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. © 2011 American Institute of Physics.

Pradeep Thalappil

Department Of Chemistry

Sarit Kumar Das

Department of Mechanical Engineering

Soujit Sen Gupta

V. Manoj Siva

CONVECTION EFFECTS

Hybrid model

MAXWELL MODELS

Nanofluids

temperature dependence

Thermal conductivity enhancement

TRANSIENT HOT WIRE METHOD

Brownian movement

Carbon nanotubes

Graphene

Heat conduction

Lead oxide

Nanofluidics

Solvents

Thermal conductivity of liquids

Fulltext

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Thermal conductivity enhancement of nanofluids containing graphene nanosheets

Journal of Applied Physics

The effects of an operating water head (OWH) and reduced graphene oxide (RGO) addition on the pervious concrete filter (PCF) and heavy metals interaction were investigated in the present study. Five simulated wastewaters containing Cd, Zn, Cu, Pb, and these four ions mixed together were filtered by PCF monoliths, considering the influence of three OWHs, viz. 30 cm, 7.5 cm, and trickling water head. The metal ions fixation degree increased with decreasing levels of OWH, which indicates that empty bed contact time influences PCF performance. Additionally, heavy metals removal and leaching from 0.06 wt% RGO modified PCF (G-PCF) and plain PCF were examined by first passing the five wastewater samples, immediately followed with strong acidic water, while maintaining the same flow and time. As a consequence of acid water passage, the fixated ions were seen to leach out from PCFs, but the RGO's strong reinforcement substantially reduced such leaching degree and additionally improved the simultaneous removal of four heavy metals. Although the estimated cost of G-PCF was relatively higher than plain PCF, both these filters appear highly efficient and inexpensive, unlike treatment units that are currently used by the electroplating industry for heavy metals removal. © 2019 American Society of Civil Engineers.

Journal of Environmental Engineering (United States)

Heavy Metal Removal and Leaching from Pervious Concrete Filter: Influence of Operating Water Head and Reduced Graphene Oxide Addition

Acid attack on cement concrete results in the development of a degraded layer surrounding the unaffected material, which causes a deterioration of mechanical properties. The effect of reduced graphene oxide, alumina and silica nanoparticles on the deterioration characteristics of 28-day cured cementitious pastes after storage in 0.5 moL/L HNO3 solution for a period of 56 days is reported in this paper. Samples were collected from the degraded pastes at different time periods and then characterised using various techniques like scanning electron microscopy with energy dispersive spectroscopy, optical microscopy, thermogravimetric analysis, mercury intrusion porosimetry, X-ray computed tomography and nanoindentation. While the porosimetry results showed that the presence of reduced graphene oxide and nano alumina decreased the amount of capillary pores (10 nm–10 μm) by up to 46% and 51% than the control paste after storage in acidic solution for 28 days, the details of the relative zones formed in the paste along with their characteristics were revealed by the microscopy and nanoindentation techniques. Overall, the results suggest that the presence of these nanomaterials refined the pore structure of the cementitious matrix and thereby increased the resistance to leaching of calcium ions from the binder phases exposed to aggressive aqueous media. © 2018 Elsevier Ltd

Cement and Concrete Composites

Effect of reduced graphene oxide, alumina and silica nanoparticles on the deterioration characteristics of Portland cement paste exposed to acidic environment

The present study describes a facile synthesis procedure for nanocomposites consisting of 1D carbon nanotubes, 2D graphene and metal-oxide nanoparticles (CuO, ZnO) referred as metal-oxide dispersed on graphene-wrapped carbon nanotubes (CuO/GC and ZnO/GC) nanocomposites. Moreover, nanofluids were prepared by dispersing the above composites and the thermal conductivity of these nanofluids has been measured. Initially, GC was synthesized by a chemical vapor deposition technique using the mixture of graphene oxide (GO) and MmNi3 as catalyst and further purified by removing catalyst impurities and amorphous carbon. Afterwards, synthesis of metal-oxide dispersed on GC nanocomposites has been done. The formation of nanocomposites has been confirmed by X-ray diffraction, scanning electron microscope, and transmission electron microscope. Further, nanofluids were prepared by dispersing nanocomposites (GC, CuO/GC, and ZnO/GC) in de-ionized (DI) water and ethylene glycol (EG) without any functionalization and surfactant. After confirming the stability of nanofluids by zeta potential analysis, thermal conductivity of these nanofluids was measured at different temperatures. GC, ZnO/GC and CuO/GC dispersed DI water based nanofluids show an enhancement of 22.6%, 24.1% and 26.0% for 0.015 vol. % at 50 °C, respectively. Further, an enhancement of 15.5%, 17.7% and 21.2% was observed for GC, ZnO/GC and CuO/GC dispersed EG based nanofluids for 0.015 vol. % at 50 °C, respectively. © 2018 Elsevier B.V.

Thermochimica Acta

An experimental study on thermal conductivity enhancement of DI water-EG based ZnO(CuO)/graphene wrapped carbon nanotubes nanofluids

This paper reports the experimental study to investigate the buoyancy-induced convective heat transfer in a square cavity using different types of nanofluids. Three types of nanofluids namely, particle based (Al2O3/Water), tube based (MWCNT/Water) and flake based (Graphene/Water) are used. A square cavity with the dimensions (40×40×200) mm3 is used as the test section. Experiments are performed for 0.1%, 0.3%, and 0.5% volume fractions with varying Rayleigh number over a range of 7 ×105 to 1 ×107. For the comparison of heat transfer characteristics of different nanofluids normalized Nusselt number is estimated. It has been observed that an enhancement in natural convective heat transfer is obtained using MWCNT/Water and Graphene/Water nanofluids at 0.1% volume fraction. However for higher concentration, a deterioration in heat transfer is observed. The relative changes in thermophysical properties of nanofluids are insufficient to explain the reported results. It is proposed that the contribution of various slip mechanisms between the nanoparticle suspension and basefluid are responsible for the enhancement. © 2017 Elsevier Masson SAS

International Journal of Thermal Sciences

Buoyancy induced convective heat transfer in particle, tubular and flake type of nanoparticle suspensions

Experiments and numerical simulation of natural convection heat transfer with nanosuspensions are presented in this work. The investigations are carried out for three different types of nanosuspensions: namely, spherical-based (alumina/water), tubular-based (multi-walled carbon nanotube/water), and flake-based (graphene/water). A comparison with in-house experiments is made for all the three nanosuspensions at different volume fractions and for the Rayleigh numbers in the range of 7 × 105-1 × 107. Different models such as single component homogeneous, single component non-homogeneous, and multicomponent non-homogeneous are used in the present study. From the present numerical investigation, it is observed that for lower volume fractions (∼0.1%) of nanosuspensions considered, single component models are in close agreement with the experimental results. Single component models which are based on the effective properties of the nanosuspensions alone can predict heat transfer characteristics very well within the experimental uncertainty. Whereas for higher volume fractions (∼0.5%), the multi-component model predicts closer results to the experimental observation as it incorporates drag-based slip force which becomes prominent. The enhancement observed at lower volume fractions for non-spherical particles is attributed to the percolation chain formation, which perturbs the boundary layer and thereby increases the local Nusselt number values. © 2017 Author(s).

Physics of Fluids

Effect of particle shape and slip mechanism on buoyancy induced convective heat transport with nanofluids

One of the reasons for the controversy on the thermal conductivity enhancement of nanofluids is the lack of extensive data over a wide range of parameters. In the present study, a comprehensive experimental dataset is obtained for thermal conductivity of nanofluids with variation in nanoparticle material, base liquid, particle size, particle volume fraction and suspension temperature. Transient hot wire (THW) equipment as well as Temperature Oscillation equipment are developed for the measurement of thermal conductivity of liquids. The measurements show that, in general, thermal conductivity values of all the nanofluids are higher than that of the equivalent macro-particle suspensions. Metallic nanofluids are found to give higher enhancements than that of oxide nanofluids. Particle size is found to have a tremendous impact on the thermal conductivity of nanofluids with enhancement in the thermal conductivity increasing almost inversely with reduction in the particle size. Increase in temperature significantly increases the thermal conductivity of a nanofluid. It is also observed that the thermal conductivity of nanoparticle suspensions is relatively higher at lower volume fractions, thereby giving a non-linear dependence on the particle volume fraction.

Journal of Nanoparticle Research

An experimental investigation into the thermal conductivity enhancement in oxide and metallic nanofluids

We report for the first time, the synthesis of highly stable exfoliated graphene based nanofluids with water and ethylene glycol as base fluids with out any surfactant and the subsequent studies on their thermal and electrical conductivities. Graphene was synthesized by thermal exfoliation of graphene oxide at 1050 °C in Ar atmosphere. The as-synthesized graphene has been suitably functionalized and further dispersed it in the base fluids without any surfactant. Thermal and electrical conductivities of these nanofluids were measured for varying volume fractions and at different temperatures. An enhancement in thermal conductivity by about 14% has been achieved at 25 °C with deionized water (DI) as base fluid at a very low volume fraction of 0.056% which increases to about 64% at 50 °C. Electrical conductivity measurements for these nanofluids indicate an enormous enhancement at 25 °C for a volume fraction of 0.03%in DI water. © 2010 American Institute of Physics.

Investigation of thermal and electrical conductivity of graphene based 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 [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. © 2009 American Institute of Physics.

A benchmark study on the thermal conductivity of nanofluids

Adding a small volume fraction of carbon nanotubes (CNTs) to a liquid enhances the thermal conductivity significantly. Recent experimental findings report an anomalously wide range of enhancement values that continue to perplex the research community and remain unexplained. In this paper we present a theoretical model based on three-dimensional CNT chain formation (percolation) in the base liquid and the corresponding thermal resistance network. The model considers random CNT orientation and CNT-CNT interaction forming the percolating chain. Predictions are in good agreement with almost all available experimental data. Results show that the enhancement critically depends on the CNT geometry (length), volume fraction, thermal conductivity of the base liquid and the nanofluid (CNT-liquid suspension) preparation technique. Based on the physical mechanism of heat conduction in the nanofluid, we introduce a new dimensionless parameter that alone characterizes the nanofluid thermal conductivity with reasonable accuracy (∼±5%). © IOP Publishing Ltd.

Nanotechnology

Predicting the effective thermal conductivity of carbon nanotube based nanofluids

Increase in the specific surface area as well as Brownian motion are supposed to be the most significant reasons for the anomalous enhancement in thermal conductivity of nanofluids. This work presents a semi-empirical approach for the same by emphasizing the above two effects through micro-convection. A new way of modeling thermal conductivity of nanofluids has been explored which is found to agree excellently with a wide range of experimental data obtained by the present authors as well as the data published in literature. © Indian Academy of Sciences.

Journal	Journal of Applied Physics
ISSN	00218979
Open Access	No