Using electrical resistance tomography and computational fluid dynamics modeling to study the formation of cavern in the mixing of pseudoplastic fluids possessing yield stress

Electrical resistance tomography (ERT) provides a non-intrusive technique to examine, in three dimensions, the homogeneity and flow pattern inside the mixing tank. In this study, a 4-plane 16-sensor ring ERT system was employed to study the shape and the size of cavern generated around a radial-flow Scaba 6SRGT impeller in the mixing of xanthan gum solution, which is a pseudoplastic fluid possessing yield stress. The size of cavern measured using ERT was in good agreement with that calculated using Elson's model (cylindrical model). The 3D flow field generated by the impeller in the agitation of xanthan gum was also simulated using the commercial computational fluid dynamics (CFD) package (Fluent). The CFD model provided useful information regarding the impeller pumping capacity, flow pattern, and the formation of cavern around the impeller. CFD results showed good agreement with the experimental data and theory.     

Characterisation of the mixing of non‐newtonian fluids with a scaba 6SRGT impeller through ert and CFD

An attempt has been made to study the mixing of yield‐pseudoplastic fluids with a Scaba 6SRGT impeller using electrical resistance tomography (ERT) and computational fluid dynamics (CFD). The ERT system with four sensor planes, each containing 16 equispaced stainless steel electrodes, was used to measure the mixing time. The multiple reference frames (MRF) technique and the modified Herschel–Bulkley model were applied to simulate the impeller rotation and the rheological behaviour of the non‐Newtonian fluids, respectively. To validate the model, the CFD results for the power consumption were compared to the experimental data. The validated model was then employed to obtain further information regarding the averaged impeller shear rate, impeller circulation, and pumping capacities. The CFD and ERT data were utilised to investigate the effect of the impeller power, fluid rheology, and impeller size on the mixing time. The mixing time results obtained in this study were in good agreement with those reported in the literature. © 2011 Canadian Society for Chemical Engineering     

Improving the dynamic performance of continuous-flow mixing of pseudoplastic fluids possessing yield stress using Maxblend impeller

The strategic approach of this article is to characterize the continuous-flow mixing of pseudoplastic fluids possessing yield stress in a stirred reactor with the Maxblend impeller. Dynamic experiments were carried out through the frequency-modulated random binary input of a brine solution to determine the extent of non-ideal flows. Mixing quality was determined on the basis of the extent of channeling and fully mixed volume. The effects of important parameters such as impeller speed (25–500 rpm), absence of baffles, fluid rheology (0.5–1.5%), fluid flow rate (3.20–14.17 L min⁻¹), and the locations of inlet/outlet on the dynamic performance of the continuous-flow mixing vessel were explored. The performance of the Maxblend impeller was then compared to the performances of various types of impellers such as close-clearance (an anchor), axial-flow (a Lightnin A320), and radial-flow (a Scaba 6SRGT) impellers. It was found when the channeling approached zero and the fully mixed volume approached the total fluid volume in the vessel, the power drawn by the A320 impeller and the Scaba impeller were about 2.9 and 4.3 times greater than that of the Maxblend impeller. Thus, the Maxblend impeller was able to drastically improve the performance of continuous-flow mixing with huge power savings. The mixing quality was further improved by optimizing the impeller speed, decreasing the fluid flow rate, decreasing the fluid concentration, and using bottom inlet- top outlet configuration. The flow non-ideality of the mixing system increased in the absence of the baffles. Thus, better mixing quality and more energy savings can be achieved by employing the findings of this study.     

Design of multiple impeller stirred tanks for the mixing of highly viscous fluids using CFD

The effect of multiple Intermig impeller configuration on hydrodynamics and mixing performance in a stirred tank has been investigated using computational fluid dynamics. Connection between impeller stages and compartmentalisation has been assessed using Lagrangian particle tracking. The results show that by a rotating the Intermig impeller by 45^° with respect to its neighbours, instead of a 90^° rotation as recommended by manufacturers, enables a wider range of operating conditions, i.e., lower Reynolds number flows, can be handled. Furthermore by slightly decreasing the distance between the lower two impellers, fluid exchange between the impellers is ensured down to Re=27.     

CFD Investigation of the Mixing of Yield‐Pseudoplastic Fluids with Anchor Impellers

The study was carried out to simulate the 3D flow domain in the mixing of pseudoplastic fluids possessing yield stress with anchor impellers, using a computational fluid dynamics (CFD) package. The multiple reference frames (MRF) technique was employed to model the rotation of the impellers. The rheology of the fluid was approximated using the Herschel–Bulkley model. To validate the model, the CFD results for the power consumption were compared to the experimental data. After the flow fields were calculated, the simulations for tracer homogenization were performed to simulate the mixing time. The effects of impeller speed, fluid rheology, and impeller geometry on power consumption, mixing time, and flow pattern were explored. The optimum values of c/D (impeller clearance to tank diameter) and w/D (impeller blade width to tank diameter) ratios were determined on the basis of minimum mixing time.     

Chaotic characteristics of pseudoplastic fluid induced by 6PBT impeller in a stirred vessel

Xanthan gum solutions with different mass concentrations were used to study the chaotic characteristics induced by the impeller of perturbed six-bent-blade turbine(6 PBT) in a stirred vessel. Based on the velocity time series obtained by the experiment of particle image velocimetry(PIV), with the software MATLAB(R2016a), the distributions of the largest Lyapunov exponent(LLE) and Kolmogorov entropy(K entropy) of the system, as two important parameters for characterizing the chaotic degree, were investigated respectively. Results showed that both of the LLE and K entropy increased with the increasing speed at the beginning. As the speed was up to 200 r·min-1, the two parameters reached the maximal values meanwhile, corresponding to 0.535 and 0.834, respectively, which indicated that the chaotic degree of the flow field was up to the highest level. When the speed was increased further, both of the LLE and K entropy decreased on the contrary, which meant that the chaotic degree was decreasing. It was also observed that the chaotic characteristics of flow field were hardly affected by the fluid rheology and the detecting positions. The research results will enhance the understanding of the chaotic mixing mechanism and provide a theoretical reference for optimizing impeller structure.     

Characterization on the hydrodynamics of a covering-plate Rushton impeller

A modified Rushton impeller with two circular covering-plates mounted on the upper and lower sides of the blades was designed. There are gaps between the plates and the blades. The turbulent hydrodynamics was analyzed by the computational fluid dynamics (CFD) method. Firstly, the reliability of the numerical model and simulation method was verified by comparing with the experimental results from literature. Subsequently, the power consumption, flow pattern, mean velocity and mixing time of the covering-plate Rushton impeller (RT-C) were studied and compared with the standard Rushton impeller (RT) operated under the same conditions. Results show that the power consumption can be decreased about 18%. Compared with the almost unchanged flow field in the lower stirred tank, the mean velocity was increased at the upper half of the stirred tank. And in the impeller region, the mean axial and radial velocities were increased, the mean tangential velocity was decreased. In addition, the average mixing time of RT-C was shortened about 4.14% than the counterpart of RT. The conclusions obtained here indicated that RT-C has a more effective mixing performance and it can be used as an alternative of RT in the process industries.     

Laminar mixing of shear thinning fluids in a SMX static mixer

Flow and mixing of power-law fluids in a standard SMX static mixer were simulated using computational fluid dynamics (CFD). Results showed that shear thinning reduces the ratio of pressure drop in the static mixer to pressure drop in empty tube as compared to Newtonian fluids. The correlations for pressure drop and friction factor were obtained at Re_MR?100. The friction factor is a function of both Reynolds number and power-law index. A proper apparent strain rate, area-weighted average strain rate on the solid surface in mixing section, was proposed to calculate pressure drop for a non-Newtonian fluid. Particle tracking showed that shear thinning fluids exhibit better mixing quality, lower pressure drop and higher mixing efficiency as compared to a Newtonian fluid in the SMX static mixer.     

计算流体力学在化学工程中的应用

计算流体力学(CFD)用于求解固定几何形状设备内的流体的动量、热量和质量方程以及相关的其他方程,已成为研究化工领域中流体流动和传质的重要工具。本文概述了CFD的基本原理以及CFD在化学工程领域方面的应用,重点介绍了CFD在搅拌槽、换热器、蒸馏塔、薄膜蒸发器、燃烧等方面的应用。     

Analysis of shear-induced coagulation in an emulsion polymerisation reactor using computational fluid dynamics

Shear-dependent coagulation is a costly problem for the latex manufacturing industry, due to product degradation and reactor downtime. In this study, a method for calculating the shear-dependent coagulation rate in emulsion polymerisation is developed. The method combines simple models for coagulation (only binary collisions being considered) with the effects of rheology on the flow field, using computational fluid dynamics (CFD) to solve the detailed flow field in the reaction vessel. By using the local shear rates (LSR), the method developed provides a more detailed and system-specific assessment compared with using an average shear rate (ASR) for calculating the coagulation rate. The difference in the predictions between the ASR and the proposed LSR method was investigated. It was found that the ASR and LSR methods predict different coagulation rates, especially for more sophisticated coagulation models where the coagulation rate is not linearly dependent on the shear rate. The LSR method was also used to study the effect of the rheology of the latex, of the impeller speed and of the reactor design on the coagulation rate. It was found that the LSR method is useful for providing both visual and numerical means to identify regions with elevated coagulation rates in the modelled reaction vessel. The treatment provides estimates of the amounts of coagulum formed on the vessel walls and on the impeller.     

Effect of pitched short blades on the flow characteristics in a stirred tank with long-short blades impeller

This work focuses on the design improvement of the long-short blades (LSB) impeller by using pitched short blades (SBs) to regulate the flow field in the stirred vessel. After mesh size evaluation and velocity field validation by the particle image velocimetry, large eddy simulation method coupled with sliding mesh approach was used to study the effect of the pitched SBs on the flow characteristics. We changed the inclined angles of the SBs from 30° to 60° and compared the flow characteristics when the impeller was operated in the down-pumping and up-pumping modes. In the case of down-pumping mode, the power number is relatively smaller and vortexes below the SBs are suppressed, leading to turbulence intensification in the bottom of the vessel. Whereas in the case of up-pumping mode, the axial flow rate in the center increased significantly with bigger power number, resulting in more efficient mass exchange between the axial and radial flows in the whole vessel. The LSB with 45° inclined angle of the SBs in the up-pumping mode has the most uniform distributions of flow field and turbulent kinetic energy compared with other impeller configurations.     

Numerical and experimental investigations of liquid mixing in two-stage micro-impinging stream reactors

A two-stage micro-impinging stream reactor(TS-MISR) that combined a first pre-mixing stage with a second micro-impinging stream reacting stage for continuous multi-component reacting systems has been built from commercial T-junctions and steel micro-capillaries. Both of operating parameters and reactor configurations,such as jet Reynolds number(Rej), volumetric flow ratio(R), the first-stage junction angle(φ), the connecting capillary length(Lc) and connecting capillary diameter(dc), had significant effects on the micromixing efficiency of the reactor. Such effects were investigated for both of the two stage structures, respectively, by experimental and CFD methods and were optimized for the best micromixing performance. Intensified micromixing among at least three reacting components can be achieved in a continuous mode by using TS-MISR; therefore, it is expected that the TS-MISR will produce products of higher quality with more uniform and stable element distribution.     

Effect of swirling addition on the liquid mixing performance in a T-jets mixer

Swirling addition to the stream is beneficial for the fluid mixing. This work aims to study the mixing process intensification in a conventional T-jets mixer by the swirling addition. After experimental verification by the planar laser-induced fluorescence technique, large eddy simulation with the dynamic kinetic energy sub-grid stress model is used to predict how the swirling strength (in terms of swirling number, S_w) and swirling directions affect the mixing performance, e.g. the tracer concentration distribution, mixing time, and turbulent characteristics in the T-jets mixers. Predictions show that the swirling strength is the key factor affecting the mixing efficiency of the process. The overall mixing time, τ₉₀, can be significantly reduced by increasing S_w. Vortex analysis shows that more turbulent eddies appear in the collision zone and the turbulent kinetic energy dissipation rate increases obviously with the swirling addition. When S_w is kept constant, the mixing process can be accelerated and intensified by adding swirling to only one stream, to both streams with the opposite swirling directions, or to both streams with the same swirling directions. Amplification of the mixing process by enlarging the mixer size or increasing the flow rates is also optimized. Thus, this work provides a new strategy to improve the mixing performance of the traditional T-jets mixers by the swirling addition.     

Numerical simulations of the reactive mixing in a commercially operated stirred ethoxylation reactor

The computational fluid dynamics (CFD) technique was applied to describe the mixing and the chemical reactions in commercially operated stirred ethoxylation reactors. Two reactor sizes in the existing ethoxylation operations were studied in this work: a laboratory-scale autoclave with a single-Rushton turbine and an industrial-scale reactor with a dual-Rushton turbine. The ethoxylation reactor contents were described as an incompressible, turbulent single-phase liquid mixing regime with chemical species undergoing heat and mass transfer. Since the accurate experimental ethoxylation flow data could not be extracted from the industrial operations, the development of the CFD model for the ethoxylation process was undertaken in two stages. The first stage simulated a single-phase liquid agitation system based on the literature with experimental data on velocities, such as Wu and Patterson [1989. Laser-Doppler measurements of turbulent-flow parameters in a stirred mixer. Chemical Engineering Science 44, 2207–2221], for a Rushton stirred reactor of standard configuration. Once validated, the numerical model was applied to compute the flow field in ethoxylation reactors. The second stage integrated the ethoxylation kinetics into the numerical model and simulated the ethoxylation process. In the simulation of the mean flow field, the qualitative features of the literature data were well reproduced. The computed results of both the ethylene oxide consumption and the temperature calculation compared very well with the measurements in the laboratory-autoclave operations. Reasonably good agreement was also reached between the simulated and experimental data on the time-dependent changes of ethylene oxide mass fraction in the bulk liquid in the industrial ethoxylation operations. These demonstrate that the CFD process model was capable of predicting the reaction behaviour and would be useful for exploration of any opportunity for increasing the ethoxylation capacity in the industrial operations.     

Computational fluid dynamics simulation of a novel bioreactor for sophorolipid production

This paper describes three-dimensional computational fluid dynamics(CFD) simulations of gas–liquid flow in a novel laboratory-scale bioreactor contained dual ventilation-pipe and double sieve-plate bioreactor(DVDSB)used for sophorolipid(SL) production. To evaluate the role of hydrodynamics in reactor design, the comparisons between conventional fed-batch fermenter and DVDSB on the hydrodynamic behavior are predicted by the CFD methods. Important hydrodynamic parameters of the gas–liquid two-phase system such as the liquid phase velocity field, turbulent kinetic energy and volume-averaged overall and time-averaged local gas holdups were simulated and analyzed in detail. The numerical results were also validated by experimental measurements of overall gas holdups. The yield of sophorolipids was significantly improved to 484 g·L~(-1)with a 320 h fermentation period in the new reactor.     

Comparison of various radiation-cooled dew condensers using computational fluid dynamics

Radiation-cooled dew water condensers can serve as a complementary potable water source. In order to enhance passive dew collection water yield, a Computational Fluid Dynamics (CFD) software, PHOENICS, was used to simulate several innovative condenser structures. The sky radiation is calculated for each of the geometries. Several types of condensers under typical meteorological conditions were investigated using their average radiating surface temperature. The simulations were compared with dew yield measurements from a 1 m² 30°-inclined planar condenser used as a reference. A robust correlation between the condenser cooling ability and the corresponding dew yield was found. The following four shapes were studied: (1) a 7.3 m² funnel shape, whose best performance is for a cone half-angle of 60°. Compared to the reference condenser, the cooling efficiency improved by 40%, (2) 0.16 m² flat planar condenser (another dew standard), giving a 35% lower efficiency than the 30° 1 m² inclined reference condenser, (3) a 30 m² 30°-inclined planar condenser (representing one side of a dew condensing roof), whose yield is the same as the reference collector, and (4) a 255 m² multi-ridge condenser at the ground surface provided results similar to the reference collector at wind speeds below 1.5 m s^− 1 but about 40% higher yields at wind speeds above 1.5 m s^− 1.     

CFD modeling of gas dispersion and bubble size in a double turbine stirred tank

In the present paper, gas dispersion in a double turbine baffled stirred tank is modeled using a commercial computational fluid dynamics (CFD) code FLUENT 6.1 (Fluent Inc., USA). A bubble number density equation is implemented in order to account for the combined effect of bubble break-up and coalescence in the tank. In the proposed work, the impellers are explicitly described in three dimensions using multiple reference frame model. Dispersed gas and bubbles dynamics in the turbulent water are modeled using an Eulerian-Eulerian approach with dispersed k-ε turbulent model and modified standard drag coefficient for the momentum exchange. The model predicts spatial distribution of gas holdup, average local bubble size and flow structure. The results are compared with experimental and numerical finding reported in the literature and good agreement between the present model and measurements of Alves et al. [Gas liquid mass transfer coefficient in stirred tanks interpreted through bubble contamination kinetics. Chemical Engineering Science, 2002, 57, 487-496] is achieved.     

In situ adaptive tabulation (ISAT) to accelerate transient computational fluid dynamics with complex heterogeneous chemical kinetics

This paper extends the in situ adaptive tabulation (ISAT) algorithm for accelerating the simulation of complex heterogeneous chemical kinetics within transient, three-dimensional, computational fluid dynamics (CFD). The ISAT algorithm, initially developed for homogeneous combustion kinetics, takes advantage of the fact that initial conditions for the chemistry in a particular cell (i.e., temperature and composition) may have been present in this cell or another cell earlier in the simulation. In such cases, the solution can be extracted from a tabulation of prior solutions more efficiently than solving the local kinetics problem. The ISAT algorithm uses efficient tabulation and retrieval algorithms, greatly accelerating the solution process. Illustrative results are based on the simulation of methane reforming in a catalytic microchannel reactor, considering coupled fluid mechanics, catalytic chemistry, and conjugate heat transfer.     

Application of direct fluid flow oscillations to improve mixing in microbioreactors

This article describes an active mixing method for a microbioreactor that was designed, simulated, tested, and successfully implemented. By applying a varying pressure to a microchannel looping tangentially into a cylindrical microreactor an oscillating fluid flow was shown to occur. Such an oscillating fluid flow improved mixing, both by diffusion and convection. The oscillating fluid flow has a large impact on the ratio between the diffusion domain and the convection domain. A good match was obtained between experimental mixing results, computational fluid dynamics simulation results and the results of a simplified mixing model thus demonstrating the potential of simulation on improving the design of microreactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

CFD modeling of mixing intensification assisted with ultrasound wave in a T-type microreactor

This paper aims to demonstrate the effect of ultrasound wave on mixing in a T-type microreactor. In order to create vibration in this microreactor, a low frequency (42 kHz) piezoelectric transducer was used. A well-known parallel-competitive reaction (Villermaux–Dushman reaction) was employed to study the mixing in the microreactor and the segregation index values were found for layouts with and without sonication. Results show that the ultrasound waves have a significant favorable influence on product distribution and the segregation index at various total flow rates. In all cases, the segregation index decreased with increase in total flow rate. The results reveal that the segregation index improved up to 10–20% by consuming a low energy (2.45 W Kg⁻¹) by the piezoelectric transducer. Finally, the computational fluid dynamics (CFD) modeling was carried out to explain the observed experimental results.