Sarit Kumar Das

Cell rolling on the vascular endothelium plays an important role in trafficking of leukocytes, stem cells, and cancer cells. We describe a semianalytical model of cell rolling that focuses on the microvillus as the unit of cell-substrate interaction and integrates microvillus mechanics, receptor clustering, force-dependent receptor-ligand kinetics, and cortical tension that enables incorporation of cell body deformation. Using parameters obtained from independent experiments, the model showed excellent agreement with experimental studies of neutrophil rolling on P-selectin and predicted different regimes of cell rolling, including spreading of the cells on the substrate under high shear. The cortical tension affected the cell-surface contact area and influenced the rolling velocity, and modulated the dependence of rolling velocity on microvillus stiffness. Moreover, at the same shear stress, microvilli of cells with higher cortical tension carried a greater load compared to those with lower cortical tension. We also used the model to obtain a scaling dependence of the contact radius and cell rolling velocity under different conditions of shear stress, cortical tension, and ligand density. This model advances theoretical understanding of cell rolling by incorporating cortical tension and microvillus extension into a versatile, semianalytical framework. © 2010 by the Biophysical Society.

Present study depicts the transient response of multi-pass plate heat exchangers (PHEs) considering flow maldistribution from port to channels. Apart from the flow maldistribution, fluid axial dispersion is also considered to take care of the fluid backmixing and other deviations from plug flow. It is assumed that each multi-pass PHE is a combination of single-pass PHEs and in each heat exchanger the fluid is distributed non-uniformly amongst channels. The fluid velocity varies from channel to channel within each module of the heat exchanger so also the heat transfer coefficient. The solution techniques have been presented here for 1-2 pass and 2-2 pass arrangements. The first one is solved by analysing successive modules of the heat exchanger and the next one by iterating the responses between two heat exchanger modules. The solution for each module has been obtained analytically by using Laplace transform followed by numerical inversion from frequency domain. The results show the effect of flow maldistribution and its effect combined with the conventional heat exchanger parameters in the transient regime. It is observed that the transient characteristics such as response delay, asymptotic value and time constant are strongly dependent on the multi-pass flow arrangement, maldistribution and backmixing characterised by axial dispersion. © 2007 Elsevier B.V. All rights reserved.

The flow of fuel and oxidant through a PEMFC is analyzed for prediction of maldistribution. Flow distribution of both fuel and oxidant from the port to the individual cells critically control the performance of a PEMFC stack in combination. The distribution of fluids was simulated by analytical approach utilizing flow channeling model of a manifold. A detailed numerical modeling is also carried out considering flow in each cell between the electrodes as flow through an equivalent porous medium offering identical resistance. The results show a close match between the analytical and numerical results. The parametric study reveals that flow rate and port size plays major role determining maldistribution of the fluids, which can be considerably skewed when large numbers of cells are stacked for larger power output.

Offset strip fins are used in Compact Heat Exchangers. A new technique for the prediction of heat transfer characteristics of these surfaces has been established. The method utilizes an analogy between heat and momentum transfer proposed for short thermally developing laminar flow. The present work exploits the intuition that the analogy can be utilized for any repeated flow structure with heat transfer from surfaces that are also repetitive in array. In the offset strip fin geometry, the original form of this analogy shows a constant bias, which depends on the fin geometry. This leads to the incorporation of the fin geometrical parameters to a new analogy equation resulting in a generalized form of equation for the entire range of offset strip fins. The results clearly indicate that the heat transfer characteristics of offset strip fin surfaces could be predicted by measuring pressure drop across them alone. © 2006 R.T. Edwards, Inc.

The villi structures found in intestinal tract, infused with mucus, can attain different configurations by elongation and lashing movements. The present work analyzes the different configurations of villi separately and the associated drag behavior that may aid in movement of food through the tract. Using numerical simulations, the variation of drag in intestinal tract with respect to different configurations, especially the inclination of villi, has been studied, with quasi-steady approximations that consider the villi as solid objects. Extending this intestinal tract surface with villi and mucus, to a normal ridge textured surface infused with a viscoelastic fluid, significant changes in drag, owing to the viscoelastic nature of the infused fluid, have been found. The extent of drag reduction or drag enhancement, which is dependent on texture of the surface, is also found to be contingent on rate of deformation of the viscoelastic fluid along the patterned surface, and the density of ridges or solid fraction. Such control over drag on these surfaces, through Weissenberg number and texture of the surface, indicates the possibility of using viscoelastic liquid infused engineered surfaces for bio-medical and industrial applications.

The influence of nanostructures concentration, morphology, permittivity and size on the augmentation of the dielectric breakdown characteristics of nano insulating oils has been experimentally examined and demonstrated in detailed for the first time. Various dielectric nanoparticles/structures, viz. zinc oxide (ZnO), zirconium oxide (ZrO2) and aluminium oxide (Al2O3), of different sizes over a range of 25–125 nm have been employed to investigate the influence of nanoparticles concentration as well as size on the dielectric performance of nano insulating oils. Bismuth oxide (Bi2O3), magnesium oxide (MgO) and copper oxide (CuO) have been utilized to investigate the effect of nanoparticles concentration and all the nanoparticles contribute to the detailed study on the effects of morphology on the breakdown characteristics. Experimental findings reveal that particle size, permittivity as well as concentration affects the dielectric performance of nano insulating oils to large extents. Particles with smaller size offer higher enhancements in the dielectric breakdown voltage compared to the bigger size particles and its mechanism and role in hampering streamer development and growth has been deliberated. The influence of temperature and moisture content has been found to have major effects on the BD performance and experimentally examined and reported. The present work reveals the potential of nanomaterials towards designing more robust power systems employing liquid dielectrics by engineering their breakdown characteristics.

A second law analysis is presented for thermally dispersive flow through a plate heat exchanger. It is well known that in plate or plate fin type heat exchangers the backmixing and other deviations from plug flow contribute significantly to the inefficiency of the heat exchanger, which is of importance to heat exchangers working in the cryogenic regime. The conventional axial heat dispersion model which is used so far is found to be better than 'plug flow' model but still unsatisfactory where the timescale related to heat transfer is comparable with the thermal relaxation time for the propagation of dispersion. The present work therefore considers dispersion as a wave phenomenon propagating with a finite velocity. The study discusses the nature of variation of different contributions to total exergy loss in the heat exchanger with respect to dispersion parameters of the Peclet number and propagation velocity of the dispersive wave. The practical example of the single-pass plate heat exchanger demonstrates how a second law optimization can be carried out for heat transfer equipment under such conditions. © 1998 Elsevier Science Ltd. All rights reserved.

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. © 2011 Fang et al.

The current study investigates the flow of nanofluids in microchannels experimentally and numerically. For this purpose, two microchannels of hydraulic diameters of 211 and 300 μm are used with alumina(45nm)-water nanofluids. The nanofluids with the concentrations 0.25, 0.50 and 1 vol% are used to observe the effect of volume fraction in the present analysis. With regard to the numerical simulation of nanofluids in microchannels, two approaches have been chosen in the current work. First one considers the nanofluids as single phase fluid and applies the mixture rule for evaluating properties for the simulation. The second type of modeling is done using the discrete phase approach which involves Eulerian-Lagrangian considerations. The fluid phase is assumed to be continuous and governed by Navier-Stokes equation. The movement of discrete nanoparticles is determined by the Newton's second law which takes into account the body force, hydrodynamic forces, the Brownian and thermophoresis forces. The predictions are validated against experimental results obtained for nanofluid flow in a chemically etched silicon wafer channel. It is found that the discrete phase modeling is more accurate with regard to the prediction of nanofluids behavior in microchannels, as compared to the single phase model. The results also show the non-uniformity of nanoparticle distribution across the channel cross-section. This non-uniformity in distribution can be attributed to the shear induced particle migration. This can also be the reason for the difference in pressure drop and heat transfer from the single phase model. The pressure drop with 0.25 and 0.5 vol% of alumina is more or less same as that of water and thus, makes it a suitable cooling liquid. However, an enhancement in heat transfer is observed from the computational predictions. © 2010 by ASME.

A water drop impacting a superhydrophobic surface (SHS) rebounds completely with remarkable elasticity. For a given drop size, the time of contact on a flat SHS remains constant. However, recent studies show that the contact time can be reduced further by triggering an asymmetry in the hydrodynamics of impact. This can be achieved in different ways; an example being the impact on a cylindrical SHS with a curvature comparable to the drop. Here, the anisotropic flow generated from the tangential momentum and elliptical footprint of the drop before the crash leads to the formation of lobes. In the present work, we perform drop impact experiments on a bathtub-like SHS and show that the radial anisotropy can be triggered even in the absence of both the tangential momentum and non-circular footprint. This is shown to be a consequence of lamella deflection during the drop spreading. The reduction in contact time is quite clearly evident in this experimental regime.