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[en] Nanofluid slip flow with distinct solid particles past a wedge with convective surface and high order slip is discussed in this paper. The wedge model is modified by considering the effects of Brownian motion and thermophphoresis together with the high order velocity slip and temperature jump. In this study, the governing fundamental equations are first transformed into third-order ordinary differential equations and solved by using the homotopy analysis method (HAM). Through error analysis and comparison with previous research, the effectiveness of HAM is ascertained, and the crucial influence of nanoparticles and high-order slip on the fluid skin-friction coefficient and heat transfer coefficient is analysed. Thermophphoresis parameter and suction/injection parameter are found to cause an increase in velocity and temperature. The rate of heat transfer in the Cu-water nanofluid is found to be higher than the others. (author)
[en] 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. (topical review)
[en] This study examines the thermal behavior of a hemispherical electronic component subjected to a natural nanofluidic convective flow. During its operation, this active dome generates a high power, leading to Rayleigh number values reaching 4.56×109 . It is contained in a hemispherical enclosure and the space between the dome and the cupola is filled with a monophasic water-based copper nanofluid whose volume fraction varies between 0 (pure water) and 10%. According to the intended application, the disc of the enclosure may be tilted at an angle ranging from 0° to 180° (horizontal disc with dome facing upwards and downwards, respectively). The numerical solution has been obtained by means of the volume control method. The surface average temperature of the dome has been determined for many configurations obtained by combining the Rayleigh number, the cavity’s tilt angle and the nanofluid volume fraction which vary in wide ranges. The temperature fields presented for several configurations confirm the effects of natural convection. The results clearly highlight the effects of these influence parameters on the thermal state of the assembly. The study shows that some combinations of the Rayleigh-tilt angle-volume fraction are incompatible with a normal operating system at steady state and that a thermoregulation is required. The correlation of the temperature-Rayleigh-Prandtl-angle type proposed in this work allows to easily carry out the thermal dimensioning of the considered electronic assembly.
[en] In the present work, an investigation on the relationship between clustering phenomenon and thermal conductivity of nanofluids is presented. Particularly, an experimental campaign was carried out to correlate mean dimension of cluster, ranging from 168 to 20,933 nm, to nanofluid’s thermal behavior. A further objective of this study was to evaluate how the stability of nanofluid can affect thermal conductivity measurements, carried out by means of hot-wire technique. Experimental results showed that thermal conductivity, measured at constant volume concentration of nanoparticles as a function of cluster dimension, first decreases and then augments, as it was under a dual effect: negative in case of small clusters and positive with big clusters within nanofluid. Actually, further measurements of zeta potential and backscattered light demonstrated that clustering reduces nanofluid’s thermal conductivity, while its increment can be related to sedimentation of clusterized particles, which produces convective motion around the hot wire, generating overestimated measurements.
[en] Highlights: • Nanofluid samples of ZnO/EG-water were prepared by a two-step method. • Presenting effects of temperature and volume fraction on thermal conductivity. • Thermal conductivity increases uniformly with increasing concentration and temperature. • Maxwell model was unable to predict the thermal conductivity of the present nanofluid. • Proposing a new correlation for predicting thermal conductivity of the nanofluid. In this study, the effects of temperature (20 °C
[en] One of the challenging points in the simulation of a nanofluid flowing through a porous medium is modeling the surface heat flux in the presence of nanoparticles and internal solid matrix. The question is how much energy is absorbed by the solid phase, fluid phase, and particles at the surface of imposing heat flux? To reach a suitable answer, a local thermal nonequilibrium approach (including three energy equations) is presented in this paper and three heat flux models are proposed for the first time. The proposed models are compared and analyzed. The effects of interstitial heat transfer coefficients on the heat transfer in a porous channel are completely studied. The fluid temperature distributions and heat transfer rate obtained by homogenous and nonhomogenous approaches (for the proposed models) are completely studied and compared. The results show that the nonhomogeneous approach experiences larger Nusselt number than the homogeneous one for all the recommended heat flux models. (author)
[en] Highlights: • Synthesis of XC-treated GNPs using simple and economical method was successful. • High stability of XC-treated GNPs nanofluids were recorded after 15 days. • XC-treated GNPs nanofluids shows good performance for thermophysical properties. • A potentially material to be used in thermal applications was introduced.
[en] Highlights: • Au, Au/Pt and Au/Pt/Ag nanofluids were prepared by microwave irradiation method. • The Au/Pt/Ag nanofluid revealed an alloy core surrounded by metallic shell. • Antibacterial activity was evaluated by MIC and zone inhibition. • Trimetallic Au/Pt/Ag showed superior biocidal activity compared to Au and Au/Pt.
[en] This paper reports the numerical results of the mixed convection and entropy generation of Cu-water nanofluid flow in an open cavity heated from different sides with non-uniform temperature distribution. The finite volume method is used to solve the governing equations. The analysis is carried out by a range of Richardson numbers, 0.01 ≤ Ri ≤ 10, at a nanoparticle volume fraction of 0 ≤ φ ≤ 0.1, and Reynolds number Re = 200, with a cavity aspect ratio of L/H = 2. Three heating modes are considered: (A) the left wall is heated (inflow side, assisting flow); (B) the horizontal bottom wall is heated; and (C) the right wall is heated (outflow side, opposing flow). The results show that the heat transfer and the entropy generation increase with increasing Richardson number and nanoparticle volume fraction. The highest heat transfer and entropy generation are obtained with heating mode C (opposing flow). The contribution of heat transfer and fluid friction irreversibilities in the entropy generation depends on Richardson number and the heater position. The present investigation shows that the configuration with non-isothermal heater located at the bottom wall (B) has the highest performance in terms of heat transfer enhancement with minimum entropy generation. (author)