Slab defects like internal and surface cracks, oscillation marks, pencil streaks, slivers and different types of chemical segregations are directly related with all transport phenomena (momentum, heat and mass transfer) which work coupled and simultaneously in the mold. Specifically heat transfer in molds has been simulated mathematically conditioning the heat flux to the residence time of steel in the mold 1)"31 using empirical functions obtained from plant data. Other researchers have performed mold temperature measurements by embedded thermocouples in the central line of the narrow face and have solved the inverse problem of heat conduction through the mold thickness to determine heat fluxes in situ 4)"6). Steel casters have been also concerned about the meniscus stability because a high standing wave will enhance flux entrapment into the metal bulk decreasing steel cleanliness. Meniscus topography, to evaluate flux entrapment, has been successfully simulated using a combination of a turbulence model with computational techniques of interface tracking or interface capturing methods 7). Mass transfer in molds has been also simulated to evaluate the influence of operating mold parameters and steel grade on chemical segregation S). However, in spite of the large amount of mold related papers it is clear that fluid dynamics must first be thoroughly understood in order to find solutions for slab defects problems. For instance, the first approaches to understand fluid dynamics considered symmetric single-phase flows at both halves of the mold with simulations performed using the k-e model of turbulence in computational meshes that involved one half or even one quarter of a mold overlooking the turbulent nature of the flow 9)"U). It was not until the work of Gupta and Lahiri that a biased flow, promoted by the fluctuating velocities under a turbulent regime, was detected in a water model. Nature of turbulence in single-phase flows, characterized by the authors through measurements of velocity, vorticity and Reynolds stresses fields have, indeed, indicated that fluid flow is not completely symmetric. The random behavior of Reynolds stresses provide unevenness in the flow which magnitudes differs with casting speed. Approaches using a swirling SEN to control turbulence and biased single-phase flows have been also reported I6H8). Bai and Thomas ascribed biased flows to the opening degree of the sliding valve and gas load in the submerged entry nozzle (SEN) among other factors.
展开▼
