18.104.22.168. Production of Nanofluids The two-step method is the most widely used technique for making nanofluids. Nanoparticles, nanotubes, nanofibers, and other nanomaterials used in this method are initially produced as dry powders, by inert gas condensation 4, chemical vapor deposition, particularly the multiwalled carbon nanotubes (MWCNT) 5. The nanometer dimensioned particles are then uniformly dispersed into a fluid in second step with the help of intensive magnetic stirring, ultrasonication, high-shear mixing, homogenizing, and ball milling, used to minimize particle agglomeration and for uniform distribution. Surfactants may be used for stabilizing the nanoparticles. However, the functionality get affected with the use of surfactants under high temperature, especially for high-temperature applications. The Two-step method is the most economic method for producing nanofluids in large scale, because nanopowder synthesis techniques have already been scaled up to industrial production levels. A key problem of two-step processes is that nanoparticles are in an agglomerated state after many hours of sonication. Due to the barriers in preparing stable nanofluids by two-step method, several new techniques are evolved to produce nanofluids, including one-step method. Different techniques are used for preparation by a single-step method: physical vapor deposition (PVD) technique, liquid chemical technique, and vacuum evaporation onto a running oil substrate (VEROS) technique. To reduce the agglomeration of nanoparticles, the one-step physical vapor condensation method to prepare Cu/ethylene glycol nanofluids was developed 7. The one-step process comprises of simultaneously making and dispersing the particles in the conventional base fluid. In this method the processes of drying, storage, transportation, and dispersion of nanoparticles are avoided, so the agglomeration of nanoparticles is minimized and the stability of fluids is increased 8. The one-step method produces uniformly dispersed nanoparticles and the particles are stably suspended in the base fluid. However the one-step method has the disadvantages in that the residual reactants are left in the nanofluids due to incomplete reaction or stabilization and that only low vapor pressure fluids are compatible with the process. 1.4. Applications of Nanofluids Nanofluids have an unrivalled integration of the four attributes desired in thermal and energy management systems: Enhanced thermal conductivity at low nanoparticle concentrations, Temperature-dependent thermal conductivity, non-linear increase in thermal conductivity with nanoparticle concentration and increase in boiling critical heat flux. These distinguishing characteristics intensify nanofluid’s potential applications to improve heat transfer and energy efficiency in industrial and engineering areas 9. 1.4.1. Industrial Coolants According to Routbort et al. 10, the use of nanofluids for industrial cooling will result in great energy savings and emissions reductions. For instance, in tyre plants, the production yield of many industrial processes is limited by the lack of potential to cool the rubber efficiently as it is being processed, and as a result lots of heat transfer fluids are required. The use of water-based nanofluids can reduce the cost of production and results in high profit margins. For US industry, the substitution of cooling and heating water with nanofluids has the potential to save 1 trillion Btu of energy 10. Generally, industrial coolants are used in public utilities; the oil and gas industry; the food and beverage processing industry; the chemicals and plastics industry; solar power plants and in buildings for heating, ventilation, and air conditioning (HVAC) systems. 1.4.2. Nuclear Reactors Nuclear reactors are used for producing electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for carrying out research. Kim et al. 11, 12, at the Nuclear Science and Engineering Department of the Massachusetts Institute of Technology (MIT), carried feasibility tests on the use of nanofluids in nuclear energy systems as a substitute to improve the performance of water-cooled nuclear system for heat removal. The studies have shown possible applications in pressurized water reactor (PWR) primary coolant, standby safety systems, accelerator targets, plasma diverters, and so forth 13. Nanofluids can be used as the main reactor coolant for PWRs, thus enabling significant power uprates of the PWRs and magnify their economic performance. The experiments revealed that, the use of nanofluids with at least 32% higher critical heat flux (CHF) enabled a 20% power density uprate in current plants without changing the fuel assembly design and without reducing the margin to CHF. Nanofluids can also be used as a coolant for the emergency core cooling systems (ECCSs) of both PWRs and boiling water reactors, where they could cool down overheat surfaces more rapidly, ensuring the power plant safety. 1.4.3. Geothermal Power Extraction When extracting energy from the earth’s crust that varies in length between 5 to 10 km and temperature between 500oC and 1000oC, nanofluids can be employed to cool the pipes exposed to such high temperatures. When drilling, nanofluids can serve in cooling the machinery and equipment working in high friction and high temperature environment. 1.4.4. Electronic Applications Nanofluids are used for cooling of microchips in computers as well as in other electronic applications which use microfluidic applications. Due to rapid development in modern technology, current electronic systems generate a huge amount of heat, which deteriorates the performance of the devices and decreases their reliability 14. So nanofluids are employed in cooling of personal computers, microelectromechanical systems (MEMS) and microchips. 1.4.5. Automotive Applications Nanofluids have been used in automobile industry for applications such as coolant, fuel additives, lubricant, shock absorber and refrigerant. The present engine oils, automatic transmission fluids, coolants, lubricants, and other synthetic high-temperature heat transfer fluids found in conventional truck thermal systems radiators, engines, heating, ventilation and air-conditioning (HVAC)—have inherently poor heat transfer properties. These could benefit from the high thermal conductivity offered by nanofluids. Nanofluids are commonly used in cooling radiators for automobiles and trucks, and power electronics for hybrid electric vehicles. 1.4.6. Biomedical Applications Optical filters can be created from specialized nanoparticle dispersions, for example, nanofluid-based filters for hybrid solar photovoltaic/thermal (PV/T) applications. This particular application is suitable because nanofluids can be utilized as both volumetric solar absorbers and heat transfer fluids.