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The aim of this paper is to investigate in depth whether adding nanoparticles or nanotubes to a fluid enhances its heat transfer capabilities. For this reason, the thermal conductivities and viscosities of a selection of nanofluids were thoroughly examined. The systems studied were (a) ethylene glycol with added CuO, TiO2, or Al2O3 nanoparticles and (b) water with TiO2 or Al2O3 nanoparticles or multiwall carbon nanotubes (MWCNTs). All of the measurements were conducted at 298.15 K. In a very recent paper, it was shown that instruments employing the transient hot-wire technique can produce excellent measurements when a finite element method (FEM) is employed to describe the instrument for the geometry of the hot wire. Furthermore, it was shown that an approximate analytic solution can be employed with equal success, over the time range from 0.1 to 1 s, provided that four specific criteria are satisfied. Subsequently a transient hot-wire instrument was, designed, constructed, and employed for the measurement of the thermal conductivities of nanofluids with an uncertainty of about 2%. A second, validated technique, namely, a hot-disk instrument, was also employed to conduct measurements on some of the systems to provide mutual support for the tesults of the thermal conductivity measurements. To investigate the effect of any enhancement Of the thermal conductivity of the fluids on their application in practical heat transfer, the viscosities of typical concentrations of:several of the nanofluids were also measured. A parallel-plate rotational rheometer, able to measure the viscosities of Newtonian and non-Newtonian liquids with an uncertainty of better than 5%, was employed for, these measurements because most of the nano fluids considered showed behavior comparable to a Bingham plastic. All of these measurements have:allowed an investigation of the change in the heat transfer capability of the base fluid when nanoparticles or MWCNTs are added to it for a typical heat exchanger. It is shown that in general the combined changes in physical properties that accompany suspension of nanoparticles in fluids mean that the heat transfer benefits are all rather modest, even when they are achieved.

cis-1,2,3,3,3-Pentafluoroprop-1-ene (R1225ye(Z)) is a non-flammable potential working fluid possessing low global warming potential. Toxicity testing has uncovered some concerns with sub-chronic and chronic exposures. In this paper, ninety-six vapor pressure data points for a saturation temperature range from approximately 233 K to 366 K from two different laboratories are presented. These data are regressed using Wagner and extended Antoine correlations. The root mean square errors of the Wagner and Antoine correlations relative to the measured data are 0.084% and 0.094%, respectively. The vapor pressure curve of R1225ye(Z) is shown to be similar to trans-1,3,3,3-tetrafluoroprop-1-ene (R1234ze(E)). The normal boiling temperature, critical temperature, critical pressure, critical density, acentric factor, and ideal gas specific heat at constant pressure at a temperature of 300 K of R1225ye(Z) are estimated to be 253.62 K, 380.05 K, 3529 kPa, 517.17 kg m−3, 0.3256, and 101.9 kJ kmol−1 K−1, respectively.

Energy demand in the world is continuously increasing and fossil fuels resources must be replaced by renewable resources with lower environmental risk factors, in particular CO 2 emissions. Concentrating solar collectors appear to be very promising for that purpose. Thus, this work presents a numerical analysis for the evaluation of the yearly yield of a low-enthalpy parabolic trough solar collector (PTC). To increase the thermal efficiency of such systems, six water-based nanofluids at different weight concentrations are investigated: Fe 2 O 3 (5, 10, 20 wt%), SiO 2 (1, 5, 25 wt%), TiO 2 (1, 10, 20, 35 wt%), ZnO (1, 5, 10 wt%), Al 2 O 3 (0.1, 1, 2 wt%), and Au (0.01 wt%). The simulation environment was validated by experimental tests using water as heat transfer fluid, in two prototypes of PTC located in the city of Ancona (central Italy), while the convective heat transfer coefficient of nanofluids was measured through a dedicated apparatus. A typical meteorological year was built to perform the simulation, which presents a time-resolution of one hour. A specific arrangement for the PTC was defined, while different inlet fluid temperatures were considered at a mass flow rate of 0.50 kg/s: 40, 50, 60, 70, and 80 °C. For this last temperature, the variation in flow rate was also studied (at 1 kg/s and 1.5 kg/s). Results show that only Au, TiO 2 , ZnO, and Al 2 O 3 nanofluids at the lower concentrations, present very small improvements compared to the use of water, while increasing the concentration of nanoparticles no advantage with respect to water appears.

tConsolidation treatment is a common practice in the field of conservation. However, when consideringcalcareous materials, there is a lack of efficient and durable consolidants.Colloidal dispersions of Ca(OH)2nanoparticles, commonly known as nanolimes, can effectively recoverthe superficial loss of cohesion. However, they do not always guarantee in-depth mass consolidation.The aim of this paper is to give directions for improving in-depth deposition of nanolime dispersionswhen applied on limestone. A conceptual model, correlating the drying rate and the kinetic stability ofnanolimes dispersed in different solvents, to the porosity of the limestone to be treated, is conceived.This model can help to select a suitable nanolime solvent depending on the substrate.Nanolimes were synthetized and dispersed in different solvents (ethanol, isopropanol, butanol andwater). The morphology and size of the lime nanoparticles were studied by dynamic light scattering(DLS) and scanning electron microscopy (SEM-EDS). The kinetic stability of the nanolime was assessed byUv–vis spectroscopy. The porosity of the limestones were determined by mercury intrusion porosimetry(MIP), measuring as well their moisture transport properties.

Enhancing the tribological performance of lubricants with nanoparticle additives is a recent challenge. The purpose of this study was to investigate the potential advantage of nanolubricants, a new class of advanced lubricants integrating nano-sized materials, to reduce friction and wear processes, in view of applications in compressors for refrigeration. An investigation of tribological and thermal properties of nanolubricants for vane-on-roller systems was carried out through ball-on-disk wear tests and thermal diffusivity photo-acoustic measurements from room temperature to 70 °C. Nanofluids containing different concentrations of graphene based nanostructures in oil were tested. Poly-alkylene glycol was selected as the oil, being a lubricant suitable for compressors operating with CO2 refrigerants. The suspensions resulted stable with time. The dispersed nanostructures played an important role in protecting surfaces against wear phenomena and in improving the friction properties and load carrying capability of raw oil. A maximum decrease of 18% for friction coefficient and over 70% for worn volume were estimated in boundary lubrication conditions, the most severe for tribological couplings. Finally, durability experiments and Stribeck tests confirmed the benefits produced by nano-additives in different lubrication regimes. Thermal investigation proved also the advantage of using nanolubricants in heat exchange properties in laminar flow conditions, e.g. during compressor power-up phases.

The addition of nanoparticles in lubricating oils recently demonstrated to reduce the coefficient of friction and to increase the load-carrying capability of lubricant in coupled surfaces. In this work, different kinds of nanoparticles were tested as additives to engine oil to improve lubrication: copper and titanium oxide nanoparticles and single walled carbon nanohorns (SWCNHs). Two nanoparticle sizes were also tested in case of copper. The tribological properties of these nanofluids were evaluated by Stribeck tests, in order to compare the effect of nanoparticles on friction coefficient and electric contact resistance in different lubrication regimes. Stribeck curves showed that the coefficient of friction was reduced, compared to raw oil, by the action of Cu nanoparticles having 130 nm diameter, leading to a mean decrease of about 17%, and by SWCNHs, with a mean decrease of about 12%. Conversely, no significant changes were detected in presence of Cu nanoparticles having 50 nm diameter or of TiO2. The suspension viscosity and stability were also tested. Wear tests were also carried out, showing a reduction of wear rate up to nearly 50% for Cu nanoparticles (150 nm diameter) and around 30% for SWCNHs. The measurements showed that nanoparticles having size comparable to the mean roughness of coupled surfaces significantly improved the tribological properties of bare oil. An explanation of nanoparticle action is proposed.

In the last years, various applications have been proposed for nanofluids in the Heating, Ventilation, Air Conditioning and Refrigeration (HVACR) field; their use as primary and secondary fluids, also as lubricants, has been kept into account to improve systems performance.The present work was developed to test the applicability of nanofluids as lubricants in the compressors of heat pump systems, with the purpose to experimentally detect the possible positive effects of nanolubricants.Several nanolubricants, formed by Polyolester (POE) or mineral oil, as base fluid, and titanium oxide (TiO2) or single wall carbon nano-horns (SWCNH), as nanoparticles, were studied in a dedicated test rig. In contrast with some published data, no improvement was detected with 0.05 to 0.5 wt% of TiO2 or 0.1 wt% of SWCNH in tested commercial oils. All results will be deeply discussed in the paper.

The present work aims at investigating a new challenging use of aluminum oxide (Al2O3) nanoparticles to enhance the thermal properties (thermal conductivity, specific heat, and latent heat) of pure paraffin waxes to obtain a new class of phase change materials (PCMs), the so-called nano-PCMs. The nano-PCMs were obtained by seeding 0.5 and 1.0 wt % of Al2O3 nanoparticles in two paraffin waxes having melting temperatures of 45 and 55 °C, respectively. The thermophysical properties such as specific heat, latent heat, and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCMs. Furthermore, a numerical comparison between the use of the pure paraffin waxes and the nano-PCMs obtained in a typical electronics passive cooling device was developed and implemented. A numerical model is accomplished to simulate the heat transfer inside the cavity either with PCM or nano-PCM. Numerical simulations were carried out using the ansys-fluent 15.0 code. Results in terms of solid and liquid phase fractions and temperatures and melting time were reported and discussed. They showed that the nano-PCMs determine a delay in the melting process with respect to the pure PCMs.

The work illustrates the preparation of stable suspensions of single-walled carbon nano-horns (SWCNHs) in engine oil and shows their tribological behavior at different temperatures. The suspension stability was verified by dynamic light scattering, and the viscosity of suspensions was measured. Stribeck test was used to evaluate the tribological performances of nano-lubricants in the 25-80 °C temperature range. The coefficient of friction was reduced with the addition of SWCNHs at all temperatures and concentrations. In particular, at T = 25 °C, the coefficient friction was reduced up to about 12 % in boundary regime even with a very low SWCNH concentration (0.01 vol%). Wear tests were carried out to verify results obtained from Stribeck characterization. A decrease in mean wear rate was observed at each temperature, between 25 and 30 %. In particular, at 25 °C, a decrease of about 30 % in wear rate was estimated. Friction coefficient decrease was also observed during all wear tests, probably due to the ball-bearing effect played by SWCNHs spherical aggregates. Therefore, SWCNHs revealed the potential to be promising candidates as additives, to develop a new class of lubricants that are suitable and effective in different operating environments. © 2014 Springer Science+Business Media New York.

Cu nanoparticles (NPs) are widely studied to understand how they work in lubricant oils to improve its tribological properties. This paper describes the synthesis and characterization of Cu NPs of different size (φ = 60 and 130 nm) carried out in EG and the dispersion procedure used to prepare nanolubricants containing different amounts of NPs (0.005 vol% -0.01 vol% -0.02 vol%). The base oil was a commercially available lubricant oil for internal combustion engines. In order to study the influence of different parameters such as particles size and concentration on tribological properties of nanolubricants, the Stribeck curves were recorded at 25°C in the three lubrication regions (elasto-hydrodynamic, mixed lubrication, and boundary lubrication) and the results discussed. It was found that Cu NPs with a 130-nm mean diameter were more effective in reducing the coefficient of friction in all the lubrication regimes with respect to smaller ones.

This research aims at studying the stability and thermophysical properties of nanofluids designed as dispersions of sulfonic acid-functionalized graphene nanoplatelets in an (ethylene glycol + water) mixture at (10:90)% mass ratio. Nanofluid preparation conditions were defined through a stability analysis based on zeta potential and dynamic light scattering (DLS) measurements. Thermal conductivity, dynamic viscosity, and density were experimentally measured in the temperature range from 283.15 to 343.15 K and nanoparticle mass concentrations of up to 0.50% by using a transient plate source, a rotational rheometer, and a vibrating-tube technique, respectively. Thermal conductivity enhancements reach up to 5% without a clear effect of temperature while rheological tests evidence a Newtonian behavior of the studied nanofluids. Different equations such as the Nan, Vogel-Fulcher-Tamman (VFT), or Maron-Pierce (MP) models were utilized to describe the temperature or nanoparticle concentration dependences of thermal conductivity and viscosity. Finally, different figures of merit based on the experimental values of thermophysical properties were also used to compare the heat transfer capability and pumping power between nanofluids and base fluid. Ministerio de Economía y Competitividad | Ref. ENE2014-55489-C2-2-R

The European Project Cheap-GSHPs, recently approved, aims at reducing the total cost of shallow geothermal systems by 20-30%, also improving actual drilling/installation technologies and designs of Ground Source Heat Exchangers (GSHEs), in combination with a holistic engineering approach to optimize the entire systems for building and district heating and cooling applications, across the different underground and climate conditions existing within the EU. In this frame, the efficiency of the system heat pump/GSHE will be an important task and a part of this project will be on the analyses of possible alternative secondary fluids to be used in the geothermal probe. Here, two different commercial suspensions of fumed Al2O3 nanoparticles in water will be considered as possible efficient secondary fluid. Four different concentrations will be studied for each nanofluid, i.e. 3%, 5%, and 30% or 40%, and the stability, thermal conductivity and dynamic viscosity of these fluids will be characterized, in order to analyse their possible employment as thermal fluids.

The solid-liquid phase change process represents an interesting possibility to reject even high heat loads, especially when they are intermittent. The term Phase Change Materials (PCMs) commonly refers to those materials that use the solid-liquid phase change process to adsorb and then release heat loads. Amongst the available PCMs, paraffin waxes have been found to exhibit many desirable characteristics, such as high latent heat, low vapour pressure in the melt, they are chemically inert and stable, and non-toxic. However, they also have a very low thermal conductivity and a high volume change during the melting process. Thus, heat transfer enhancement techniques are required for their possible implementation in PCM passive electronics cooling devices, especially in case of intermittent operations. The present work aims at investigating a new challenging use of aluminium oxide (Al2O3) nanoparticles to enhance the thermal properties (thermal conductivity, specific heat, and latent heat) of pure paraffin waxes to obtain a new class of PCMs, the so-called nano-PCMs. The nano-PCMs were obtained by seeding different amounts of oxide nanoparticles in two paraffin waxes having melting temperatures of 45 and 55 °C. The thermophysical properties such as specific heat, latent heat, and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCMs. Finally, a numerical comparison between the use of pure paraffin waxes and the nano-PCMs obtained in a typical electronics passive cooling device was developed and implemented. The heat sink is an aluminium box of 40x40 mm and 50 mm high, which is filled up with the PCMs and then heated from the bottom to simulate a hot spot to be cooled. Different heat flow rates were imposed 10, 20 and 30 W; the results are presented in terms of melting time and maximum junction temperature. A numerical model is accomplished to simulate the heat transfer inside the cavity with PCM and nano-PCM. Numerical simulations were carried out using the Ansys-Fluent 15.0 code. Results in terms of solid and liquid phase temperatures and stream function were reported. Moreover, a comparison with experimental results was also performed.

In this paper an experimental and numerical study to investigate the convective heat transfer characteristics of fully developed turbulent flow of a water-Al2O3 nanofluid in a circular tube is presented. The numerical simulations are accomplished on the experimental test section configuration. In the analysis, the fluid flow and the thermal field are assumed axial-symmetric, two-dimensional and steady state. The single-phase model is employed to model the nanofluid mixture and k-? model is used to describe the turbulent fluid flow. Experimental and numerical results are carried out for different volumetric flow rates and nanoparticles concentration values. Heat transfer convective coefficients as a function of flow rates and Reynolds numbers are presented. The results indicate that the heat transfer coefficients increase for all nanofluids concentrations compared to pure water at increasing volumetric flow rate. Heat transfer coefficient increases are observed at assigned volumetric flow rate for nanofluid mixture with higher concentrations whereas Nusselt numbers present lower values than the ones for pure water.

The present work aims at investigating a new challenging use of aluminium oxide (Al2O3) nanoparticles and Carbon black (CB) nano-aggregates to enhance the thermal properties of pure paraffin waxes to obtain a new class of PCMs, the so-called nano-PCMs. The nano-PCMs were obtained by seeding suitable amounts of Al2O3 nanoparticles and CB nano-aggregates in a paraffin wax with a melting temperatures of 70 °C to obtain 1%wt dispersions. The thermophysical properties such as specific heat, latent heat, and thermal conductivity were measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCMs and then compared with the ones obtained for the pure paraffin wax. Finally, a numerical comparison between the use of pure paraffin wax and the nano-PCMs obtained in a typical hybrid water TES was developed and implemented. The modelled hybrid TES is a typical 70 L water tank, where a certain number of pipes filled up with PCMs are located to improve its heat storage capabilities. The results are presented in terms of loading and unloading time, and total amount of energy stored.

This paper deals with the characterization and modelling of water-based nanofluids containing zinc oxide (ZnO) nanoparticles in concentrations ranging between 1 and 10 wt%. Low concentrations were chosen to reduce fouling and excessive pressure drops. First of all, the stability was verified by means of an instrument, based on the dynamic light scattering (DLS) technique, measuring mean nanoparticle diameters and Zeta potential. Moreover, nanofluids pH was measured. Then, thermal conductivities and dynamic viscosities were measured, analysing their dependence on temperature and nanoparticle concentration. Thermal conductivity was measured by means of a hot disk apparatus in the temperature range between 10 and 70 degrees C, while viscosity was measured by a magnetic suspension rheometer in the same range of temperatures. Finally, the heat transfer capability of these fluids was studied measuring their heat transfer coefficients in a dedicated apparatus between 18 and 40 degrees C. Heat transfer coefficient was evaluated at different Reynolds number, in turbulent flow regime. Reynolds and Nusselt numbers were deduced by using previously measured thermal conductivity and viscosity values. Moreover, numerical simulations in two-dimensional turbulent and steady state flow were carried out. No increase in heat transfer coefficient in the temperature range between 18 and 40 degrees C was found. Comparison between experimental and numerical simulation data, in terms of wall temperature profiles, showed a good agreement.

The present work aims at investigating a new challenging use of oxide (TiO2, Al2O3, etc.) nanoparticles to enhance the thermal properties: thermal conductivity, specific heat, and latent heat of pure paraffin waxes to obtain a new class of Phase Change Materials (PCMs), the so-called nano-PCMs. The nano-PCMs were obtained by seeding different amounts of oxide nanoparticles in a paraffin wax having a melting temperature of 45°C. The thermophysical properties such as latent heat and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCM. Finally, a numerical comparison between the use of the pure paraffin wax and the nano-PCM in a typical electronics passive cooling device was implemented. Numerical simulations were carried out using the Ansys-Fluent 15.0 code. Results in terms of solid and liquid phase temperatures, melting time and junction temperature were reported. Moreover, a comparison with experimental results was also performed.

It is well known that the heat transfer associated with a phase change process is much higher than sensible enthalpy change even in forced convection. In particular, the vaporization process has been widely studied because it exploits the highest heat transfer coefficient; this heat transfer mechanism is used in both passive (i.e. heat pipes) and active (i.e. refrigerating machines) cooling devices. However, the solid-liquid phase change process is another interesting possibility to reject even high heat loads, especially when they are intermittent. The term Phase Change Materials (PCMs) commonly refers to those materials, which use the solid-liquid phase change process to adsorb and then release heat loads (Mancin et al., 2015). The present work aims at investigating the feasibility of a new challenging use of Aluminum Oxide (Al 2 O 3 ) and Carbon Black (CB) nanoparticles to enhance the thermal properties: thermal conductivity, specific heat, and latent heat of pure paraffin waxes to obtain a new class of PCMs, the so-called nano-PCMs. The nano-PCMs were obtained by seeding 1 wt% of nanoparticles in paraffin waxes with melting temperatures of 20 °C and 25 °C. The thermophysical properties were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCM. These new nano-PCMs can represent a feasible and interesting way to mitigate or eliminate the intrinsic limitations in the use of paraffin waxes as PCMs for both energy storage and passive cooling applications.

New nanofluids as dispersions of dry ZnO nanopowder in ethylene glycol + water (50/50% in volume) were investigated at three mass nanoparticle concentrations up to 5%. Several sonication parameters were selected with the aim to optimize nanofluid stability, which was analysed by Dynamic Light Scattering (DLS). Thermal conductivity and dynamic viscosity were experimentally determined with a TPS 2500S hot disk device and an AR-G2 rotational rheometer, respectively. Temperature and concentration dependences on these two properties were studied by analysing the base fluid and nanofluids at 1%, 2.5% and 5% mass concentrations within the temperature range from 283.15 to 343.15 K. Mouromtseff numbers were calculated to evaluate the effectiveness of the presented nanofluids comparing with the base fluid. The heat transfer performances of the nanofluid at 1% mass concentration and base fluid were experimentally determined by using an assembly, which includes a testing section based on a uniform-heat-flux boundary condition. Local and overall heat transfer coefficients were obtained at the inlet temperatures of 293 and 314 K for different power supplies and flowing speeds, corresponding to laminar and/or laminar-to-turbulent transition Reynolds numbers. The results were expressed in a dimensionless way through the Nusselt number. Finally, the goodness of different semi-empirical equations for laminar, laminar-to-turbulent or turbulent flow conditions was tested.

Single walled carbon nanohorns (SWCNHs) are a black nanoscale spherical aggregate of cylindrical tubes of graphitic carbon which differ from nanotubes in their "horn-like" shape. Their peculiar structure makes them one of the best electronconductors at a nanoscale level. Although not commercially exploited, their rapid environmental diffusion is expected to rise significantly in the next few years. Therefore, we appraised the ecotoxicology of SWCNH powders by taking into account the ecological role of the two species that were deployed in exposure experiments: polychaetes, Hediste diversicolor, and mussels, Mytilus galloprovincialis. Adult mussels and polychaetes were exposed to three SWCNH concentrations: 1, 5, and 10 mg L-1 and acute effects were measured after 24 and 48 h. Sub-lethal effects were estimated at level of physiological functions such as digestion in mussels (i.e. variations in lysosomal parameters and lipofuscin content) and the antioxidant system in both species (i.e. glutathione peroxidase activity and malondialdehyde content). SWCNH suspension in sea water was also characterised, highlighting the formation of aggregates the size of which was related to SWCNH concentrations and their resident time in the medium. The results showed that SWCNH affected the oxidative and lysosomal systems on the hepatopancreas and led to lysosomal alterations on haemocytes in mussels. The biological responses were less clear in polychaetes. This preliminary investigation highlighted the need of focusing future research efforts on possible physiological impairments caused by long-term exposure to SWCNHs in marine species. (C) 2013 Elsevier B.V. All rights reserved.

With the aim of preparing stable nanofluids for heat exchange applications and to study the effect of surfactant on the aggregation of nanoparticles and thermal diffusivity, stable silver colloids were synthesized in water by a green method, reducing AgNO₃ with fructose in the presence of poly-vinylpyrollidone (PVP) of various molecular weights. A silver nanopowder was precipitated from the colloids and re-dispersed at 4 vol% in deionized water. The Ag colloids were characterized by UV-visible spectroscopy, combined dynamic light scattering and ζ-potential measurements, and laser flash thermal diffusivity. The Ag nanopowders were characterized by scanning electron microscopy and thermal gravimetric analysis. It was found that the molecular weight of PVP strongly affects the ζ-potential and the aggregation of nanoparticles, thereby affecting the thermal diffusivity of the obtained colloids. In particular, it was observed that on increasing the molecular weight of PVP the absolute value of the ζ-potential is reduced, leading to increased aggregation of nanoparticles. A clear relation was identified between thermal diffusivity and aggregation, showing higher thermal diffusivity for nanofluids having higher aggregation. A maximum improvement of thermal diffusivity by about 12% was found for nanofluids prepared with PVP having higher molecular weight.

An experimental investigation on water-based nanofluids containing iron oxide (Fe2O3) in concentrations ranging between 5 and 20% in mass is presented. The purpose of this study is to measure thermal conductivity and dynamic viscosity of these fluids, as a starting point to study the heat transfer capability. The stability of the nanofluids was verified by pH and Zeta potential measurements. A dynamic light scattering (DLS) technique was used to obtain the mean nanoparticle diameters. It was found that thermal conductivity of these nanofluids improved with temperature and particles concentration. The temperature and nanoparticle concentration effects on viscosity were analyzed, obtaining a significant increase with respect to water. All the fluids exhibited a Newtonian behaviour. The experimental values were compared with some theoretical models for both thermal conductivity and dynamic viscosity.

This paper deals with the characterization of water-based nanofluids containing zinc oxide (ZnO) nanoparticles in concentrations ranging between (1 and 10) wt%. Low concentrations have been chosen to reduce fouling and excessive pressure drops. First of all, the stability was verified by means of a Zetasizer Nano ZS (Malvern), based on the dynamic light scattering (DLS) technique, measuring mean nanoparticle diameters and Zeta potential. Moreover, nanofluids pH was tested. Then, nanofluids thermal conductivities and dynamic viscosities were measured, analysing their dependence on temperature and nanoparticle concentration. Thermal conductivity was measured by a hot disk apparatus TPS 2500 S in the temperature range between (10 and 70)°C, while viscosity was measured by a magnetic suspension AR-G2 rheometer (TA Instruments) in the same range of temperatures. Finally, the heat transfer capability of these fluids was examined measuring their heat transfer coefficients in a dedicated apparatus between (18 and 40)°C. The fluid was forced to flow inside a smooth horizontal pipe at controlled flow rate and with uniform heat flux along the tube. Heat transfer coefficient was evaluated at different Reynolds number, for turbulent flow regimes. Reynolds and Nusselt numbers were deduced by using thermal conductivity and viscosity values previously measured.

A total of 13 796 compressed liquid density measurements for 2,3,3,3-tetrafluoroprop-1-ene (R1234yf) for eight isotherms evenly spaced from (283.15 to 353.15) K for pressures from near saturation to 35 MPa are presented. In addition, a saturated liquid density correlation and a Tait correlation are developed.

At present, literature data on viscosity of nanofluids are still scarce and controversial. The possible non-Newtonian behaviour of these fluids is frequently neglected and the problems related to the nanofluids stability and the actual composition are often not considered. In this paper, viscosity data for nanofluids formed by water, as base fluid, and solid nanoparticles of two different materials – single wall carbon nanohorn (SWCNH) or titanium dioxide (TiO2) – are presented. Viscosity was measured by using a rheometer and obtained as a function of the nanoparticles mass fraction and the shear rate, thus allowing evaluation of the possible non-Newtonian behaviour for the nanofluid. Both the studied nanofluids showed a Newtonian behaviour. The viscosity data were correlated by different equations and here an empirical correlation is proposed.

In the recent years, great interest has been devoted to the unique properties of nanofluids. The dispersion process and the nanoparticle suspension stability have been found to be critical points in the development of these new fluids. For this reason, an experimental study on the stability of water-based dispersions containing different nanoparticles, i.e. single wall carbon nanohorns (SWCNHs), titanium dioxide (TiO2) and copper oxide (CuO), has been developed in this study. The aim of this study is to provide stable nanofluids for selecting suitable fluids with enhanced thermal characteristics. Different dispersion techniques were considered in this study, including sonication, ball milling and high-pressure homogenization. Both the dispersion process and the use of some dispersants were investigated as a function of the nanoparticle concentration. The high-pressure homogenization was found to be the best method, and the addition of n-dodecyl sulphate and polyethylene glycol as dispersants, respectively in SWCNHs-water and TiO2-water nanofluids, improved the nanofluid stability.

Nanofluids, i.e. suspensions of nanoparticles in liquids, seem to be very promising as thermal vectors in systems where secondary fluids are applied. Here, the characterisation of water-based nanofluids containing titanium oxide (TiO2) in concentrations ranging between 1 and 35% in mass is presented. First of all, the mean nanoparticle diameters are studied by dynamic light scattering (DLS) technique and the nanofluid stability is analysed by Zeta potential measurements. Then, the behaviour of viscosity as a function of composition and temperature is evaluated by means of a rheometer, in the temperature range between 283 K and 343 K. Furthermore, the thermal conductivity of these nanofluids is measured, in order to understand its possible enhancement as a function of temperature and nanoparticle concentration.

The optical characterization is reported of a new fluid consisting of single-wall carbon nanohorns and ethylene glycol for solar energy applications. Carbon nanohorns play a significant role in enhancing sunlight absorption with respect to the pure base fluid. The obtained results are compared with those obtained for fluids suspending more conventional carbon forms, i.e. carbon-black particles. We found that nanohorn spectral features are far more favorable than those of amorphous carbon for the specific application. This result shows that carbon nanohorn-based nanofluids can be useful for increasing the efficiency and compactness of thermal solar devices, reducing both environmental impact and costs.

Heat transfer fluids, such as water, mineral oil, and ethylene glycol, always have an important role in many industrial processes. The poor heat transfer properties of these common fluids hamper their effectiveness as heat exchangers. The conversion into nanofluids is considered a suitable solution to increase the heat transfer efficiency of such fluids. Ever since the report of the abnormal thermal conductivity k enhancement of nanofluids by Choi et al. [1], many researchers have tried to explain the mechanisms leading to extraordinarily high thermal conductivity. Accordingly, several theories with an emphasis on different thermal nanofluid mechanisms have appeared to predict enhanced conductivity measurements. The predictive models were developed using the experimentally nanofluid measured thermal conductivity data, provided by many research groups. There are many ways to measure the thermal conductivity of fluids, such as the cylindrical cell method, temperature oscillation method, steady-state parallel-plate method, 3?? method, thermal constants analyser method, thermal comparator method, and transient hot-wire or hot disk method. However, the experimental findings have been controversial and theories do not fully explain the mechanisms of elevated thermal conductivity. Some researchers argued that the anomalous k enhancement data are caused by inaccuracies of thermal measurement methods. In this paper, measurements on thermal conductivities of nanofluid mixtures (alumina/water) by means of two different methods are accomplished, i.e. the flash and the hot disk technique. In the first method, a NETZSCH model LFA 447 NanoFlash is employed, while in the second one a Hot Disk model TPS 2500 S is used. A comparison between the results obtained from the different measurement techniques is done. Two-step method is used to prepare nanofluids mixtures with a nanoparticles volumetric concentration from 0.1% to 4%. Each mixture, at assigned volumetric concentration, is treated with a sonicator for different times and thermal conductivity is measured in the range of temperature from 20°C to 50°C. Moreover, for assigned volumetric concentration and sonication, the stability analysis is performed and thermal conductivity measurements are carried out to determine the effect of sonication time. Results show the thermal conductivity dependence on sonication time and an asymptotic value is evaluated for each volumetric concentration.