Fluid Dynamics of Thin Steel Slab Continuous Casting Secondary Cooling Zone Air Mists

Spray cooling technology plays a prominent role in the secondary cooling of continuously cast thin steel slabs. The prevailing cooling conditions arise from the nature of the intermittent contact of drops with the hot metallic surface, which is affected primarily by the size, velocity and water flux of the drops approaching the surface. This study reports laboratory and computational work carried out to determine these properties for air-mist jets generated by two different fan nozzles, operating over a wide range of
water and air flow rates and pressures. A Particle/Droplet Image Analysis (PDIA) technique was used to measure simultaneously the droplet size and velocity spectra and their correlation. Room temperature measurements were carried out over small probe volumes adjacent and perpendicular to the plane where the steel strand would stand in the actual process. Furthermore, the distributions of impacting water flux and mist impact pressure were measured to obtain a fuller characterization of the mists. On the theoretical side, a computational fluid-dynamic (CFD) model based on the solution of the 3-D turbulent Navier-Stokes equation for the air coupled to the motion equation or the water drops was used for predicting the mist dynamics. The model requires exclusively the design and operating characteristics of the particular nozzle and its corresponding drop-size distribution. The PDIA measurements showed that the number and volume distributions of droplet size followed log-normal and Nukiyama-Tanasawa distributions, respectively.
Additionally, the distributions were correlated well to the water- and air-nozzle pressures over the wide range of conditions studied. Input of these results to the CFD model led to an excellent prediction of the mist dynamics, as the extensive validation results demonstrated. The increase in the proportion of finer and faster drops that was found with the increase in air-nozzle pressure for a given water flow rate, suggests
that the intensification in boiling heat transfer reported in other studies, may be related to a more frequent and intimate contact of finer drops with the surface.