Showing papers by "Jacopo Buongiorno published in 2009"

A benchmark study on the thermal conductivity of nanofluids

Abstract: This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Experimental Study of Flow Critical Heat Flux in Alumina-Water, Zinc-Oxide-Water, and Diamond-Water Nanofluids

Abstract: It is shown that addition of alumina, zinc-oxide, and diamond particles can enhance the critical heat flux (CHF) limit of water inflow boiling. The particles used here were in the nanometer range (<100 nm) and at low concentration (≤0.1 vol %). The CHF tests were conducted at 0.1 MPa and at three different mass fluxes (1500 kg/m 2 s, 2000 kg/m 2 s, and 2500 kg/m 2 s). The thermal conditions at CHF were subcooled. The maximum CHF enhancement was 53%, 53%, and 38% for alumina, zinc oxide, and diamond, respectively, always obtained at the highest mass flux. A postmortem analysis of the boiling surface reveals that its morphology is altered by deposition of the particles during boiling. Additionally, the wettability of the surface is substantially increased, which seems to correlate well with the observed CHF enhancement.

Nuclear magnetic resonance-based study of ordered layering on the surface of alumina nanoparticles in water

Abstract: Layering of water molecules on the surface of alumina nanoparticles in an alumina/water nanofluid is studied using nuclear magnetic resonance (NMR). The data suggest that a thin ordered layer (∼1.4 nm) of water molecules surrounds each nanoparticle. This ordered layer increases the nanoparticle effective volumetric fraction; however, the nanofluid thermal conductivity appears to be unaffected by this layer, and in good agreement with Maxwell’s effective medium theory. Furthermore, the NMR data suggest that the nanoparticles do not enhance, but rather stifle micromixing in the base fluid.

Nanofluid Heat Transfer Enhancement for Nuclear Reactor Applications

Abstract: Colloidal dispersions of nanoparticles are known as ‘nanofluids’. Such engineered fluids offer the potential for enhancing heat transfer, particularly boiling heat transfer, while avoiding the drawbacks (i.e., erosion, settling, clogging) that hindered the use of particle-laden fluids in the past. At MIT we have been studying the heat transfer characteristics of nanofluids for the past five years, with the goal of evaluating their benefits for and applicability to nuclear power systems (i.e., primary coolant, safety systems, severe accident mitigation strategies). This paper will summarize the MIT research in this area with particular emphasis to boiling behavior, including, prominently, the Critical Heat Flux limit and quenching phenomena.

Nanofluid heat transfer enhancement for nuclear reactor applications

Abstract: Colloidal dispersions of nanoparticles are known as ‘nanofluids’. Such engineered fluids offer the potential for enhancing heat transfer, particularly boiling heat transfer, while avoiding the drawbacks (i.e., erosion, settling, clogging) that hindered the use of particle-laden fluids in the past. At MIT we have been studying the heat transfer characteristics of nanofluids for the past five years, with the goal of evaluating their benefits for and applicability to nuclear power systems (i.e., primary coolant, safety systems, severe accident mitigation strategies). This paper will summarize the MIT research in this area with particular emphasis to boiling behavior, including, prominently, the Critical Heat Flux limit and quenching phenomena.

Measurement of nucleation site density, bubble departure diameter and frequency in pool boiling of water using high-speed infrared and optical cameras

Abstract: A high-speed video and IR thermometry based technique has been used to obtain time and space resolved information on bubble nucleation and boiling heat transfer. This approach provides a fundamental and systematic method for investigating nucleate boiling in a very detailed fashion. Data on bubble departure diameter and frequency, growth and wait times, and nucleation site density are measured with relative ease. The data have been compared to the traditional decades-old and poorly-validated nucleate-boiling models and correlations. The agreement between the data and the models is relatively good. This study also shows that new insights into boiling heat transfer mechanisms can be obtained with the present technique. For example, our data and analysis suggest that a large contribution to bubble growth comes from heat transfer through the superheated liquid layer in addition to micro layer evaporation. (author)

Alumina Nanoparticle Pre-coated Tubing Ehancing Subcooled Flow Boiling Cricital Heat Flux

Abstract: Nanofluids are engineered colloidal dispersions of nano-sized particle in common base fluids. Previous pool boiling studies have shown that nanofluids can improve critical heat flux (CHF) up to 200% for pool boiling and up to 50% for subcooled flow boiling due to the boiling induced nanoparticle deposition on the heated surface. Motivated by the significant CHF enhancement of nanoparticle deposited surface, this study investigated experimentally the subcooled flow boiling heat transfer of pre-coated test sections in water. Using a separate coating loop, stainless steel test sections were treated via flow boiling of alumina nanofluids at constant heat flux and mass flow rate. The pre-coated test sections were then used in another loop to measure subcooled flow boiling heat transfer coefficient and CHF with water. The CHF values for the pre-coated tubing were found on average to be 28% higher than bare tubing at high mass flux G = 2500 kg/m2 s. However, no enhancement was found at lower mass flux G = 1500 kg/m2 s. The heat transfer coefficients did not differ much between experiments when the bare or coated tubes were used. SEM images of the test sections confirm the presence of a nanoparticle coating layer. The nanoparticle deposition is sporadic and no relationship between the coating pattern and the amount of CHF enhancement is observed.Copyright © 2009 by ASME

Effect of Nanoparticle Deposition on Rewetting Temperature and Quench Velocity in Experiments With Stainless Steel Rodlets and Nanofluids

Abstract: Quenching of small stainless steel rods in pure water and nanofluids with alumina and diamond nanoparticles at low concentrations (0.1 vol%) was investigated experimentally. The rods were heated to an initial temperature of ∼1000 °C and then plunged into the test fluid. The results show that the quenching behavior of the nanofluids is nearly identical to that of pure water. However, due to nanofluids boiling during the quenching process, some nanoparticles deposit on the surface of the rod, which results in much higher quenching rate in subsequent tests with the same rod. It is likely that particle deposition destabilizes the film-boiling vapor film at high temperature, thus causing the quenching process to accelerate, as evident from the values of the quench front speed measured by means of a high-speed camera. The acceleration strongly depends on the nanoparticle material used, i.e., the alumina nanoparticles on the surface significantly improve the quenching, while the diamond nanoparticles do not. The possible mechanisms responsible for the quench front acceleration are discussed. It is found that the traditional concept of conduction-controlled quenching cannot explain the acceleration provided by the nanoparticle layer on the surface.Copyright © 2009 by ASME

Alumina Nanoparticle Pre-Coated Tubing Enhancing Subcooled Flow Boiling Critical Heat Flux

Abstract: Nanofluids are engineered colloidal dispersions of nano-sized particle in common base fluids. Previous pool boiling studies have shown that nanofluids can improve critical heat flux (CHF) up to 200% for pool boiling and up to 50% for subcooled flow boiling due to the boiling induced nanoparticle deposition on the heated surface. Motivated by the significant CHF enhancement of nanoparticle deposited surface, this study investigated experimentally the subcooled flow boiling heat transfer of pre-coated test sections in water. Using a separate coating loop, stainless steel test sections were treated via flow boiling of alumina nanofluids at constant heat flux and mass flow rate. The pre-coated test sections were then used in another loop to measure subcooled flow boiling heat transfer coefficient and CHF with water. The CHF values for the pre-coated tubing were found on average to be 28% higher than bare tubing at high mass flux G = 2500 kg/m2 s. However, no enhancement was found at lower mass flux G = 1500 kg/m2 s. The heat transfer coefficients did not differ much between experiments when the bare or coated tubes were used. SEM images of the test sections confirm the presence of a nanoparticle coating layer. The nanoparticle deposition is sporadic and no relationship between the coating pattern and the amount of CHF enhancement is observed.Copyright © 2009 by ASME