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.

A Benchmark Study on the Thermal Conductivity of Nanofluids

We describe an optical beam deflection technique for measurements of the thermal diffusivity of fluid mixtures and suspensions of nanoparticles with a precision of better than 1%. Our approach is tested using the thermal conductivity of ethanol-water mixtures; in nearly pure ethanol, the increase in thermal conductivity with water concentration is a factor of 2 larger than predicted by effective medium theory. Solutions of C60–C70 fullerenes in toluene and suspensions of alkanethiolate-protected Au nanoparticles were measured to maximum volume fractions of 0.6% and 0.35vol%, respectively. We do not observe anomalous enhancements of the thermal conductivity that have been reported in previous studies of nanofluids; the largest increase in thermal conductivity we have observed is 1.3%±0.8% for 4nm diam Au particles suspended in ethanol.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-001994.pdf

Thermal conductivity of nanoparticle suspensions

Stability of nanofluid: A review

Heat transfer improvement of water/single-wall carbon nanotubes (SWCNT) nanofluid in a novel design of a truncated double-layered microchannel heat sink

On the nanofluids applications in microchannels: A comprehensive review

The characteristics of convective heat transfer in microchannel heat sinks using Al2O3 and TiO2 nanofluids

Orientation effects on liquid-vapor phase change heat transfer on nanoporous membranes

A microchannel-nanoporous composite structure evaporator of a substrate level of a GaN HEMT device belongs to the technical field of microelectronic device cooling. Cooling at the substrate level of the GaN HEMT device reduces the use of interface bonding materials, greatly reduces junction temperature and extends the service life of the GaN HEMT device. The evaporator provided by the applicationconsists of an upper substrate (6) and a lower substrate (5), wherein the upper substrate comprises a nanoporous region (2), a fluid inlet (1) and a fluid outlet (8), and the front surface of the lower substrate is engraved with a microchannel region (4), an inlet liquid storage tank (3) and an outlet liquid storage tank (9). The upper substrate (6) and the lower substrate (5) are subjected to a packaging process by a bonding technique to ensure better contact between the nanoporous structure region (2) and the microchannel region (4). The device utilizes phase change evaporation of the liquidin the nanopore for heat dissipation, and has the characteristics such as stable operation, uniform temperature distribution, less required working medium and low operating pressure.

Microchannel-nanoporous composite structure evaporator of substrate level of GaN HEMT device

This study aims to satisfy the thermal management of gallium nitride (GaN) high-electron mobility transistor (HEMT) devices, microchannel-cooling is designed and optimized in this work.,A numerical simulation is performed to analyze the thermal and flow characteristics of microchannels in combination with computational fluid dynamics (CFD) and multi-objective evolutionary algorithm (MOEA) is used to optimize the microchannels parameters. The design variables include width and number of microchannels, and the optimization objectives are to minimize total thermal resistance and pressure drop under constant volumetric flow rate.,In optimization process, a decrease in pressure drop contributes to increase of thermal resistance leading to high junction temperature and vice versa. And the Pareto-optimal front, which is a trade-off curve between optimization objectives, is obtained by MOEA method. Finally, K-means clustering algorithm is carried out on Pareto-optimal front, and three representative points are proposed to verify the accuracy of the model.,Each design variable on the effect of two objectives and distribution of temperature is researched. The relationship between minimum thermal resistance and pressure drop is provided which can give some fundamental direction for microchannels design in GaN HEMT devices cooling.

Numerical simulation and microchannels parameters optimization for thermal management of GaN HEMT devices

As ultrasonic rolling mainly affects the residual stress and surface morphology of Ti6Al4V, it is of great significance to study different process parameters, such as amplitude, static pressure, time, and step length, thereby improving the performance of Ti6Al4V. However, unstable bouncing caused by the elastic relationship between the indenter and the sample is inevitable. Based on finite-element analysis and Hertz contact theory, the concept of critical damping was proposed to quickly stabilize the system. Therefore, a three-dimensional finite-element model was established using the ABAQUS software, after which the ultrasonic-rolling process was simulated by applying static pressure on the sample surface using a spring. Then, the distribution of residual stress under different amplitudes, static pressures, times, and step lengths was analyzed. Based on simulation and experimental results showed that the maximum residual stress of −689.802 MPa was obtained when the amplitude, static pressure, and time were 10 µm, 600 N, and 0.064 s, respectively. When the step length was 1/2 of the indentation width, the surface stress concentration was obvious, convex defects were large, and surface roughness Ra increased to 1.076 µm. Investigations also revealed that when the step length was 1/8, Ra decreased by 39.68% compared with that when the step length was 1/2, the thickness of the plastic-deformation layer reached 56.07 µm. When the step length was 1/10, the maximum compressive residual stress reached −888.007 MPa and Ra increased. On the basis of the abovementioned optimization parameters, the influence of step length on the properties of Ti6Al4V alloy was further explored, and direction for the strengthening process of Ti6Al4V alloy by studying the pattern of surface strengthening was presented.

Wang Jiahao

Papers

The characteristics of convective heat transfer in microchannel heat sinks using Al2O3 and TiO2 nanofluids

Orientation effects on liquid-vapor phase change heat transfer on nanoporous membranes

Microchannel-nanoporous composite structure evaporator of substrate level of GaN HEMT device

Numerical simulation and microchannels parameters optimization for thermal management of GaN HEMT devices

Analysis of the residual stress and surface morphology of TI6AL4V using elastoplastic contact ultrasonic rolling