BITS Faculty Publications
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Item Interplay of high zeta potential and steric factor on the slip thermofluidics of power law fluids through narrow confinements(Elsevier, 2022-04) Harikrishnan, A.R.Thermal transport in a non-Newtonian fluid flow through a parallel plate microchannel is investigated with an emphasis on the effects of high zeta potential and steric factor. The fluid viscosity is modelled using the power law and the flow is driven by a combination of electric field and pressure gradient in the form of a dimensionless flow actuation coefficient. The effects of joule heating, viscous dissipation and slip length at the wall are considered. A semi-analytical formulation for the temperature profile is developed which is solved numerically using the Galerkin Finite Element method. We find that the Nusselt number increases as the flow behaviour index is increased. Velocity slip is also found to have a positive impact on convective heat transfer. However, as the zeta potential and steric factor increase, the Nusselt number begins to decline as there is an increase in viscous dissipation. We highlight the importance of considering variable electrical conductivity in evaluating the effect of joule heating at high zeta potential and moderate steric factor. In this regime, joule heating dominates advective transport and heat flow is reversed which may be detrimental or even damaging to microfluidic devices. A comprehensive thermal map is developed for a wide range of zeta potentials and steric factors.Item Electrohydrodynamic fibrillation governed enhanced thermal transport in dielectric colloids under a field stimulus(RSC, 2018-05) Harikrishnan, A.R.Electrorheological (ER) fluids are known to exhibit enhanced viscous effects under an electric field stimulus. The present article reports the hitherto unreported phenomenon of greatly enhanced thermal conductivity in such electro-active colloidal dispersions in the presence of an externally applied electric field. Typical ER fluids are synthesized employing dielectric fluids and nanoparticles and experiments are performed employing an in-house designed setup. Greatly augmented thermal conductivity under a field's influence was observed. Enhanced thermal conduction along the fibril structures under the field effect is theorized as the crux of the mechanism. The formation of fibril structures has also been experimentally verified employing microscopy. Based on classical models for ER fluids, a mathematical formalism has been developed to predict the propensity of chain formation and statistically feasible chain dynamics at given Mason numbers. Further, a thermal resistance network model is employed to computationally predict the enhanced thermal conduction across the fibrillary colloid microstructure. Good agreement between the mathematical model and the experimental observations is achieved. The domineering role of thermal conductivity over relative permittivity has been shown by proposing a modified Hashin–Shtrikman (HS) formalism. The findings have implications towards better physical understanding and design of ER fluids from both ‘smart’ viscoelastic as well as thermally active materials points of view.Item Slip hydrodynamics of combined electroosmotic and pressure driven flows of power law fluids through narrow confinements(Elsevier, 2021-10) Harikrishnan, A.R.Electroosmotic flows (EOF) in microfluidic devices can be greatly enhanced over superhydrophobic surfaces because the high shear rates within the electrical double layer can drive large slip velocities at the interface. Using the power law fluid model, we derive a novel formulation for the Helmholtz–Smoluchowski (HS) velocity and use it to examine the effect of slip on the hydrodynamics of a coupled pressure driven and EOF. Semi analytical relations for the velocity gradient are obtained for cases of a favourable pressure gradient but exact solutions of the velocity can be found only for certain power law indices. Cases of adverse pressure gradient and fractional power law indices are investigated using numerically using the Galerkin Finite Element Method. The validity of the semi analytical relations verified by comparison with the numerical method. The presence of velocity slip at the wall leads to an enhancement of the HS velocity that is most pronounced in shear thinning fluids. Adverse pressure gradients are observed to generate an inflection in the velocity profile and even a two-way flow for certain flow parameters. The strength of the adverse pressure gradient needed to setup a reverse flow at the channel centre reduces as the slip length is increased. The location of the point of inflection is found to depend on the channel height, pressure gradient, electric field, slip length, Debye length and non-Newtonian behaviour.