Peter J. Witt

A numerical model for predicting bubble formation in a 3D fluidized bed

Abstract Fluidized bed systems have the potential to be widely used in the power generation, mineral processing and chemical industries. One factor limiting their increased use is the lack of adequate design techniques for scaling such systems. A model has been developed for simulating gas–solid fluidized bed plant. The model uses a multiphase Eulerian–Eulerian technique to predict the transient behaviour of fluidized bed systems. The commercial CFD code CFX is used as the computational framework for solving the discretized equations. To overcome the problem of accurate geometrical representation experienced in previous models a body fitted grid system is employed. The model is used to predict isothermal flow in a three-dimensional bubbling fluidized bed. Predictions of the three-dimensional model show bubble formation with gas bubbles or voids preferentially moving along the centre of the bed. Predicted behaviour is qualitatively consistent with experimental observations.

CFD simulation of free surface flow and heat transfer of liquid slag on a spinning disc for a novel dry slag granulation process

Molten slag is atomised by a spinning disc into droplets that are quenched via cold air to produce glassy slag granules. Steady-state two-dimensional CFD simulations of the multiphase flow and heat transfer in the slag and air were carried out. Also considered in the CFD model is conjugate heat transfer between liquid slag, solidified slag and the disc. The thickness and temperature of the slag film prior to its rupture were predicted. Influence of factors including the slag pouring rate, temperature and the disc spinning speed on the slag film thickness and temperature were investigated.

Prediction of dust loss from conveyors using computational fluid dynamics modelling

Abstract Dust lift-off from conveyors forms a significant environmental and operational problem for operators in the mining, power generation and process industries. One means of reducing dust lift-off is to provide airflow deflectors or other aerodynamic modifications to the conveyor. A computational fluid dynamics (CFD) model has been developed to take into account the effect of wind direction, velocity and conveyor guarding on the dust loss from conveyors. The model is developed in the framework of CFX4. Experimental measurements of dust lift-off from the surface of a bed of ore in a wind tunnel at different wind velocities are used to characterise the dust. Based on the experimental data a model for predicting the mass and particle size distribution lifted from the bed surface at different air velocities is developed. The dust loss model is coupled to a Lagrangian particle-tracking model to predict particle trajectories. Validation of the model is undertaken by comparing CFD predictions against wind tunnel test work and shows good agreement. Results are presented for a typical conveyor design. The combination of experimental and CFD modelling is found to be a powerful tool for analysing dust loss from conveyors and can be extended to stockpiles and other situations where dust loss is a problem. The model can readily be extended to account for heat and moisture transfer in beds of porous materials.

The swirling flow structure in supersonic separators for natural gas dehydration

The supersonic separator is a novel compact tubular device for natural gas dehydration. The separation mechanism is not well understood for the complicated fluids with a delta wing located in the supersonic flows. We investigated the gas swirling separation characteristics in supersonic velocities using the Reynolds stress turbulence model. The results showed that the Laval nozzle designed with the cubic polynomial and Foelschs analytical methods formed an extremely stable and uniform supersonic flow. The delta wing generated a strong swirling flow with the centrifugal acceleration of around 107 m s−2 to remove the condensed liquids from the mixture. However, the supersonic flow was quite sensitive to the delta wing, which led to the disturbance and non-uniformity of the gas dynamic parameters. This violent variation in the supersonic flow had a secondary action on the condensation, even resulting in the re-evaporation of the condensed liquids.

Tube erosion modelling in a fluidised bed

Abstract The paper presents the results of a computational model of erosion in a fluidised bed and a corresponding erosion experiment. The experiment has been simulated using the CFX [CFX-F3D, Version 4.1, Flow Solver User Manual, Computational Fluid Dynamics Services, AEA Industrial Technology, Harwell Laboratory, Oxfordshire, UK, 1995] code with computational models of hydrodynamics (hydrodynamic model A and kinetic theory model) and erosion (Finnie and kinetic theory). The experiment has been conducted at room temperature using a horizontal acrylic tube immersed in a rectangular fluidised bed for a total of 126 h of run. Erosion measurements were made every 14 h at eight equally spaced positions around the tube. The results show an induction period of 42 h. Most of the wear occurred around the bottom of the tube with the maximum at an angle of about 45° from the tube bottom. The kinetic theory model predictions are in good agreement with the experimental results.

Comparison of Two-Equation Turbulence Models in Simulation of a Non-Swirl Coal Flame in a Pilot-Scale Furnace

The capability of six two-equation Reynolds-averaged Navier-Stokes (RANS) models for simulation of a non-swirl coal flame in a pilot-scale furnace has been investigated. These turbulence models—the standard k-ϵ model, re-normalization group (RNG) k-ϵ model, modified k-ϵ model, Wilcox k-ω model, Menter k-ω model (also called BSL model), and Shear-stress transport (SST) model—are assessed with the use of measured gas phase velocity, temperature, oxygen, and carbon dioxide volume fraction data from the literature. Predictions of the standard k-ϵ model, RNG k-ϵ model, BSL, and SST model are generally in good agreement with the experimental data. The Wilcox k-ω model generally overpredicts O2 volume fraction and underpredicts CO2 volume fraction. The modified k-ϵ model yields results that have large discrepancies from measurements.

Numerical simulation and validation of gas-particle rectangular jets in crossflow

Abstract This paper presents a numerical study of a gas-particle flow in three inclined rectangular jets in crossflow. The predicted gas phase velocities and particle phase velocities are validated against previously reported experimental data. Two turbulence models, the standard k–ɛ model and Shear Stress Transfer (SST) model, are used to model the gas phase turbulence. This work shows that both models provide acceptable predictions of the gas flow and mixing generated by the three jets. Neither model could accurately reproduce the jet core and the flow near bottom wall. The particle phase in this flow comprises a large number of small particles. Thus particles follow the gas phase flow closely and any errors in the turbulence model and gas flow predictions are passed on to the particle phase simulation. This paper also includes a literature review on rectangular jets in crossflow and gas-particle laden jets in crossflow.

CFD Modelling of the Effects of Operating Parameters on the Spreading of Liquids on a Spinning Disc

A novel dry slag granulation process based on a spinning disc is being developed by CSIRO. This process utilises centrifugal force to break up molten slag into droplets, which are then quenched into solidified granules by a flow of cold air. In this process the sensible heat of slag is recovered as hot air. In the present work, a previously developed steady-state, two-dimensional and multiphase CFD model was applied to perform parametric numerical experiments to investigate the effects of a number of parameters on the liquid film thickness at the disc edge, which included liquid mass feeding (pouring) rate, disc spinning speed, disc radius, liquid viscosity, density and surface tension. The modelling results were compared with experimental data and were found to be in good agreement. To reduce the number of simulations needed, Box and Behnkens fractional factorial design of numerical experiment was adopted. Furthermore, in order for the modelling results to be applicable to atomisation of different liqui...

Modeling Issues in CFD Simulation of Brown Coal Combustion in a Utility Furnace

This paper describes the mathematical formulation and modelling issues of a computational fluid dynamics (CFD) model of a 375 MW utility furnace. This tangentially-fired furnace is fuelled by high moisture content brown coal from coal mines at Latrobe Valley in Victoria, Australia. The influences of different turbulence models, particle dispersion, and radiation models on the CFD prediction are investigated. Two turbulence models, standard k-ϵ model and Shear-Stress Transport (SST) model, provide similar predictions that are in good agreement with the plant data. The effect of particle dispersion on the prediction is found to be insignificant for this high-volatile brown coal. The predicted wall incident radiation flux based on two radiation models, namely, discrete transfer (DT) model and P-1 model are compared against power plant measurements. The comparison reveals that the DT model provides good prediction of the radiation profiles, while the P-1 model considerably underpredicts the wall incident radi...

Numerical Modeling of Flow Dynamics in The Aluminum Smelting Process: Comparison Between Air–Water and CO2–Cryolite Systems

Air–water models have been widely applied as substitutes for CO2–cryolite systems in the study of the complex bubble dynamics and bubble-driven flow that occurs in the molten electrolyte phase in the aluminum electrolytic process, but the detailed difference between the two systems has not been studied. This paper makes a numerical comparison between the bubble dynamics for the two systems. Simulations of both single bubble and continuous bubbling were conducted using a three-dimensional computational fluid dynamics (3D CFD) modeling approach with a volume of fluid (VOF) method to capture the phase interfaces. In the single bubble simulations, it was found that bubbles sliding under an anode in a CO2–cryolite system have a smaller bubble thickness and a higher sliding velocity than those in the air–water system for bubbles of the same volume. Dimensionless analysis and numerical simulation show that contact angle is the dominant factor producing these differences; the effects of kinematic viscosity, surface tension, and density are very small. In the continuous bubbling simulations, the continuous stream of air bubbles detaches from the anode sidewall after a period of climbing, just as it does in the single bubble simulation, but bubbles have less tendency to migrate away from the wall. Quasi-stable state flow characteristics, i.e., time-averaged bath flow pattern, turbulence kinetic energy, turbulence dissipation rate, and gas volume fraction, show a remarkable agreement between the two systems in terms of distribution and magnitude. From the current numerical comparisons, it is believed that the air–water model is a close substitutive model for studying bubble-driven bath flow in aluminum smelting processes. However, because of the difference in bubble morphologies between the two systems, and also the reactive generation and growth of bubbles in the real system, there will likely be some differences in bubble coverage of the anode in the anode–cathode gap.