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This study presents the application of a modified single-phase method by thermal dispersion model incorporating heterogeneous distribution of nanoparticle concentration for evaluating thermal performance of a nanofluid in a circular porous metal foam tube. Numerical approach was conducted for Re = 200–1000, mean concentration of 0.5–2%, and metal foam porosity of 0.7–0.9. It is observed that the predicted data by application of the thermal dispersion approach are in satisfactory agreement with those obtained experimentally, whereas applying the general homogeneous model results in an underestimation. The results reveal that the heterogeneity of the concentration distribution is directly proportional to nanoparticle mean concentration, Reynolds number, and the metal foam porosity. The velocity and temperature profiles at a cross section have been found to be flatter in dispersion model compared to those obtained from the homogeneous model. Furthermore, it is achieved that the Nusselt number varies directly relative to mean concentration and Reynolds number, whereas it inversely alters relative to the porosity. This reduction is found to be more profound at lower porosities. [ABSTRACT FROM AUTHOR]

In this work, a two-phase mixture model for evaluation of flow and heat transfer performance of SiO 2 /water nanofluids under turbulent flow was proposed by considering the heterogeneity of concentration due to crossed effect and the influences of shear rate, viscosity gradient, thermophoresis and Brownian motion on the diffusion of the nanoparticles. The effects of Peclet number, Reynolds number, nanoparticle size and nanofluid mean concentration on the distribution of nanoparticles have been evaluated. The values of thermal conductivity and viscosity as the main thermophysical properties of nanofluids changed across different layers of the liquid due to the heterogeneous distribution of concentration. It was observed that an increase in the Peclet number caused heterogeneity in the distribution of the properties. The achieved nanoparticle distribution has been implemented for analysis of nanofluid using two-phase mixture model. It was found that the effect of nanofluid concentration on the Nusselt number was more noticeable in lower Reynolds numbers due to the insignificant effect of flow momentum on heat transfer. The maximum of 43.9% enhancement in convection heat transfer was achieved by dispersion of 4% SiO 2 nanoparticles inside DI-water at Re = 25,000. [ABSTRACT FROM AUTHOR]

The enhancement of heat transfer between parallel surfaces, including parallel plates, parallel disks, and two concentric pipes, is vital because of their wide applications ranging from lubrication systems to water purification processes. Various techniques can be utilized to enhance heat transfer in such systems. Adding nanoparticles to the conventional working fluids is an effective solution that could remarkably enhance the heat transfer rate. No published review article focuses on the recent advances in nanofluid flow between parallel surfaces; therefore, the present paper aims to review the latest experimental and numerical studies on the flow and heat transfer of nanofluids (mixtures of nanoparticles and conventional working fluids) in such configurations. For the performance analysis of thermal systems composed of parallel surfaces and operating with nanofluids, it is necessary to know the physical phenomena and parameters that influence the flow and heat transfer characteristics in these systems. Significant results obtained from this review indicate that, in most cases, the heat transfer rate between parallel surfaces is enhanced with an increase in the Rayleigh number, the Reynolds number, the magnetic number, and Brownian motion. On the other hand, an increase in thermophoresis parameter, as well as flow parameters, including the Eckert number, buoyancy ratio, Hartmann number, and Lewis number, leads to heat transfer rate reduction. [Display omitted] • Nanofluid flow and heat transfer between parallel surfaces are studied. • Parallel plates, parallel disks, and concentric pipes are considered. • Physical phenomena and influential parameters are evaluated. [ABSTRACT FROM AUTHOR]

Irreversibility factors for the TiO2–water nanofluid flow are evaluated through entropy generation rates caused by friction and heat transfer by taking into account nanoparticle migration. Effects of three factors of viscosity gradient, non-uniform shear rate and Brownian diffusion are considered. The investigations are performed in both conventional channels and minichannels since the channel size has the considerable effects of on entropy generation. Influence of particle migration on the irreversibility factors is more noticeable in smaller channels such that with considering particle migration, the maximum thermal entropy generation rate increases about 51% for 0.2 mm channel, while this increment is almost 16% for 1 mm channel. Unlike results are obtained for conventional channels and minichannels, such that total entropy generation intensifies in minichannels and reduces in conventional channels by particle concentration increment. Friction is main factor in entropy generation for minichannels, while heat transfer demonstrates a higher contribution in conventional channels. The optimal channel size where minimum entropy generation occurs is obtained which is dependent on Reynolds number. Therefore, for lower Reynolds number, optimal case occurs at smaller channel size. Furthermore, diagrams of total entropy generation show minimum (optimal) point for 3 mm channel, while they demonstrate the constant trends for other channels. [ABSTRACT FROM AUTHOR]

Highlights • Transient characteristics of nanofluid in a minichannel heat sink with MEPCM layer in its ceiling are studied. • The wall temperature rises sharply after sudden heat-pulse and then tend to rise slowly. • The dispersion of nanoparticles inside water leads to the clear decrement of wall and temperature. Abstract In this work, transient thermal characteristics of pure water or Al 2 O 3 /water nanofluid with 2–8 wt% concentrations as coolant flowing through a minichannel heat sink with MEPCM layer in its ceiling are experimentally investigated. The heat sink consists of eight minichannels with length, height, and width of 50 mm, 3 mm, and 1 mm for each minichannel, respectively. A uniform heat flux in the range of 1.2–6.8 W/cm2 is imposed to heat the heat sink and a heat pulse amplitude of 8 W/cm2 is suddenly applied for 10 min after steady-state heating condition and then the heating power is cut off immediately. The dimensionless wall temperature distribution, temperature of MEPCM layer, maximum wall temperature suppression, and heat transfer effectiveness of the system are assessed under sudden heat-pulse conditions. The measured results reveal that the wall temperature and MEPCM layer temperature rise sharply after sudden heat-pulse and then tend to rise slowly. Moreover, it is observed that the latent heat absorbed by the MEPCM layer during melting process can reduce the severe temperature variation caused by sudden heat-pulse. Furthermore, it is concluded that the dispersion of alumina nanoparticles inside pure water and the increase of its concentration lead to the significant decrement of wall temperature and MEPCM layer temperature and increment of the maximum wall temperature suppression. Finally, the comparison of the results obtained from the employment of nanofluid and pure water in the heat sink shows that the nanofluid-cooled heat sink with MEPCM layer in its ceiling outperforms the water-cooled one with/without MEPCM layer. [ABSTRACT FROM AUTHOR]

Abstract This work investigates experimentally the cooling performance of Al 2 O 3 /water nanofluid with 8 wt% concentration flowing through a minichannel heat sink with a MEPCM layer in its ceiling. The hydrothermal characteristics of the nanofluid-cooled minichannel heat sink are analyzed and the dimensionless wall temperature distribution, Nusselt number, heat transfer effectiveness, coefficient of performance, figure of merit, and wall temperature thermal resistance are assessed. The measured results show that the dispersion of alumina nanoparticles inside pure water declines the wall temperature and consequently enhances the heat transfer effect and Nusselt number augments. The maximum values of average heat transfer effectiveness and FOM at the inlet of the MCHS are achieved 1.4 and 1.27 by flowing nanofluid with 8 wt% nanoparticle concentration and Re = 1549, respectively. The results revealed that embedding MEPCM layer in the ceiling of the MCHS has little influence on the cooling performance of Al 2 O 3 /water nanofluid in the minichannel heat sink. [ABSTRACT FROM AUTHOR]