Showing papers on "Nanofluid published in 2011"

A review on applications and challenges of nanofluids

Abstract: Nanofluids are potential heat transfer fluids with enhanced thermophysical properties and heat transfer performance can be applied in many devices for better performances (i.e. energy, heat transfer and other performances). In this paper, a comprehensive literature on the applications and challenges of nanofluids have been compiled and reviewed. Latest up to date literatures on the applications and challenges in terms of PhD and Master thesis, journal articles, conference proceedings, reports and web materials have been reviewed and reported. Recent researches have indicated that substitution of conventional coolants by nanofluids appears promising. Specific application of nanofluids in engine cooling, solar water heating, cooling of electronics, cooling of transformer oil, improving diesel generator efficiency, cooling of heat exchanging devices, improving heat transfer efficiency of chillers, domestic refrigerator-freezers, cooling in machining, in nuclear reactor and defense and space have been reviewed and presented. Authors also critically analyzed some of the applications and identified research gaps for further research. Moreover, challenges and future directions of applications of nanofluids have been reviewed and presented in this paper. Based on results available in the literatures, it has been found nanofluids have a much higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional fluids. This can be considered as one of the key parameters for enhanced performances for many of the applications of nanofluids. Because of its superior thermal performances, latest up to date literatures on this property have been summarized and presented in this paper as well. However, few barriers and challenges that have been identified in this review must be addressed carefully before it can be fully implemented in the industrial applications.

Finite difference solution of mhd radiative boundary layer flow of a nanofluid past a stretching sheet

Abstract: Unsteady heat and mass flow of a nanofluid past a stretching sheet with thermal radiation in the presence of magnetic field is studied. To obtain non-similar equation, continuity, momentum, energy and concentration equations have been non-dimensionalised by usual transformation. The non-similar solutions are presented here which depends on the Magnetic parameter M respectively . The obtained equations have been solved by explicit finite difference method. The temperature and concentration profiles are discussed for the different values of the above parameters with different time steps.

A review of nanofluid stability properties and characterization in stationary conditions

Abstract: A new engineering medium, called nanofluid attracted a wide range of researches on many cooling processes in engineering applications, which are prepared by dispersing nanoparticles or nanotubes in a host fluid. In this paper, the stability of nanofluids is discussed as it has a major role in heat transfer enhancement for further possible applications. It also represents general stabilization methods as well as various types of instruments for stability inspection. Characterization, analytical models and measurement techniques of nanofluids after preparation by a single step or two-step method are studied.

Empirical correlating equations for predicting the effective thermal conductivity and dynamic viscosity of nanofluids

Abstract: In this paper, two empirical correlations for predicting the effective thermal conductivity and dynamic viscosity of nanofluids, based on a high number of experimental data available in the literature, are proposed and discussed. It is found that, given the nanoparticle material and the base fluid, the ratio between the thermal conductivities of the nanofluid and the pure base liquid increases as the nanoparticle volume fraction and the temperature are increased, and the nanoparticle diameter is decreased. Additionally, also the ratio between the dynamic viscosities of the nanofluid and the pure base liquid increases as the nanoparticle volume fraction is increased, and the nanoparticle diameter is decreased, being practically independent of temperature. The ease of application of the equations proposed, and their wide regions of validity (the ranges of the nanoparticle diameter, volume fraction and temperature are 10–150 nm, 0.002–0.09 and 294–324 K for the thermal conductivity data, and 25–200 nm, 0.0001–0.071 and 293–323 K for the dynamic viscosity data), make such equations useful by the engineering point of view, for both numerical simulation purposes and thermal design tasks.

A critical synthesis of thermophysical characteristics of nanofluids

Abstract: A critical synthesis of the variants within the thermophysical properties of nanofluids is presented in this work. The experimental results for the effective thermal conductivity and viscosity reported by several authors are in disagreement. Theoretical and experimental studies are essential to clarify the discrepancies in the results and in proper understanding of heat transfer enhancement characteristics of nanofluids. At room temperature, it is illustrated that the results of the effective thermal conductivity and viscosity of nanofluids can be estimated using the classical equations at low volume fractions. However, the classical models fail to estimate the effective thermal conductivity and viscosity of nanofluids at various temperatures. This study shows that it is not clear which analytical model should be used to describe the thermal conductivity of nanofluids. Additional theoretical and experimental research studies are required to clarify the mechanisms responsible for heat transfer enhancement in nanofluids. Correlations for effective thermal conductivity and viscosity are synthesized and developed in this study in terms of pertinent physical parameters based on the reported experimental data.

Synthesis of Al2O3–Cu/water hybrid nanofluids using two step method and its thermo physical properties

Abstract: In the present work, Al2O3–Cu hybrid particles have been synthesized by hydrogen reduction technique from the powder mixture of Al2O3 and CuO in 90:10 weight proportions obtained from a chemical route synthesis. Al2O3–Cu/water hybrid nanofluids with volume concentrations from 0.1% to 2% were then prepared by dispersing the synthesized nanocomposites powder in deionised water. The experimental results have shown that both thermal conductivity and viscosity of the prepared hybrid nanofluids increase with the nanoparticles volume concentration. The thermal conductivity and viscosity of nanofluids have been measured and it has been found that the viscosity increase is substantially higher than the increase in thermal conductivity. The experimental measurement of thermal conductivity showed a maximum enhancement of 12.11% for a volume concentration of 2%. The experimental results have been compared with the classical theoretical models available in literature.

Nanofluid optical property characterization: towards efficient direct absorption solar collectors.

Abstract: Suspensions of nanoparticles (i.e., particles with diameters < 100 nm) in liquids, termed nanofluids, show remarkable thermal and optical property changes from the base liquid at low particle loadings. Recent studies also indicate that selected nanofluids may improve the efficiency of direct absorption solar thermal collectors. To determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known. That is, their absorption of the solar spectrum must be established. Accordingly, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (0.25 to 2.5 μm). A simple addition of the base fluid and nanoparticle extinction coefficients is applied as an approximation of the effective nanofluid extinction coefficient. Comparisons with measured extinction coefficients reveal that the approximation works well with water-based nanofluids containing graphite nanoparticles but less well with metallic nanoparticles and/or oil-based fluids. For the materials used in this study, over 95% of incoming sunlight can be absorbed (in a nanofluid thickness ≥10 cm) with extremely low nanoparticle volume fractions - less than 1 × 10-5, or 10 parts per million. Thus, nanofluids could be used to absorb sunlight with a negligible amount of viscosity and/or density (read: pumping power) increase.

Experimental and theoretical studies of nanofluid thermal conductivity enhancement: a review

Abstract: Nanofluids, i.e., well-dispersed (metallic) nanoparticles at low- volume fractions in liquids, may enhance the mixture’s thermal conductivity, knf, over the base-fluid values. Thus, they are potentially useful for advanced cooling of micro-systems. Focusing mainly on dilute suspensions of well-dispersed spherical nanoparticles in water or ethylene glycol, recent experimental observations, associated measurement techniques, and new theories as well as useful correlations have been reviewed. It is evident that key questions still linger concerning the best nanoparticle-and-liquid pairing and conditioning, reliable measurements of achievable knf values, and easy-to-use, physically sound computer models which fully describe the particle dynamics and heat transfer of nanofluids. At present, experimental data and measurement methods are lacking consistency. In fact, debates on whether the anomalous enhancement is real or not endure, as well as discussions on what are repeatable correlations between knf and temperature, nanoparticle size/shape, and aggregation state. Clearly, benchmark experiments are needed, using the same nanofluids subject to different measurement methods. Such outcomes would validate new, minimally intrusive techniques and verify the reproducibility of experimental results. Dynamic knf models, assuming non-interacting metallic nano-spheres, postulate an enhancement above the classical Maxwell theory and thereby provide potentially additional physical insight. Clearly, it will be necessary to consider not only one possible mechanism but combine several mechanisms and compare predictive results to new benchmark experimental data sets.

Magnetic field effect on natural convection in a nanofluid-filled square enclosure

Abstract: This paper examines the natural convection in an enclosure that is filled with a water-Al2O3 nanofluid and is influenced by a magnetic field. The enclosure is bounded by two isothermal vertical walls at temperatures Th and Tc and by two horizontal adiabatic walls. Based upon numerical predictions, the effects of pertinent parameters such as the Rayleigh number (103 ≤ Ra ≤ 107), the solid volume fraction (0 ≤ ϕ ≤ 0.06) and the Hartmann number (0 ≤ Ha ≤ 60) on the flow and temperature fields and the heat transfer performance of the enclosure are examined. Prandtl number is considered to be Pr = 6.2. The results show that the heat transfer rate increases with an increase of the Rayleigh number but it decreases with an increase of the Hartmann number. An increase of the solid volume fraction may result in enhancement or deterioration of the heat transfer performance depending on the value of Hartmann and Rayleigh numbers.

Enhancement of specific heat capacity of high-temperature silica-nanofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications

Abstract: In this study, we report the anomalous enhancement of specific heat capacity of high-temperature nanofluids. Alkali metal chloride salt eutectics were doped with silica nanoparticles at 1% mass concentration. The specific heat capacity of the nanofluid was enhanced by 14.5%. Dispersion behavior of the nanoparticles in the eutectic was confirmed by scanning electron microscopy (SEM). Three independent competing transport mechanisms are enumerated to explain this anomalous behavior.

Applicability of nanofluids in high flux solar collectors

Abstract: Concentrated solar energy has become the input for an increasing number of experimental and commercial thermal systems over the past 10–15 years [M. Thirugnanasambandam et al., Renewable Sustainable Energy Rev. 14 (2010)]. Recent papers have indicated that the addition of nanoparticles to conventional working fluids (i.e., nanofluids) can improve heat transfer and solar collection [H. Tyagi et al., J. Sol. Energy Eng. 131, 4 (2009); P. E. Phelan et al., Annu. Rev. Heat Transfer 14 (2005)]. This work indicates that power tower solar collectors could benefit from the potential efficiency improvements that arise from using a nanofluid working fluid. A notional design of this type of nanofluid receiver is presented. Using this design, we show a theoretical nanofluid enhancement in efficiency of up to 10% as compared to surface-based collectors when solar concentration ratios are in the range of 100–1000. Furthermore, our analysis shows that graphite nanofluids with volume fractions on the order of 0.001% or l...

Nanofluid for enhanced oil recovery

Abstract: An experimental study of nanofluids intended for enhanced oil recovery is presented in this work. An aqueous solution of anionic surface-active agents with addition of light non-ferrous metal nanoparticles was used as the focus of the study. It is shown that the use of the nanofluid permitted a 70–90% reduction of surface tension on an oil boundary in comparison with surface-active agent aqueous solution and is characterized by a shift in dilution. Use the developed nano-suspension results in a considerably increase EOR.

Graphene-Based Engine Oil Nanofluids for Tribological Applications

Abstract: Ultrathin graphene (UG) has been prepared by exfoliation of graphite oxide by a novel technique based on focused solar radiation. Graphene based engine oil nanofluids have been prepared and their frictional characteristics (FC), antiwear (AW), and extreme pressure (EP) properties have been evaluated. The improvement in FC, AW, and EP properties of nanofluids is respectively by 80, 33, and 40% compared with base oil. The enhancement can be attributed to the nanobearing mechanism of graphene in engine oil and ultimate mechanical strength of graphene.

Review of Heat Conduction in Nanofluids

Abstract: Nanofluids—fluid suspensions of nanometer-sized particles—are a very important area of emerging technology and are playing an increasingly important role in the continuing advances of nanotechnology and biotechnology worldwide. They have enormously exciting potential applications and may revolutionize the field of heat transfer. This review is on the advances in our understanding of heat-conduction process in nanofluids. The emphasis centers on the thermal conductivity of nanofluids: its experimental data, proposed mechanisms responsible for its enhancement, and its predicting models. A relatively intensified effort has been made on determining thermal conductivity of nanofluids from experiments. While the detailed microstructure-conductivity relationship is still unknown, the data from these experiments have enabled some trends to be identified. Suggested microscopic reasons for the experimental finding of significant conductivity enhancement include the nanoparticle Brownian motion, the Brownian-motion-induced convection, the liquid layering at the liquid-particle interface, and the nanoparticle cluster/aggregate. Although there is a lack of agreement regarding the role of the first three effects, the last effect is generally accepted to be responsible for the reported conductivity enhancement. The available models of predicting conductivity of nanofluids all involve some empirical parameters that negate their predicting ability and application. The recently developed first-principles theory of thermal waves offers not only a macroscopic reason for experimental observations but also a model governing the microstructure-conductivity relationship without involving any empirical parameter.

Significant thermal conductivity enhancement for nanofluids containing graphene nanosheets

Abstract: We developed a facile technique to produce ethylene glycol based nanofluids containing graphene nanosheets. The thermal conductivity of the base fluid was increased significantly by the dispersed graphene: up to 86% increase for 5.0 vol % graphene dispersion. The 2D structure and stiffness of graphene and graphene oxide help to increase the thermal conductivity of the nanofluid. The thermal conductivity of graphene oxide and graphene in the fluid were estimated to be ∼4.9 and 6.8 W/m K, respectively.

Experimental study of heat transfer enhancement using water/ethylene glycol based nanofluids as a new coolant for car radiators

Abstract: Traditionally forced convection heat transfer in a car radiator is performed to cool circulating fluid which consisted of water or a mixture of water and anti-freezing materials like ethylene glycol (EG). In this paper, the heat transfer performance of pure water and pure EG has been compared with their binary mixtures. Furthermore, different amounts of Al2O3 nanoparticle have been added into these base fluids and its effects on the heat transfer performance of the car radiator have been determined experimentally. Liquid flow rate has been changed in the range of 2–6 l per minute and the fluid inlet temperature has been changed for all the experiments. The results demonstrate that nanofluids clearly enhance heat transfer compared to their own base fluid. In the best conditions, the heat transfer enhancement of about 40% compared to the base fluids has been recorded.

Stagnation-point flow of a nanofluid towards a stretching sheet

Abstract: This communication reports the flow of a nanofluid near a stagnation-point towards a stretching surface. The effects of Brownian motion and thermophoresis are further taken into account. The analytic solutions are developed by homotopy analysis method (HAM). Special emphasis has been given to the parameters of physical interest which include stretching ratio a / c , Prandtl number Pr , Lewis number Le , Brownian motion number Nb and thermophoresis number Nt . It is observed that reduced Nusselt number is an increasing function of ratio a / c . The comparison of the present results with the existing numerical solutions in a liming sense is also shown and this comparison is very good.

Viscosity and thermal conductivity of nanofluids containing multi-walled carbon nanotubes stabilized by chitosan

Abstract: Thermal conductivity, viscosity, and stability of nanofluids containing multi-walled carbon nanotubes (MWCNTs) stabilized by cationic chitosan were studied. Chitosan with weight fraction of 0.1%, 0.2 wt%, and 0.5 wt% was used to disperse stably MWCNTs in water. The measured thermal conductivity showed an enhancement from 2.3% to 13% for nanofluids that contained from 0.5 wt% to 3 wt% MWCNTs (0.24 to 1.43 vol %). These values are significantly higher than those predicted using the Maxwell's theory. We also observed that the enhancements were independent of the base fluid viscosity. Thus, use of microconvection effect to explain the anomalous thermal conductivity enhancement should be reconsidered. MWCNTs can be used either to enhance or reduce the fluid base viscosity depending on the weight fractions. In the viscosity-reduction case, a reduction up to 20% was measured by this work. In the viscosity-enhancement case, the fluid behaved as a non-Newtonian shear-thinning fluid. By assuming that MWCNT nanofluids behave as a generalized second grade fluid where the viscosity coefficient depends upon the rate of deformation, a theoretical model has been developed. The model was found to describe the fluid behavior very well.

Improving the cooling performance of automobile radiator with Al2O3/water nanofluid

Abstract: In this paper, forced convective heat transfer in a water based nanofluid has experimentally been compared to that of pure water in an automobile radiator. Five different concentrations of nanofluids in the range of 0.1–1 vol.% have been prepared by the addition of Al2O3 nanoparticles into the water. The test liquid flows through the radiator consisted of 34 vertical tubes with elliptical cross section and air makes a cross flow inside the tube bank with constant speed. Liquid flow rate has been changed in the range of 2–5 l/min to have the fully turbulent regime (9 × 103

Experimental investigation of forced convective heat transfer coefficient in nanofluids of Al2O3/EG and CuO/EG in a double pipe and plate heat exchangers under turbulent flow

Abstract: Nanofluid is the term applied to a suspension of solid, nanometer-sized particles in conventional fluids; the most prominent features of such fluids include enhanced heat characteristics, such as convective heat transfer coefficient, in comparison to the base fluid without considerable alterations in physical and chemical properties. In this study, nanofluids of aluminum oxide and copper oxide were prepared in ethylene glycol separately. The effect of forced convective heat transfer coefficient in turbulent flow was calculated using a double pipe and plate heat exchangers. Furthermore, we calculated the forced convective heat transfer coefficient of the nanofluids using theoretical correlations in order to compare the results with the experimental data. We also evaluated the effects of particle concentration and operating temperature on the forced convective heat transfer coefficient of the nanofluids. The findings indicate considerable enhancement in convective heat transfer coefficient of the nanofluids as compared to the base fluid, ranging from 2% to 50%. Moreover, the results indicate that with increasing nanoparticles concentration and nanofluid temperature, the convective heat transfer coefficient of nanofluid increases. Our experiments revealed that in lower temperatures, the theoretical and experimental findings coincide; however, in higher temperatures and with increased concentrations of the nanoparticles in ethylene glycol, the two set of results tend to have growing discrepancies.

A critical review on convective heat transfer correlations of nanofluids

Abstract: Nanofluids are engineered colloids made of a base fluid and nanoparticles, which become potential candidate for next generation heat transfer medium. Nanofluids have higher thermal conductivity and single-phase heat transfer coefficients than their base fluids. The heat transfer coefficient increases appear to go beyond the mere thermal conductivity effect, and cannot be predicted by traditional pure fluid correlations. This review summarizes the correlations development for fluid flow and heat transfer characteristics of nanofluids in forced and free convection flows. The review shows that most of the investigations recommended conventional friction factor correlation of base fluid for pressure drop prediction of the nanofluids for both laminar and turbulent flows in minichannel as well as in microchannel. However, the conventional correlation is not suitable for heat transfer coefficient of nanofluid and hence various correlations have been suggested for the Nusselt number for both laminar and turbulent flow. However, the large deviation of predicted values for proposed correlations has been observed may be due to strong influence of particle properties and nanofluid composition on flow and heat transfer characteristics, lack of common understanding on basic mechanism of nanofluid flow and insufficient experimental data on nanofluid heat transfer. Hence, a general framework for heat transfer correlation needs to be developed.

Analytical solution of natural convection flow of a nanofluid over a linearly stretching sheet in the presence of magnetic field

Abstract: In this paper, we examine the convective flow and heat transfer of an incompressible viscous nanofluid past a semi-infinite vertical stretching sheet in the presence of a magnetic field. The governing partial differential equations with the auxiliary conditions are reduced to ordinary differential equations with the appropriate corresponding conditions via scaling transformations. The analytical solutions of the resulting ODEs are obtained, and from which the analytical solutions of the original problem are presented. The influence of pertinent parameters such as the magnetic parameter, the solid volume fraction of nanoparticles and the type of nanofluid on the flow, heat transfer, Nusselt number and skin friction coefficient is discussed. Comparison with published results is presented.

Investigation of viscosity and thermal conductivity of SiC nanofluids for heat transfer applications

Abstract: Nanofluids are nanotechnology-based colloidal dispersions engineered by stably suspending nanoparticles. Transmission electron microscopy and scanning electron microscope images are acquired to characterize the shape and size of SiC nanoparticles, because the properties of the nanofluids depend on the morphologies of nanoparticles. The dispersion behavior for SiC/deionized water (DIW) nanofluids were investigated under different pH values and characterized with the zeta potential values. The isoelectric point of SiC/DIW nanofluid was identified in terms of colloidal stability. Then their viscosity and thermal conductivity were investigated as a function of volume fraction to evaluate SiC/DIW nanofluids’ potential to function as more effective working fluids in heat transfer applications.

Eulerian–Eulerian two-phase numerical simulation of nanofluid laminar forced convection in a microchannel

Abstract: In this paper, laminar forced convection heat transfer of a copper–water nanofluid inside an isothermally heated microchannel is studied numerically. An Eulerian two-fluid model is considered to simulate the nanofluid flow inside the microchannel and the governing mass, momentum and energy equations for both phases are solved using the finite volume method. For the first time, the detailed study of the relative velocity and temperature of the phases are presented and it has been observed that the relative velocity and temperature between the phases is very small and negligible and the nanoparticle concentration distribution is uniform. However, the two-phase modeling results show higher heat transfer enhancement in comparison to the homogeneous single-phase model. Also, the heat transfer enhancement increases with increase in Reynolds number and nanoparticle volume concentration as well as with decrease in the nanoparticle diameter, while the pressure drop increases only slightly.

The wetting and spreading of nanofluids on solids: Role of the structural disjoining pressure

Abstract: The wetting and spreading behavior of pure liquids over solid surfaces changes if liquids contain nanosized spherical particles or surfactant micelles, globular proteins and macromolecules. Recent studies on the spreading of nanofluids have demonstrated the inadequacy of well-known concepts of the spreading and adhesion of pure liquids on solid surfaces in understanding nanofluid spreading behavior. This paper reviews the progress made in the wetting and spreading of nanofluids over solid surfaces with an emphasis on the complex interactions between the particles in the nanofluid and with the solid substrate, as well as the spreading of thin nanofluid films containing nanoparticles on hydrophilic surfaces driven by the structural disjoining pressure gradient. The spreading droplet advances as a series of distinct nanoparticle layers.

Numerical investigation on nanofluids turbulent convection heat transfer inside a circular tube

Abstract: In this paper, turbulent forced convection flow of water–Al2O3 nanofluid in a circular tube, subjected to a constant and uniform heat flux at the wall, is numerically analyzed. Two different approaches are taken into account: single and two-phase models, with particle diameter equal to 38 nm. It is observed that convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement increases with the particle volume concentration and Reynolds number. Comparisons with correlations present in literature are accomplished and a very good agreement is realized.

A review of boiling and convective heat transfer with nanofluids

Abstract: Nanofluids have evoked immense interest from researchers of multi-disciplines from around the globe due to their fascinating thermophysical properties and numerous potential benefits and applications in important fields such as microelectronics, microfluidics, transportation, and biomedical. However, there are many controversies and inconsistencies in reported arguments and experimental results on various thermal characteristics such as effective thermal conductivity, convective heat transfer coefficient and boiling heat transfer rate of nanofluids. As of today, researchers have mostly focused on anomalous thermal conductivity of nanofluids. Although investigations on boiling, droplet spreading, and convective heat transfer are very important in order to exploit nanofluids as the next generation coolants, considerably less efforts have been made on these major features of nanofluids. In this paper, these important cooling features—boiling, spreading, and convective heat transfers of nanofluids are presented together with exhaustive review of research and development made in these areas of nanofluids.

Effect of aggregation on the viscosity of copper oxide–gear oil nanofluids

Abstract: Results on viscosity of the stable nanofluids, prepared by dispersing 40 nm diameter spherical CuO nanoparticles in gear oil are presented. Viscosity of the studied nanofluids displays strong dependence both on CuO loading in the base fluid, as well as, on temperature between 10 and 80 °C. Presence of aggregated CuO nanoparticles in the fluid, with average cluster size ∼7 times the primary diameter of CuO nanoparticles, have been confirmed by DLS data. Viscosity of the nanofluids is enhanced by ∼3 times of the base fluid with CuO volume fraction of 0.025, while it decreases significantly with the rise of temperature. Newtonian behavior of the gear oil changes to non-Newtonian with increase of CuO loading. Shear thinning is observed for nanofluids containing CuO volume fraction >0.005. CuO volume fraction dependence of the viscosity of CuO–gear oil nanofluids is predicted well using the modified Krieger–Dougherty equation derived taking into account the aggregation mechanism. Temperature variation of the nanofluid viscosity agrees very well with the modified Andrade equation, reported by Chen et al.

A review on the performance of nanoparticles suspended with refrigerants and lubricating oils in refrigeration systems

Abstract: Recently scientists used nanoparticles in refrigeration systems because of theirs remarkable improvement in thermo-physical, and heat transfer capabilities to enhance the efficiency and reliability of refrigeration and air conditioning system. In this paper thermal–physical properties of nanoparticles suspended in refrigerant and lubricating oil of refrigerating systems were reviewed. Heat transfer performance of different nanorefrigerants with varying concentrations was reviewed and review results are presented as well. Pressure drop and pumping power of a refrigeration system with nanorefrigerants were obtained from different sources and reported in this review. Along with these, pool boiling heat transfer performance of CNT refrigerant was reported. Moreover, challenges and future direction of nanofluids/nanorefrigerants have been reviewed and presented in this paper. Based on results available in the literatures, it has been found that nanorefrigerants have a much higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional refrigerant. This can be considered as one of the key parameters for enhanced performance for refrigeration and air conditioning systems. Because of its superior thermal performances, latest upto date literatures on this property has been summarized and presented in this paper as well. The results indicate that HFC134a and mineral oil with TiO2 nanoparticles works normally and safely in the refrigerator with better performance. The energy consumption of the HFC134a refrigerant using mineral oil and nanoparticles mixture as lubricant saved 26.1% energy with 0.1% mass fraction TiO2 nanoparticles compared to the HFC134a and POE oil system. It was identified that fundamental properties (i.e. density, specific heat capacity, and surface tension) of nanorefrigerants were not experimentally determined yet. It may be noted as well that few barriers and challenges those have been identified in this review must be addressed carefully before it can be fully implemented in refrigeration and air conditioning systems.

The Cheng–Minkowycz problem for the double-diffusive natural convective boundary layer flow in a porous medium saturated by a nanofluid

Abstract: The paper presents an analytical treatment of double-diffusive nanofluid convection in a porous medium. The problem treated is natural convection past a vertical plate when the base fluid of the nanofluid is itself a binary fluid such as salty water. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis, while the Darcy model is used for the porous medium. In addition the thermal energy equations include regular diffusion and cross-diffusion terms. A similarity solution is presented.