Thermal properties of liquid have a vital role in cooling as well as heating purposes in industry. Heat transfer performance of liquid depends on thermal conductivity and thermal conductivity is a crucial physical property. The thermal conductivity of conventional fluids is poor so it is inadequate for very high cooling applications. Solid additives and Maxwell theory (1873) are used to improve the thermal conductivities of conventional fluids and the Maxwell theory is also known as classical effective medium theory used for effective characteristics of mixtures. These methods are used by the scientists to get the better thermal conductivity.
Good tuning of the proportions of these solid suspensions to micrometer and millimeter ranges for receiving improved heat transfer performance have unsuccessful because of the disadvantages like as particle sedimentation, low thermal conductivity, corrosion of components of machines, excessive pressure drop particle clogging, etc. Downscaling of element sizes are used for searching of latest types of liquid suspensions which have improved heat transfer performance as well as thermal properties. Altering the properties of material completely when the physical mechanism below its critical scale. Modern nanotechnology provides chemical and physical routes which are used for the preparation of nano structured materials or nanometer sized particles and engineered to the molecular or atomic scales with improved thermo-physical properties comparison to their relevant bulk forms. Coolant fluids like as minerals oil, ethylene glycol and water have an vital role in numerous industrial sectors with chemical production, power generation, air-conditioning, microelectronics and transportation. Even if diverse techniques have been useful to improve their capabilities such as heat transfer and their performance is frequently restricted by their poor thermal conductivities which slow down the improvement of performance and heat exchangers compactness. Due to growing demand of recent technology for device efficiency and process amplification, there was a requirement to build up latest variety of fluids which are more valuable in terms of performance of heat exchange. To achieve this, researchers established that if we scatter tiny amounts of 1-100 nm size of nanometer solid elements in base fluids, these fluids provide superior thermal conductivity comparison to conventional coolants.
Nano word is taken from the Latin word “nano” which is employ as a prefix and it is signify the 10-9 fraction of a unit. The nano elements are very fine so the nano-fluids show to execute more such as a single-phase liquid than solid–liquid combination. The normally materials is used for nano elements are metals (Cu, Al, Ag, Fe, Au), nonmetals (carbon, graphite), ceramics carbides (SiC), oxides (CuO, Al2O3, SiO2, TiO2), layered (Cu+C, Al+ Al2O3,), nitrides (AiN, SiN), functionalized nano particles and PCM. The base liquid is generally a conductive liquid, like as oil (or other lubricants), water (and other coolants), bio-fluids, polymer solutions, and other ordinary liquids like as paraffin. Analysis have exposed that nanofluids have improved thermo physical properties like as thermal diffusivity, thermal conductivity, coefficients of convective heat transfer and viscosity comparison to base fluids such as water or oil.
FEATURES OF NANOFLUIDS
When the elements are accurately scattered, these characteristics of nanofluids are predictable to provide the following advantages:
- Heat conduction is higher: Due to more surface area, these nanoparticles have high heat transfer. Because of their petite dimension, they may have micro convection and mobile. So the irregular raise in thermal conductivity of nanofluids.
- Stability: Because of the partical size are tiny and weight is less so the sedimentation problem is solved.
- Reduced erosion: Nanoparticles are tiny therefore the momentum pass on a wall is very low. Due to this reason it has low erosion when close with nanoparticles.
- Pumping power is decrease: Pumping power ought to be improved via a factor of ten, heat transfer in conventional fluid is amplified via a factor of two. In this situation of nanofluids, the essential raise in pumping power would very judicious but there is increase in viscosity.
PREPARATION OF NANOFLUIDS
It must be noticed that production of Nanofluids is not only uncomplicated mixing and scattering the solid molecules in a base fluid. It is most momentous stage in the employ of some nano structured metals or Nanoparticle to improve the thermal properties of conventional heat transfer liquids. The cause is that assortment of solid molecules could occur in base fluid media if production of Nanofluids are not proper so get the low thermo-physical characteristics of Nanofluids. There are two main methods which are normally employed for Nanofluids production such as one-step and two-step methods.
METHODS FOR SYNTHESIS THE NANOMATERIALS
Materials scientists are performing the research to produce the novel materials have improved characteristics, great functionality and smaller cost compare to existing one. Numerous chemical and physical techniques have been created to improve the nanomaterials performance and showing enhanced characteristics with goal to have superior manage over distribution and particle size. In common, bottom-up and top-down are two key approaches for synthesis of nanomaterials.
ESTIMATION OF NANOPARTICLE VOLUME CONCENTRATION
The quantity of nanoparticles which is need for production of nanofluids is determined by the formula which is known as law of mixture. A sensitive equilibrium by 0.1mg resolution is employed for weight the nanoparticles precisely. The mass of nano-particles which is required for production of 3000 ml nanofluid with meticulous volume concentration, employing base fluid is water and determined by employing the given relation.
% volume concentration = [(Wn⁄ρn)/⌊Wn⁄ρn+Wb⁄ρb⌋ ×100]
Here, Wn = Nanoparticle weight
ρn = Nanoparticle density
Wb = Base fluid weight
ρb = Base fluid density
Modern progress on nano technology have permitted to improvement of latest kind of fluids which is known as nanofluids, to explain fluid suspension which including nanometer dimension particles consisting metal oxides (for example bismuth, alumina, silica, oxide, zircon and titanium),numerous carbon allotropes (for example diamond, multi-walled and single walled carbon nano pipes) and chemically stable materials (for example gold, copper and silver) with thermal conductivities, ranges of magnitudes is more than base fluids and with dimension appreciably lesser than 100nm. The raise in the amount of research articles which devoted to this subject so future illustrate a visible development and significance of heat transfer improvement technology. So, this part presents the topical investigation in convective heat transfer, consisting the numerical and experimental investigations. Nanofluid is novel type of heat transfer medium including nano molecules which are stably and homogeneously distributed in base liquid. These strewn nano particles normally a metal oxide or metal really improve the nanofluid thermal conductivity, raises convection and conduction coefficients, offering for high heat transfer of Nanofluids which have been contemplate for purposes as superior heat transfer liquids for nearly two decades. Though, because of the complexity and broad diversity of nanofluid devices, no conformity has been attained on the value of potential advantages due to employing nanofluids for heat transfer purposes. It is compared by conventional solid fluid suspensions for intensifications of heat transfer; nanofluids having appropriately dissolved nanoparticles which have many benefits which are given below:
- More specific surface area so more high transfer surface among fluids and particles.
- More dispersion stability through main Brownian motion of molecules.
- Decreased pumping power comparison to pure fluid to attain comparable heat transfer escalation.
- Decreased particle clogging comparison to conventional slurries so supporting system miniaturization.
- Regulating characteristics consisting surface watt capability and thermal conductivity via changeable concentrations of particle for suiting diverse applications.The primary test with nano fluids offered high encouraging characteristics than they were consideration to seize. The four inimitable features were observed which are given below.
Irregular improvement of thermal conductivity:
The most significant feature attained in nanofluids was irregular increase in thermal conductivity which far away from opportunity and much greater than any theory might be forecast.
Nanofluids have been testimony to be unwavering over months which employing a stabilizing agent.
Newtonian behavior and less concentration:
More improvement of conductivity was attained with a extremely low concentration of molecules that entirely preserved the Newtonian manner of the liquid. The increase in viscosity was ostensible so pressure drop was raised only slightly.
Dependence on particles size:
Unlike the circumstances by micro slurries, the improvement of conductivity was attained which depends not only particle size but also on particle concentration. In common, with reducing the particle dimension so raise in improvement was observed.
The above potentials is offer the required thrust to start the research in nanofluids and hope that these liquids have an significant role in producing the next cohort of cooling technology. The consequence can be stable and greatly conducting nanofluid with exhilarating novel purposes in the future.