Size and Velocity Characteristics of Droplets Generated by Thin Steel Slab Continuous Casting Secondary Cooling Air-Mist
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Y cooling takes place when a dispersion of fine droplets impinges on a heated surface to remove large amounts of energy by evaporation as well as by convection.[1] Depending on the application, the temperature of the surface Tw can considerably exceed the saturation temperature of the cooling liquid Ts or be slightly above or below it. Therefore, all or some cooling regimes of a conventional boiling curve[2] could appear. Cooling with sprays can offer a good balance of high heat removal capability, surface isothermality, and fluid accounting. Because of these characteristics, the technology of spray cooling has become ubiquitous. Cases J.I. MINCHACA M., Ph.D. Student, A.H. CASTILLEJOS E., Professor, and F.A. ACOSTA G., Associate Professor, are with the Centre for Research and Advanced Studies, CINVESTAV – Unidad Saltillo, Coahuila, Mexico. Contact e-mail: humberto.castillejos@ cinvestav.edu.mx Manuscript submitted December 30, 2010. Article published online March 8, 2011. 500—VOLUME 42B, JUNE 2011
with a large surface superheat (i.e., with large Tw-Ts) appear in metallurgical processes (e.g., continuous casting [CC] of steel[3,4] and heat treatment of alloys[5,6]), in safety systems for emergency cooling and pressure control of nuclear reactors,[7,8] for fire suppression/ control in electrical transformers, in flammable material storage tanks,[9,10] and so on. More recently, spray cooling has awoken a great deal of interest in the removal of high heat fluxes from surfaces at relatively low temperatures, such as the surface of electronic devices[11] or the human skin.[12,13] In 2009, world crude steel production reached more than 1220 million tons, whereas the proportion of continuously cast steel rose to 92 pct.[14] Hence, considering that spray cooling is vital to this process, its importance is evident in this as well as many other applications in which it participates. In steel CC, a solidified shell leaves the mold or primary cooling system at around 1473 K (1200 C) and enters a containment-spray cooling arrangement called the secondary cooling system, which is made of closely spaced support rolls, between which METALLURGICAL AND MATERIALS TRANSACTIONS B
nozzles are interspaced to achieve a spray cooling pattern that ensures the strand solidifies completely through its thickness before it reaches the system exit. Of the different heat extraction modes occurring in the secondary cooling system of CC machines (i.e., direct spray impingement, roll contact, radiation, and convection to draining water)[4] spray cooling accounts for ~ 60 pct of the heat extracted through the system, but this contribution can be much larger in its upper zones.[3,15] These facts are important because they suggest that the strand surface temperature and its solidification can be controlled through the judicious selection of the nozzle type, nozzle position with respect to the strand, and nozzle operating conditions. In a typical CC machine, the nozzle types and positions are fixed and the only degrees of freedom left to the operator are the oper
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