The Fluid Dynamics of Secondary Cooling Air-Mist Jets
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secondary cooling system for continuous casting of steel should be able to provide a heatextraction distribution on the strand surface that grants optimum levels of quality and productivity. If this is not the case, bulging, cracks, segregation, and, ultimately, poor metallurgical quality and operating practice will result. A review of defects linked to spray-cooling practice in billets and slabs was presented by Brimacombe et al.[1] In their excellent article, the authors pointed out that both the intensity and uniformity of the heat removal should be controlled to avoid low ductility and large tensile strains, which are potential sources of quality problems. In continuous casting, air-mist (pneumatic) nozzles are gaining acceptance over hydraulic nozzles I. HERNA´NDEZ C., Doctoral Student, F.A. ACOSTA G., Associate Professor, A.H. CASTILLEJOS E., Professor, and J.I. MINCHACA M., Master of Science Student, are with CINVESTAV – Unidad Saltillo, Carr. Saltillo-Monterrey Km. 13, Saltillo, 25000, Coahuila, Me´xico. Contact e-mail: humberto.castillejos@cinvestav. edu.mx Manuscript submitted on January 28, 2008. Article published online October 2, 2008. 746—VOLUME 39B, OCTOBER 2008
owing mainly to two claimed advantages: (a) a more uniform heat extraction and (b) a broader range of heatremoval intensities. It could be suggested that the first arises from the even dispersion of drops that results from the trajectories imparted to them by the nozzle-orifice geometry and the accompanying air, and that the second is a consequence of the wide turndown ratio (ratio between the minimum and maximum water flow) and the controllability of the air/water voluminic-flows ratio, A/ W. Recently, the control of this last ratio (at approximately 10) was found to give the possibility of increasing productivity (casting speed) of thin slabs without a detriment on quality and despite that cooling occurs in the transition- boiling regime.[2] In this regime, the heat flux and heat-transfer coefficient are a strong function of surface temperature in contrast to the film-boiling regime that is prevalent and is sought out in the continuous casting of conventional slabs.[3] When the solid-surface superheat (i.e., the difference between the surface temperature and the liquid-saturation temperature) is ‘‘sufficient,’’ the film-boiling state is reached, where the liquid drops are not in direct contact with the hot surface but are separated by a continuous film of vapor. Because the superheats in conventional- and thin-slab casting are
METALLURGICAL AND MATERIALS TRANSACTIONS B
similar, the different boiling behavior of the drops must be associated with the dynamic parameters affecting the chances of drop contact with the surface, i.e., drop size, drop-normal and tangential velocities, and dropletimpact flux. This article presents a study of these parameters for the case of fan air-mist nozzles, which produce flat impact footprints and are widely used in continuous casting of steel. It has been reported that there is a direct dependency of the heat transfer on th
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