Steady Two‐Dimensional Non‐Newtonian Flow Past an Array of Long Circular Cylinders up to Reynolds number 500: A Numerical Study

The free surface and zero vorticity cell models have been combined with the equations of motion to investigate numerically the steady flow of incompressible power‐law (shear‐thinning and shear‐thickening) fluids across banks of long cylinders. The equations of motion in the stream function/vorticity formulation have been solved numerically using a second order accurate finite difference method to obtain extensive information on the behaviour of the drag coefficient, surface vorticity distribution, streamlines and iso‐vorticity patterns, for high Reynolds numbers (Re = 50 500) and using a wide range of power‐law index (0.3 ≤ n ≤ 2.0), and porosity (0.4 ≤ e ≤0.9) values. The behaviour of the aforementioned parameters at low Reynolds numbers has also been investigated and validated using theoretical and numerical work from the literature. The results reported here enable extension of the limits of creeping flow behaviour up to Re = 50 for fluids with highly shear‐thickening characteristics under low porosity conditions.     

Power‐Law Fluid Flow Over a Sphere: Average Shear Rate and Drag Coefficient

Based on the consideration of the rate of mechanical energy dissipation, an expression for the average shear rate for a sphere falling in a power‐law fluid in the creeping flow regime has been deduced. The average shear rate in a power‐law fluid (n<1) appears to be higher than that in an equivalent Newtonian fluid. This in turn has been combined with the numerical predictions of drag coefficient (up to Reynolds number of 100) of a sphere to develop a generalized drag correlation for power‐law liquids encompassing both n > 1 and n < 1 which appears to apply up to much higher values of the Reynolds number. The available experimental data have been used to demonstrate the reliability and accuracy of the new correlation for shearthinning liquids. Also, in the limit of n = 1, this expression reproduces the standard drag curve with a very high accuracy.     

Construction of a Holliday Junction in Small Circular DNA Molecules for Stable Motifs and Two‐Dimensional Lattices

Design rules for DNA nanotechnology have been mostly learnt from using linear single‐stranded (ss) DNA as the source material. For example, the core structure of a typical DAO (double crossover, antiparallel, odd half‐turns) tile for assembling 2D lattices is constructed from only two linear ss‐oligonucleotide scaffold strands, similar to two ropes making a square knot. Herein, a new type of coupled DAO (cDAO) tile and 2D lattices of small circular ss‐oligonucleotides as scaffold strands and linear ss‐oligonucleotides as staple strands are reported. A cDAO tile of cDAO‐c64nt (c64nt: circular 64 nucleotides), shaped as a solid parallelogram, is constructed with a Holliday junction (HJ) at the center and two HJs at both poles of a c64nt; similarly, cDAO‐c84nt, shaped as a crossed quadrilateral composed of two congruent triangles, is formed with a HJ at the center and four three‐way junctions at the corners of a c84nt. Perfect 2D lattices were assembled from cDAO tiles: infinite nanostructures of nanoribbons, nanotubes, and nanorings, and finite nanostructures. The structural relationship between the visible lattices imaged by AFM and the corresponding invisible secondary and tertiary molecular structures of HJs, inclination angle of hydrogen bonds against the double‐helix axis, and the chirality of the tile can be interpreted very well. This work could shed new light on DNA nanotechnology with unique circular tiles.     

Numerical Investigation of the Gas‐Solid Flow Characteristics in a Three‐Dimensional Spouted Bed with a Draft Tube

Gas‐solid motions in a three‐dimensional conical spouted bed with a draft tube are investigated based on a simulation carried out by the coupling approach of computational fluid dynamics combined with the discrete‐element method. The distribution properties of the velocity, the concentration, and the flux of the solid phase are discussed. The vertical solid velocity in the central region initially increases, diminishes gradually, and finally decreases sharply in the region above the draft tube. Vigorous lateral solid motion occurs in the periphery of the fountain and the spout‐annulus interface. In addition, the vertical solid flux shows a large value in the spout. A larger vertical velocity but a more dilute solid concentration can be detected along the axial direction when enlarging the gas flow rate.     

Investigation of Fluid and Coarse‐Particle Dynamics in a Two‐Dimensional Spouted Bed

The aerodynamics of particles and gas flow in a two‐dimensional spouted bed (2DSB) with draft plates is investigated with the aid of the discrete element method. The geometry of the 2DSB with draft plates is set as close as possible to the experimental apparatus of Kudra [1] and Kalwar [2]. The physical properties of the coarse particles are similar to those of shelled corn. The calculated minimum spouting velocity and pressure drop agree well with the correlations of Kudra [1] and Kalwar [2]. In the spout region, the particle vertical velocities are found to decrease as the height increases. The fluid velocity in the downcomer region decreases as the superficial gas velocity increases. The particle circulation rate increases when the friction coefficient decreases or the separation height increases. At the minimum spouting velocity, the bed height does not affect the particle circulation rate in the 2DSB with draft plates. The draft plates not only reduce the minimum spouting velocity and pressure drop but also increase the maximum spoutable bed height. The effect of taking out the draft plates on the spouting phenomenon is investigated and the effect of putting in a deflector on the possible breakage of the particles is also estimated.     

Two‐Dimensional Model for Oxidative Coupling of Methane in a Packed‐Bed Membrane Reactor

A two‐dimensional (2D) model of a packed‐bed membrane reactor was developed to describe ethylene production by oxidative coupling of methane (OCM). The model covers all relevant energy and mass transport processes in the reactor and allows a more precise prediction of the temperature and conversion patterns. It is demonstrated that the fast OCM reaction leads to oxygen depletion in the vicinity of the membrane, causing strong radial concentration gradients in the bed. Further results indicate that the detailed 2D model can provide more accurate predictions of experimental data than the simplified one.     

Asymmetric Homogeneous Hydrogenation in Flow using a Tube‐in‐Tube Reactor

In this update, the asymmetric homogeneous hydrogenation of a number of trisubstituted olefins utilizing the recently developed tube‐in‐tube gas‐liquid flow reactor is described. A number of chiral iridium‐ and rhodium‐based catalysts and other parameters such as pressure, solvent, temperature and catalyst loading were screened. The advantage of the flow set‐up for rapid screening and optimization of reaction parameters is illustrated. Furthermore, a comparative study using batch conditions aided in the optimization of the flow reaction set‐up. The set‐up was further modified to recycle the catalyst which prolonged catalytic activity.     

Gas Flow Behavior in a Two‐Phase Reactor Stirred with Triple Turbines

Gas behavior was studied in a gas‐liquid reactor stirred with multiple turbines. Triple PBTs pumping either down or up and BT‐6's were used. The behavior of coalescing (water) and non‐coalescing (sodium sulfate solution) systems was investigated. The gas phase behavior was characterized by means of the RTD and modeled with the axial dispersion model. This model was confirmed to interpret the experimental data well for water and satisfactorily for the coalescence‐inhibiting solutions. The influence of the operating conditions, turbine type and system coalescing behavior on the model parameters is discussed. Comparison is also made with similar data regarding Rushton turbines and high‐solidity ratio hydrofoils, as well as gas holdup.     

Phase Distribution Measurements during Transient Bubbly Two‐Phase Flow in a Vertical Pipe

An experimental procedure for investigating transient bubble flow for an adiabatic air/water system with vertical upward flow in a pipe is presented. The results of the measured local transient two‐phase flow parameters are shown along a pipe length of approx. four meters. From the measured radial phase distributions under steady and under transient conditions one can draw conclusions about the interfacial forces. Here, the effects indicate the action of forces such as a transverse lift force and a time dependent force like the virtual mass force during the transient. For modelling the transverse lift force which seems to play a dominant role for that flow regime the formulation of Zun was chosen and it was implemented into the commercial CFD‐Code Fluent Release 4.4.4 via user‐defined subroutines. Finally, results from the simulation of the steady states of start and end conditions of an experimental measured transient are shown.     

CO2 Absorption Using Nanofluids in a Wetted‐Wall Column with External Magnetic Field

A new method for enhancing the mass transfer coefficient in the gas absorption process is reported. CO₂ absorption experiments were carried out in a wetted‐wall column using different aqueous nanofluids as the solvent. The mass transfer characteristics were found to increase by applying Al₂O₃/water nanofluid. The mass transfer coefficient decreased with TiO₂/water nanofluid. In the case of Fe₃O₄/water nanofluid, the mass transfer rate was enhanced by increasing the nanoparticle volume fraction, but the mass transfer coefficient was lower than that obtained with water for all experimental conditions studied. Finally, applying a downward magnetic field resulted in higher mass flux and mass transfer coefficient in comparison with experiments without a magnetic field.     

Residence Time Distribution of Steady and Pulsed Flow in a Parallel‐Plate Channel with Staggered Fins

The residence time distribution (RTD) in a parallel‐plate channel with staggered fins for both steady and pulsed flow conditions was experimentally determined. Dispersion and tank‐in‐series models were also adopted to characterize the system. The process fluid was water and the experiments were performed at room temperature. A steady Reynolds number Re ranging from 100 to 1000 was studied. The pulsating flow was generated using a frequency f of 6–20 Hz and an amplitude A of 0–2.3 mm. A pulse injection of sodium chloride solution was used as a tracer and the response in the form of electrical conductivity was measured at the outlet stream. The flow in the staggered finned channel approaches nearly plug‐flow behavior with either higher steady‐flow velocity or superposition of oscillation at low Re.     

Numerical Simulation of the Hydrodynamics of Gas/Solid Two‐Phase Flow in a Circulating Fluidized Bed with Different Inlet Configurations

The gas/solid flow characteristics in a circulating fluidized bed with two different inlet configurations were investigated by numerical simulation based on an Eulerian approach. In order to describe the interaction between the gas phase and the solid phase and the influence of the solid phase on the gas turbulence, a source term formulation with a more reasonable physical meaning was introduced. The simulation results were validated by the experimental data; then, the model was employed to examine the effect of the inlet configuration on the gas and solid feeding. The simulation results showed that, using the side feeding system, the distributions of solid flow and concentration were highly variable both over the column cross‐section and along the column height. However, such variations can be improved by using the elbow inlet system where the gas and solid are fed from the bottom.     

Effects of Ionic Environments on Bovine Serum Albumin Fouling in a Cross‐Flow Ultrafiltration System

The influence of electrostatic interactions on membrane fouling during the separation of bovine serum albumin (BSA) from solution was studied in a cross‐flow ultrafiltration system. Experiments were carried out at different pH values between 3.78 and 7.46; and for different ionic strengths between 0.001 M and 0.1 M. The changes in permeate flux, cake layer resistance, zeta potentials of BSA and polyether sulfone (PES) membranes, and electrostatic interaction energies, were evaluated. At all of the ionic conditions studied, PES membranes are negatively charged. However, BSA molecules are either negatively or positively charged depending on the ionic environment. Whereas the cake layer resistance decreased with increasing pH and ionic strength, the permeate fluxes increased. The calculated electrostatic energy was a minimum at the isoelectric point (IEP) of BSA. However, at this point, the cake resistances corresponding to fouling at each ionic strength, were not minimized. Below the IEP of BSA, the electrostatic forces were attractive, while above the IEP, repulsive electrostatic forces were dominant.     

Effects of Distribution Channel Dimensions on Flow Distribution and Pressure Drop in a Plate‐Fin Heat Exchanger

A numerical investigation on flow distribution and pressure drop characteristics in a plate‐fin heat exchanger is presented. Influences from length and width of the distribution channel were particularly investigated. Flow distribution in the studied model was improved by increasing the length or reducing the width of the distribution channel, but at the cost of an increased pressure drop. The relationship of flow distribution and pressure drop was analyzed by the porous medium approach. A dynamic balance phenomenon was observed and further studied. Based on the results, a novel strategy for the attainment of flow uniformity, which causes negligible pressure drop variation, was proposed. Finally, a performance effectiveness factor was developed for predicting the effect of the proposed strategy on the performance of plate‐fin heat exchangers.     

A New Pulsation Policy in a Disk and Doughnut Pulsed Column Applied to Solid‐Liquid Extraction of Andrographolide from Plants

Classically, sinusoidal oscillations are imposed to enhance mixing and mass transfer between two phases contacted. But, in any case of solid‐liquid contact, it was noticed that this pulsation mode was not efficient enough to allow a controlled behavior of the solid phase. The problem is particularly met during the treatment of raw plants or polydispersed populations with complex physical properties. The objective of this study is to demonstrate the viability of using a nonsinusoidal pulsation in a continuous contactor to replace a traditional batch sinusoidal mode. A review of the different pulsation techniques is firstly presented. The example of solid‐liquid extraction of andrographolide from plants has then been chosen to bring out the advantages of the new pulsation mode. The development of this application as a continuous process in a column has indeed encountered difficulties due to the important heterogeneity of the matter: one of these classes tends to float and the other to sink, which always leads to a definitive flooding in classical operations. Typically, the proposed signal is composed of two different periods: on the one hand, a classical sinusoidal pulsation step used to mix the liquid and solid phases in the active part of the column and allowing an optimal mass transfer and, on the other hand, an impulsion phase, generally used for the transport of solid. The extraction is carried out in a disk and doughnut column of 54 mm in diameter and 3.5 m in height. Liquid and solid are flowing concurrently and downwardly. Experiments have been performed to know the global characteristics of the process in steady state and to suggest some elements for industrial design. The results showed that an optimal tuning of the geometric characteristics of the column, the level of interface and the parameters of the pulsation could increase the operated domain where flooding is avoided.