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

Recent developments in phase change materials for energy storage applications: A review

A concentrating solar power (CSP) system converts sunlight into a heat source which can be used to drive a conventional power plant. Thermal energy storage (TES) improves the dispatchability of a CSP plant. Heat can be stored in either sensible, latent or thermochemical storage. Commercial deployment of CSP systems have been achieved in recent years with the two-tank sensible storage system using molten salt as the storage medium. Considerable research effort has been conducted to improve the efficiency of the CSP system and make the cost of electricity comparable to that of the conventional fossil-fuel power plant. This paper provides a comprehensive summary of CSP plants both in operation and under construction. It covers the available technologies for the receiver, thermal storage, power block and heat transfer fluid. This paper also reviews developments in high temperature TES over the past decade with a focus on sensible and latent heat storage. High temperature corrosion and economic aspects of these systems are also discussed.

Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies

http://web.mit.edu/nnf/publications/GHM137.pdf

A modified Cassie-Baxter relationship to explain contact angle hysteresis and anisotropy on non-wetting textured surfaces.

Evaporation of pure liquid sessile and spherical suspended drops: a review.

Evaporation of sessile drops on micro-patterned surfaces is investigated over a range of heterogeneity length scales and solid area fractions. The surface topology is generated by a uniform arrangement of square pillars or square holes. The evaporation process is captured using high resolution imaging techniques and later post-processed for such information as contact angle, contact circle diameter and drop volume. It is observed that two distinct phases of evaporation existed for all substrate characteristics: pinned triple line (TL) phase and moving TL phase. In both phases, the process follows a linear decrease of surface area. The dimensionless evaporation rate constant is found to be higher during the moving TL phase in comparison with the pinned TL phase. In addition, it is found that the triple line topology has no effect on the evaporation rate constant.

/pdf/evaporating-drops-on-patterned-surfaces-transition-from-2jbn8w0ka6.pdf

Evaporating drops on patterned surfaces: Transition from pinned to moving triple line

Encapsulated phase change material for high temperature thermal energy storage – Heat transfer analysis

Encapsulated phase change materials for energy storage – Characterization by calorimetry

Cassie-Baxter theory has traditionally been used to study liquid drops in contact with microstructured surfaces. The Cassie-Baxter theory arises from a minimization of the global Gibbs free energy of the system but does not account for the topology of the three-phase contact line. We experimentally compare two situations differing only in the microstructure of the roughness, which causes differences in contact line topology. We report that the contact angle is independent of area void fraction for surfaces with microcavities, which correspond to situations when the advancing contact line is continuous. This result is in contrast with Cassie-Baxter theory, which uses area void fraction as the determining parameter, regardless of the type of roughness.

/pdf/effect-of-three-phase-contact-line-topology-on-dynamic-58v2v9l8ac.pdf

Effect of three-phase contact line topology on dynamic contact angles on heterogeneous surfaces.

The present study demonstrates the importance of actual agglomerated particle size in the nanofluid and its effect on the fluid properties. The current work deals with 5 to 100 nm nanoparticles dispersed in fluids that resulted in 200 to 800 nm agglomerates. Particle size distributions for a range of nanofluids are measured by dynamic light scattering (DLS). Wet scanning electron microscopy method is used to visualize agglomerated particles in the dispersed state and to confirm particle size measurements by DLS. Our results show that a combination of base fluid chemistry and nanoparticle type is very important to create stable nanofluids. Several nanofluids resulted in stable state without any stabilizers, but in the long term had agglomerations of 250 % over a 2 month period. The effects of agglomeration on the thermal and rheological properties are presented for several types of nanoparticle and base fluid chemistries. Despite using nanodiamond particles with high thermal conductivity and a very sensitive laser flash thermal conductivity measurement technique, no anomalous increases of thermal conductivity was measured. The thermal conductivity increases of nanofluid with the particle concentration are as those predicted by Maxwell and Bruggeman models. The level of agglomeration of nanoparticles hardly influenced the thermal conductivity of the nanofluid. The viscosity of nanofluids increased strongly as the concentration of particle is increased; it displays shear thinning and is a strong function of the level of agglomeration. The viscosity increase is significantly above of that predicted by the Einstein model even for very small concentration of nanoparticles.

Sudhakar Neti

Papers

Evaporating drops on patterned surfaces: Transition from pinned to moving triple line

Encapsulated phase change material for high temperature thermal energy storage – Heat transfer analysis

Encapsulated phase change materials for energy storage – Characterization by calorimetry

Effect of three-phase contact line topology on dynamic contact angles on heterogeneous surfaces.

Particle agglomeration and properties of nanofluids