Pawan K. Singh

A benchmark study on the thermal conductivity of nanofluids

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.

Thermal conductivity enhancement of nanofluids containing graphene nanosheets

This paper envisages a mechanism of heat conduction behind the thermal conductivity enhancement observed in graphene nanofluids. Graphene nanofluids have been prepared, characterized, and their thermal conductivity was measured using the transient hot wire method. The enhancements in thermal conductivity are substantial even at lower concentrations and are not predicted by the classical Maxwell model. The enhancement also shows strong temperature dependence which is unlike its carbon predecessors, carbon nanotube (CNT) and graphene oxide nanofluids. It is also seen that the magnitude of enhancement is in-between CNT and metallic/metal oxide nanofluids. This could be an indication that the mechanism of heat conduction is a combination of percolation in CNT and Brownian motion and micro convection effects in metallic/metal oxide nanofluids, leading to a strong proposition of a hybrid model.

Effect of temperature on turbulent and laminar flow efficacy analysis of nanofluids

The present study investigates the temperature-dependent efficacy analysis of nanofluids for both laminar and turbulent flow applications. Characterizations for the thermo-physical properties of nanofluids are carried out at different temperatures for the viability analysis. Thermal conductivity of yttria nanofluids shows strong temperature dependence compared to copper nanofluids with an increase of 7.5% to 21% for a temperature range of 25u2009°C to 85u2009°C. Moreover, the effective viscosity of yttria nanofluids is found to decrease gradually with temperatures in spite of copper nanofluid insensitivity. A theoretical study is carried out to compute the efficacy of yttria and copper nanofluids for laminar and turbulent flow regimes over a range of temperatures with the help of their thermo-physical properties. Yttria nanofluids are found viable for both laminar and turbulent flow for temperatures above 37u2009°C and 45u2009°C, respectively. On the other hand, copper nanofluids are found applicable only in laminar flow condition for temperatures above 75u2009°C. The stability and superior figure of merit for yttria nanofluids with temperature make it a potential coolant for higher-temperature convective cooling systems working on both laminar and turbulent flows.

Anomalous Size Dependent Rheological Behavior of Alumina Based Nanofluids

Rheological characteristics of alumina (Al 2 O 3 ) nanofluids (NFs) were found to exhibit an unexpected behavior. Two base-fluids viz, water and ethylene glycols (EG) with particles of average diameter of 11, 45 and 150 nm were examined. An anomalous reduction in viscosity compared to that of the base fluid was seen for EG based NFs. However, viscosity reduction was absent in water based NFs. The inter-related effects of particle size, concentration and mode of dispersion (mono or poly-dispersed) were investigated. Particle migration under shear is attributed to the reduction of viscosity. The increase in bulk viscosity with particle size reduction is attributed to the surface forces acting between the particles and the medium in a suspension and the increase of effective volume with size.

Hydrodynamic Study of Nanofluids in Microchannel

Present study tries to put light on hydrodynamics of nanofluids in microchannels. For the present hydrodynamic study, the microchannels of hydraulic diameters of 212 and 301 μm are used. Present study also uses nanofluids in microchannel. To observe the hydrodynamic effect of nanofluids in microchannel, the alumina nanoparticles with sizes 45 nm are chosen with the water as base fluid. The nanofluids with the dilute concentrations 0.25 vol% are used to observe the effect of volume fraction. From the study of base fluid flow in microchannel, it is found that the axial pressure drop is linear thus showing the incompressible behaviour of fluid. For all microchannels, early transition to turbulence was observed. Also for the same Re the pressure drop was higher for smaller channel. However, the usage of nanofluids in these microchannels shows different behavior from normal fluids. The axial pressure drop was again linear thus proving that even though these fluids are different from normal fluids; they follow the behaviour of incompressible Newtonian fluids. Surprisingly, the friction factor was similar for these fluids as compared to base fluids. This can be attributed to dilute concentration of nanofluids, which make them a homogeneous fluid. It suggests that the use of dilute nanofluids in microchannel results in no or little penalty in pressure drop. It also suggests that if nanofluids have to be used as a better coolant, the hydrodynamics and heat transfer characteristics has to be studied as higher concentrations.Copyright