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The demand for efficient and sustainable energy is continuously increasing. Among the many technologies with great potential within this field are nanofluids. Nevertheless, there is still a considerable lack of information regarding their erosive effects on systems materials. In this research, the tribological behaviour of aqueous 1.33 wt% TiO2 nanofluid was investigated when jet-impinged with an average velocity of 0.8 m/s at flat targets of various materials (plastic, copper, rubber). The target surfaces were analysed using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and X-ray diffraction (XRD). It was found that impinging TiO2 nanofluid caused erosion of 282 g/( yr⋅mm2) for copper and 212 g/( yr⋅mm2) for plastic. In addition, a deposition of nanoparticles was found for rubber at rate of 2.7 kg/(yr ⋅mm2).
Molten salts-based nanofluids have been widely considered for Thermal Energy Storage (TES) applications due to their enhanced thermophysical properties. However, the application of such fluids faces many challenges, among which are the correct determination of their properties, stability, compatibility with construction materials and the overall environmental impact. In this work, we attempt to provide a comprehensive analysis of nanofluids based on nano-alumina and molten carbonate salt for the benefit of next-generation high-temperature TES applications. In particular, considerable statistics, cross-verification, novel preparation and characterization methods were applied to record ~12% increase of thermal conductivity, ~7% increase of heat capacity and ~35% increase of viscosity. It was demonstrated that such nanofluids have poor dispersion stability under static conditions; however, the enhanced thermophysical properties can be maintained by mechanical stimuli, e.g. mixing or redistribution. We show that some nanoparticles interact with typical construction materials such as stainless steel 310 by forming mixed oxides and considerably reducing the corrosion rates. An erosion study has been performed demonstrating negligible effect of nanoparticles even in the case of their strong agglomeration. Finally, life cycle analysis revealed that viscosity and preparation method of such nanofluids must be targeted to minimize the environmental impact.
Molten salts-based nanofluids are attractive candidates for thermal energy storage applications due to their enhanced thermophysical properties. However, their stability remains an open issue. In the present work, the size effect of SiO2 nanoparticles on the stability and thermophysical properties of molten binary nitrate salt was studied. For that purpose, the effect of SiO2 based nanofluids was systematically studied by using in-situ high-temperature observations and zeta potential experiments. From the analysis, the nanofluids having nanoparticles larger than 450 nm demonstrate superior stability compared to the ones with nanoparticles of 27 nm. Moreover, in contrast to the case of 27 nm particles increase of viscosity was shown to be negligible for particles larger than 450 nm. The absence of specific heat capacity (Cp) or thermal conductivity enhancement for the cases of larger nanoparticles suggests that the development of molten salts-based nanofluids is bounded by the compromise between the stability and improvement of thermophysical properties, depending on the particle size. These results open a pathway for the development of stable molten salt-based nanofluids with enhanced thermophysical properties where the size of the nanoparticles must be optimized.
A numerical investigation on foams in forced convection with Kelvin cell structure is carried out on water/Al2O3 nanofluids. The three-dimensional governing equations are written considering the single-phase model for the nanofluid. The analyzed foams have a porosity of ɛ = 0.95 and four different pore density values, with cells per inch (CPI) equal to 5, 10, 20 and 40, with assigned heat flux on the surface. The nanofluid velocity is in the range 0.003–0.1 m/s and the Reynolds number ranges from 0.6 to 5 × 10². It is observed that thermal development occurs first for the lowest velocities and for highest CPI values whereas the maximum temperature is obtained for the 5 CPI configuration and v = 0.003 m/s. Results show that the highest local entropy generation values are recorded near the points at higher temperature in correspondence with the inlet section and close to the geometric variation of the cells. The local entropy generation density reaches the maximum value at 40 CPI for the maximum velocity considered of 0.1 m/s. Global dimensionless entropy generation decreases for Reynolds number increases and the opposite is related to the dimensionless viscous entropy generation. Thermal dimensionless entropy generation is significantly greater than the dimensionless viscous entropy generation.
The worldwide increasing of thermal energy consumption fosters new technological solutions based on nanomaterials. The use of nanofluids enhances energy efficiency leading to eco-friendlier devices. Thus, researchers are encouraged to understand how modified thermophysical properties enhance heat transfer capability. Magnesium oxide based n-tetradecane nanofluids are designed in terms of stability for cold storage application. Thermal conductivity, viscosity, density, and isobaric heat capacity were determined by transient hot wire, rotational rheometry, mechanical oscillation U-tube, and differential scanning calorimetry. Furthermore, a useful relationship on thermal conductivity and viscosity of nanofluids is proposed based on Andrade, Osida and Mohanty theories. Its reliability is checked with the here reported results and literature data of different nanofluids: titanium oxide within water, silver within poly(ethylene glycol), and aluminium oxide within (1-ethyl-3-methylimidazolium methanesulfonate + water). Similar trends have been found for all nanofluids excepting titanium oxide aqueous nanofluids, this differentiated behaviour being expected by the proposed relationship.
The article focuses on the physical properties of nanofluids based on ethylene glycol (EG) with dispersed zirconium dioxide (ZrO2) nanoparticles. For this purpose, the two-step method was applied to prepare samples with five different nanoadditive volume fractions (0.002, 0.004, 0.006, 0.0081, 0.0102). No surfactants were used in the sample preparation process. All materials are commercially available and were used without any modification. To determine the physical properties of ZrO2-EG nanofluids, various techniques were used. Oscillating U-tube method, Du Noüy ring method Ohm law, and dielectric spectroscopy were applied to obtain the mass density, surface tension, electrical conductivity and permittivity, respectively. All measurements were performed at a constant temperature of 298.15 K. The effect of nanoparticles volume fraction on the physical properties of the prepared nanofluids was determined. The mass density, surface tension, electrical conductivity and permittivity increase with the increasing content of ZrO2 in ethylene glycol.
The lack of a standard experimental procedure to determine thermal conductivity of fluids is noticeable in heat transfer processes from practical and fundamental perspectives. Since a wide variety of techniques have been used, reported literature data have huge discrepancies. A common practice is using manufactured thermal conductivity meters for nanofluids, which can standardize the measurements but are also somewhat inaccurate. In this study, a new methodology to perform reliable measurements with a recent commercial transient hot-wire device is introduced. Accordingly, some extensively studied fluids in the literature (water, ethylene glycol, ethylene glycol:water mixture 50:50 vol%, propylene glycol, and n-tetradecane) covering the range 0.100 to 0.700 W m−1 K−1 were used to check the device in the temperature range 283.15 to 333.15 K. Deviations between the collected data and the theoretical model, and repeatabilities and deviations between reported and literature values, were analyzed. Systematic deviations in raw data were found, and a correction factor depending on the mean thermal conductivity was proposed to operate with nanofluids. Considering all tested effects, the expanded (k = 2) uncertainty of the device was set as 5%. This proposed methodology was also checked with n-hexadecane and magnesium-oxide-based n-tetradecane nanofluids.
Research on nanofluids has increased markedly in the last two decades. Initial attention has focused on conventional or mono nanofluids, dispersions of one type of solid nano-sized particles in a base fluid. Despite various challenges such as dispersion stability or increased pumping power, nanofluids have become improved working fluids for various energy applications. Among them, convective heat transfer has been the main research topic since the very beginning. Hybrid nanofluids, dispersions of two or more different nanoadditives in mixture or composite form, have received attention more recently. Research on hybrid nanofluids aims to further enhance the individual benefits of each single dispersion through potential synergistic effects between nanomaterials. Multiple experimental studies have been conducted independently analysing the convective heat transfer performance of mono or hybrid nanofluids for single-phase and two-phase convective heat transfer applications. However, there are still no general conclusions about which nanofluids, mono or hybrid, present better prospects. This review summarizes the experimental studies that jointly analyse both hybrid and mono nanofluids for these applications and the results are classified according to the heat transfer device used. Based on this criterion, three large groups of devices were noticed for single-phase convective heat transfer (tubular heat exchangers, plate heat exchangers and minichannel heat exchangers/heat sinks), while one group was identified for two-phase convective heat transfer (heat pipes). The main outcomes of these studies are summarized and critically analysed to draw general conclusions from an application point of view.
In this study, the thermal conductivity of aqueous nanofluids containing clusters of iron oxide (Fe3O4/γ-Fe2O3) nanoparticles has been investigated experimentally for the first time, with the aim of assessing the role of a controlled aggregation of nanoparticles in these final nanofluids. For that, clusters of iron oxide nanoparticles of different cluster size (46-240 nm diameter range) were synthesized by a solvothermal method and fully characterized by transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The rheological behavior of the optimal nanofluids was also studied by rotational rheometry. The nanofluids were obtained by dispersing the clusters of iron oxide nanoparticles in water taking into account different solid volume fractions (from 0.50 to 1.5 wt%) and the experiments were conducted in the temperature range from 293.15 K to 313.15 K. The study reveals and quantifies enhancements in the thermal conductivity of nanofluid with increase of cluster size and temperature. Furthermore, a 0.50 wt% concentration of clusters of iron oxide nanoparticles within the whole range of proposed nanofluids offers great stability and improved thermal conductivity for heat transfer applications with an small dynamic viscosity increase. In addition, the larger the size of the clusters of iron oxide nanoparticles, the greater the increase in thermal conductivity for the designed Fe3O4/γ-Fe2O3 cluster-based nanofluids, with thermal conductivity values following a constant upward trend and reaching a maximum increase of 4.4% for the largest synthesized clusters (average size of 240 nm). These results open the door for the development of iron oxide-based nanofluids on which taking advantage of an optimized aggregation of nanoparticles by using size-customized clusters.
Hybrid nanofluids aim to further improve the characteristics of mono nanofluids. However, experimental studies that jointly explore the physical properties of hybrids and the corresponding mono nanofluids are missing. In this work, mono B4C and TiB2 and hybrid TiB2:B4C nanoadditives are used for the first time to design nanofluids based on propylene glycol:water 20:80 wt%. The density, isobaric heat capacity, and thermal conductivity of the nanofluids are determined by the oscillating U-tube, differential scanning calorimetry, and transient hot wire methods, respectively. The rheological behaviour is investigated through rotational rheometry. Additionally, surface tension and electrical conductivity are investigated. The B4C mono nanofluid shows the highest improvements of thermal conductivity (6.0%) and electrical conductivity (70 times higher), but also the highest viscosity increases (51–54%). The hybrid nanofluid presents intermediate values between those of the mono nanofluids for all the properties except dynamic viscosity. Interactions between spherical and sheet-like nanoparticles explain this behaviour.
Ionic liquids have been suggested as new engineering fluids, specifically in the area of heat transfer, and as alternatives to current biphenyl and diphenyl oxide, alkylated aromatics and dimethyl polysiloxane oils, which degrade above 200 °C, posing some environmental problems. Addition of nanoparticles to produce stable dispersions/gels of ionic liquids has proved to increase the thermal conductivity of the base ionic liquid, potentially contributing to better efficiency of heat transfer fluids. It is the purpose of this paper to analyze the prediction and estimation of the thermal conductivity of ionic liquids and IoNanofluids as a function of temperature, using the molecular theory of Bridgman and estimation methods previously developed for the base fluid. In addition, we consider methods that emphasize the importance of the interfacial area IL-NM in modelling the thermal conductivity enhancement. Results obtained show that it is not currently possible to predict or estimate the thermal conductivity of ionic liquids with an uncertainty commensurate with the best experimental values. The models of Maxwell and Hamilton are not capable of estimating the thermal conductivity enhancement of IoNanofluids, and it is clear that the Murshed, Leong and Yang model is not practical, if no additional information, either using imaging techniques at nanoscale or molecular dynamics simulations, is available.
A study on impinging slot jets in a confined channel filled with metal foam, saturated with nanofluids, and lower wall partially heated is numerically accomplished. The partially heated wall is at assigned temperature and buoyancy effects are taken into consideration. A two-dimensional domain is analyzed considering the local thermal non-equilibrium for the metal foam and the single-phase model is assumed to simulate the nanofluids at assigned Rayleigh number for several Peclet numbers. The governing equation solutions are accomplished using the Ansys-Fluent code and simulations are carried out for aluminum foams with nanofluids, Al2O3/water based, as working fluids. Several porosity values, from 0.90 to about 0.97, for different pore density, pore per inches (PPI), from 5, to 40 PPI, are used. The volumetric concentrations change from 0% to 5% with a particle diameter of 30 nm. The distance between the slot jet outlet section and the lower heated surface is five times the slot jet width. The presented results point out that the dimensionless convective heat transfer coefficients increase with increase in values of Peclet number, and decrease with the volumetric concentration. It is noted that the surface heat transfer coefficient shows different behavior varying porosity for assigned Peclet and Rayleigh numbers. Some correlations for the average total Nusselt number in terms of porosity and volumetric concentration are proposed in quadratic forms for assigned pore density and Peclet number.
Nanofluids are obtained by dispersing nanoparticles and dispersant, when present, in a base fluid. Their properties, in particular their stability, however, are strictly related to several other parameters, knowledge of which is important to reproduce the nanofluids and correctly interpret their behavior. Due to this complexity, the results appear to be frequently unreliable, contradictory, not comparable and/or not repeatable, in particular for the scarcity of information on their preparation. Thus, it is essential to define what is the minimum amount of information necessary to fully describe the nanofluid, so as to ensure the possibility of reproduction of both their formulation and the measurements of their properties. In this paper, a literature analysis is performed to highlight what are the most important parameters necessary to describe the configuration of each nanofluid and their influence on the nanofluid’s properties. A case study is discussed, analyzing the information reported and the results obtained for the thermophysical properties of nanofluids formed by water and TiO2 nanoparticles. The aim is to highlight the differences in the amount of information given by the different authors and exemplify how results can be contradictory. A final discussion gives some suggestions on the minimum amount of information that should be given on a nanofluid to have the possibility to compare results obtained for similar nanofluids and to reproduce the same nanofluid in other laboratories.
The paper summarized results of experimental studies on thermophysical and electrical properties of ethylene glycol based nanofluids containing carbon black nanoparticles. Two types of nanoparticles differing in size and specific surface area were used to develop nanofluids. During examination the thermal conductivity, isobaric heat capacity, mass density, nanofluid-air surface tension, dynamic viscosity, refractive index and electrical conductivity were measured in strict controlled temperature 298.15 K. Obtained results indicate that the specific surface area have great influence on these fundamental properties of nanofluids developed with carbon nanoparticles. Finally, the enhancement of electrical conductivity described in the paper is one of the highest reported for the nanofluids in available literature.
Nanofluids’ thermophysical properties and heat transfer performance has been investigated for many years, while research on their surface tension (ST) and wetting behavior is very limited. To assess nanofluids potential as industrial products, a complete picture is required to prove their performance in a specific application. Boiling heat transfer, microfluidics and drug development are among the applications where ST is a variable. ST of water-based ZnO nanofluids were measured in the presence and absence of direct ultrasonication. The experiments covered variation of ST with ZnO concentration (0.05-0.4 vol.%), ultrasonication amplitude (40% and 100%) and duration. To the best of the authors’ knowledge, this is the first report of ST– ultrasonication process relation for a nanofluid. Results showed that after direct ultrasonication, nanofluids ST is strongly affected by the temperature raise, and in those cases relative ST may provide a clearer picture. A nano-film over individual and agglomerated nanoparticles spotted via TEM imaging was affected from the ultrasonication. Such a nano-film can play a key role in the anomalous thermal transport and wettability of nanofluids. Statistical analyses revealed that changes in ultrasonication amplitude resulted in a statistically significance difference on nanofluid ST and relative ST. Changes in nanoparticle concentration caused a significant difference on the nanofluid ST while the difference in relative ST was insignificant. Variation of ultrasonication duration caused significant variations on the relative ST while the difference in nanofluid ST was not significant. This work highlights that based on specific applications ST and other related features of any nanofluid can be adjusted employing proper ultrasonication conditions.
Experimental results of an oscillating heat pipe (OHP) operated with a ferronanofluid are reported. It is found, that the ferronanofluid increases the thermal performance of the OHP compared to the pure base fluid deionised water. Applying an outer magnetic field reduces the amount of heat transferred significantly. However, after removing the magnets the original situation is restored again. These findings give hope to employ ferronanofluids in combination with switchable magnetic fields to control the thermal performance of OHP's.
Oxidized carbon nanohorn water based nanofluids are attracting more and more attention for solar harvesting applications because of their peculiar features: stable suspension without any surfactant, higher solar absorbance values as compared to the base fluid, etc. However, these nanofluids are still matter of research because not all their properties have been comprehensively studied yet. This paper is focused on the wettability properties of the carbon nanohorn water based nanofluids on different metal surfaces. The static contact angle was measured for ten nanofluids with concentrations ranging from 0.002%wt to 0.1% wt, five of those used non-oxidized carbon nanohorns while the other five used oxidized carbon nanohorns. Moreover, this work investigates a novel use of the oxidized carbon nanohorn nanofluids to deposit an ultra-thin layer via pool boiling. The realized coating was proved to be stable and found to slightly affect the wettability of the nanofluids. Finally, the solar reflectance of the coated aluminum sample was measured and found to be from 2 to 10 times lower as compared to a reference aluminum surface.
Nanofluids have been used in several thermal management systems, as they contribute to the augmentation of the heat dissipation rates in many applications, especially those where heat pipes, pulsating heat pipes and loop heat pipes are used. Long term use of this type of device is required in order to increase their reliability, which requires chemical compatibility between the base fluid and the nanoparticle. The verification of the non-condensable gases (NCGs) generation is required as the nanofluid undergoes thermal cycling and chemical reactions are present, along with the interaction with the device's housing material. The prediction of the amount of NCGs is necessary for each base fluid and nanofluid combination, which contributes to the evaluation of the life time expectancy for those devices. For this purpose, the use of the Arrhenius model is applied, which indicates the amount of NCG generated and the time related for which the HP must undergo the ageing process. As the stability of nanofluids is strongly connected to various thermal properties, devices' thermal efficiency as well as their longevity, it is also important to employ stable nanofluids in this type of thermal control device.
This review discusses, in terms of practical aspects and future application, about heat transfer fluids properties with special focus on the possible use of ionic liquids in heat transfer applications. Even if some data were identified in the open literature, most of the ionic liquids studies on thermophysical properties are yet at the beginning and fully described chemicals are scarce. The most relevant thermophysical properties of ionic liquids (density, specific heat, thermal conductivity, and viscosity) are paralleled with the properties of several commercial heat transfer fluids. A consistent attention is paid to the analysis of heat transfer coefficient and friction factor, as well as Prandtl number. Concluding, ionic liquids can be seen as an alternative to regular heat transfer oils, however, a consistent experimental approach using real-life geometries and conditions is mandatory to be developed to fully understand all the processes.
Hybrid nanofluids are novel fluids developed as a consequence of the nanofluids research, involving pairs of nanoparticles or nanocomposite to overcome some barriers from simple nanofluids properties. This chapter summarizes some research on stability, pH, pumping power, and selection of nanoparticles to be able to develop ready to be implemented hybrid nanofluids. The research on stability of these new fluids acknowledges different testing methods and the most relevant ones are pH control and Zeta potential. In regard to thermophysical properties measurement techniques one can affirm that there is no standard approach and every group used their own facilities and the same is applying to pumping power studies, as well. If it talks about the selection of nanoparticles and base fluids, the research needs to be extended and a systematic approach is needed. If it summarizes some drawbacks one can outline the differences between different approaches (i.e., numerical or experimental), the inexistence of a nanoparticle selection method and the most important drawback is the insufficient understanding of the mechanisms related to nanofluids heat transfer and fluid flow. As a general conclusion it may affirm that a tremendous and coordinated effort is needed to overcome the implementation barriers of hybrid nanofluids in real life applications.
We report in this study the experimental investigation of the dynamic viscosity of purified multi-walled carbon nanotubes (MWCNT) water and water-propylene glycol based nanofluids in the temperature range 10-80 °C. Four weight concentrations of MWCNTs are considered, between 0.005 and 0.1 wt.%. Triton X-100, a common nonionic surfactant, is used to disperse the nanotubes and stabilize the nanofluids as evidenced by optical characterization. Purified and non-damaged MWCNTs are used for nanofluid preparation by the two-step method. MWCNT characterization is deeply investigated from a set of complementary techniques such as thermogravimetric analysis, transmission electron microscopy and Raman spectroscopy. The studied nanofluids behave as Newtonian fluids for low nanotube content while a shear-thinning behavior is noticed for higher concentration. Finally, the viscosity enhancement of nanofluids with MWCNT loading is compared to the modified Maron-Pierce model considering the presence of aggregates and their size obtained from optical observations.
The development of advanced heat transfer fluids (HTF) with enhanced heat transfer properties has been identified as a key target to increase the efficiency of industrial processes. In this work, heat transfer performance improvements of a novel nanofluid, consisting of metallic nanoparticles dispersed in a commercial thermal oil, were investigated. Nanofluids combining tin nanoparticles (1 mass %) with Therminol 66 (TH66) were synthesised using the two step-method and experimentally analysed. The effectiveness of biosurfactant addition and nanoparticle polyethylene terephthalate (PET) nanocoating for high temperature nanofluid stabilisation were independently investigated. The PET nanoscale coatings were grown by molecular layer deposition, which has been used for the first time in this field. The thermal conductivity, dynamic viscosity and specific heat capacity of the stable, oil-based nanofluids were characterised at high temperatures, and the results were compared and in good agreement with models found in the relevant literature. Finally, the heat transfer performance of the nanofluids with respect to their base fluids was evaluated, employing empirical values for the thermophysical properties of the involved materials. In this way, increments of the heat transfer coefficients up to 9.3% at 140 °C, relevant to industrial applications were obtained.
Nanoparticles (NPs) exhibit remarkable photothermal conversion efficiency under optical illumination. This light-induced heating on NPs is interesting in many different applications, such as solar radiation absorption in nanofluids, which the present work focuses on. Consequently, mastering the temperature increase undergone by NPs and the surrounding media is extremely relevant today. As nanothermometry measurements of a single NP are hard to obtain, numerical simulations can contribute to better understand the physical phenomena involved in light-induced heating. In this vein, the current work presents theoretical and numerical formulations to predict the heating of optically excited NPs. Theoretically, a thermodynamic approach is conducted to obtain balance and constitutive equations. These equations are numerically discretised in the finite element method and implemented into a research code. The main novelty of the present work lies in developing, from a multiphysics perspective, a time domain formulation capable of modelling instantaneous dissipation that can be easily extended to account for more physical phenomena. Finally, the numerical model is validated by comparing analytical and numerical results, and maximum values of 0.0014 (%) of relative error between them are reached. Then some different analysis are performed for gold, silver and graphite NPs of 20 (nm) in diameter to characterise the temperature increase they produce in the surrounding medium (water) when optically excited at a wavelength of 400 (nm) and a laser intensity of 5 × 104(W/cm2) –silver NPs exhibiting the most significant temperature increase. The influence of NP concentration on the increase of temperature in nanofluids is numerically assessed as well by testing values of NP concentration up to a maximum of 0.052 (%), which considerably enhances temperature increase. In conclusion, the present numerical tool could be used to predict light-induced heating in NPs, which could complement and reduce the number of experiments for optimising the photothermal efficiency of solar nanofluids.
A total of five Working Group (WG) meetings have been held in the frame of Nanouptake COST Action (www.nanouptake.eu) during the whole duration of the Action, with the following dates and places: • 1st WG Meetings (October 2016) Castellon (Spain) • 2nd WG Meetings (March 2017) Lisbon (Portugal) • 3rd WG Meeting (October 2017) Lisbon (Portugal) • 4th WG Meeting (May 2018) Naples (Italy) • 5th WG Meeting (June 2019) Castellon (Spain) A report from each of the four WG (Heating, Cooling, Storage, Boiling-Solar-Modelling) in each WG meeting has been summarized by the corresponding WG Leaders and Vice-leaders of the COST Action. The compilation of the different presentations of the WG meetings have been shared in open access in different documents uploaded in the website http://repositori.uji.es/xmlui/handle/10234/182627 During the first three Working Groups Meeting, each of the four WG (Heating, Cooling, Storage, Boiling-Solar-Modelling) had different locations and timetables. In those meetings, each Nanouptake participant had to choose to select which WG to attend. However, there was an increasing interest from different members to attend all the different WG, as many participants work in more than one of these applications. Following this demand, the agendas of the last two WG meetings were arranged so that all participants could attended all the WG meetings. Taking this into account, the first 3 WG meetings are split in this document in the different WG reports (sections 2 to 5 of this document) and the last 2 meetings have joint reports in section 6.
The idea of this handbook was to open a call for possible contributions in the field of nanofluids industrial applications in the energy sector among the Nanouptake network. A similar structure has been followed in all contributions with a limit of two p: a) title, authors, affiliation, b) short description of the device c) effects following from employing nanofluids, d) design rules correlations, e) photos and plots, f) references and g) contact person. A total of 27 contributions were received and distributed in different chapters covering various energy sectors: two-phase and heatpipes (4), heating & cooling (13), storage (1) and solar applications (7). An additional chapter has been dedicated to measurement of thermophysical properties (2). The handbook will be distributed among the Nanouptake network and also to relevant industrial partners, with the objective of accelerating transfer of nanofluids knowledge from fundamental research to suitable and attractive industrial application in the energy sector.
The idea behind the Workshops Proceedings document is to collect in an eBook the information of all the Nanouptake Working Group (WG) Workshops before April 2019 where the participants have been presenting their last research work in nanofluids. This has been the case of three Working Group (WG) workshops, with the following dates, locations and participants: • October 2016, Castellón (Spain), 83 participants • October 2017, Lisbon (Portugal), 85 participants • May 2018, Naples (Italy), 76 participants After this summary, a general index of the research contributions in the field of nanofluids (including title and author/s), followed by the contributions from the four different Working Groups of Nanouptake will be presented WG1: Heating WG2: Cooling WG3: Storage WG4. Boiling, Solar Application, Modelling and others To finalize, general conclusions for each of the Working Group close the document
This paper presents the preparation and thermal/physical characterization of phase change materials (PCMs) based on poly(ethylene glycol) 400 g·mol−1 and nano-enhanced by either carbon black (CB), a raw graphite/diamond nanomixture (G/D-r), a purified graphite/diamond nanomixture (G/D-p) or nano-Diamond nanopowders with purity grades of 87% or 97% (nD87 and nD97, respectively). Differential scanning calorimetry and oscillatory rheology experiments were used to provide an insight into the thermal and mechanical changes taking place during solid-liquid phase transitions of the carbon-based suspensions. PEG400-based samples loaded with 1.0 wt.% of raw graphite/diamond nanomixture (G/D-r) exhibited the lowest sub-cooling effect (with a reduction of~2 K regarding neat PEG400). The influences that the type of carbon-based nanoadditive and nanoparticle loading (0.50 and 1.0 wt.%) have on dynamic viscosity, thermal conductivity, density and surface tension were also investigated in the temperature range from 288 to 318 K. Non-linear rheological experiments showed that all dispersions exhibited a non-Newtonian pseudo-plastic behavior, which was more noticeable in the case of carbon black nanofluids at low shear rates. The highest enhancements in thermal conductivity were observed for graphite/diamond nanomixtures (3.3-3.6%), while nano-diamond suspensions showed the largest modifications in density (0.64-0.66%). Reductions in surface tension were measured for the two nano-diamond nanopowders (nD87 and nD97), while slight increases (within experimental uncertainties) were observed for dispersions prepared using the other three carbon-based nanopowders. Finally, a good agreement was observed between the experimental surface tension measurements performed using a Du Noüy ring tensiometer and a drop-shape analyzer.
The study gives a comprehensive overview of ferrohydrodynamic pipe flow experiments under the influence of magnetic fields carried out in the last decade. Both cases laminar and turbulent flow are considered. The compilation confirms that significant enhancement of heat transfer is possible as long as the magnetic field has the same orientation as the heat flux. Various effects, among them chain-like aggregation of particles, generation of secondary motion by magnetic field, and pinning of large amounts of particles on the inner pipe wall are discussed as the primary causes. The study depicts that occurrence and intensity of these effects depend on the relative strength of the imposed magnetic force compared to the inertial and viscous forces characterising the flow. Based on the compiled experimental and numerical findings, it seems to be possible to build large-scale heat transfer devices with switchable heat transfer capabilities.
In this study, a novel rotating permanent magnetic actuator (PMA) system is proposed to manipulate magnetic nanofluids to pump of chemicals inside micro-sized channels with a circular path. The PMA consists of two permanent magnet pairs and a rotor-like structure. A semicircular-shaped microchannel with a square cross-section area is located at the top of the actuator in order to investigate the performance of the PMA. Fe3O4-water magnetic nanofluid is employed as a working fluid for the manipulation inside the microchannel. In the first stage of this work, a numerical survey is conducted to determine the most suitable angular distance between permanent magnets of a pair in terms of generated magnetic field form in the microchannel region and velocity distribution of magnetic nanofluid within the semicircular microchannel when the permanent magnets are stationary. Preliminary experiments are then carried out for the stationary permanent magnets to validate the predicted flow field results. Performance tests for different PMA speeds (7.5-30 rpm) and particle concentrations (1-3% by vol.) indicate that it is possible to manipulate the magnetic nanofluid inside the semicircular channel within a velocity range of 58.7-940 µm/s, which corresponds to a flow rate range of 0.56-9.02 µL/min. The results confirm that the proposed PMA system provides flow rate requirements in analytical microfluidic applications such as low flow drug delivery (1-10 µL/min), cell sorting (6.1 µL/min), and pathogen detection (3-5.83 µL/min).
In this paper a numerical investigation in metal porous structures with Kelvin cell model is carried out on water/Al2O3 nanofluids using the single-phase model. The examined foams are characterized by a porosity ɛ = 0.95 and different values of cells per inch (CPI), equal to 10 and 20 CPI. Three different values of Al2O3 nanoparticle volume concentrations ϕ, equal to 0, 2 and 4% are used. Results are presented and discussed in terms of pressure drop between the inlet and outlet sections. Furthermore, it has been possible to obtain a correlation used to find the value of permeability and drag coefficient for metal porous structures under investigation. It has been possible to notice how these two properties of the foam do not depend on the type of fluid but only on the geometric properties. In addition, the interfacial heat transfer coefficient between the fluid phase and the solid matrix has been estimated. In conclusion, the results in terms of Nusselt number as a function of Reynolds number based on the struct diameter show that it does not change varying the CPI values.
Nanoencapsulated phase change materials (nePCMs) – which are composed of a core with a phase change material and of a shell that envelopes the core – are currently under research for heat storage applications. Mechanically, one problem encountered in the synthesis of nePCMs is the failure of the shell due to thermal stresses during heating/cooling cycles. Thus, a compromise between shell and core volumes must be found to guarantee both mechanical reliability and heat storage capacity. At present, this compromise is commonly achieved by trial and error experiments or by using simple analytical solutions. On this ground, the current work presents a thermodynamically consistent and three-dimensional finite element (FE) formulation considering both solid and liquid phases to study thermal stresses in nePCMs. Despite the fact that there are several phase change FE formulations in the literature, the main novelty of the present work is its monolithic coupling – no staggered approaches are required – between thermal and mechanical fields. Then, the FE formulation is implemented in a computational code and it is validated against one-dimensional analytical solutions. Finally, the FE model is used to perform a thermal stress analysis for different nePCM geometries and materials to predict their mechanical failure by using Rankine’s criterion.
In the present study, the pool boiling process for the refrigerant R141b and its mixtures with Span 80 surfactant and TiO2 nanoparticles has been examined. The results for the heat transfer coefficient (HTC) were taken at various boiling pressures (0.2, 0.3, 0.4 MPa) in the range of the heat fluxes 5.8–56.4 kW m−2 and for the internal boiling characteristics (IBC) such as the bubble departure diameter, frequency and velocity of bubble growth at atmospheric pressure in the range of the heat fluxes 29.6–57.0 kW m−2. We found that the additives of Span 80 and Span 80/TiO2 nanoparticles enhance the HTC at the lower heat flux densities and pressures. However, at higher values of the heat flux and pressure the HTC was deteriorated by the additives. At the same time, no significant impact was obtained for the IBCs. An analysis of the Rensselaer Polytechnic Institute model performance for the case when experimental data on the nucleation sites density is unavailable has revealed no qualitative agreement between experimental and predicted data on the HTC. Thus, we proposed a new approach that combines limited set of the experimental data (LSED) with correlations of the IBC’s versus heat flux and pressure. Finally, the LSED allowed to achieve both qualitative and quantitative agreement (within ± 10%) between predicted and experimental data on the HTC.
In the present work, Fe3O4 nanoparticles produced by the ultrasonic precipitation method and characterized by XRD, SEM, and BET methods are used to produce nanofluids using a mixture of water and ethylene glycol (WEG 50:50) as a base fluid and both sodium dodecyl sulfonate and oleic acid as surfactants. The thermal conductivity, dynamic viscosity and surface tension of these Fe3O4 nanofluids are experimentally evaluated for temperatures ranging from 253.15 to 293.15 K and different volume concentrations of nanoparticles, 0.01, 0.05 and 0.1% respectively. Experiments indicate that the thermophysical properties of nanofluids are strongly dependant on concentrations of nanoparticles and temperatures, particularly at sub-zero temperatures. Actually, it is shown that the thermal conductivity of nanofluids increases with almost 9.5%, and 14.3%, at 263.15 K and 293.15 K respectively, with 0.1 vol%. The thermal conductivity enhancement of nanofluids with concentration and temperature is compared to some relevant theoretical models. A good agreement is achieved with a comprehensive model taking into consideration effective medium theory, the nanolayer effect of molecules around the solid particle, Brownian motion of nanoparticles encompassing aggregation and nano-convection. It is also found that the dynamic viscosity of nanofluids decreases with nanoparticle content in particular below 273.15 K, up to 40% at 0.1% in volume. Surface tension decreases by adding the surfactant to the base fluid and then increases with Fe3O4 concentration with nearly 38% and 33% with 0.1% in nanoparticle volume fraction at 253.15 and 293.15 K, respectively. Finally, these results are promising in view of Fe3O4 nanofluids use in cooling applications.
The potential use of molten salt-based nanofluids as thermal energy storage material in Concentrated Solar Power plants has gained attention over the last years due to their enhanced storage capacity. The possible effects of the salt-based nanofluid production at industrial scale have not been yet investigated, as this could influence the nanoparticles agglomeration and therefore their thermal and flow properties. Four methods were evaluated for the production of solar salt-based nanofluids containing 1 wt% of silica nanoparticles. The particle size distribution, the stability, the rheological behaviour and the specific heat of the samples were measured. Nanofluids prepared by means of a dry mixing method presented the lowest viscosity, trimodal particle size distribution and lack of stability. The commonly used dissolution method coupled with oven drying in a petri dish as well as the ball milling method presented non-Newtonian behaviour and intermediate values of particle size and stability. The new spray drying method proposed provided a monomodal particle size distribution with high stability but the highest viscosity and shear thickening behaviour. Results suggest that the four methods evaluated are appropriate for specific heat enhancement (up to 21.1%) but a commitment between stability and viscosity has to be achieved.
This review discusses exclusively the recent research on electrical conductivity of nanofluids, correlations and mechanisms and aims to make an important step to fully understand the nanofluids behavior. Research on nanoparticle-enhanced fluids’ electrical conductivity is at its beginning at this moment and the augmentation mechanisms are not fully understood. Basically, the mechanisms for increasing the electrical conductivity are described as electric double layer influence and increased particles’ conductance. Another idea that has resulted from this review is that the stability of nanofluids can be described with the help of electrical conductivity tests, but more coordinated research is needed. The purpose of this article is not only to describe the aforementioned studies, but also to fully understand nanofluids’ behavior, and to assess and relate several experimental results on electrical conductivity. Concluding, this analysis has shown that a lot of research work is needed in the field of nanofluids’ electrical characterization and specific applications.
The thermal system design strongly depends on material selection. Nanofluids offer design flexibility and fine-tuning of properties by incorporation of nanoparticles in base fluids. This flexibility is guided by particle-particle communication, which may be beneficial in creating ballistic routes in heat transfer but also detrimental due to affecting nanofluid properties. The transition of nanofluids to industrial use requires application-based examinations. For this purpose, different types of nanofluids were investigated in this work in terms of their thermal efficiency in a flat plate solar collector (FPSC) and some figure-of-merits (FOMs), under laminar and turbulent flow conditions. Investigation of both aims at clarifying the correlation between FOMs and FPSC thermal efficiency, and further reporting on the validity of FOMs in assessing thermal efficiency. Results indicate that nanofluids’ eligibility as a heat transfer fluid depends on the flow condition, since a base fluid could outperform a nanofluid under turbulent flow. Nanofluid type and nanoparticle shape affects thermal performance, as suspensions of nanoplatelets/nanotubes in low concentrations (<0.04 vol%/0.25 vol%) are shown to outperform certain spherical metal-oxide nanoparticles (<3 vol%), according to some FOMs. It is shown that performance evaluation criteria (PEC), overall energetic efficiency, and energy ratio (ER) do not capture FPSC thermal efficiency trends, e.g., for graphene nanoplatelet nanofluid, as Mouromtseff number-based comparisons do for laminar and turbulent conditions. It must be highlighted that the FOM type to indicate thermal efficiency should be chosen depending on the application, and simultaneous consideration of thermal and hydrodynamic characteristics is required.
Recent efforts in using ionic liquids for the dissolution of wood constituents, including cellulose, concluded that 1-ethyl-3-methylimidazolium acetate ([emim][OAc]) is one of the most promising ionic liquids to dissolve cellulose. However, in order to make possible any industrial application, thermophysical properties of the pure liquid and of its mixtures with water are needed. In this study the influence of water on the thermophysical properties of [emim][OAc] is discussed. Density, speed of sound, viscosity, electrical conductivity, thermal conductivity, refractive index and infinite dilution diffusion coefficients were measured for mixtures of [emim][OAc] and water, in the temperature range 293.15–333.15 K, at 0.1 MPa, and full composition range. The rheological behaviour was also tested. The mixture was found to be completely miscible. Water free values for the properties of pure ionic liquid were extracted from mixture data, using new methodologies. Results show that the mixture is highly structured, dominated by hydrogen bonding interactions between the water molecules and the ionic liquid acetate anions. The values of the properties establish a consistent and accurate data set for chemical plant cellulose dissolution and other future applications of these mixtures.
New systems using solid particles for solar energy capturing, heat transfer and thermal energy storage have been proposed and analysed in direct particle solar receivers. In this work, black coloured silica sand was investigated as a possible solid particle for such combined systems. Two different methods based on a carbon coating approach were implemented to black colour the initial material to improve their solar absorption characteristics. The morphology of the raw and coloured sands was analysed by scanning electron microscopy (SEM), particle size characterisation and porosity measurements. The coating of the black-coloured silica sands was evaluated by thermogravimetry. Solar absorption was characterised in a double-beam UV-VIS spectrophotometer combined with an integrating sphere, and with enhancements of approximately 100%, found for both coloured sands. The thermal storage and heat transfer capabilities of the initial and coated sands were measured at different temperatures. Some improvements in the specific heat capacity and reductions in thermal conductivity due to porosity changes were observed.
Ionic liquids have proved to be excellent heat transfer fluids and alternatives to common HTF’s used in industry for heat exchangers and other heat transfer equipment. However, its industrial utilization depends on the cost/kg of its production, to be competitive for industrial applications with biphenyl and diphenyl oxide, alkylated aromatics and dimethyl polysiloxane oils, which degrade above 200ºC and possess some environmental problems. The efficiency as a heat transfer fluid depends on the fundamental thermophysical properties necessary to model convective heat transfer (density, heat capacity, thermal conductivity and viscosity) and calculate the heat transfer coefficients in given heat exchangers geometries. 1-ethyl-3-methylimidazolium methanesulfonate [C2mim][CH3SO3] (CAS Number 145022-45-3), (ECOENG™ 110) is actually produced by BASF, under the trade name of Basionics® ST 35, with an assay ≥ 97% with ≤ 0.5% water and ≤ 2% chloride (Cl⁻). Density, speed of sound, heat capacity, viscosity, electrical and thermal conductivity of the industrial product, and after drying under vacuum, were obtained for the temperature range 283.15 – 363.15 K, at atmospheric pressure. Values of the thermal properties studied showed an interesting phase behavior below the melting point (303 K), which suggest the existence of second order-disorder transitions (λ-type) before reaching the freezing point. A study was carried out to investigate the toxicity of [C2mim][CH3SO3] in saltwater crustaceous. The lethal concentration was determined with Artemia and the LC50 found to be higher than with an organic solvent, such as ethylene glycol.
The effective thermal conductivity and convective cooling performance of nanoparticles-loaded fluids (i.e., nanofluids) in mini- and micro-channels systems are presented. Research findings on the direct application of these new fluids in cooling systems of electronic devices are also discussed and analyzed. Results showed that nanofluids possess significantly higher thermal conductivity compared to their base conventional fluids. These new fluids also exhibit enhanced convective heat transfer coefficients and reduced thermal resistance in mini- and micro-channels as well as microchannel heat sinks demonstrating their potential in cooling high heat generating electronic devices and systems. Research using nanofluids in electronic devices like computational/central processing units (CPU) confirmed that these emerging nanoparticles-dispersed fluids can outperform conventional coolants in cooling such modern electronics. In addition, future needs and challenges of these new coolants are also highlighted.
We use molecular dynamics simulation to study the effect of suspended carbon nanomaterials, nanotubes and graphene sheets, on the thermal conductivity of ionic liquids, an issue related to understanding the properties of nanofluids. One important aspect that we develop is an atomistic model of the interactions between the organic ions and carbon nanomaterials, so we do not rely on existing force fields for small organic molecules nor assume simple combining rules to describe the interactions at the liquid-material interface. Instead, we use quantum calculations with a density functional suitable for non-covalent interactions to parameterize an interaction model, including van der Waals terms and also atomic partial charges on the materials. We fitted a n-m interaction potential function with n values 9 or 10 and m values between 5 and 8, so a 12-6 Lennard-Jones function would not fit the quantum calculations. For the atoms of ionic liquids and carbon nanomaterials interacting among themselves we took existing models from the literature. We studied the imidazolium ionic liquids [C4C1im][SCN], [C4C1im][N(CN)2], [C4C1im][C(CN)3] and [C4C1im][(CF3SO2)2N]. Attraction is stronger for cations (than for anions) above and below the π-system of the nanomaterials, whereas anions show stronger attraction for the hydrogenated edges. The ordering of ions around and inside (7,7) and (10,10) single-walled nanotubes, and near a stack of graphene sheets, is analysed in terms of density distribution functions. We found that anions are found, as well as cations, in the first interfacial layer interacting with the materials, which is surprising given the interaction potential surfaces. The thermal conductivity of the ionic liquids and of composite systems containing one nanotube or one graphene stack in suspension was calculated using non-equilibrium molecular dynamics. Thermal conductivity was calculated along the axis of the nanotube and across the planes of graphene, in order to see the anisotropy. In the composite systems containing the nanotube there is an enhancement of the overall thermal conductivity, with calculated values comparing well with experiments on nanotube suspensions, namely in terms of the order of the different ionic liquids. In the systems containing the graphene stack, the interfacial region of ionic liquid near the surface of the material has an enhanced thermal conductivity with respect to the bulk liquid, but no significant discontinuity in the temperature profiles were observed. This is important information for models of thermal conduction in nanofluids.
The application of nanofluids in direct solar absorption, heat transfer or direct solar steam generation entails carrying out a comprehensive study taking into account several physical quantities. Long-term stability, rheological, thermophysical and optical properties of dispersions must be known to assess their potential for envisaged applications. Two low-concentration nanofluids, 0.005 and 0.05 wt%, of sulfonic acid-functionalized and polycarboxylate chemically modified graphene nanoplatelets in water were considered in this work. Elemental analyses of the nanopowders and pH evaluations of the colloids were carried out. The rheological behaviour of dispersions at different temperatures was studied by rotational rheometry. Thermal conductivities were measured by the transient hot wire method and densities by the oscillating U-tube technique. Additionally, a brief report of the optical properties was included to provide a comprehensive physical analysis.
The exceptional thermal properties of graphene are maintained in a few orders of magnitude in its multi-layered structure, graphene nanoplatelets, with a considerably lower production cost. Ethylene glycol and water binary mixtures are commonly used as working fluids in different applications, such as the automotive sector, industrial cooling systems, renewable domestic heating systems, or solar and wind power plants, due to the protection provided against low freezing temperatures. In this work, the heat transfer performance of different-loaded sulfonic acid functionalized graphene nanoplatelet nanofluids (0.25, 0.50 and 1.0 wt%) based on an ethylene glycol:water 50:50 vol% mixture was experimentally assessed. Firstly, thermophysical and rheological evaluations for base fluid and nanofluids were developed in a wide temperature range (293.15 to 333.15 K). Densities, isobaric heat capacities, thermal conductivities and dynamic viscosities for base fluid and nanofluids were determined by vibrating tube, differential scanning calorimetry, transient hot wire and rotational rheometry methods, respectively. Subsequently, convective heat transfer coefficients and pressure drops/pumping powers were experimentally determined through a test rig with a concentric tube heat exchanger as main element at different working temperatures (from 298.15 to 318.15 K) and flow rates (from 300 to 700 dm3·h−1). Furthermore, a dimensionless analysis from the obtained results was carried out. The proposed new 0.25 wt% nanofluid achieves the higher convective heat transfer performance. Finally, a summary and a comparison of the results for different glycolated water-based carbon nanofluids evaluated in the same experimental facility are reported.
The main objective of this study is to design and characterize silver suspensions based on poly(ethylene glycol) PEG400, Ag/PEG400, as energy storage media for low-temperature applications. A polyvinylpyrrolidone (PVP) treatment was applied to ~22 nm silver nanoparticles to ensure good stability in poly(ethylene glycol). An array of different experimental techniques was utilized to analyze the molecular mass and purity of base poly(ethylene glycol), morphology of dry PVP-capped Ag nanoparticles, hydrodynamic average size of dispersed Ag particles, as well as thermal stability of PEG400 and Ag/PEG400 dispersions. Samples exhibited good temporal stabilities with average hydrodynamic diameter around 50 nm according to dynamic light scattering analyses. Melting and solidification transitions were investigated in terms of temperature and enthalpy from differential scanning calorimeter (DSC) thermograms. The thermophysical characterization was completed with thermal conductivity (k), dynamic viscosity (η), isobaric heat capacity (Cp), density (ρ), and surface tension (σ) measurements of designed materials using a Hot Disk thermal conductivimeter, a rotational rheometer, a DSC calorimeter working with a quasi-isothermal modulated method, a U-tube densimeter and a drop shape analyzer, respectively. For a nanoparticle loading of only 1.1% in mass, sub-cooling reduced by 7.1% and thermal conductive improved by 3.9%, with almost no penalization in dynamic viscosity (less than 5.4% of increase). Maximum modifications in Cp, ρ, and σ were 0.9%, 2.2%, and 2.2%, respectively. Experimental results were compared with the values provided by using different theoretical or semi-empirical equations. In particular, good descriptions of dynamic viscosity as functions of temperature and nanoparticle volume concentration were obtained by using the Vogel–Fulcher–Tammann equation and a first-order polynomial η( ϕ v , n p ) correlation, with absolute average deviations of 2.2% and 0.55%, respectively.
Hybrid nanofluids, suspensions of nanocomposites or nanoparticle-decorated complex structures in conventional fluids, are focusing the attention of the research in the recent years because of the potential synergistic effects among particles. Nevertheless, deep studies based on systematic experimental investigations are missing. Thus, in this work, a propylene glycol:water 10:90 wt% mixture was proposed as base fluid while various ratios (0:1, 1:4, 1:1, 4:1 and 1:0) between the constituents present in the hybrid nanoadditive (silver/graphene nanoplatelet with different functionalizations) were swept. Eleven nanofluids with three total nanoadditive concentrations of 0.05, 0.1 and 1 wt% were designed. The stability of the samples was assessed by dynamic light scattering measurements over the time. Experimental optical, rheological and thermophysical profiles jointly were analysed with the aim to produce a comprehensive elucidation of the new hybrid nanofluids. Optical properties were investigated through optical spectroscopy, rheological properties through rotational rheometry, thermal conductivity by means of transient hot-wire method, and density through oscillating U-tube technique. The optical transmittance results for all analysed nanofluids confirmed that more than 90% sunlight extinction is achieved for a path length lower or equal to 6 mm. Its spatial distribution in the nanofluid volume can be tailored acting on the concentration of each nanoadditive, allowing the design of customized miniaturized systems. Newtonian behaviour was evidenced for all samples, with viscosity increases lower that 9% for all 0.1 wt% nanofluids, those with optical applications. Thermal conductivity results showed practically no variations with respect the base fluid for all samples with this total concentration.
Continuous technological development in automotive industries has increased the demand for high efficiency engines. Optimizing design and size of a radiator in order to reduce a vehicle weight is a requirement for making the world green. Using of fins is one of the techniques to increase the cooling rate of the radiator. However, traditional approach of enhancing the cooling rate by using fins and microchannels has already showed to their limit. Furthermore, heat transfer fluids such as water and ethylene glycol have very low thermal conductivity. As result there is an urgency for new and innovative heat transfer fluids for increasing heat transfer rate in an automotive cooling circuit. Nanofluids represent potential substitute of conventional coolants in engine cooling system. In this paper a cooling circuit is modelled in TRNSYS (version 17) and investigated in transient regime. The circuit is composed by a heating system, a pump and a heat exchanger. The heat exchanger represents a car radiator in the circuit. The nanofluid is a mixture of water and Al2O3 nanoparticles. The study is conducted for three different engine operating conditions (Low, Medium and High) and different volume concentrations of nanoparticles.
Convective flow with chemical reaction in a vertical duct using third kind boundary conditions is investigated. The heat is exchanged from the walls of the duct and the external fluid. The equations of balance are written in dimensionless form taking into account the effect of viscous dissipation. The inclusion of viscous dissipation reflects the transformation of original balance equations into nonlinear equations. Hence the closed form solutions are not possible. Therefore, the balance equations are resolved by the classical explicit Runge–Kutta method combined with shooting method which can be employed for various magnitudes of Brinkman number. The solutions obtained numerically are justified by solving the balance equations analytically using the perturbation technique which is applicable only for Brinkman numbers to be less than one. The flow profiles are shown graphically for different values of the thermal and solutal Grashof numbers, Biot numbers, Brinkman number, and chemical reaction parameter. The impacts of physical characteristics on the friction factor and Nusselt number are also evaluated and the results are tabulated. The solutions received by perturbation technique and Runge-Kutta method are equal when Br=0 and as the Brinkman number increases, the error also increases.
Renewable energy has become of great interest over the past years in order to mitigate Global Warming. One of the actions gaining attention is the enhancement of the thermal energy storage capacity of Concentrated Solar Power plants. The addition of nanoencapsulated phase change materials (core-shell nanoparticles) to the already used materials has been proposed for that purpose, due to the possibility of increasing thermal storage through the contribution of both core latent heat and sensible heat. In this work, Atomic Layer Deposition has been used to synthesise SiO2 and Al2O3 nanoscale coatings on tin nanoparticles. The multi-encapsulated phase change materials have been characterised in terms of chemical composition, crystalline structure, particle size, thermal stability and thermal storage capacity. Sn@Al2O3 nanoparticles present the best thermal behaviour as they show the lowest reduction in the phase change enthalpy over 100 cycles due to the oxidation barrier of the coating. Moreover, the specific heat of both nanoparticles and solar salt-based nanofluids is increased, making the nanoencapsulated phase change material suitable for thermal energy storage applications.
Nanofluids are considered as promising alternative in heat exchange processes to the classical fluids, which usually present poor thermal properties. One interesting application for nanofluids is as heat transfer fluid in solar thermal applications plants. Boron nitride nanotubes present interesting thermophysical properties for use in nanofluids. Therefore, nanofluids based on boron nitride nanotubes were prepared by a two-step method, dispersing this nanomaterial in a heat transfer fluid typically used. Stability, rheological and thermal properties of the nanofluids were analysed. To check the stability, ultraviolet–visible spectroscopy and particle size and ζ-potential measurements were performed for a month, obtaining that the nanofluids were stable. Furthermore, surface tension was measured and no significant differences were observed with regard to the base fluid. In a variable range of temperature, nanofluids show Newtonian behaviour with a slight increase in viscosity. Besides, the boron nitride nanotubes caused an increase in thermal conductivity of up to 33% with regard to the base fluid. The use of these nanofluids also led to an improvement in the heat transfer coefficient under turbulent flow conditions of up to 18%. Finally, the analysis of the outlet temperature in solar thermal applications shows that these nanofluids are a promising alternative in this application.
Thermosyphons working with nanofluids and surfactant solutions demonstrate potential to improve the efficiency of heat transfer and to reduce the risk of high mechanical load caused by geyser boiling. This paper presents the summary of the experimental study on this phenomenon and proposes a methodology of data analysis. Two carbon-based nanofluids, surfactant solution and water were tested. Nanohorns nanofluid and sodium dodecyl sulfate solution supressed the geysering for high temperatures of evaporator.
Magnetic nanofluids are colloidal mixtures of ferromagnetic nanoparticles dispersed in a base fluid. They can be actuated and manipulated under the influence of the external magnetic field. This makes them especially attractive to be employed in microfluidics and nanofluidics. In the presence of the external magnetic field, thermal conductivity and viscosity of the magnetic nanofluids can be tuned, hence magnetic field dependent thermal conductivity and viscosity measurements have become a hot topic for the researchers. In this paper, studies in the available literature on the thermal conductivity and the viscosity of the magnetic nanofluids in the presence of the magnetic field have been collected, compared and discussed. The observations reveal that there is a contradiction between the results which were presented in the literature. The differences between the available experimental results which may be caused by the application of the external magnetic field have been discussed by categorizing and comparing the studies which investigated the influence of the similar parameters by using most similar samples. Additionally, magnetic field dependent thermal conductivity and viscosity models available in the literature have been reviewed.