Flow dynamics in thin slab caster moulds

Research Doctorate - Doctor of Philosophy (PhD)This study investigated the dynamic fluid flow phenomena that occurred in water models of a thing slab caster mould. The flow of fluid in the thin slab water models was observed to be strongly oscillatory in nature, leading to oscillating surface waves. The magnitude and the period of oscillation of these waves represent quality constraints on thin slab casting. Limited fundamental knowledge is available on the fluid flow conditions in thin slab casting, as it is an emerging technology which lies outside the dimensional and operating range of conventional slab casters. This study used the results of laboratory experiments to elucidate observations made in the industrial situation. Fluid flow conditions were investigated using both water and mathematical models: The water models were used to characterise the oscillatory flow phenomenon and to investigate the effects of system variables. Results were compared with data from the slab casting literature. ; The mathematical model was used to investigate the mechanism of oscillation. The mould studied was rectangular in cross section, 1500 mm wide by 60 mm thick. Both full and 1/6 scale water models were constructed. The maximum wave height was observed to increase with increasing casting speed, and was very high in comparison with values established from slab casting practice. Wave heights were decreased by modifying the submerged entry nozzle design to direct more flow down into strand. The reduction in wave height achieved was of the same order as that reported for the application of electro-magnetic forces in a pilot caster. A dominant flow oscillation was observed with a period of around 5s at 4m/minute casting speed. This oscillation was dependent on the free interaction between the left and right eddies in the upper recirculating region. A secondary oscillation period was observed with a period of around 10s, and was associated with the lower recirculating region. Flow in the thin slab models oscillated despite centreline geometric symmetry and steady inlet flows. The period of oscillation was significantly higher than the natural period for gravity waves in this geometry. It was also shown experimentally that the oscillation was not influenced by the non-symmetrical flow control device. The thin slab caster geometry is a narrow, deep, rectangular tank, supplied with fluid by one or more submerged jets. These jets are confined by the mould walls and the free surface, and therefore re-entrain the fluid in the mould, setting up a pattern of recirculating eddies. It was concluded that the oscillation was an inherent feature of jets in a confined fluid volume. A mathematical model was developed to investigate this particular type of flow. The mathematical model simulated the transient flow of a single confined jet in a geometry where oscillation was known to occur and date was available for validation. The simulation was performed without gravity to removed any possibility of gravity wave or u-tube type oscillations. Despite steady inlet flow and symmetrical boundary conditions, the model was able to demonstrate the oscillation phenomenon. The oscillation dynamic was explained in terms of the balance between the pressure gradient across the jet and the opposing momentum of the recirculating, entrained flow. The success of the mathematical model to reproduce the oscillating flow behaviour required the simulation of the entire flow domain under transient conditions. The common approach of dividing the flow domain with symmetry planes, and running steady state simulations, would have been inadequate for this purpose. The period of oscillation was closely related to the average residence time of flow in the upper recirculating region. This relationship was followed by operating casters, full and 1/6 scale water models, and the mathematical model, and was consistent with Strouhal number similarity. For casters this simplifies to: [formula could not be replicated]. The knowledge of the mechanism of confined jet oscillation in the thin slab geometry may prove to be useful in the optimisation of SEN design and electro-magnetic flow control, or the development of novel metal delivery systems