Showing papers by "Jacopo Buongiorno published in 2008"

Experimental Investigation of Turbulent Convective Heat Transfer and Pressure Loss of Alumina/Water and Zirconia/Water Nanoparticle Colloids (Nanofluids) in Horizontal Tubes

Abstract: The turbulent convective heat transfer behavior of alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) nanoparticle dispersions in water is investigated experimentally in a flow loop with a horizontal tube test section at various flow rates (9000

Nanofluids for Enhanced Economics and Safety of Nuclear Reactors: An Evaluation of the Potential Features, Issues, and Research Gaps

Abstract: Nanofluids are engineered colloidal suspensions of nanoparticles in water and exhibit a very significant enhancement (up to 200%) of the boiling critical heat flux (CHF) at modest nanoparticle conc...

Alumina Nanoparticles Enhance the Flow Boiling Critical Heat Flux of Water at Low Pressure

Abstract: Many studies have shown that addition of nanosized particles to water enhances the critical heat flux (CHF) in pool boiling. The resulting colloidal dispersions are known in the literature as nanofluids. However, for most potential applications of nanofluids the situation of interest is flow boiling. This technical note presents first-of-a-kind data for flow boiling CHF in nanofluids. It is shown that a significant CHF enhancement (up to 30%) can be achieved with as little as 0.01% by volume concentration of alumina nanoparticles in flow experiments at atmospheric pressure, low subcooling 20° C, and relatively high mass flux 1000 kg/ m 2 s. DOI: 10.1115/1.2818787

Transient and Steady-State Experimental Comparison Study of Effective Thermal Conductivity of Al2O3∕Water Nanofluids

Abstract: Nanofluids are being studied for their potential to enhance heat transfer, which could have a significant impact on energy generation and storage systems. However, only limited experimental data on metal and metal-oxide based nanofluids, showing enhancement of the thermal conductivity, are currently available. Moreover, the majority of the data currently available have been obtained using transient methods. Some controversy exists as to the validity of the measured enhancement and the possibility that this enhancement may be an artifact of the experimental methodology. In the current investigation, Al 2 O 3 /water nanofluids with normal diameters of 47 nm at different volume fractions (0.5%, 2%, 4%, and 6%) have been investigated, using two different methodologies: a transient hot-wire method and a steady-state cut-bar method. The comparison of the measured data obtained using these two different experimental systems at room temperature was conducted and the experimental data at higher temperatures were obtained with steady-state cut-bar method and compared with previously reported data obtained using a transient hot-wire method. The arguments that the methodology is the cause of the observed enhancement of nanofluids effective thermal conductivity are evaluated and resolved. It is clear from the results that at room temperature, both the steady-state cut-bar and transient hot-wire methods result in nearly identical values for the effective thermal conductivity of the nanofluids tested, while at higher temperatures, the onset of natural convection results in larger measured effective thermal conductivities for the hot-wire method than those obtained using the steady-state cut-bar method. The experimental data at room temperature were also compared with previously reported data at room temperature and current available theoretical models, and the deviations of experimental data from the predicted values are presented and discussed.

Measurement of Key Pool Boiling Parameters in Nanofluids for Nuclear Applications

Abstract: Nanofluids, colloidal dispersions of nanoparticles in a base fluid such as water, can afford very significant Critical Heat Flux (CHF) enhancement. Such engineered fluids potentially could be employed in reactors as advanced coolants in safety systems with significant safety and economic advantages. However, a satisfactory explanation of the CHF enhancement mechanism in nanofluids is lacking. To close this gap, we have identified the important boiling parameters to be measured. These are the properties (e.g., density, viscosity, thermal conductivity, specific heat, vaporization enthalpy, surface tension), hydrodynamic parameters (i.e., bubble size, bubble velocity, departure frequency, hot/dry spot dynamics) and surface conditions (i.e., contact angle, nucleation site density). We have also deployed a pool boiling facility in which many such parameters can be measured. The facility is equipped with a thin indium-tin-oxide heater deposited over a sapphire substrate. An infra-red high-speed camera and an optical probe are used to measure the temperature distribution on the heater and the hydrodynamics above the heater, respectively. The first data generated with this facility already provide some clue on the CHF enhancement mechanism in nanofluids. Specifically, the progression to burnout in a pure fluid (ethanol in this case) is characterized by a smoothly-shaped and steadily-expanding hot spot. By contrast, in the ethanol-based nanofluid the hot spot pulsates and the progression to burnout lasts longer, although the nanofluid CHF is higher than the pure fluid CHF. The presence of a nanoparticle deposition layer on the heater surface seems to enhance wettability and aid hot spot dissipation, thus delaying burnout.

Experimental study on quenching of a small metal sphere in nanofluids

Abstract: The objective of this research is to systematically investigate the quenching characteristics of a hot sphere in nanofluids. The experiments are carried out with a small (9.5 mm) stainless steel sphere with initial temperatures near 1000 °C. Alumina nanofluids and deionized water are tested at low volume concentrations (less than 0.1% by volume) and saturated conditions (100 °C). The results show that the quenching behavior in nanofluids is nearly identical to that in pure water. Moreover it is found that some nanoparticles accumulate on the sphere surface during the quenching process. Such accumulation of nanoparticles on the surface promotes the destabilization of the vapor film in subsequent quenching experiments, thus accelerating the return to nucleate boiling at higher temperature than that in the clean surface case.Copyright © 2008 by ASME

Surface Modifications Using Nanofluids for Nucleate Boiling Heat Transfer and CHF Enhancements

Abstract: Colloidal dispersions of nanoparticles, also known as nanofluids, have shown to result in significant Critical Heat Flux (CHF) enhancement. The CHF enhancement mechanism in nanofluids is due to the buildup of a layer of nanoparticles which occurs upon boiling. Some studies have shown that this layer may also lead to increase in nucleate boiling heat transfer. Therefore, this paper discusses how key surface parameters such as surface wettability and surface roughness can be manipulated and optimized by coating nanoparticles in colloidal dispersions onto the desired surface, to enhance the nucleate boiling heat transfer and CHF.Copyright © 2008 by ASME

Experimental Study of Flow Critical Heat Flux in Low Concentration Water-Based Nanofluids

Abstract: Nanofluids are known as dispersions of nano-scale particles in solvents. Recent reviews of pool boiling experiments using nanofluids have shown that they have greatly enhanced critical heat flux (CHF). In many practical heat transfer applications, however, it is flow boiling that is of particular importance. Therefore, an experimental study was performed to verify whether or not a nanofluid can indeed enhance the CHF in the flow boiling condition. The nanofluid used in this work was a dispersion of aluminum oxide particles in water at very low concentration (≤0.1 v%). CHF was measured in a flow loop with a stainless steel grade 316 tubular test section of 5.54 mm inner diameter and 100 mm long. The test section was designed to provide a maximum heat flux of about 9.0 MW/m2 , delivered by two direct current power supplies connected in parallel. More than 40 tests were conducted at three different mass fluxes of 1,500, 2,000, and 2,500 kg/m2 sec while the fluid outlet temperature was limited not to exceed the saturation temperature at 0.1 MPa. The experimental results show that the CHF could be enhanced by as much as 45%. Additionally, surface inspection using Scanning Electron Microscopy reveals that the surface morphology of the test heater has been altered during the nanofluid boiling, which, in turn, provides valuable clues for explaining the CHF enhancement.Copyright © 2008 by ASME

Experimental Study of Laminar Convective Heat Transfer and Viscous Pressure Loss of Alumina-Water Nanofluid

Abstract: Laminar convective heat transfer and viscous pressure loss were investigated for alumina-water nanofluid in a flow loop with a vertical heated tube. The experimental results are in good agreement with traditional model predictions for laminar flow, if the loading- and temperature-dependent thermophysical properties are utilized. No abnormal heat transfer enhancement was observed. The heat transfer coefficients in the entrance region and in the fully-developed region are estimated to increase by 17% and 27%, respectively, for alumina nanofluid at 6 vol%. Measured pressure loss of the nanofluid is within 20% of theory. It is concluded that the nanofluid laminar convective heat transfer and viscous pressure loss behavior can be predicted by existing models as long as the correct mixture properties are used. This finding is consistent with our previous observation for alumina nanofluid tested in the fully-developed turbulent flow regime.Copyright © 2008 by ASME

Experimental observation of the dynamic micro-and macro-layer during pool boiling

Abstract: This paper presents the results of an experimental study on nucleate pool boiling. Experiments were performed using vapor-deposited thin films which were electrically heated. High-speed infrared and visible cameras simultaneously observed bubble growth from the heater surface. Possible experimental confirmation of microlayer dynamics is presented.Copyright © 2008 by ASME

The Nuclear Renaissance in the U.S.

Abstract: Nuclear power currently provides 20% of the electricity generation in the U.S. and about 16% worldwide. As a carbon-free energy source, nuclear is receiving a lot of attention by industry, lawmakers and environmental groups, as they attempt to resolve the issue of man-made climate change. For the first time in 30 years several U.S. electric utilities have applied for construction and operation licenses of new nuclear power plants. This talk will review the safety, operational and economic record of the existing U.S. commercial reactor fleet, will provide an overview of the reactor designs considered for the new wave of plant construction, and will discuss several research projects being conducted at the Massachusetts Institute of Technology to support the expansion of nuclear power in the U.S. and overseas.