An experimental study of novel nanofluids based on deep eutectic solvents (DESs) by Choline chloride and ethylene glycol

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGIn this work, the preparation and characterization of some new nanofluids based on deep eutectic solvents (DESs) consisting of a hydrogen bond acceptor, ethylene glycol (EG), and a hydrogen bond donor, choline chloride (ChCl), as well as water, are presented. The nanofluids were designed by the dispersion of spherical MgO nanoparticles in four different DESs, ChCl:EG (molar ratio of 1:2), 1ChCl:5EG, 1ChCl:2EG:2Water, and 1ChCl:5EG:2Water. The stability of nanofluids was carried out by measuring size distribution for five days, which discovered the best results obtained with nanofluids of dispersed MgO in DES 1ChCl:5EG. Thermophysical properties (thermal conductivity and density) were measured and the influence of nanoparticles’ mass fraction, temperature, and water content all were examined. The acquired outcomes revealed that the trend of density was reducing by increment in temperature since for pure base fluids and DES-based nanofluids 1.3% decrement were recorded, averagely (the decline in density was sharper in the case of DES 1ChCl:5EG and its based nanofluids). The thermal conductivity was almost constant during the range of 283.15–333.15 K. It confirmed that the thermal conductivities of prepared nanofluids based on DESs with water were higher in comparison to the ones based on DESs without water and nanoparticles concentration could promote thermal conductivity. The greatest enhancement was gained at 10 wt% of MgO suspended in DES 1ChCl:2EG. The isobaric thermal expansivity was also determined at different temperatures. Eventually, the general conclusions were drawn and concerning the results, the MgO/DES 1ChCl:2EG 10 wt% nanofluids was introduced as the most efficient.Agencia Estatal de Investigación | Ref. PID2020-112846RB-C21Agencia Estatal de Investigación | Ref. PDC2021-121225-C21Agencia Estatal de Investigación | Ref. ENE2017-86425-C2-1-

Phase change characterization of eco-friendly isopropyl palmitate-based graphene nanoplatelet nanofluid for thermal energy applications

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGThermal energy storage (TES) facilitates the integration of renewable energy by decoupling production and consumption, mitigating intermittence issues. However, current TES systems use fossil fuel-based products as storage media, so the development of sustainable and efficient materials should be investigated. This work aims to the design and characterization of new eco-friendly nanoenhanced phase change materials (NePCMs) based on dispersions of graphene nanoplatelets of two different lateral sizes (7.2 and 40 μm) in isopropyl palmitate for cold storage applications. The stability of the NePCMs was analysed by dynamic light scattering, selecting Span® 80 as surfactant to improve the stabilization of both nanomaterials within the base material. The influence of the nanoadditive concentration on the temperature transitions and on the solid–liquid phase change were comprehensively studied by differential scanning calorimetry, finding out that the dispersed nanoadditives do not alter the polymorphism of the isopropyl palmitate. Additionally, reductions in the sub-cooling effect higher than 2 K for both nanoplatelets were found, with increases in the latent heat up to 5.9 and 3.9% for the shorter and the longer nanoplatelets, respectively. Isobaric heat capacities were also determined by temperature-modulated differential scanning calorimetry, reporting maximum reductions of 12% compared to the base material.Agencia Estatal de Investigación | Ref. PID2020-112846RB-C21Agencia Estatal de Investigación | Ref. PDC2021-121225-C21Agencia Estatal de Investigación | Ref. ENE2017-86425-C2-1-RXunta de Galicia | Ref. ED481A-2018/287Xunta de Galicia | Ref. ED431C 2020/06Xunta de Galicia | Ref. ED481A-2021/28

Hybrid or mono nanofluids for convective heat transfer applications. A critical review of experimental research

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.Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGAgencia Estatal de Investigación | Ref. PID2020-112846RB-C21Ministerio de Ciencia e Innovación | Ref. PDC2021-121225-C21Xunta de Galicia | Ref. ED481A-2018/28

Thermophysical, rheological and dielectric behaviour of stable carbon black dispersions in PEG200

Phase change materials can store or release large amounts of energy during phase change. An increasing number of authors are studying the influence of the dispersion of nanometric particles on these materials. This article presents the design and experimental characterization of temporal stability, thermal conductivity, isobaric heat capacity, phase change transitions, rheological behaviour, and dielectric properties of nano-enhanced phase change materials based on carbon black (CB) dispersions in polyethylene glycol (PEG200) by using polyvinylpyrrolidone (PVP) as surfactant. We studied the temporal stability of carbon black nanoparticles dispersed in PEG200 using dynamic light scattering and spectrophotometry techniques. All the samples showed good temporal stability, since the measurements of the hydrodynamic size of the nanoparticles are practically constant over time and the wavelength observed by UV–vis shows a small variation of around 4% for static conditions. We observed small changes in thermal conductivity and isobaric heat capacity. Nevertheless, the thermograms evidence how the latent heat clearly increases with the load of carbon black nanoparticles up to four times that of the PEG200. The viscosity studies do not show variation with shear rate, indicating a Newtonian behaviour, excluding the 2.0 wt% CB/PVP + PEG200 nanofluid. Additionally, we noticed frequency dependent and independent regions for permittivityAgencia Estatal de Investigación | Ref. PID2020-112846RB-C21Agencia Estatal de Investigación | Ref. PDC2021-121225-C21Ministerio de Universidades | Ref. 33.50.460A.752European Cooperation in Science and Technology | Ref. IG15119Universidade de Vigo/CISU

A comprehensive study of the thermophysical and rheological properties of ZrO2 based nanofluids as geothermal fluids

Geothermal heat pump systems in residential and commercial applications have become popular in many countries over the past years. The heat transfer performance of the ground heat exchangers in these systems has still room for improvement since they have huge influence on the overall efficiency. Likewise, new heat transfer fluids with enhanced properties, known as nanofluids, have been proposed as a potential solution to substitute the conventional working fluids and to improve the heat transfer processes and performance. A reliable and appropriated proposal of nanofluids for a particular application must include a complete fluid dynamic characterization including thermophysical, rheological, heat transfer coefficients, and pressure drops analysis, as well as physical or chemical characterization of the nanomaterial. In this study, a novel proposal of propylene glycol:water (10:90 vol%)-based zirconium oxide nanofluids of different nanoparticle mass concentrations (0.25, 0.50, 0.75, 1.0, and 5.0 wt%) as possible geothermal working fluids and their thermophysical and rheological characterization are performed. Thus, the nanopowder was extensively investigated by means of Transmission Electron Microscopy, High Resolution Transmission Electron Microscopy, X-Ray diffraction, and Ultraviolet visible spectroscopy obtaining the shape, size distribution, d-spacing, electron diffraction pattern, and crystallinity. Then, thermal conductivities, dynamic viscosities, densities, and isobaric heat capacities for base fluid and nanofluids were measured by transient hot wire, rotational rheometry, vibrating tube, and differential scanning calorimetry methods, respectively. Increases in thermal conductivity, dynamic viscosity, and density of the nanofluids up to 2.8%, 13%, and 4.1% were found, respectively, while decreases in heat capacity reached 11% in comparison to the base fluid. Different models and equations were also employed to analyse the experimental data.Agencia Estatal de Investigación | Ref. PID2020-112846RB-C21Agencia Estatal de Investigación | Ref. PDC2021-121225-C21European Cooperation in Science and Technology | Ref. CIG15119Fundação para a Ciência e a Tecnologia | Ref. UIDB/50022/2020Agencia Estatal de Investigación | Ref. PRE2021-097589Xunta de Galicia | Ref. ED481A-2021/284Universidade de Vigo/CISU

Preparation and characterization of stable methyl myristate−in−water nanoemulsions as advanced working fluids for cooling systems

Phase change material emulsions (PCME) have gained increasing scientific interest due to their potential to enhance the storage capability of thermal facilities. Herein we present the design and characterization of oil−in−water (O/W) nanoemulsions by employing a dispersed phase mixture (2–12 wt%) enriched in methyl myristate as phase change material. The emulsifier and dispersed phase compositions were optimized based on dynamic light scattering and calorimetric analyses. A two−surfactant formulation composed of sodium dodecyl sulfate and BrijTM S2 (20:49 in weight) was selected to produce stable colloidal dispersions of a methyl stearate:n–hexadecane:methyl myristate mixture (at a mass proportion of 1:3:36) in water. No phase separation or significant growth in emulsified droplet size was detected under storage conditions or when the slurries were subjected to different heating−cooling cycles. The melting/crystallization transitions, rheological behavior, thermal conductivity and density of optimized nanoemulsions were experimentally investigated in order to further understand how the concentration and physical state of suspended droplets may influence those thermal and physical properties. According to differential scanning calorimetry studies, slurries showed moderate subcooling degrees (∼3 °C), even though their solid−liquid transitions extended over a slightly wider range of temperatures than the same mixture used as the dispersed phase but in bulk−form. The shear−thinning character observed for developed nanoemulsions at low temperatures disappeared with the melting of suspended droplets. Considering an operating temperature interval of 15 °C around melting−crystallization phase changes, the 12 wt% optimized suspension presented a storage capacity 18 % higher than that of water under the same conditions. Furthermore, thermal reliability tests verified that phase change characteristics did not significantly changed after 8 months of storage and throughout 500 thermal cycles.Agencia Estatal de Investigación | Ref. PID2020-112846RB-C21Agencia Estatal de Investigación | Ref. PID2022-136443OB-I00Agencia Estatal de Investigación | Ref. IJC2020-043779-

Experimental methodology to determine thermal conductivity of nanofluids by using a commercial transient hot-wire device

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.Ministerio de Ciencia e Innovación | Ref. PID2020-112846RB-C21Ministerio de Ciencia e Innovación | Ref. PDC2021-121225-C21Xunta de Galicia | Ref. ED481A-2018/287Ministerio de Economía y Competitividad | Ref. ENE2017-86425-C2-1-

A new relationship on transport properties of nanofluids. Evidence with novel magnesium oxide based n-tetradecane nanodispersions

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGThe 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 improve 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.Agencia Estatal de Investigación | Ref. ENE2017-86425-C2-1-RAgencia Estatal de Investigación | Ref. PID2020-112846RB-C21Agencia Estatal de Investigación | Ref. PDC2021-121225-C21Xunta de Galicia | Ref. ED431C 2020/06Xunta de Galicia | Ref. ED481A-2018/28

Thermal conductivity and viscosity measurements of ethylene glycol-based Al2O3 nanofluids

The dispersion and stability of nanofluids obtained by dispersing Al2O3 nanoparticles in ethylene glycol have been analyzed at several concentrations up to 25% in mass fraction. The thermal conductivity and viscosity were experimentally determined at temperatures ranging from 283.15 K to 323.15 K using an apparatus based on the hot-wire method and a rotational viscometer, respectively. It has been found that both thermal conductivity and viscosity increase with the concentration of nanoparticles, whereas when the temperature increases the viscosity diminishes and the thermal conductivity rises. Measured enhancements on thermal conductivity (up to 19%) compare well with literature values when available. New viscosity experimental data yield values more than twice larger than the base fluid. The influence of particle size on viscosity has been also studied, finding large differences that must be taken into account for any practical application. These experimental results were compared with some theoretical models, as those of Maxwell-Hamilton and Crosser for thermal conductivity and Krieger and Dougherty for viscosity.Ministerio de Educación y Ciencia | Ref. CTQ2006-15537-C02Xunta de Galicia | Ref. PGIDIT07PXIB314181P

A numerical approach in the assessment of a new class of fluids performance in laminar flow

The present paper focuses on the analysis of the thermal conductivity for both base fluid and their ionanofluids. Sequently, an analysis of the new heat transfer fluids behavior in laminar flow was performed to compare the experimental and theoretical results. In terms of experimental, the results showed a higher thermal conductivity for the ionanofluids with the highest nanoparticles concentration. Plus, the thermal conductivity is decreasing slightly with nanoparticles mass fraction while it is almost constant with increasing temperature. On the other hand, in the case of thermal conductivity theoretically determined, this is decreasing with increasing temperature. Furthermore, Mouromtseff number was used to evaluate the relative heat transfer capacity of different fluids. Higher Mouromtseff indicates a better heat transfer capacity of a certain fluid compared to a regular one. Concluding, the present study compared the thermal conductivity for base fluid and Al2O3 dispersed in [C 2 mim][CH 3 SO 3 ]:H 2 O in the particle concentration range of 1–10 % wt and temperatures between 283.50 and 333.38 K. The results demonstrate that the thermal conductivity enhances with concentration increase. The maximum thermal conductivity enhancement of 10.2% was found for 10 % wt of ([C 2 mim][CH 3 SO 3 ]:H 2 O) + Al 2 O 3 ionanofluids. An increased Mouromseff number was noticed for theoretical thermal conductivity