Sliding Mesh Approach Research Articles

Experimental Characterisation and Modelling of Homogeneous Solid Suspension in an Industrial Stirred Tank

In this work, we study the conditions needed to reach homogeneous distribution of aluminium salts particles in water inside a torispherical bottom shaped stirred tank of 70 L equipped with a Pfaudler RCI type impeller and three equispaced vertical baffles. The aim of the present study is to develop a CFD model describing the quality of particle distribution in industrial scale tanks. This model, validated with experimental data, is used afterwards to develop scale-up and scale-down correlations to predict the minimum impeller speed needed to reach homogeneous solid distribution Nhs. The commercial CFD software Fluent 14 is used to model the fluid flow and the solid particle distribution in the tank. Sliding Mesh approach is used to take the impeller motion into account. Assuming that the discrete solid phase has no influence on the continuous liquid phase behaviour, the fluid flow dynamics is simulated independently using the well-known k-∊ turbulence model. The liquid-solid mixture behaviour is then described by implementing the Eulerian Mixture model. Computed liquid velocity fields are validated by comparison with PIV measurements. Computed Nhs were found to be in good agreement with experimental measurements. Results from different scales allowed correlating Nhs values to the volumetric power consumption.

Experimental and Numerical Investigation of Wheel Housing Aerodynamics on Heavy Trucks

Wheel and underbody aerodynamics have become important topics in the search to reduce the aerodynamic drag of the heavy trucks. This study aims to investigate, experimentally as well as numerically, the local flow field around the wheels and in the wheel housing on a heavy truck; and how different approaches to modelling the wheel rotation in CFD influences the results. Emphasis is on effects due to ground simulation, and both moving ground and wheel rotation were requirements for this study. A 1:4-scale model of part of a heavy truck geometry has been developed. During the model design numerical simulations were used to optimise the shape, in order to replicate the flow field near the wheel of a complete truck. This was done by changing the flow angles of the incoming and exiting flows, and by keeping the mass flow rates in to, and out of, the wheel housing at the same ratios as in a reference full size vehicle. To reduce blockage effects, the model was sectioned to reduce both height and width. In the experiments, pressure sensors and static pressure taps located in the wheel housing were utilised, and the simulations replicated the boundary conditions of the wind tunnel experiments, both in terms of the geometry of the model and wind tunnel as well as the modelling of the ground simulation. It was found that the wheel wake structures changed significantly when ground simulation was utilised. The main outflow through the wheel housing was influenced by the wheel rotation and took place further upstream, which resulted in large differences in the flow field downstream of the wheel. The influence of different strategies for modelling the wheel rotation in CFD was investigated and it was found that the Sliding Mesh approach was the most accurate method.

Hydrodynamic and kinetic study of cellulase production by Trichoderma reesei with pellet morphology

Numerical simulations and experimental validation were performed to understand the effects of hydrodynamics on pellet formation and cellulase production by filamentous T. reesei. The constructed model combined a steady-state multiple reference frame (MRF) approach describing mechanical mixing, oxygen mass transfer, and non-Newtonian flow field with a transient sliding mesh approach and kinetics of oxygen consumption, pellet formation, and enzyme production. The model was experimentally validated at various agitation speeds in a two-impeller Rushton turbine fermentor. Results from simulation and experimentation showed that higher agitation speeds led to increases in the pellet diameter and the proportion of pelletized (vs. filamentous) forms of the biomass. It also led to increase in dissolved oxygen mass transfer rate in shear-thinning fluid and cellulase productivity. The extent of these increases varied considerably among agitation speeds. Pellet formation and morphology were presumably affected within a viscosity-dependent shear-rate range. Cellulase activity and cell viability were shown to be sensitive to impeller shear. A maximum cellulase activity of 3.5 IU/mL was obtained at 400 rpm, representing a twofold increase over that at 100 rpm.

미끄럼 격자를 이용한 HAWT 시스템 주위의 비정상 유동장 해석

An unsteady RANS analysis study of the 3-D flow around the NREL Phase VI horizontal axis wind turbine(HAWT) was performed using sliding mesh approach. Two different analysis models such as rotor-only and rotor with tower/nacelle were constructed to investigate the blade/tower interaction. Analysis results for the rotor with tower/nacelle were compared with the corresponding NREL`s experimental data which produced fairly good validation of the present CFD model. Comparison of flows around those two models also clearly showed the blade/tower interaction even it was small for upwind configuration. Other visualization results and integrated aerodynamic loads including torque of the blade demonstrated the effective unsteady flow simulation capability of the present CFD model.

CFD simulation of stirred tanks: Comparison of turbulence models. Part I: Radial flow impellers

AbstractA critical review of the published literature regarding the computational fluid dynamics (CFD) modelling of single‐phase turbulent flow in stirred tank reactors is presented. In this part of review, CFD simulations of radial flow impellers (mainly disc turbine (DT)) in a fully baffled vessel operating in a turbulent regime have been presented. Simulated results obtained with different impeller modelling approaches (impeller boundary condition, multiple reference frame, computational snap shot and the sliding mesh approaches) and different turbulence models (standard k − ε model, RNG k − ε model, the Reynolds stress model (RSM) and large eddy simulation) have been compared with the in‐house laser Doppler anemometry (LDA) experimental data. In addition, recently proposed modifications to the standard k − ε models were also evaluated. The model predictions (of all the mean velocities, turbulent kinetic energy and its dissipation rate) have been compared with the experimental measurements at various locations in the tank. A discussion is presented to highlight strengths and weaknesses of currently used CFD models. A preliminary analysis of sensitivity of modelling assumptions in the k − ε models and RSM has been carried out using LES database. The quantitative comparison of exact and modelled turbulence production, transport and dissipation terms has highlighted the reasons behind the partial success of various modifications of standard k − ε model as well as RSM. The volume integral of predicted energy dissipation rate is compared with the energy input rate. Based on these results, suggestions have been made for the future work in this area.

Verification and validation for RANS simulation of KCS container ship without/with propeller

The free surface flow of a modern container ship KCS without propeller was firstly simulated using three sets of grids. The computed results including resistance, wave elevation and flow field on propeller disk were compared with the experimental data in detail. Verification and validation of resistance and wave profile were performed using recommended procedures proposed by ITTC. Then the viscous flow around KCS with operating propeller behind was also simulated. Both body force approach and sliding mesh approach were applied to consider for the effect of propeller. The results of these two approaches were compared with the measured data. These numerical investigation shows that accurate prediction of propeller/hull interaction using CFD method is becoming feasible and the huge potential of CFD application in ship hydrodynamics performance prediction is demonstrated.

CFD Simulation of a Solvent Extraction Pump Mixer Unit: Evaluating Large Eddy Simulation and RANS Based Models

Mixer-settler equipment is widely used for solvent extraction (SX) operations. The pump mixer is the heart of an SX process. Any improvement in understanding of hydrodynamics and flow instabilities within a SX pump mixer unit would enable effective design of the mixer -settler equipment. In this direction, the present work investigates the predictive performance of the Large Eddy Simulation (LES) model vis-à-vis the PIV experimental results and RANS based model. Comparisons have been made initially for single phase operation of a Mixer unit, and then for the multiphase operation. The ANSYS/CFX modelling package has been used to set-up a transient three-dimensional CFD model using the sliding mesh approach for impeller motion and Eulerian-Eulerian approach for multi-phase flows. The present paper compares the flow patterns predicted by the LES model and compares them to RANS model prediction and PIV data. The prediction of flow structures and turbulence intensities will eventually pave the way for determination of droplet size and mass transfer rates, which are required in designing these systems.

From the ideal to the real induction machine: Modelling approach and experimental validation

The aim of this paper is to discuss the ability of a numerical code in reproducing a real machine behavior, with particular reference to the cage currents of a four-poles 50 Hz induction motor. The used 2D Finite Element method (FEM) is based on voltage driven field formulation, handling the nonlinearity by the fixed point technique and the rotor movement by the sliding mesh approach. The numerical outcomes are validated by experiments performed on a dedicated laboratory setup, able to provide the instantaneous cage currents. Then, a spectral algorithm has been applied to the experimental and computed variables and the results have been interpreted in terms of magneto-motive forces. This approach allows us to determine possible machine eccentricities or other asymmetries not introduced in the simulations. Discussing the computed and measured spectra and excluding in the last ones the lines corresponding to non-idealities, an evaluation on how the modelling approach reproduces accurately the real machine is possible. The validated model will be able to reproduce the machine behavior of usual induction machines whose rotor currents are not measurable.

CFD modeling of pump‐mix action in continuous flow stirred tank

AbstractThe present work involves Computational Fluid Dynamics (CFD) modeling and validation of head and power characteristics of a Top Shrouded Turbine with Nonradial Rectangular Blades (TSTNRB) operated in backswept mode. Following the successful validation reported here and in two previous studies by the authors. CFD has been used to simulate the performance of different kinds of impellers viz. Straight Blade Paddle (SBP), Rushton Turbine (RT), Top Shrouded Turbine with Radial Rectangular Blades (TSTRRB), Top Shrouded Turbine with Radial Trapezoidal Blades (TSTRTB), Top Shrouded Turbine with Non‐radial Rectangular Blades (TSTNRB), Top Shrouded Turbine with Curved Blades (TSTCB), and Both side Shrouded Turbine with Radial Trapezoidal Blades (BSTRTB), to compare them for pump‐mix action. Baffle‐impeller interaction has been modeled using the sliding mesh approach. Standard k‐ε model has been used for turbulence modeling. The power number, head number, and flow number have been computed for each impeller. The impellers have been rated on the basis of head generated by them for equal specific power input. An attempt has been made to explain why different impellers operating at the same speed and handling the same inlet flow rate should give different heads. How this insight can be used to conceptualize better designs of pump‐mix impeller is illustrated. © 2007 American Institute of Chemical Engineers AIChE J, 2008

Computational Fluid Dynamics Modeling of a Bench-scale Pump−Mixer: Head, Power and Residence Time Distribution

The present work involves several single-phase computational fluid dynamics (CFD) simulations of a continuous-flow bench-scale pump−mixer that uses a top-shrouded turbine with trapezoidal blades. Baffle−impeller interaction has been modeled using the sliding-mesh approach. The standard k−e model has been used for turbulence modeling. CFD simulations have been used to predict power consumption and the head generated by the pump-mix impeller, as well as to conduct virtual tracer experiments. Results from CFD simulations have been validated with the experimental data obtained on a physical counterpart. Virtual residence time distribution (RTD) curves have been used to perform compartment modeling of the pump−mixer. A significant difference in the hydrodynamic behavior between the low clearance design and the high clearance design has been observed.

Large Eddy Simulation of Flow Fields in Vessels Stirred by Dual Rushton Impeller Agitators

Three typical hydrodynamic characteristics (parallel, merging and diverging flow) in a baffled vessel stirred by a dual Rushton impeller agitator are investigated by large eddy simulation (LES) using two subgrid scale (SGS) models: the standard and dynamic Smagorinsky–Lilly models applied with the commercial CFD code FLUENT 6. The agitator motion is modeled using the sliding mesh approach. The results are compared with experimental and simulated data from the literature. It is concluded both SGS models used for LES can reproduce the complicated flow in the stirred vessel, while there are no remarkable differences between the results obtained by two SGS models though the dynamical Smagorinsky–Lilly model needs about 17% additional computing time.

CFD modeling of pilot-scale pump-mixer: Single-phase head and power characteristics

The present work involves single-phase computational fluid dynamics (CFD) simulations of continuous flow pump-mixer employing top-shrouded Rushton turbines with trapezoidal blades. Baffle—impeller interaction has been modeled using sliding mesh and multiple reference frame approaches. Standard k – ε model has been used for turbulence modeling. Several CFD runs representing different combinations of geometric and process parameters have been carried out. Results of CFD simulations have been used to find out two macroscopic performance parameters of pump-mixer—power consumption and head generated by the impeller. The simulation results have been compared with the experimental data obtained on a pilot-scale setup. Good agreement between CFD predictions and experimental results is observed. In most cases, sliding mesh approach is found to perform better than multiple reference frame approach. Details from CFD simulations have been used to have an insight into the pumping action of the impeller.

Modeling and Experimental Study of a Stirred Tank Reactor

This study proposes a comparison between CFD (Computational Fluid Dynamics) and experimental results for a cylindrical stirred tank reactor. The stirred tank reactor considered here is 0.7 m high, 0.4 m in diameter, baffled and has four impeller blades on a single shaft, placed at the symmetry axis. The tank is filled with water and serves as a model for molten aluminium purifying reactors.The experimental part of the study compares the use of PIV and LDV techniques to determine the flow field inside the tank. We use PIV to get a complete picture of the flow field in two vertical sections of the tank (one radial and one eccentric). LDV is then used to check and compare the velocities obtained by PIV along horizontal lines belonging to the two vertical sections considered under the PIV-measurements. Good overall agreement between the two methods is obtained.The experimental results are compared to simulation of the tank using an alternative model that we have implemented in the commercial CFD software Fluent. The modeling approach we propose here avoids the classical sliding mesh technique and its tedious use (especially for pre- and post-processing). Our model does not physically represent the impeller blades. It rather introduces them as a time-dependent source term in the momentum equation. This term is only accounted for at the cells that would have been swept by a blade if the blades were present. The modeling of the tank and the set-up of the case are faster and easier. The computation of the flow field using this modeling approach is compared to the sliding mesh approach and to the experimental results discussed above. Results show good agreement between experimental data and both modeling approaches (sliding mesh technique and no impeller model). The model we propose does not bring more accuracy but more convenience in the modeling work.

Influence of Stator Clocking on the Unsteady Three-Dimensional Flow in a Two-Stage Turbine

To give insight into the influence of the clocking and the stator–rotor interaction, the unsteady three-dimensional (3D) flow through a two-stage turbine is simulated numerically, using a time marching Navier–Stokes computer code with a sliding mesh approach. A stator clocking is applied to the second stator vane over several circumferential positions. The numerical results are compared with the experimental one to check the availability of the code. Clocking effects on the turbine performance, wake trajectories, and outlet flow field are focused. A relative efficiency variation of about 0.52% is concluded among clocking positions. A link between the turbine efficiency and the wake trajectories on the midspan is shown based on the presented clocking analysis in the 3D unsteady flow field. The detailed illustration of the outlet flow field shows that the influence of the clocking at the outlet is focused on the temperature distribution.

Investigation of the Optimum Clocking Position in a Two‐Stage Axial Turbine

A frozen rotor approach in a steady calculation and a sliding mesh approach in an unsteady simulation are performed in a stator clocking investigation. The clocking is executed on the second stator in a two‐stage axial turbine over several circumferential positions. Flow field distributions as well as the estimated performances from two approaches are compared with each other. The optimum clocking positions are predicted based on the estimated efficiency from the two approaches. The consistence of the optimum clocking positions is discussed in the paper. The availability and the limit of the frozen rotor approach in predicting the optimum clocking position is analyzed. It is concluded that the frozen rotor approach is available to search the optimum clocking position in the preliminary design period, although it misses some features of the unsteady flow field in the multistage turbines.

Numerical Evaluation of Mixing Time in a Tank Reactor Stirred by a Magnetically Driven Impeller

In this work, we investigate numerically the mixing characteristics of a tank reactor stirred by a low off-bottom clearance magnetically coupled impeller. We calculate the fully three-dimensional, time-dependent flow field using the Reynolds average form of the Navier−Stokes equations and adopting the sliding mesh approach. Mixing of a scalar species is computed in the Eulerian framework. We compute power consumption, pumping capability, fluid dynamic efficiency, and mixing time to homogeneous distribution from pointwise release of a scalar. We identify relationships useful to optimize the choice of operational parameters. Numerical results are in qualitative agreement with empirical correlations available from the literature and may be used to improve understanding of the mixing processes. The numerical procedure used mimics possible experimental approaches with the advantage of a lower cost. The methodology outlined can be a reference method to derive guidelines for optimization of impeller/tank design ...

The Influence of the Addition Position of a Tracer on CFD Simulated Mixing Times in a Vessel Agitated by a Rushton Turbine

Previous papers on simulated mixing times in stirred vessels using CFD have sometimes given predictions in good agreement with empirical equations based on experiments and some have not. In this study, mixing times have been measured for a vessel agitated by a Rushton turbine and compared with those predicted by CFD. The flow field was developed using the sliding mesh approach and computational parameters and the point of addition of the tracer have been varied. The simulations were very insensitive to the former whilst the radial distance from the wall of the latter had a very profound effect on both the mixing time and the development of the concentration field. When the addition point was close to the sliding mesh surface, the simulation was in good agreement with experimental values and empirical predictions whilst that for a point close to the wall was much too long. This finding may explain the contradictions in the literature.