Enhancing thermal conductivity of fluids with nanoparticles Prepared by: Smith R Kashid T.E. Mech. Roll no:344

Introduction: Modern nanotechnology provides new opportunities to process and produce materials with average crystallite sizes below 50 nm. Fluids with nanoparticles suspended in them are called nanofluids. A term proposed by Choi in 1995 of the Argonne National Laboratory, U.S.A.

Synthesis of nanofluids: There are mainly two techniques used to produce nanofluids: i) The single-step method. ii) The two-step method. Some special requirements are essential e.g. even and stable suspension, durable suspension, negligible agglomeration of particles, no chemical change of the fluid, etc. Nanofluids are produced by dispersing nanometer-scale solid particles into base liquids such as water, ethylene glycol (EG), oils, etc.

The single-step method: 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. A modified VEROS process was proposed by Wagener et al. Eastman et al. also developed a another modified VEROS technique Zhu et al. presented a novel one-step chemical method A vacuum-SANSS (submerged arc nanoparticle synthesis system) method has been employed by Lo et al. Recently, a Ni nanomagnetic fluid was also produced by Lo et al. using the SANSS method.

The two-step method:. In this method, nanoparticles was first produced and then dispersed in the base fluids. Generally, ultrasonic equipment is used to intensively disperse the particles and reduce the agglomeration of particles. For example, Eastman et al. , Lee et al. , and Wang et al used this method to produce Al2O3 nanofluids. Also, Murshed et al. Prepared TiO2 suspension in water using the two-step method.

Aadvantage: Disadvantage: one-step technique :- nanoparticle agglomeration is minimized. the two-step technique works well for oxide nanoparticles. Disadvantage: One-step techniqe :- only low vapor pressure fluids are compatible with such a process. Two step tehnique:- less successful with metallic particles.

Potential benefits of nanofluids: enhancement of heat transfer due to increased pumping power can be estimated from the following equation: h/h0 = (P/P0)^0.29 For a nanofluid flowing in the same heat transfer equipment at a fixed velocity, enhancement of heat transfer due to increased thermal conductivity can be estimated from the equation: hnf/h0 = (knf/k0)2/3

In heat exchangers that use conventional fluids, heat transfer can only be improved by significantly increasing flow rates.

Liu et al. (1988) have studied the influence of particle loading and size on the pressure drop of slurry. the potential benefits of nanofluids could provide tremendous performance,size/weight, and cost advantages.

The use of nanofluids in a wide variety of applications Future challenges : The use of nanofluids in a wide variety of applications appears promising, but the development of the field is hindered by: (i) the lack of agreement between results obtained in different laboratories (ii) the often poor characterization of the suspensions (iii) the lack of theoretical understanding of the mechanisms responsible for the observed changes in properties.

FUTURE RESEARCH PLANS: The research effort to produce and characterize the heat transfer behavior of nanofluids will consist of following main tasks. 1. Nanophase metal powders will be produced in existing state-of-the-art gas-condensation preparation systems at ANL. The particle size and agglomeration behavior of nanophase powders in liquids will be studied. 2. Technology for production of nanoparticle suspensions will be developed and the stability, dispersion, and rheological/transport properties of these nanofluids will be investigated. 3. Practical applications of nanofluids will be Investigated.

Conclusion: The concept of nanofluids is an innovative idea. One of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power. The use of nanofluids in a wide range of applications appears promising, but the development of the field faces several challenges A critical review of the state-of-the-art nanofluids research for heat transfer application was conducted in this work, which showed that our current understanding on nanofluids is still quite limited.

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