Abhijit P Deshpande

Bubble induced liquid circulation is important in applications such as bubble columns and air-lift reactors. In this work, we describe an experimental and numerical investigation of liquid circulation induced by a bubble plume in a tank partitioned by a baffle. The baffle divides the tank into two compartments. Liquid can flow from one compartment to the other through openings at the top and the bottom of the baffle. Gas (air) was injected in the riser section in the form of bubbles at one corner of the tank. The temporal and spatial variation of velocity field in the liquid as a function of the gas flow rate was measured using particle image velocimetry (PIV). At a constant gas flow rate, the liquid flow field is unsteady due to the interaction with the bubbles. The time scales associated with the velocity-time series and the bubble plume thickness variation were calculated. The time averaged-velocity field was used to quantify the variation of the liquid circulation rate with gas flow rate. The turbulence in the liquid was measured in terms of turbulent intensities. These were calculated from the experimental data and were observed to be less than 3 cm/s. A 2-d Euler-Euler two-fluid model with buoyancy and drag as the interaction terms was used to simulate the flow. The parameters chosen for the simulations were selected from literature. It is shown that inclusion of turbulence model such as k - ε{lunate} is necessary to capture the overall flow behavior. Good agreement was observed between experimentally obtained velocity profiles and the recirculation rates with the simulation results. © 2007 Elsevier Ltd. All rights reserved.

Microencapsulation is a widely used method for making healing agents used in self-healing composites. In this study, a novel two-stage process was used to make double-walled microcapsules. Dicyclopentadiene–urea formaldehyde (DCPD–UF) microcapsules were synthesized by in situ polymerization of oil-in-water emulsion followed by siloxane coating through ‘sol–gel process’ (DCPD–UF–siloxane microcapsules). Average diameter of microcapsules, UF shell thickness and siloxane coating thickness were found to be 300, 1.4 and 16 µm, respectively. The effect of addition of microcapsules on rheological properties of epoxy was studied. Breaking pattern of single-walled and double-walled microcapsules immersed in epoxy was analyzed by continuous monitoring of the deformation behavior through a rheometer–microscope arrangement, confirming improved mechanical properties of the double-walled microcapsules. In this study, epoxy resin cast specimens with and without microcapsules were prepared and the effect of microcapsules on mechanical properties was examined. Epoxy specimens with double-walled microcapsules were found to be having improved mechanical properties compared to those with single-walled microcapsules. Finally, healing efficiency of DCPD–UF–siloxane microcapsules in epoxy was observed to be marginally higher, and therefore, this double-walled microcapsule system is shown to be a promising candidate for further self-healing composite investigations.

The time-dependent fluid flow in a square cavity was studied using model fluids of glyceml-water solution at different frequencies and amplitudes of motion of the top plate. The range of Reynolds numbers in our investigation varied from 5 to 3700. The experiments were carried out in a square cavity with a periodically driven lid, and planar velocity measurements were obtained using particle image velocimetry. The flow was driven by moving the top surface of the cavity in a simple harmonic motion. The aspect ratio, defined as the ratio of cavity length to the cavity height, is unity. The ratio of cavity spanwise width to the length of the cavity is 0.2. The temporal variation of velocity at fixed locations in the cavity exhibits a periodic variation. The basic frequency of the fluid motion at a point in the flow domain was observed to be the same as that of plate motion for low Reynolds number Re. However, existence of dominant secondary frequencies was observed along the central vertical plane. The velocity variation as a function of time at a fixed position and the velocity profiles along horizontal and vertical planes are also quantitatively described. These were compared to computational fluid dynamics (CFD) simulations based on the finite volume technique. Comprehensive details of the flow as a function of Reynolds number are analyzed. The evolution of secondary vortices at different plate positions as a function of Reynolds number is also presented. The planar velocity measurements acquired are indicative of the flow behavior in a periodically driven cavity with a narrow span width even at high Re. At very low Re, the flow throughout the periodically driven cavity qualitatively resembles the classical steady lid-driven cavity flow. At high Re, the entire cavity is occupied with multiple vortices. The qualitative features of the bulk flow observed are valid even for cavities with infinite span width. Copyright © 2006 by ASME.

In this work, we investigate experimentally as well as numerically a drainage displacement system; i.e., a non-wetting fluid displacing a wetting fluid in a porous medium. Experiments were carried out in a horizontal rectangular channel packed with a monolayer of glass beads. The displacement of a higher viscosity wetting fluid (silicone oil) by a lower viscosity non-wetting fluid (air) is studied. Similarly, the displacement of a lower viscosity wetting fluid (silicone oil) by a higher viscosity non-wetting fluid (glycerol) is also studied. Flow structures, such as viscous fingering and stable displacement, were obtained. The behavior of the flow in the experiments was simulated using a pore network model. The model consists of a network of tubes of equal lengths inclined at 45°. The radius of the tubes is assumed to follow a random distribution to ensure a realistic representation of a porous medium. The pressure distribution across the network is obtained by assuming laminar flow in each tube. The Hagen-Poiseuille equation is used after including the effect of capillary pressure to determine the flow velocity in each tube. The displacement of the interface for each time step is restricted to 2.5-5.0% of the tube length and the maximum velocity in the network is used to calculate this time interval. The movement of the interface inside the tube is calculated using a second-order Runge-Kutta method. Once the interface reaches a node, the volume of the fluid entering the neighboring tubes is determined by the pressure drop across them. We have varied the capillary number, Ca (μv/σA), and viscosity ratio, M, and have obtained two different flow regimes, viscous fingering and stable displacement. The residual amount of defending fluid present in the network model is calculated for the two regimes of drainage displacements. It is found that when stable displacement occurs, the system has significantly less amount of defending fluid present for the same duration of time as compared with the case when viscous fingering is exhibited. The fronts of the invading fluid during viscous fingering at different capillary numbers are self-similar with a fractal dimension of 1.3 that matches with the experimental results. © 2011 by Begell House, Inc.

Mixing of fluids is important in many industrial processes. In this paper we discuss the process of kinematic mixing in Newtonian fluid and quantify the intensity of mixing. A cavity with a periodically driven lid was chosen for the experimental and numerical analysis. It is shown that the system behavior depends on two dimensionless parameters, the geometric parameter (Af) and the Stokes number (St). The mixing is quantified by the Planar Laser Induced Fluorescence (PLIF) technique using Rhodamine-B as the dye. Here the emitted fluorescent light intensity was related to the concentration of the dye present in the system. The extent of mixing was determined by calculating the deviation of intensity fluctuations. In addition, numerical analysis using the Particle Separation approach based on the calculation of stretch rates were done and applied to study the effect of two dimensionless parameters on mixing. © 2006 WIT Press.

Understanding of the flow of fluids through beds of fibrous media is extremely important for composites processing. In this work, we have investigated a steady flow of a Newtonian fluid through two-dimensional porous media using lattice Boltzmann methods. The porous domains studied in this work represent different types of porous media encountered in composite processing. Initially, the methodology was validated with a simulation of flow through random porous media. Flow through porous media with circular and elliptical inclusions was simulated with different geometric arrangements. Simulations were also carried out with anisotropic porous media. The permeability was estimated as a function of porosity, geometric arrangements and the degree of anisotropy. The simulation results agree well with those from analytical, empirical and experimental studies. The results demonstrate that such a method will be very useful in simulating composite processing.

Rheology of Complex Fluids Abhijit P. Deshpande, J. Murali Krishnan, P. B. Sunil Kumar This book provides an in-depth treatment of complex fluids. A clear understanding of the flow and rheological behavior of such fluids is crucial while carrying out the processing operations, designing equipments which handle/transport these fluids, and in their end-use applications. This book: • Discusses multicomponent-multiphase systems of which most of complex fluids are examples •Covers a wide variety of application areas from polymers to biological systems. •Introduces active fluids, with internal energy generation, and their rheology •Involves multidisciplinary tools, and brings together contributors from different backgrounds Rheology of Complex Fluids is a must-read for researchers and practicing engineers in the complex fluid industry. Selected portions of the book can be used as supplementary teaching material. © Springer Science+Business Media, LLC 2010.

In this work, the rheological properties, thermal stability and the lap shear strength of epoxy adhesive joints reinforced with different carbon nano-fillers such as multi-walled carbon nanotubes (CNT), graphene nanoplatelets (GNP) and single-walled carbon nanohorns (CNH) have been studied. The nano-fillers were dispersed homogeneously using Brabender® Plasti-Corder®. The epoxy pre-polymer with and without the nano-fillers exhibited shear thinning behavior. The nano-filler epoxy mixtures exhibited a viscoplastic behavior which was analyzed using Casson's model. Thermo-gravimetric analysis indicated an increase in the thermal stability of the epoxy with the addition of carbon nano-fillers. Carbon nano-fillers resulted in increased lap shear strength having high Weibull modulus. The joint strength increased by 53%, 49% and 46% with the addition of 1 wt.% CNT, 0.5 wt.% GNP and 0.5 wt.% CNH, respectively. The strength of the joints having high filler content (>1 wt.%) was limited by mixed mode type of failure.

Microcapsules are widely used by researchers in self-healing composites. In this study, multi-walled carbon nanotubes (CNT) were incorporated into the core of the microcapsules, along with the self-healing agent. Dicyclopentadiene (DCPD) and urea-formaldehyde (UF) were chosen as the core and shell materials respectively, and DCPD-CNT-UF based dual core microcapsules were synthesized. Two types of microcapsules, namely, DCPD-UF and DCPD-CNT-UF were successfully synthesized by the in situ polymerization technique. The novelty of this work is the development of dual core microcapsules with DCPD-CNT-UF combination. Surface morphology characterization and elemental analysis of the microcapsules were carried out using a scanning electron microscope (SEM-EDX). TGA and DSC analysis show that DCPD-CNT-UF microcapsules have better thermal stability than DCPD-UF microcapsules. These novel DCPD-CNT-UF microcapsules were found to be compatible with epoxy base resin for making resin castings. The presence of CNT is found to improve the mechanical, thermal and electrical properties of the resin cast specimens without compromising on self-healing efficiency.

The micellar characteristics of a non-ionic, natural surfactant, saponin obtained from the soapnut tree, Sapindus mukorossi, were studied in aqueous solution. Critical micelle concentration of Sapindus saponin determined using conductivity measurements and UV absorption studies was 0.045 wt%. Increase in temperature and salt concentration led to decrease in the critical micelle concentration of Sapindus saponin. The critical micelle concentration was found to increase with increase in hardness of water and increase in pH. The micellar aggregation number was determined using cyclic voltammetry and was found to be between 13 and 21. The size of the Sapindus saponin micelles was determined using intrinsic viscosity measurements and was found to be independent of saponin concentration for concentrations above the CMC. Solubilisation of two types of crude oils and a vegetable oil was studied using micellar solubilisation technique. At lower concentrations of the surfactant, the micellar solubilisation of crude oils in saponin was better than synthetic surfactants like Triton X100® and SDS where as, the solubilisation of vegetable oil was better in synthetic surfactants. © Carl Hanser Publisher.