Developments in Nano-fluids

Developments in Nano-fluids : A Review
Dinesh kumar1, Gurpreet Singh Sokhal2
1., Mechanical Engineering Department, Chandigarh University, Gharuan, Punjab
2. Assistant professor, Mechanical Engineering Department, Chandigarh University, Gharuan, Punjab
Abstract
Purpose of Present study is to obtain status of research and development in the field of nanofluid. Aim of this paper is to understand various techniques being used for producing nanofluids and then obtaining the effect of various factors on thermal conductivity of nanofluids over their base fluid. Various works related to this field are studied throughout the process and then analyzed to understand the development being done in order to produce nanofluids in economic way. It was concluded at the end of this study that two step techniques have potential to be used in mass production of nanofluids.

Introduction
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Nanotechnology is manipulation of matter at a scale of atomic and molecular level. It is field of science which deals with study of various small substances, their relations and applications. Particles at this super-molecular level are known as nanoparticles. These particles exist between 1nm to 100nm and are surrounded by an interfacial layer consisting of ions inorganic and organic molecules. During last two or three decades this field has shown great potential in the field of heat transfer. Nanofluid study is a part of nanotechnology.

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Nanofluids are defined as colloidal suspension of nano-sized particles into a base fluid like water ethylene glycol; water etc. size of these particles exists between 1nm to 100nm.
Nanofluids have different heat transfer properties from that of their base fluids. There are cases where nanofluids have shown more than 100% increase in heat transfer coefficient from that of their base fluids. Many fluids used in heat transfer application nowadays are very costly or hazardous to environment, which highlights need to find alternative to these heat transfer fluids. This technology provides an alternative method to improve thermal properties of fluids which can be obtained easily and at minimum cost. Nanofluids are mostly made of metal, oxides (Al2O3, CuO), carbides, nitrides (SiN, AlN). Common base fluids used to form nanofluids are water, ethyl glycol and oil. One of biggest challenge in the field of nanofluids is found to be production of Nanoparticles at larger scale in cost efficient way. Production techniques in use are not of production standard to be used for mass production. Term nanofluid was introduced by Choi 1 in 1995 at the Argonne National Laboratory. Various applications of nanofluids include transportation, electronics cooling, defense, space, nuclear system cooling, biomedicine and automobiles.

Figure1. Applications of nanofluids 2
Preparation of nanofluids
Two techniques used for developing nanofluids commercially are two step technique and one step technique. In two step technique nanofluid particles are produced separately and then immersed in the base fluids. In this case nanoparticles are produced with physical or chemical process and then these particles are dispersed in fluid. Most widely used method is two step techniques because of huge scalability and cost effective nature of process. This process have disadvantage of containing huge amount of agglomeration due to high surface area. Bonneman et. al 3 suggested a method for producing palladium particles(2.2nm) using chemical reduction pathways. Water/TiO2 nanofluid was prepared by Murshed et. al. 4 through ultrasonic dispersion method.

In case of one step technique both dispersion and production of nanoparticles take place together in single process. One step technique minimizes problem of agglomeration which plays an important role in reducing heat transfer performance of nanofluids. Disadvantage of one step technique is that impurities (residuals) are left in nanofluids because of incomplete reaction. Some of the one step techniques used are described throughout the study below.

Direct evaporation technique:
Direct evaporation method was reported by akoh et. Al.5 for production of nanofluids. This method involves direct evaporation and then further condensation of nanoparticles into base fluids. Choi et. al.6 was the one who developed direct evaporation technique in which source material is evaporated in a cylinder and then condensed directly into base fluid.

Chemical reduction:
Nanoparticles are prepared by reducing metal salts in to nano?uids in the presence of various solvents. The CuO nano?uids were produced by the chemical reduction of copper acetate while sodium lauryl sulfate solution was present, in paper published by sandhya et. al.7. Kumar et al. 8 prepared copper (Cu) nano?uids by chemical reduction of copper sulfate pent hydrate in the presence of SLS in water.

Submerged arc nanoparticles synthesis system:
This technique is used to produce stable CuO nanofluid. Pure Cu rod is submerged in dielectric fluid (in vacuum chamber) where temperature in between 6000 to 12000? to melt and vaporize copper rod. Size of nano fluid produced by Lo et. al. 9 lies in range 38.9nm-49.1nm. Similar technique was used by Lo. Et.al.10 to develop nanofluid containing nanoparticles of silver having size of 12.5nm.

Laser ablation:
Au/water nano?uid were prepared by kim et. al.11 using pulse laser ablation method in liquids.kim et. al. used Q-switched Nd:YAG laser to produce these nanofluids. Variation of the irradiation time was varied between 1h and18 h. laser beam was focused on Au tablet submerged in pure water using a biconvex lens.

Figure2. Laser Ablation
Polyol process :
In this process a metal precursor is dissolved in ployol (liquid) and reduction in metallic precursor is obtained by adjusting experimental conditions 12. Water based nanofluids are produced with the help of this method. Coating of hydrophilic polyol takes place on nanofluids in this method. One other technique used for development of nanofluids are physical vapor condensation method.

Factors affecting Thermal conductivity of nanofluids
1. Particle size:
Wang and majumdar 13 reported in his study that decrease in nanoparticle size results in increase in thermal conductivity due to increase in ratio of surface area to volume. Further Teng et al. 14 suggested that in case of Al2 O3 enhancement of 5.3% and 12.8% with nanoparticles of 100nm and 20 nm sizes at room temperature. It was found in the study conducted by Beck et. al.15. that enhancement decreases when size of Al2O3 was increased from 2nm to 280nm. Photon scattering was suggested to be reason for this effect.

2. Operating Temperature:

Studies conducted by various groups have reported that heat transfer characteristics improve when temperature is increased. Faiza M. et. al.16 have shown in his study that heat transfer increases with increase in temperature. Average increase in heat transfer was found to be in range of 20 to 150%.

3. Ph- Value:
Yo et. al. 17 have shown that enhancement of thermal conductivity of nanofluids can be improved by adjustment of ph value of nanofluid. Researchers found 2.57% higher increase in thermal conductivity when ph value of Al2o3/water was changed from neutral ph-7 to ph-10.volume concentration taken for study was 1.5% with particle size of 50nm.

4. Particle volume concentration:
Works of various groups have shown trend where thermal conductivity of nanofluids increases with increase in nanofluids. Jahanshahi et.al. 18 have demonstrated 32% increase in thermal conductivity as volume concentration of SiO2/water (12nm) was varied from 1% to 10%. For Al2O3/ water nanofluids enhancement of 8% was reported by kim et. al. 19 when volume concentration of fluid was changed from 0 to 3%.

Conclusion:
Two step techniques are more economical due to its large scalability and cost effective nature and high productivity.

Two step techniques have potential to become main stream industrial process if its limitations like aggregation which causes low enhancement in heat transfer are minimized.

One step technique has less control over parameters like size of nanoparticles and is more economical in batch mode production rather than continuous mode production.

Thermal conductivity increases with decrease in size of nanoparticles, increase in operating temperatures and particle volume concetration.

References
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