Vivek V. Ranade

Dynamics of gas-liquid flow in a rectangular bubble column: Experiments and single/multi-group CFD simulations

Abstract Several flow processes influence overall dynamics of gas–liquid flow and hence mixing and transport processes in bubble columns. In the present work, we have experimentally as well as computationally studied the effect of gas velocity, sparger design and coalescence suppressing additives on dynamics of gas–liquid flow in a rectangular bubble column. Wall pressure fluctuations were measured to characterize the low frequency oscillations of the meandering bubble plume. Bubble size distribution measurements were carried out using high-speed digital camera. Dispersed gas–liquid flow in bubble column was modelled using Eulerian–Eulerian approach. Bubble population was represented in the model with a single group or multiple groups. Bubble coalescence and break-up processes were included in the multi-group simulations via a suitable population balance framework. Effect of superficial gas velocity and sparger configurations was studied using single-group simulations. Model predictions were verified by comparison with the experimental data. Role of bubble size in determining plume oscillation period was studied. Multi-group simulations were carried out to examine evolution of bubble size distribution. An attempt is made to understand the relationship between local and global (over all the dispersion volume) bubble size distribution. The models and results reported here would be useful to develop and to extend the applications of multi-group CFD models.

Fluid mechanics and blending in agitated tanks

A flow and mixing model can provide a sound fundamental basis for the quantitative and optimum design of impeller and tank geometries. This paper describes the results of detailed numerical simulations using such a model for the flow generated by a downflow-pitched blade turbine in a fully baffled cylindrical vessel and the subsequent bulk mixing. Comparisons of predicted flow characteristics with experimental data (measured in a 0.3 m i.d. vessel using a laser Doppler anemometer) show good agreement. The results of this flow model were then used to simulate the mixing of an inert tracer (introduced in pulse mode) in agitated tanks. Various mixing time definitions were studied and compared with published correlations. Some numerical experiments were performed to generate guidelines for the development of new impellers.

FLOW GENERATED BY PITCHED BLADE TURBINES II: SIMULATION USING κ-ε MODEL

Experimental data on average velocity and turbulence intensity generated by pitched blade downflow turbines (PTD) were presented in Part I of this paper. Part II presents the results of the simulation of flow generated by PTD The standard κ-e model along with the boundary conditions developed in the Part 1 have been employed to predict the flow generated by PTD in cylindrical baffled vessel. This part describes the new software FIAT (Flow In Agitated Tanks) for the prediction of three dimensional flow in stirred tanks. The basis of this software has been described adequately. The influence of grid size, impeller boundary conditions and values of model parameters on the predicted flow have been analysed. The model predictions successfully reproduce the three dimensionality and the other essential characteristics of the flow. The model can be used to improve the overall understanding about the relative distribution of turbulence by PTD in the agitated tank

An efficient computational model for simulating flow in stirred vessels: A case of Rushton turbine

Abstract Computational tools are being increasingly used to analyse flow and mixing in baffled stirred vessels. In a baffled stirred vessel, flow around the rotating impeller blades interacts with stationary baffles and generates a complex, three-dimensional, recirculating turbulent flow. We have developed an efficient computational model, in which a quasi-steady flow is computed for any momentary impeller position. This model adequately captures most of the significant details of the flow both within and outside the impeller without requiring any empirical input/ adjustable parameter. The method was applied to the flow generated by a standard Rushton turbine, for which detailed experimental data are available. A case of fully baffled vessel with standard Rushton turbine (DT) was simulated using FLUENT code. The impeller rotation was modelled in terms of appropriate source terms at the blade surfaces. The laminar and turbulent flow generated by DT were simulated using this model. The model predictions were validated by comparisons with the published experimental data. Overall impleller performance characteristics like pumping number and power number were also compared with the experimental data for both, laminar and turbulent flow regimes. The approach presented here can be used as a general purpose, mixer design tool.

Trailing Vortices of Rushton Turbine: PIV Measurements and CFD Simulations with Snapshot Approach

Understanding fluid dynamic characteristics of trailing vortices behind impeller blades and the capability to computationally simulate these vortices is essential for reliable design and scale-up of stirred reactors. In this paper, trailing vortices behind the blades of a standard Rushton turbine were studied using particle image velocimetry (PIV). Angle resolved and angle averaged flow fields near the impeller blades were measured and the structure of trailing vortices was studied in detail. A computational snapshot approach of Ranade and Dommeti was extended and used to simulate flow generated by the Rushton turbine in baffled stirred vessels. The approach was implemented using the commercial CFD code, FLUENT (of Fluent Inc, USA). Two turbulence models, namely, standard k – ɛ model and renormalization group version (RNG) of k – ɛ model were used for simulating the flow in stirred vessels. Predicted results were compared with the angle resolved PIV measurements to examine whether the computational model captures the flow structures around impeller blades. Predicted results were also compared with the angle averaged PIV data. Predicted gross flow characteristics like pumping number were also compared with the present and previously published experimental data. The results and conclusions drawn from this study will have important implications for extending the applicability of CFD models for simulating flow near impeller blades.

A COMPUTATIONAL SNAPSHOT OF GAS-LIQUID FLOW IN BAFFLED STIRRED REACTORS

Abstract In a stirred reactor, flow around the rotating impeller blades interacts with the stationary baffles and generates a complex, three-dimensional, recirculating turbulent flow. When gas is sparged in such a reactor, gas tends to accumulate in the low pressure region behind the impeller blades forming so-called gas cavities, which significantly alter the flow and turbulence in the reactor. In this paper, a computational technique is developed to predict the turbulent gas—liquid flow in a stirred reactor. The technique is also able to predict the flow around the impeller blades and the accumulation of gas behind these blades. Unlike the past efforts, no empirical (in the form of impeller boundary conditions) is required. A computational snapshot approach has been used to model the gas—liquid flow in a stirred reactor with a disc turbine. This approach essentially boils down to capturing the flow characteristics of a stirred vessel at one time instant from the solution of steady-state equations with boundary conditions corresponding to that particular time instant. A mathematical model is developed for turbulent, dispersed gas—liquid flow. The time-averaged two-phase momentum equations are solved by using a finite volume algorithm. The turbulent stresses are simulated using ak—ɛ model. The distribution of gas around the impeller blades is predicted for the first time. The model also enables an a priori prediction of the drop in the power dissipated by the impeller in the presence of gas. Predicted flow characteristics of the gas—liquid reactor show good agreement with the experimental data.

Modelling of fluid dynamics and mixing in shallow bubble column reactors: influence of Sparger design

Fluid dynamics and mixing in shallow bubble column reactors is controlled by sparger configuration. In this paper, we evaluate the applicability of using computational fluid dynamics (CFD)-based models to simulate transient fluid dynamics and mixing in shallow bubble columns. A two-fluid model along with the standard k–e turbulence model was used to simulate flow generated by gas sparged through two different spargers in a bubble column with height to diameter ratio of 2. Different mixing time definitions were evaluated using the CFD-based simulations of mixing of passive tracer. The role of unsteady fluid dynamics and implications of possible simplifications in the geometry of bubble columns for simulating mixing in bubble columns were studied. The simulated results were analysed with reference to the experimental data of Haque et.al. (1986. Chemical Engineering Journal, 33, 63–69). The presented results and analysis will be useful to extend the application of CFD models for screening alternative sparger designs.

Gas-liquid flow in stirred reactors: Trailing vortices and gas accumulation behind impeller blades

In a gas–liquid stirred reactor, gas tends to accumulate in low-pressure regions behind the impeller blades. Such gas accumulation significantly alters impeller performance characteristics. We have computationally investigated gas–liquid flow generated by a Rushton (disc) turbine. Rotating Rushton turbine generates trailing vortices behind the blades, which influence the gas accumulation in the impeller region. Characteristics of these trailing vortices were first investigated by considering a model problem of flow over a single impeller blade. Predicted results were compared with the published experimental data. Circulation velocity and turbulent kinetic energy of the trailing vortices were found to scale with blade tip velocity. Several numerical experiments were carried out to understand interaction of gas bubbles and trailing vortices. Gas–liquid flow in stirred vessel was then simulated by extending the computational snapshot approach of Ranade and Dometti (Chem. Engng Res. Des., 74, 476–484, 1996). The approach was able to capture the main features of gas–liquid flow in stirred vessels. The detailed analysis of predicted results with reference to experimental data and the results obtained for flow over a single impeller blade will be useful for extending the scope of computational fluid dynamics (CFD) based tools for engineering gas–liquid stirred reactors.

Flow in bubble columns : some numerical experiments

Bubble columns are being widely used in the chemical and petrochemical industry. It has become increasingly important to develop theoretically sound models for scaling up and designing of these reactors for more complicated reaction systems. Such reactor models would require detailed knowledge of the flow characteristics of the reactor. This paper presents a numerical simulation model for predicting the details of turbulent gas—liquid flow encountered in bubble columns. The time-averaged Navier—Stokes equations for the liquid phase are solved, using a finite-volume solution algorithm. The turbulent stresses are obtained from the κ—e model. The bubble slip velocities were specified externally to the solution, avoiding the need for the solutions of the gas-phase momentum equations. The results of various numerical experiments are described to highlight the influences of key parameters on the characteristics of flow in bubble columns.

Modelling of Turbulent Flow in a Bubble Column Reactor

A comprehensive mathematical model for describing flow, turbulence and gas hold-up distribution in a bubble column reactor is developed. For large diameter bubble columns (>0.1m), operated with the zero or low liquid throughput, sparged gas tends to pass preferentially through the central portion of the column. This non-uniform gas hold-up distribution leads to macroscale circulation and turbulence in the reactor. Two new mechanisms are proposed and modelled to explain such a non-uniform gas hold-up distribution in bubble columns. The influence of accompanying wakes and of the column wall on the motion of gas bubbles is accounted for for the first time. Turbulent, dispersed gas-liquid flow is described by the time averaged two phase momentum equations. The turbulent stresses are simulated using a k-ɛ model. Extra terms arising from the gas hold-up fluctuations and pressure gradients are included in the model. The predicted flow characteristics of the bubble column reactor are verified by comparison with the published experimental data over a wide range.