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Nanofluids, with Special Emphasis on Ferrofluids

By T.Jayakumar and John Philip
Non Destructive Evaluation Division Indira Gandhi Centre for Atomic Research, Kalpakkam 603102

Nanofluids are suspensions of nanoparticles (e.g. carbon, metals, and metal oxides) in the size range of about 10 to 50 nm in a carrier fluid (e.g. water, ethylene glycol and lubricants).  One of the promising applications of such fluids is their use as media for heat transfer with improve energy efficiency and single-phase convective heat transfer as their thermal conductivity can be as high as 40% of the pure base fluid.

The nanofluid technology is expected to create opportunities for many applications such as refrigerant chillers, electronic manufacturing, chemical processes, cosmetics, pharmaceuticals, power generation, heating, ventilation, air-conditioning, textiles, and paper production. In nuclear industry, the use of water-based nanofluids could improve the performance of the water cooled nuclear systems. Automobile motor systems with nanofluid based oils and coolants enable improving the engine efficiency. In this particular case, copper or copper oxides in water or ethylene glycol (radiator fluid) are evaluated.

In automobiles, use of nanofluids with improved heat transfer coefficient can lower pollution and reduce operating costs as well as facilitate the miniaturisation of the systems. Even at very low volume fractions, the nanoparticles dramatically increase thermal conductivity and the critical heat flux of the fluid. Due to the small size of the particles, settling, abrasion and clogging issues are eliminated, enabling the nanofluid to be immediately incorporated into existing thermal management systems.

Nanofluids are also found to improve the efficiency of high heat-flux devices like supercomputer circuits and high power microwave tubes that enable development of smaller chips with rapid heat dissipation. Nanofluids will also reduce the quantity of required coolant resulting in weight reduction of ground and space based instrumentation.

Nanofluids can be employed for extraction of useful components like oil from sand and rock. Use of nanofluids significantly improves drilling speeds for drilling processes in oil exploration, thus leading to extraction of more oil at higher speeds. Nanofluids can also be employed for removal of harmful components i.e. for environmental cleaning.

Rocket fuel containing small quantity of nano carbon (diamond/carbon nanotubes) enables the fuel to be a better coolant in rocket thrust chambers. This would allow the engines to operate at higher pressures without excessive wall heating. If aluminum nanoparticles are used in the fuels, the specific impulse of the engine would increase even without a change in chamber pressure or geometry.

In medical field, nanofluids can be used during critical surgeries to cool the brain so that it requires less oxygen and thereby enhances the patient’s chance of survival and reduce the risk of brain damage.

Ferrofluid is a magnetic nanofluid in which magnetic nanoparticles are dispersed in a carrier fluid. The particles having an average size of about 10 nm are coated with a stabilizing dispersing agent (surfactant), which prevents particle agglomeration even when a strong magnetic field gradient is applied to the ferrofluid. Ferrofluids are used for many applications such as dynamic mechanical seals, airborne seals for protection of optical devices and sensitive electronic instrumentation in military and surveillance aircraft, stimulant for enhancing chemical reactions and medical diagnostics and therapy. In medical applications, for example, ferrofluid provides new cancer treatment techniques by employing iron based nanoparticles as delivery vehicles for drugs or radiation.

We have a dedicated programme on synthesis, characterization and application of magnetic ferrofluids required for realization of  mechanical seals for sodium pumps of fast breeder reactors. Magnetic seal takes advantage of the response of a magnetic nanofluid to an applied magnetic field. The basic seal components include ferrofluid, a permanent magnet, two pole pieces and a magnetically permeable shaft.  The magnetic circuit of a seal is completed by the stationary pole pieces and the rotating shaft concentrates magnetic flux in the radial gap under each pole piece. When the fluid is applied to this gap, it assumes the shape of a liquid O-ring and produces a hermetic seal. Ferrofluid vacuum rotary feedthroughs utilize multiple rings of ferrofluid contained in stages formed by grooves machined into either the shaft or the pole pieces. Typically a single stage can sustain a pressure differential of 0.2 atmospheres (200 mbar). The pressure capacity of the entire feedthrough is approximately equal to the sum of the pressure capacities of the individual stages.

We have been working on the development of ferrofluid based magnetic seals for the last few years. We have now established methodologies to synthesis high quality magnetite based ferrofluid with very good stability.  Our core expertise in the ferrofluid based sealant technology includes, synthesis, characterization, and interfacial modification of nanoparticles suitable for ferrofluid based seals, in-depth understanding of stability of magnetic nanofluids and innovative stabilization methodologies, design and development of mechanical assemblies for leak free sealants. We have successfully tested ferrofluids developed by us using static and dynamic test assemblies.  The vacuum sealing capabilities of the ferrofluid developed by us is under study. The ultimate goal is to replace the conventional mechanical seals used in the sodium pumps of fast breeder reactors with the ferrofluid based seals

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