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A nanofluid is a fluid containing nanometer-sized particles, called . These fluids are engineered colloid of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or . Common base fluids include water, ethylene glycol, and oil.
Nanofluids have many potentially heat transfer applications,Minkowycz, W., et al., Nanoparticle Heat Transfer and Fluid Flow, CRC Press, Taylor & Francis, 2013 including microelectronics, , pharmaceutical processes, and hybrid-powered engines, engine cooling/vehicle thermal management, domestic refrigerator, chiller, heat exchanger, in grinding, machining and in boiler flue gas temperature reduction. They exhibit enhanced thermal conductivity and convective heat transfer coefficient compared to the base fluid. Knowledge of the Rheology behaviour of nanofluids is critical in deciding their suitability for convective heat transfer applications. Nanofluids also have special acoustical properties and in ultrasonic fields display shear-wave reconversion of an incident compressional wave; the effect becomes more pronounced as concentration increases.
In computational fluid dynamics (CFD), nanofluids can be assumed to be single phase fluids; however, almost all academic papers use a two-phase assumption. Classical theory of single phase fluids can be applied, where physical properties of nanofluid is taken as a function of properties of both constituents and their concentrations. An alternative approach simulates nanofluids using a two-component model.
The spreading of a nanofluid droplet is enhanced by the solid-like ordering structure of nanoparticles assembled near the contact line by diffusion, which gives rise to a structural disjoining pressure in the vicinity of the contact line. However, such enhancement is not observed for small droplets with diameter of nanometer scale, because the wetting time scale is much smaller than the diffusion time scale.
A 2013 study considered the effect of an external magnetic field on the convective heat transfer coefficient of water-based magnetite nanofluid experimentally under laminar flow regime. It obtained up to 300% enhancement at Re=745 and magnetic field gradient of 32.5 mT/mm. The effect of the magnetic field on pressure was not as significant.
Such responsive nanofluids can detect and image defects in ferromagnetic components. The so-called photonic eye is based on a magnetically polarizable nano-emulsion that changes colour when it comes into contact with a defective region in a sample. The device could monitor structures such as rail tracks and pipelines.
MoS2 and graphene work as third body lubricants, essentially acting as ball bearings that reduce the friction between surfaces. (2016). 9783319448633, Springer. ISBN 9783319448633 (2006). 9781420017861, CRC Press. ISBN 9781420017861 This mechanism requires sufficient particles to be present at the contact interface. The beneficial effects diminish because sustained contac pushes away the third body lubricants.
Other nanolubricant approaches, such as magnesium silicate hydroxides (MSH) rely on nanoparticle coatings by synthesizing nanomaterials with adhesive and lubricating functionalities. Research into nanolubricant coatings has been conducted in both the academic and industrial spaces.Rudenko P (Washington SU, Chang Q, Erdemir A (Argonne NL. Effect of Magnesium Hydrosillicate on Rolling Element Bearings. In: STLE 2014 Annual Meeting; 2014.Chang Q, Rudenko P (Washington SU, Miller D, et al. Diamond like Nanocomposite Boundary Films from Synthetic Magnesium Silicon Hydroxide (MSH) Additives.; 2014. Nanoborate additives as well as mechanical model descriptions of diamond-like carbon (DLC) coating formations have been developed. Companies such as TriboTEX provide commercial formulations of synthesized MSH nanomaterial coatings for vehicle engine and industrial applications.
The nanofluid particles undergo redox reactions at the electrode. Particles are engineered to remain suspended indefinitely, comprising up to 80 percent of the liquid’s weight with the viscosity of motor oil. The particles can be made from inexpensive minerals, such as ferric oxide (anode) and gamma manganese dioxide (cathode). The nanofluids use a nonflammable aqueous suspension. As of 2024 DARPA-funded Influit claimed to be developing a battery with an energy density of 550-850 wh/kg, higher than conventional lithium-ion batteries. A demonstration battery operated successfully between −40 °C and 80 °C.
Discharged nanofluids could be recharged while in a vehicle or after removal at a service station. Costs are claimed to be comparable to lithium ion. An EV-battery sized fuel reservoir (80 gallons) was expected to provide range comparable to a conventional gasoline vehicle. Fluids that escape, e.g., following a crash, turn into a pastelike substance, which can be removed and reused safely. Flow batteries also produce less heat, reducing their thermal signature for military vehicles.
Despite these apparently conclusive experimental investigations theoretical papers continue to claim anomalous enhancement, particularly via Brownian and thermophoretic mechanisms. Brownian diffusion is due to the random drifting of suspended nanoparticles in the base fluid which originates from collisions between nanoparticles and liquid molecules. Thermophoresis induces nanoparticle migration from warmer to colder regions, again due to such collisions. A 2017 study considered the mismatch between experimental and theoretical results. It reported that Brownian motion and thermophoresis effects have no significant effects: their role is often amplified in theoretical studies due to the use of incorrect parameter values. Experimental validation of these assertions came in 2018 Brownian diffusion as a cause for enhanced heat transfer is dismissed in the discussion of the use of nanofluids in solar collectors.
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