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Summary Due to the growing needs of super coolant for industrial applications, in this article, we investigate experimentally as well as numerically the possible best heat transfer fluid. In this respect, we considered three nanofluids mixtures of Al2O3/water (concentrations of 0.2%vol and 0.3%vol) and 0.2%vol Fe3O4/water and 0.2%vol ND‐Fe3O4/water, and a hybrid fluid of 0.3%vol MWCNT‐Fe3O4 in water. First, we experiment with forced convection in three porous channels using two concentrations of 0.2 and 0.3%vol of Al2O3 nanofluid, respectively. We validated the experimental results with the numerically simulated data and confirmed satisfactory agreement among them. Second, using the numerical code, we predicted the performance of 0.2%vol Fe3O4 nanofluid, 0.2%vol (ND‐Fe3O4) nanofluid, and a hybrid fluid of 0.3%vol (MWCNT(26%)‐Fe3O4(74%)) for heat extraction. With such configuration, the usefulness of using the proposed nanofluid and hybrid fluid enlarge the knowledge for heat enhancement. Our experimental results show that 0.2%vol Al2O3 efficiently transfers heat compared to the 0.3%vol Al2O3. The simulated results show that 0.2%vol Fe3O4 nanofluid is the best fluid for heat extraction among the experimentally and numerically studied fluids. [ABSTRACT FROM AUTHOR]

The following work experimentally and numerically investigated the thermal performance of a hybrid nanofluid, prepared by decorating a nanostructured aluminum oxide support with copper oxide nanostructures, in a flow system of porous open-cell foam metals. The porous medium was comprised of 6061-T6 aluminum with a porosity of 0.91 and a permeability of 9.54788 × 10−7 m2. Experiments were performed under variable heat flux, using a hybrid nanofluid consisting of a 0.1 mass% aqueous solution of CuO@Al2O3 nanocomposite particles 28 ± 11 nm in size. Thermal performance was evaluated with respect to the Nusselt number and the index of performance with pressure. Remarkably, the implementation of a copper oxide/alumina nanocomposite with the use of porously filled channels resulted in significant thermal enhancement (6–11%) relative to commercial alumina nanofluid, despite a total copper concentration of only 0.0001 mass% in the hybrid nanofluid. Increased performance is attributed to a combination of ultralow copper content and the approach to hybrid nanofluid design. Specifically, a small amount of copper significantly increased the local Nusselt number, indicative of superior heat extraction. At the same time, a numerical model of the system was also developed and agreed with experimental measurements within an error of 5%. Numerical results predicted a slightly higher pressure drop for the hybrid nanofluid, but also showed higher absolute pressures for the hybrid fluid all along the channel in the three-channel configuration. Simulation also produced an interesting discrepancy between the performances of the hybrid nanofluid as a function of heat flux, possibly related to different channel pressures inherent to the two heat sink models under investigation. This could point to a heightened pressure sensitivity of the thermal properties of hybrid nanofluids as well as a greater need to consider experimental design in the comparison of heat enhancement across nanofluidic systems. In terms of material design, decorating alumina nanoparticles with copper nanoparticles rather than mixing two individual nanostructured components appears to have been a beneficial strategy. The photochemical methodology used to prepare the nanocomposite material may also have improved thermal performance by yielding smaller (< 5 nm) copper oxide nanoparticles and provided access to the synergistic properties of a true nanocomposite material. This study demonstrates that heat enhancement by nanofluids can be achieved using a much smaller amount of copper than previously described in the literature and further highlights that synthetic methodology and material characterization can have a dramatic impact upon the performance of applied nanocomposite materials. Additionally, this work delivers a practical example of how progressive nanofunctionalization of materials can enhance thermal functionality of nanofluids. [ABSTRACT FROM AUTHOR]

Summary: The electronic industry is increasingly investigating different approaches for the cooling of electronic equipment. The use of bulk phase change materials is also a promising approach for energy storage. The introduction of microencapsulated phase change materials combined with nanofluids can be beneficial. The combined use of a nanofluid and a metallic porous material can be used to mitigate problems resulting from small thermal conductivity. This study investigated a ternary mixture of water with a nanofluid and a microencapsulated phase change material in a porous medium. The model was previously validated with experimental data using a 0.5%vol concentration nanofluid in water. The results revealed that heat storage capability can be achieved as long as the microencapsulated phase change materials, which consists of encapsulated eicosane, is at a concentration of 20%. Because the melting temperature of microencapsulated phase change materials is approximately 36°C, energy storage at a low flow rate and heat flux is recommended. [ABSTRACT FROM AUTHOR]

The nanoparticles in fluid enhance the rate of heat transfer because of higher thermal conductivity of nanoparticles. In literature, researchers have used different numerical methods to simulate this phenomenon considering the fluid as a single phase or as a two phase system. In some cases, the numerical results were compared with experimental results and good agreements were achieved; while for other no comparison was done. On the other hand, there is no identical numerical research that compares various numerical approaches that can be used to study thermo-fluidic flow in nanofluid. Accordingly, in this study we have numerically solved a benchmark heat transfer nanofluid problem using three different widely used numerical approaches: Finite Element Method (FEM), Lattice Boltzmann Method (LBM) and Finite Difference Method (FDM). The numerical results were compared with experimental results in literature for validation purposes. Subsequently, a FEM simulation is carried out for a three dimensional domain to investigate the three dimensional effects of the enclosure’s walls on heat transfer in the nanofluid. The simulations were performed for water (H 2 O)-aluminum oxide(Al 2 O 3 ) nanofluid at a particle concentration range of 1%–3% volume fraction. Numerical results in the forms of temperature, stream function and velocity variations as well as average Nusselt number at the walls have been presented for a range of Rayleigh numbers (for different particles concentration). While, a suitable combination of the governing control parameters resulted in an acceptable CFD outcomes for all three numerical methods, some differences have been observed between different numerical approaches which are reported in this study. Eventually, the strength and weakness of various numerical approaches are discussed in details. [ABSTRACT FROM AUTHOR]

Nanofluids are a new class of engineered fluids consisting of nanoparticles dispersed in a base fluid. Different base liquids have been used with the most common one being water. The concentration of these nanoparticles can range from 0.01 vol% to 5 vol% or greater promising a wide range of attributes including an improved thermal conductivity and heat transfer properties. In previous studies, different numerical modeling approaches have been proposed assuming in some schemes the fluid as a single phase whereas in others as a two-phase system consisting of a liquid phase and a solid phase. So far, large number of experimental and empirical work has been conducted to study the role of the nanoparticles in the properties of the nanofluid. However, there is no general agreement on the validity of the results, due to the complexity of the configuration of the experimental models. Recently Ho et al. [International Journal of Thermal Sciences, Vol 49, 2010] addressed this shortcoming by performing a set of experiments on a simple cavity as model geometry, to isolate the effect of material properties on the heat transfer characteristics. Due to the simplicity of the cavity geometry, the interaction of the fluid with surfaces is well known. In this paper, we are proposing a cavity-based approach for the numerical modeling of thermo fluidic flow in a nanofluid. The numerical results have been compared with the specific condition of the experimental data of Ho et al., resulting in 1% discrepancy when using the single phase model. However, higher discrepancy with up to 10% when using the two phase model was found. The numerical simulation was performed using the finite element technique for a range of nanoparticles of aluminum oxide concentration from 1 vol% to 3 vol% in water. Results in the form of temperature and velocity variations as well as a comparison of average Nusselt number have been presented for a range of Rayleigh numbers and particle concentration. It was observed that while the multi-phase technique provide deep insight about the liquid and solid phase in the mixture, single-phase approach predicts the heat transfer rate with better accuracy. [ABSTRACT FROM AUTHOR]

Abstract: Thermophoresis plays an important role in forced and natural convection in channels and enclosures when nanofluids are used instead of pure fluids. One objective of this work, therefore, is to investigate whether reliable expressions exist for the estimation of thermophoresis data. As the second objective of this work we will show similarities between thermophoresis of nanoparticles and macromolecule dispersed in a base fluid with thermodiffusion of species in binary mixtures. To this end, a nonequilibrium thermodynamics-based expression primarily developed for the estimation of thermodiffusion factor in binary mixtures is extended and applied to thermophoresis in nanofluids. A hydrodynamics-based expression and the nonequilibrium thermodynamic-based expression developed here, are used to estimate the thermophoretic velocity in nanofluids. Validation results suggest that the general form of the hydrodynamics-based equation is valid for thermophoresis of nano-sized and even sub-nanometer particles in liquids; however, the correct prediction of the matching parameter is still unresolved. Also, the nonequilibrium thermodynamics combined with the concept of activation energy of viscous is somewhat capable of estimating thermophoresis coefficient of inert particles and macromolecules of about 1 nm or smaller. The agreement, however, is qualitative. [Copyright &y& Elsevier]

The use of nanofluids for heat transfer has been examined in recent years as a potential method for augmentation of heat transfer in different systems. Often, the use of nanoparticles in a working fluid does not disrupt the system in significant ways. As a result of this general improvement of a system's heat transfer capabilities with relatively few detrimental factors, nanofluids and hybrid nanofluids have become an area of considerable research interest. One subcategory of this research area that has been under consideration is the concentration of each of the nanoparticles, leading to either successful augmentation or hindrance. The focus of the current experimental investigation was to examine the resulting impact on heat transfer performance as a result of each nanofluid implemented in an identical three-channel heat exchanger. This work examined the experimental impacts of 0.5 wt% titania (TiO2), 1 wt% titania, a mixture of 0.5 wt% titania and 0.5% silica, and a 0.5 wt% hybrid nanofluid of titania synthetically modified with copper-based nanostructures (Cu + TiO2). The experimental work examined a range of heat flux densities from 3.85 W cm−2 to 7.51 W cm−2, and varying flow rates. Each of the nanoparticles were suspended in distilled water and then mixed using an ultrasonic water bath. The performances of each nanofluid were determined using the local Nusselt number to evaluate the possible thermal enhancement offered by each nanofluid mixture. While the 0.5 wt% Cu + TiO2 hybrid nanofluid did significantly increase performance, the use of a 0.5 wt% TiO2/SiO2 double nanofluid in a three-channel heat exchanger exhibited the greatest performance enhancement, with an average increase of 37.3% as compared to water. [ABSTRACT FROM AUTHOR]

Nanofluids have produced a wide range of researches for various cooling/heating purposes, owing to the enhanced thermophysical properties they bring by suspending nanoparticles in the base fluid. This work proposes a detailed computational fluid dynamic (CFD) study of heat transfer enhancement in microchannel solar collectors coupled with nanofluid. The accuracy of the numerical model is ensured through a reliable finite element analysis considering the complexity of the three‐dimensional structure of microchannel solar collector. The thermophoretic motion induced by the suspension of Al2O3 nanoparticles was also evaluated to further understand the thermal enhancement observed in forced convection regimes. The accuracy of the model was first validated with respect to propylene glycol/water fluid, and then applied to evaluate the performance for Al2O3/water nanofluid. A detailed comparison of the performance of the two fluids with an assessment of the temperature and velocity profiles, was adopted to evaluate the thermal efficacy of adding nanofluids. A further investigation of the effect of solar collector inclination angles (0, π/6, π/4,and π/3) at the optimal volumetric concentration of the nanofluid was also done to determine the impact of the system geometry on the efficacy of the heat removal. It was established that the optimal heat removal is achieved at 2% nanoparticle concentration. Finally, it was also detected that increasing the inclination angle of the solar collector (from 0 to π/3) obstructed the heat removal efficiency. [ABSTRACT FROM AUTHOR]