A variety of projects involving hydrodynamic and transport phenomena in liquid metal processing operations have been studied at the MMPC over the years. The fluid, heat, and mass transfer, components deal with basic aspects of the mathematical and physical modelling of multi-phase, multi-dimensional flows in systems of metallurgical interest (e.g., furnaces, ladles, tundishes, twin roll casters, single belt casters, settling tanks, etc.). Thanks to the full-scale ladle-tundish-mould facility, much of the reduced scale modeling work was successfully applied at the full scale.
Transient Steel Flow Studies in Ladle Shrouds during Start-up Operations, with or with no, air Infiltration
The ladle shroud is an essential device in modern Steelmaking, connecting the ladle to the tundish, thereby protecting the de-oxidized molten steel from the atmosphere, to prevent re-oxidation. The connection between the ladle’s slide-gate nozzle, or lower nozzle, and the ladle shroud, involves an overlap between the two parts. This overlap usually involves a gasket seal through which argon is injected to displace the air.
Schematic of the ladle-ladle shroud connection and the lower nozzle-ladle shroud joint.
The main purpose for injecting argon gas into the ladle shroud, is to prevent air being sucked into the joint, leading to re-oxidation of the steel, and thereby generating billions of alumina inclusions in the 1-2 micron size range. These can then accumulate, and lead to clogging of the exit port(s). A novel transient flow visualization of the initial filling stage of a reverse-tapered Ladle Shroud was done through mathematical modelling. This allowed us to create a multi-phase flow visualization of the transient phenomena occurring within the ladle shroud-tundish system. Phenomena such as steel splashing and steel-air-argon interactions were predicted for the first time using our CFD modelling work.
Predicted steel volume fraction 3D contours at different times in isometric view.
CFD Flow Modeling and Mixing in an Elliptical Ladle
Recently, a quasi-single phase, isothermal, three-dimensional, incompressible, turbulent flow model was re-developed, to simulate and understand flow dynamics inside an elliptical ladle, and to calculate liquid mixing times occurring within it, numerically. The calculations were performed by Rohit Kumar Tiwari, under steady state conditions. He is performing this research as a PhD student in the MMPC group. The geometry of an elliptical ladle operating in an industrial set-up was developed using ANSYS SpaceClaim software. For the simulation of the quasi-single-phase method, the top layer of air is removed, and the gas-liquid plume region is introduced to incorporate the effect of argon bubbling.
Elliptical ladle geometry created in ANSYS SpaceClaim for a) single plug b) dual plug configuration
Modelling of Slag Entrainment during the Emptying of Metallurgical Vessels
The role of various operating parameters in the formation of “vortexing” and “non-vortexing” funnels, which entrain slag during ladle-teeming operations, has been studied experimentally. Models have been developed to predict slag entrainment behaviour in other vessel geometries and drainage conditions, in keeping with the vessels used in the continuous casting of steel. In collaboration with Vesuvius and Foseco, the role of inclined bottom surfaces of ladles in the formation of late forming vortexing funnels has been studied. Results suggest that off-centred nozzles placed in sloping ladles bottoms to help increase steel yield led to higher critical heights for vortexing funnels, i.e. impaired steel quality. The performance of Dofasco’s 340 ton-ladle shroud/slide gate system was studied, at the full scale, to determine critical vortexing heights and means to improve steel quality.
Modelling of Two-Phase Flows in Ladle Shroud Systems
The entrainment of air into steel flowing in the ladle shroud to the tundish was modelled, for water velocities ranging between 0.4 and 2 m/s through a 53 mm ID ladle shroud, 1.2 metres long. It was demonstrated that at low percentage volumes of gas within the shroud, small bubbles form whose terminal rising velocities are less than the downflow velocity of the liquid. This gives rise to a uniformly dispersed array of downwards moving bubbles, termed the ‘bubbly flow’ regime. At higher air entrainment ratios and low water velocities of 0.4 m/s (corresponding to a three-quarter way closed slide gate nozzle setting for example), flow through the shroud separates from the walls, a large gas cavity filling the upper half of the shroud. At liquid velocities of 2 m/s and fractional gas volumes of about 5%, the bubbly flow regime transforms into a bubbly-slug flow condition, while at still higher rates of air entrainment in the order of 10% fractional volume (corresponding to a cracked shroud for example), an air core forms, with liquid draining down the sidewalls of the shroud. These observations are important with respect to the placement of wire inserts into the sidewalls of ladle shrouds, used for the detection of slag carryover. The new water modelling facilities now allow us to re-study at the full scale, and at equivalent vacuum levels for air aspiration into the shroud, using the 10m high “pop-up” facility, and the ladle shroud/slide gate nozzle set up.
Physical Modelling of Advanced Tundish Vessels
The aim of this comprehensive project is to develop detailed physical and supporting mathematical models incorporating 3D turbulent flow, mixing, heat transfer and inclusions distribution, in tundishes. Two full-scale tundishes have been built (four and six strand delta designs) in order to test the performance of the various flow control devices used for assuring metal quality. Experimental work to be mentioned includes critical vortexing heights, during tundish emptying and refilling operations, together with residence time distribution curves as a function of flow rates, with various impact pad designs, nozzle’s well-blocks, gas entrainment levels in ladle shroud, etc. (Ben Kim, Shamik Ray, Luis Calzado, Kinnor Chattopadhyay, Ken Morales, Kaiser Hamid, Roger Ren, Sheng Chang, Justin Lee, and Jason Hsin, under the supervision of Dr. M. Isac), were involved in this work over the years.
These studies at the full scale allow for better design and control of secondary steel processing vessels, in order to design vessels that prevent harmful impurities entering the casting mould. It is particularly helpful, in this regard, to be able to install geometrically equivalent metering nozzles, ladle shrouds and SEN’s, as well as tundish “furniture.” The flow system is capable of handling flows of up to 600 litres/min, as will as being able to model thermal variations in the entering stream. The work on modelling slag entrainment in tundishes during ladle changes was continued with 2 phase flows in tundishes, and the potential role of a swirling ladle shroud as an alternative to tundish furniture for good control and helping float-out inclusions.
Modelling of Two-Phase Flows in Ladle Shroud and SEN Systems.
Over the years, we have carried out a detailed study on how the effects of gas entrainment in ladle shrouds can affect bulk flows of steel within the tundish. While the amounts of argon gas entrainment is unknown by the industry, and is not even measured, it is used so as to prevent ingress of any air to the fully killed steels during their transfer from the ladle to the tundish. Any air ingress will cause absorption of nitrogen into the steel, and the reaction of oxygen with dissolved aluminum in the liquid steel. As such, we have studied potential argon flow rates ranging from 2-10% argon/liquid steel (water) volume ratios. The bubbles formed are about 6 mm in diameter in water and spread uniformly throughout the entering jet. Using the FLUENT-ANSYS code, we were able to successfully model this two-phase flow system entering the tundish, and to demonstrate how the bubble-liquid plume was formed. It resulted in the gas bubbles decoupling from the entering jet of water (steel). The decoupling bubbles then cause a flow reversal around the core of the entering jet, particularly at higher gas/liquid ratios, which removes the slag cover from around the ladle shroud, causing slag entrainment possibilities, and allowing air ingress to the exposed liquid steel.
Dye mixing at different times following dye injection at a casting speed of 2.0 m/min. a) at an immersion depth of 133 mm. b) at an immersion depth of 200 mm.
Modelling of Micro-Bubble Swarms in a Full-Scale Water Model Tundish
Following the fundamental research study by Roger Ren (2015), we were able to produce micro-bubble swarms in our full-scale, four-strand, delta shaped tundish, located at the MMPC (McGill Metals Processing Centre). The objective of the study was to investigate the effectiveness of micro-bubbles in removing inclusions smaller than 50μm. Air was injected into a ladle shroud with twelve, laser-drilled orifices. The bubbles generated using different gas injection protocols were recorded using a high-speed camera, and the bubble images were post-processed using the commercial software, Image J. With this new ladle shroud, bubble sizes could be reduced dramatically, to as small as a 675 µm average diameter. A three-dimensional, CFD model simulation was developed, using parameters obtained from the corresponding water model experiments, in order to predict the behavior of these micro-bubbles within the tundish and their potential influence on flow patterns and inclusion float-out capability. Sheng Chang, Justin Lee, and Jason Hsin, undergraduates Ajay Panicker, Karim Selim, Jad Makarem, Zhongxun Liu, When Chen, Denguang Lu, and Nuoheng Zhang, under the supervision of Dr. M. Isac, were involved in this work over the years.
Recordings of bubble sizes for various injection generation positions
Modelling Shrouded Supersonic Jets in Metallurgical Reactor Vessels
In steelmaking applications, supersonic gas jets are preferred over subsonic jets so as to increase gas penetration into the molten bath. A supersonic jet has a greater force (jet momentum per unit time) associated with it vs a subsonic jet, owing to its higher velocity and higher gas density. During oxygen blowing, if the lance is held some distance above the surface of the bath, the oxygen jet will entrain some of the furnace atmosphere thus lowering the oxygen concentration. It may be desirable in some processes to withdraw the lance as far as possible from the melt to be processed, so as to avoid mechanical and chemical erosion problems. As such, some knowledge of what effect this may have on oxygen concentration at the surface is essential6.
In order to solve this problem and to promote the characteristics of a supersonic jet, co-jet lance technology has recently been introduced to the steelmaking industry. In our research, a supersonic jet lance technology named shrouded jet has been used (Dr. L. Calzado). This technology is based on generating a jet envelope (shroud) around the main supersonic jet flowing from the nozzle output opening.