Process Metallurgy Influence of Aspect Ratio on Fluid Flow and Heat Transfer in Mould when Using Swirl Flow during Casting S. Kholmatov'', S. Takagi 2l , P. Jonsson 1 ), L. Jonsson", S. Yokoya" 1) Division of Applied Process Metallurgy, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm 10044, Sweden 2) Department of Mechanical Engineering, Nippon Institute of Technology, Miyashiromachi, Saitama-ken 345-8501, Japan Mathematical modelling was used to study the effect of a changed aspect ratio of a continuous casting mould on the resulting flow field in the upper part of the mould when using a swirl flow in the nozzle. Model predictions were initially compared to physical modelling data. More specifically, the predicted axial velocities were found to differ only at the most -3 mm/s from the measured data. Thus, the model was concluded to be sound. By changing the aspect ratio of a billet mould from 1 to 3 systematically, a numerical analysis of the mould region of a billet continuous caster was performed with a novel injection concept using swirling flow in the immersion nozzle in order to control the heat and mass transfer in the continuous casting mould. The predictions showed that the aspect ratio of the mould has a large influence on the flow field in the upper part of the mould. The meniscus temperature was found to increase with an increasing aspect ratio from 1 to 2, but the maximum temperature was found to decrease when the aspect ratio was increased above 2. Keywords: swirl flow, aspect ratio, billet mould, immersion nozzle, continuous casting of steel DOl: 10.2374/SRI07SP116-79-2008-698; submitted on 20 September 2007, accepted on 18 January 2008 Water Inlet • x h"'----+--- Physical Modelling Computer LDV Probe divergent angle of the nozzle has a large influence on the flow development and heat transfer at the meniscus. The findings mentioned above are very useful for controlling the flow pattern in continuous cast billets, blooms and rectangular slabs. The focus of the current study is the effect on heat and mass transfer in the mould when a swirling flow is used by changing the aspect ratio, from square to rectangular billet. In the first part of the paper the physical and mathematical models are described. Thereafter, the results are presented and discussed. Figure 1. Overview of the experimental set-up. The physical modelling setup includes an experimental mould as well as visualization and measurement equipments as shown in Figure 1. More specifically, the Introduction In the continuous casting processes, it is well known that the fluid flow pattern in the mould has a key effect both on the surface quality and on the internal quality of the ingots. Furthermore, gas bubbles and non-metallic inclusions must be removed from the liquid steel at the meniscus as well as cooling of the mould wall must be carried out to solidify the steel. These examples indicate some fundamen- tal aspects of the continuous casting operation in which the meniscus temperature must be kept high and at the same time cooling of the mould walls needs to take place. A technology of swirling flow formation in a submerged entry nozzle has been proposed as an effective measure for controlling the flow pattern, especially at the meniscus, in a continuous casting mould [1-7]. Previous research has showed the positive effects of the swirl motion such as: • For water modelling: o By changing swirl strength and outlet shape of the immersion nozzle, it is easy to control the flow pattern as well as the direction of the flow. o Uniform velocity distribution can be obtained within a short distance from the outlet of the immersion nozzle. o Heat and mass transfer is superior and controlled both better and more easily compared to the conventional immersion nozzle. o The swirling motion suppresses disturbances near the meniscus, etc. • For the real steel casting stage: o The submerged entry nozzle with a swirl blade for steel casting improves the process productivity and the surface quality of rectangular billets and coils. In addition, the authors have studied the following related topics in two previous studies: the effect of changing the divergent angle of the nozzle on the flow field and temperature distribution in square [8] and round [9] billets. The previous research has showed that the 698 steel research into 79 (2008) NO.9