Modeling Turbulent Flow in Stirred Tanks with CFD: The Influence of the Modeling Approach, Turbulence Model and Numerical Scheme

Single phase turbulent flow in a tank stirred by a down- and an up-pumping pitched blade turbine has been simulated using CFD. The effect of the modeling approach, discretization scheme and turbulence model on mean velocities, turbulent kinetic energy and global quantities, such as the power and circulation numbers, has been investigated. The results have been validated by LDV data. The stationary and time-dependent modeling approaches were found to have little effect on the turbulent flow, however the choice of the numerical scheme was found to be important, especially for the predicted turbulent kinetic energy. A first order method was found to highly underestimate LDV data compared with higher order methods. The type of the turbulence model was limited to the k-e and RNG models due to convergence difficulties encountered with a Reynolds Stress Model (RSM) and there was found to be little effect of these models on the mean flow and turbulent kinetic energy. This latter quantity was found to be largely under predicted in the discharge region of the down-pumping impeller in comparison with LDV data. Better agreement was found for the up-pumping pitched blade turbine. Estimated power numbers were found generally to be in good agreement for the down- and up-pumping data. However, the circulation number tended to be over predicted by about 30% and 40% for the down- and uppumping agitators, respectively

An experimental and CFD study of liquid jet injection into a partially baffled mixing vessel: a contribution to process safety by improving the quenching of runaway reactions

Thermal runaway remains a problem in the process industries with poor or inadequate mixing contributing significantly to these incidents. An efficient way to quench such an uncontrolled chemical reaction is via the injection of a liquid jet containing a small quantity of a very active inhibiting agent (often called a stopper) that must be mixed into the bulk of the fluid to quench the reaction. The hazards associated with such runaway events mean that a validated computational fluid dynamics (CFD) model would be an extremely useful tool. In this paper, the injection of a jet at the flat free surface of a partially baffled agitated vessel has been studied both experimentally and numerically. The dependence of the jet trajectory on the injection parameters has been simulated using a single-phase flow CFD model together with Lagrangian particle tracking. The comparison of the numerical predictions with experimental data for the jet trajectories shows very good agreement. The analysis of the transport of a passive scalar carried by the fluid jet and thus into the bulk, together with the use of a new global mixing criterion adapted for safety issues, revealed the optimum injection conditions to maximise the mixing benefits of the bulk flow pattern

Transient hydrodynamics and free surface capture of an under-baffled stirred tank during stopping

The transient hydrodynamics and the free surface shape have been numerically predicted by CFD for an under-baffled agitated vessel during the stopping phase of the agitator, including the inertial period after the agitator has completely stopped. The simulations were carried out in a fully transient manner using a gas/liquid inhomogeneous two phase flow model coupled with a k–1 turbulence model. The time dependence of the system studied reveals that the history of the fluid evolution during the impeller slowing phase determines the instantaneous results, implying that the resulting hydrodynamics cannot be determined via a classical steady-state approach. The numerical prediction of the free surface shape during stopping is in agreement with experimental data

An experimental and computational study of the vortex shape in a partially baffled agitated vessel

The vortex shape in a non-standard partially baffled agitated vessel in the form of a glass-lined, under-baffled stirred vessel has been investigated using both experimental and numerical approaches for an air/water system for different rotation speeds of the agitator. A simple and flexible experimental strategy was developed for determination of the time-averaged location of the unstable free surface using a process involving superimposition of images. CFD simulations were made to predict the vortex shape by using an Eulerian–Eulerian multiphase model coupled with a homogenous turbulence model. The simplifying assumptions of a constant bubble size, a constant drag coefficient and use of the k–ε turbulence model were made. An assessment of the capability of the numerical method to predict the vortex shape was carried out through comparison between experimental data and numerical results. Considering for comparison purposes a water isosurface volume fraction equal to 0.9, to account for the existence of air/water mixture present at the interface in the experiments, instead of the classical value of 0.5, gave very good agreement with the experimental data

Single and multiphase CFD approaches for modelling partially baffled stirred vessels: comparison of experimental data with numerical predictions

Whilst the use of CFD to study mixing vessels is now common-place, there are still many specialised applications that are yet to be addressed. Here we present CFD and PIV results for a hydrodynamic study of a partially baffled vessel with a free surface. The standard k.ε and SSG Reynolds Stress turbulence models are used and the numerical predictions of the mean flow field are compared with experimental data for single phase modelling. At low rotation rates a flat free surface is observed and the flow is simulated using a single phase model, whilst at high rotation rates an Eulerian–Eulerian multiphase model is used to capture the free surface location, even under conditions when gas is drawn down to the impeller. It is shown that there are significant transient effects that mean many of the “rules of thumb” that have been developed for fully baffled vessels must be revisited. In particular such flows have central vortices that are unsteady and complex, transient flow-induced vortical structures generated by the impeller–baffle interactions and require a significant number of simulated agitator rotations before meaningful statistical analysis can be performed. Surprisingly, better agreement between CFD and experimental data was obtained using the k.ε than the SSG Reynolds stress model. The multiphase inhomogeneous approach used here with simplified physics assumptions gives good agreement for power consumption, and with PIV measurements with flat and deformed free surfaces, making this affordable method practical to avoid the erroneous modelling assumption of a flat free surface often made in such cases

Alternate operating methods for improving the performance of a continuous stirred tank reactor

The effect of the pumping direction of an axial flow impeller, the feeding rate and the number of feed inlets on the operation of a continuously-fed stirred tank has been studied using CFD. The flow patterns generated by the up-pumping and down-pumping impeller, under both ‘typical’ and ‘intensified’ operating conditions, are compared. The effect of various tank configurations on the performance of the vessel is assessed by analysing the flow and power numbers, as well as the concentration field of a non-reactive tracer. Furthermore, the inlet feed jets are reduced using traditional jet similarity analysis and are compared with that of a typical round jet. The results show that up-pumping impellers improve circulation in the upper part of the tank and reduce shortcircuiting of the feed stream with only a small increase in power consumption. Furthermore, by using multiple feed inlets to increase the total throughput capacity, the amplitude of torque fluctuations is decreased and impeller bypassing is also decreased. The ensemble of conclusions suggest that the throughput capacity and mixing quality of a CSTR can be improved, without problems of short-circuiting, by employing up-pumping impellers coupled with multiple surface feed points

Jet injection studies for partially baffled mixing reactors: a general correlation for the jet trajectory and jet penetration depth

This paper is devoted to the analysis of the jet trajectories, obtained using computational fluid dynamics (CFD), at two different scales (laboratory and industrial) with application to quenching of runaway reactions. One of the goals was to describe how the jet penetrates the fluid in the stirred vessel and to build an easy to use correlation for research and industrial purposes. A model of the jet trajectory based on the analogy with a jet in a cross-flow has been used to predict the jet trajectory at the pilot and industrial scales. The correlation, built using a statistical analysis, has shown that the jet in a cross-flow model performs very well to describe the jet trajectories. A very interesting conclusion is that the correlation constants were found to be independent of scale. Finally, the authors proposed a definition of the penetration depth and use it in its dimensionless form to predict how the jet penetrates in the industrial vessel with the current injection conditions

PIV measurements in an aerated tank stirred by a down- and up-pumping axial flow impeller

Liquid phase hydrodynamics in an aerated tank stirred by a down- and an up-pumping pitched blade turbine have been investigated using Particle Image Velocimetry. The effect of agitator configuration and the gas phase on the mean velocity fields and turbulent quantities in the vessel have been investigated. The global mean gas holdup has also been evaluated for the two pumping conditions. For the gas flow rate used, the presence of gas only slightly alters the liquid flow patterns produced by both the down- and up-pumping configurations and causes a general decrease in the mean liquid velocities. The turbulent kinetic energy in the impeller discharge region was not affected by the presence of gas, but in the bulk of the tank, aeration caused a decrease in this value. Global gas holdup was found to be ~36% greater for the up-pumping impeller and a large amount of gas was found to be entrained by the primary circulation loop

An evaluation of risk from sea level rise and storm surge on subpopulations of Brachytrupes megacephalus (Lefèbvre, 1827) on the island of Malta

Brachytrupes megacephalus (Lefèbvre, 1827) is a relatively large gryllid with predominant but not exclusive distribution across northern Africa, favouring sandy habitats in coastal and Saharan hyper-arid regions. The crepuscular species is also known to occur in certain areas of Europe’s central Mediterranean littoral. The species’ distribution in the Maltese Islands is restricted to fragmented populations on northern coastal sites in Malta and one site in Gozo. The present contribution focuses on the vulnerability of the species’ habitat due to sea level rise and storm surges and estimates that habitat loss on the basis of three modelled inundation scenarios at 5 m, 10 m and 15 m contour heights would be 28.1%, 49.7% and 65.9% respectively. Furthermore, the models developed serve to provide insights for conservation management, specifically through elucidating the potential for linkages that ensure ecological connectivity for subpopulations on elevated terrain.peer-reviewe

A novel method to include the free surface in a CFD model of jet injection into partially-baffled mixing vessels

International audienceA novel methodology that can be used to perform simulations of fluid jet injection into a mixing vessel with a deformed free surface is presented and validated. The idea is to first use a two-fluid model to determine the location and shape of the free surface, and then results from the simulation are used to generate a single-phase model that has the same free surface shape. Example simulations are performed using ANSYS CFX for a pilot-scale partially-baffled vessel. Application of the methodology in the study of reaction quenching is presented but it is believed that this methodology has many other possible applications involving fluid or solid injection