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Mechanical Seminar Topics -Abstract

The present scenario of high thermal loading coupled with high flux levels demands exploration of new heat transfer augmentation mechanisms. In this context, ‘Nanofluids’ may emerge as alternative heat transfer fluids. The term ‘Nanofluids’ is used to indicate a special class of heat transfer fluids that contain stabilized nanoparticles (≤50 nm) of metallic/non metallic substances uniformly and stably suspended in an engineering fluid. This Mechanical Seminar topics paper presentation deals with the  property characterization,  performance  and potential applications of nanofluids .

Keywords Nano size, Properties, Potential benefits, Applications

INTRODUCTION

Heat transfer technology stands at the cross roads of miniaturization on one hand and astronomical increase in heat flux on the other. The usual enhancement techniques for heat transfer can hardly meet the challenge of ever increasing demand of heat removal in processes involving electronic chips, laser applications or similar high energy devices. The factors which limit the usual techniques are many folded. One major limitation is the poor thermal characteristics of usual heat transfer fluids. Metals in solid form have orders-of-magnitude higher thermal conductivities than those of fluids. This inherent inadequacy of these fluids makes the heat removal mechanism less effective even with the best utilization of their flow properties. For example, the thermal conductivity of copper at room temperature is about 700 times greater than that of water and about 3000 times greater than that of engine oil, as shown in Figure.

Mechanical Seminar Topics
Mechanical Seminar Topics -THERMAL CONDUCTIVITY OF TYPICAL MATERIALS

Therefore, in this mechanical seminar topics paper presentation the thermal conductivities of fluids that contain suspended solid metallic particles could be expected to be significantly higher than those of conventional heat transfer fluids. The idea of increasing thermal conductivity of fluids with conducting particles suspended on them is not new. Ahuja and Liu et al carried out the studies on practical implication of hydrodynamics and heat transfer of slurries.  The major problems with  such suspensions  are the rapid settling of these particles, the abrasive action of the particles and clogging in small flow passages Thus even though the slurries have higher conductivities, they are hardly useable as heat transfer fluids. These problems can be overcome by using nano sized particles. Nanofluids are new class of heat transfer fluids and are engineered by suspending nanometer-sized particles like copper oxide, carbon nanotubes etc. in conventional heat transfer fluids such as water, ethylene glycol, or engine oil. The average size of particles used in nanofluids is below 50 nm. Modern nanotechnology provides great opportunities to process and produce materials with average crystallite sizes below 50 nm.

Nanofluids have   single-phase heat transfer coefficients than their base fluids. In particular, the heat transfer coefficient increases appear to go beyond the mere thermal conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter’s.

Potential Benefits of Nanofluids as a coolant

There is now great industrial interest in nanofluids. Some of the specific potential benefits of nanofluids are described below.

  • Improved Heat Transfer and Stability: Because heat transfer takes place at the surface of the particle, it is desirable to use a particle with a large surface area. Nanoparticles provide extremely high surface areas for heat transfer and therefore have great potential for use in heat transfer. The much larger relative surface areas of nanophase powders, when compared with those of conventional micrometer-sized powders, should markedly improve the heat transfer capabilities and stability of the suspensions.
  • Reduced Pumping Power: In heat exchangers that use conventional fluids, the heat transfer coefficient can be increased only by significantly increasing the velocity of the fluid in the heat transfer equipment. However, the requited pumping power increases significantly with increasing velocity. 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. For example, to improve the heat transfer of a conventional fluid by a factor of 2, pumping power must be increased by a factor of about 10. However, if a nanoparticle-based fluid with a thermal conductivity =3 times that of a conventional fluid were used in the same heat transfer equipment, the rate of heat transfer would be doubled .Therefore, the potential savings in pumping power is significant with nanofluids.
  • Minimal Clogging:   Nanophase metals are believed to be ideally suited for applications in which fluids flow through small passages, because the metallic nanoparticles are small enough that they are expected to behave like molecules of liquid. This will open up the possibility of using nanoparticles even in microchanels for many envisioned high-heat-load applications.
  • Miniaturized Systems: Nanofluid technology will support the current industrial trend toward component and system miniaturization by enabling the design of smaller and lighter heat exchanger systems. Miniaturized systems will reduce heat transfer fluid inventory.
  • Cost and Energy Savings: Successful employment of nanofluids will result in significant energy and cost savings because heat exchange systems can be made smaller and lighter, existing system for such fluids but also to develop a method for direct evaporation of nanoparticles into high-vapor-pressure fluids such as water.

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