Sasidhar Kondaraju

Study of aggregational characteristics of emulsions on their rheological properties using the lattice Boltzmann approach

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.

Two-phase numerical model for thermal conductivity and convective heat transfer in nanofluids

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.

A lattice Boltzmann model for substrates with regularly structured surface roughness

Superhydrophobic surface characteristics are important in many industrial applications, ranging from the textile to the military. It was observed that surfaces fabricated with nano/micro roughness can manipulate the droplet contact angle, thus providing an opportunity to control the droplet wetting characteristics. The Shan and Chen (SC) lattice Boltzmann model (LBM) is a good numerical tool, which holds strong potentials to qualify for simulating droplets wettability. This is due to its realistic nature of droplet contact angle (CA) prediction on flat smooth surfaces. But SC-LBM was not able to replicate the CA on rough surfaces because it lacks a real representation of the physics at work under these conditions. By using a correction factor to influence the interfacial tension within the asperities, the physical forces acting on the droplet at its contact lines were mimicked. This approach allowed the model to replicate some experimentally confirmed Wenzel and Cassie wetting cases. Regular roughness structures with different spacing were used to validate the study using the classical Wenzel and Cassie equations. The present work highlights the strength and weakness of the SC model and attempts to qualitatively conform it to the fundamental physics, which causes a change in the droplet apparent contact angle, when placed on nano/micro structured surfaces.

Accelerated Lattice Boltzmann Model for Colloidal Suspensions: Rheology and Interface Morphology

Colloids are ubiquitous in the food, medical, cosmetics, polymers, water purification, and pharmaceutical industries. The thermal, mechanical, and storage properties of colloids are highly dependent on their interface morphology and their rheological behavior. Numerical methods provide a convenient and reliable tool for the study of colloids. Accelerated Lattice Boltzmann Model for Colloidal Suspensions introduce the main building-blocks for an improved lattice Boltzmannbased numerical tool designed for the study of colloidal rheology and interface morphology. This book also covers the migrating multi-block used to simulate single component, multi-component, multiphase, and single component multiphase flows and their validation by experimental, numerical, and analytical solutions. Among other topics discussed are the hybrid lattice Boltzmann method (LBM) for surfactant-covered droplets; biological suspensions such as blood; used in conjunction with the suppression of coalescence for investigating the rheology of colloids and microvasculature blood flow. The presented LBM model provides a flexible numerical platform consisting of various modules that could be used separately or in combination for the study of a variety of colloids and biological flow deformation problems.

Hybrid Turbulence Modeling of Liquid Spray Impingement on a Heated Wall with Arbitrary Lagrangian Eulerian Method

Numerical simulation of two phase spray impingement on a heated wall was carried out. Hybrid turbulence modeling was used for analysis where large eddy simulation was employed away from the wall, and a k-epsilon model was employed near the wall. The effect of vortex motion on turbulent heat flux values was analyzed using different Reynolds numbers of impingement and at different angles. It was observed that the turbulent heat flux attained maximum values with high vortex formation. The ejection of hot fluid from the surface was predominant when compared to the down sweep motion of the cold fluid. The Nusselt number plot indicated high heat transfer rates for higher Reynolds number.

Capillary Displacement of Viscous Liquids in Geometries with Axial Variations

Axial variations in geometry and presence of viscous displaced fluid are known to alter the diffusive-dynamics of capillary imbibition of a wetting liquid. We here show that the coupled effect of axially varying capillary geometry and finite viscosity of the displaced fluid can lead to significant variations in both short and long time dynamics of imbibition. Based on a theoretical model and lattice Boltzmann simulations, we analyze capillary displacement of a viscous liquid in straight and diverging capillaries. At short times, the imbibition length scales proportionally with time as opposed to the diffusive-dynamics of imbibition of a single wetting liquid. Whereas, at long times, geometry-dependent power-law behavior occurs which qualitatively resembles single liquid imbibition. The distance at which the crossover between these two regimes occurs depends strongly on the viscosities of the imbibing and the displaced liquid. Additionally, our simulations show that the early time imbibition dynamics are also affected by the dynamic contact angle of the meniscus.

Mathematical model for dropwise condensation on a surface with wettability gradient

We present a mathematical model for dropwise condensation (DWC) heat transfer on a surface with wettability gradient. We adapt well-established population balance model for DWC on inclined surfaces to model DWC on a surface with wettability gradient. In particular, our model takes into account the effect of wettability gradient and energy released during drop coalescence to determine the drop departure size. We validate our model with published experimental data of DWC heat flux and drop size distribution. Based on various experimental studies on drop motion, we also propose a mechanism that explains how the energy released during drop coalescence on a surface with wettability gradient and in a condensation environment aids drop motion. The mechanism correctly explains the shift of center of mass of two coalescing drops on a surface with wettability gradient toward the drop on high wetting region. Using the model, we analyze the effect of wettability gradient on the DWC heat flux. Our model predictions show that the optimal choice of wettability gradient is governed by differential variations in population density and heat transfer through a drop with change in wettability of the surface. We also demonstrate that contact angle at which there is maximum heat transfer through a drop varies with thickness of coating layer leading to change in optimal wettability gradient. [DOI: 10.1115/1.4039014]

Effect of Surface Wettability on Dropwise Condensation Using Lattice Boltzmann Method

Understanding the condensation mechanism is crucial to enhance the heat transfer performance of numerous industrial applications such as power generations, fog harvesting, water desalination, cooling of nuclear reactor, and thermal management of electronic device. In the present study, simulations are performed to investigate the effect of surface wettability on droplet growth dynamics during dropwise condensation. To simulate droplet growth dynamics involving phase change heat transfer, thermal lattice Boltzmann method has been employed with two distribution function for fluid and temperature field. Simulations performed in this work are used to analyze the effect of surface wettability on nucleation time and the evolution of average droplet radius, height, base diameter, and contact angle of the droplet. It is observed that nucleation time increases exponentially with the contact angle. The growth rate of droplet is higher for smaller droplets compared to larger droplets.Copyright