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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]

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]

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]

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]

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]

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: 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: 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]

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: 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 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]