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This study investigated the convective heat transfer coefficient and friction coefficient in a copper helical double pipe heat exchanger utilizing biosynthetic aqua-based silver (Ag) nanofluids prepared with Histiopteris incisa leaf extract. The nanofluids were characterized using UV–Vis spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy. The experimental approach involved the constant heat flux method under laminar flow conditions with flow rates ranging from 100 to 160 litres per hour (LPH). Biosynthesized aqua-based silver nanofluids were synthesized using a one-step biosynthesis method, resulting in volume concentrations ranging from 0.3 to 0.9%. Our findings demonstrated that the biosynthesized aqua-based silver nanofluids exhibited enhanced convective heat transfer compared to pure water. Additionally, the heat transfer coefficient showed an increasing trend with higher volume concentrations of biosynthesized aqua-based silver nanofluids. Notably, the most significant improvement in convective heat transfer, reaching 40%, was achieved with a 0.9% volume concentration of biosynthesized aqua-based silver nanofluids, a flow rate of 140 LPH, and a Dean number of 1400. Moreover, the friction coefficients observed for biosynthesized aqua-based silver nanofluids were 35% higher than those of water in the Dean number range of 1400–2400. [ABSTRACT FROM AUTHOR]

In this experimental work, a comparative study of convective heat transfer and drop in pressure characteristics of Alumina-Cu/water and Alumina-CNT/water hybrid nanofluids in fully developed laminar flow in a uniformly heated tube with circular cross section is presented. For this, Alumina-Cu hybrid nanoparticles (90:10 proportion was taken. Below this proportion for, e.g., 80:20, the nanofluid was not stable. Coagulation was happening) were prepared by using a hydrogen reduction technique. Characterization was done for the prepared powder through XRD and scanning electron microscope (SEM) to validate the chemical constitution, to establish the size of the particle and to investigate the morphology of surface. The synthesized Alumina-Cu hybrid nanopowder is dispersed in deionized water in an ultrasonic bath to form a well-dispersed 0.1% volume concentration of hybrid nanofluid. Carbon nanotube (CNT) functionalization was done by acid treatment method to convert the CNT surface from hydrophobic to hydrophilic. Scanning electron microscope was used for characterization of the aluminum oxide particles and functionalized multi-walled carbon nanotubes. To prepare 0.1% volume fraction of Alumina-CNT/water nanofluid, required quantities of Alumina powder and functionalized CNT were dispersed in deionized water in 90:10 proportion. Experiments were conducted in the laminar flow range with water, Alumina/water and hybrid nanofluids, and the results obtained have been compared. The convective heat transfer experimental results showed greater enhancement with Alumina-CNT/water hybrid fluid compared to Alumina-Cu/water hybrid fluid. The maximum enhancements of 30.65% and 20.48% in Nusselt number have been obtained for 0.3% volume fraction of Alumina-CNT/water hybrid nanofluid and Alumina-Cu/water hybrid nanofluid, respectively, at Reynolds number of 1350 as compared to the Nusselt number of water. [ABSTRACT FROM AUTHOR]

Nanofluids are advanced heat transfer fluids that exhibit thermal properties superior than that of the conventional fluids such as water, oil etc. This paper reports the experimental study on convective heat transfer characteristics of water based titanium dioxide nanofluids in fully developed flow through a uniformly heated pipe heat exchanger fitted with modified butterfly inserts. Nanofluids are prepared by dispersing TiO2 nanoparticles of average particle size 29 nm in deionized water. The heat transfer experiments are performed in laminar regime using nanofluids prepared with 0.1% and 0.3% volume fractions of TiO2 nanoparticles. The thermal performance characteristics of conventional butterfly inserts and modified butterfly inserts are also compared using TiO2 nanofluid. The inserts with different pitches 6 cm, 9 cm and 12 cm are tested to determine the effect of pitch distance of inserts in the heat transfer and friction. The experimental results showed that the modification made in the butterfly inserts were able to produce higher heat transfer than conventional butterfly inserts. [ABSTRACT FROM AUTHOR]

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]

An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and dimpled tube under laminar flow with constant heat flux is carried out with distilled water and CuO/water nanofluids. For this, CuO nanoparticles with an average size of 15.3 nm were synthesized by sol-gel method. The nanoparticles are then dispersed in distilled water to form stable suspension of CuO/water nanofluid containing 0.1, 0.2 and 0.3% volume concentration of nanoparticles. It is found that the experimental Nusselt numbers for 0.1, 0.2 and 0.3% volume concentration of CuO nanoparticles are about 6, 9.9 and 12.6%, respectively higher than those obtained with distilled water in plain tube. However, the experimental Nusselt numbers for 0.1, 0.2 and 0.3% volume concentration of CuO nanoparticles are about 3.4, 6.8 and 12%, respectively higher than those obtained with distilled water in dimpled tube. The friction factor of CuO/water nanofluid is also increased due to the inclusion of nanoparticles and found to increase with nanoparticle volume concentration. The experimental results show that there exists a difference in the enhancement levels of Nusselt numbers obtained with nanofluids in plain tube and dimpled tube. Hence it is proposed that the mechanism of heat transfer enhancement obtained with nanofluids is due to particle migration from the core of fluid flow to tube wall. [ABSTRACT FROM AUTHOR]

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]

Heat transfer characteristics of water-based nanocrystalline alumina (Al2O3) nanofluids flowing through a uniformly heated tube under a fully developed laminar and turbulent flow regime is investigated experimentally in the present work to explore the heat transfer mechanism in nanofluids. In a laminar flow, the increase in Nusselt number was attributed to the thermophysical properties of the nanofluid. The movement of nanoparticles, along with the turbulent eddies in the turbulent core region and diffusion mechanism, such as thermophoresis, in the laminar sublayer are believed to be the reasons for enhanced heat transfer in turbulent region. The compatibility of Al2O3/water nanofluids was also examined by monitoring its color. [ABSTRACT FROM AUTHOR]

Heat transfer and friction factor characteristics in a circular tube fitted with wire coil inserts are investigated experimentally using Al2O3/water nanofluid as the working fluid. The effects of the pitch ratio and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with fully developed transition flow for Reynolds number in the range of 2500 to 5000. The experiments were performed using wire coil inserts having different pitch ratios with Al2O3/water nanofluid having 0.1% volume concentration of nanoparticles as the working fluid. Experiments using the plain tube and with wire coil inserts were also carried out with distilled water as the working fluid for experimental setup validation and comparison. The experimental results reveal that the use of nanofluids increases the heat transfer rate with negligible increase in friction factor in the plain tube and the tube fitted with wire coil inserts. In addition, empirical correlations are proposed based on the experimental results of the present study, which are found to be sufficiently accurate for prediction of the heat transfer and friction factor characteristics. [ABSTRACT FROM AUTHOR]

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]

Due to superior thermal properties and many potential applications, nanofluids have recently attracted the attention of the research community. Though the experiments in nanofluids have revealed the spectacular heat transfer enhancement in them, there are many inconsistencies in the reported experimental values and controversies in the mechanisms proposed for heat transport in nanofluids. This indicates that it is crucial to be aware of the different mechanisms contributing to superior thermal properties of nanofluids and discuss the existing controversies in them. Therefore, in this article, we provide an overview of the various possible mechanisms reported in the literature that contribute to enhanced thermal conductivity of nanofluids. The other factors affecting the nanofluids thermal conductivity and the mathematical models for estimating nanofluids thermal conductivity with their limitations are also discussed. [ABSTRACT FROM AUTHOR]

An experimental investigation was conducted to study the effect of relative waviness (amplitude-to-wavelength ratio) on heat transfer and pressure drop characteristics of wavy minichannel array heat sinks. Hydrodynamically fully developed flows of de-ionized (DI) water, and 0.5% and 0.8% concentrations of Al2O3/water nanofluid were introduced into two heat sinks, each with an array of 15 rectangular wavy minichannels of amplitude-to-wavelength ratio of 0.100 and 0.133. The minichannels had a width of 0.9 mm and a depth of 1.3 mm and were machined on a 30 × 30 mm2 Aluminum substrate of 11 mm thickness. Reynolds number was varied from 700 to 2300 and a constant heat flux of 45,000 W/m2 was applied. Analysis of experimental results suggests increases in both Nusselt number and pressure drop with an increase in relative waviness in the laminar and transitional flow regimes. A maximum performance factor (PF) of 2.6 is achieved for a Reynolds number of 1900 using DI water in the wavy minichannels of higher relative waviness. Temperature distributions and new Nusselt number correlations for both geometries are also presented. [ABSTRACT FROM AUTHOR]

The present experimental study focuses on the energy storage performance of Therminol 55-TiO2 nanofluids for the absorption of solar energy. Photothermal conversion efficiency is enhanced using Fresnel lens and secondary reflectors with a glass-type evacuated absorber tube. The focal length of the Fresnel lens is 150 mm, and that of the secondary reflector is 70 mm. The optical absorbance, extinction coefficient, and thermal conductivity of nanofluids at 100, 250, 350, and 500 ppm are reported. The optical path length of the energy storage medium is 1 cm. The optical performance of the nanofluids is analyzed in the range of 400 to 800 nm. Compared to base fluid, the prepared concentrations show higher absorbance in the measured range of wavelength. The optimum concentration is found to be 250 ppm, and its specific heat is measured in the temperature range of 27 to 117°C and is found to vary from 1.85 to 2.19 J/g °C. The thermal conductivity of the maximum concentration of nanofluid is 0.134 W/mK. The optical absorbance test confirms the stability of nanofluids. Maximum temperature and photothermal conversion efficiency are obtained. [ABSTRACT FROM AUTHOR]

The present study aims to develop a compact experimental facility to trap solar energy. Line focusing concentrators, i.e., Fresnel lens and secondary reflectors, are coupled to enhance the photothermal conversion efficiency. Two types of receiver tubes are used, a plain copper tube and an evacuated glass tube embedded with a copper tube. Surfactant-free multiwalled carbon nanotubes-Therminol55 nanofluid with concentrations of 25, 50, 75, and 100 ppm are used in this study. The characterization of the nanoparticles and nanofluids is presented. In the visible range, a maximum absorbance and extinction coefficient of 0.75 and 1.7 cm-1 are obtained for 100 ppm concentration. The thermal conductivity is also enhanced by 6.29% compared to base fluid. A maximum fluid temperature of 78.15 and 89.58 °C is observed for plain receiver tube and receiver tube in evacuated space, respectively, and the corresponding efficiencies are 12.65 and 17.36%. [ABSTRACT FROM AUTHOR]

Convective heat transfer and friction factor studies are experimentally carried out in a smooth and five helically corrugated tubes of different heights and pitches of corrugation with spiraled rod inserts. The experiment is conducted under turbulent flow (Re = 4800–8900) and constant wall heat flux conditions. Deionized (DI) water and titanium dioxide (TiO2)/DI water nanofluids are used as working fluids. The average size of TiO2 nanoparticles is 32 nm. Two volume concentrations of nanofluids (0.25 and 0.5%) are used in this study. The combined effects of nanofluids, inserts and corrugation in tubes on Nusselt number and friction factor are investigated. The results indicate that (i) the addition of TiO2 nanoparticles in DI water upsurges the heat transfer rate, which increases with nanofluids volume concentrations; (ii) use of inserts and corrugation in tubes enhances the heat transfer rate further; (iii) among the corrugated tubes, the tube having highest corrugation height (hc = 1 mm) and lowest pitch (pc = 8 mm) with spiraled rod insert of smaller pitch (pi = 30 mm) shows the maximum thermal performance factor of 1.56 for 0.5% volume concentration of nanofluids. [ABSTRACT FROM AUTHOR]

An experimental study on the convective heat transfer and friction factor characteristics of TiO2/DI water nanofluids in uniformly heated plain and helically corrugated tubes (HCT) with and without spiraled rod inserts (SRI) under laminar flow regime is presented in this paper. TiO2 nanoparticles with an average size of 32 nm are dispersed in deionized (DI) water to form stable suspensions containing 0.1, 0.15, 0.2, and 0.25% volume concentrations of nanoparticles. It is found that the inclusion of nanoparticles to DI water ameliorated Nusselt number which increased with nanoparticles concentration upto 0.2%. Two spiraled rod inserts made of copper with different pitches (pi = 50 mm and 30 mm) are inserted in both plain and corrugated tubes and it is found that the addition of these inserts increased the Nusselt number substantially. For Helically corrugated tube with lower pitch and maximum height of corrugation (pc = 8 mm, hc = 1 mm) with 0.2% volume concentration of nanoparticles, a maximum enhancement of 15% in Nusselt number is found without insert and with insert having lower pitch (pi = 30 mm) the enhancement is 34% when compared to DI water in plain tube. The results on friction factor show a maximum penalty of about 53.56% for the above HCT. [ABSTRACT FROM AUTHOR]

The purpose of this study is to determine the optimum design parameters of the heat exchanger with circular ring inserts and Al2O3/water nanofluid using Taguchi experimental design method. The basic principle of using nanofluid and circular ring inserts is to enhance the heat transfer characteristics of a heat exchanger, thus reducing the size and cost of it. The effects of the three design parameters, such as Reynolds number (Re), pitch to diameter ratio (p/d) of circular ring inserts, and volume concentration (ϕ) of nanofluid on heat transfer characteristics are investigated. In the Taguchi experimental design method, Nusselt number and friction factor are considered as performance parameters. An L9 orthogonal array is chosen as an experimental plan for the design parameters. The results are then analyzed by using ANOVA statistical tools. The optimum configurations of control factors for Nusselt number and friction factor are A3B3C3 and A3B1C2, respectively. The results also proved that the Taguchi experimental design method has been successfully applied in the study. Numerical and experimental results are in very good agreement with each other. [ABSTRACT FROM AUTHOR]

An experimental work had been carried out to investigate the heat transfer and friction factor characteristics of Al2O3 /water nanofluid with circular ring inserts of various pitch to diameter ratios at constant heat flux condition. Al2O3 /water nanofluids with two different volume concentrations were used as the working fluid. The experiments were performed in the turbulent flow regime of 5000 to 25 000 Reynolds number. The results showed that the heat transfer characteristics of Al2O3 /water nanofluids were intensified compared to water and it increases when utilised with circular ring insert. The Nusselt number enhancement of 0.3% volume concentration of nanofluid in the tube without insert was 8.61% and with insert was 38.71% compared to water. Based on the investigation, two correlations among various parameters namely, Nusselt number (Nu), friction factor(f), Prandtl number (Pr), pitch to diameter ratio (p/d), Reynolds number (Re) and particle volume fraction (f) were presented. Over the range investigated, the maximum thermal performance factor of 1.26 is found with the simultaneous employment of the insert and Al2O3/water nanofluid at 0.3% volume concentration. [ABSTRACT FROM AUTHOR]

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]

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]

In this study, the heat transfer and friction factor of a shell and helically coiled tube heat exchanger using AlO / water nanofluid at 0.1%, 0.4% and 0.8% particle volume concentration were tested. The test was conducted under laminar flow condition at 5100 < Re < 8700. It is found that the overall heat transfer coefficient, inner heat transfer coefficient and experimental inner Nusselt number are 24%, 25% and 28%, respectively, higher than water at 0.8% particle volume concentration of nanofluid. It is observed that the presence of nanoparticles further intensify the formation of secondary flow and proper mixing of fluid when nanofluid passes through the helically coiled tube. Apart from further flow intensification, higher thermal conductivity of nanofluid and random movement of nanoparticles contribute to the enhanced heat transfer coefficient. Also found that the friction factor increases over particle volume concentration and this is due to increased nanofluid viscosity while increasing particle volume concentration. [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]

In this experimental investigation, the heat transfer coefficients of a shell and helically coiled tube heat exchanger using /water nanofluid under laminar flow condition were studied. The nanoparticles were characterized by X-Ray diffraction (XRD). The /water nanofluid at 0.1%, 0.4% and 0.8% particle volume concentration were prepared by using two step method. The prepared nanofluid was characterized by scanning electron microscope (SEM). It is observed that the overall heat transfer coefficient, inner heat transfer coefficient and experimental inner Nusselt number increase while increasing particle volume concentration and increasing inner Dean number. The enhancement of overall heat transfer coefficient was found to be 7%, 16.9% and 24.2% at 0.1%, 0.4% and 0.8% /water nanofluid respectively when compared with water. The enhancement of tube side experimental Nusselt number was found to be 17%, 22.9% and 28% at 0.1%, 0.4% and 0.8% particle volume concentration of /water nanofluid respectively when compared with water at fixed Dean number. The tests were conducted in the range of 1600 < < 2700, and 5200 < < 8600 under laminar flow condition and counter flow configuration. These enhancements are due to higher thermal conductivity of nanofluid while increasing particle volume concentration and Brownian motion of nanoparticles. It is studied that there is no negative impact on formation of secondary flow and mixing of fluid when nanofluid passes through the helically coiled tube. [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]

In this experimental investigation, the heat transfer and pressure drop analysis of a shell and helically coiled tube heat exchanger by using AlO / water nanofluids have been carried out under turbulent flow condition. The AlO/ water nanofluids of 0.1%, 0.4%, and 0.8% particle volume concentration have been prepared by using two step method. The tube side experimental Nusselt number of 0.1%, 0.4% and 0.8% nanofluids were found to be 28%, 36% and 56%, respectively higher than water. These enhancements are due to higher thermal conductivity of nanofluid, better fluid mixing and strong secondary flow formation in coiled tube. The pressure drop of 0.1%, 0.4% and 0.8% were found to be 4%, 6%, and 9%, respectively higher than water. The increase in pressure drop is due to increase in nanofluid viscosity while adding nanoparticles. The measurement of nanofluid thermal performance factor is found to be greater than unity. It is concluded that the AlO nanofluid can be applied as a coolant in helically coiled tube heat exchanger to enhance heat transfer with negligible pressure drop. [ABSTRACT FROM AUTHOR]