A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. [1] [2] The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol, [3] and oil.
Nanofluids have many potentially heat transfer applications, [4] including microelectronics, fuel cells, pharmaceutical processes, and hybrid-powered engines, [5] 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. [6] Knowledge of the rheological behaviour of nanofluids is critical in deciding their suitability for convective heat transfer applications. [7] [8] 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. [9]
In computational fluid dynamics (CFD), nanofluids can be assumed to be single phase fluids; [10] [11] 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. [12] An alternative approach simulates nanofluids using a two-component model. [13]
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. [14] 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. [15]
Thermal conductivity, viscosity, density, specific heat, and surface tension are significant thermophysical properties of nanofluids. Parameters such as nanoparticle type, size, shape, volume concentration, fluid temperature, and nanofluid preparation method affect thermophysical properties. [16]
Nanofluids are produced by several techniques:
Base liquids include water, ethylene glycol, and oils have been used. Although stabilization can be a challenge, on-going research indicates that it is possible. Nano-materials used so far in nanofluid synthesis include metallic particles, oxide particles, carbon nanotubes, graphene nano-flakes and ceramic particles. [19] [20]
A biologically-based, environmentally friendly approach for the covalent functionalization of multi-walled carbon nanotubes (MWCNTs) using clove buds was developed. [21] [22] No toxic/hazardous acids are typically used in common carbon nanomaterial functionalization procedures, as employed in this synthesis. The MWCNTs are functionalized in one pot using a free radical grafting reaction. The clove-functionalized MWCNTs are then dispersed in distilled water (DI water), producing a highly stable MWCNT aqueous suspension (MWCNTs Nanofluid).
Nanofluids are primarily used for their enhanced thermal properties as coolants in heat transfer equipment such as heat exchangers, electronic cooling system(such as flat plate) and radiators. [23] Heat transfer over flat plate has been analyzed by many researchers. [24] However, they are also useful for their controlled optical properties. [25] [26] [27] [28] Graphene based nanofluid has been found to enhance Polymerase chain reaction [29] efficiency. Nanofluids in solar collectors is another application where nanofluids are employed for their tunable optical properties. [30] [31] [32] Nanofluids have also been explored to enhance thermal desalination technologies, by altering thermal conductivity [33] and absorbing sunlight, [34] but surface fouling of the nanofluids poses a major risk to those approaches. [33] Researchers proposed nanofluids for electronics cooling. [35] Nanofluids also can be used in machining. [36]
One project demonstrated a class of magnetically polarizable nanofluids with thermal conductivity enhanced up to 300%. Fatty-acid-capped magnetite nanoparticles of different sizes (3-10 nm) were synthesized. It showed that the thermal and rheological properties of such magnetic nanofluids are tunable by varying magnetic field strength and orientation with respect to the direction of heat flow. [37] [38] [39] Such response stimuli fluids are reversible and have applications in miniature devices such as micro- and nano-electromechanical systems. [40] [41]
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. [42]
A nanofluid-based ultrasensitive optical sensor changes its colour on exposure to low concentrations of toxic cations. [43] The sensor is useful in detecting minute traces of cations in industrial and environmental samples. Existing techniques for monitoring cations levels in industrial and environmental samples are expensive, complex and time-consuming. The sensor uses a magnetic nanofluid that consists of nano-droplets with magnetic grains suspended in water. In a fixed magnetic field, a light source illuminates the nanofluid, changing its colour depending on the cation concentration. This color change occurs within a second after exposure to cations, much faster than other existing cation sensing methods.
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. [44] [45]
Nanolubricants modify oils used for engine and machine lubrication. [46] Materials including metals, oxides and allotropes of carbon have supplied nanoparticles for such applications. The nanofluid enhances thermal conductivity and anti-wear properties. Although MoS2, graphene, and Cu-based fluids have been studied extensively, fundamental understanding of underlying mechanisms is absent.
MoS2 and graphene work as third body lubricants, essentially acting as ball bearings that reduce the friction between surfaces. [47] [48] 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. [49] [50] Nanoborate additives as well as mechanical model descriptions of diamond-like carbon (DLC) coating formations have been developed. [51] Companies such as TriboTEX provide commercial formulations of synthesized MSH nanomaterial coatings for vehicle engine and industrial applications. [52] [47]
Many researches claim that nanoparticles can be used to enhance crude oil recovery. [53]
Magnetic nanoparticle clusters or magnetic nanobeads of size 80–150 nanometers form ordered structures along the direction of an external magnetic field with a regular interparticle spacing on the order of hundreds of nanometers resulting in strong diffraction of visible light. [54] [55]
Nanoelectrofuel-based flow batteries ((NFB) have been claimed to store 15 to 25 times as much energy as traditional flow batteries. The Strategic Technology Office of the U.S. Defense Advanced Research Projects Agency (DARPA) is exploring military’s deployment of NFB in place of conventional lithium-ion batteries. [56]
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. [56]
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. [56]
A 30-lab study reported that "no anomalous enhancement of thermal conductivity was observed in the limited set of nanofluids tested in this exercise". [57] The COST funded research programme, Nanouptake (COST Action CA15119) was conducted with the intention "develop and foster the use of nanofluids as advanced heat transfer/thermal storage materials to increase the efficiency of heat exchange and storage systems". One 5-lab study reported that "there are no anomalous or unexplainable effects". [58]
Despite these apparently conclusive experimental investigations theoretical papers continue to claim anomalous enhancement, [59] [60] [61] [62] [63] [64] [65] particularly via Brownian and thermophoretic mechanisms. [2] 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. [66] Experimental validation of these assertions came in 2018 [67] Brownian diffusion as a cause for enhanced heat transfer is dismissed in the discussion of the use of nanofluids in solar collectors.[ citation needed ]
Boiling or ebullition is the rapid phase transition from liquid to gas or vapor; the reverse of boiling is condensation. Boiling occurs when a liquid is heated to its boiling point, so that the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Boiling and evaporation are the two main forms of liquid vapourization.
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.
Thermoelectric materials show the thermoelectric effect in a strong or convenient form.
Ferrofluid is a liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended in a carrier fluid. Each magnetic particle is thoroughly coated with a surfactant to inhibit clumping. Large ferromagnetic particles can be ripped out of the homogeneous colloidal mixture, forming a separate clump of magnetic dust when exposed to strong magnetic fields. The magnetic attraction of tiny nanoparticles is weak enough that the surfactant's Van der Waals force is sufficient to prevent magnetic clumping or agglomeration. Ferrofluids usually do not retain magnetization in the absence of an externally applied field and thus are often classified as "superparamagnets" rather than ferromagnets.
A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At the lowest range, metal particles smaller than 1 nm are usually called atom clusters instead.
A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.
Nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm) or structures having nano-scale repeat distances between the different phases that make up the material.
Micro heat exchangers,Micro-scale heat exchangers, or microstructured heat exchangers are heat exchangers in which fluid flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are microchannels, which are channels with a hydraulic diameter below 1 mm. Microchannel heat exchangers can be made from metal or ceramic.
There are a number of possible ways to measure thermal conductivity, each of them suitable for a limited range of materials, depending on the thermal properties and the medium temperature. Three classes of methods exist to measure the thermal conductivity of a sample: steady-state, time-domain, and frequency-domain methods.
Magnetic nanoparticles (MNPs) are a class of nanoparticle that can be manipulated using magnetic fields. Such particles commonly consist of two components, a magnetic material, often iron, nickel and cobalt, and a chemical component that has functionality. While nanoparticles are smaller than 1 micrometer in diameter, the larger microbeads are 0.5–500 micrometer in diameter. Magnetic nanoparticle clusters that are composed of a number of individual magnetic nanoparticles are known as magnetic nanobeads with a diameter of 50–200 nanometers. Magnetic nanoparticle clusters are a basis for their further magnetic assembly into magnetic nanochains. The magnetic nanoparticles have been the focus of much research recently because they possess attractive properties which could see potential use in catalysis including nanomaterial-based catalysts, biomedicine and tissue specific targeting, magnetically tunable colloidal photonic crystals, microfluidics, magnetic resonance imaging, magnetic particle imaging, data storage, environmental remediation, nanofluids, optical filters, defect sensor, magnetic cooling and cation sensors.
Exfoliated graphite nano-platelets (xGnP) are new types of nanoparticles made from graphite. These nanoparticles consist of small stacks of graphene that are 1 to 15 nanometers thick, with diameters ranging from sub-micrometre to 100 micrometres. The X-ray diffractogram of this material would resemble that of graphite, in that the 002 peak would still appear at ~26o 2 theta. However, the peak would appear considerably smaller and broader. These features indicate that the interplanar distance in exfoliated graphite is similar to that of the parent graphite, but the stack size is small. Since xGnP is composed of the same material as carbon nanotubes, it shares many of the electrochemical characteristics, although not the tensile strength. The platelet shape, however, offers xGnP edges that are easier to modify chemically for enhanced dispersion in polymers.
Nanofluid-based direct solar collectors are solar thermal collectors where nanoparticles in a liquid medium can scatter and absorb solar radiation. They have recently received interest to efficiently distribute solar energy. Nanofluid-based solar collector have the potential to harness solar radiant energy more efficiently compared to conventional solar collectors. Nanofluids have recently found relevance in applications requiring quick and effective heat transfer such as industrial applications, cooling of microchips, microscopic fluidic applications, etc. Moreover, in contrast to conventional heat transfer like water, ethylene glycol, and molten salts, nanofluids are not transparent to solar radiant energy; instead, they absorb and scatter significantly the solar irradiance passing through them. Typical solar collectors use a black-surface absorber to collect the sun's heat energy which is then transferred to a fluid running in tubes embedded within. Various limitations have been discovered with these configuration and alternative concepts have been addressed. Among these, the use of nanoparticles suspended in a liquid is the subject of research. Nanoparticle materials including aluminium, copper, carbon nanotubes and carbon-nanohorns have been added to different base fluids and characterized in terms of their performance for improving heat transfer efficiency.
Potential graphene applications include lightweight, thin, and flexible electric/photonics circuits, solar cells, and various medical, chemical and industrial processes enhanced or enabled by the use of new graphene materials.
Space Engine Systems Inc. (SES) is a Canadian aerospace company and is located in Edmonton, Alberta, Canada. The main focus of the company is the development of a light multi-fuel propulsion system to power a reusable spaceplane and hypersonic cruise vehicle. Pumps, compressors, gear boxes, and other related technologies being developed are integrated into SES's major R&D projects. SES has collaborated with the University of Calgary to study and develop technologies in key technical areas of nanotechnology and high-speed aerodynamics.
A chemiresistor is a material that changes its electrical resistance in response to changes in the nearby chemical environment. Chemiresistors are a class of chemical sensors that rely on the direct chemical interaction between the sensing material and the analyte. The sensing material and the analyte can interact by covalent bonding, hydrogen bonding, or molecular recognition. Several different materials have chemiresistor properties: semiconducting metal oxides, some conductive polymers, and nanomaterials like graphene, carbon nanotubes and nanoparticles. Typically these materials are used as partially selective sensors in devices like electronic tongues or electronic noses.
In materials science, vertically aligned carbon nanotube arrays (VANTAs) are a unique microstructure consisting of carbon nanotubes oriented with their longitudinal axis perpendicular to a substrate surface. These VANTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes and possess a morphology that may be precisely controlled. VANTAs are consequently widely useful in a range of current and potential device applications.
Nanoparticles have been studied extensively for their antimicrobial properties in order to fight super bug bacteria. Several characteristics in particular make nanoparticles strong candidates as a traditional antibiotic drug alternative. Firstly, they have a high surface area to volume ratio, which increases contact area with target organisms. Secondly, they may be synthesized from polymers, lipids, and metals. Thirdly, a multitude of chemical structures, such as fullerenes and metal oxides, allow for a diverse set of chemical functionalities.
The transient hot wire method (THW) is a very popular, accurate and precise technique to measure the thermal conductivity of gases, liquids, solids, nanofluids and refrigerants in a wide temperature and pressure range. The technique is based on recording the transient temperature rise of a thin vertical metal wire with infinite length when a step voltage is applied to it. The wire is immersed in a fluid and can act both as an electrical heating element and a resistance thermometer. The transient hot wire method has advantage over the other thermal conductivity methods, since there is a fully developed theory and there is no calibration or single-point calibration. Furthermore, because of the very small measuring time there is no convection present in the measurements and only the thermal conductivity of the fluid is measured with very high accuracy.
A nanoparticle interfacial layer is a well structured layer of typically organic molecules around a nanoparticle. These molecules are known as stabilizers, capping and surface ligands or passivating agents. The interfacial layer has a significant effect on the properties of the nanoparticle and is therefore often considered as an integral part of a nanoparticle. The interfacial layer has an typical thickness between 0.1 and 4 nm, which is dependent on the type of the molecules the layer is made of. The organic molecules that make up the interfacial layer are often amphiphilic molecules, meaning that they have a polar head group combined with a non-polar tail.
Kambiz Vafai is a mechanical engineer, inventor, academic and author. He has taken on the roles of Distinguished Professor of Mechanical Engineering and the Director of Bourns College of Engineering Online Master-of-Science in Engineering Program at the University of California, Riverside.
European projects: