Stein Tore Johansen

Fluid dynamics in bubble stirred ladles: Part II. Mathematical modeling

A mathematical model which describes the fluid flow in a bubble stirred ladle is presented. The model predicts mean flow, turbulent characteristics, bubble dispersion, and gas-liquid interaction from fundamental principles. Numerical predictions for a water model of a ladle show very satisfactory quantitative agreement with experimental results for all regions of the ladle. The model is applied to the study of refractory wear and yields results that are in qualitative agreement with practical experience.

Fluid dynamics in bubble stirred ladles: Part I. experiments

Air is supplied through a porous plug placed in the center axis of a cylindrical perspex-water model of a ladle. A Laser-Doppler system is employed to measure radial and axial mean and fluctuating velocities. Velocities in the two-phase bubbly region can also be determined. Velocities are measured near the bottom, half-way up, and near the free surface. It is shown that the bubbles contribute to production of turbulence. The ladle has recirculation zones near the bottom, where the mean velocities are very low. Close to the free surface the radial mean and turbulent velocities are high, promoting mass transfer through the interface. The present measured velocity profiles cannot be reduced to a single profile by employing similarity scaling.

A mechanistic model to determine the critical flow velocity required to initiate the movement of spherical bed particles in inclined channels

Abstract This study presents a mechanistic model that predicts the critical velocity, which is required to initiate the movement of solid bed particles. The model is developed by considering fluid flow over a stationary bed of solid particles of uniform thickness, which is resting on an inclined pipe wall. Sets of sand bed critical velocity tests were performed to verify the predictions of the model. An 80 mm flow loop with recirculation facilities was constructed to measure the critical velocities of the sand beds. The tests were carried out by observing the movement of the bed particles in a transparent pipe while regulating the flowrate of the fluid. Water and aqueous solutions of PolyAnoinic Cellulose were used as a test fluid. The critical velocities of four sand beds with different particle size ranges were measured. The model was used to predict the critical velocities of the beds. The model predictions and experimentally measured data show satisfactory agreement. The results also indicated that the critical velocity is influenced by the properties of the fluid, flow parameters, and particle size.

Mechanistic model for cuttings removal from solid bed in inclined channels

This paper presents the results and analysis of a set of erosion rate experiments, designed to investigate the removal rate of stationary sand bed particles in an inclined channel. The erosion rate tests of three beds with different bed particle-size ranges show that beds with intermediate average particle size have the maximum erosion rate. The theoretical analysis using a mechanistic model supports this observation. The instantaneous acceleration of bed particles at the beginning of transportation is correlated with particle removal rate. It is shown that the mechanistic model can predict optimum operating parameters to improve the efficiency of hole cleaning.

Some considerations regarding optimum flow fields for centrifugal air classification

Abstract Centrifugal air classifiers, or classifiers with some sort of centrifugal field, are used in classifying particulate materials in the cut size range between a few hundred microns to below one micron. The sharpness of cut in these devices depends on adequate dispersion of the feed material and the maintenance of identical forces on the particles, regardless of their position, in the classifying zone. This paper demonstrates, on the basis of computer simulations, that the latter condition is not as easy to realize as one might wish.

A two-phase model for particle local equilibrium applied to air classification of powders

Abstract A mathematical model has been devised to study air classification of fine powders. The model assumes that the particles have short relaxation times in the carrier gas. Hence, the particles are regarded locally as stationary, settling particles. The particles are allowed to exchange momentum with the fluid and they can alter the turbulent structure of the carrier gas. The present work considers only steady operation. A version of a continuous ACUCUT air classifier has been studied. The turbulent flow pattern in the classifier is analysed, and grade efficiency curves are predicted. The results show that increased particle loading in the classifier can improve its performance, by damping the turbulence. However, high loading may change the macroscopic flow pattern due to momentum coupling between the phases. The analysis also suggests that vortex shedding may occur in the classifier. Such unsteady flows may impair classification and should be avoided. The present model can be used to modify the classifier design to obtain steady flow.

Flow pattern and alloy dissolution during tapping of steel furnaces

Alloying of steel during tapping from BOFs and EAFs has been studied by computational fluid dynamics in two-dimensional axisymmetric models of two ladles. The flow patterns and particle trajectories have been computed for six different levels of steel in the tapping ladle, five different alloy sizes, two alloy injection points, and three types of bulk alloy (FeMn, SiMn, and FeSi75). Based on the fluid dynamics in the ladle and a definition of good alloying practice, conclusions with regard to alloy sizing and timing of alloy addition have been established. The computational results support findings in plant tests, which show the benefit of using small sized alloys. Furthermore, a method that allows us to estimate the optimum feeding rate for alloy additions during steelplant operation has been developed. Results from full scale tests in steelplants are shown.

Modelling of bubble behaviour in aluminium reduction cells

A phenomenological model for the creation and transport of anodic gas bubbles in aluminium reduction cells is presented. The proposed model is a multiscale approach in which molecular species are produced and transported through a supersaturated electrolyte. Sub–grid bubbles are allowed to form through nucleation and the resulting bubble population evolves through mass transfer and coalescence. As sub–grid bubbles reach a certain size, they are transferred to a macroscopic phase, which evolution is governed by a volume of fluid method. The current work describes the underlying theory and motivation for the proposed model and it is used to describe a laboratory–scale electrolysis cell, showing the potential of the suggested approach. The influence of selected properties of the model is identified by means of a factorial analysis.

Acoustic Wave Propagation in Low Mach Flow Pipe

Flow induced vibrations and resulting sound are observed in many situations, with applications ranging from music and human sciences to process and aerospace industry. In the case of flow through corrugated pipes extremely strong acoustic field can be produced, which in extreme cases can result in structural and component damage. In the present paper we analyze the physical phenomena resulting in sound production in corrugated pipes. The flow over single cavities is computed and it is shown that a given corrugation geometry can cause self sustained pressure oscillations caused by interactions between vortices shed from neighboring corrugation cavities. In a larger computation the flow and resulting acoustics in a corrugated, 0.614 m long, pipe is analyzed. Now, a large scale acoustic field is captured, leading to formation of standing sound waves in the pipe. The simulations of the singing frequencies compared well to measured frequencies versus flow velocities. Based on the mechanisms revealed through this analyzes a general feedback loop mechanism for flow induced sound in corrugated pipes is proposed.

Filter-Based Unsteady RANS Computations for Single-Phase and Cavitating Flows

The widely used Reynolds-Averaged Navier-Stokes (RANS) approach, such as the k-e two-equation model, has been found to over-predict the eddy viscosity and can dampen out the time dependent fluid dynamics in both single- and two-phase flows. To improve the predictive capability of this type of engineering turbulence closures, a consistent method is offered to bridge the gap between DNS, LES and RANS models. Based on the filter size, conditional averaging is adopted for the Navier-Stokes equation to introduce one more parameter into the definition of the eddy viscosity. Both time-dependent single-phase and cavitating flows are simulated by a pressure-based method and finite volume approach in the framework of the Favre-averaged equations coupled with the new turbulence model. The impact of the filter-based concept, including the filter size and grid dependencies, is investigated using the standard k-e model and with the available experimental information.Copyright