Retrospect and Perspective of Micro-mixing Studies in Stirred Tanks

Mixing problems are most likely encountered and sometimes can be severe in scaling-up projects. Micro-mixing is an important aspect for fast or quasi-instantaneous reactions. Poor micro-mixing might produce more undesired by-products, leading to higher purification costs. This paper gives an extensive review and analysis of micro-mixing studies in single- and multi phase stirred tanks. The relevant experiment techniques, micro-mixing models and nurherical approaches are critically reviewed and analyzed with remarks and perspectives. The reported studies on two-phase micro-mixing experiments and on the impact of the presence of the dispersed phases on turbulence have been limited to a narrow range of conditions. More importantly, disparities widely exist among different reports. Both Lagrangian and Eulerian models are based on oversimplified assumptions, which may lead to uncertainties or even unrealistic results. A heuristic model, which is from the perspective of CFD （computational fluid dynamics） and can cover the whole spectrum of scales and also focus on every subrocess, is desired in the future.

Numerical simulation of micro-mixing in gas–liquid and solid–liquid stirred tanks with the coupled CFD-E-model

The coupled CFD-E-model for multiphase micro-mixing was developed,and used to predict the micro-mixing effects on the parallel competing chemical reactions in semi-batch gas–liquid and solid–liquid stirred tanks.Based on the multiphase macro-flow field,the key parameters of the micro-mixing E-model were obtained with solving the Reynolds-averaged transport equations of mixture fraction and its variance at low computational costs.Compared with experimental data,the multiphase numerical method shows the satisfactory predicting ability.For the gas–liquid system,the segregated reaction zone is mainly near the feed point,and shrinks to the exit of feed-pipe when the feed position is closer to the impeller.Besides,surface feed requires more time to completely exhaust the added H+solution than that of impeller region feed at the same operating condition.For the solid–liquid system,when the solid suspension cloud is formed at high solid holdups,the flow velocity in the clear liquid layer above the cloud is notably reduced and the reactions proceed slowly in this almost stagnant zone.Therefore,the segregation index in this case is larger than that in the dilute solid–liquid system.

Characterization of micro-mixing in a novel impinging streams reactor

This paper presents an experimental investiga-tion of a novel impinging stream reactor(ISR)with the aim of high mixing intensity.The integral mixing quality in the reactor was measured with the iodide-iodate reaction and showed excellent mixing performance.The impact of the operating parameters,such asfluxes,circulation and inter-nozzle distances,was investigated in terms of segregation index.The results showed that the increase offlux,the decrease of inter-nozzle distance and a suitable circulation can improve the micro-mixing efficiency.Based on turbulence theory,it was estimated that the characteristic micro-mixing time was 0.002–0.02 s,which was much shorter than that in the stirred tank reactor.The micro-mixing time was related to the segregation index,which was in good agreement with those in the literature.

Computational and experimental investigations of a microfluidic mixer for efficient iodine extraction using carbon tetrachloride enhanced with gas bubbles

Numerous studies have been conducted on microfluidic mixers in various microanalysis systems, which elucidated the manipulation and control of small fluid volumes within microfluidic chips. These studies have demonstrated the ability to control fluids and samples precisely at the microscale. Microfluidic mixers provide high sensitivity for biochemical analysis due to their small volumes and high surface-to-volume ratios. A promising approach in drug delivery is the rapid microfluidic mixer-based extraction of elemental iodine at the micro level, demonstrating the versatility and the potential to enhance diagnostic imaging and accuracy in targeted drug delivery. Micro-mixing inside microfluidic chips plays a key role in biochemical analysis. The experimental study describes a microfluidic mixer for extraction of elemental iodine using carbon tetrachloride with a gas bubble mixing process. Gas bubbles are generated inside the microcavity to create turbulence and micro-vortices resulting in uniform mixing of samples. The bubble mixing of biochemical samples is analyzed at various pressure levels to validate the simulated results in computational fluid dynamics(CFD). The experimental setup includes a high-resolution camera and an air pump to observe the mixing process and volume at different pressure levels with time. The bubble formation is controlled by adjusting the inert gas flow inside the microfluidic chip. Microfluidic chip-based gas bubble mixing effects have been elaborated at various supplied pressures.

Mixing Characteristics and Bubble Behavior in an Airlift Internal Loop Reactor with Low Aspect Ratio

The present study summarizes the results of macro-and micro-mixing characteristics in an airlift internal loop reactor with low aspect ratio(H/D≤5) using the electrolytic tracer response technique and the method of parallel competing reactions respectively. The micro-mixing has never been investigated in airlift loop reactors. The dual-tip electrical conductivity probe technique is used for measurement of local bubble behavior in the reactor. The effects of several operating parameters and geometric variables are investigated. It is found that the increase in superficial gas velocity corresponds to the increase in energy input, liquid circulation velocity and shear rate, decreasing the macro-mixing time and segregation index. Moreover, it is shown that top clearance and draft diameter affect flow resistance. However, the bubble redistribution with a screen mesh on the perforated plate distributor for macro-mixing is insignificant. The top region with a high energy dissipation rate is a suitable location for feeding reactants. The analysis of present experimental data provides a valuable insight into the interaction between gas and liquid phases for mixing and improves the understanding of intrinsic roles of hydrodynamics upon the reactor design and operating parameter selection.

Enhancing heat transfer at the micro-scale using elastic turbulence

Small concentrations of a high-molecular-weight polymer have been used to create so-called ＂elastic tur- bulence＂ in a micro-scale serpentine channel geometry. It is known that the interaction of large elastic stresses created by the shearing motion within the fluid flow with streamline curvature of the serpentine geometry leads initially to a purely-elastic instability and then the generation of elastic turbulence. We show that this elastic turbulence enhances the heat transfer at the micro-scale in this geometry by up to 300% under creeping flow conditions in comparison to that achieved by the equivalent Newtonian fluid flow.

EFFECTIVE SOLUTION METHOD OF CHEMICAL REACTION KINETICS WITH DIFFUSE

The time integration method with four-order accuracy, self-starting and implicit for the diffuse chemical reaction kinetics equation or the transient instantaneous temperature filed equation was presented. The examples show that both accuracy and stability are better than Runge-Kutta method with four-order. The coefficients of the equation are stored with sparse matrix pattern, so an algorithm is presented which combines a compact storage scheme with reduced computation cost. The computation of the competitive and consecutive reaction in the rotating packed bed, taken as examples, shows that the method is effective.

Pilot Study on Adjustable Head Fall Mixer with Grids for Rapid Mixing

The performance of adjustable head fall mixer is studied in a pilot experiment. Experimental results indicate that the dispersal efficiency increases with the space between grids and mixing pipe, which could be up to 96%. When the flow rate increases more than 31%, the dispersal efficiency decreases a little but still above 96%. It is demonstrated that special coagulant injection head makes significant Contribution to improving the mixer efficiency. The almost same turbidity of sedimentation effluent could be achieved with less coagulant, thus 30% chemical can be saved. Dissolved oxygen increases 2.5 mg/L in the mixing unit, which will benefit the following processes.

Microwave Induced Ethanol Bath Bonding for PMMA Microfluidic Device

High bonding strength,low deformation and convenient procedure are all very important aspects in the microfluidic device fabrication process. In this paper,an improved microwave induced bonding technology is proposed to fabricate microfluidic device based on methyl methacrylate( PMMA). This method employs pure ethanol as the bonding assisted solvent. The ethanol not only acts as the microwave absorbing material,but also works as the organic solvent in bath. The presented research work has shown that the bonding process can be completed in less than 45 s. Furthermore,the convenient bonding only applies microwave oven,beakers and binder clips. Then,we discuss effects of microwave power,bonding time on bonding strength and deformation of microstructures on PMMA microfluidic device. Finally,a 4 layers micro-mixer has been fabricated using the proposed bonding technique which includes 15 trapezoid micro-channels,9 T-type mix units and an X-type mix unit. Experimental results show that the proposed bonding method have some advantages compared with several traditional bonding technologies,such as hot pressing bonding,ultrasonic bonding and solvent assisted bonding methods in respect of bonding strength,deformation and bonding process. The presented work would be helpful for low coat mass production of multilayer polymer microfluidic devices in lab.

Microfluidic fuel cells integrating slanted groove micro-mixers to terminate growth of depletion boundary layer thickness

Because of potential high energy densities,microfluidic fuel cells can serve as micro-scale power sources.Because microfluidic fuel cells typically operate in the co-laminar flow regime to enable a membrane-less design,they generally suffer from severe mass transfer limitations with respect to diffusion transport.To address this issue,a novel channel design that integrates slanted groove micro-mixers on the side walls of the channel is proposed.Numerical modeling on the design of groove micro-mixers and grooveless design demonstrates a mass transfer enhancement that has a 115%higher limiting current density and well-controlled convective mixing between the oxidant and the fuel streams with the use of slanted groove micro-mixers.Moreover,the growth of the thickness of the depletion boundary layer is found to be terminated within approximately 2 mm from the channel entrance,which is distinct from the constantly growing pattern in the grooveless design.In addition,a simplified mass transfer model capable of modeling the mass transfer prFocess with the presence of the transverse secondary flow is developed.Further,a dimensionless correlation is derived to analyze the effects of the design parameters on the limiting current density.The present theoretical study paves the way towards an optimal design of a microfluidic fuel cell integrating groove micro-mixers.

NUMERICAL EVALUATION OF TWO-FLUID MIXING IN A SWIRL MICRO-MIXER

A collaborative investigation of two-fluid mixing in a swirl micro-mixer was carried out by the Shanghai Jiao Tong University and the Tokyo Denki University. Pure water and a mixture of glycerol and water were separately injected into branch channels and they were subsequently mixed in the central chamber. The two-fluid flow pattern was numerically modeled, in which the dependence of the mixture viscosity and density on the mass fraction of glycerol in the mixing fluid was carefully taken into consideration. The mixing performance of the two fluids was evaluated by varying the Reynolds numbers and the mass fractions of glycerol in water. The mixing process was extensively analyzed using streamline maps and contour plotting distributions of pressure and glycerol concentration. The numerical results show that the acceptable uniformity of mixing at Re = 0.1 is primarily attributed to the time-consuming molecular diffusion, whereas the cost-effective mixing at Re 〉 500 was obtained because of the generation of the swirling flow. The increasing mass fraction of glycerol in water was found to attenuate the mixing performance. The preliminary microscopic visualization of the two-fluid mixing at Re=1300 demonstrated the consistence with the numerical results.

Machine learning for integrating combustion chemistry in numerical simulations

A strategy based on machine learning is discussed to close the gap between the detailed description of combustion chemistry and the numerical simulation of combustion systems.Indeed,the partial differential equations describ-ing chemical kinetics are stiffand involve many degrees of freedom,making their solving in three-dimensional unsteady simulations very challenging.It is discussed in this work how a reduction of the computing cost by an order of magnitude can be achieved using a set of neural networks trained for solving chemistry.The ther-mochemical database used for training is composed of time evolutions of stochastic particles carrying chemical species mass fractions and temperature according to a turbulent micro-mixing problem coupled with complex chemistry.The novelty of the work lies in the decomposition of the thermochemical hyperspace into clusters to facilitate the training of neural networks.This decomposition is performed with the Kmeans algorithm,a local principal component analysis is then applied to every cluster.This new methodology for combustion chemistry reduction is tested under conditions representative of a non-premixed syngas oxy-flame.