An Overview of Nanofluids in Different Heat Transfer Mechanisms

An Overview of Nanofluids in Different Heat Transfer Mechanisms

Rekha Devi

Assistant Prof. Government College Jhandutta, Distt. Bilaspur (H.P)

ABSTRACT:

The enhanced thermophysical properties of nanofluids and their capacity to be incorporated into a variety of thermal applications, including improving the efficiency of heat exchangers used in industries and solar energy harvesting for renewable energy production, are the main reasons for the growing interest in these materials. Nanfluids have various thermo-physical properties such as thermal conductivity, thermal diffusivity, viscosity etc. Finally, a conclusion on the merits and demerits of nanofluids is presented along with nanofluids models. The conclusion on the benefits and drawbacks of nanofluids is offered together with nanofluids models in the end. Nanofluids have enormous applications in sciences and technology. Commonly, Tiwari and Das nanofluid model and Buongiorno nanofluids model are utilized to examine the convective heat transmission in nanofluids.

INTRODUCTION:

The suspension of nanoparticles in fundamental fluids is known as nanofluids. The base fluid's thermal characteristics are improved by the inclusion of nanoparticles. In 1995, Choi and Eastman[1] developed the concept of dispersing nanoparticles in carrier fluids and coined the name "nanofluids." The base fluids include water, glycol, oils, and lubricants, whereas the nanomaterials utilised to make nanofluids include non-metals, single- or multiwall carbon nanotubes, and oxidized ceramics. Adding millimeter- or micrometer-sized solid particles to base fluids has been used to try to boost the thermal conductivity of the fluid. The viscosity of an alumina-water nanofluid increases with an increase in particle concentration, according to research by Das et al. When it was noticed that nanofluid containing smaller CuO particles showed enhancement in thermal conductivity with temperature [2], an experiment was conducted utilising Al₂O₃-water nanofluid and CuO water nanofluid. Devi Reddy experimented on a car radiator employing water-based TiO₂ ethylene glycol nanofluid as the coolant at various nanoparticle concentration levels The results showed that there is enhancement in heat transfer coefficient compared to the base fluid. An experiment performed on automobile radiator using ethylene glycol and water based TiO₂ nanofluid as the Coolant at different concentration levels of nanoparticles Devi Reddy et al [3] . The outcomes demonstrated an improvement in heat transfer coefficient as compared to the base fluid. Ju. examined the thermal conductivity of aqueous alumina nano and found that it exhibits an abnormal enhancement [4]. The results of an investigation employing Ni-Diamond nanoparticles dispersed in water and ethylene glycol combination in various ratios and water alone came to the conclusion that the thermal conductivity was greatest for the nanoparticles [5]. Shyam Sundar et al. Ethylene glycol and TiO₂ were used in an experiment by Abdul Hamid et al. [6]. The thermal conductivity of Cu -water and Al-water nano fluids was experimentally evaluated by a thermal property analyzer by M. Tajik Jamal-Abadi et al. [7] According to the aforementioned finding, particle concentration is a crucial characteristic because as it rises, thermal conductivity of nanofluids also does. CuO NPs' thermal stability and weight loss were investigated, and experimental results showed that viscosity reduces as temperature rises [8]. Zirconia-water nanofluid was permitted to flow through horizontal tubes at various flow rates by Wesley Williams et al., and it was discovered that there was no abnormal increase in heat transfer rate [9]. Heat transfer performance of a vehicle radiator was studied by S.M. Bhaskar et al. The model was shown to be unable to predict the thermal conductivity of the nanofluids [10] when they employed silicon dioxide-water nanofluid and compared the experimental results with Hamilton and Crosser model. By dispersing silver nanoparticles coated with polyvinyl pyrolidone in distilled water, S. Iyahraj et al. created nanofluid. Thermal analyzer was used to measure the nanofluid's thermal conductivity[11]. These investigations by several researchers have led to the conclusion that the augmentation of thermal conductivity is dependent on concentration, temperature, base fluids, nanoparticle size and shape, and nanoparticle volume fraction.

Types of Nanofluids: 

1.                   Pure Metals, Metal Oxides and Carbide Based Nanofluids:

In 1995, Choi proposed using nanoparticles to enhance the thermophysical characteristics of thermo-fluid. [12]. Since then there has been considerable research into developing nanofluids with unique thermo-physical properties such as thermal conductivity, thermal diffusivity and viscosity [13]. Researches have revealed the thermo-physical properties of several fluids can be improved by the addition of small concentrations of nanoparticles. Pure metals (Au, Ag, Cu, Al, and Fe), metal oxides (Al₂O₃ CuO, Fe₃O₄, SiO₂, TiO₂, and ZnO), Carbides (SiC, TiC) and a variety of carbon materials (diamond, graphite, single/multi wall carbon nanotubes) are different types of nanoparticles investigated. Eastman et al. have used combinations of Cu nanoparticles and ethylene glycol to produce nanofluids.

One of the key steps that uses nanoparticles to increase the thermal conductivity of the base fluid is the preparation of nanofluid. To create high-quality nanofluid, the nanoparticles must be uniformly dispersed and suspended in the base fluid. Nanofluids are being prepared using one-step and two-step processes, respectively. Synthesis and dispersion of nanoparticles happen simultaneously in a one-step technique. Sol-gel technique was employed by Siva Eswara et al. to create alumina nanofluid [13]. Direct condensation of the metallic vapour into the nano particles with a flowing low pressure liquid is the one-step approach utilised by Eastman et al. To create Cu/ethylene glycol nanofluids, they created a one-step physical vapour condensation procedure. [14]. Akanksha et al. produced a stable nanofluid through direct evaporation. [15]. CuO-based nanoparticles were prepared using the Submerged Arc Nanoparticle Synthesis System by Lo et al. [16] and then dispersed into deionized water. The one-step procedure involves creating and distributing the particles in the fluid simultaneously in order to decrease the aggregation of nanoparticles The one-step procedure involves creating and distributing the particles in the fluid simultaneously in order to decrease the aggregation of nanoparticles. This method avoids the drying, storing, transportation, and dispersion of nanoparticles. As a result, nanoparticle agglomeration is reduced and fluid stability is increased [17]. One-step physical preparation of nanofluids is quite expensive. Since nanofluids cannot produce nanofluids on a wide scale, one-step chemical methods are fast evolving. The literature leads to the conclusion that both strategies have certain benefits and drawbacks.. One-step procedure has an advantage here because it is challenging to produce a stable nanofluid using a two-step method. Numerous cutting-edge methods, including the one-step method, have been developed to manufacture nanofluids due to the difficulties in producing stable nanofluids using the two-step method. The one-step approach does a good job of preventing unwanted particle aggregation.

 Two Step Method: This technique involves the preparation of the nanoparticles as dry powders separately using physical or chemical methods, followed by their dispersion into the base fluid utilising magnetic force agitation, ultrasonic agitation, and high-shear mixing. Alumina nanoparicles were dispersed in the base fluid (water) in a two-step process by Akanksha et al. [18]. In order to create copper oxide nano fluid, scientists combined nanopowder with stabilisers, water, and an ultrasonic combination.

Factors which affect the heat transfer rates in nanofluids:

1.                   Thermal conductivity:

The addition of nanoparticles to the base fluid results in an increase in thermal conductivity. According to published research, nanofluids have a greater thermal conductivity than base fluids [19], [20]. The hot-wire transient device [22] and thermal property analyzer [21] are two common techniques for measuring thermal conductivity. After a thorough analysis, it was discovered that temperature, base fluid composition, particle size, shape, and material, and base fluid composition all affect thermal conductivity.

2.                   Viscosity:

The viscosity of nanofluids would not be altered by the inclusion of nanoparticles, according to Choi and Eastman [18], although this is not always true. Fluids' internal resisting force is known as viscosity. After analysing the literature, it is concluded that temperature, morphology, volume concentration, and shear rate are some of the elements that affect viscosity.

3.                   Surface tension:

In the phenomena of boiling and two-phase heat transfer, surface tension is still another crucial factor. When compared to viscosity and thermal conductivity, it has, however, attracted less attention lately. Pantzali et. al experimental .'s investigation of a nanofluid's performance in a commercial herringbone-type plate heat exchanger (PHE) and comparison to water, whose surface tension was determined using the "pendant drop" method, took place in plate heat exchangers [23,24]. It was found that the surface tension of Al2O3 and TiO2 nanofluids without the addition of a surfactant was equal to or higher than that of water. Surface tension was considerably reduced when a surfactant was used to stabilise CuO and CNT nanofluids. The following information from scientific effort is very relevant, despite the fact that there are few data on the surface tension of nanofluids. (i) Temperature has a significant impact on how tight a nanofluid's surface tension is. (ii) The influence of nanoparticle concentration on the surface tension of nanofluids is nonlinear (iii It is important to look into how the size of nanoparticles affects the surface tension of nanofluids. (iv)A crucial element in the boiling heat transfer and the mechanism of critical heat flux augmentation is the combined influence of the nanoparticles and surfactant utilised on the surface tension of the nanofluid.

Models of Nanofluids:

Tiwari and Das nanofluid model:

Because nanofluids behave like liquids, Tiwari and Das [25] gave this nanofluid model. It is also known as a homogeneous nanofluid flow model or a single component flow model. Tiwari and Das came to the conclusion that this model's increased thermal conductivity of fluids is primarily responsible for the improvement in the heat transfer characteristics of nanofluids.

Buongiorno nanofluid model:

The seven slip mechanisms that contribute to the development of the relative velocities between carrier fluids and nanoparticles were discussed by Buongiorno. But he came to the conclusion that only Brownian motion [26] and thermophoresis, out of these seven slip mechanisms, can generate relative velocity.

Application of Nanofluids:

Heat Transfer Intensification:

A new cooler created by Jang and Choi integrated nanofluids and microchannel heat sinks [27]. When compared to the device using pure water as the working medium, a higher cooling performance was attained. The thermal resistance and temperature difference between the heated microchannel wall and the coolant were both decreased by nanofluids. By swapping out the distilled water for a nanofluid made up of distilled water and Al₂O₃ nanoparticles at varying concentrations, Nguyen et al. created a closed liquid-circuit to study the improvement of heat transfer in a liquid cooling system [28]. According to measured data, adding nanoparticles to distilled water significantly improved the cooling block's convective heat transfer coefficient. According to these studies, nanofluids could improve performance compared to utilising pure water as the coolant. The improvement was caused by the thermal dispersion effect of nanoparticles and an increase in coolant's thermal conductivity.

Transportation:

By boosting efficiency, lowering weight, and simplifying thermal management systems, nanofluids offer a huge potential to enhance automotive and heavy-duty engine cooling rates. Higher horsepower engines with the same size cooling system can remove more heat thanks to the better cooling rates for automobile and truck engines. Due to their low-pressure functioning compared to ethylene glycol and water mixtures, which are utilised as automobile coolants, ethylene glycol-based nanofluids have gained a lot of attention in the application as engine coolant [29]. Because the nanofluids have a high boiling point, they can be utilised to raise the normal coolant operating temperature and then reject additional heat through the current coolant system [30]. Nanofluids were utilised by Tzeng et al. [31] to cool automatic transmissions. Transmission from a four-wheel drive car served as the experimental platform. By dispersing CuO and Al₂O₃ nanoparticles in engine transmission oil, the used nanofluids were created. The findings demonstrated that both at high and low rotating speeds, CuO nanofluids produced the lower transmission temperatures. The usage of nanofluid in the transmission offers a certain advantage in terms of thermal performance.

Cooling of Electronic Components:

Higher heat transfer coefficients are made possible by nanofluids with improved thermal conductivity. According to research done by Jang et al. employing micro channel heat sinks and diamond-water nanofluid, cooling performance was 10% better than it would have been with a micro channel heat sink made of just water. The temperature differential between the heated micro channel wall and the coolant was decreased as a result of using nanofluid [59]. When Al₂O₃-water nanofluids were used in place of water as the base fluid in a closed-circuit experiment by Nguyen et al. to study the improvement of a liquid cooling system, it was discovered that the convective heat transfer coefficient was improved by 23% for a volume concentration of 4.5% [51]. According to Chen et al., who used flat heat pipes with silver nanofluid and various particle concentrations, the boiling point was lowered as a result of an increase in effective liquid conductance and the effective thermal conductivity of the wick structure in heat pipes, which led to a decrease in thermal resistance [29]. The findings of these researches inspired scientists to create nanofluids for cooling electronic components. Diamond nanoparticles were added to the water used in high performance liquid chromatography (HPLC) by Ma et al. [57]. The efficiency of the cooling device is increased because the OHP's mobility prevents the nanoparticles from settling. When Lin et al. [30] used silver nanoparticles to study nanofluids in pulsing heat pipes, they found encouraging outcomes. The heat pipes' ability to transport heat was increased by the silver nanofluid. In order to use Al₂O₃Al₂O₃-water nanofluid in a closed cooling system intended for microprocessors or other electronic devices, Nguyen et al. [28] examined the heat transfer enhancement and behaviour of the material. Researchers are encouraged to create nanofluids for cooling electronic components as a result of the findings of their experiments.

Mass Transfer Enhancement:

The increase of mass transfer by nanofluids has been examined in many studies. First, Kim et al. [32] looked at how nanoparticles affected the bubble type of absorption for the NH₃/H₂O absorption system. The performance of the absorption is improved by the addition of nanoparticles by a factor of 3.21. Thermal gradient causes mass diffusion to occur. Diffusionthermo suggests that a concentration gradient causes heat transmission [33]. Using CNTs-ammonia nanofluids as their working medium, Ma et al. investigated the mass transfer process of absorption [34].

Energy Applications:

With a focus on the effective use and conservation of waste heat and solar energy in industry and buildings, thermal energy storage in the form of sensible and latent heat has grown in importance [35]. One of the most effective methods for storing thermal energy is latent heat storage. Al₂O₃-H₂O nanofluids were investigated by Wu et al. for its potential as a brand-new phase-change material for cooling system thermal energy storage. The thermal response test revealed that adding Al₂O₃ nanoparticles significantly lowered the water's supercooling degree, increased the freezing point's initial time, and decreased the overall freezing point [36]. A new class of nanofluid phase transition materials were created by Liu et al. by floating a small quantity of TiO₂ nanoparticles in a saturated BaCl2 aqueous solution. When compared to the base material, the thermal conductivities of the nanofluidsphase change materials were remarkably high. The heating and cooling speeds of phase change materials can be increased by adding copper nanoparticles [37]. The increased thermal conductivities of nanofluids, which improve heat transmission, and the absorption qualities of nanofluids are two outstanding properties of nanofluids that are used for energy applications. The development of storage systems was required due to energy requirements and the temporal differences between energy sources.

Solar Absorption:

The experimental findings on nanofluid-based solar collectors using CNTs, graphite, and silver [38] are described. Utilizing nanofluids as the absorption medium increased efficiency in solar thermal collectors by up to 5%. The experimental and numerical findings showed a rapid initial increase in efficiency with volume percent, followed by a plateau in efficiency as volume fraction grew further. According to theoretical research on the viability of utilising a nonconcentrating direct absorption solar collector, the presence of nanoparticles boosted incoming light absorption by more than nine times over pure water [39].  The efficiency of an absorption solar collector using nanofluid as the working fluid was found to be up to 10% greater (on an absolute basis) than that of a flat-plate collector under the same operating conditions. Studies revealed that a nanofluid-based solar collector had a slightly longer payback period for the current cost of nanoparticles but had the same economic savings at the end of its useful life as a traditional solar collector.

Friction Reduction:

Nanoparticles have garnered a lot of attention recently due to their outstanding load-carrying capability, good extreme pressure resistance, and friction-reducing qualities. On a four-ball machine, Zhou et al. assessed the tribological behaviour of Cu nanoparticles in oil. Studies revealed that zinc dithiophsphate was inferior to copper nanoparticles in terms of friction-reduction and antiwear characteristics, especially at high applied loads. The base oil's ability to carry loads could be significantly improved by the nanoparticles [40].

It was discovered that the dispersion of solid particles was crucial, particularly when a slurry layer developed. In the minimal quantity lubrication (MQL) grinding process of cast iron, water-based Al₂O₃ and diamond nanofluids were used. The formation of a dense and hard slurry layer on the wheel surface during the nanofluid MQL grinding process could improve grinding performance. The advantages of using nanofluids for minimising grinding forces, enhancing surface roughness, and preventing workpiece burning were demonstrated. MQL grinding could considerably lower the grinding temperature as compared to dry grinding [41]. Cu nanoparticles with changed surfaces used as 50CC oil additives have their wear and friction qualities examined. The tribological characteristics of Cu nanoparticles improved as oil temperature was raised. The improved tribological performances of Cu nanoparticles were likely caused by the formation of a thin copper protective coating with reduced elastic modulus and hardness on the worn surface, especially when the oil temperature was greater [42]. Due to their great dispersibility in ionic liquid, multiwalled carbon nanotube composite at room temperature was initially described by Yu et al. as a potential lubricant additive [43]. Additionally, the composite showed good characteristics for antiwear and friction reduction. According to Wang et alstudy .'s of the tribological characteristics of ionic liquid-based nanofluids containing functionalized MWNTs under loads ranging from 200 - 800 N [44], the nanofluids showed excellent friction-reduction properties under 800 N and outstanding antiwear properties when used in reasonable concentrations[45].

Industrial Cooling Applications:

There will be significant energy savings and emissions reductions through the use of nanofluids in industrial cooling. Nanofluids have the potential to save 1 trillion Btu of energy for US industry by replacing cooling and heating water [46]. Using nanofluids in closed loop cooling cycles could save the US electric power industry between 10 and 30 trillion Btu annually (equivalent to the annual energy consumption of about 50,000–150,000 households). About 5.6 million metric tonnes of carbon dioxide, 8,600 metric tonnes of nitrogen oxides, and 21,000 metric tonnes of sulphur dioxide will be reduced as a result [48]. In order to investigate the cooling capabilities of polyalphaolefin nanofluids containing exfoliated graphite nanoparticle fibres, experiments were carried out utilising a flow-loop system [49]. The specific heat of nanofluids was found to be 50% greater for nanofluids compared with polyalphaolefin, and it increased with temperature, according to these observations. It was discovered that nanofluids had a 4 times better thermal diffusivity. Nanofluids improved convective heat transmission by 10% when compared to polyalphaolefin. In order to create the most conductive coolant with a few dozen times greater thermal conductivity than that of water, Ma et al. presented the idea of nanoliquid-metal fluid [50].

The liquid metal with low melting point is anticipated to be an idealistic foundation fluid for creating superconductive solutions, which could result in the ultimate coolant in numerous heat transfer improvement fields. More conductive nanoparticles can be added to increase the liquid-metal fluid's thermal conductivity.

Nanofluid Coolant:

Due to their smaller size and better location of the radiators, nanofluids are employed as coolants. Researchers at Argonne have found that radiators' frontal areas can be reduced when high-thermal conductive nanofluids are used. [51] Singh et al. As a result of the use of nanofluid, friction and wear were reduced, parasitic losses were minimised, pumps and compressors operated more efficiently, and more than 6% less fuel was used. In the wet, dry, and lowest quantity lubrication grinding of cast iron, Shen et al. investigated the wheel wear and tribological properties. In the minimal quantity lubrication grinding method, water-based alumina and diamond nanofluids were used, and the results of the grinding were compared to those of pure water [52].  With the usage of nanofluids in engines, components would be smaller and lighter, resulting in greater gas mileage, cost savings for consumers, and less pollutants for a cleaner environment, as detailed in the literature on nanofluids.

Brake and Other Vehicular Nanofluids:

The heat generated while braking is transported throughout the brake fluid in the hydraulic braking system, dispersing the kinetic energy of the vehicle. The hydraulic system's ability to disperse the heat produced by braking is delayed if the heat raises the boiling point of the brake fluid. Arc-submerged nanoparticle synthesis is the technique used to create copper-oxide brake nanofluid. In a vacuum working environment, bulk copper metal is melted and utilised as the electrode. The electrode is submerged in dielectric liquid, and the vaporised metals are condensed in the dielectric liquid.

The plasma charging arc technique is used to create aluminum-oxide brake nanofluid. This is carried out in a manner that is quite similar to the ASNSS method. High-temperature plasma electric arcs melt aluminium metal while extensively combining it with the dielectric fluid [22]. CuO and Al2O3Al2O3 nanoparticles were mixed with motor transmission oil by Tzeng et al. [31]. A four-wheel-drive vehicle's transmission served as the experimental configuration. A cutting-edge rotary blade coupling in the transmission produced high local temperatures at rapid rotational rates. The findings showed that both at high and low rotating speeds, CuO nanofluids produced the lowest transmission temperatures. From the perspective of thermal performance, using nanofluid in the transmission provides a clear advantage. Zhang showed that surface-modified nanoparticles steadily disseminated in mineral oils are beneficial in lowering wear and increasing load-carrying capacity in automobile lubrication applications.

From the many studies, it can be inferred that adding nanoparticles to lubricants improves tribological characteristics including load carrying capacity, wear resistance, and the ability to reduce friction between moving mechanical components. The use of nanofluids to increase heat transfer rates in automotive systems is encouraged by these results.

LIMITATIONS OF NANOFLUIDS:

Mass transfer enhancement's underlying process is currently unknown. The amount of study already done on mass transfer in nanofluids is insufficient. To better understand certain crucial impacting aspects, a lot of simulation and experimental work needs to be done. Many constraints, including the long-term stability of nanofluid in suspension, prevent advancement in the field of nanofluid application. Nanofluids also have significant drawbacks. Because of the aggregation of nanoparticles caused by extremely strong vander wall interactions, which causes the suspension to be non-homogeneous, long-term physical and chemical stability of nanofluids is a significant practical concern. To create stable nanofluids, physical or chemical techniques have been used, such as I adding surfactant, (ii) altering the surface of the suspended particles, and (iii) exerting high force on the clusters of suspended particles.

Al2O3 nanofluids stored for longer than 30 days than fresh nanofluids show some settling, according to Lee and Choi [45]. Because it could result in coolant pipes becoming clogged, particle settling needs to be thoroughly studied. The efficiency of applying nanofluids depends on the pressure drop development and needed pumping power during coolant flow. The relationship between density and viscosity and pressure drop and pumping power is well recognized. Numerous studies demonstrate a considerable increase in the pressure drop of nanofluids relative to base fluid.. For a specific flow rate, one of Choi's experimental studies (2009) estimated a 40% increase in pumping power compared to water. In order for a fluid to exchange more heat, an ideal heat transfer fluid should have a greater value of specific heat. According to earlier research, nanofluids have a lower specific heat than base fluid. One step or two step processes are used to prepare nanofluids. Both techniques involve expensive and cutting-edge equipment. As a result, nanofluids cost more to produce. As a result, a disadvantage of nanofluid applications is their high cost.

Challenges for nanofluid applications in heat transfer technology:

The primary area of interest that is first investigated is the thermal conductivity of nanofluids. The results of current studies have shown that the thermal conductivity of nanofluid is greater than that of base fluid. The increase in thermal conductivity varies based on a variety of factors, including temperature, nanoparticle type, shape, and size, nanofluid concentration, and nanoparticle type. Different research teams are investigating the prediction of nanofluid thermal conductivity by experimental study and theoretical modelling. Nanofluid has the potential to be used in many different industries, including as a coolant for microchips and other devices, due to its increased thermal conductivity and promising heat transfer performance. The study of nanofluid has revealed that there are still many difficulties in using nanofluid as a heat transfer fluid, though. This study aims to evaluate a number of difficulties in using nanofluid as an alternative coolant, including nanofluid thermophysical properties assessment, heat transfer characteristics analysis, and nanofluid stability factor.

CONCLUSION:

The present paper provided an overview of a thorough investigation of nanofluid, including its preparation procedures, characteristics, stability evaluation procedures, and possible applications in the sectors of energy, biomedicine, heat transfer intensification, and mass transfer enhancement. An extensive mathematical equation for electrical conductivity for nanofluids has been proposed after analysis of the thermal conductivity, viscosity, specific heat, and electric conductivity of nanofluids. The current literature has covered the several uses for nanofluids. Nanofluids have a wide range of interesting uses, including cooling and heat transmission in a number of different industries. The experimental results from several groups did not agree with one another, so it is crucial to systematically identify these elements. Additionally, one of the possible solutions is to improve the compatibility between nanomaterials and the base fluids by altering the parameters of the interface between two phases. An essential stage in examining the properties of nanofluids is the production of the fluids. The ability of additives to influence the tiny characteristics of nanofluids makes this a crucial area for future research.

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