Abstract
The primary objective of this study is to investigate the effect of slip mechanisms in nanofluids through scaling analysis. The role of nanoparticle slip mechanisms in both water- and ethylene glycol-based nanofluids is analyzed by considering shape, size, concentration, and temperature of the nanoparticles. From the scaling analysis, it is found that all of the slip mechanisms are dominant in particles of cylindrical shape as compared to that of spherical and sheet particles. The magnitudes of slip mechanisms are found to be higher for particles of size between 10 and 80 nm. The Brownian force is found to dominate in smaller particles below 10 nm and also at smaller volume fraction. However, the drag force is found to dominate in smaller particles below 10 nm and at higher volume fraction. The effect of thermophoresis and Magnus forces is found to increase with the particle size and concentration. In terms of time scales, the Brownian and gravity forces act considerably over a longer duration than the other forces. For copper-water-based nanofluid, the effective contribution of slip mechanisms leads to a heat transfer augmentation which is approximately 36% over that of the base fluid. The drag and gravity forces tend to reduce the Nusselt number of the nanofluid while the other forces tend to enhance it.
Highlights
Nanofluid was first proposed by Choi and Eastman [1] about a decade ago, to indicate engineered colloids composed of nanoparticles dispersed in a base fluid
Drag force is proportional to the relative velocity between the base fluid and nanoparticle and is expressed by [12]: FD = −β vbf − vp mp where vbf is the velocity of the base fluid, vp is the particle velocity, mp is the mass of the particle, b is the interphase momentum exchange coefficient: β=
Gravity Gravity force is proportional to the volume of the particle, and the relative density of nanoparticle and base fluid is expressed as: FG = −Vp ρp − ρbf g where Vp is the volume of the particle, rp is the density of the nanoparticle, rbf is the density of the base fluid, and g is acceleration due to gravity
Summary
Nanofluid was first proposed by Choi and Eastman [1] about a decade ago, to indicate engineered colloids composed of nanoparticles dispersed in a base fluid. Many studies show that there is an abnormal increase in single phase convective heat transfer coefficient relative to the base fluid [2] Such an increase mainly depends on factors such as the form and size of the particles and their concentration, the thermal properties of the base fluid as well as those of the particles, kinetics of particle in flowing suspension, and nanoparticle slip mechanisms. Buongiorno [11] estimated the relative importance of different nanoparticle transport mechanisms through scaling analysis for water/alumina nanofluid and concluded that Brownian diffusion and thermophoresis are the two most important slip mechanisms. He ascertained that these results hold good for any nanoparticle size and nanofluid combination.
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