A thermal transport mechanism leading to the enhanced thermal conductivity of graphene nanofluids has been proposed. The graphene sheet size is postulated to be the key to the underlying mechanism. Based on a critical sheet size derived from Stokes-Einstein equation for the poly-dispersed nanofluid, sheet percolation and Brownian motion assisted sheet collisions are used to explain the heat conduction. A collision dependant dynamic conductivity considering Debye approximated volumetric specific heat due to phonon transport in graphene has been incorporated. The model has been found to be in good agreement with experimental data. © 2013 AIP Publishing LLC.

Arvind Pattamatta

Department of Mechanical Engineering

Sarit Kumar Das

Soujit Sen Gupta

Saikat Chakraborty

DYNAMIC CONDUCTIVITY

Enhanced thermal conductivity

Experimental datum

Graphene sheets

PHONON TRANSPORT

SHEET SIZE

STOKES EINSTEIN EQUATIONS

THERMAL TRANSPORT

Brownian movement

Graphene

Heat conduction

Nanofluidics

Solvents

Thermal conductivity

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

Applied Physics Letters

The present paper explores the concept of improving the AC dielectric breakdown strength of insulating mineral oils by the addition of graphene or carbon nanotubes (CNTs) to form stable dispersions. Experimental observations of graphene and CNT nano-oils show that not only improved average breakdown voltage, but also significantly improved reliability and survival probabilities of the oils under AC high voltage stressing is achieved. Improvement of the tune of ∼70-80% in the AC breakdown voltage of the oils has been obtained. The study examines the reliability of such nano-colloids using a two-parameter Weibull distribution and the oils show greatly augmented electric field bearing capacity. The fundamental mechanism responsible for such observed outcomes is reasoned to be delayed streamer development and reduced streamer growth rates due to effective electron scavenging. A mathematical model based on the principles of electron scavenging is proposed to quantify the amount of electrons scavenged by the nanostructures. © EDP Sciences, 2019.

EPJ Applied Physics

Streamer evolution arrest governed amplified AC breakdown strength of graphene and CNT colloids

Electrophoresis has been shown as a novel methodology to enhance heat conduction capabilities of nanocolloidal dispersions. A thoroughly designed experimental system has been envisaged to solely probe heat conduction across nanofluids by specifically eliminating the buoyancy driven convective component. Electric field is applied across the test specimen in order to induce electrophoresis in conjunction with the existing thermal gradient. It is observed that the electrophoretic drift of the nanoparticles acts as an additional thermal transport drift mechanism over and above the already existent Brownian diffusion and thermophoresis dominated thermal conduction. A scaling analysis based on the thermophoretic and electrophoretic velocities from classical Huckel-Smoluchowski formalism is able to mathematically predict the thermal performance enhancement due to electrophoresis. It is also inferred that the dielectric characteristics of the particle material is the major determining component of the electrophoretic amplification of heat transfer. Influence of surfactants has also been probed into and it is observed that enhancing the stability via interfacial charge modulation can in fact enhance the electrophoretic drift, thereby enhancing heat transfer calibre. Also, surfactants ensure colloidal stability as well as chemical gradient induced recirculation, thus ensuring colloidal phase equilibrium and low hysteresis in spite of the directional drift in presence of electric field forcing. The findings may have potential implications in enhanced and tunable thermal management of micro-nanoscale devices and in thermo-bioanalysis within lab-on-a-chip devices. © 2019 Elsevier Inc.

Experimental Thermal and Fluid Science

Amplifying thermal conduction calibre of dielectric nanocolloids employing induced electrophoresis

The present paper illustrates experimentally the impact of non-uniform heat generation in a microprocessor on the hot spot distribution and the method to cool the same, efficiently employing parallel microchannel cooling configurations. It is often assumed during design of microprocessor cooling systems that the heat load emitted by the device is uniform, however real time tracking of the device shows that such assumptions are far from the reality. An Intel® Core™ i7–4770 3.40 GHz quad core processor has been experimentally mimicked using heat load data retrieved from a real microprocessor with non-uniform core activity. Parallel microchannel based heat spreader configurations using U, I and Z type flow configurations have been employed to mitigate the thermal load from the mimicked device. The observations clearly show that the microchannel cooling system experiences two forms of hot spots, one due to the flow maldistribution within the system and the other due to the additional non-uniform heat generation by the device. The hot spots have been shown to exhibit drastically different shapes and core temperatures and this has been verified through simulations and infrared thermography. To efficiently cool hot spot core temperatures, nanofluids have been employed and ‘smart cooling’ has been observed and the same has been explained based on nanoparticle slip mechanisms. The present work shows that the notion that high flow maldistribution leads to high thermal maldistribution, is not always true and existing maldistribution can be effectively utilized to tackle specific hot spot location. The present work can be important to design cooling mechanisms for real microprocessors with high core activity leading to non-uniform hot spot formation probabilities. © 2017 Elsevier Masson SAS

International Journal of Thermal Sciences

Mitigating non‐uniform heat generation induced hot spot(s) in multicore processors using nanofluids in parallel microchannels

In recent times, convective heat transfer using nanofluids has been an active field of research. However experimental studies pertaining to buoyancy induced convective heat transfer using various nanofluid is relatively scarce. In the present study, a square enclosure of dimensions (40 × 40 × 200) mm is used as test section. Initially, Al2O3/Water nanofluid with volume percentage of 0.1%, 0.3%, 1% and 2% and Rayleigh numbers ranging from 7 × 105 to 1 × 107 are studied. These results are then compared with Ho et al. (Int J Therm Sci 49(8):1345–1353, 2010) experimental data. Nusselt number (Nu) is calculated based on the thermophysical properties that are measured in-house for the given conditions. Further, MWCNT/Water nanofluid with volume percentage 0.1%, 0.3% and 0.5% is formulated and are studied for various Rayleigh numbers. Comparison of Al2O3/Water and MWCNT/Water nanofluid have been made for different volume fractions and for various range of Rayleigh numbers. It is observed that MWCNT/Water nanofluid when compared with Al2O3/Water nanofluid yields higher values of the Nusselt number for a given volume fractions. All the existing experimental studies using particle based nanofluid concluded a deterioration in natural convective heat transfer. This study for the first time demonstrates an enhancement in natural convection using MWCNT/Water nanofluid. Such enhancement cannot be simply explained based only on the relative changes in the thermophysical properties. Factors such as percolation network in MWCNT/Water nanofluid which increases the heat transfer pathway between two walls and the role of slip mechanisms might be the possible reasons for the enhancement. © 2017, Springer-Verlag GmbH Germany.

Heat and Mass Transfer/Waerme- und Stoffuebertragung

Enhancement of natural convection heat transfer in a square cavity using MWCNT/Water nanofluid: an experimental study

While parallel microchannel based cooling systems (PMCS) have been around for quite a period of time, employing the same and incorporating them for near–active cooling of microelectronic devices is yet to be implemented and the implications of the same on thermal mitigation to be understood. The present article focusses on a specific design of the PMCS such that it can be implemented at ease on the heat spreader of a modern microprocessor to obtain near-active cooling. Extensive experimental and numerical studies have been carried out to comprehend the same and three different flow configurations (U, I and Z) of PMCS have been adopted for the present investigations. Additional to focussing on the thermofluidics due to flow configuration, nanofluids (as superior heat transfer fluids) have also been employed to achieve the desired essentials of mitigation of overshoot temperatures and improving uniformity of cooling. Two modelling methods, Discrete Phase Modelling (DPM) and Effective Property Modelling (EPM) have been employed for numerical study to model nanofluids as working fluid in micro flow paths and the DPM predictions have been observed to match accurately with experiments. To quantify the thermal performance of PMCS, an appropriate Figure of Merit (FoM) has been proposed. From the FoM It has been perceived that the Z configuration employing nanofluid is the best suitable solutions for uniform thermal loads to achieve uniform cooling as well as reducing maximum temperature produced with in the device. The present results are very promising and viable approach for futuristic thermal mitigation of microprocessor systems. © 2017 Elsevier Ltd

International Journal of Heat and Mass Transfer

Heat spreader with parallel microchannel configurations employing nanofluids for near–active cooling of MEMS

The role of percolation and sheet dynamics during heat conduction in poly-dispersed graphene nanofluids

Journal	Applied Physics Letters
ISSN	00036951
Open Access	Yes