metal nanoparticles. However, some surface treated nanoparticles showed excellent dispersion. The first materials tried for nanofluids preparation were oxide particles, mainly because they are easy to make and chemically stable in solution.

EXPERIMENTAL PROCEDURE:

PREPARATION OF NANOPARTICLES:

· In a clean 100ml beaker take 50ml of distilled water and add 5g of copper acetate to it. And then in a separate beaker prepare a fresh PVA solution by adding 1g of PVA (polyvinyl alcohol) to a 50ml of distilled water.

· Then to the freshly prepared copper acetate solution 5ml of the freshly prepared PVA solution is added with continuous stirring in a magnetic stirrer. Blue coloured copper acetate solution was formed.

· Stirring is carried out for about 10-20 mins and then 1ml of hydrazine hydrate solution is slowly added to the copper acetate solution. While adding the reducing agent there will be a colour change from blue colour to greenish yellow colour. Then after stirring for more than a minute greenish colour vanishes and brown coloured solution is obtained.

· Then the above brown colour solution is stirred with heating at a temperature of 50-60^*C to a duration of 30 minutes.

· Then the solution is centrifuged at 6000rpm in a centrifuge. After the completion of centrifugation copper nanoparticle set to settle on the side of the centrifuge tube along with remaining solution, which is then poured out.

· Centrifuge tubes are dried in a digital hot air oven at 30-40^*C to a duration of 2hrs. After completion of drying dried copper nanoparticle are scraped off and stored in a vile for further use.

· Alumina nanopowder was purchased.

PREPARATION OF COPPER AND ALUMINA NANOFLUIDS:

· 10ml of base fluid (water) was taken in a properly cleaned test tube. The required amount of water was filled in the ultra sonicator instrument with proper cleaning. The required amount of the Nanoparticle was added for the preparation of nanofluid. The amount of particle to be taken are given as follows:-

a. Copper nanoparticle (vol% = 0.1%) = 0.063g.

b. Alumina nanopowder (vol% = 0.1%) = 0.039g.

c. Base fluid (distilled water) = 10ml.

· Then in a clean test tube the required copper nanoparticle was introduced into 10ml of distilled water. This solution was sonicated in an Ultrasonicator. Sonication is done for proper dispersion of the nanoparticle in the base fluid. Sonication process was done for 30min. After 30mins of Sonication copper nanofluids was obtained.

· For the preparation of alumina nanofluid the above same procedure is followed.

PREPARATION OF BI – METALLIC NANOFLUIDS:

· Here the bi-metallic nanofluid consist of the following compounds as given below:-

a. Copper nanoparticle.

b. Alumina nanoparticle.

· For the preparation of core – shell bimetallic nanoparticle, core copper solution is prepared by dispersing required amount of copper nanoparticle in a 250ml of distilled water. The required amount of nanoparticle is determined by using data of Cu (0.033mol in 250ml of water).

· Then to the prepared core copper nanoparticle solution, add different volume proportions of alumina (shell) nanoparticle solution of (1,3,5 and 6ml).

· After mixing both the solutions were further boiled for 60mins with good stirring and then left to cool at room temperature.

ANALYSIS:

1. X – RAY DIFFRACTION:-

X-Ray Diffraction analysis is the most useful method by which X-Rays of a known wavelength are passed through a sample to identify the crystalline structure. The X-Rays are diffracted by the lattice of crystal to give a unique pattern of peaks of 'reflections' at different angles and of different intensity, just as light can be diffracted by a grating of suitably spaced lines. The phase identification of the copper nanoparticles was also carried out by X-ray diffraction method. The sample was grounded using a mortar and pestle into powder. X-ray powder diffraction measurement was carried out. The X-Ray detector moves around the sample and measures the intensity of these peaks and the position of these peaks [diffraction angle 2θ]. The highest peak is defined as the 100% peak and the intensity of all the other peaks are measured as a percentage of the 100% peak. The powder XRD data were obtained in the 2q range from 35° to 45° which agrees well with the standard range of copper (2θ=37.5). The X-ray diffractometer was calibrated by means of external silicon standard, SRM 640a. The diffraction pattern indicated that the sample is the copper nanoparticles. The XRD pattern of the mixture is shown in the Figure

Fig.1 XRD analysis for copper

2. SCANNING ELECTRON MICROSCOPE (SEM):

Morphology of the entire nanoparticle Copper and alumina nanoparticles was characterized by SEM analysis we gain further insight into the features of the copper and alumina nanoparticles. The freeze-dried copper and alumina nanoparticles were mounted on specimen stubs with double-sided adhesive carbon tape, coated with Au/Pd alloy to make the surface conducting in a sputter coater (BAL-TEC SCD-005), and examined under a Philips XL-30 SEM at 12-16 kV with a tilt angle of 45^o. Scanning electron microscopy provided the morphology and size details of the copper and alumina nanoparticles. The experimental results showed that the diameters of prepared nanoparticles were in the range of copper of around 200 nm to 500 nm and alumina nanoparticle was 100nm – 150nm.

Fig 2 : SEM image of copper nanoparticle

Fig 3: SEM image of alumina nanoparticle

3. UV - SPECTROSCOPHY:

The figures show the UV spectra of copper in the range 495 nm – 510 nm. The absorption band in visible light region (495 nm – 510 nm, peak at 501 nm) is typical for copper nanoparticles. To monitor the stability of copper nanoparticle, the absorption of the particle was measured after different periods of time. And UV analysis was done for bi – metallic nanofluid, the UV spectra in the range of 200 – 600nm. There was two peaks in the bi – metallic nanofluid the peaks are (1^st peak – 210nm, 2^nd peak – 501nm) which is the range of copper and alumina. There was no change in peak position for some days, except for the decrease of absorbance. The particles settlement increase with increase in time. So, there was change in the absorbance

Fig.4–UV Spectroscopy analysis of copper, alumina and Bi – metallic nanofluid.

EXPERIMENTAL INVESTIGATION:

There are many factors which influence the enhancement of heat transfer, but thermal conductivity is the most important parameter responsible for enhanced heat transfer. Many experimental works have been reported on this aspect in this field of research. Some of the well known instruments used for measuring thermal conductivity are as follows: the temperature oscillation technique, the steady-state parallel-plate technique and the transient hot wire line method has been employed to measure the thermal conductivity of nanofluids. Among them the transient hot wire method has been used most extensively, because in general nanofluids are electrically conductive. A modified hot-wire cell and electrical system was introduced by coating the hot wire with an epoxy adhesive which has excellent electrical insulation and heat conduction. Here KD2PRO was used to measure the thermal conductivity of the nanofluid which is to be measured.Comparison with various data indicated that the thermal conductivity of nanofluids increases with decreasing particles size.

Results demonstrated 12% improvement of the effective thermal conductivity at 3 vol% of nanoparticles. Below are the results which are obtained from the copper and alumina nanoparticle, most important to measure the thermal conductivity of bi – metallic nanofluid.

Fig.7 – Graphs for Bi metallic nanofluids behaviour against temperature

From the above figure we can find that the thermal conductivity of the nanofluid increases with the raise in temperature. During thermal conductivity analysis they were measured in room temperature. It’s a graphical representation of behavior of thermal conductivity of nanofluid along with volume% of the nanoparticle introduced in the base to prepare nanofluid by two – step method. It was found that there is an increase in the thermal conductivity of the nanofluid with increase in the volume percentage concentration in the base fluid. The graphs below are the plot between vol% vs thermal conductivity:-

Fig 8. Plot between volume % vs nanofluid thermal conductivity

MAXWELL’S CORRELATION:

The conventional understanding of the effective thermal conductivity of mixtures originates from continuum formulations which typically involve only the particle size/shape and volume fraction and assume diffusive heat transfer in both fluid and solid phases. This method can give a good prediction for micrometer or lager-size solid/fluid systems. For particle–fluid mixtures, numerous theoretical studies have been conducted. The Maxwell model for thermal conductivity for solid–liquid mixtures of relatively large particles (micro-/minimize) is good for low solid concentrations. The effective thermal conductivity, k_eff, is given by

Table-Thermal conductivity of Maxwell’ correlation and experimental values.

S.No	VOL CONC (%)	TEMP (⁰C)	Copper Nanofluid, K (W/m.K)		Alumina Nanofluid, K (W/m.K)
S.No	VOL CONC (%)	TEMP (⁰C)	K (Exp)	K (Theo)	K (Exp)	K (Theo)
1	0.2	33.47	0.664	0.601	0.633	0.602
2	0.4	34.53	0.624	0.609	0.705	0.642
3	0.6	34.88	0.646	0.605	0.714	0.606

Fig 9. Comparison of experimental Thermal Conductivity and Theoretical Thermal Conductivity with Volume Concentration %

CONCLUSIONS:

From the systematic experimental study conducted in the present research, a new combination of bi – metallic nanofluid was prepared. Before the preparation of nanofluid, nanoparticle was prepared and verified by some methods like: x-ray diffraction, UV-Spectroscopy and SEM analysis. And in UV-spectroscopy, wavelength of the nanoparticle was found to be around 525nm, and by scanning electron microscope the size of the nanoparticles was in the range of 200-500nm and in X-ray diffraction showed a peak value of 2θ = 36.8. Then by using two – step method nanofluids were prepared. After preparation of nanofluid they were measured using KD2PRO thermal conductivity analysis devices and for copper Nanofluid the thermal conductivity was increased by 7% than the basefluid and from alumina, thermal conductivity was increased by 10% than basefluid. And Bi – metallic nanofluid, thermal conductivity was measured with the help of KD2PRO and there was 22% increase in the thermal conductivity than the base fluid. Also by this study we can conclude that thermal conductivity depends upon the temperature, as the temperature increases thermal conductivity of the nanofluid also increases and it also depends upon the volume % of the nanoparticle which is used for the preparation of the nanofluid, as the volume % of nanoparticle increasing which resulted in the increase in thermal conductivity. The use of nanofluids in a wide range of applications appears promising, but the development of the field faces several challenges: the poor performance of suspensions, lack of theoretical understanding of the mechanism. Further theoretical and experimental research investigations are needed to understand the heat transfer characteristics of nanofluids.

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Received on 19.08.2013 Accepted on 01.09.2013

Research J. Engineering and Tech. 4(4): Oct.-Dec., 2013 page 272-278