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.
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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.
•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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•Comprehensive approach to develop plasmonic nanofluid for direct solar absorption.•Blended plasmonic nanoparticles morphologies for broadband high absorptivity.•Finite element method ...obtains the optical properties of plasmonic nanoparticles.•Radiative transfer equation obtains the performance of direct solar collector.•Very low concentrations of plasmonic nanoparticles can attain an efficiency of 85%.
Direct absorption solar collectors were introduced to overcome the limitations of conventional surface absorber collectors. The advances in nanotechnology accompanied with phenomenological discoveries at the nanoscale have allowed the appearance of plasmonic nanofluids, which utilize localized surface plasmon resonance phenomenon that multiplies the extinction efficiency of the plasmonic nanoparticle several times at the resonance wavelength. Silver nanoparticles exhibit a high intensity of the localized surface plasmon, which can be fine-tuned within the broadband 350–1200 nm by tailoring their shape, size and aspect ratio. In this paper, we have numerically investigated the effects of silver nanoparticles' morphology on the localized surface plasmon resonance and on the extinction peaks. Numerical results allow determining the effective morphologies at every band of the solar spectrum. Thus, nanofluids composed of blended Ag nano-morphologies were designed, which can expand the absorbance over the entire solar spectrum. By means of the radiative transfer equation, we found that blended plasmonic nanofluids have the potential to raise the efficiency of the direct solar collector to more than 85% at a very low concentration below 0.001 wt%. Utilization of the blended plasmonic nanofluids are not limited to solar thermal and concentrated solar power applications, but also can be extended into the optical filters in PV/thermal applications.
Solar energy is one of the cleanest forms of energy sources and considered as a green source of energy. Solar energy benefit ranges from low carbon emission, no fossil fuel requirement, long term ...solar resources, less payback time and other. However like other power generation sources, solar energy has also some Safety, Health and Environmental (SHE) concerns. This paper presents the overview of solar energy technologies and addresses the SHE impact of solar energy technologies to the sustainability of human activities. This paper will also recommend the possible ways to reduce the effect of potential hazards of widespread use of solar energy technologies.
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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.
The turbulent forced convection heat transfer of water/functionalized multi-walled carbon nanotube (FMWCNT) nanofluids over a forward-facing step was studied in this work. Turbulence was modeled ...using the shear stress transport K-
ω
model. Simulations were performed for Reynolds numbers ranging from 10,000 to 40,000, heat fluxes from 1,000 to 10,000 W/m
2
, and nanoparticle volume fractions of 0.00% to 0.25%. The two-dimensional governing equations were discretized with the finite volume method. The effects of nanoparticle concentration, shear force, heat flux, contraction, and turbulence on the hydraulics and thermal behavior of nanofluid flow were studied. The model predictions were found to be in good agreement with previous experimental and numerical studies. The results indicate that the Reynolds number and FMWCNT volume fraction considerably affect the heat transfer coefficient; a rise in local heat transfer coefficient was noted when both Reynolds number and FMWCNT volume fraction were increased for all cases. Moreover, the contraction of the channel passage leads to the formation of two recirculation regions with augmented local heat transfer coefficient value.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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.
•Finite volume method based on shear stress transport (SST) K–ω Model employed in this paper.•Enhancement of heat transfer with the increase of nanoparticle concentration and Reynolds number ...observed.•The effect of expansion ratio was clearly observed at the downstream inlet region.
This paper presents a numerical study of heat transfer to turbulent and laminar Cu/water flow over a backward-facing step. Mathematical model based on finite volume method with a FORTRAN code is used to solve the continuity, momentum, energy and turbulence equations. Turbulence was modeled by the shear stress transport (SST) K–ω Model. In this simulation, three volume fractions of nanofluid (0%, 2% and 4%), a varying Reynolds number from 50 to 200 for the laminar range and 5000 to 20,000 for the turbulent range, an expansion ratio of 2 and constant heat flux of 4000W/m2 were considered. The results show the effect of nanofluid volume fraction on enhancing the Nusselt number in the laminar and turbulent ranges. The effect of expansion ratio was clearly observed at the downstream inlet region where the peak of the Nusselt number profile was referred to as enhanced heat transfer due to the generated recirculation flow. An increase of pressure drop was evident with an increasing Reynolds number and decreasing nanofluid volume fraction, while the maximum pressure drop was detected in the downstream inlet region. A rising Reynolds number caused an increasing Nusselt number, and the highest heat transfer augmentation in the present investigation was about 26% and 36% for turbulent and laminar range, respectively compared with pure water.
