Unbaffled Tank Research Articles

CFD simulations of the transition between non-aerated and aerated conditions in uncovered unbaffled stirred tanks

The transition between non-aerated and aerated regimes in uncovered unbaffled stirred tanks (UUSTs) was investigated using computational fluid dynamics (CFD). The Volume of Fluid (VOF) method was employed to model the free surface dynamics under various operational conditions. The simulations were able to predict the peculiar experimentally observed behavior of UUSTs and revealed that at velocities below a critical threshold (N_<_Ncrit), the system remains non-aerated, while exceeding this threshold (N_≥_Ncrit) induces bubble ingestion, leading to significant changes in power consumption and flow patterns. The CFD simulations accurately predicted the behaviour of the Power Number (Np) as well as the vortex shape inside the tank both in subcritical and supercritical regimes and showed good agreement with original experimental data and correlations from the literature. Additionally, the modeling of the aerated regime successfully predicted the vortex shape, the bubble dispersion within the tank, and the cavities formed behind the blades.

Numerical Investigation of the Fuel Tank Sloshing Condition of a Commercial Vehicle

This study investigates the impact of baffles on fuel sloshing behavior within truck fuel tanks using numerical simulations. The volume of the fluid multiphase model is employed to analyze the flow dynamics of 25% diesel fuel in a 250 L tank, modeled in a 3D domain. Two configurations were compared: a tank with baffles and one without. The primary focus is to analyze fuel distribution within the intake port region during vehicle acceleration and deceleration maneuvers. The simulated scenario mimics a realistic driving situation. The vehicle accelerates from 0 km/h to 60 km/h over 10 s, followed by a 3-s braking period to reach a complete stop (0 km/h) at the 13-s mark. The simulation then observes the fuel behavior within the tank for an addi-tional 7 s while the vehicle remains stationary. Results reveal significant differences in fuel behavior between baffled and unbaffled tanks. In the absence of baffles, the sloshing motion is substantial, leading to a complete depletion of fuel in the intake port region for a duration of 3 s during both the acceleration and deceleration phases (between 10 and 13 s). Compared to a standard tank, the presence of baffles significantly reduced the sloshing amplitude by approximately 70%. Furthermore, baffles led to a 50% decrease in pressure variations on the tank walls. Temporary fuel starvation can negatively impact engine performance and combustion efficiency. Conversely, the presence of baffles within the tank effectively mitigates sloshing and ensures continuous fuel presence at the intake port the entire simulation. This suggests that baffles play a crucial role in maintaining a stable and consistent fuel supply to the engine, even during dynamic vehicle maneuvers.

Mitigation of vibration and free-surface instabilities by using anti-vortex baffles in an industrial stirred vessel

Inadequate understanding of mixing dynamics can result in compromised product quality, increased production expenses and revenue loss. This study utilizes vibrometer measurements in both baffled and unbaffled eccentric flat-bottomed stirring vessels, containing a trisodium phosphate–water mixture, to investigate the impact of baffles on reducing vibrations. Through examining the effect of different configurations of baffles, this research effectively eliminates vortices and vibrations generated during agitation, consequently reducing maintenance frequency. Additionally, computational fluid dynamics (CFD) analysis is conducted on baffled and unbaffled stirred tanks, revealing streamlined patterns and mean velocities. By solving the Reynolds-averaged Navier–Stokes (RANS) equations with the k–[Formula: see text] turbulence model, obtained results reveal the importance of baffles in reducing vibrations and vortices while stabilizing the free-surface. However, balancing the number of baffles with impeller blades is crucial to prevent vibration regeneration.

Flow stirred by open turbine: A modal analysis of multiscale flow characteristics

AbstractThe turbulence observed in the stirred tank is a result of the superposition of flows at different scales, each exerting various influences on the operational processes. To understand the flow characteristics associated with stirring, this paper undertakes particle image velocimetry (PIV) experimental research on an unbaffled tank stirred by an open turbine featuring four distinct blade deflection angles. The flow field in the unbaffled tank manifests as a double‐circulation flow pattern; however, it is worth noting that the blade deflection angle exerts a significant impact on the stirring process. Specifically, when the blade deflection angle reached 90°, the flow discharge aligned closely with the radial direction, accompanied by pronounced fluctuations in the high‐velocity region. The Lagrangian coherent structures (LCS) formed were subsequently extracted and subjected to finite‐time Lyapunov exponent (FTLE) analysis. Notably, a more regular wake pattern emerged in proximity to the blade tip, while a larger scale flow structure persisted along the flow direction. To systematically analyze the flow field stirred by the open turbine, modal analysis was employed. By reconstructing the flow field based on the principle of 50% energy content, the decomposition of flows at different scales was achieved. The turbulent kinetic energy (TKE) generated by the various scale flows was subsequently calculated, revealing that the high TKE values primarily originate from the large‐scale flow structure. Conversely, the TKE generated by the small‐scale flow exhibits a uniform distribution across different regions. Notably, in regions characterized by higher velocities, the peak TKE associated with the small‐scale flow also manifests; however, it is significantly smaller in magnitude compared to that generated by the original flow or the large‐scale flow.

An Experimental and Computational Approach to Development of Mixing Processes for Miscible Liquids in Unbaffled Tanks.

Mixing of liquids is a critical unit operation in the biopharmaceutical drug product manufacturing. It commonly consists of mixing miscible liquids to dilute bulk drug substance (DS) or pool multiple lots of drug substance. In the past, at-scale mixing studies have been conducted to determine the mixing parameters, namely mixing speed, and mixing time. At-scale studies have historically been utilized to overcome the challenges associated with geometric dissimilarity of mixing systems found when scaling up. In addition, such studies are quite costly, as they often use actual DS to overcome a lack of representativeness associated with simple salt trace models often employed. As a result, there is a significant need for alternative cost-effective methods that can predict mixing parameters with close agreement to actual experiments and operations. At-scale mixing experiments were conducted using full-sized tanks and surrogate solutions. Several computational fluid dynamic (CFD) simulation methods were conducted and compared with the experiments to determine the most reliable computational techniques. The experiments demonstrate that surrogate solutions can be used reliably to determine mixing parameters in at-scale studies instead of the valuable drug products. Studying different CFD methods also showed that transient simulations that use a large eddy simulation (LES) viscous modeland a sliding mesh can correctly predict the mixing parameters. Results of this study establish a practical and reliable methodology to perform mixing studies for miscible liquids with different kinematic viscosities. The methods discussed herein greatly reduce the routine mixing study costs in the biopharmaceutical industry and increase efficiency and accuracy of the results, allowing proactive scale-up of mixing operations.

Effect of aspect ratio of elliptical stirred vessel on mixing time and flow field characteristics in the absence of baffles

Elliptical tanks were used as an alternative to circular tanks in order to improve mixing efficiency and reduce mixing time in unbaffled stirred tanks (USTs). Five different aspect ratios of elliptical vessels were designed to compare their mixing time and flow field. Computational fluid dynamics (CFD) simulations were performed using the k–ε model to calculate the mixing time and simulate turbulent flow field features, such as streamline shape, velocity distribution, vortex core region distribution, and turbulent kinetic energy (TKE) transfer. Visualization was also carried out to track the tinctorial evolution of the liquid phase. Results reveal that elliptical stirred tanks can significantly improve mixing performance in USTs. Specifically, the mixing time at an aspect ratio of 2.00 is only 45.3% of the one of a circular stirred tank. Furthermore, the secondary flow is strengthened and the vortex core region increases with the increase of aspect ratio. The axial velocity is more sensitive to the aspect ratio than the circumferential and radial velocity. Additionally, the TKE transfer in elliptical vessels is altered. These findings suggest that elliptical vessels offer a promising alternative to circular vessels for enhancing mixing performance in USTs.

Blending in unbaffled and baffled mechanically agitated vessels – Effect of Reynolds number and liquid height

The standard vessel for mixing in the process industry is a baffled tank. This design is very common especially for low viscosity fluids. Nonetheless, unbaffled vessels are found in industry and the laboratory. The reasons given for using unbaffled vessels are primarily concerns around cleaning the vessel between batches and the possibility of stagnation zones in high viscosity fluids particularly when properties change over the processing time. In this paper we will examine the effect of baffles on the blend time using one vessel: with and without baffles. The blend time was measured using the iodine color change method using a range of viscosities and Reynolds Numbers. The depth of the vortex was also measured. The same axial flow-down pumping impeller is used. In this work we observed that the flow patterns are dramatically different between unbaffled and baffled tanks. At high Reynolds Numbers with the baffled configuration, we see the familiar turbulent diffusion throughout the vessel. Without the baffles the fluid segregates into two zones: a swirling inner zone near the shaft and a more turbulent outer zone. The overall blend time is longer in the baffled tank at the same rotational speed compared to the baffled tank. This difference decreases with increasing Reynolds Number. We identified two blend times in using unbaffled tanks, a short blend time for the outer zone and a much longer time for the inner vortex centered zone which defines the overall blend time for the vessel. At low Reynolds Numbers the baffled blending is in the transitional flow regime between turbulent and laminar with a combination of eddies and striations. The last region of the tank to become completely mixed is located behind the baffles. Unbaffled blend times are shorter and as the Reynolds Number decreases the difference increases. The vortex is also reduced with decreasing Reynolds number. We consider this work just a beginning step in a long study and hope this work will be picked up by other researchers as happened so many times with Professor Nienow’s work.

A parallel and adaptative Nitsche immersed boundary method to simulate viscous mixing

In this study, we present a parallel immersed boundary strategy that uses Nitsche's method (noted NIB) to weakly impose on a given fluid the boundary conditions associated with a solid of arbitrary shape and motion. Specific details of the software implementation, as done in the software Lethe, are discussed. We verify the NIB method and compare it with other methods in the literature on the well-established test cases of Taylor-Couette flow and von Karman vortex street. Then, we validate the NIB method through simulations of the mixing of fluid in a stirred tank, which is a process central to industries as diverse as polymer manufacturing, food processing, pharmaceutical or chemicals. Simulation results show excellent agreement with experimental data available in the literature for a large range of Reynolds numbers (▪), for baffled and unbaffled tanks with a pitched-blade turbine (PBT) impeller. Lastly, the versatility of the NIB method is demonstrated with simulations of a mixing rig with two off-centered impellers with overlapping swept volumes, a case that is either unpractical or impossible to simulate with many other techniques. The software as well as all the files that we used for the simulations are available in the public domain for ease of reproducibility.

Hydrodynamics performance of Newtonian and shear-thinning fluids in a tank stirred with a high shear impeller: Effect of liquid height and off-bottom clearance

The hydrodynamics performance of a ring-style Hockmeyer© high-shear impeller (HSI) in an unbaffled tank with Newtonian and shear-thinning fluids at Reynolds numbers (Re) between 20 and 300 is addressed. Power measurements of this study were used to validate computational fluid dynamics simulations. Key hydrodynamic parameters for the HSIs performance, i.e., pumping and power numbers, and viscous dissipation are analyzed. The effect of the off-bottom clearance (C), working fluid height (Z) (in the range 0.18 ≤C/T ≤ 0.38 and 0.8 ≤Z/T ≤ 1, where T is the tank diameter), and fluid rheology, on these parameters were investigated. Results show that the examined C/T and Z/T ratios do not significantly affect the power number. The fluid rheology, C/T and Z/T had, on the contrary, a significant impact on the remaining hydrodynamic parameters. For 45 ≤Re≤ 300, the shear-thinning fluid showed better performance for dispersion processes, i.e., higher pumping number and viscous dissipation, and lower power number than the Newtonian fluid.

Artificial neural network to predict the power number of agitated tanks fed by CFD simulations

AbstractThe power consumption of the agitator is a critical variable to consider in the design of a mixing system. It is generally evaluated through a dimensionless number known as the power number . Multiple empirical equations exist to calculate the power number based on the Reynolds number and dimensionless geometrical variables that characterize the tank, the impeller, and the height of the fluid. However, correlations perform poorly outside of the conditions in which they were established. We create a rich database of 100 k computational fluid dynamics (CFD) simulations. We simulate paddle and pitched blade turbines in unbaffled tanks from 1 to 100 and use an artificial neural network (ANN) to create a robust and accurate predictor of the power number. We perform a mesh sensitivity analysis to verify the precision of the values given by the CFD simulations. To sample the 100 k mixers by their geometrical and physical properties, we use the Latin hypercube sampling (LHS) method. We then normalize the data with a MinMax transformation to put all features in the same scale and thus avoid bias during the ANN's training. Using a grid search cross‐validation, we find the best architecture of the ANN that prevents overfitting and underfitting. Finally, we quantify the performance of the ANN by extracting 30% of the database, predicting the using the ANN, and evaluating the mean absolute percentage error. The mean absolute error in the ANN prediction is 0.5%, and its accuracy surpasses correlations even for untrained geometries.

Hydrodynamic Comparative Study on the Pumping Effects of a Square Tank Equipped with Single-Stage and Bi-Stage Impellers

Abstract A computational fluid dynamics simulation is done for comparative study from the pumping effect on the four surfaces of the stirred tank. The flow field generated by one-stage and bi-stage six-bladed Rushton turbine in the unbaffled square tank was studied. The Reynolds-averaged Navier-Stokes equation with steady-state multi-reference frame approach (MRF) is used to simulate hydrodynamic flow in the tank. The turbulent viscosity, the turbulent kinetic energy and mean velocity distributions obtained in vertical and horizontal plans are analyzed and discussed. We can deduce that the additional Rushton turbine in the upper part of the square tank improves the quality of the mixture.

Modeling of free-surface shape in unbaffled tanks

This paper aims to present a numerical investigation of the two-phase flow in unbaffled uncovered stirred tanks equipped with a Rushton turbine. The shaft is positioned eccentrically. As simulation software, the Simcenter Star CCM+ 2021.2 is used to carry out multiphase unsteady simulations. Next, the numerical results are compared with results obtained on a test bench.

A Numerical Study on the Effects of Perforated and Imperforate Baffles on the Sloshing Pressure of a Rectangular Tank

Sloshing has many industry applications, namely in offshore engineering, aerospace, ship building, and manufacturing. Sloshing simulation is essential to better understand the sloshing pattern and consequently to improve the tank design to reduce noise levels, stresses on the structure, and optimize the baffle configurations and arrangements. Thus, the aim of this study is to determine the effects of perforated (porous) and imperforate (solid) baffles on the sloshing pressure using ANSYS FLUENT software based on Volume of Fluid (VOF) method where a rectangular tank with 25% and 60% filling ratios was considered. In the first case, an unbaffled rectangular tank with 60% filling ratio was used for the validation purpose, while in the second case, a 25% filling ratio was investigated considering two scenarios, namely a unbaffled tank and a baffled tank case with perforated and imperforate baffles. The outcomes of the results indicate that perforated baffle can significantly reduce the sloshing pressure in the tank. The validation of the results also shows a good agreement with the published experimental results.

Effects of baffle on flow structure and cyclic variation in stirred tanks with Rushton turbine

In order to reveal the effect of baffles on flow structure and cyclic variation stirred by Rushton turbines, particle image velocimetry experiments were performed in baffled and unbaffled stirred tanks. In a baffled tank, the peak value of time-averaged velocity obviously increases, the high velocity region enlarges significantly, and the fluctuation of fluid elements near the turbine increases significantly. In the 20° phase plane of the baffled tank, the high swirl strength region slightly enlarges, but the peak of phase-resolved swirl strength is lower. The peak value and high value regions of turbulent kinetic energy are larger in the baffled stirred tank at various phase planes. Using quadruple proper orthogonal decomposition, the flow field is decomposed into a small-scale part, a transition part, a large-scale part, and a mean part with different spatial scales. Adding the baffle, the mean part with largest spatial scale obviously increases (it is favorable for mass transfer at different liquid levels) and the transition part, which mainly includes the random fluctuation of a turbulent cascade, slightly enhances (it is conducive to diffusion). The main influence of cyclic variation for various blade passages is the flow with large-scale, and the standard deviation of time coefficients decreases by 3%–4% for the mean part and the large-scale part by adding the baffle.

CFD simulation of hydrodynamics characteristics in a tank with forward-reverse rotating impeller

BackgroundA forward-reverse rotating impeller was used in an unbaffled tank to improve the mixing efficiency. Flow field and power characteristics in the tank were numerically studied. MethodsComputational fluid dynamics(CFD) simulations were performed to investigate hydrodynamics in the unbaffled tank. The Reynolds stress turbulence (RST) model along with a sliding mesh approach was used to predict instantaneous flow fields in the stirred tank. Significant findingsFluid in the unbaffled tank flows periodically with the forward-reverse impeller agitation. Zonal flow phenomenon was found for the fluid affected by agitation in different directions. The effects of operating conditions on instantaneous fluid velocity vary with positions in the tank. Flow field characteristics in the forward-reverse agitated unbaffled tank are very different from those in constant agitated unbaffled and baffled tanks. Generally, transient power consumption is only the function of its corresponding transient rotational speed. The averaged impeller power number is a constant and is not affected by maximum rotational speed and cycle time. The averaged power number is the largest in the unbaffled tank with forward-reverse agitation, followed by the baffled tank at a constant agitation, and the smallest in the unbaffled tank at a constant impeller speed.

Suspended particles behavior in aqueous [Bmim]BF4 solution by novel on-line electrical sensing zone method

Ionic liquids (ILs) and aqueous ionic liquid solutions have been investigated as excellent solvents or catalysts in a few multiphase systems. To optimize the reactor design and operation conditions, it is significant to study the suspended particles behaviors in ILs involved solid–liquid mixing processes. A novel on-line electrical sensing zone (ESZ) method is developed to achieve the measurement of local solids concentration in aqueous [Bmim]BF4 conductive solutions. The suspension degrees of solid particles stirred by different impellers at varied agitation speeds have been obtained. The mean fluid velocities and turbulent fluctuating velocities close to the bottom of the unbaffled tanks have been determined by stereoscopic particle image velocimetry (SPIV). The results showed that the aqueous [Bmim]BF4 solutions are more efficient on solids than normal saline solution. Radical flow impeller with wider blades at C/T = 0.2 presents its better performance on solids suspension. The variations of solids suspension curves are more consistent with that of mean radial velocities at axial height of z/H = 0.99. It indicated that the solids suspension in unbaffled stirred tanks is mainly related to the mean flow rather than to turbulent fluctuations.

Modeling mixing dynamics in uncovered baffled and unbaffled stirred tanks

AbstractThis paper presents results of measurements performed in uncovered baffled and unbaffled stirred tanks. The Laser Doppler Anemometry (LDA) set consists of a 300 mW argon‐ion laser generating two pairs of blue and green beams. Additionally, three turbulence models are verified to simulate mixing dynamics in uncovered baffled and unbaffled stirred tanks. Simulations are performed using both the steady‐ and the transient‐state approach. The unbaffled tank was modeled using both the single‐ and the multiphase approaches. The cited correlations of the central vortex depth are used to verify the multiphase calculations. In order to reduce the computational time, a simplified numerical model is proposed. It assumes a single‐phase simulation without the central vortex. The flow fields below the central vortex level are compared between the single‐ and the multiphase approach. The simplified model can be used both to design and optimize the mixing process.

CFD simulations of early- to fully-turbulent conditions in unbaffled and baffled vessels stirred by a Rushton turbine

Laboratory scale unbaffled tanks provided with a top cover and a baffled tank both stirred by a Rushton turbine were simulated by carrying out RANS simulations. Three different turbulence models were adopted (k-ωSST, k-ε and the SSG Reynolds stress model) to predict the flow field and the relevant performance parameters (power and pumping numbers) of the tank operated from early to fully turbulent conditions. CFD results were compared with literature experimental data and DNS simulation results to validate and properly compare the models. In the range of Reynolds numbers investigated, results showed that, for the unbaffled tank, the SSG model based on Reynolds stresses is a better choice at larger Re, while the k-ω SST model better reproduces the experiments at lower values. Conversely, no significant differences between the predictions of the three models were found in the baffled vessel.

The effect of baffle on flow structures and dynamics stirred by pitch blade turbine

•In order for a better understanding of how baffle impacts on the flow field stirred by pitched blade turbine (PBT), the Particle Image Velocimetry (PIV) experiment was conducted in baffled and unbaffled tank, with time-averaged method, phase-resolved method and Proper Orthogonal Decomposition (POD) method applied to analyze the turbulent characteristics. The POD quadruple decomposition method was adopted to decompose the flow structures of different scales. As indicated by the experimental results, in the measured area, an independent circulation area develops under the impeller, thus forming a pattern of double circulation flow in the unbaffled tank. There is a relatively concentrated distribution of incoherent flow structures in the stirred flow field, and these small-scale flow structures hinder transfer to the whole stirred tank, which makes it easy for the difference in local concentration to arise. With the addition of baffle, the axial discharge of fluid is enhanced, and the independent circulation area under the turbine shows change, and the large-scale convection of main body develops. The stirring-induced incoherent flow structure shows a wider range of distribution, while the small-scale structure under the stirring tank creates a more favorable condition for the diffusion into the whole stirred tank.

Parametric studies on sloshing in a three-dimensional prismatic tank with different water depths, excitation frequencies, and baffle heights by a Cartesian grid method

This paper aims to numerically investigate violent sloshing in a partially filled three-dimensional (3D) prismatic tank with or without a baffle, further to clarify the suppressing performance of the baffle and the damping mechanism of sloshing. The numerical model is based on a Cartesian grid multiphase flow method, and it is well validated by nonlinear sloshing in a 3D rectangular tank with a vertical baffle. Then, sloshing in an unbaffled and baffled prismatic tank is parametrically studied. The effects of chamfered walls on the resonance frequency and the impact pressure are analyzed. The resonance frequencies for the baffled prismatic tank under different water depths and baffle heights are identified. Moreover, we investigated the effects of the baffle on the impact pressure and the free surface elevation. Further, the free surface elevation, pressure and vortex contours are analyzed to clarify the damping mechanism between the baffle and the fluid.