Arnab Atta

Computational studies on flow maldistribution of Newtonian liquids in periodic packed beds

Abstract Packed bed, one of the most important reactors in several chemical processes and petroleum refinery operations, has been a field of interest for many researchers. Familiarized with the steady state hydrodynamics over the past two decades, researchers are now engaged in exploring the efficacy of unsteady state operation of packed bed reactors (PBRs) to extract even more from its efficiency and performance (Atta et al., 2014). The present work deals with CFD studies on cyclic modes of single phase flow operation in structured packed beds having face centered cubic (FCC) and modified simple cubic (SC) packing arrangements which are modeled by unit cell approach. Max-min and on-off flow operations at three different particle Reynolds number ( Re p ) are carried out for five different splits. Liquid distribution is quantitatively examined at the mid-plane of both geometries by evaluating mass fluxes. The mass flux analysis reveals that in either case of on-off and max-min operation, liquid distribution is found to improve than for continuous flow. A distribution coefficient, calculated using the standard deviation concept, indicates that the FCC packing shows the best distribution for max-min operation with the split having maximum peak flow duration. However, exactly opposite trend is observed in modified SC, where the split with lowest peak flow rate duration shows the best distribution among the five splits. This work using a single phase is not only relevant to various practical application but also sets the foundation in exploring multiphase flow hydrodynamics of cyclic packed beds.

Controlled Nanoscale Electrohydrodynamic Patterning Using Mesopatterned Template

We report the path for a possible fabrication of an array of nanogrooves, by electro-hydrodynamic instability-mediated patterning of a thin polymer film using a patterned stamp with much larger features. Using a predictive computational model based on finite element method, we find the route to control the coalescence of initial instabilities that arise with the onset of spatially varying DC electric field generated through topographical patterns in the top electrode. These quasi-steady structures are shown to evolve with the electrostatic and geometric nature of the two-electrode system and are of a stable intermediate during the process of feature replication, under each electrode feature. We identify conditions to obtain nanogrooves for a range of operating conditions. Such simulations are likely to guide experiments, where simultaneous optimization of multiple parameters to fabricate features with lateral dimension smaller than that of the electrode patterns is challenging.

Re-entrant structural evolution using electrically heterogeneous patterned electrode

Abstract The pattern morphology in electrohydrodynamic (EHD) instability systems is extremely sensitive toward several design parameters, such as initial film thickness (h0), electrode spacing (d), pattern periodicity (Lp), and geometry of electrodes. This initiates a need for numerical simulations to account for these structural evolutions. In this work, a 2D computational model is developed for EHD instability in ultra-thin films (

Numerical investigation of viscous effect on Taylor bubble formation in co-flow microchannel

Abstract In recent years, studies on two–phase flow in microchannels have attracted huge interest due to its wide range of application in lab–on–a–chip devices and microreactors. Regarding gas–liquid two–phase flow in microchannels, most of the previous research dealt with gas and Newtonian liquid phase. However, several fluids in practical application exhibit non–Newtonian behavior, as well. In this study, gas and non-Newtonian liquid phase flow in a circular co–flow microchannel has been numerically investigated. The developed CFD model is initially validated with the literature data, and thereafter the model is employed for non–Newtonian studies. Polyacrylamide (PAAm) aqueous solutions with different mass concentration, which exhibit shear thinning behavior are used as non–Newtonian liquid. The effects of PAAm concentration, gas and liquid inlet velocities, and surface tension on Taylor bubble have been systematically explored. The results show that Taylor bubble length decreases with increasing PAAm concentration. The bubble velocity is found to increase with increasing PAAm concentration due to increase in liquid film thickness around the bubble. The film around the Taylor bubble is precisely captured. It is observed that the rheological properties of continuous phase have significant effect on bubble shape and liquid film thickness. Squeezing break up mechanism is observed at higher liquid inlet velocity for lower concentration of PAAm. The bubble formation frequency is found to reach maximum with increasing liquid velocity and PAAm concentration. Different flow patterns are observed namely, Taylor bubble, and non–Taylor bubble, where the bubble length is smaller than the capillary diameter of the channel. Additionally, flow pattern maps are also reported based on inlet velocities. These understandings motivate for new predictive modeling approaches in design and applications demanding the use of non–Newtonian fluids.