| Slide gate control system, with the advantages of accurate control precision of molten steel flow rate, automatic control being easily realized, simple operation and safety, consequently, it is widely employed in continuous casting of steel. However, due to the suppression and extrusion effect of slide gate to the molten steel, the sudden expansion phenomenon appears after extrusion by slide gate, and some flow patterns are emerged, such as contraction flow, sudden expansion flow, separated flow, and so on. Thus, the flow in the SEN, the jet characteristic at nozzle port and the flow in the mold become more complex. The asymmetric and unsteady flow state in the mold directly affects the removal efficiency of inclusions and gas bubbles, and it has a great influence to the formation and uniform growth of initial solidification shell (rule-less surface crack or breakout). Based on pouring technique of slide gate, the adaptability of turbulence models for the fluid flow in the SEN and nozzle port has been discussed; the flow state, velocity distribution and physical characteristic of jet exiting from nozzle port under different process parameters have been studied. And then, the prediction ability of turbulence models for the flow behavior and flow states in the SEN and mold have been discussed, and the basic hypothesis and specialty of standard k-εmodel and RSM are displayed, together with the adaptability and accuracy of these two turbulence models for the simulation of fluid flow in the continuous casting system. Meanwhile, by means of physical simulation of water model and velocity measurement using Ultrasonic Doppler Velocimeter (UDV), the effect of process (casting speed, etc.) and structural (SEN length under slide gate and so on) parameters on the jet flow status (swirl flow or smooth flow), unsteady/asymmetric flow in the mold, surface vortex and its formation rules are studied quantitatively when slide gate is not fully opened and/or nozzle port is clogged, where the evaluation indicators include velocity distribution in the mold, surface flow velocity, and the intensity and position of impingement and scour of jet to the mold narrow wall. On the basis of above research results, the influence rules of different process and structural parameters on the unsteady/asymmetric flow in the mold are summarized, and some theoretical guidance and references are provided to optimize process and improve slab quality.In the first part of the paper, under the condition of slide gate moving perpendicular to the mold wide wall, the adaptability of turbulence models for the fluid flow in the SEN and nozzle port has been discussed. In addition, the influence rules of inlet velocity at SEN, opening ratio of slide gate and SEN length on the swirl characteristic of nozzle jet have been studied. The study indicates that, compared to standard k-εmodel, RSM can predict the swirl characteristic of jet at nozzle port accurately, and the simulated result agrees to the UDV measurement, so that the calculation accuracy of RSM is superior to the former, and it has strong adaptability; and it is found that, the velocity at nozzle inlet cannot make effect on the jet swirl characteristic at nozzle port. When the opening ratio of slide gate is smaller than 35 pct, a secondary flow and a separated flow caused by slide gate appear under the slide gate, in the circumstances of a longer SEN (600 mm), a swirl flow appears at nozzle port, with the swirl direction of that from the clogging side via nozzle bottom to the opening side of the slide gate, and the swirl direction is affected by the SEN length, when the length of SEN is decreased to 400 mm, the swirl direction is changed to the opposite mode; when the opening ratio of slide gate is in the range of 35 pct to 40 pct, a dynamic equilibrium point of inertia force of molten steel, fluctuation of turbulence and expansion effect of streamline is emerged, the jet presents a symmetry/smooth outflow state; and then, with the opening ratio of slide gate increasing, the jet swirl direction is changed to the opposite swirl direction, as is described when the opening ratio of slide gate is in the range of 40 pct to 65 pct, the swirl direction of jet is from the clogging side via nozzle bottom to the opening side of the slide gate, and it is not affected by the SEN length; when the opening ratio of slide gate is in the range of 65 pct to 70 pct, the second dynamic equilibrium point is emerged, and then, with the SEN length decreasing, the influence of velocity component towards to the SEN wall of the slide gate clogging side is gradually strengthened, in the condition of shorter SEN, the jet swirl direction will change to the pattern as described when the opening ratio of slide gate is smaller than 35 pct; when the opening ratio of slide gate is larger than 90 pct, the swirl phenomenon almost disappears, and the jet presents to a quasi-symmetry/smooth outflow stare.In the second part of the paper, the prediction ability of turbulence models for the flow of molten steel in the mold has been discussed under different slide gate opening mode, and the physical simulation and UDV measurement are conducted to study the effect of process and operation parameters on unsteady flow of molten steel in the mold when slide gate is not full opened or outlet is clogged, where the asymmetry fluid flow in mold is obtained by SEN slide gate and clogging of outlet, and the influence rules of opening ratio of slide gate and nozzle port clogging on the asymmetric flow are analyzed and summarized based on above experimental results. (1) Under the condition of slide gate moving perpendicular to the mold wide wall, the swirl flow at nozzle port will cause asymmetric velocity distributions near the mold wide wall, and the RSM can predict the flow state in the upper part of mold, but the simulation result in the lower part of mold disagrees to the measurement result; and under the same opening pattern of slide gate, the clogging at one nozzle port has serious effect on the bias flow in the mold. (2) In the circumstances of slide gate moving parallel to the mold wide wall, the partial opening slide gate bring to unbalanced flow rate of molten steel at two nozzle ports, presenting impact depth and jet angle of two streams asymmetry, which will cause non-homogeneous fluid flow in mold; for the flow status in the mold, both standard k-εmodel and the RSM can predict the basic flow characteristic in the mold, but the result simulated by RSM is close to the UDV result, it is shown that RSM has an advantage to simulate such kind flow. (3) The SEN immersion depth and slab width has little influence on the asymmetric flow in the mold; with the increasing of casting speed, the impingement points of two jet shift up and the asymmetry of fluid flow become weakened. If the SEN length is long enough, the asymmetry outflow of two nozzle port almost disappears. In addition, increasing the SEN length under slide gate, it is useful to eliminate the asymmetric phenomenon at nozzle port when the slide gate is partial opened.In the last part of the paper, physical experiment and UDV measurement have been used to study the rules and distributions of surface vortex, and the influences of operating process on the vortex. In slab continuous casting mold, if the surface streams, coming from two upper recirculation zones, are imbalance, they encounter and shear at a point on one side of the SEN, and cause a swirl flow at free surface. It may bring to circular motion of around fluid and on the effect of descent force a spiral eddies of water called vortex is forming, and it is believed to have significant contribution to mold powder entrapment, air entrapment and re-oxidation of molten steel. Various experiments with different parameters have been done in lab to analyze the effect of these parameters on vortex. The study indicates that the cause of vortex lies in two aspects, one is that the turbulence of surface flow gives rise to asymmetry of flow momentum near the SEN, and the other is operating parameters cause asymmetry of the surface flow besides the SEN. There are three types of vortex which appears on surface, a single vortex on one side of SEN, two-vortex on one side of SEN, and two-vortex in diagonal position of SEN, but they are all related to the surface flow near the SEN. The vortex strength (vortex tube diameter and working depth) and frequency increase as the raise of surface flow kinetic energy. The increase of SEN immerging depth and slab width will decrease the strength and frequency of vortex.
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