Showing papers on "Nanofluid published in 2010"

Applications of Nanofluids: Current and Future:

Abstract: Nanofluids are suspensions of nanoparticles in fluids that show significant enhancement of their properties at modest nanoparticle concentrations. Many of the publications on nanofluids are about u...

Nanofluid-based direct absorption solar collector

Abstract: Solar energy is one of the best sources of renewable energy with minimal environmental impact. Direct absorption solar collectors have been proposed for a variety of applications such as water heating; however the efficiency of these collectors is limited by the absorption properties of the working fluid, which is very poor for typical fluids used in solar collectors. It has been shown that mixing nanoparticles in a liquid (nanofluid) has a dramatic effect on the liquid thermophysical properties such as thermal conductivity. Nanoparticles also offer the potential of improving the radiative properties of liquids, leading to an increase in the efficiency of direct absorption solar collectors. Here we report on the experimental results on solar collectors based on nanofluids made from a variety of nanoparticles (carbon nanotubes, graphite, and silver). We demonstrate efficiency improvements of up to 5% in solar thermal collectors by utilizing nanofluids as the absorption mechanism. In addition the experiment...

Enhancement of heat transfer using nanofluids—An overview

Abstract: A colloidal mixture of nano-sized particles in a base fluid, called nanofluids, tremendously enhances the heat transfer characteristics of the original fluid, and is ideally suited for practical applications due to its marvelous characteristics. This article addresses the unique features of nanofluids, such as enhancement of heat transfer, improvement in thermal conductivity, increase in surface volume ratio, Brownian motion, thermophoresis, etc. In addition, the article summarizes the recent research in experimental and theoretical studies on forced and free convective heat transfer in nanofluids, their thermo-physical properties and their applications, and identifies the challenges and opportunities for future research.

Enhanced thermal conductivity of nanofluids: a state-of-the-art review

Abstract: Adding small particles into a fluid in cooling and heating processes is one of the methods to increase the rate of heat transfer by convection between the fluid and the surface. In the past decade, a new class of fluids called nanofluids, in which particles of size 1–100 nm with high thermal conductivity are suspended in a conventional heat transfer base fluid, have been developed. It has been shown that nanofluids containing a small amount of metallic or nonmetallic particles, such as Al2O3, CuO, Cu, SiO2, TiO2, have increased thermal conductivity compared with the thermal conductivity of the base fluid. In this work, effective thermal conductivity models of nanofluids are reviewed and comparisons between experimental findings and theoretical predictions are made. The results show that there exist significant discrepancies among the experimental data available and between the experimental findings and the theoretical model predictions.

An experimental investigation into the thermal conductivity enhancement in oxide and metallic nanofluids

Abstract: One of the reasons for the controversy on the thermal conductivity enhancement of nanofluids is the lack of extensive data over a wide range of parameters. In the present study, a comprehensive experimental dataset is obtained for thermal conductivity of nanofluids with variation in nanoparticle material, base liquid, particle size, particle volume fraction and suspension temperature. Transient hot wire (THW) equipment as well as Temperature Oscillation equipment are developed for the measurement of thermal conductivity of liquids. The measurements show that, in general, thermal conductivity values of all the nanofluids are higher than that of the equivalent macro-particle suspensions. Metallic nanofluids are found to give higher enhancements than that of oxide nanofluids. Particle size is found to have a tremendous impact on the thermal conductivity of nanofluids with enhancement in the thermal conductivity increasing almost inversely with reduction in the particle size. Increase in temperature significantly increases the thermal conductivity of a nanofluid. It is also observed that the thermal conductivity of nanoparticle suspensions is relatively higher at lower volume fractions, thereby giving a non-linear dependence on the particle volume fraction.

Techniques for measuring the thermal conductivity of nanofluids: A review

Abstract: There has been a rapid progress in research activities concerning nanofluids since a large enhancement in their thermal conductivity has been reported a decade ago. While this extraordinary thermal conductivity of nanofluids deserves scientific investigation, the inconsistency and controversy of the results reported by different groups for identical nanofluids across the world raises fundamental doubts and poses a hindrance in the potential applications of nanofluids. This paper presents a critical review of the several techniques for the measurement of thermal conductivity of nanofluids employed by the researchers. Additionally, a detailed description of a unique thermal conductivity measurement device based on the thermal comparator principle, developed by the present authors has been described. Besides the principle of this measurement device, the constructional details have been elaborated. Finally, some suggestions have been made for improving the reliability of the measurement of thermal conductivity.

Effects of nanoparticle charging on streamer development in transformer oil-based nanofluids

Abstract: Transformer oil-based nanofluids with conductive nanoparticle suspensions defy conventional wisdom as past experimental work showed that such nanofluids have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. This paradoxical superior electrical breakdown performance compared to that of pure oil is due to the electron charging of the nanoparticles to convert fast electrons from field ionization to slow negatively charged nanoparticle charge carriers with effective mobility reduction by a factor of about 1×105. The charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities shows that this electron trapping is the cause of the decrease in positive streamer velocity, resulting in higher electrical breakdown strength. Analysis derives the electric field in the vicinity of the nanoparticles, electron trajectories on electric field lines that charge nanoparticles, and expressions for the char...

Mixed convection flow in a lid-driven inclined square enclosure filled with a nanofluid

Abstract: This work is focused on the numerical modeling of steady laminar mixed convection flow in a lid-driven inclined square enclosure filled with water–Al2O3 nanofluid. The left and right walls of the enclosure are kept insulated while the bottom and top walls are maintained at constant temperatures with the top surface being the hot wall and moving at a constant speed. The developed equations are given in terms of the stream function–vorticity formulation and are non-dimensionalized and then solved numerically subject to appropriate boundary conditions by a second-order accurate finite-volume method. Comparisons with previously published work are performed and found to be in good agreement. A parametric study is conducted and a set of graphical results is presented and discussed to illustrate the effects of the presence of nanoparticles and enclosure inclination angle on the flow and heat transfer characteristics. It is found that significant heat transfer enhancement can be obtained due to the presence of nanoparticles and that this is accentuated by inclination of the enclosure at moderate and large Richardson numbers.

Experimental Investigation on the Thermal Conductivity and Viscosity of Silver-Deionized Water Nanofluid

Abstract: This article presents an experimental investigation where the thermal conductivity and viscosity of silver-deionized water nanofluid is measured and studied. The mixture consists of silver nanoparticles of 0.3, 0.6, and 0.9% of volume concentrations and studied for temperatures between 50°C and 90°C. The transient hot-wire apparatus and Cannon-Fenske viscometer are used to measure the thermal conductivity and kinematic viscosity of nanofluid, respectively. The thermal conductivity increases with the increase in temperature and particle concentrations. A minimum and maximum enhancement of 27% at 0.3 vol% and 80% at 0.9 vol% are observed at an average temperature of 70°C. The viscosity decreases with the increase in temperature and increases with the increase in particle concentrations. The effect of Brownian motion and thermophoresis on the thermo-physical properties is discussed. Thus, an experimental correlation for thermal conductivity and viscosity, which relates the volume concentration and t...

Preparation and Melting/Freezing Characteristics of Cu/Paraffin Nanofluid as Phase-Change Material (PCM)

Abstract: A new sort of nanofluid phase-change material (PCM) is developed by suspending a small amount of nanoparticles in melting paraffin. Cu, Al, and C/Cu nanoparticles were selected to add to the meltin...