Influence of straight nozzles on fluid flow in mold and billet quality

Abstract

Flux entrainment defects in a billet mold with two straight nozzles (outer radius of 60 mm, S60, and 73 mm, S73, and same interior radius of 36 mm) are studied using mathematical simulations and experimental techniques including particle image velocimetry (PIV), tracer injection, and water-oil modeling. Experimental findings indicated that using either of these nozzles at the deepest position of 135 mm (measured from the nozzle tip to the meniscus level) induces a flow near the meniscus, characterized by low turbulence yielding very stable water-oil interfaces. Simultaneously, owing to the mold radius, the entry jet induces a nonsymmetric flow downstream in the mold. At the shallow position of 90 mm, nozzle S73 induces a flow characterized by cyclic velocity spikes of low frequency and short duration close to the meniscus making the metal-flux interface highly instable. As the gap between the nozzle outer wall and the mold face is narrower and the nozzle position is shallower, the fluid close to the meniscus develops zones of high vorticity gradients leading to rotating flows, which are responsible for that instability. Water-oil experiments indicated that during the generation of this phenomenon, water flow entrains oil droplets in the side of the mold inner radius. Instability of metal-flux interface is then a consequence of the fluctuating nature of the recirculating flows in the mold, which induce upper rotating flows with large vorticity gradients. This mechanism explains actual flux entrainment defects in a billet caster. © 2008 The Minerals, Metals & Materials Society and ASM International.