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Nanofluids are colloidal suspensions constituted of nanoparticles and typical heat transfer fluids which have shown potential in yielding enhanced heat transport for many applications. Significant attention has been paid to their thermal conductivity enhancement which has been alleged, in some cases, to exceed theoretical limits classifying the enhancement as "anomalous". The present study aims to quantitatively investigate the nature of the enhancements reported in the literature and classify their alignment with theoretical predictions. To do so, a rigorous and objective mathematical analysis method has been employed. The novelty and value of the present work lies in the deeper characterisation and understanding of the anomalous observations reported. The present analytical study focuses on (spherical) Al2O3–water nanofluids. It was discovered that studies involving low nanoparticle concentrations (ϕ ≤ 0.2 vol%) and the use of electrostatic stabilisation (through pH control) as opposed to steric stabilisation (using surfactants) as suspension stability control methods are likely to report anomalous effects. An exceptional case was observed for d < 15 nm, where to achieve anomalous enhancement, surfactants and pH controllers should not be used to prevent significant interfacial resistance. The shared characteristics of these anomalous observations indicate that nanofluid preparation effects are linked to the underlying physical mechanisms of heat transfer involved and those should be further investigated. The failure of studies attempting to replicate anomalous thermal conductivity enhancement in the literature could hence be understood, as these did not satisfy the conditions required to lead to an anomalous enhancement. The role of measurement errors was also considered. [ABSTRACT FROM AUTHOR]

The energy crisis, Greenhouse Gas (GHG) emissions, and Chlorofluorocarbon (CFC) emissions are major environmental issues at present. It is critical to achieve and reduce emissions and energy consumption through the use of environmentally friendly refrigerants. Utilizing an environmentally friendly refrigerant such as HFC-32 may offer a viable solution to the ozone depletion potential (ODP) and global warming issues. This study examines the effects of aluminium oxide (Al2O3) nanoparticles at volume concentrations of 0.06, 0.08, 0.1, 0.12, and 0.14% in pure refrigerants such as HFC-32 and R-410a used in air-conditioning systems based on the vapour compression refrigeration cycle. The thermophysical properties of pure and nanorefrigerants have been determined using REFPROP (NIST properties of fluid Reference) and a theoretical formulation model using MATLAB software. The important outcomes of HFC-32 nanorefrigerant show the maximum performance with 0.14% alumina nano additives which results in a 46.14% increase in the coefficient of performance (COP) and massive power savings upto 31.59%. Thermal conductivity exhibited an increase with an increment in nanoparticle concentration. Maximum thermal conductivity of 0.172 W/m-K is recorded in the case of HFC-32/Al2O3 nanorefrigerant with 0.14% volume concentration. The net refrigeration effect of pure refrigerants (R410a and HFC-32) is 77% and 79% and on addition of nanorefrigerants to the pure the net refrigeration effect increases to 81.2% and 83.5% for R410a and HFC-32 respectively. [ABSTRACT FROM AUTHOR]

Solar water heaters are one of the most popular applications of renewable energy. So, many modifications have been performed on them to boost their performance. The novelty of the current experimental work is to simultaneous employment of porous medium inside water reservoir and oil-based nanofluid as circulating fluid. Accordingly, TiO2 nanoparticles with two concentrations of 0.2 and 0.4 mass% were dispersed in engine oil as-based fluid. As results, when concentration of 0.4 mass% TiO2 in engine oil is integrated with porous medium, the temperature is increased by 6.4 °C. This temperature enhancement leads to boost the thermal efficiency more than 41%. Moreover, thermal efficiency increments of 5.4% and 19% are occurred, for the cases of employing porous media and simultaneous application of 0.4 mass% TiO2/oil with porous media, respectively. The exergy efficiency was assessed by means of the modified approach. Accordingly, utilization of only porous medium creates 19.6% improvement in the exergy efficiency of the system. Furthermore, the entropy generation in the system was calculated, too, and the base case showed 14.7% more entropy generation compared with TiO2/oil integrated with porous medium. Lastly, a comparative study has been done to show the superiority of this work in comparison with the others. [ABSTRACT FROM AUTHOR]

A wide range of studies was conducted to increase the heat transfer rate and reduce the size and cost of shell and tube heat exchangers (STHE). The paper's contributions lie in its ability to provide a comprehensive, up-to-date, and systematic overview of the various methods available for heat transfer enhancement in STHEs, making it an essential resource for researchers, engineers, and practitioners in the field of heat transfer. The studies that researched the overall heat transfer coefficient (U), number of transfer units, exergy efficiency, pressure drop, and thermal–hydraulic performance were reviewed. There are some advantages of the passive method such as no external needed power and lower operating cost compared to the active methods. The studies broadly support the view that heat transfer enhancement in STHE is heading toward considerable progress. A total of 47.8% of studies have focused on the passive approach, the air injection method, enhancing heat transfer utilizing nanofluids, and compound methods have percentages of studies 20.2, 22.3, and 9.7%, respectively. The air bubble injection causes the rise of the U ratio where the maximum value was indicated at 452% compared to only water flow. Swirl vane, corrugated tube, and wire coil insert have U ratio values of 130, 161, and 264%, respectively. Nanofluid results in a growth in the heat transfer where the TiO2 has the maximum U ratio (175.9%) compared to traditional fluid. The combination of air injection and passive heat augmentation methods, which was shown to be a substantial solution to several issues, needs to be the focus of more work in the future. Geometrical changes in tube surfaces in STHE are too required in the future with the use of materials coating to enhance heat transfer. The theoretical analysis of heat transfer techniques still needs to be improved, especially for pertinent empirical formulations. Also, since there aren't many relevant numerical simulations, more attention is required. [ABSTRACT FROM AUTHOR]

Nanofluids are generally utilized in providing cooling, lubrication phenomenon, and controlling the thermophysical properties of the working fluid. In this paper, nanoparticles of Al2O3 are added to the base fluid, which flows through the counterflow arrangement in a turbulent flow condition. The fluids employed are ethylbenzene and water, which have differing velocities on both the tube and the shell side of the cylinders. A shell tube-type heat exchanger is used to examine flow characteristics, friction loss, and energy transfer as they pertain to the transmission of thermal energy. The findings of the proposed method showed that the efficiency of a heat exchanger could be significantly improved by the number, direction, and spacing of baffles. With the inclusion of nanoparticles of 1% volume, the flow property, friction property, and heat transfer rate can be considerably improved. As a result, the Nusselt number and Peclet numbers have been increased to 261 and 9.14 E +5. For a mass flow rate of 0.5 kg/sec, the overall heat transfer coefficient has also been increased to a maximum value of 13464. The heat transfer rate of the present investigation with nanoparticle addition is 4.63% higher than the Dittus–Boelter correlation. The friction factor is also decreased by about 17.5% and 11.9% compared to the Gnielinski and Blasius correlation. The value of the friction factor for the present investigation was found to be 0.0376. It is hence revealed that a suitable proportion of nanoparticles along with the base fluids can make remarkable changes in heat transfer and flow behavior of the entire system. [ABSTRACT FROM AUTHOR]

In this work, a heat pipe solar collector (HPSC) utilization system is proposed. An experimental system was designed to analyze the effect of nanofluid concentration on the efficiency of the solar collector. The HPSC system was investigated with low concentration of water-based Al2O3 and MgO as the working medium. To examine the structural and optical properties, the X-ray diffraction (XRD) and Fourier transform-infrared (FT-IR) were performed. The HSPC exhibited higher efficiency when they treated with MgO nanofluids. As the concentration of the nanofluid increases, the efficiency of the solar collector is increased irrespective of the working medium. Nevertheless, compared to the Al2O3, MgO reported superior efficiency irrespective of the concentration. Second, the entropy generation for MgO was lower and highest for the neat water. Further, the flow rate of the nanofluids increases the collector efficiency spikes. Experimental system shows that the maximum performance of the HPSC is possible by employing the nanofluids to the existing system in an optimized concentration. [ABSTRACT FROM AUTHOR]

A parallel flow heat exchanger containing conical sectional inserts with three different pitch ratio (i.e., 1, 2, and 3) in a different orientation to the flow direction of the working fluid had been fabricated. Al2O3 with different volume concentration, i.e., 0%, 2%, and 4% nanoparticles (90 nm size) were added to De-ionized water as the working fluid. The combined effect of the Nanofluid and the inserts in different orientations was studied. It was concluded that the insert, flow direction, and Nanofluid volume concentration provided most augmented heat transfer and enhanced the thermal properties. [ABSTRACT FROM AUTHOR]

Enhanced thermal conductivity of nanofluids has proven importance in enhancing heat transfer for many application. In this study, thermal conductivity of graphite nanopowder dispersed in water at different temperatures was studied experimentally. Stable nanofluids of different concentrations (0.2 vol%, 0.5 vol%, 0.8 vol%, 1 vol% and 1.5 vol%) are prepared using ultrasonic cleaner by sonicating for 3 hour. Thermal conductivity was measured from temperature 25 to 55 ºC with an interval of 5 ºC using KD2 Pro thermal properties analyser. Experimental results showed that thermal conductivity increases with increase in temperature and volume concentration. Thermal conductivity of Water -- Graphite nanofluid showed enhancement of 5.6% to 20.42% for 0.2 vol% to 1.5 vol% of concentration at 25 ºC respectively. However, the maximum improvement of 39.72% was found at 1.5% of concentration at 55 ºC compared to water. A correlation was developed considering the effect of temperature and concentration using the regression method. The proposed correlation effectively predicts the thermal conductivity of Water -- Graphite nanofluids with an accuracy of ±2.8%. [ABSTRACT FROM AUTHOR]

Environmental friendly insulating oils are sought after as an alternative to polychlorinated biphenyls and mineral oil. The ester‐based insulating oils are the industry preferred choice as they are fire resistant and non‐toxic in nature. They also have moisture tolerance capability and oxidation stability are quite higher in synthetic ester due to its stable chemical structure than seed‐based natural ester oil. In this work, attempts were made to develop un‐inhibited synthetic‐ester‐based high dielectric permittivity CaCu3Ti4O12 (CCTO) nanofluids. These nanofluids were investigated for the critical parameters such as AC breakdown voltage, dielectric permittivity, loss tangent, DC resistivity, interfacial tension, viscosity, flash point and acidity properties as per the procedure outlined in IEC and ASTM standards. It has been observed that the addition of nanopowder in un‐inhibited oil results decrement in overall critical properties. These results suggest that these un‐inhibited ester oils are well suitable for high‐voltage transformer application in the present form and the combination of oxidation inhibitor and nanoceramic in the ester oils would give rise to enhanced critical parameters. [ABSTRACT FROM AUTHOR]

Nanofluids have opened the doors towards the enhancement of many of today's existing thermal applications performance. This is because these advanced working fluids exhibit exceptional thermophysical properties, and thus making them excellent candidates for replacing conventional working fluids. On the other hand, nanomaterials of carbon-base were proven throughout the literature to have the highest thermal conductivity among all other types of nanoscaled materials. Therefore, when these materials are homogeneously dispersed in a base fluid, the resulting suspension will theoretically attain orders of magnitude higher effective thermal conductivity than its counterpart. Despite this fact, there are still some challenges that are associated with these types of fluids. The main obstacle is the dispersion stability of the nanomaterials, which can lead the attractive properties of the nanofluid to degrade with time, up to the point where they lose their effectiveness. For such reason, this work has been devoted towards providing a systematic review on nanofluids of carbon-base, precisely; carbon nanotubes, graphene, and nanodiamonds, and their employment in thermal systems commonly used in the energy sectors. Firstly, this work reviews the synthesis approaches of the carbon-based feedstock. Then, it explains the different nanofluids fabrication methods. The dispersion stability is also discussed in terms of measuring techniques, enhancement methods, and its effect on the suspension thermophysical properties. The study summarizes the development in the correlations used to predict the thermophysical properties of the dispersion. Furthermore, it assesses the influence of these advanced working fluids on parabolic trough solar collectors, nuclear reactor systems, and air conditioning and refrigeration systems. Lastly, the current gap in scientific knowledge is provided to set up future research directions. [ABSTRACT FROM AUTHOR]