Back Mixing Research Articles

A study of particle motion in Kwerk-type twisted blades: Effect of configuration on particle mixing performance by CFD–DEM

Static mixers could promote the particle mixing in industries. Experimental and numerical investigations of particle mixing and flow characteristics in the riser with static mixers are carried out. The instantaneous microscopic characteristics and energy loss in Kwerk-type static mixer (K-KSM) are evaluated by Computational Fluid Dynamics coupled with Discrete Element Method (CFD–DEM). A more suitable calculation method is proposed, which utilizes particle coordinates and the coordination number of central particles to calculate the instantaneous mixing degree of particles (PK). The PK, Residence Time Distribution (RTD) and collision correlation are analyzed at different superficial gas velocities (Ug) and mass flow rates (Ms). The results show that the performance of K-KSM is superior to that of KSM under current conditions. Back mixing and aggregation phenomena are eliminated at Ug > 5 m/s and Ms < 1.73 g/s, and the recommended regimes are Ug ≥ 6.0 m/s and Ms < 1.5 g/s.

Preparation of inorganic salt particles by reactive crystallization in an annular swirling flow reactor

This study investigates the performance of an annular conical swirling flow reactor in liquid–liquid reactions for the preparation of inorganic salt particles. Utilizing Particle Image Velocimetry (PIV) measurements and Computational Fluid Dynamics (CFD) simulations, it was observed that the conical design effectively inhibited the attenuation of swirling flow intensity along the axial direction, with the primary mass transfer region close to the central conical wall. In comparison to a stirred reactor, calcium carbonate particles synthesized in the swirling flow reactor exhibited more uniform morphology, smoother and defect-free surface. Notably, the average particle size remained consistently within 5–6 μm (excluding the 0.3 M feedstock), whereas stirring led to particles with an average size ranging from 8-11 μm. Analysis of the residence time distribution curve revealed minimal back mixing in the swirling flow reactor, ensuring nearly equal growth time for salt particles, thus yielding particles with a uniform size distribution. As a result, the swirling flow reactor presents compelling prospects for applications in reactive crystallization.

Characterizing passive flow in nuclear prismatic modular reactor core channels: Temperature, velocity, and heat transfer analysis

This study aims to advance understanding and visualization of the passive flow inside the core channels of a dual-channel plenum-to-plenum test facility designed as representative geometry of nuclear prismatic modular reactors core. Simultaneous experimental measurements of local surface and coolant fluid temperatures and flow fields in addition to local heat transfer coefficients were provided by integrating sophisticated flow dynamics and heat transfer techniques at different axial locations (Z/L=0.04,0.28,0.41,0.59,0.77,0.96) along the facility heated channel. Results emphasize the significant dependency of thermal and flow fields, of naturally driven gas, on heating intensity and configuration. Interestingly, although the entire length of the channel is subjected to heating, an inflection in surface and gas temperature in addition to changes in velocity distribution were observed through the top section of the heated channel. This highlights the significance of the complexity of the facility which causes severe interactions, and potential recirculation and back mixing, between rising hot gas with the cold gas in the upper plenum. This is validated by the negative heat flux measurements collected at the top section of the channel indicating heat reversal where heat is transferred from air to the channel. Moreover, densitometric Froude number for different heating profiles were estimated and found to range from 0.66 to 0.75 indicating the existence of a jet flow at the hot channel outlet. The obtained results provide valuable benchmark data for validating further investigations by computational fluid dynamics (CFD) and to perform further investigations of thermal hydraulics in prismatic block reactors.

Reliability-based design optimization of screw shaft for continuous high-pressure hydrothermal co-liquefaction process

Hydrothermal co-liquefaction (HTCL) is the prominent process for producing bio-products with a higher conversion rate. It is performed at high temperatures and pressure in the presence of water. Earlier, it was mostly conducted in batch reactors, but it has major limitations including operating volume, back mixing and tedious process for high productivity. With that, the present investigation is performed on designing the screw shaft for the high-pressure HTCL process. The dimensional factors including flight length, pitch, helix angle and depth were considered to design the optimal screw shaft. Likewise, principal stresses, shear stress, bending stress, bending moment and total deformation were regarded as inevitable response variables to analyze the internal strength of the shaft. In this regard, the Taguchi approach provides the L9 (34) orthogonal array as an experimental design. Then, the numerical results from the transient structural analysis were analyzed with the assistance of statistical methods such as Grey Relational Grade (GRG), Grey Fuzzy Reasoning Grade, Analysis of Variance (ANOVA) and Taguchi method to find the most influential dimensions for minimizing the response variable. Consequently, the results from both GRG and Taguchi optimization were compared and selected the most optimum parameters.

Solid Phase Macromixing Study in a Pilot-Scale Geldart Group B Circulating Fluidized Bed Riser Using Single Particle RTD and RPT Measurements

Solid flow in a Geldart’s group B circulating fluidized bed (CFB) riser is complex, and it exhibits backflow and recirculation in the riser. A single radioactive tracer particle is used to measure the overall and sectional residence time distribution in a CFB riser at a gas velocity of 7.6–9.2 m/s and a solid flux of 100–200 kg/m2s. At the same time, radioactive particle tracking (RPT) data are used to measure the trajectories of the tracer particle and its length distribution at the bottom and middle sections of the riser. Both residence time distribution (RTD) and trajectory length distribution data obtained from RPT and RTD experiments are processed and compared. Results show that the bottom section has higher back mixing than the middle section. The results also show that back mixing in both the sections reduces with an increase in the gas inlet velocity and reduces marginally with an increase in the solid flux. Results confirm that RPT and RTD data are highly correlated and can be used with the same accuracy to quantify the macromixing behavior of any process vessel/reactor.

In-situ CdS nanowires on g-C3N4 nanosheet heterojunction construction in 3D-Optofluidic microreactor for the photocatalytic green hydrogen production

Herein, we reported a simple and cost-effective fabrication method to develop an effective corrugated serpentine OFMR (C-SOFMR) with advanced features, such as expansion/contraction and wavy microstructure. A laminar flow with no back mixing was observed in plain serpentine OFMR (P-SOFMR). While, stretching and folding of fluid along with back mixing was observed in C-SOFMR. Further, the CdS nanowires on g-C3N4 nanosheet (CN/CdS) heterojunction was synthesized in situ both P-SOFMR and C-SOFMR and utilized the device for the photocatalytic green hydrogen generation. The CN/CdS heterojunction endowed with narrow band gap energy (2.01 eV). The longer CdS nanowires (∼110 nm) benefit the electronic interface with CN in the CN/CdS heterojunction and lead to the spatial separation (reduced recombination) of excitons along the CdS axial direction. The charges generated were utilized efficiently for the HER reaction in both P-SOFMR and C-SOFMR at higher flow rates attributing to the rapid micro-mixing and mass transfer. The CN/CdS heterojunction showed the highest photocatalytic activity (6.38 μmol h−1 in C-SOFMR and 6.16 μmol h−1 in P-SOFMR at 1.0 mL min−1) due to its good optronic properties. This study is a path forward for the utilization of advanced optofluidic devices to produce green hydrogen directly from solar energy.

Continuous hydrogenation of nitriles to primary amines with high selectivity in flow

Hydrogenation of nitriles is considered to be an atom-economic route in the pharmaceutical industry to synthesize amines, crucial building blocks in the fine chemical industry. However, high selectivity of primary amines and high efficiency are difficult to achieve in conventional batch reactors due to that severe back mixing and poor mass transfer performance producing a mixture of secondary, tertiary amines and hydrogenolysis byproducts. In this study, a continuous flow system based on a micro-packed bed is developed for the hydrogenation of nitriles to primary amines and the hydrogenation of benzonitrile (BN) is selected as the model reaction. With the optimal reaction conditions, maximal conversion of BN (100%) and selectivity to benzylamine (99.1%) are obtained. The reaction kinetics is also determined after the internal and external diffusion limitations are eliminated. Moreover, the continuous flow system based on the micro-packed bed exhibited remarkable substrate suitability in other nitrile hydrogenation systems.

Forming and breaking the ceiling of inlet gas velocity regarding to separation efficiency of cyclone

The maximum-efficiency inlet velocity (MEIV) is a ceiling of inlet gas velocity that defines separation efficiency during cyclone design and operation. Experiment and computational fluid dynamics (CFD) simulation exhibited that an apex cone at the dust outlet can break the ceiling and improve the separation efficiency. The phenomenon is closely related to the effect of excessive high inlet gas velocity on the back-mixing escape of fine particles, which is the final result of back mixing, entrainment by the rapid upward airflow, and secondary separation of the inner vortex. In the center of the inner vortex, the airflow rotates slowly and moves rapidly upward. This elevator type of airflow delivers re-entrained particles to the vortex finder. A higher inlet gas velocity accelerates the elevator, causing more entrained particles to escape. This explains the decrease in efficiency at an excessively high inlet gas velocity. When an apex cone is installed at the dust outlet, the back-mixing is significantly weakened because the vortex core is bounded to the center of separator, while the transport effect of rapid upward airflow is weakened by the decrease in axial velocity in the center. Therefore, particle escape is weakened even at excessive high inlet gas velocities. Instead, the centrifugal effect is enhanced because of increased tangential velocity of the gas and particles. Consequently, the ceiling of inlet gas velocity is broken.

Insights into the effect mechanism of back-mixing inoculation on sewage sludge biodrying process: Biodrying characteristics and microbial community succession

Back mixing was frequently used to replace conventional bulking agenting, however, however, the internal effect mechanism was unclear. This study compared four bulking agents: mushroom residue (MR), MR + primary BM (BM-P), BM-P, and secondary BM (BM-S). The effect mechanism of back mixing (BM) inoculation was assessed based on biodrying performance and microbial community succession. Four trials (Trial A, Trial B, Trial C, and Trial D) reached maximum temperatures of 61.9, 68.8, 73.7, and 69.9 °C on days 6, 3, 2, and 2, respectively. Application of BM increased pile warming rate and resulted in higher temperatures. Temperature changes and microbial competition lead to decline in microbial diversity and richness during the biodrying process. Microbial diversity increased of four biodried products. The number of microorganisms shared by Trial A, Trial B, Trial C, and Trial D were 90, 119, 224, and 300, respectively. The addition of BM improved microbial community stability, and facilitating the initiation of biodrying process. Microbial genera that played an important role in the biodrying process included Ureibacillus, Bacillus, Sphaerobacter, and Tepidimicrobium. Based on these results, it was concluded that BM was efficient method to enhanced the microbial activity and reduced the usage of bulking agent.

Modeling of slurry-type photocatalytic reactors containing core-shell particles for predicting transient behaviours based on Langmuir-Hinshelwood kinetics

Governing equations were derived for photocatalytic batch reactors and CSTR (Continuously Stirred Tank Reactor) containing photocatalytic pellets to obtain numerical solutions of the concentration in bulk medium and intra-particle concentration based on Langmuir-Hinshelwood kinetics. Numerical solutions could be obtained by finite element method under proper initial and boundary conditions to solve coupled reaction-diffusion equations using partial and ordinary differential equations. Effects of Thiele modulus, Biot number, Langmuir—Hinshelwood parameter and photocatalyst loading were investigated by predicting the performance of batch reactor. Effects of residence time and initial reactant concentration were studied for CSTR. In addition to conventional morphologies of pellets such as sphere, cylinder and slab, mathematical modeling of core-shell particles was also conducted to study the effect of thickness of inert core. Although the performance of spherical pellets was the best, cylindrical pellets also showed good activity for removing reactant by photocatalytic decomposition. In core-shell spherical pellets, there was little effect of thickness of inert core on the reduction rate of reactant concentration and steady-state concentration in batch and CSTR, respectively, compared to core-shell cylindrical and slab-type particles. The cascade connection of photocatalytic CSTRs and fixed bed reactors with back mixing could be also analysed by finite element method. Experimental data about the removal of methylene blue using silica-titania core-shell particles in batch photocatalytic reactor were compared with numerical solutions to estimate reaction parameters.

Liquid-phase mass-transfer processes in packings with a segregated flow model

Liquid back mixing behavior has significant influences on the liquid-side mass-transfer coefficient in packed columns and rotating packed beds(RPBs). And the liquid-side mass-transfer coefficient without the liquid back mixing behavior is called a liquid-side intrinsic mass-transfer coefficient. A ratio of the liquid-side mass-transfer coefficient to the liquid-side intrinsic mass-transfer coefficient is defined as a liquid-side mass-transfer coefficient factor. The lower coefficient factor means the bigger reduction in the liquid-side mass-transfer coefficient. A segregated flow model in packings is developed. From the model, the liquid back mixing effect is first separated from the liquid-side mass-transfer coefficient and is then involved in the coefficient factor, and equations of the liquid-side intrinsic mass-transfer coefficient and coefficient factor are derived. Values of the liquid-side intrinsic mass-transfer coefficient and coefficient factor of oxygen desorption from water in Flexipac 2 structure packing in a packed column were calculated. The values of the intrinsic mass-transfer coefficient were in good agreement of a deviation of 0.58 % with the predictions by theoretical mass-transfer model. Values of the coefficient factor of oxygen desorption from water in a RPB with wire mesh packing were also obtained. Values of the coefficient factor in the RPB rotor were much lower than those in the packed column due to a highly intensified mass-transfer and a higher liquid back mixing in the RPB rotor as compared with those in the packed column. The best method to increase the coefficient factor is to reduce the liquid back mixing and to increase the liquid flow rates in packed columns and RPBs.

Correlation of microbial dynamics to odor production and emission in full-scale sewage sludge composting

Odor is inevitably produced during sewage sludge composting, and the subsequent pollution hinders the further development of composting technologies. Third-generation high-throughput sequencing was used to analyze microbial community succession, and the correlations between odor and microbial communities were evaluated. Hydrogen sulfide (47.5–87.9 %) and ammonia (9.4–49.9 %) contributed majorly to odor emissions, accounting for 93.7–98.5 % of the emissions. Volatile sulfur compounds were mainly produced in the mesophilic and pre-thermophilic phases (43.0–83.4 %), whereas ammonia was mainly produced in the thermophilic phase (52.1–59.4 %). Microorganisms dominant in the mesophilic and thermophilic phases correlated positively with odor production in the following order: Rhodocyclaceae > Clostridiaceae_1 > Hyphomicrobiaceae > Acidimicrobiales > Family_XI, whereas those dominant in the cooling phase showed negative correlations with odor production in the following order: Bacillus > Sphingobacteriaceae > Pseudomonadaceae > DSSF69 > Chitinophagaceae. The back mixing of mature compost is expected to serve as an economical measure for controlling odor during sewage sludge composting.

Simulation and Experimental Investigation of the Radial Groove Effect on Slurry Flow in Oxide Chemical Mechanical Polishing

Slurry flow on the pad surface and its effects on oxide chemical mechanical polishing (CMP) performance were investigated in simulations and experiments. A concentric groove pad and the same pad with radial grooves were used to quantitatively compare the slurry saturation time (SST), material removal rate (MRR), and non-uniformity (NU) in polishing. The monitored coefficient of friction (COF) and its slope were analyzed and used to determine SSTs of 25.52 s for the concentric groove pad and 16.06 s for a certain radial groove pad. These values were well correlated with the simulation prediction, with around 5% error. Both the laminar flow and turbulent flow were included in the sliding mesh model. The back mixing effect, which delays fresh slurry supply, was found in the pressure distribution of the wafer–pad interface.

Improvement of an annular thin film UV-C reactor by fluid guiding elements

In this paper special elements are inserted into an annular thin film UVC reactor to guide the laminar flow. These “Fluid Guiding Elements” (FGE) divide the flow into three separated laminar currents that are alternatingly directed through a small gap of 0.6 mm facing the UV source by turns. Computational fluid dynamic simulations have been used to visualize the flow dynamics inside the reactor.The residence time distribution and the Bodenstein number show an increase of back mixing in the reactor with the elements. An Iodide/iodate actinometry comparison has shown a better dose distribution throughout the treated volume based on the FGE. The inactivation of E. coli DH5α as an exemplary organism could be improved by more than 4 log levels after 5 passes through the reactor using the FGE independent of the flow rate. The absorbance of a model solution was varied by different dye concentrations and the use of FGE improve the inactivation of Saccharomyces cerevisiae. Industrial relevanceThe stabilization of liquid foods before bottling is a crucial aspect. The consumers often do not accept a chemical additive like sulphites in wine or dimethyl carbonate in juices. A physical alternative is the UV-C Treatment. In contrast to a turbulent flow a laminar flow has a far lower pressure drop. Therefore, a lower amount of energy is needed to pump the liquid. This study shows that with fluid guiding elements a sufficient inactivation can be achieved with laminar flow.

Effects of Inlet Types and Lengths on the Flow Field of Cyclone Separators

The effect of inlet type and length on the flow field was considered computationally for seven cyclone separators. The turbulent model was described by the Reynolds Stress Model (RSM). The air-water interface in underflow pipe and the spatial distribution of particles were tracked by the Volume of Fluid (VOF) model and Discrete Phase Model (DPM), respectively. Comparison investigations showed that inlet type and length had important impacts on flow field of cyclone. The instability flow field and back mixing phenomena were eliminated in symmetric double-inlet cyclone. The turbulent dissipation is obvious with a short inlet length. When it increased to 1.25D/2, the area and intensity of the turbulent dissipation tended to be stable. The optimum cyclone is the symmetric double-inlet with inlet length of 1.25D/2. When the particle diameter was larger than 5 μm, the complete separation could be realized.

An experimental and numerical study on the modelling of fluid flow, heat transfer and solidification in a copper continuous strip casting process

An experimental and “numerical study (first part of this study study) was carried out to investigate the solidification process in a copper continuous strip casting process. The model has been tuned by experimental results (i.e. cooling water flow measurements, temperature measurements and metallographic analysis). Further, the results have been used to study the possibility of improved productivity. In this report (second of the study) the flow pattern of the molten copper during a strip casting process as a manufacturing method has been studied using a full-scale water model. The dynamic similarity between model and real system has been studied. Six different types of inlet system to the mould have been studied: inlet nozzle jets with free stream, submerged nozzle jets, slot-submerged inlet system, semi slot-submerged inlet system, submerged-slot inlet nozzle jets and finally submerged-slot inlet nozzle jets with jet killer. Moreover, the effects of nozzle angle, nozzle diameter, casting speed, tundish adjustment and misalignment of the inlet nozzle jets on the flow pattern have been investigated. The vortex formation and bubble entrainment, depending upon the nozzle configuration, immersion depth and the fluid level in the mould have also been studied. It was found that the slot-submerged inlet nozzle jets with jet killer arrangement showed an obvious improvement of the fluid flow characteristics, yielding better tracer distribution in the flow pattern, lower values of back mixing flow, lower turbulence and lower vortex and recirculation flow.

Investigation of Flow of the Disc-and-Doughnut Baffles and 40% Cut Segmental Baffles

Heat exchanger is usually used in manufacturing process. At present, many researchers have efforts to increase the performance of the heat exchanger with less of the cost. This research discussed about the performance of heat exchanger using 40% cut segmental baffles compared with modified double segmental baffles disc-and-doughnut type. In this study, the investigation of the computational results consisted of heat flux, velocity profile along the heat exchanger, pressure distribution and, theoretical heat transfer coefficient and heat exchanger effectiveness. The model was calculated using finite difference method forward modeling with Multiphysics Software and focuses on the performance evaluation of the small shell-and-tube heat exchanger (STHE) – laboratory type. The tubes are composed of 14 tubes with 0.583 m length, triangular 30° rotated pitch. The pipe radius of shell and tube are 0.055 m and 0.00635 m, respectively. The baffle radius of disc and doughnut are 0.055 m and 0.025 m, respectively, and the baffle radius of 40% cut segmental baffles are 0.055 m. Both types of the baffle have a distance of 0.116 m which is evenly distributed along the shell. The generalized minimal residual (GMRES) method for the fluid flow case used as an iterative method for solving some of the complex linear equations shows good performance as in reliability and validity. For the 40% cut segmental baffles, fluid flow makes a zigzag pattern, an Eddy or swirling of a fluid, and there was some back mixing of fluid stream which caused several dead zones along the shell. The occurrence of the dead zones caused the heat transfer to be ineffective and gave lower value compared to the double segmental disc-and-doughnut baffles. The 40% cut segmental baffles was also seen to have a higher pressure at the outlet region than the double segmental disc-and-doughnut baffles. The disc-and-doughnut baffles leads to a turbulent fluid flow which causes an increase in heat transfer characteristics and also lower pressure drop than the 40% cut segmental baffles. Based on the theoretical, both types of disc-and-doughnut baffles and the 40% cut segmental baffles of heat exchanger investigated have highest effectiveness at the lowest mass flow rate of the hot fluids (tube).

Computational study of particle distribution development in a cold-flow laboratory scale downer reactor

The use of downer reactors (gas-solid co-current downward flow) in the Fluid Catalytic cracking (FCC) process for the upgrading of heavy crude oil into more valuable products has gradually become more common in the last decades. This kind of reactor is characterized by having homogeneous axial and radial flow structures, no back mixing, and shorter residence times as compared with the riser reactor type. Although downer reactors were introduced a long time ago, available information in literature about the multiphase hydrodynamic behavior at FCC industrial scale is scarce. Therefore, it is necessary to conduct experimental and computational studies to enhance the understanding of the hydrodynamics of two-phase co-current downward flow. The Computational Fluids Dynamics (CFD) software, Ansys Fluent, is used to study two-dimensional gas (air) and solid (catalyst particle) flow in a downer section of a cold-flow circulation fluidized bed (CFB) system at a laboratory scale. The implemented computational model is validated by comparing numerical results for solid velocity and volume fraction with measurements carried out on a CFB system using a fiber-optic probe laser velocimeter. According to numerical results obtained for different gas velocity and solid flux, flow development cannot only be estimated by considering solid axial velocity changes along the reactor; it is also necessary to take into account solid volume fraction axial variations as radial profiles can change even when velocity profiles are developed.

Study on velocity field analysis and back-mixing of fluid in reactor based by simulation

During the operation of the reactor, the degree of back mixing directly reflects the concentration distribution, temperature distribution, reaction process of the fluid in the reactor, and so on, thereby affecting the practical value of the reactor. Therefore, this article uses computer simulation to analyze the influence of inlet flow rate on the degree of back mixing. It can provide data support for the preparation of organic semiconductor materials under different reaction conditions. The research results show that when the inlet flow velocity reaches 0.3 m/s, the stirring effect of the fluid weakens, the jetting effect of the inlet pipe is dominant, and the model parameters are the largest.

Potential of anammox process towards high-efficient nitrogen removal in wastewater treatment: Theoretical analysis and practical case with a SBR

Anammox process has been widely applied in the wastewater nitrogen removal for its high rate and low cost. However, few researches reported the process potential in treating low-strength nitrogen wastewater. In this study, a sequencing batch reactor (SBR) was taken to explore the feasibility of low-strength nitrogen wastewater treatment by anammox process in theory and practice. The practical operation indicated that the effluent with satisfactory quality (1.90 ± 0.70 mg-TN·L−1) could be achieved, when the SBR was fed with low-strength nitrogen influent (6.20 ± 0.45 mg-NH4+-N·L−1 and 7.96 ± 0.59 mg-NO2--N·L−1). The hydraulic retention time (HRT), nitrogen removal efficiency, nitrogen removal rate (NRR) and hydraulic loading rate of SBR were 5.42 h, 86.5%, 0.054 kg-N·m−3·d−1 and 4.43 m3·m−3·d−1 during the 79-day operation, respectively. The theoretical analysis revealed the potential of anammox SBR. When SBR is stably operated, the maximum NRR would be 0.062 kg-N·m−3·d−1 if the effluent nitrogen was required to be as low as 3 mg·L−1. The NRR value is feasible for engineering. However, considering the lower specific substrates utilization rate in practice, the maximum stable NRR was calibrated and found inefficient afterwards. In order to improve the potential of anammox process, the reactors without back mixing and with periodic bioaugmentation should be taken in priority for the engineering applications. In particular, the bioaugmentation frequency and single addition amount were calculated as 7 d and 0.3 g-VSS·L−1, respectively. The results may provide guidance for the development of high-efficient and stable nitrogen removal process under low-strength condition.