Prediction and control of powder entrapping at meniscus of molten steel in a continuous casting mold

本文首先分析了连铸结晶器内钢液的流场，重点分析了自由表面轮廓和速度的非定常行为。对不可压缩流体流动的三维Navier-Stokes方程的有限差分近似，采用mac型解法进行了计算。这里采用了非稳态拟合体坐标系。数值结果表明，浇注速度和喷嘴类型对钢液流动结构的影响是显著的。同时，用水代替钢水进行了表面速度分布的测量。计算结果与实验数据吻合较好。接下来，我们数值分析了自由表面波动对放置在半月板下方的假想球形材料（而不是熔化的粉末）运动的影响。在双流体模型的基础上，将颗粒轨迹法引入方程组，建立了液-颗粒两相流的数值模型。数值结果表明，在浮力作用下，直径为1mm左右的大颗粒在短时间内上升到液体表面。另一方面，较小颗粒的运动跟随液体流动。流场随浇口流出角和浇注速度的变化而变化。另外，在自由表面附近形成两个螺旋结构的情况下，由于剪切应力的作用，粉末会发生包绕。

First, we have analyzed the flow field of molten steel in a continuous casting mold, focusing upon the unsteady behavior of the free surface profile and velocity. The computation has been performed using the MAC-type solution method to a finite differencing approximation of the three-dimensional Navier-Stokes equations for incompressible fluid flow. Here, the non-steady body-fitted coordinate system has been used. According to the numerical results, the effects of the casting speed and the kind of nozzle on the flow structure of molten steel have been found to be remarkable. Also, the experiment has been undertaken to measure the surface velocity distribution, using water instead of molten steel. It has been confirmed that the numerical results are in good agreement with the experimental data.Next, we have numerically analyzed the influence of the free surface fluctuation on the movement of hypothetical spherical materials (instead of melted powder) which are placed just below meniscus. The numerical model for liquid-particles two-phase flow has been built up by incorporating the particle trajectory method into the system of equations on the basis of the two-fluid model. According to the numerical results, the large particles whose diameter are of the order of 1mm rise to the liquid surface in a short time owing to the buoyancy force. On the other hand, motion of smaller particles follows the liquid flow. The flow field is changeable depending on the outflow angle of immersion nozzle as well as the casting speed. Also, it has been found that the powder entrapping occurs owing to the shear stress on condition that two vortical structures are formed near the free surface.