The Fluid Dynamics of Secondary Cooling Air-Mist Jets

Abstract

For the conditions of thin-slab continuous casting, air-mist secondary cooling occurs in the transition-boiling regime, possibly as a result of an enhanced intermittent contact of highmomentum water drops with the hot metallic surface. The dynamics of the intermittent contact or wetting/dewetting process should be primarily dependent on the drop size, drop impactvelocity and -angle and water-impact flux, which results from the nozzle design and the interaction of the drops with the conveying and entrained air stream. The aim of this article was to develop a model for predicting the last three parameters based on the design and operating characteristics of air-mist nozzles and on experimentally determined drop-size distributions. To do this, the Eulerian fluid-flow field of the air in three dimensions and steady state and the Lagrangian velocities and trajectories of water drops were computed by solving the turbulent Navier-Stokes equation for the air coupled to the motion equation for the water drops. In setting this model, it was particularly important to specify appropriately the air-velocity profile at the nozzle orifice, as well as, the water-flux distribution, and the velocities (magnitude and angle) and exit positions of drops with the different sizes generated, hence special attention was given to these aspects. The computed drop velocities, water-impact flux distributions, and airmist impact-pressure fields compared well with detailed laboratory measurements carried out at ambient temperature. The results indicate that under practical nozzle-operating conditions, the impinging-droplet Weber numbers are high, over most of the water footprint, suggesting that the droplets should establish an intimate contact with the solid surface. However, the associated high mean-droplet fluxes hint that this contact may be obstructed by drop interference at the surface, which would undermine the heat-extraction effectiveness of the impinging mist. The model also points out that a large proportion of fine drops would be prevented by the air-flow pattern from reaching the surface. The numerical analysis of air-mist jets under conditions relevant to secondary cooling had not been addressed before, and it constitutes a first step in an effort to develop a model to describe the dynamic and thermal interaction of dense-drop media with hot metallic surfaces.

An Experimental and Computational Study of the Fluid Dynamics of Dense Cooling Air-Mists

“…3.1, the initial velocity, (u i ) k , of the drops was prescribed by assuming that the drops exit with the terminal velocity that they reach in the mixing chamber (Minchaca et al, 2010). As suggested by the observed drop trajectories (Hernández et al, 2008), according to the position assigned to the drop the angle of the velocity varied from 0 deg to  deg in the x-direction and from 0 deg to  deg in the y-direction. A schematic representation of the velocity vectors of the drops is displayed in Fig.…”

“…The predicted liquid fraction contours indicate that the increase in p a at constant W causes a redistribution of the liquid in the free jet that could affect the water impact density. In previous w measurements no effect was detected (Hernández et al, 2008). Thus, it was decided to refine the patternator to try to reveal if there was an influence of p a on the water impact density for a constant W. Figure 10 shows measured and computed water impact density maps for p a of 257 kPa and 320 kPa with W= 0.50 L/s.…”

“…1, the water splatters against a deflector surface and the resulting splashes are further split by the shear forces exerted by the axial air-stream, which also accelerates the drops as they move along the mixing chamber toward the exit port. Thus, the liquid emerges in the form of drops with different sizes and velocities and with a non-uniform spatial distribution (Hernández et al, 2008). In addition to w, the size, d d , and velocity, u, of the drops in dense air-mists play a crucial role in the cooling of highly superheated surfaces (Bendig et al, 1995;Jenkins et al, 1991;Hernández et al, 2011).…”

“…To the best knowledge of the authors only PDIA has been used for the characterization of dense sprays and mists. Particle image velocimetry, PIV, has been employed for measuring the velocity of drops in dense mists, but the technique did not allow the simultaneous determination of size (Hernández et al, 2008). Recent works have presented a detailed experimental characterization of the local variation of w, d d and u obtained with typical air-mist nozzles, operating over a wide range of conditions of practical interest Hernández et al, 2011).…”

“…It was found that drops smaller than 100 µm would experience large deflections due to gravity, and would never reach a plane beyond 0.25 m. The sprays and mists that have been studied experimentally and computationally are far apart from those used in important metallurgical processes. In recent studies the authors presented a 3-D computational fluid dynamic (CFD) Eulerian-Lagrangian model for free dense air-mist jets (Hernández et al, 2008). However, since new and rigorous experimental information has been generated the model has been refined in regard to the size distribution imposed at the nozzle orifice.…”