Nanofluids are engineered colloids made of a base fluid and nanoparticles (1-100 nm) Nanofluids have higher thermal conductivity' and single-phase heat transfer coefficients than their base fluids In particular the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter's In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid These are inertia, Brownian diffusion, thermophoresis, diffusioplwresis, Magnus effect, fluid drainage, and gravity We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids Based on this finding, we developed a two-component four-equation nonhomogeneous equilibrium model for mass, momentum, and heat transport in nanofluids A nondimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases Furthermore, a comparison of the nanoparticle and turbulent eddy time and length scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies so an effect on turbulence intensity is also doubtful Thus, we propose an alternative explanation for the abnormal heat transfer coefficient increases: the nanofluid properties may vary significantly within the boundary layer because of the effect of the temperature gradient and thermophoresis For a heated fluid, these effects can result in a significant decrease of viscosity within the boundary layer, thus leading to heat transfer enhancement A correlation structure that captures these effects is proposed

Convective Transport in Nanofluids

Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.

/pdf/investigation-on-convective-heat-transfer-and-flow-features-3a5qwo5efi.pdf

Investigation on Convective Heat Transfer and Flow Features of Nanofluids

Turbulent friction and heat transfer behaviors of dispersed fluids (i.e., uttrafine metallic oxide particles suspended in water) in a circular pipe were investigated experimentally. Viscosity measurements were also conducted using a Brookfield rotating viscometer. Two different metallic oxide particles, γ-alumina (Al2O3) and titanium dioxide (TiO2), with mean diameters of 13 and 27 nm, respectively, were used as suspended particles. The Reynolds and Prandtl numbers varied in the ranges l04-I05 and 6.5-12.3, respectively. The viscosities of the dispersed fluids with γ-Al2O3 and TiO2 particles at a 10% volume concentration were approximately 200 and 3 times greater than that of water, respectively. These viscosity results were significantly larger than the predictions from the classical theory of suspension rheology. Darcy friction factors for the dispersed fluids of the volume concentration ranging from 1% to 3% coincided well with Kays' correlation for turbulent flow of a single-phase fluid. The Nusselt n...

Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles

Effective thermal conductivity of mixtures of e uids and nanometer-size particles is measured by a steady-state parallel-plate method. The tested e uids contain two types of nanoparticles, Al 2O3 and CuO, dispersed in water, vacuum pump e uid, engine oil, and ethylene glycol. Experimental results show that the thermal conductivities of nanoparticle ‐e uid mixtures are higher than those of the base e uids. Using theoretical models of effective thermal conductivity of a mixture, we have demonstrated that the predicted thermal conductivities of nanoparticle ‐e uid mixtures are much lower than our measured data, indicating the dee ciency in the existing models when used for nanoparticle ‐e uid mixtures. Possible mechanisms contributing to enhancement of the thermal conductivity of the mixtures are discussed. A more comprehensive theory is needed to fully explain the behavior of nanoparticle ‐e uid mixtures. Nomenclature cp = specie c heat k = thermal conductivity L = thickness Pe = Peclet number P q = input power to heater 1 r = radius of particle S = cross-sectional area T = temperature U = velocity of particles relative to that of base e uids ® = ratio of thermal conductivity of particle to that of base liquid ¯ = .® i 1/=.® i 2/ ° = shear rate of e ow Ω = density A = volume fraction of particles in e uids Subscripts

/pdf/thermal-conductivity-of-nanoparticle-fluid-mixture-54tj4jnmls.pdf

Thermal Conductivity of Nanoparticle -Fluid Mixture

Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review

The efficacy of gliding arc (GA) discharge for the generation of hydrogen peroxide (H2O2) and water with a low pH was studied because H2O2 combined with low-pH environment is known as a strong oxidizer that can be used for the bacterial inactivation. The ability of the GA discharge to inactivate Escherichia coli in water was tested experimentally, and the inactivation was found to increase with the plasma treatment time and rate of water injection flow to the GA discharge system. The best result showed a 2-log reduction of the number of colony-forming units of E. coli from 104 to 102 at a water injection flow rate of 180 mL/min. Furthermore, pH in the plasma-treated water was decreased from 6.0 to 3.55 after 25

Effects of H2O2 and Low pH Produced by Gliding Arc Discharge on the Inactivation of Escherichia Coli in Water

The turbulent hydrodynamic behavior of a drag-reducing viscoelastic fluid moving across a sudden-expansion step in a circular pipe is investigated with expansion ratios, D/d , of 1.39 and 1.90. Aqueous solutions of polyacrylamide AP-273 (200, 500, and 1000 ppm) are used to represent the viscoelastic fluid. Pressure distributions before and after the sudden-expansion step are measured, from which the loss coefficient for the sudden-expansion step is calculated for water and the polyacrylamide solutions. In general, the loss coefficients for the polyacrylamide solutions are consistently smaller than those for water over a range of Reynolds number ( Re D = 4840–107,000), which may be attributed to their elasticity. With the expansion ratio ( D/d ) increasing from 1.39 to 1.90, the loss coefficient also increases by 100% for water and by approximately 150–200% for the polyacrylamide solution. While the effect of the Reynolds number on the loss coefficient is negligibly small for water, the loss coefficient for the polyacrylamide solution is found to increase with increasing Reynolds numbers. From an instantaneous pressure fluctuation, recorded at each pressure tap location, it is discovered that the flow instability produced at a reattachment point is quickly attenuated for water but amplified for the polyacrylamide solution in the downstream of the test section.

Turbulent hydrodynamic behavior of a drag-reducing viscoelastic fluid in a sudden-expansion pipe

A l'aide d'une technique d'ombroscopie a large champ, on visualise la trajectoire et le sillage tourbillonnaire de rotors d'helicoptere. Les trajectoires de tourbillons d'extremite sont visibles pour des nombres de Mach de 0,38 a 0,60. Influence de l'angle d'attaque

Young I. Cho

Papers

Effects of H2O2 and Low pH Produced by Gliding Arc Discharge on the Inactivation of Escherichia Coli in Water

Turbulent hydrodynamic behavior of a drag-reducing viscoelastic fluid in a sudden-expansion pipe

Wide-field shadowgraphy of tip vortices from a helicopter rotor

A study of wall effect for viscoelastic fluids in the falling ball viscometer

A study of scale formation around air bubble attached on a heat-transfer surface