CFD simulation model for mixing tank using multiple reference frame (MRF) impeller rotation

An agitated two-phase flow is studied numerically and experimentally in a mixing vessel agitated with two six-blade Rushton turbines. In Computational Fluid Dynamics (CFD), the full Eulerian multiphase approach coupled with the standard turbulence model is performed to deal with two-phase flow. The impeller rotation was modelled by the Multiple Reference Frame (MRF) approach. The simulation was used to investigate the flow field, power and mixing time in single and two-phase cases. The results of the calculations have been verified with the data that was measured by particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) technique. The predicted results show good agreement with the experimental data. The computational model presented in this study could be useful for explaining the two-phase flow patterns on the mixing process and extending the applications of multiphase stirred reactors.

Although significant advances have been achieved in emulsion polymerization in recent decades, the effect of mixing on this type of polymerization has not been fully delineated yet. In fact, mixing plays a significant role in the performance of an emulsion polymerization reaction. For instance, in case of a very low agitation rate, larger droplets are generated and phase separation, which limits the diffusion mechanism, may occur. In contrast, vigorous agitation can result in reduced nucleation of particles. Therefore, the main objective of this study is to investigate the impact of mixing parameters (e.g. impeller speed, impeller type, impeller number, and baffles) on the monomer conversion, the polymer average molecular weight, particle size and size distributions, transition glass temperature, and number of particles. To achieve this objective, the emulsion polymerization of methyl methacrylate (MMA) was carried out in a lab-scale reactor equipped with a top-entry agitator, 4 wall baffles, a U shaped cooling coil, and a temperature controller. To analyze the reactive flow inside the polymerization reactor, a novel computational fluid dynamics (CFD) model was developed. The multiple reference frames (MRF) technique, k-ε model, and mixture model approach were employed to model the impeller rotation, turbulence, and multiphase flow, respectively. The particle number density distribution within the reactor was also estimated by means of the population balance approach, which employs a discrete method to describe the nucleation and growth of the polymer particles. The experimental data and CFD results showed that the installation of the baffles enhanced the particle size and molecular weight but reduced the conversion and particle number. The number density achieved using the Rushton impeller was higher than that for the pitched blade impeller. The results revealed that the effect of the impeller speed on the characteristics of the polymer attained using the pitched-blade turbine was more prominent than that for the Rushton turbine. It was also found that the impact of the impeller speed on the polymer characteristics was much more pronounced for the double pitched-blade turbines rather than for the double Rushton turbines.

This work focuses on the development of efficient computational tools for the simulation of turbulent multiphase polydispersed flows. In terms of methodologies we focus here on the use of Large Eddy Simulation (LES) and Quadrature-Based Methods of Moments (QBMM). In terms of applications the work is finalised, in order to be applied in the future, to particle production processes (precipitation and crystallisation in particular). An important part of the work concerns the study of the flow field in a Confined Impinging Jets Reactor (CIJR), frequently used in particle production processes. The first part is limited to the comparison and analysis of micro Particle Image Velocimetry (μPIV) experiments, carried out in a previous work, and Direct Numerical Simulation (DNS), carried out in this thesis. In particular the effects of boundary and operating conditions are studied and the numerical simulations are used to understand the experimental predictions and demonstrate the importance of unavoidable fluctuations in the experimental inlets. This represents a preparatory work for the LES modelling of the CIJR. Before investigating the accuracy of LES predictions for this particular application, the model and the implementation are studied in a more general context, represented by a well-known test case such as the periodic turbulent channel flow: the LES model implementation in TransAT, the code used in this work, is compared with DNS data and with predictions of other codes. LES simulations for the CIJR, provided with the proper boundary conditions obtained by the previous DNS/μPIV study, are then performed and compared with experiments, validating the model in a more realistic test case. Since particle precipitation and crystallization often result in complex interactions between particles and the continuous phase, in the second part of the work particular attention has been paid in the modelling of the momentum transfer and the resulting velocity of the particles (relative to the fluid). In particular the possibility of describing poly-disperse fluid-solid systems with QBMM together with LES and Equilibrium Eulerian Model (EEM) is assessed. The study is performed by comparing our predictions with DNS Lagrangian data in the turbulent channel flow previously described, seeded with particles corresponding to a realistic Particle Size Distribution (PSD). The last part of the work deals with particle collisions, extending QBMM to the investigation of non-equilibrium flows governed by the Boltzmann Equation with a hard-sphere collision kernel. The evolution of the particle velocity distribution is predicted and compared with other methods for kinetic equations such as Lattice Boltzmann Method (LBM), Discrete Velocity Method (DVM) and Grad's Moment Method (GM). The overall results of this thesis can be extended to a broad range of other applications of single-phase, dispersed multiphase and non-equilibrium flows

A turbulent jet emanating from an unconfined, premixed burner is investigated by using large eddy simulation (LES) and direct numerical simulation (DNS) as well as experimentally by means of optical (OH* chemiluminescence), acoustic (microphone), and laser-optical measurement techniques (Particle Image Velocimetry). Comparison of the results obtained through experiments, LES, and DNS indicate a reasonable agreement. In order to analyze the impact of mesh refinement on the resolved flame properties and acoustic radiations, computational grids with varying resolutions are used for the LES. As large coherent flow motion exists in the considered flow case, due to an over-predicted diffusion the flame length calculated with LES is underestimated. On the other hand, DNS exhibits a similar intensity distribution for OH* as the experiment and, hence, the flame length is predicted accurately by DNS. The emitted noise spectrum has a tonal shape with peaks at the burner’s resonance frequency for the non-reacting flow which changes to broadband noise and, in general, is raised in amplitude for reacting flows. In addition, it is shown that an increase in Reynolds number, preheat temperature, or a decrease in equivalence ratio close to stoichiometric ratios yields more noise being emanated from the burner. The latter indicates the fact that direct combustion noise is linked to interactions of turbulent fluctuations with the flame front. When using an equivalence ratio closer to stoichiometric ratio, a thinner reaction zone is expected and the intrinsic interaction between the flame and turbulent flow is more pronounced.

High-fidelity experimental measurements for modeling and simulation of nuclear engineering applications

Oxidation ditches (OD) are one of the most common aeration units or bioreactors in the activated sludge process. Flow and hydrodynamic characteristics play an important role in successful operation of an OD. It is difficult to study the rheology in a detailed way because it’s time-consuming and expensive. However, due to the development of computational fluid dynamics (CFD), such a study has become easier and less expensive. CFD is a powerful tool to simulate the hydrodynamics. Since in mixing operations, the generation of fluid flow is due to the input of mechanical energy in form of rotation of the impeller, the one of very important consideration is the choice of a model that describes impeller rotation. The flow features associated with multiple rotating parts can be analyzed using the multiple reference frame (MRF) capability. Based on previous studies on OD in the field of wastewater treatment (few numbers has been done) there is no direct mention of the size of rotating zone for the simulation so a fundamental analysis is required and can be helpful for further studies in this field. In this study the effect of rotating zone dimensions on the accuracy of oxidation ditch simulation has been analyzed and four model of moving frame have been set and simulated. Based on the analysis the bigger the radius of rotating zone is the higher the accuracy of modelling is. Moreover, the rotating zone in multiple reference approach cannot be semi-circle.

In this paper, a new error concealment algorithm for the new coding standard H.264 is presented. The algorithm consists of a block size determination step to determine the size type of the lost block and a motion vector recovery step to find the lost motion vector from multiple reference frames. The main feature of this algorithm are as follows. In the block size determination step, we propose a criterion to determine the size type of the lost block from the current frame. In the motion vector recovery step, the optimal motion vector for the lost block chosen from multiple previous reference frames with the minimum value of the side match distortion. The proposed algorithm not only can determine the most correct mode for the lost block, but also can save much more computation time for motion vector recovery. Experimental results show that the proposed algorithm achieves 0.4/spl sim/7 dB improvement over the conventional VM method.

Using positron emission particle tracking (PEPT) to study the turbulent flow in a baffled vessel agitated by a Rushton turbine: Improving data treatment and validation

One of the common bioreactors in activated sludge process is oxidation ditch (OD). Since aeration and mixing are the main causes of alteration in flow field, study on hydrodynamics is essential for effective operation and optimization of an OD. Such study has become less-difficult and more cost-effective because computational fluid dynamics as a dominant tool to simulate the hydrodynamics has increasingly improved. Rotation of the impeller and mixer generates fluid flow, so selecting a model that outlines impeller rotation plays a vital role. Based on the proficiency of multiple reference frame (MRF) approach, the flow characteristics associated with several rotational parts can be investigated. However, in most of the previous studies on OD as a wastewater treatment equipment, there is no clear comment on the dimension of rotational zone required for the simulation; therefore, an exclusive investigation is essential and can be supportive for additional studies on processes with moving parts. In this paper, the influence of rotational zone proportions on the precision of OD simulation was analysed and four models were proposed and simulated by (or with) MRF approach. As a result of this investigation, the more the radius of rotating zone is, the greater the exactness of modelling is. Additionally, the semi-circle rotating zone in MRF approach is infeasible.

Experimental and numerical studies of the effects of the contraction ratios on the swirling flow characteristics of the model combustor outlet in lean gas turbines

Solid-liquid mixing plays a significant role in crystallization, suspension polymerization, leaching, solid-catalyzed reaction and adsorption. In this study, a computational fluid dynamic (CFD) model was developed for solid-liquid mixing in a cylindrical tank equipped with a top-entering impeller. The multiple reference frame (MRF) technique, k-ε model and Eulerian-Eulerian approach were employed to simulate the impeller rotation, turbulent flow and multiphase flow, respectively. The effects of impeller speed, solid concentration, particle size, solid density and impeller clearance on the mixing performance of four different impellers (A310, marine propeller, pitched blad turbine and A320) were investigated. The CFD results were in good agreement with experimental data measured using electrical resistance tomography (ERT). In order to investigate the mixing quality in this study, the impeller speed required for maximum homogeneity, clouding height, and just-suspended impeller speed were investigated.

A styrene polymerization in a lab-scale CSTR equipped with a pitched blade turbine impeller was simulated using the computational fluid dynamics (CFD) approach. The impeller motion was integrated in the geometry using the multiple reference frame (MRF) technique. The presence of non-linear source term and the highly coupled nature of transport equations of the polymerization, made the convergence difficult to achieve. The effects of the impeller speed, the input-output locations and the residence time on the polymerization in the CSTR were investigated. The CFD simulation shows that good mixing remained limited to the impeller region. Regions far from the impeller remained unmixed due to high viscosity of the polymer mass. The path lines of the particles, released at the inlet, were also generated to analyze the reaction progress as the chemicals travel throughout the reactor. The monomer conversion computed using the CFD model was compared to data reported in the literature. Conversion predicted using the CFD model is in good agreement with that obtained from the CSTR model at low residence time. However, the CFD predicted coversions were higher than those calculated from the CSTR model, at high residence time. It was found that the input-output locations had significant effect on the conversion and the homogeneity in the CSTR.

Rotating geometries are prevalent in various engineering applications and significantly influence turbulence generation and heat transfer. Accurately modeling these geometries in computational fluid dynamics (CFD) is challenging due to the presence of swirling flows and strong rotor–wake interactions. This study focuses on simulating propeller–body interactions in turboprop aircraft configured in tractor layout. Three numerical models are considered for predicting propeller-induced upwash and downwash with varying fidelity and computational cost: (i) Multiple Reference Frame (MRF), (ii) Blade Element Model (BEM), and (iii) Virtual Blade Model (VBM). These models are validated and compared using well-characterized two-bladed and four-bladed propellers. Both MRF and VBM predict thrust accurately, but VBM offers over an order-of-magnitude reduction in computational cost. VBM also outperforms BEM in accuracy, particularly due to its compatibility with turbulence models. Reliable performance is observed for advance ratios up to J =0.85, beyond which prediction errors increase due to transitional flow phenomena. Finally, the validated VBM approach is employed within a Reynolds-Averaged Navier–Stokes (RANS) framework to simulate turboprops in tractor configuration. The effects of the propeller slipstream on wing aerodynamics are analyzed in detail. The results demonstrate that the VBM–RANS approach captures wing–propeller interaction effects with good accuracy, yielding lift and drag coefficients within 6% of experimental values for cruise conditions. This level of agreement establishes VBM as a valuable tool for the aerodynamic design and optimization of propeller-driven aircraft, including turboprops and unmanned aerial vehicles (UAVs).

Assessment of the methodology for the CFD simulation of the flight of a quadcopter UAV

CFD simulations of two opposing plane wall jets in a generic empty airplane cabin: Comparison of RANS and LES