Flow fields control for bubble refinement induced by electromagnetic fields

Large bubbles seriously reduce the efficiencies of the interactions between the bubbles and the molten steel, such as energy transfer, momentum transfer, mass transfer and chemical reaction. To reduce the size of the bubbles and increase the gas–liquid interface area, a novel non-intrusive method of bubble refinement was proposed, which only depends on the molten steel flow field controlled by the rotating electromagnetic field. The flow fields of the molten steel for bubble refinement were analyzed, and the corresponding bubble refinement was investigated. It was found that the molten steel formed obvious rotating turbulent flow for bubble refinement under the unidirectional rotating electromagnetic field. However, the large vortex in the center of the molten pool caused by the rotating flow made the bubbles aggregate and coalesce again, resulting in formation of larger bubbles and gas cavity. To suppress the central vortex formation and enhance the bubble refinement, the forward-reverse rotating electromagnetic field for bubble refinement was proposed. The irregular and chaotic flow occurred repeatedly because of alternating forward and reverse rotating in a short period, so that the turbulent kinetic energy and turbulent dissipation of the flow field always remained at a high level which favors bubble refinement. As a result, the bubble diameter can decrease by more than 50% compared to that without electromagnetic field. Furthermore, it is important that this non-intrusive kind of bubble refinement method completely avoids the introduction of non-metallic inclusions caused by intrusive configuration.