Showing papers by "Jacopo Buongiorno published in 2006"
TL;DR: In this article, the authors considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid and concluded that only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids.
Abstract: 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
5,329 citations
TL;DR: In this paper, a porous layer of nanoparticles on the heated surface occurs upon boiling of nanofluids containing alumina, zirconia, or silica nanoparticles.
Abstract: Buildup of a porous layer of nanoparticles on the heated surface occurs upon boiling of nanofluids containing alumina, zirconia, or silica nanoparticles. This layer significantly improves the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. The contact angle reduction is attributed to changes in surface energy and surface morphology brought about by the presence of the nanoparticle layer. The high surface wettability can plausibly explain the boiling critical heat flux enhancement in nanofluids.
359 citations
TL;DR: The supercritical water-cooled nuclear reactor (SCWR) concept offers potential for superior economics due to its high thermal efficiency and plant simplification as discussed by the authors, however, design of a thermal-spect...
Abstract: The supercritical water-cooled nuclear reactor (SCWR) concept offers potential for superior economics due to its high thermal efficiency and plant simplification. However, design of a thermal-spect...
18 citations
07 May 2006
TL;DR: In this paper, three methods of generating these nanofluids are available: creating them from chemical precipitation, purchasing the nanoparticles in powder form and mixing them with the base fluid, and direct purchase of prepared nanoparticles.
Abstract: As part of an effort to evaluate water-based nanofluids for nuclear applications, preparation and characterization has been performed for nanofluids being considered for MIT’s nanofluid heat transfer experiments. Three methods of generating these nanofluids are available: creating them from chemical precipitation, purchasing the nanoparticles in powder form and mixing them with the base fluid, and direct purchase of prepared nanofluids. Characterization of nanofluids includes colloidal stability, size distribution, concentration, and elemental composition. Quality control of the nanofluids to be used for heat transfer testing is crucial; an exact knowledge of the fluid constituents is a key to uncovering mechanisms responsible for heat transport enhancement.
13 citations
01 Jul 2006
TL;DR: In this paper, the authors evaluated water-based nano-fluids for nuclear applications, preparation and characterization has been performed for nanofluids being considered for MIT's nanofluid heat transfer experiments, and three methods of generating nano-fluids are available: creating them from chemical precipitation, purchasing the nano-particles in powder form and mixing them with the base fluid, and direct purchase of prepared nano-fluid.
Abstract: As part of an effort to evaluate water-based nano-fluids for nuclear applications, preparation and characterization has been performed for nano-fluids being considered for MIT's nano-fluid heat transfer experiments Three methods of generating these nano-fluids are available: creating them from chemical precipitation, purchasing the nano-particles in powder form and mixing them with the base fluid, and direct purchase of prepared nano-fluids Characterization of nano-fluids includes colloidal stability, size distribution, concentration, and elemental composition Quality control of the nano-fluids to be used for heat transfer testing is crucial; an exact knowledge of the fluid constituents is essential to uncovering mechanisms responsible for heat transport enhancement Testing indicates that nano-fluids created by mixing a liquid with nano-particles in powder form are often not stable, although some degree of stabilization is obtainable with pH control and/or surfactant addition Some commercially available prepared nano-fluids have been found to contain unacceptable levels of impurities and/or include a different weight percent of nano-particles compared to vendor specifications Tools utilized to characterize and qualify nano-fluids for this study include neutron activation analysis (NAA), inductively-coupled plasma spectroscopy (ICP), transmission electron microscopy (TEM) imaging, thermogravimetric analysis (TGA) and dynamic light scattering (DLS) Preparation procedures and characterization results for selected nano-fluidsmore » will be discussed in detail (authors)« less
1 citations