Arvind Pattamatta

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

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.

Scaling analysis for the investigation of slip mechanisms in nanofluids

The primary objective of this study is to investigate the effect of slip mechanisms in nanofluids through scaling analysis. The role of nanoparticle slip mechanisms in both water- and ethylene glycol-based nanofluids is analyzed by considering shape, size, concentration, and temperature of the nanoparticles. From the scaling analysis, it is found that all of the slip mechanisms are dominant in particles of cylindrical shape as compared to that of spherical and sheet particles. The magnitudes of slip mechanisms are found to be higher for particles of size between 10 and 80 nm. The Brownian force is found to dominate in smaller particles below 10 nm and also at smaller volume fraction. However, the drag force is found to dominate in smaller particles below 10 nm and at higher volume fraction. The effect of thermophoresis and Magnus forces is found to increase with the particle size and concentration. In terms of time scales, the Brownian and gravity forces act considerably over a longer duration than the other forces. For copper-water-based nanofluid, the effective contribution of slip mechanisms leads to a heat transfer augmentation which is approximately 36% over that of the base fluid. The drag and gravity forces tend to reduce the Nusselt number of the nanofluid while the other forces tend to enhance it.

A Comparative Study of Two-Temperature and Boltzmann Transport Models for Electron-Phonon Nonequilibrium

Ultrashort-pulsed laser irradiation on metals creates a thermal nonequilibrium between electrons and the phonons. Previous computational studies used the two-temperature model and its variants to model this nonequilibrium. However, when the laser pulse duration is smaller than the relaxation time of the energy carriers or when the carriers mean free path is larger than the material dimension, these macroscopic models fail to capture the physics accurately. In this article, the nonequilibrium between energy carriers caused by a laser pulse interaction with a metal film is modeled via a numerical solution of the Boltzmann transport model (BTM) for electrons and phonons. A comparative assessment of the two-temperature model and its variants is carried out relative to the BTM. The higher order Runge-Kutta discontinuous Galerkin (RKDG) method is used for numerical discretization of the models. In this study, the gold film thickness is varied between 2–2000 nm, and the laser pulse duration and fluence are varied between 5 fs to 10 ps and 10–2000 J/m2, respectively. It is found that BTM shows the best agreement with the experimental data compared to the two-temperature models for the electron and phonon temperature profiles and the melting threshold fluence.

Particle–fluid interactivity reduces buoyancy-driven thermal transport in nanosuspensions: A multi-component Lattice Boltzmann approach

ABSTRACT Severe contradictions exist between experimental observations and computational predictions regarding natural convective thermal transport in nanosuspensions. The approach treating nanosuspensions as homogeneous fluids in computations has been pinpointed as the major contributor to such contradictions. To fill the void, inter-particle and particle–fluid interactivities (slip mechanisms), in addition to effective thermophysical properties, have been incorporated within the present formulation. Through thorough scaling analysis, the dominant slip mechanisms have been identified. A Multi-Component Lattice Boltzmann Model (MCLBM) approach is proposed, wherein the suspension has been treated as a non-homogeneous twin component mixture with the governing slip mechanisms incorporated. The computations based on the mathematical model can accurately predict and quantify natural convection thermal transport in nanosuspensions. The role of slip mechanisms such as Brownian diffusion, thermophoresis, drag, Saffman lift, Magnus effect, particle rotation, and gravitational effects has been accurately described. A comprehensive study on the effects of Rayleigh number, particle size, and concentration revealed that the drag force experienced by the particles is primarily responsible for the reduction of natural convective thermal transport. In essence, the dominance of Stokesian mechanics in such thermofluidic systems is established in the present study. For the first time, as revealed though a thorough survey of the literature, a numerical formulation explains the contradictions observed, rectifies the approach, predicts accurately, and reveals the crucial mechanisms and physics of buoyancy-driven thermal transport in nanosuspensions.

Investigation on Flow Maldistribution in Parallel Microchannel Systems for Integrated Microelectronic Device Cooling

This paper brings out the phenomenon of flow maldistribution in parallel microchannel systems, which is supposed to have an adverse effect on hot spot formation and temperature distribution in microelectronic devices. An extensive experimental study is carried out where in the parameters affecting the flow maldistribution such as number of channel, area of cross section of the manifold, channel hydraulic diameter, and Reynolds number are varied to study their effect on the pressure drop across the parallel channels designed for liquid cooling of a CPU. It is observed that the flow distribution among the channels improves significantly with a decrease in the channel hydraulic diameter due to higher pressure drop offered by each individual channels simultaneously. This results in a considerable reduction in both the peak temperature and the average temperature of the device with decreasing channel diameters. It is also inferred that the flow maldistribution is relatively invariant with Reynolds number for the microchannel system, which is not the case for the macrochannels. Flow maldistribution is found to increase with increase in number of channels and with a decrease in the manifold area relative to the channel area. The “I” type flow configuration is found to have the least maldistribution while the “U” type shows the maximum and Z type falls in between. A simple force analysis of the governing equation in the manifold of the parallel microchannel system reveals a strong dominance of the frictional force over the inertial force and both the forces contribute to the uniform flow distribution at smaller hydraulic diameters, where the 1-D theoretical models failed to achieve a concurrence with the present experimental results. Also the present 3-D numerical simulations give a satisfactory agreement with the experimental results projecting it as an effective tool in the design and analysis of microchannel cooling system. The potential zones of hot spots are identified as low fluid velocity zones or low pressure drop zones among the channels resulting from flow maldistribution.

Superior dielectric breakdown strength of graphene and carbon nanotube infused nano-oils

Nano-oils comprising stable and dilute dispersions of synthesized Graphene (Gr) nanoflakes and carbon nanotubes (CNT) have been experimentally observed for the first time to exhibit augmented dielectric breakdown strengths compared to the base transformer oils. Variant nano-oils comprising different Gr and CNT samples suspended in two different grades of transformer oils have yielded consistent and high degrees of enhancement in the breakdown strength. The apparent counter-intuitive phenomenon of enhancing insulating caliber of fluids utilizing nanostructures of high electronic conductance has been shown to be physically consistent thorough theoretical analysis. The crux mechanism has been pin pointed as efficient charge scavenging leading to hampered streamer growth and development, thereby delaying probability of complete ionization. The mathematical analysis presented provides a comprehensive picture of the mechanisms and physics of the electrohydrodynamics involved in the phenomena of enhanced breakdown strengths. Furthermore, the analysis is able to physically explain the various breakdown characteristics observed as functions of system parameters, viz. nanostructure type, size distribution, relative permittivity, base fluid dielectric properties, nanomaterial concentration and nano-oil temperature. The mathematical analyses have been extended to propose a physically and dimensionally consistent analytical model to predict the enhanced breakdown strengths of such nano-oils from involved constituent material properties and characteristics. The model has been observed to accurately predict the augmented insulating property, thereby rendering it as an extremely useful tool for efficient design and prediction of breakdown characteristics of nanostructure infused insulating fluids. The present study, involving experimental investigations backed by theoretical analyses and models for an important dielectric phenomenon such as electrical breakdown can find utility in design of safer and more efficient high operating voltage electrical drives, transformers and machines.

Large electrorheological phenomena in graphene nano-gels

Large-scale electrorheology (ER) response has been reported for dilute graphene nanoflake-based ER fluids that have been engineered as novel, readily synthesizable polymeric gels. Polyethylene glycol (PEG 400) based graphene gels have been synthesized and a very high ER response (∼125 000% enhancement in viscosity under influence of an electric field) has been observed for low concentration systems (∼2 wt.%). The gels overcome several drawbacks innate to ER fluids. The gels exhibit long term stability, a high graphene packing ratio which ensures very high ER response, and the microstructure of the gels ensures that fibrillation of the graphene nanoflakes under an electric field is undisturbed by thermal fluctuations, further leading to mega ER. The gels exhibit a large yield stress handling caliber with a yield stress observed as high as ∼13 kPa at 2 wt.% for graphene. Detailed investigations on the effects of graphene concentration, electric field strength, imposed shear resistance, transients of electric field actuation on the ER response and ER hysteresis of the gels have been performed. In-depth analyses with explanations have been provided for the observations and effects, such as inter flake lubrication/slip induced augmented ER response. The present gels show great promise as potential ER gels for various smart applications.

A Comparative Study of Submicron Phonon Transport Using the Boltzmann Transport Equation and the Lattice Boltzmann Method

The subcontinuum energy transport mechanism in solids can be explained by the Lattice Boltzmann Method (LBM), a discrete representation of the Boltzmann Transport Equation (BTE). The present study focuses on a detailed comparison of the LBM and BTE. Results reveal that at continuum scale, the LBM follows the BTE almost precisely. However, as the device dimensions are reduced, approaching the ballistic limit, the LBM deviates from the BTE results in terms of thermal property estimation. The inherent nonisotropic lattice configuration has a dominant contribution to the performance of the LBM. A threshold length scale is also proposed for successful implementation of the LBM solver.

Cascaded collision lattice Boltzmann model (CLBM) for simulating fluid and heat transport in porous media

ABSTRACT The present paper reports a cascaded collision lattice Boltzmann model for the simulation of an incompressible two-dimensional fluid flow in a porous media regime. The cascaded model is first validated for the nonporous regime using limiting conditions against previous finite element model reports. Subsequently, the cascaded collision model is applied to the lid-driven porous-filled cavity to demonstrate the largely augmented numerical stability of the model against the more common Bhatnagar–Gross–Krook and multiple relaxation time collision models. Finally, the cascaded model is applied to an inflow–outflow case of flow and heat transfer over a porous bluff body to showcase its efficiency in capturing the complex fluid and heat transport phenomenon through porous media.

Rayleigh–Benard convection in water-based alumina nanofluid: A numerical study

ABSTRACT Rayleigh–Benard (R-B) convection in water-based alumina (Al2O3) nanofluid is analyzed based on a single-component non-homogeneous volume fraction model (SCNHM) using the lattice Boltzmann method (LBM). The present model accounts for the slip mechanisms such as Brownian and thermophoresis between the nanoparticle and the base fluid. The average Nusselt number at the bottom wall for pure water is compared to the previous numerical data for natural convection in a cavity and a good agreement is obtained. The parameters considered in this study include the Rayleigh number of the nanofluid, the volume fraction of alumina nanoparticle and the aspect ratio of the cavity. For the Al2O3/water nanofluid, it is found that heat transfer rate decreases with an increase of the volume fraction of the nanoparticle. The results are demonstrated and explained with average Nusselt number, isotherms, streamlines, heat lines, and nanoparticle distribution. The effect of nanoparticles on the onset of instability in R-B convection is also analyzed.