NANOFLUIDS Abstract Suspended nano particles in conventional fluids are called nanofluids...recent development of nanotechnology brings out a new heat transfer coolant called 'nanofluids'. These fluids exhibit larger thermal properties than conventional coolants Nanofluids can be considered to be the next-generation heat transfer fluids because they offer exciting new possibilities to enhance heat transfer performance compared to pure liquids. Micrometer-sized particle-fluid suspensions exhibit no such dramatic enhancement. Nanofluids are expected to have superior properties compared to conventional heat transfer fluids, as well as fluids containing micro-sized metallic particles. The much larger relative surface area of nanoparticles, compared to those of conventional particles, not only significantly improves heat transfer capabilities, but also increases the stability of the suspensions. In this paper an attempt has been made to discuss about the enhancement of thermal properties in nanofluids especially about the thermal conductivity. The increase in thermal conductivity of the nanofluids in comparison with the conventional heat transfer fluids is assumed to be because of the following mechanisms. 1) Brownian motion 2) Interfacial layer(nano layer) 3) Volume fraction INTRODUCTION Suspended nanoparticles in various base fluids can alter the fluid flow and heat transfer characteristics of the base fluids. These suspensions of nano sized particles in the base fluids are called nanofluids. Nanofluids can be considered to be the next-generation heat transfer fluids because they offer exciting new possibilities to enhance heat transfer performance compared to pure liquids. The study of nanofluids has lately gained scientific interest, due to their enhanced thermal conductivity, which would significantly improve the performance of heat transfer equipment.
In the past, the thermal conductivity of working fluids has been augmented by suspending Millimeter- or micrometer-sized particles in a base fluid. However, it has not been of interest for practical applications due to problems such as sedimentation, erosion, clogging, fouling and increased pressure drop of the flow channel. Lately, technological progress has led to the development and production of metal particles in nanometer scale, which, when dispersed in a conventional base fluid, appreciably enhance its thermal conductivity. Water, ethylene glycol and various kinds of oils are usually employed as base fluids. It seems that these suspensions, called nanofluids, can possibly overcome the aforementioned problems, because the particles are ultra-fine and are usually used at low particle concentrations. Also, nanofluids can improve abrasion-related properties as compared to the conventional solid/fluid mixtures. Successful employment of nanofluids will support the current trend toward component miniaturization by enabling the design of smaller and lighter heat exchanger systems.
From the fig it is evident the suspensions of nano sized particles in conventional heat transfer fluids i.e. nanofluids help in the better functioning of the heat transfer equipment for the following reasons. Better dispersion Behavior Less clogging and Abrasion Much larger Surface area-to volume Ratio SYNTHESIS In general, there are two methodologies used to produce nanofluids, 1. single-step method, In the single step method the nanoparticles are produced and dispersed simultaneously into the base fluid 2. Two step method Here the two aforementioned processes are accomplished separately. A single-step method is usually employed for metal nanofluid preparation, while a twostep method applies better for nanofluids containing oxide nanoparticles. The main advantage of the single-step technique is the minimization of nanoparticles agglomeration. The most well-known single-step methods are the direct evaporation approach. Direct Evaporation 10 nm dia CuO in ethylene glycol
THERMAL CONDUCTIVITY OF NANO FLUIDS The fluids that have been traditionally used for heat transfer applications have a rather low thermal conductivity. Taking into account the rising demands of modern technology, it has been recently proposed that dispersion of small amounts of nanometer-sized solids in the fluid called nanofluids can enhance the thermal conductivity of the fluids. This increase in the thermal conductivity is predicted to be because of the following reasons: 1. Brownian motion 2. Interfacial layer (nanolayer) 3. Volume fraction of particles Brownian motion It has been found that the Brownian motion of nanoparticles at the molecular and nanoscale level is a key mechanism governing the thermal behavior of nanoparticle fluid suspensions ("nanofluids"). The enhancement in the effective thermal conductivity of nanofluids is due mainly to the localized convection caused by the Brownian movement of the nanoparticles. It is postulated that the enhanced thermal conductivity of a nanofluid, when compared to conventional predictions, is mainly due to Brownian motion which produces micro-mixing. This effect is additive to the thermal conductivity of a static dilute suspension. keff = kstatic + kbrownian Since the speed of thermal wave propagation is much faster than the particle Brownian motion, the static part cannot be neglected.
Brownian motion of nanoparticles Interfacial layer (nanolayer) Although liquid molecules close to a solid surface are known to form layered structures, little is known about the connection between this nanolayer and the thermal properties of solid/liquid suspensions. It is assumed that the solid-like nanolayer acts as a thermal bridge between a solid nanoparticle and a bulk liquid and so is key to enhancing thermal conductivity. From this thermally bridging nanolayer idea, a structural model of nanofluids that consists of solid was suggested. nanoparticles, a bulk liquid, and solid-like nanolayers.
Conventional pictures of solid/liquid suspensions do not have this nanolayer. The thermal conductivity of the nanolayer on the surface of the nanoparticle is not known. However, because the layered molecules are in an intermediate Physical state between a bulk liquid and a solid the solid-like nanolayer of liquid molecules would be expected to lead to a higher thermal Conductivity than that of the bulk liquid. Schematic cross section of nanofluid structure consisting of nanoparticles, bulk liquid, and nanolayers at solid/liquid interface.
Single spherical particle with interfacial layer in a fluid medium. Volume fraction Highly conductive nanoparticles of very low volume fractions distributed in a quiescent liquid (called nanofluids ) may measurably increase the effective thermal conductivity of the suspension when compared to the pure liquid.
CONCLUSIONS Many studies have observed significantly improved heat transfer Properties in nanofluids (thermal conductivity, Heat flux, heat transfer coefficient) many conflicts exist between different studies Different sample preparation techniques, particle size, surface Treatment, fluid and nanoparticle materials, measurement techniques May be important Degree of interaction between particles appears to be important New theories are needed that take into account all important Characteristics of nanofluids. The development of nanofluids is hindered by the lack of theoretical understanding of the mechanisms.
REFERENCES Mechanisms of Enhanced Heat Transfer in Nanofluids by J.A. Eastman, Materials Science Division, Argonne National Laboratory Document By SANTOSH BHARADWAJ REDDY Email: help@matlabcodes.com 1.Engineeringpapers.blogspot.com 2.www.matlabcodes.com 3.microcontroller-project-codes.blogspot.com 4.microcontroller-library.blogspot.com 5.arduino-projects-here.blogspot.com 6.labview-projects.blogspot.com 7.java-basics.blogspot.com 8.itsnanoworld.blogspot.com More Papers,Projects and Presentations available on above sites.