Because of higher density chips, the design of more compact electronic components makes heat dissipation even more difficult. All advanced electrical or electronic devices are facing heat management challenges due to the increased levels of heat generation and the reduction in the surface area for heat rejection or dissipation. So a reliable heat management system is very important for the continuous and smooth working of these modern electronic devices.
The traditional heat transfer fluids used in today’s thermal management systems, such as, water, oils, and ethylene glycol, have inherently poor thermal conductivities which is the one of the major limiting factors for the low heat transfer performance of these fluids. The thermal conductivity of metallic liquids is much higher than those of non metallic liquids. As a consequence, 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.
Nanofluid is a new type of heat transfer fluid, having nanoparticles (1–100 nm) which are evenly distributed in the base fluid. These uniformly distributed nanoparticles are generally metal or metal oxides which have a great enhancing effect on the thermal conductivity of the nanofluid, thus increasing conduction and convection coefficients and allowing for higher heat transfer.
The emergence of nanofluids as a new field of nano scale heat transfer in liquids is related directly to miniaturization trends and nanotechnology. Nanofluids, a suspension of nano-sized particles in liquids, have emerged as a potential candidate for the design of heat transfer fluids. The enhanced thermal conductivity of nanofluids offer several benefits such as higher cooling rates, decreased pumping power, smaller and lighter cooling systems and improved wear resistance. The novel nanofluids enable a more efficient, effective and uniform heat removal capability for systems requiring highly accurate temperature control at high heat fluxes.
It is quite interesting to note that nanoparticles can be dispersed not only in coolants and engine oils, but also in transmission fluids, gear oils, and other fluids and lubricants. Actually nanofluids provide better overall thermal management and better lubrication.
The cooling applications of nanofluids include Crystal Silicon Mirror Cooling, Electronics cooling, Vehicle cooling, Transformer cooling, Space and Nuclear systems cooling, Defense applications and so on. One of the first applications of research in the field of nanofluids is for developing an advanced cooling technology to cool crystal silicon mirrors used in high-intensity x-ray sources.
Types of Nanofluids
There are various metallic, non metallic nanoparticles and multiwalled carbon nanotubes (MWCNT) which are currently used with base fluids to enhance the thermal performance of the cooling systems.
Nanoparticles used in nanofluids have been made of various materials such as oxide ceramics (Al2O3, CuO), nitride ceramics (AlN, SiN), carbide ceramics (SiC, TiC), metals (Cu, Ag, Au), semiconductors (TiO2, SiC), carbon nanotubes, and composite materials such as alloyed nanoparticles Al70Cu30 or nanoparticle core-polymer shell composites.
In addition to the nonmetallic, metallic and other materials for nanoparticles, completely new materials and structures such as materials ‘doped’ with molecules in their solid liquid interface structure, may also have desirable characteristics.
Common base fluids are water, ethylene glycol and oil. The metallic nanoparticles like Cu, Fe, Au, Ag etc. and non metallic particles or compounds like Al2O3 (Alumina), CuO, SiC, TiO2, Fe3O4 (Iron Oxide), ZrO2 (Zirconia), WO3 (Tungsten trioxide), ZnO, SiO2 etc. are generally used with base fluids.
Nanofluids thermal management
Nanofluids are expected to be a promising coolant candidate for thermal management system of next generation high heat dissipation electronic systems. Generally, there are two alternatives for improving the heat dissipation for the electronic equipment. First one is to find the best geometry for cooling devices; second one is to increase their capability to transfer heat. Nanofluids with very high thermal conductivities also have high convective heat transfer coefficients when compared to their base fluids. Recent reviews showed that nanofluids can increase the heat transfer coefficient as well as the thermal conductivity of a fluid or coolant.
Nanofluids have very high potential in improving automotive industry and cooling rates of heavy-duty engine by increasing efficiency, reducing the weight and complexity of heating/cooling systems.
Continuous technological development in automotive industries has increased the demand for high efficiency engines. Most internal combustion engines are fluid cooled using either air or a liquid coolant run through a heat exchanger (radiator) cooled by air. Nanofluids seem to be potential replacement of conventional coolants in engine cooling system. Recently there has been considerable research findings reported which highlights superior heat transfer performances of Nanofluids. Nanofluids are potential heat transfer fluids with enhanced thermo physical properties and heat transfer performance.
The power generation industry is interested in transformer cooling application of nanofluids for reducing transformer size and weight. The ever-growing demand for greater electricity production will require upgrades of most transformers at some point in the near future at a potential crest of millions of dollars in hardware retrofits. If the heat transfer capability of existing transformers can be increased, many of the upgrades may not be necessary.
In Space and Defense sectors, because of the restriction of space, weight, and available energy in space stations and aircrafts, there is a very strong demand for highly efficient heating/cooling systems which are as small in size as possible. The Nanofluids with very high heat fluxes are capable of providing the necessary cooling/heating rates in such applications and in other systems of the military or defense and space sectors, which may include military vehicles and submarines or even high-power laser. Therefore, the applications of nanofluids range widely especially in the fields where density of power is very high and the equipment needs to be smaller and lighter.
There are a number of military devices and systems such as high-powered military electronics, military vehicle components, radars and lasers which require high-heatflux cooling. In reality, cooling with conventional heat transfer fluid is difficult for such conditions. Some specific examples of potential military applications include power electronics and directed-energy weapons cooling. Nanofluids provide advanced cooling technology for military vehicles, submarines and high-power laser diodes.
Nanofluid research for defense application considers multifunctional nanofluids with added thermal energy storage or energy harvesting through chemical reactions. The novel projected applications of nanofluids include sensors and diagnostics that instantly detect chemical warfare agent in water or water or food borne contamination; biomedical applications include cooling medical devices, deleting unhealthy substances in the blood, cancer treatment or drug delivery; and development of advanced technologies such as advanced vapour compression refrigeration systems. It is clear that nanofluids will be increasingly important for high-value added niche applications as well as for high- volume applications
Synthesis of Nanofluids
Nanofluid is defined as a colloidal solvent containing dispersed nanometer-sized particles (~1-100 nm). Researchers have found out many materials that can be used as base fluids and nano particles. The goal of nanofluids is to achieve the highest possible thermal properties at the smallest possible concentrations by uniform dispersion and stable suspension of nanoparticles in host fluids.
Nanofluid cannot be considered simple liquid-solid mixture. It is important to achieve agglomeration-free suspension for considerably long time periods without the possibility of any changes in chemical composition of the base fluid. This can only be achieved by reducing the density difference between liquids and solids or by increased viscosity of base fluid. For two-phase systems one of the major issues is the stability of these nanofluids, and till date it has remained a challenge.
Therefore, synthesis and suspension of nearly non agglomerated or mono dispersed nanoparticles in liquids is the key to significant enhancement in thermal properties of nanofluids.
The preparation of nanofluids from nanoparticles can be broadly categorized by one step and two step production methods. Stable and highly conductive nanofluids are produced by. Both approaches for creating nano particle suspensions suffer from agglomeration of nano particles, which is a key issue in all the technology involving nano powder.
The Two-step method is the most commonly used method for preparing nanofluids. Nanoparticles, nanotubes (carbon nanotubes), nanofibers and other nanomaterials utilized in this method are produced as dry powders first by the means of chemical and physical methods. After that the powder is be dispersed into a base fluid in the second step, with the help of external mixing or stirring methods like magnetic agitators, ultrasonic agitators, high-shear mixers, homogenizing or ball milling.
The Two-step method is the most commonly used and economic method to prepare nanofluids in large quantities because the nanopowder manufacturing techniques have already started providing up to required industrial production levels. Because of the high surface area and surface related activity, the nanoparticles have a tendency to accumulate together. One of the important methods to improve the stability of nanoparticles in base fluids is to use surfactants which reduce surface tension of base fluids.
Because of the difficulties faced regarding stability during the mixing process in preparing nanofluids by Two-step method, the One-step method was developed. In order to reduce the accumulation of nanoparticles, Eastman et al. suggested the one-step physical vapor condensation process for preparing Cu/ethylene glycol nanofluids.
In this one-step method, it involves the simultaneous synthesis and dispersion of the nanoparticles in the base fluid. By this method, the drying, storage and transportation processes are removed, so the accumulation of nanoparticles is kept at a minimum. Thus the stability of fluids is greatly increased. The one-step method can be used to prepare fluids with uniformly dispersed nanoparticles and these particles can be kept suspended in a stable manner. The nanoparticles so prepared have needlelike, square, polygonal or circular morphological shapes. The One step process avoids the unwanted particle aggregation quite well.
Although the two-step method works well for oxide nanoparticles, it is not as effective for metal nanoparticles such as copper. For nanofluids containing high-conductivity metals, it is clear that the single-step technique is preferable to the two-step method.
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