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Two-phase numerical model for thermal conductivity and convective heat transfer in nanofluids
Kondaraju, Sasidhar,Lee, Joon Sang Springer 2011 NANOSCALE RESEARCH LETTERS Vol.6 No.1
<P>Due to the numerous applications of nanofluids, investigating and understanding of thermophysical properties of nanofluids has currently become one of the core issues. Although numerous theoretical and numerical models have been developed by previous researchers to understand the mechanism of enhanced heat transfer in nanofluids; to the best of our knowledge these models were limited to the study of either thermal conductivity or convective heat transfer of nanofluids. We have developed a numerical model which can estimate the enhancement in both the thermal conductivity and convective heat transfer in nanofluids. It also aids in understanding the mechanism of heat transfer enhancement. The study reveals that the nanoparticle dispersion in fluid medium and nanoparticle heat transport phenomenon are equally important in enhancement of thermal conductivity. However, the enhancement in convective heat transfer was caused mainly due to the nanoparticle heat transport mechanism. Ability of this model to be able to understand the mechanism of convective heat transfer enhancement distinguishes the model from rest of the available numerical models.</P>
Investigation of heat transfer in turbulent nanofluids using direct numerical simulations.
Kondaraju, Sasidhar,Jin, E K,Lee, J S Published by the American Physical Society through 2010 Physical review. E, Statistical, nonlinear, and so Vol.81 No.1
<P>A numerical study has been performed by using a combined Euler and Lagrangian method on the convective heat transfer of Cu and Al2O3 nanofluids under the turbulent flow conditions. The effects of volume fraction of nanoparticles, nanoparticle sizes, and nanoparticle material are investigated. The mechanism of convective heat transfer enhancement in nanofluids has also been investigated, by studying the influence of particle dispersion and two-way interaction between fluid and particle temperature. The results show significant enhancement of heat transfer of nanofluids. The numerical data are compared with the correlation data of the experiments and reasonably good agreement is achieved.</P>
Kondaraju, Sasidhar,Farhat, Hassan,Lee, Joon Sang The Royal Society of Chemistry 2012 Soft matter Vol.8 No.5
<P>Predicting the rheological properties of emulsions is one of the most challenging and complicated problems in material and fluid sciences. Substantial complications in prediction of rheology arise due to the deformability and aggregation of emulsions. Thus a better understanding of deformation and aggregation of emulsions can lead to a better understanding of the shear thinning region of emulsions. Though numerous experimental and theoretical studies were performed to obtain rheological correlations of emulsions, their inability to visualize and understand the droplet deformation in the presence of large volume fractions has stagnated our understanding of the shear thinning behavior of emulsions. With the aid of a numerical tool, which can help in visualizing the droplet deformation and correlate it to rheological behavior of emulsions, we have made an attempt to understand the physics behind the shear thinning behavior and also predict its rheological characteristics for emulsions at different volume fractions. In this article, we try to obtain a theoretical understanding of the influence of deformation and de-aggregation of droplets on the emulsion rheology. Simulations performed in this article using a multi-component lattice Boltzmann model are used to quantify (a) relative viscosity of emulsions with change in shear rate, (b) relative viscosity of emulsions with change in time, (c) effect of deformation of droplets on the shear thinning region in emulsions, and (d) relative viscosity of emulsions with change in volume fraction.</P> <P>Graphic Abstract</P><P>This work discusses the effects of deformation and aggregational characteristics of droplets on the rheological behavior of emulsions. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1sm06193c'> </P>
Yoon, Hong Min,Kondaraju, Sasidhar,Lee, Jung Shin,Suh, Youngho,Lee, Joonho H.,Lee, Joon Sang Elsevier 2017 APPLIED SURFACE SCIENCE - Vol.409 No.-
<P><B>Abstract</B></P> <P>Recent studies have revealed that contact line forces play an important role in the droplet spreading process. Despite their significance, the physics related to them has been studied only indirectly and the effect of contact line forces is still being disputed. We performed a molecular dynamics simulation and mimicked the droplet spreading process at the nanoscale. Based on the results of the simulation, the contact line forces were directly calculated. We found that the forces acting on the bulk and the contact line region showed different trends. Distinct positive and negative forces, contact line spreading, and friction forces were observed near the contact line. We also observed a strong dependency of the atomic kinetics in the contact line region on the variations in the contact line forces. The atoms of the liquid in the contact line region lost their kinetic energy due to the contact line friction force and became partially immobile on the solid surface. The results of the current study will be useful for understanding the role of the contact line forces on the kinetic energy dissipation in the contact line region.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Contact line forces, including friction and spreading forces are directly calculated. </LI> <LI> Overall trends of variations in contact line forces during droplet spreading process show characteristics of contact line forces. </LI> <LI> Detail relations of contact line forces and atomic kinetics in the contact line provide a clear evidence of the possible energy dissipation mechanism in droplet spreading process. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Yoon, Hong Min,Jung, Youngmo,Jun, Seong Chan,Kondaraju, Sasidhar,Lee, Joon Sang The Royal Society of Chemistry 2015 Nanoscale Vol.7 No.14
<P>A sliding object on a crystal surface with a nanoscale contact will always experience stick-slip movement. However, investigation of the slip motion itself is rarely performed due to the short slip duration. In this study, we performed molecular dynamics simulation and frictional force microscopy experiments for the precise observation of slip motion between a graphene layer and a crystalline silicon tip. The simulation results revealed a hierarchical structure of stick and slip motion. Nanoscale stick and slip motion is composed of sub-nanoscale stick and slip motion. Sub-nanoscale stick and slip motion occurred on a timescale of a few ps and a force scale of 10<SUP>−1</SUP>nN. The relationship between the trajectories of the silicon tip and stick-slip peak revealed that in-plane and vertical motions of the tip provide information about stick and slip motion in the sub-nanoscale and nanoscale ranges, respectively. Parametric studies including tip size, scan angle, layer thickness, and flexibility of the substrate were also carried out to compare the simulation results with findings on lateral force microscopy.</P>