Jong-Leng Liow

A model of bubble growth in wetting and non-wetting liquids

Abstract A mathematical model for predicting bubble growth, bubble volume, bubble shape and frequency of bubbling for bottom gas injection in wetting and non-wetting liquids is presented. The model takes into account the slip of the bubble and the contact angle. The contact angle, together with the chamber volume and gas flow rate are found to play an important role in determining the size of the bubble formed. The calculated results of the bubble volume for argon-iron and argon-cast iron systems are in good agreement with experimental data across the surface tension to the inertia-controlled regimes. The model highlights the importance of the effect of a refractory surface on bubble formation and establishes a quantitative basis for on-line pressure monitoring of gas injection in metallurgical processes.

Numerical modelling of free surface flows in metallurgical vessels

Abstract A numerical model for simulating the transient behaviour of multi-fluid problems defined in 2D rectangular and cylindrical geometries is presented. The model uses a piecewise linear volume tracking scheme, and maintains sharp interfaces and captures fine-scale flow phenomena such as fragmentation and coalescence. The numerical model was applied to four problems of pyrometallurgical relevance – entrainment of matte in the flow of slag during skimming operations, splash resulting from a drop impinging on a bath, bubble rise in a liquid bath, and top-submerged gas injection. The numerical predictions are in good agreement with the published experimental results. The simulation of top-submerged gas injection showed, in detail, the phenomena of bubble formation, bubble rise, and splash drop formation and recoalescence with the bath. Data useful for engineering purposes such as pressure traces and time-averaged flow fields were obtained, allowing assessment of splash behaviour for given gas injection conditions. The numerical model has been shown to be versatile in being able to adapt to a wide range of multi-phase flow problems.

Quasi-equilibrium bubble formation during top-submerged gas injection

Abstract The size of a bubble formed by top-submerged gas injection at very low air flow rates into water was studied and shown not to be related to the size of an equilibrium bubble at 180° contact with the lance. The growth of the bubble was initially governed by the inner lance diameter but after the maximum bubble pressure had been exceeded, the bubble size was governed by intermediate diameters between the inner and outer lance diameters. The bubble became unstable when the water–air–solid contact line moved up the outside of the lance and detachment followed quickly. For the formation of argon bubbles in pig iron, the bubble volume was determined by the outer lance diameter when the contact angle between the bubble and lance surface is at 90°. Application of the results to the maximum bubble pressure method for determining surface tension of non-wetting liquids showed that for large-diameter lances, two pressure peaks can be obtained if the chamber volume behind the lance tip is small. The first peak is related to the maximum equilibrium bubble at the inner lance diameter and the second peak is related to the outer lance diameter, both peaks can be used to determine the surface tension.

Slopping resulting from gas injection in a Peirce-Smith converter : water modeling

The formation of standing waves in a water model of a Peirce-Smith converter was studied experimentally. The occurrence of a standing wave was found to be determined by the bath depth and tuyere submergence. The results showed that it is possible to obtain regions in the bath depth and tuyere angle/tuyere submergence plots where no standing waves were found and spitting was minimal. It is considered that the wave steepness is responsible for the changes in standing wave mode found in the water model and can contribute to the splashing of liquid from the first asymmetric standing wave if the amplitude of the standing wave is large. The results showed that the lower gas flow limit of the first asymmetric standing wave can be calculated from the power per unit mass required to sustain the standing wave. An experimental value of 0.7 to 1.0 watt/kg was obtained for the power per unit mass from the potential power supplied. It was shown that the contribution of the kinetic energy of the gas flow to the power per unit mass to form a first asymmetric standing wave can be obtained as a function of the tuyere angle.

Peer assessment in thesis oral presentation

Peer assessment has been studied in various situations and actively pursued as a means by which students are given more control over their learning and assessment achievement. This study investigated the reliability of staff and student assessments in two oral presentations with limited feedback for a school-based thesis course in engineering where the different disciplines ran their own assessment. Staff scores were generally found to be more reliable than student scores, but it was not consistent. The engineering disciplines displayed widely varying differences between the staff and student scores. A large variation in reliability was found for the different disciplines making it difficult to reconcile to a standard school-based overall grading scheme for the course when the reliability of marking was low. The results also showed that the average scores for oral presentations are generally much higher than for written examinations, based on the grade point average. Future research will need to investigate how best to develop a consistent assessment framework for oral presentations and moderation of scores for consistency, particularly for large classes where assessment are done by different groups of staff and students and collated into a single set of marks for the class.

LES of Turbulent Bubble Formation and Break-Up Based on Interface Tracking

A 3D finite-difference method featuring piecewise planar volume tracking and explicit sub-grid scale modeling is used to simulate the violent, turbulent bubbling resulting from air venting into a water pool through a downcomer pipe. A moderate-sized mesh and domain decomposition-based parallelism were used for a O(105) − O(106)-timestep computation, in order to extract turbulence statistics representative of long-time simulation of fully-developed flow. Volume tracking is seen to be a robust basis for super-grid scale simulation of the bubble rise, fragmentation and coalescence phenomena; it captures the kinematics of interfaces that are adequately resolved on the grid, and preserves the existence of bubbles and liquid jets breaking up to grid-scale (1–2 cells) size. Interface deformations are seen to strongly correlate with the large-scale structures forming at the front in the gas phase. Enhanced energy decay according to the 8/3 power law seen in bubbly flow is generally attained, with also some tendency towards the Kolmogorov K41 slope. The bubbling is turbulent on the gas side, and the main sources of turbulence are bubble break-up and gas jetting from the downcomer tip.

Study of refractory wear in the tuyere region of a peirce-smith nickel convertor

Abstract Profiles of the composition variation with distance from the hot face were mapped out by SEM analysis of chrome-magnesite tuyere refractory bricks from a Peirce-Smith nickel converter. The presence of magnesium sulphate was confirmed by XRD, SEM and wet analysis. The work showed that periclase was highly susceptible to attack by sulphur dioxide and formed magnesium sulphate in the cooler regions of the refractory brick where the reaction was favourable. Thermal cycling caused the periclase to alternate between the sulphate and the oxide leading to increased pore formation and decreased brick strength. This combined with tuyere punching contributed to the enhanced loss of refractory at the tuyere line. The accretion layer had a high nickel ferrite content which agreed with the high oxygen potential expected at the tuyere region. An interface region between the accretion layer and the refractory showed that counterdiffusion of iron and magnesium assisted in removing periclase from the refractory. Nickel and nickel compounds were limited to the first two mm of the hot face and no infiltration was found. The chrome-spinel is more resistant to attack than the periclase. The pore structure in the refractory brick was identified as the key variable in resisting sulphate formation. An ability to close or fill the pores will reduce the possibility of sulphur dioxide diffusion into the cooler regions for magnesium sulphate formation.

Experimental Study of Cutting Forces in Micro End-Milling

Micro-end-milling is an efficient and economical manufacturing operation that is capable of accurately producing high aspect ratio features and parts. It is important to study the cutting forces in micro-milling for the planning and control of the process. This paper presents an experimental study of the cutting forces in micro-end-milling of a 6160 aluminum alloy. The measured cutting forces are presented and discussed for different cutting conditions, such as various feeds per tooth, cutter diameters, and cutting speeds. It is found that the peak cutting forces increase with increasing cutting speed and feed rate. The effects of tool runout on the cutting forces were also analyzed based on the experimental results, from which the influences of feed rate and cutting speed are found to be obvious.

Slopping resulting from gas injection in a peircesmith converter: The period of the standing wave

The periods of the transverse standing waves occurring in the Peirce-Smith converter geometry as a function of bath depth were solved using linear wave theory by formulating an integral equation of the second kind. The eigenvalues of the integral equation gave the wave periods which were calculated to four-figure accuracy. The eigenvalues were approximated using Chebyshev polynomials for the first four wave modes. They include the complete range of eigenvalues for the first two symmetric standing waves, which has not been reported in the literature. The asymptotic eigenvalues for the transverse standing waves are given. Experimental results from a water model agreed with the theoretical predictions over a wide range of bath depths. The first symmetric standing wave was enhanced by the presence of the longitudinal standing wave, which showed that the three-dimensional geometry of the model was important in the modeling of the Peirce-Smith converter. Data from a Peirce-Smith copper converter were in reasonable agreement with the theory. An analysis of the standing wave frequency showed that the presence of standing waves could be minimized by a change in the geometry.

Low-cost design of 35 mm drum camera for high-resolution, high-speed image analysis

A design for a continuous access high-speed 35 mm photographic imaging system with a maximum frame rate of 3000 frames/s is presented. The design is based on a readily available 35 mm camera body and other inexpensive components and can operate with f numbers of f5.6 and above. To demonstrate the system, results are recorded of a water drop impacting a free surface with a resolution per frame of 1200×2500 grayscale pixels and 152 third frame images per run.