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Abstract: In this paper experimental investigations on convective heat transfer and pressure drop characteristics in a novel electronic heat sink fitted with dynamic mixers using water and Al2O3/water nanofluid are presented. For this, Al2O3 nanoparticles of average size 47nm are synthesized using microwave assisted chemical precipitation method and characterized using XRD and SEM. Heat transfer and pressure drop characteristics of water and nanofluid are studied in the electronic heat sink of base size 60mm×60mm made of copper with and without the dynamic mixer. The enhancements in the heat transfer and pressure drop results are compared with water and the heat sink without any mixer. The results reveal that there is significant increase in the convective heat transfer when the dynamic mixer is used for enhancing the heat transfer and further increase in heat transfer is found for the nanofluid. The rise in pressure drop compared with the increase of heat transfer is less. [Copyright &y& Elsevier]

Abstract: Experimental investigations on thermophysical properties and forced convective heat transfer characteristics of various nanofluids are reviewed and the mechanisms proposed for the alteration in their values or characteristics due to the addition of nanoparticles are summarized in this review. A comprehensive review on the experimental works on specific application of nanofluids is also presented. As the literature in this area is spread over a span of two decades, this review could be useful for researchers to have an accurate screening of wide range of experimental investigations on thermophysical properties, forced convective heat transfer characteristics, the mechanisms involved and applications of various nanofluids. [Copyright &y& Elsevier]

Abstract: Heat dissipation in the electronic components is being a critical issue due to the faster increase in the components’ heat flux and increasing demand for the miniature in features’ size. In the present work CuO/water nanofluids of volume fractions 0.1% and 0.2% are prepared by dispersing the nanoparticles in deionised water. A thin channelled copper water block of overall dimension 55×55×19mm is used for the study. The interface temperature of the water block is measured and a maximum reduction of 1.15°C is observed when nanofluid of 0.2% volume fraction is used as the working fluid compared to deionised water. Convective heat transfer coefficient of water block is found to increase with the volume flow rate and nanoparticle volume fraction and the maximum rise in convective heat transfer coefficient is observed as 29.63% for the 0.2% volume fraction compared to deionised water. Pumping power for the deionised water and nanofluids are calculated based on the pressure drop in the water block and the average increase in pumping power is 15.11% for the nanofluid volume fraction of 0.2% compared to deionised water. A correlation is proposed for Nusselt number which fits the experimental Nusselt number with in ±7.5%. [Copyright &y& Elsevier]

Abstract: A comparison of thermal characteristics of Al2O3/water and CuO/water nanofluids in transition flow through a straight circular duct fitted with helical screw tape inserts was made in this study. The helical screw tape inserts with twist ratios Y =1.78, 2.44 and 3 were used in the experimental study using 0.1% volume concentration Al2O3/water and CuO/water nanofluid. The average enhancements in Nusselt number for water with twist ratios of 1.78, 2.44 and 3 were 156.24%, 122.16% and 89.22% respectively when compared to plain tube. The average increase in Nusselt number corresponding to the twist ratios of 1.78, 2.44 and 3 were 166.84%, 128.67% and 89.22% respectively for Al2O3/water nanofluid. In the case of CuO/water nanofluid, the enhancements in Nusselt number were 179.82%, 144.29% and 105.63% for twist ratios 1.78, 2.44 and 3 respectively. Thermal performance analysis based on the constant pumping power criteria shows that helical screw tape inserts give better thermal performance when used with CuO/water nanofluid than with Al2O3/water nanofluid. [Copyright &y& Elsevier]

Abstract: In this experimental work, a fully developed laminar convective heat transfer and pressure drop characteristics through a uniformly heated circular tube using Al2O3–Cu/water hybrid nanofluid is presented. For this we synthesized Al2O3–Cu nanocomposite powder in a thermo chemical route that involves a hydrogen reduction technique and then dispersed the prepared hybrid nano powder in deionised water to form a stable hybrid nanofluid of 0.1% volume concentration. The prepared powder was characterized by X-ray diffraction and Scanning Electron Microscope to confirm the chemical composition, to determine the particle size and to study the surface morphology. The convective heat transfer experimental results showed a maximum enhancement of 13.56% in Nusselt number at a Reynolds number of 1730 when compared to Nusselt number of water. The experimental results also show that 0.1% Al2O3–Cu/water hybrid nanofluids have slightly higher friction factor when compared to 0.1% Al2O3/water nanofluid. The empirical correlations proposed for Nusselt number and friction factor are in good agreement with the experimental data. [Copyright &y& Elsevier]

Abstract: An experimental investigation on the convective heat transfer and friction factor characteristics in circular tube with spiraled rod inserts (pitch=15mm, 30mm) under turbulent flow with constant heat flux is carried out with distilled water and Al2O3/water nanofluids. For this purpose, Al2O3 nanoparticles were synthesized by using chemical precipitation method. The average size of particle is found to be 40.3nm. The nanoparticles are then dispersed in distilled water to form stable suspension of Al2O3/water nanofluids with 0.3, 0.4 and 0.5% volume concentration of nanoparticles. It is found that (i) heat transfer enhancement is caused by suspending nanoparticles and becomes more pronounced with the increase of the particle volume concentration (ii) the Nusselt number for spiraled rod inserts under turbulent flow showed an increase of about 10–48% compared to the Nusselt numbers obtained with plain tube (iii) the isothermal pressure drop for turbulent flow with spiraled rod inserts were found to be between 2 and 8% higher than the plain tube. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Abstract: An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and helically dimpled tube under turbulent flow with constant heat flux is presented in this work using CuO/water nanofluid as working fluid. The effects of the dimples and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with a fully developed turbulent flow for the Reynolds number in the range between 2500 and 6000. The height of the dimple/protrusion was 0.6mm. The effect of the inclusion of nanoparticles on heat transfer enhancement, thermal conductivity, viscosity, and pressure loss in the turbulent flow region were investigated. The experiments were performed using helically dimpled tube with CuO/water nanofluid having 0.1%, 0.2% and 0.3% volume concentrations of nanoparticles as working fluid. The experimental results reveal that the use of nanofluids in a helically dimpled tube increases the heat transfer rate with negligible increase in friction factor compared to plain tube. The experimental results showed that the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% (for 0.1%, 0.2% and 0.3% volume concentrations respectively) higher than the Nusselt number obtained with plain tube and water. The experimental results of isothermal pressure drop for turbulent flow showed that the dimpled tube friction factors were about 2–10% higher than the plain tube. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds number, pitch ratio and volume concentration fits with the experimental data within ±15%. [ABSTRACT FROM AUTHOR]

Abstract: Experimental investigations and theoretical determination of effective thermal conductivity and viscosity of Al2O3/H2O nanofluid are reported in this paper. The nanofluid was prepared by synthesizing Al2O3 nanoparticles using microwave assisted chemical precipitation method, and then dispersing them in distilled water using a sonicator. Al2O3/water nanofluid with a nominal diameter of 43nm at different volume concentrations (0.33–5%) at room temperature were used for the investigation. The thermal conductivity and viscosity of nanofluids are measured and it is found that the viscosity increase is substantially higher than the increase in thermal conductivity. Both the thermal conductivity and viscosity of nanofluids increase with the nanoparticle volume concentration. Theoretical models are developed to predict thermal conductivity and viscosity of nanofluids without resorting to the well established Maxwell and Einstein models, respectively. The proposed models show reasonably good agreement with our experimental results. [Copyright &y& Elsevier]

Abstract: In this paper, fully developed laminar flow convective heat transfer and friction factor characteristics of Al2O3/water nanofluid flowing through a uniformly heated horizontal tube with and without wire coil inserts is presented. For this purpose, Al2O3 nanoparticles of 43nm size were synthesized, characterized and dispersed in distilled water to form stable suspension containing 0.1% volume concentration of nanoparticles. The Nusselt number in the fully developed region were measured and found to increase by 12.24% at Re=2275 for plain tube with nanofluid compared to distilled water. Two wire coil inserts made of stainless steel with pitch ratios 2 and 3 were used which increased the Nusselt numbers by 15.91% and 21.53% respectively at Re=2275 with nanofluid compared to distilled water. The better heat transfer performance of nanofluid with wire coil insert is attributed to the effects of dispersion or back-mixing which flattens the temperature distribution and make the temperature gradient between the fluid and wall steeper. The measured pressure loss with the use of nanofluids is almost equal to that of the distilled water. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds/Peclet number, pitch ratio and volume concentration fits with the experimental data within ±15%. [Copyright &y& Elsevier]

The present work analyzes the thermal performance of a cylindrical copper mesh wick heat pipe using water based CuO nanofluids. The studies are extended further by varying the heat pipe inclination angle and heat input. Thermophysical properties of CuO/DI water nanofluids are also effectively analyzed. The reduction in thermal resistance ratio is about 23.83% and 10.43% respectively for 1.0 and 1.5 wt.% of CuO nanofluid compared with low concentration. Evaporation and condensation heat transfer coefficient ratios are improved by the use of CuO nanofluid and the maximum enhancement obtained is 30.50% and 23.54% respectively for an optimum tilt angle of 60°. Thermal efficiency of heat pipe tends to increase with the heat load and inclination angle and an improvement of 33.34% is observed for 120 W heat load at 60° inclination angle compared with the horizontal heat pipe. After the experimentation, characterizations of mesh wick structures are carried out. It is found that the deposition of CuO nanoparticles creates a thin coating layer on the wick surfaces in evaporator section. This increases the surface wettability and enhances the thermal performance of heat pipe. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a comparison of heat transfer and pressure drop characteristics of CuO/water nanofluids in a helically coiled heat exchanger held in horizontal and vertical positions is presented. Experiments were conducted in the turbulent flow regimes using water and CuO/water nanofluids of 0.1% and 0.2% volume concentrations. The experimental results show that there is no much difference between horizontal and vertical arrangements in the enhancement of convective heat transfer coefficient and friction factors of nanofluids compared to water. Irrespective of the positions of the helically coiled heat exchanger, the enhancement in internal Nusselt numbers is high for higher concentration nanofluids at turbulent flow. Also the experimental friction factors obtained are high for higher concentration nanofluids at low flow rates. Based on the experimental data, correlations are proposed for Nusselt number and friction factor. [Copyright &y& Elsevier]

Directly solar radiation absorbing nanofluids have the potential to absorb a wide spectrum of solar radiation and displace selectively coated metallic receivers in solar thermal collectors. Parameters including nanoparticle concentration, synthesis and storage conditions, can influence their long-term usage. In this study, 60 min was found to be optimal sonication duration to synthesise a uniform suspension of nanofluid containing amorphous-carbon nanoparticles and ethylene glycol as base fluid. Nanoparticle concentration can be used to tune extinction coefficient of nanofluid in the range of 75–400 m−1 for wavelength range of 320–1000 nm. Long-term stability and high temperature studies showed a time and temperature dependent increase in transmittance of nanofluid which is restored by 5 min of stirring. Computational modelling highlighted the role of incident intensity, nanoparticle concentration as well as inlet flow rate on receiver exit temperature. A ray-optics model employing weather data for Delhi (India) can predict the optical efficiency of an Asymmetric Compound Parabolic Concentrator solar collector. This combined approach can enable to predict the flow rate required to achieve a desired supply temperature at target locations. This rational framework combining experimental and computational approaches can be used to identify design parameters relevant for application of nanofluids in thermal collectors. • Synthesis parameters for nanofluids containing amorphous carbon in ethylene glycol. • Nanofluid optical properties can be tuned by changing nanoparticle concentration. • Temperature and time dependent settling can be restored by 5 min of stirring. • Extinction coefficient of nanofluid varies linearly with nanoparticle concentration. • Performance of concentrating solar collectors using directly absorbing nanofluid. [ABSTRACT FROM AUTHOR]

Abstract This paper reports the design and construction of a modified active solar distillation system (MSDS) and comparison of its performance parameters with that of a conventional solar still (CSS). MSDS consists of a solar still, a Fresnel lens concentrator with an evacuated receiver tube and a serpentine loop type heat exchanger. Al 2 O 3 –Therminol-55 nanofluid (nHTF) is used as the heat transfer fluid in the solar collector loop. Various performance parameters of MSDS and CSS such as saline water temperature, hourly yield and total yield have been compared under the ambient and solar conditions prevailing at Trichy. The experimental results indicate the hourly yield of MSDS with 0.1% nHTF is 45–250.27% more than CSS, and the total yield of MSDS with 0.1% nHTF is 12.190 L/m2 day compared to 3.48 L/m2 day for CSS. The daily efficiency of MSDS is found to vary with volumetric concentration of nanofluid, and maximum efficiency of 53.55% is obtained for 0.1% nHTF. Estimated cost for production of one litre-distilled water is approximately 1.54 INR for CSS and 1.41 INR for MSDS. MSDS developed in this study is found to be a better alternative to the CSS still due to its high productivity and cost-effectiveness. Highlights • Performance of the modified solar distillation system (MSDS) is evaluated experimentally. • Total freshwater yield enhanced by 250.27% for MSDS compared to conventional solar still. • The daily average efficiency of MSDS is found varying with the concentration of nano fluid. • MSDS can lead to better alternatives to replace the conventional solar still. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the enhancement of thermal performance characteristics in a plain tube fitted with helical screw louvered rod inserts using water and carbon nanotube (CNT)/water nanofluids of 0.1%, 0.2% and 0.5% volume concentration under turbulent flow condition. In the experiments, the swirling flow was introduced by using helical screw louvered rod inserts arrangements (forward and backward) inside the inner test tube with different twist ratios, Y = 1.78, 2.44 and 3.0. The experimental results revealed that the increase in heat transfer rate of the helical screw louvered rod insert was found to be strongly influenced by turbulence or vortex motion. The use of helical louvered rod inserts in plain tube causes intensification in heat transfer compared to plain tube for a given Reynolds number, while the Friction Factor was higher which increases with decrease in twist ratio. The maximum thermal performance factor value of 1.23 was found with the use of CNT/water nanofluid of 0.5% volume concentration with twist ratio of 1.78 in backward arrangement. Thermal performance analysis based on the constant pumping power criteria showed that helical louvered rod inserts with backward arrangement led to better thermal performance than that with forward arrangement. [ABSTRACT FROM AUTHOR]

Nanofluids that directly absorb solar radiation have been proposed as an alternative to selectively coated metallic receivers in solar thermal collectors. Given the expense of characterising a potential nanofluid experimentally methods for comparing nanofluids virtually are needed. This paper develops a computational wave optics model using COMSOL to simulate the absorption of nanoparticles suspended in a fluid for solar radiation (380–800 nm) and compares it to experimental results using reflectance and transmission spectrometry. It was concluded that while both yielded data with matching trends, the exact absorption of some fluids differed by up to 1 AU. Optical characteristics of nanofluids comprising ethylene glycol (melting point −12.99 °C and boiling point range 195–198 °C at 1013 h Pa) and graphene oxide (sheets size 5 nm × 19 nm × 19 nm, volume fraction 0.004–0.016%) have been experimentally measured. An optimum volume fraction of 0.012% of graphene oxide has been identified achieving a minimum reflectance and highest absorbance over the visible spectral range. [ABSTRACT FROM AUTHOR]