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In this paper, we address the morphology of nanoparticles in a base fluid as a variable to see how different shapes can be used to control thermal conductivity along with the types of nanoparticles. Hybrid and mono particle suspensions, including TiO2, Al2O3, and hybrid TiO2–Al2O3, have been used. Water and ethylene glycol are used as base fluids, and nanoparticles range from 0 to 4% in a volume fraction. The Nusselt number, average heat transfer, thermal resistance, pressure drop, and pumping power as a function of Reynolds number are studied for different nanofluids in microchannels. Overall performance is characterized by the analysis of pressure drop, pumping power, thermal resistance, and maximum power. Simulation results show that the thermal conductivity increases linearly with particle morphology. By adding 4% of HyNF (50% Al2O3-50% TiO2) with a morphology of n1 = 7 and n2 = 3, respectively, it results in a substantial 18.6% enhancement in thermal conductivity, compared to 12% and 10.04% enhancement for mono-nanofluids with Al2O3 and TiO2 nanoparticles, respectively. The type of base liquid has a negligible effect on the thermal conductivity of the same kind of nanoparticles. Nevertheless, the findings of this study provide valuable guidance for cooling microelectronic cooling components via nanofluid-based thermal management. [ABSTRACT FROM AUTHOR]

• Effect of nanofluids in solar steam and pure water generation was investigated. • The performance of MWCNT, SWCNT, GO and GNP nanomaterials was compared. • The sensible heat storage of nanofluids during the light on and off were studied. • High quality of distilled water was proved by seawater desalination using nanofluids. In the present study, the performance of water based-nanofluids containing four different types of carbon nanostructures for solar vapor generation has experimentally been investigated. The considered nanofluids are multi-walled carbon nanotube, single-walled carbon nanotube, graphene nanoplate and graphene oxide. Nanofluids with three nanoparticle mass concentrations of 0.001%, 0.002%, and 0.004% were prepared and their evaporation characteristics were investigated for solar radiation intensities in the range of 1.5 and 3.5 Suns. The results indicated that the multi-walled carbon nanotube nanofluid with a mass concentration of 0.004% provides the highest evaporation rate with the maximum total efficiency of 94% at 3.5 Suns. The total efficiencies for nanofluids containing the single-walled carbon nanotube, graphene nanoplate and graphene oxide at similar conditions and that of pure water, respectively, are about 91%, 90%, 81% and, 54%. Furthermore, it is observed that for the mass concentration of 0.004%, the evaporation rate with respect to water increases to about 72%, 63%, 58% and 36%, respectively, for the multi-walled carbon nanotube, the single-walled carbon nanotube, graphene nanoplate and graphene oxide nanofluids. As far as the sensible heat is concerned, graphene oxide shows the most considerable sensible heating efficiency at the mass concentration of 0.004% with the maximum fluid bulk temperature rise of 20.5 °C, which is 40% higher than that of water. Finally, it was found that increasing the radiation intensity from 1.5 to 3.5 Suns improves the evaporation rate; however, the sensible heat efficiency is more influenced such that the evaporation efficiency declines. The quality of the produced water from a seawater sample is also examined and it was shown that it is well below the limitations of the drinking water standards. [ABSTRACT FROM AUTHOR]

In the present paper, the performance of the graphene oxide in the solar steam generation has experimentally been examined. For this purpose, a setup was built for measuring the evaporation rate, which consists of a solar simulator with a xenon lamp as a radiation source, a pyranometer for light intensity measuring, and a sensory system for measuring the temperature and the mass. Then, the nanofluid with three nanoparticle mass concentrations of 0.001, 0.002, and 0.004% was prepared and exposed to the light intensity of 3.5 Suns (3.5 kW m−2). Finally, the effects of the light intensity variations on the solar steam generation were studied at the steady and transition conditions. The results showed that the examined carbon nanostructure is efficiently capable of direct solar energy harvesting, such that a maximum total efficiency of 78.9% at 3.5 Suns can be obtained, while the corresponding value for the case of pure water is about 54%. Furthermore, it was found that increasing the light intensity from 1.5 to 3.5 Suns enhances the evaporation flux rate, yet, reduces the evaporation efficiency. [ABSTRACT FROM AUTHOR]

Abstract: This article presents a numerical investigation on heat transfer performance and pressure drop of nanofluids flows through a straight circular pipe in a laminar flow regime and constant heat flux boundary condition. Al2O3, CuO, carbon nanotube (CNT) and titanate nanotube (TNT) nanoparticles dispersed in water and ethylene glycol/water with particle concentrations ranging between 0 and 6vol.% were used as working fluids for simulating the heat transfer and flow behaviours of nanofluids. The proposed model has been validated with the available experimental data and correlations. The effects of particle concentrations, particle diameter, particles Brownian motions, Reynolds number, type of the nanoparticles and base fluid on the heat transfer coefficient and pressure drop of nanofluids were determined and discussed in details. The results indicated that the particle volume concentration, Brownian motion and aspect ratio of nanoparticles similar to flow Reynolds number increase the heat transfer coefficient, while the nanoparticle diameter has an opposite effect on the heat transfer coefficient. Finally, the present study provides some considerations for the appropriate choice of the nanofluids for practical applications. [Copyright &y& Elsevier]

One of the innovative methods of improving heat transfer characteristics of heat exchangers in solar systems is applying nanofluids as the heat transfer media. In this study, laminar convective heat transfer of water-based TiO 2 nanofluid flowing through a uniformly heated tube has been investigated via experiments and numerical modeling. The thermal conductivity and dynamic viscosity of the prepared nanofluids have also been measured and modeled at different temperatures and nanoparticle concentrations. Based on the results, a maximum enhancement of 21% in average heat transfer coefficient has been obtained using TiO 2 /water nanofluids. For the numerical section, the single-phase model was compared with the common two-phase numerical approaches. The numerical investigation indicated that the predicted heat transfer coefficients using single-phase and common two-phase approaches, even based on experimental thermophysical properties of nanofluids, underestimate and overestimate the experimental data, respectively. Therefore, some modifications are implemented to the common two-phase model in order to obtain more accurate predictions of the heat transfer characteristics of nanofluids. This modified model investigated the effects of particle concentration, particle diameter, and particle and basefluid material on the heat transfer rate at different Reynolds numbers. The results indicated that the convective heat transfer coefficient increases with an increase in nanoparticle concentration and flow Reynolds number, while particle size has an inverse effect. The obtained results can be very useful to the investigation of the potential application of nanofluid-based solar collectors. [ABSTRACT FROM AUTHOR]

Application of nanofluids in thermal energy devices such as solar collectors is developing day by day. This paper reports the results of experiments on a flat plate solar collector where the working fluid is SiO 2 /ethylene glycol (EG)–water nanofluid with volume fractions up to 1%. The thermal efficiency and performance characteristics of solar collector are obtained for mass flow rates between 0.018 and 0.045 kg/s. The curve characteristics of solar collector indicate that the effects of particle loading on the thermal efficiency enhancement are more pronounced at higher values of heat loss parameter. The results of this work elucidate the potential of SiO 2 nanoparticles to improve the efficiency of solar collectors despite its low thermal conductivity compared to other usual nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract: The momentum and forced convection heat transfer for a laminar and steady free stream flow of nanofluids past an isolated square cylinder have been studied numerically. Different nanofluids consisting of Al2O3 and CuO with base fluids of water and a 60:40 (by mass) ethylene glycol and water mixture were selected to evaluate their superiority over conventional fluids. Recent correlations for the thermal conductivity and viscosity of nanofluids, which are functions of particle volumetric concentration as well as temperature, have been employed in this paper. The simulations have been conducted for Pe =25, 50, 100 and 200, with nanoparticle diameters of 30 and 100nm and particle volumetric concentrations ranging from 0% to 4%. The results of heat transfer characteristics of nanofluid flow over a square cylinder showed marked improvement comparing with the base fluids. This improvement is more evident in flows with higher Peclet numbers and higher particle volume concentration, while the particle diameter imposes an adverse effect on the heat transfer characteristics. In addition, it was shown that for any given particle diameter there is an optimum value of particle concentration that results in the highest heat transfer coefficient. [Copyright &y& Elsevier]

Abstract Modeling and simulation of nanofluid flows is crucial for applications ranging from the cooling of electronic devices to solar water heating systems, particularly when compared to the high expense of experimental studies. Accurate simulation of a thermal-fluid system requires a deep understanding of the underlying physical phenomena occurring in the system. In the case of a complex nanofluid-based system, suitable simplifying approximations must be chosen to strike a balance between the nano-scale and macro-scale phenomena. Based on these choices, the computational approach – or set of approaches – to solve the mathematical model can be identified, implemented and validated. In Part I of this review (Mahian et al., 2019), we presented the details of various approaches that are used for modeling nanofluid flows, which can be classified into single-phase and two-phase approaches. Now, in Part II, the main computational methods for solving the transport equations associated with nanofluid flow are briefly summarized, including the finite difference, finite volume, finite element, lattice Boltzmann methods, and Lagrangian methods (such as dissipative particle dynamics and molecular dynamics). Next, the latest studies on 3D simulation of nanofluid flow in various regimes and configurations are reviewed. The numerical studies in the literature mostly focus on various forms of heat exchangers, such as solar collectors (flat plate and parabolic solar collectors), microchannels, car radiators, and blast furnace stave coolers along with a few other important nanofluid flow applications. Attention is given to the difference between 2D and 3D simulations, the effect of using different computational approaches on the flow and thermal performance predictions, and the influence of the selected physical model on the computational results. Finally, the knowledge gaps in this field are discussed in detail, along with some suggestions for the next steps in this field. The present review, prepared in two parts, is intended to be a comprehensive reference for researchers and practitioners interested in nanofluids and in the many applications of nanofluid flows. [ABSTRACT FROM AUTHOR]

Abstract It has been more than two decades since the discovery of nanofluids-mixtures of common liquids and solid nanoparticles less than 100 nm in size. As a type of colloidal suspension, nanofluids are typically employed as heat transfer fluids due to their favorable thermal and fluid properties. There have been numerous numerical studies of nanofluids in recent years (more than 1000 in both 2016 and 2017, based on Scopus statistics). Due to the small size and large numbers of nanoparticles that interact with the surrounding fluid in nanofluid flows, it has been a major challenge to capture both the macro-scale and the nano-scale effects of these systems without incurring extraordinarily high computational costs. To help understand the state of the art in modeling nanofluids and to discuss the challenges that remain in this field, the present article reviews the latest developments in modeling of nanofluid flows and heat transfer with an emphasis on 3D simulations. In part I, a brief overview of nanofluids (fabrication, applications, and their achievable thermo-physical properties) will be presented first. Next, various forces that exist in particulate flows such as drag, lift (Magnus and Saffman), Brownian, thermophoretic, van der Waals, and electrostatic double layer forces and their significance in nanofluid flows are discussed. Afterwards, the main models used to calculate the thermophysical properties of nanofluids are reviewed. This will be followed with the description of the main physical models presented for nanofluid flows and heat transfer, from single-phase to Eulerian and Lagrangian two-phase models. In part II, various computational fluid dynamics (CFD) techniques will be presented. Next, the latest studies on 3D simulation of nanofluid flow in various regimes and configurations are reviewed. The present review is expected to be helpful for researchers working on numerical simulation of nanofluids and also for scholars who work on experimental aspects of nanofluids to understand the underlying physical phenomena occurring during their experiments. [ABSTRACT FROM AUTHOR]