Laminar and turbulent mixed convection heat transfer of water/Cu nanofluids in a rectangular shallow cavity was studied utilizing a two-phase mixture model. The upper movable lid of the cavity was at ...a lower temperature compared to the bottom wall. Simulations were performed for Grashof numbers of 105 (laminar flow) and 1010 (turbulent flow) for Richardson numbers from 0.03 to 30, and nanoparticle volume fractions of 0.00–0.04. The two-dimensional governing equations were discretized using a finite volume method. The effects of nanoparticle concentration, shear and buoyancy forces, and turbulence on flow and thermal behavior of nanofluid flow were studied. The model predictions for very low solid volume fraction (φ ≈ 0) were found to be in good agreement with earlier numerical studies for a base fluid. It is shown that for specific Grashof (Gr) and Richardson (Ri) numbers, increasing the volume fraction of nanoparticles enhances the convective heat transfer coefficient and consequently the Nusselt number (Nu) while having a negligible effect on the wall shear stress and the corresponding skin friction factor.
•Mixed convection of nanofluids is studied utilizing a two-phase mixture model.•Increasing the nanoparticles augments mixed convection.•Effects of nanoparticles on skin friction and wall shear stress is insignificant.•Increasing nanoparticles results in slightly lower turbulence intensities.•At low Richardson numbers a primary clockwise eddy forms inside the enclosure.
•Previous studies on nanofluid-based flat-plate solar collectors are presented.•Nanofluids effectively enhanced performance of flat-plate solar collectors.•Carbon nanostructure-based nanofluids ...reached higher collector performance.•More research needed on surfactant stability in nanofluids at high temperatures.•Main challenges of nanofluids are cost, stability, viscosity, and pumping power.
Continuous escalation of the cost of generating energy is preceded by the fact of scary depletion of the energy reserve of the fossil fuels and pollution of the environment as developed and developing countries burn these fuels. To meet the challenge of the impending energy crisis, renewable energy has been growing rapidly in the last decade. Among the renewable energy sources, solar energy is the most extensively available energy, has the least effect on the environment, and is very efficient in terms of energy conversion. Thus, solar energy has become one of the preferred sources of renewable energy. Flat-plate solar collectors are one of the extensively-used and well-known types of solar collectors. However, the effectiveness of the collector’s absorber plate to absorb solar energy limits the efficiency of this type of collector, as does the inefficient transfer of the solar energy via heat transfer to the fluid in the collector’s flow channels. To improve its efficiency and performance, “nanofluids,” synthesized by mixing solid, nanometer-sized particles at low concentrations with the base fluid, have been used with remarkable effects on the thermophysical properties, such as thermal conductivity. The use of nanofluids as an advanced kind of fluids is a comparatively recent development. In this paper, the previous investigations of the performance of flat-plate solar collectors using nanofluids as working fluids are covered in detail. Then, some conclusions and recommendations are presented concerning the use of nanofluids in flat-plate solar collectors.
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•A novel, green method for covalent functionalization of MWCNTs was introduced.•Highly stable clove-treated MWCNTs aqueous suspension was synthesized.•The success of functionalization ...process was validated by characterization techniques.•The solubility of the synthesized MWCNTs nanofluid was verified by zeta potential and UV–vis spectra.•The synthesized clove-treated MWCNTs nanofluid showed remarkable thermo-physical properties.
In this study, we propose an innovative, bio-based, environmentally friendly approach for the covalent functionalization of multi-walled carbon nanotubes using clove buds. This approach is innovative because we do not use toxic and hazardous acids which are typically used in common carbon nanomaterial functionalization procedures. The MWCNTs are functionalized in one pot using a free radical grafting reaction. The clove-functionalized MWCNTs (CMWCNTs) are then dispersed in distilled water (DI water), producing a highly stable CMWCNT aqueous suspension. The CMWCNTs are characterized using Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The electrostatic interactions between the CMWCNT colloidal particles in DI water are verified via zeta potential measurements. UV–vis spectroscopy is also used to examine the stability of the CMWCNTs in the base fluid. The thermo-physical properties of the CMWCNT nano-fluids are examined experimentally and indeed, this nano-fluid shows remarkably improved thermo-physical properties, indicating its superb potential for various thermal applications.
•Thermal conductivity and viscosity models of nanofluids were reviewed.•Metallic oxides nanofluids (Al2O3, CuO, SiO2 and ZnO) were considered.•Various shapes of nanoparticles were selected.•Thermal ...conductivity increased as temperature and concentration increased.•Nanoparticles shape has sufficient impact on thermophysical properties of nanofluids.
For over ten years, investigators focused on determining and modelling the effective thermal conductivity and viscosity of nanofluids. Lately, many theoretical and experimental investigations on convective heat transfer have been performed on the augmentation of heat transfer by utilizing suspensions of nanometer-sized solid particle materials (metallic or nonmetallic) in base fluids. The main purpose of this work is to determine the thermal conductivity and viscosity of various types of metallic oxides (Al2O3, CuO, SiO2 and ZnO) for nanoparticle concentrations of 1–5 vol% at temperatures of 300–320 K and nanoparticle shapes (blades, platelets, cylindrical, bricks, and spherical). The results illustrate that the effective thermal conductivity and thermal conductivity ratio of metallic oxide nanofluids increase with temperature and nanoparticles volume fraction but decreases nanoparticle size intensifies. Besides that, the results of effective viscosity and viscosity ratio obtained indicate a considerable rise with the increase of nanoparticles concentration. Thus, optimum nanoparticle concentration is essential to be determined in forming nanofluids that can enhance thermal systems performance. Finally, it is found that nanoparticles shape has great impact on the thermophysical properties of nanofluids.
Water and ethylene glycol as conventional coolants have been widely used in an automotive car radiator for many years. These heat transfer fluids offer low thermal conductivity. With the advancement ...of nanotechnology, the new generation of heat transfer fluids called, “nanofluids” have been developed and researchers found that these fluids offer higher thermal conductivity compared to that of conventional coolants. This study focused on the application of ethylene glycol based copper nanofluids in an automotive cooling system. Relevant input data, nanofluid properties and empirical correlations were obtained from literatures to investigate the heat transfer enhancement of an automotive car radiator operated with nanofluid-based coolants. It was observed that, overall heat transfer coefficient and heat transfer rate in engine cooling system increased with the usage of nanofluids (with ethylene glycol the basefluid) compared to ethylene glycol (i.e. basefluid) alone. It is observed that, about 3.8% of heat transfer enhancement could be achieved with the addition of 2% copper particles in a basefluid at the Reynolds number of 6000 and 5000 for air and coolant respectively. In addition, the reduction of air frontal area was estimated.
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.
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•The prepared water-based pristine GNPs nanofluids in this research were not stable.•All the surfactants investigated, SDBS, GA, CTAB, and SDS, increased the viscosity.•Thermal ...conductivity of nanofluids enhanced in the presence of GA, SDBS, and CTAB.•Highest nanofluid stability was obtained using an ultrasonication time of 60min.•(1–1) SDBS–GNPs nanofluid with 60min ultrasonication showed the highest stability.
A pioneering idea for increasing the thermal performance of heat transfer fluids was to use ultrafine solid particles suspended in the base fluid. Nanofluids, synthesized by mixing solid nanometer sized particles at low concentrations with the base fluid, were used as a new heat transfer fluid and developed a remarkable effect on the thermophysical properties and heat transfer coefficient. For any nanofluid to be usable in heat transfer applications, the main concern is its long-term stability. The aim of this research is to investigate the effect of using four different surfactants (sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAB), and gum Arabic (GA)), each with three different concentrations, and five ultrasonication times (15, 30, 60, 90, and 120min) on the stability of water-based graphene nanoplatelets (GNPs) nanofluids. In addition, the viscosity and thermal conductivity of the highest stability samples were measured at different temperatures. For this aim, nineteen different nanofluids with 0.1wt% concentration of GNPs were prepared via the two-step method. An ultrasonication probe was utilized to disperse the GNPs in distilled water. UV–vis spectrometry, zeta potential, average particle size, and Transmission Electron Microscopy (TEM) were helpful in evaluating the stability and characterizing the prepared nanofluids. TEM and zeta potential results were in agreement with the UV–vis measurements. The highest nanofluid stability was obtained at 60-min ultrasonication time. The prepared water-based pristine GNPs nanofluids were not stable, and the stability was improved with the addition of surfactants. The presence of SDBS, SDS, and CTAB surfactants in the nanofluids resulted in excessive foam. The best water-based GNPs nanofluid was selected in terms of better stability, higher thermal conductivity, and lower viscosity. From all the samples that were prepared in this research, the (1–1) SDBS–GNPs sample with 60-min ultrasonication showed the highest stability (82% relative concentration after 60days), the second better enhancement in the thermal conductivity of the base fluid (8.36%), and nearly the lowest viscosity (7.4% higher than distilled water).
•The dispersion of TEA-GNPs nanoparticles in base fluid increased FPSC’s efficiency.•Efficiency of FPSC increased with specific surface area of TEA-GNPs nanoparticles.•As weight concentration of ...TEA-GNPs increased, efficiency improved up to 10.53%.•Good agreement was obtained between experimental data and MATLAB code predictions.•TEA-GNPs nanofluid can efficiently be used in FPSCs for enhanced energy efficiency.
The effects of using aqueous nanofluids containing covalently functionalized graphene nanoplatelets with triethanolamine (TEA-GNPs) as novel working fluids on the thermal performance of a flat-plate solar collector (FPSC) have been investigated. Water-based nanofluids with weight concentrations of 0.025%, 0.05%, 0.075%, and 0.1% of TEA-GNPs with specific surface areas of 300, 500, and 750 m2/g were prepared. An experimental setup was designed and built and a simulation program using MATLAB was developed. Experimental tests were performed using inlet fluid temperatures of 30, 40, and 50 °C; flow rates of 0.6, 1.0, and 1.4 kg/min; and heat flux intensities of 600, 800, and 1000 W/m2. The FPSC’s efficiency increased as the flow rate and heat flux intensity increased, and decreased as inlet fluid temperature increased. When using nanofluids in the FPSC, the measured temperatures of absorber plate and tube wall decreased down to 3.35% and 3.51%, respectively, with the increase in weight concentration and specific surface area, while the efficiency increased up to 10.53% for 0.1- wt% TEA-GNPs nanofluid with specific surface area of 750 m2/g, in comparison with water. When using water as heat transfer fluid, very good agreement was obtained between the experimental and predicted values of absorber plate temperature, tube wall temperature, and collector’s efficiency with maximum differences of 3.02%, 3.19%, and 3.26%, respectively. While, when using nanofluids, higher differences were found, up to 4.74%, 4.7%, and 13.47% for TEA-GNPs nanofluid with specific surface area of 750 m2/g, respectively. Accordingly, the MATLAB code was capable of simulating the thermal performance of FPSCs utilizing nanofluids as their heat transfer fluids with acceptable accuracy. Values of performance index were all greater than 1, and increased as weight concentration increased up to 1.104 for 0.1- wt% TEA-GNPs nanofluid with specific surface area of 750 m2/g, implying higher positive effects on efficiency than negative effects on pressure drop. Accordingly, the investigated nanofluids can efficiently be used in FPSCs for enhanced energy efficiency, and the 0.1- wt% water-based TEA-GNPs nanofluid with specific surface area of 750 m2/g was comparatively the superior one.
Nanofluids are fluid nanoparticle suspensions that exhibit enhanced properties at modest nanoparticle concentrations. Nanofluids have unique heat transfer properties and are utilized in high heat ...flux systems (e.g., electronic cooling systems, heat exchanger liquids, solar collectors, and nuclear reactors). However, suspension stability is critical in the development and application of these heat transfer fluids. Reynolds number, mass concentration, and particle size control the heat transfer behavior of fluids. Sedimentation and agglomeration of nanoparticles in nanofluids and their dispersion have rarely been investigated. Therefore, this paper explains the parameters that affect the stability of nanofluids and the different techniques used to evaluate the stability of nanofluids. This paper also presents an updated review of properties of nanofluids, such as physical (thermal conductivity) and rheological properties, with emphasis on their heat transfer enhancement characteristics. Studies on zeta potential as a function of pH are discussed and extended further to identify opportunities for future research.
•Comprehensive review of nanofluids and latest methods of preparation.•Parameters that affect the stability of nanofluids and the different techniques are discussed.•Effect of different surfactants on the rheological properties of nanofluids has been presented.•Sedimentation and agglomeration of nanoparticles in nanofluids are discussed in detail.•zeta potential as a function of pH is discussed and opportunities for future research.
Bio-fuel has come under consideration due to the effect of fossil oil crisis. Bio-fuels are acting as a renewable replacement of petroleum fuels due to some environmental and economic benefits. ...Bio-fuel can be produced from different kinds of raw materials. Researchers have seen that absolute utilization of bio-fuel is not appreciable as it will affect the food chain but the blend of bio-fuel with conventional fuel could precisely reduce its use and become beneficial to green house effect. It has been inferred that in the hot and cold environment bio-fuel is not fully convenient to replace fossil fuel. In the controlled environment with modified combustion equipment, biodiesel can be used as an alternate fuel. Research results reveal that bio-fuel has lower heating value in comparison to diesel fuel so it is consumed more in fuel-break mean effective power ratio and emits more NOx in comparison to the diesel fuel. Thus there remains a compromise between GHG emission and saving of fossil fuel energy by introducing bio-fuel either totally or as a blending component of engine fuel. Finally, bio-fuel could be considered as a replenishable energy source which might pave the future pathway management and planning of energy.
