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In the last five years, a new generation of advanced numerical methods was developed to reduce computational costs and improve the prediction process. The combination of Artificial Intelligence and traditional computational methods is the best sample of this generation. Employing this method can expand the horizon of numerical modeling. This method is also suitable for complex processes such as natural convection and oscillating heat transfer. This case study tries to peruse the effects of variable boundary conditions such as magnetic field, angle, and nanofluids volume fraction on the heat transfer. Also, the impact of oscillation has been studied. To accomplish these aims, numerical modeling of natural convection based on Boussinesq approximation was used. Techniques from machine learning were employed to develop a predictive tool that utilizes the data generated by computational analysis of the problem. The Ra and Ha were considered in the ranges of 1036 and 0

We investigate the electroosmosis of nanofluid in a rotating microfluidic channel under the influence of an applied magnetic field. We bring out the rotation-induced complex flow dynamics in the channel as modulated by the nanoparticle driven modifications in the viscous drag. In particular, we observe the flow reversal at the center of the channel, emerging from an intricate competition among different forcings under consideration. We identify the critical rotation Reynolds number, signifying the critical strength of channel rotation relative to the viscous resistance to the flow, for which the flow reversal at the channel center sets in. We demonstrate that the strength of the flow reversal for higher rotation Reynolds number decreases, since higher rotation Reynolds number breaks the interparticle interactions, leading to an enhancement in the effective viscosity of the fluid. Finally, we explain the consequential effects of colloidal suspensions of nanoparticle as realized through the particle concentration and agglomeration size on the alterations in the volume transport rates in the channel. [ABSTRACT FROM AUTHOR]

The thermal conductivity of TiC/water nanofluid is found out at different weight fractions of 0.1%, 0.2% and 0.3% and the results are compared with water (base fluid). It has been shown that the thermal conductivity of TiC/water nanofluid is improved when compared with water. Then, an inexpensive cryogenic treatment is adopted to find the thermal conductivity of cryo-TiC/water nanofluid. It has been found that the cryogenic treatment on TiC nanopowder is further enhancing the thermal conductivity of TiC/water nanofluid. The lattice vibration of the particles is responsible for the improvement of thermal conductivity. [ABSTRACT FROM AUTHOR]

Deep Cryogenic treatment (DCT) is a cooling treatment which may be applied to materials at a low temperature of −196 °C, and in the present work is subjected to Silicon carbide nanopowder. Thermal diffusivity of SiC before and after cryogenic treatment is measured using the flash method. Thermal conductivity of SiC nanopowders is also determined. The result shows that there is a drastic improvement of 33% of thermal diffusivity and 45% of thermal conductivity of SiC nanopowder due to DCT. The thermal conductivity for the SiC/Water nanofluid is also determined. The result shows that the thermal conductivity is enhanced by dispersing conventional and cryogenically treated SiC nanopowder with water. Brownian motion of nano particles is responsible for the enhancement of thermal conductivity. [ABSTRACT FROM AUTHOR]

The present work investigated thermal performance and heat transfer characteristics of a solar parabolic trough collector using Al2O3/DI‐H2O nanofluids. Nanofluids of varying concentrations (0.05% ϕ ≤ 0.5%) with mass flows (0.0083–0.05 kg/s) were considered for turbulent regime (2401 ≤ Re ≤ 7202) analysis. Experiments were carried out as per ASHRAE 93(2010) standards. By thermal performance analysis using Al2O3 nanofluid, it was understood that the collector efficiency improved up to 56% at a maximum volume concentration of ϕ = 0.5% and flow rate of 0.05 kg/s. The heat transfer study comparing Al2O3 nanofluid with pure water showed appreciable reduction in temperature gradient and surface temperature of the absorber. The heat transfer characteristics such as Nusselt number and friction factor relating to Reynolds number fits the experimental and predicted data and found within the limits of ±5.35% and ±9.61% for Nusselt number and friction factor respectively. Moreover, a similar empirical correlation was developed for collector efficiency, which was identified to be within the limit of ±1.02%. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 1149–1159, 2018 [ABSTRACT FROM AUTHOR]

This research reports the thermal performance and flow characteristics of nanofluid flows in two different types of microchannel heat sink (MCHS) with multiple zigzag flow channel structures experimentally with regard to the continuous zigzag flow channel (CZ-HS) and the single cross-cutting zigzag flow channel (CCZ-HS). SiO 2 nanoparticles with particle loadings of 0.3, 0.6, and 0.8 vol.% and dispersed in deionized water (DI water) are used as the working medium. Both CZ-HS and CCZ-HS are made from copper material. Their dimensions are approximately 28 × 33 mm. Hydraulic diameter and number of flow channels are equally designed as seven 1-mm flow channels, respectively. The heat transfer area of CZ-HS is approximately 1176 mm 2 and that of CCZ-HS is 1238 mm 2 . The effects of single cross-cutting of the flow channel, Reynolds number, and particle concentration on the Nusselt number and pressure drop characteristics are investigated. The experimental data indicate that the nanofluid-cooled heat sink provided larger thermal performance than the heat sink cooled by water of approximately 3–15%. Similarly, the results indicated that the thermal performances of the CCZ-HS are larger than those of the CZ-HS by an average of 2–6%. For the pressure drop, the measured data showed that particle concentration and cross-cutting of the flow channel have a small effect on the pressure-drop data. [ABSTRACT FROM AUTHOR]

In this article, the viscosity of two common nanofluids including Al2O3/water and TiO2/water is measured at high temperatures, and high concentrations of the nanofluids. The range of temperature is 15–60°C where the volume fraction of nanoparticles varies from 1 to 8%. Next, comparisons have been done with the most well-known theoretical and experimental reports in the literature. Finally, using the experimental data, a helpful correlation is presented. [ABSTRACT FROM PUBLISHER]

Despite massive efforts in the field of nanofluids over the last two decades, nanofluids are primarily still used in a lab scale due to numerous controllable and uncontrollable barriers that impede their effective large-scale implementation. Nanofluids market uptake can be realized only when those barriers have been overcome. These barriers must be examined for their impacts on all aspects of nanofluids market adoption. In this study, barriers to the commercial applications of nanofluids in thermal energy technologies are identified in the literature and are assessed in consultation with experts in the field using a total interpretive structural modeling approach and cross-impact matrix multiplication applied to a classification analysis. It is discovered that most of the barriers are interrelated and can influence one another. Long-term stability issue is identified as the main driver in the effective implementation of nanofluids at commercial scale. Research in this direction might be able to help R&D institutions and researchers in this field to sort out the most influential barriers to nanofluids market uptake. [Display omitted] • Analyzing barriers to commercial application of nanofluids' market uptake. • Total interpretive structural modeling approach for assessing the barriers' relationships. • Categorization of barriers by cross-impact matrix multiplication applied to a classification. • Environmental consequences of nanofluids should be mitigated to boost market demand. • Researchers and industries must collaborate to foster the widespread use of nanofluids. [ABSTRACT FROM AUTHOR]

Abstract: This article reports a comparison of the differences between using measured and computed thermophysical properties to describe the heat transfer performance of TiO2–water nanofluids. In this study, TiO2 nanoparticles with average diameters of 21nm and a particle volume fraction of 0.2–1vol.% are used. The thermal conductivity and viscosity of nanofluids were measured by using transient hot-wire apparatus and a Bohlin rotational rheometer, respectively. The well-known correlations for calculating the thermal conductivity and viscosity of nanofluids were used for describing the Nusselt number of nanofluids and compared with the results from the measured data. The results show that use of the models of thermophysical properties for calculating the Nusselt number of nanofluids gave similar results to use of the measured data. Where there is a lack of measured data on thermophysical properties, the most appropriate models for computing the thermal conductivity and viscosity of the nanofluids are the models of Yu and Choi and Wang et al., respectively. [Copyright &y& Elsevier]

This article presents the measurement of thermal conductivity and viscosity of nanofluids experimentally. Silver nanoparticles dispersed in water with volume concentrations of 0.3, 0.4, 0.6, 0.8, 0.9, and 1.2 vol% are used in the present study. A transient hot-wire apparatus is used for measuring the thermal conductivity of nanofluids and a Cannon-Fenske viscometer is used to measure the kinematic viscosity of nanofluids. The data are collected for temperatures ranging from 50 to 90°C. The results have shown an increase in the measured thermal conductivity and viscosity of nanofluids as the particle concentrations increase, and the values are higher than the values of the base liquids. The minimum enhancement of 27% for 0.3 vol% and a maximum enhancement of 115% for 1.2 vol% are observed at an average temperature of 70°C when compared with pure water for the same temperature. Further, the thermal conductivity of nanofluids increases with the increase in nanofluid temperatures and, conversely, the viscosity of nanofluids decreases with the increase in temperature of nanofluids. An experimental correlation is developed based on the experimental data for thermal conductivity and viscosity that relates the particle volume concentration and nanofluid temperature. [ABSTRACT FROM AUTHOR]

Abstract: Nanofluid is a new class of heat transfer fluids engineered by dispersing metallic or non-metallic nanoparticles with a typical size of less than 100nm in the conventional heat transfer fluids. Their use remarkably augments the heat transfer potential of the base liquids. This article presents the heat transfer coefficient and friction factor of the TiO2-water nanofluids flowing in a horizontal double tube counter-flow heat exchanger under turbulent flow conditions, experimentally. TiO2 nanoparticles with diameters of 21nm dispersed in water with volume concentrations of 0.2–2vol.% are used as the test fluid. The results show that the heat transfer coefficient of nanofluid is higher than that of the base liquid and increased with increasing the Reynolds number and particle concentrations. The heat transfer coefficient of nanofluids was approximately 26% greater than that of pure vol.%, and the results also show that the heat transfer coefficient of the nanofluids at a volume concentration of 2.0vol.% was approximately 14% lower than that of base fluids for given conditions. For the pressure drop, the results show that the pressure drop of nanofluids was slightly higher than the base fluid and increases with increasing the volume concentrations. Finally, the new correlations were proposed for predicting the Nusselt number and friction factor of the nanofluids, especially. [Copyright &y& Elsevier]

Abstract: This article reports an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2vol.% TiO2 nanoparticles. The heat transfer coefficient and friction factor of the TiO2–water nanofluid flowing in a horizontal double-tube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO2 nanoparticles of about 21nm diameter are used in the present study. The results show that the convective heat transfer coefficient of nanofluid is slightly higher than that of the base liquid by about 6–11%. The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature, and the temperature of the heating fluid has no significant effect on the heat transfer coefficient of the nanofluid. It is also seen that the Gnielinski equation failed to predict the heat transfer coefficient of the nanofluid. Finally, the use of the nanofluid has a little penalty in pressure drop. [Copyright &y& Elsevier]

Abstract: Nucleate pool boiling heat transfer of a refrigerant-based-nanofluid was investigated at different nanoparticle concentrations and pressures. TiO2 nanoparticles were mixed with the refrigerant HCFC 141b at 0.01, 0.03 and 0.05vol%. The experiment was performed using a cylindrical copper tube as a boiling surface. Pool boiling experiments of nanofluid were conducted and compared with that of the base refrigerant. The results indicate that the nucleate pool boiling heat transfer deteriorated with increasing particle concentrations, especially at high heat fluxes. At 0.05vol%, the boiling heat transfer curves were suppressed. At high pressures of 400 and 500kPa, the boiling heat transfer coefficient at a specific excess temperature was almost the same. [Copyright &y& Elsevier]

Abstract: Researches in heat transfer have been carried out over the previous several decades, leading to the development of the currently used heat transfer enhancement techniques. The use of additives is a technique applied to enhance the heat transfer performance of base fluids. Recently, as an innovative material, nanometer-sized particles have been used in suspension in conventional heat transfer fluids. The fluids with these solid-particle suspended in them are called ‘nanofluids’. The suspended metallic or nonmetallic nanoparticles change the transport properties and heat transfer characteristics of the base fluid. The aim of this review is to summarize recent developments in research on the heat transfer characteristics of nanofluids for the purpose of suggesting some possible reasons why the suspended nanoparticles can enhance the heat transfer of conventional fluids and to provide a guide line or perspective for future research. [Copyright &y& Elsevier]

The performance of thermoelectric cooling of electronic devices with nanofluid in a multiport minichannel heat exchanger is experimentally investigated. The Bismuth Telluride (BiTe 3 ) thermoelectric cooler (TEC) with a Δ T max of 67 °C is used to extract heat from the electronic devices, which is a power transistor. The power transistor in the circuit board usually operates with the electric power ranging from 20 W to 400 W which is considered as the input power to the TEC. The aluminum oxide (Al 2 O 3 )–water nanofluid with volume concentrations of 0.1% and 0.2% is used as the coolant to remove the heat from the hot side of the TEC. The Reynolds number is varied from 200 to 1000. The result showed 40% enhancement in the coefficient of performance (COP) of thermoelectric module for 0.2% of nanoparticle volume concentration. A 9.15% decrement in thermoelectric temperature difference between the hot and cold side has also been observed for nanofluids (0.2 vol.%), which enhanced the module cooling capacity. The enhancement in local Nusselt number is found to be 23.92% for 0.2% of nanoparticles volume concentration when compared with that of water at a Reynolds number of 1000 and at 400 W power input. The migration of nanoparticles due to temperature difference (thermophoresis) from the wall of the minichannel to the center is attributed to be the reason for the higher local Nusselt number at the entrance region. The thermal effectiveness of the cooling system increases with increase in volume concentration which makes the nanofluids as a promising coolant for electronic cooling applications. [ABSTRACT FROM AUTHOR]

Abstract: This study presents the improvement in heat transfer performance of a heat pipe using silver nanoparticles dispersed in DI (De-Ionized) water. The nanoparticles suspended in conventional fluids have superior heat transfer capability due to improved thermal conductivity. The heat pipes are tested for heat inputs ranging from 20W to 100W in five steps, which is suitable for removing heat from power transistors in electronics and processors in computers. The effect of various operational limits and test parameters such as heat inputs, volume fraction, vapour temperature on the thermal resistance, evaporation and condensation heat transfer coefficients, are experimentally investigated. The tested silver nanoparticles volume concentration ranged from 0.003% to 0.009% with average nanoparticle diameter of 58.35nm. The experimental results are evaluated in terms of performance metrics by direct measurement of vapour temperatures in the centre core of heat pipe. A substantial reduction in thermal resistance of 76.2% is observed for 0.009vol.% concentration of silver nanoparticles. Further an enhancement in the evaporation heat transfer coefficient of 52.7% is observed for the same concentration. The use of nanoparticles enhances the operating range of heat pipe by 21% compared with that of DI water. [Copyright &y& Elsevier]

Highlights • Effect of uniform and non-uniform magnetic field on flow as well as heat transfer. • Effect of Cu, Al, TiO2 and hybrid (Cu + TiO2) nanofluids. • Direct as well as transverse jet impingement comparison and evaluation. • Equivalent nanofluid pairs as a switching option for efficient heat transfer fluid. Abstract Effect of uniform and non-uniform magnetic field and jet impingement on the hydrodynamic and heat transfer performance of nanofluids has been investigated numerically. Five types of magnetic field with different strengths (Ha = 0–40) are applied externally to the flow domain under direct and transverse jet (cross-flow) impingement conditions. The effect of Reynolds number (Re = 200–600), nanoparticle type (Cu, Al, TiO 2 , and hybrid (Cu + TiO 2)), nanoparticle diameter (d p = 20 nm–80 nm) and concentration (Ø = 1 vol% to 3 vol%) on the hydrodynamic and heat transfer behavior under uniform and non-uniform magnetic field are predicted. The presence of magnetic field introduced a Lorentz force responsible for higher values of flow velocity particularly near the walls resulting in the enhancement of average Nusselt number. Moreover, the direct and transverse jet against the applied uniform/non-uniform magnetic field also enhanced the local flow velocity near the impingement region leading to enhancement in the local Nusselt number. Transverse jet exhibits higher average Nusselt number in comparison with direct jet. A maximum heat transfer enhancement of 173% is obtained for 3 vol% Cu nanofluid under magnetic field. Moreover, two equivalent nanofluid pairs are also identified that will provide a better switching option in thermal management of high power electronic devices and nuclear reactors. [ABSTRACT FROM AUTHOR]

• Simulation of nanofluid in a heat exchanger equipped with twisted tape using a two-phase mixture model. • Studying effect of nanofluid and twisted tape on the thermal performance of heat exchanger. • Evaluating effect of material type of twisted tape on heat transfer and pressure drop in the heat exchanger. • The use of twisted tape improved the Nusselt number up to 22%. • Alumina nanoparticles increased heat transfer and pressure drop up to 30% and 40%, respectively. The present study dealt with the numerical simulation of nanofluid flow in a double tube heat exchanger equipped with twisted tape. Alumina/water nanofluid and pure water are considered to be working fluids. A two-phase mixture model was employed for nanofluid flow simulation. The effect of nanofluid and twisted tape on the hydrodynamic and thermal performance of the heat exchanger was studied. Next, the focus was placed on the effect of the surface roughness (material type) of twisted tape on heat transfer and pressure drop in the heat exchanger. The results are presented as the temperature distribution, velocity field, Nusselt number, and pressure drop for various Reynolds numbers, nanofluid concentrations, pitch ratios, and tape insert materials. The results revealed that the use of twisted tape improved the Nusselt number up to 22%; also, adding alumina particles to water augmented heat transfer up to 30% and increased pressure drop up to 40%. Tapes with more roughness provided heat transfer enhancement of up to 16%, whereas the friction factor increased up to 21%. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • A brief introduction of nanofluid and its applications were discussed. • The role of effective parameters on the nanofluid viscosity was reported. • Viscosity increased with increasing nano-additives amounts and reducing temperature. • Nano-additives size had different effects on nanofluids viscosity have been reported. • Hybrid nanofluids can well cover nanofluid weaknesses and improve its strengths. Abstract Nanoparticles are an evolution in improving heat transfer by fluids that have the good potential for heat transfer. Thus, in the last two decades, interest in researching them has increased dramatically. It has been proved that the thermophysical properties of nanofluids are different from those of common fluids. One of these properties is viscosity, which has a significant contribution to the calculation of the fluid heat transfer. In the present study, a brief introduction of nanofluid and its applications have been discussed in the first. Then, the classical equations suggested for predicting the viscosity of nanofluids and their accuracy have been reviewed. The role of effective parameters on the nanofluid viscosity has also been reported, indicating an increase in the viscosity of the common fluids by increasing the nano-additives volume fraction and reducing it with increasing temperature. For the effect of increasing nano-additives size, different results (decreasing or increasing) on fluid viscosity have been reported. The effect of changing the type of nano-additives has also been described for a variety of metal/metal oxides nanoparticles, and nanomaterial extracted from nature. In addition, nanofluids made with carbon nanotubes are described according to the high heat transfer coefficient. Finally, the introduction of hybrid nanofluid and their comparison with mono nanofluids has been explained by the conclusion that hybrid nanofluids can well cover nanofluid weaknesses and improve its strengths. [ABSTRACT FROM AUTHOR]

The optimization of MgO-water nanofluids in order to reduce the cost and increase the heat transfer coefficient is investigated in this study. At first, the heat transfer coefficient is obtained at various values of solid volume fractions, diameters of nanoparticles, and Reynolds numbers based on empirical data. The cost amount is also determined in terms of solid volume fractions and diameters of nanoparticles. Then, the heat transfer coefficient function and the cost function are attained via RSM (Response Surface Method) and with a regression coefficient of over 0.997. The optimization is performed by the non-dominated sorting genetic algorithm which has a significant capability of achieving optimal response. Finally, the Pareto front, the optimal heat transfer coefficient, and their corresponding minimum cost have been obtained. An appropriate correlation is also provided to achieve the optimal model of the minimum cost in terms of the maximum heat transfer coefficient. Optimization results have shown that, compared to the first optimization, the cost has decreased about 38% in the best case. [ABSTRACT FROM AUTHOR]

In this paper, at first, a new correlation was proposed to predict the relative viscosity of MWCNTs-SiO 2 /AE40 nano-lubricant using experimental data. Then, considering minimum prediction error, an optimal artificial neural network was designed to predict the relative viscosity of the nano-lubricant. Forty-eight experimental data were used to feed the model. The data set was derived to training, validation and test sets which contained 70%, 15% and 15% of data points, respectively. The correlation outputs showed that there is a deviation margin of 4%. The results obtained from optimal artificial neural network presented a deviation margin of 1.5%. It can be found from comparisons that the optimal artificial neural network model is more accurate compared to empirical correlation. [ABSTRACT FROM AUTHOR]

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]

The present paper deals with the design, development and the measurement of thermal conductivity of graphene–water nanofluid using a transient hot wire technique at temperatures below and above ambient conditions ranging from 10 °C to 50 °C. The equipment is designed to measure the thermal conductivity using a single platinum wire of diameter 50 μm and 100 mm length. The platinum micro-wire acts both as a temperature sensor and heating element. Low volume concentrations (0.05, 0.1 and 0.15%) of graphene, having the size less than 100 nm, dispersed in 100 ml of water with SDBS (sodium dodecyl benzene sulfonate) as surfactant, for prolonged stability, is used in the present study. The results showed an enhancement in the thermal conductivity of 37.2% for 0.15% volume concentration of graphene at 50 °C when compared with that of the water at the same temperature. An interesting observation from this study is that the average thermal conductivity enhancement percentage with the increase in volume concentration (say from 0.05% to 0.15%) is found to be 3.3% higher when compared with that of the average enhancement with the increase in temperature from 10 °C to 50 °C. [ABSTRACT FROM AUTHOR]

In the present work, forced convective heat transfer of water/functionalized multi-walled carbon nanotube (FMWCNT) nanofluid in a two-dimensional microchannel is investigated. To solve the governing Navier–Stokes equations and discritization of the solution domain, the numerical method of finite volume and SIMPLE algorithm have been employed. Walls of the microchannel are under a periodic heat flux, and slip boundary conditions along the walls have been considered. Effect of different values of shear forces, solid nanoparticles concentration, slip coefficient, and periodic heat flux on the flow and temperature fields as well as heat transfer rate has been evaluated. In this study, changes of the variables considered to be from 1 to 100 for Reynolds number, 0–25% for weight percentage of solid nanoparticles, and 0.001–0.1 for velocity slip coefficient. Results of the current work showed good agreement with the numerical and experimental studies of other researchers. Data are presented in the form of velocity and temperature profiles, streamlines, and temperature contours as well as amounts of slip velocity and Nusselt number. Results show that local Nusselt number along the length of microchannel changes in a periodic manner and increases with the increase in Reynold number. It is also noted that rise in slip coefficient and weight percentage of nanoparticles leads to increase in Nusselt number, which is greater in higher Reynolds numbers. [ABSTRACT FROM AUTHOR]

In the present study, the thermal conductivity of CuO/EG–water nanofluid in different solid concentrations and temperatures has been experimentally investigated. Using a two-step method, the nanofluid has been produced in different solid concentrations ranging from 0.1% to 2% and temperatures up to 50 °C. The thermal conductivity of the nanofluid has been experimentally measured using the KD2 Pro instrument. Based on the experimental data, new correlations for predicting the thermal conductivity of CuO/EG–water at different temperatures have been proposed. The results show that with the increase of the solid concentration, the thermal conductivity of the nanofluid increases. Furthermore, the thermal conductivity of the nanofluid increases while the temperature increases. This increase is by far more noticeable in higher solid concentrations compared with lower solid volume fraction. This means that it is the presence of nanoparticles in the base fluid that causes the increase of the effect of temperature on the thermal conductivity. [ABSTRACT FROM AUTHOR]

In the present study, the thermal conductivity of the ZnO-EG nanofluid has been investigated experimentally. For this purpose, zinc oxide nanoparticles with nominal diameters of 18 nm have been dispersed in ethylene glychol at different volume fractions (0.000625, 0.00125, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04, and 0.05) and temperatures (24–50 °C). The two-step method is used to disperse nanoparticles in the base fluid. Based on the experimental data, an experimental model has been proposed as a function of solid concentration and temperature. Then, the feedforward multilayer perceptron neural network has been employed for modeling thermal conductivity of ZnO-EG nanofluid. Out of 40 measured data obtained from experiments, 28 data were selected for network training, while the remaining 12 data were used for network testing and validating. The results indicate that both model and ANN outputs are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

In this paper, an analytical study is performed on the entropy generation and heat transfer due to nanofluid flow in a flat plate solar collector. The working fluid considered in this work is Al 2 O 3 /water nanofluid with four different particle sizes, including 25, 50, 75, and 100 nm and volume concentrations up to 4%. Effects of tube roughness, nanoparticle size, and different thermophysical models are investigated on the Nusselt number, heat transfer coefficient, outlet temperature of the collector, entropy generation, and Bejan number. In addition, the effects of solar radiation and ambient temperature on entropy generation are examined. The results are presented for constant mass flow rates ranging from 0.1 to 0.8 kg/s. It is found that when the mass flow rate is considered to be constant for all working fluids, the Nusselt number and heat transfer coefficient have different trends. It is observed that uncertainties in thermophysical models and tube roughness have considerable effects on the values of heat transfer coefficient and Nusselt number. The findings show that with an increase in the volume fraction of nanofluid, the outlet temperature increases while with increasing the nanoparticle size a very insignificant decrease is observed in the outlet temperature. It is seen that the trend of changes in the outlet temperature is exactly in opposite to the Nusselt number trend. The analysis of entropy generation concludes that the entropy generation decreases with increasing the nanofluid concentration. It is found that the tube roughness increases the entropy generation and its effect is more visible at high mass flow rates while the effects of uncertainties in thermophysical models on entropy generation are not significant in any mass flow rate and volume fraction. Finally, a critical mass flow rate is determined under two different intensities of solar radiation and ambient temperature so that for the values higher than the critical mass flow rate the effects of roughness on entropy generation become important and should be considered. [ABSTRACT FROM AUTHOR]

An experimental investigation has been carried out to compare the enhancement in the thermal performance of sintered and mesh wick heat pipes by varying the working fluid, inclination angle and heat input. Similar geometrical specifications of 12, 330 and 1 mm respectively are selected for the outer diameter, length and wick thickness and kept constant for both sintered and mesh wick heat pipes. The study focuses on changes in surface temperature distribution, thermal resistance and effective thermal conductivity of heat pipes. The results showed that the maximum reduction in surface temperature is obtained for sintered wick heat pipe at 45° tilt angle and 60° for mesh wick heat pipe with CuO/DI water nanofluid concentration at 1.0 wt.% for both the cases. The reduction in thermal resistance of sintered wick heat pipe is 13.92% higher compared with mesh wick heat pipe for the same heat input, mass concentration and inclination angle. Presence of CuO nanoparticles in DI water and increasing heat input tremendously increases the thermal conductivity of heat pipes. An important observation from this study is the sole effect of sintered wick in heat pipe not only reduces the thermal resistance but also increases the heat transport capacity up to 20 W compared with that of mesh wick. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the natural convection in a square cavity filled with Al2O3/water nanofluid has been investigated. The flow and heat transfer characteristics of the nanofluid in the cavity are documented when bottom wall is under heterogeneous heating, the right wall is considered cold while the top and left walls are adiabatic. Nine different cases are considered for the non-uniform heat flux where the total heat flux applied to the cavity is the same for all the cases in magnitude but the profile is different. For different values of Rayleigh numbers (103–106), nanoparticle volume fractions (0–9%), and aspect ratios, the optimal profile of heat flux is determined in which Nusselt number is maximized. It is found that the trend of Nusselt number is different for the nine cases at Ra =103 where the conduction is the dominant phenomenon. The results also reveal that for high values of Rayleigh number, i.e. 106, the Nusselt number is minimized where the heat flux in the vicinity of adiabatic wall is smallest while the heat flux near the cold wall is biggest. [Copyright &y& Elsevier]

Abstract: In this investigation, a numerical model having two-dimensional equations was obtained by a CFD program and authors' experimental data were evaluated for the verification procedure of the numerical outputs. The experimental case study includes the single-phase flow of pure water in plain and micro-fin pipes whereas the numerical one has the simulated results of TiO2 particles suspended in single phase water flow in equivalent pipes at a constant heat flux. Hydrodynamics and thermal behaviors of the water–TiO2 flow were calculated by constant heat flux and temperature-dependent settings. Physical specifications of nanofluids were calculated by means of the results of authors' previous ANN analyses. This study illustrates local and average values of temperature, pressure, and velocity distributions in the tested pipes; furthermore, comparisons of pressure drop characteristics are given in terms of nanoparticle concentrations and tube types. [Copyright &y& Elsevier]

Abstract: In this paper, an analytical study of the second law of thermodynamics is performed for the flow and heat transfer of TiO2/water nanofluid in a vertical annulus with isoflux walls and under the influence of magnetohydrodynamic (MHD) field. The governing equations in cylindrical coordinates are written and then simplified with reasonable assumptions. The simplified equations are solved analytically to obtain expressions for the velocity, temperature, and entropy generation distributions. The results are presented for different values of nanofluid volume fractions , Hartmann number M and the flow parameter Gr/Re. To calculate the thermal conductivity and viscosity of TiO2/water nanofluid, relations based on experimental data are used. Finally, using the well-known theoretical models for the thermal conductivity and viscosity, the effects of Hartmann number on the results are investigated. [Copyright &y& Elsevier]

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]

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]

Using renewable energy, especially solar energy, is an essential requisite for preventing global warming and climate change. Solar-powered steam generation is one of the recent solutions to the problem of environmental pollution and can be a promising way to generate clean energy. In this study, the superficial and volumetric approaches of the zinc oxide nanoparticles (ZnO) in solar vapor generation have been compared experimentally. For the volumetric approach, the nanofluid performance at different mass concentrations (0.001%, 0.002%, and 0.004%) on the solar vapor generation was examined. For the superficial approach, the evaporative performance of delignified mulberry wood coated by ZnO nanoparticles as an absorbent (ZnO-Wood) was investigated. Moreover, the effect of nanoparticle size on optical and evaporative properties was evaluated. Finally, the economic performance of the ZnO nanofluid was compared to the other nanofluids. The results indicated that the evaporative efficiency of the nanofluids containing ZnO(50 nm) nanoparticles at 3 suns (3 kW/m2) was approximately two times greater than that of water (18%). However, the evaporative efficiency of ZnO-Wood was 3.15 (56%) times greater than that of water. In addition, the results of the nanoparticle size effect showed that increasing the size of nanoparticles from 20 nm to 50 nm in the superficial and volumetric approaches increases the evaporative efficiency by roughly 10% and 3%, respectively. Finally, the cost analysis of nanofluid for solar vapor generation illustrated that ZnO nanoparticles are cost-effective compared to other nanoparticles presented in the literature. [ABSTRACT FROM AUTHOR]

Due to their superior thermophysical properties, there is a growing body of work on nanofluids in the field of thermal systems. However, there is no specific review of the role of the nanoparticle shape, which has been found crucial to their performance adjustment. A comprehensive literature review of the effect of nanoparticle shape on the hydrothermal performance of thermal systems utilizing nanofluids was compiled. The review covered the forced, mixed, and natural convection regimes and included heat exchangers, boundary layer flows, channel flows, peristaltic flows, impinging jets, cavity flows, and flows of hybrid nanofluids. It indicated that the control of nanoparticle shape is a promising technique for the optimization of heat exchange and the required pumping power. However, no uniform conclusion was reached for the role of nanoparticle shape on the hydrothermal performance of thermal systems. In most of the previous studies in the natural and forced convection regimes, the platelet–like nanoparticle acquired the highest heat transfer rate. However, most of the works in the mixed convection regime reported the best heat transfer performance for the blade–like nanoparticle. More research studies are required in future to determine the role of nanoparticle shape for thermal management of energy systems. • Nanoparticle shape is crucial to the performance control of various nanofluids. • The effects of nanoparticle shape on thermal system performance were reviewed. • Nanoparticle shape control is a promising technique for heat transfer optimization. • More research is required for the role of nanoparticle shape in thermal systems. [ABSTRACT FROM AUTHOR]