Axial mass transfer in Taylor flow through circular microchannels

Abstract

in Wiley InterScience (www.interscience.wiley.com).A Computational Fluid Dynamics (CFD) model has been formulated to evaluate axial mixing in Taylor flow. The residence-time distribution (RTD) curves of a Taylor flow unit cell (comprised of a liquid slug and two half bubbles) is calculated numerically from a step-tracer simulation by solving the steady-state Navier-Stokes and the transient convection-diffusion equations for a variety of conditions (10 < Pe < 10 5 , 10 À5 < Ca < 10 À3 ). The concentration calculated at the outlet of a unit cell gives the cell-density function, from which its RTD curve can be found. Two different trends in the unit cell RTD curves were observed with Pe. At high Pe, the curves have characteristic peaks, while at low Pe only one peak is found that decays slowly. The size and separation of the peaks is affected by the Pe, film thickness d, and slug length, but not Re. The moments of the RTD curves are then used to assess literature unit cell models (CSTR-PFR, two-region, and the model of Thulasidas et al.1 ) For Pe d > 10 the CSTR-PFR model showed the largest difference from the CFD simulations, while the model by Thulasidas et al. showed reasonable agreement. The two-region model fitted the simulations, but only for Sh values significantly different from those found by literature correlations. For Pe d < 10, the CSTR-PFR model gave the best predictions compared to the numerical simulations. A method for calculating the residence-time distribution of a reactor, based on the residence-time distribution of a unit cell by means of a convolution method, was also introduced and gave results which compared very well with experimental data. The short length of the reactor used in the experiments, however, could not allow proper differentiation from other models investigated, which also showed good agreement with the experimental data.

Effect of interfacial mass transfer on the dispersion in segmented flow in straight capillaries

“…Higher flow rates of continuous phase also imply a relatively thicker film thereby yielding greater extent of back mixing. Similar observations are also reported by Salman et al 25 on the basis of CFD simulations segmented flow. The effect of slug/drop sizes on dispersion 25 and on mixing in the drops 15 has been studied numerically and is known to be significant.…”

“…23 Based on an early experimental analysis, Thulasidas et al 24 have reported a two-zone model where the film region and the bulk region of a liquid slug were modeled differently to take into account the mass transfer of tracer between these two zones. This approach was further extended by Salman et al 25 for understanding the effect of interslug mass transfer on axial dispersion in segmented flow. Their observations indicate that typical plug flow nature was observed at high Pe while a long tail was observed at low Pe and these observations were independent of Re.…”

Axial dispersion in segmented gas-liquid flow: Effects of alternating channel curvature

“…More recently they developed a computational fluid dynamics model for the axial dispersion and residence time distribution in a straight two-dimensional gassegmented channel flow. 25 Muradoglu et al 26 studied the axial dispersion in the gas-segmented flow using a finitevolume/front-tracking ͑FV/FT͒ method and examined the effects of the Peclet number, capillary number, and segment size in a straight two-dimensional channel. Kreutzer et al 27 studied the axial dispersion in a straight rectangular microchannel and found that the liquid-filled menisci significantly increase the leakage and the axial dispersion is the least when the liquid flow rate is greater than the gas flow rate.…”

Influence of hydrodynamics on liquid mixing during Taylor flow in a microchannel

“…The elongated bubble has a characteristic capsular shape with an equivalent diameter larger than the channel width, and the bubble itself is separated from the channel wall by a very thin liquid film. Salman et al 35 had carried out the numerical study on the axial mixing in Taylor flow through circular microchannels. It was found that increasing film thickness resulted in larger and more distinct peaks in the unit cell residence-time distribution (RTD) curve, and the separation between the peaks increased while their size decreased with an increase in the length of the liquid slug.…”