11 pág.


DisciplinaLingotamento Contínuo de Aços30 materiais72 seguidores
Pré-visualização5 páginas
velocities are high and directed upward in a plane locateding flow pattern from the entry plane until, at least, the
near the wall (Figure 13(c)) for the BT, and these velocitiesexterior outlet promoting transverse mixing in addition to
are considerably lower and slightly directed toward thelongitudinal mixing (Figures 12(a), 12(c), 13(a), and 13(c)).
tundish floor (Figure 13(d)) for the tundish with a TI&Meanwhile, using a tundish with the TI&D arrangement,
D arrangement.transverse mixing is only observed in the pouring box and,
after the dam, the fluid behaves as a plug flow.
Velocity fields in horizontal planes are shown in Figures C. Mathematical Model of Mass Transfer12(a) through (d). Figure 12(a) shows the velocity field of
water in the upper bath surface in a bare tundish and Figure Dynamics of chemical mixing of the tracer after 30 sec-
onds of its injection in the ladle nozzle are shown in Figures12(b) the corresponding field in the tundish with a TI&D
arrangement. Figures 12(c) and (d) show the same type of 14(a) and (b) as isoconcentration lines expressed in mass
fractions units for longitudinal-vertical planes. Figures 14(a)information for a plane located at half the bath depth.
Velocity fields in longitudinal-vertical fields are shown and (b) correspond to the jet entry plane for a BT and a
tundish with the TI&D arrangement, respectively. It is seenin Figures 13(a) through (d). Figure 13(a) shows the velocity
field at the outlet plane for the bare tundish and Figure 13(b) that, at this short time, the tracer has been dispersed, reaching
the interior outlet completely and the exterior one partiallyshows the corresponding field for the tundish with the TI&
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Fig. 14\u2014Front view of the isoconcentration lines of tracer at 30 s. After
Fig. 15\u2014Upper view of the isoconcentration lines of the tracer at 30 s.the injection: (a) inlet plane in BT arrangement, (b) inlet plane in TI&D
After the injection: (a) near the floor plane in BT arrangement, (b) neararrangement, (c) outlet plane in BT arrangement, and (d ) outlet plane in
the floor plane in TI&D arrangement, (c) center plane of the bath heightTI&D arrangement.
in BT arrangement, and (d ) center plane of the bath height in TI&D
in the bare tundish. In the tundish with a TI&D arrangement,
the tracer remains in a mixing process in the entry zone,
and further downstream, it has just passed over the upper
side of the dam. Figures 14(c) and (d) show the same type
of information for the outlet plane in both kinds of tundishes
where the same comments are applicable.
Horizontal views of the chemical mixing 30 seconds after
injection of the tracer can be seen in Figure 15. Figures
15(a) and (b) show the isoconcentration lines in a plane
located near the tundish floor for a bare tundish and the TI&
D arrangement, respectively. In the first case, the tracer has
already reached the position of the interior outlet and is a
little more than halfway from the exterior outlet. In the
second case, the tracer is just exiting from the hole in the
Figures 15(c) and (d) show the chemical mixing of the
tracer, 30 seconds after the injection, in the upper planes of
Fig. 16\u2014Mathematically calculated total RTD curves.a bare tundish and the TI&D tundish, respectively. The tracer
dispersion has reached the lateral wall of the bare tundish
since the momentum transfer and the turbulence are high
enough to promote transverse and longitudinal mixing pro- outlets. The tracer is still away from the lateral tundish
wall, as can be seen in Figure 15(d). The results of thecesses. In the TI&D arrangement, the tracer is driven toward
the top bath surface, but with a lower turbulence, and the mathematical simulations for the total RTD curves at both
outlets are shown in Figure 16 for the BT and with a TI&isoconcentration lines are deformed by the presence of the
Table III. Flow Characteristics Results from the 4. Tracer diffusion under turbulent conditions was simulated
Calculated Total RTD Curves acceptably well through the mathematical model using
the standard k-« model.Arrangement VDead VPlug VMixed tcalc su2 D/UL 5. These results prove that a simple arrangement composed
BT 0.1309 0.2112 0.6579 284.0 0.3196 0.1992 of a TI and a pair of dams has a better performance
TI&D 0.0785 0.370 0.5515 315.82 0.1596 0.0874 than the complex furniture usually employed in actual
tundishes such as weirs, dams, vortex killers, and sophis-
ticated impact pads.
D arrangement, and the flow quantification parameters are
presented in Table III. These values were calculated from
ACKNOWLEDGMENTSthe calculated total RTD curves in Figure 16 and following
the procedure of Sahai and Emi.[21] The experimental and The authors give thanks to the institutions SNI, CoNaCyT,
the simulated results exhibit similar flow characteristics, as COFAA, and IPN for the partial financial support. One of
can be seen by comparing Tables II and III. Therefore, us (JdeJB) gives thanks to Instituto Tecnolo´gico de Morelia
similar conclusions can be reached from the water model for allowing him a leave of absence at IPN. Thanks are also
and the mathematical simulations in a complementary fash- given to FOSECO INC., whose support was determinant to
ion. Even the calculated curves (Figure 16) and the experi- perform this study.
mental ones (Figure 9) may look physically different. This
supports the contention that the mass transfer model
LIST OF SYMBOLSdescribes adequately the turbulent chemical mixing of the
tracer inside the water model for a BT and a tundish with A cross-sectional area
rather complex furniture. C tracer concentration
From the analysis of Figures 10 through 15 and the visual Ci tracer concentration at strand i
observations of the tracer dispersion during the experimental C1, C2, constants in the turbulence model
trials, brief descriptions about the tracer dispersion in the and CD
tundish with the TI&D arrangement can be made. E empirical constant in Eq. [16]
Just after the injection, the tracer is driven upward to the Ei RTD curve at strand ibath surface without appreciable turbulence. At an interme- Dm molecular tracer diffusivitydiate time after the injection, the tracer strikes the baffle Dt turbulent tracer diffusivity
and is re-directed again toward the top bath surface and Deff effective tracer diffusivitydriven along the tundish length. When the front of the tracer G generation term (Eq. [10])
dispersion reaches the level between both outlets, the fluid k turbulent kinetic energy
has lost momentum and the tracer descends toward the tun- mi mass of tracer exiting through strand idish floor. Once there, the tracer is diffused toward the M total mass of tracer injected into the vessel
outlets obtaining, in this way, a very similar mass of tracer P pressure
in both outlets. Moreover, the hole in the dam, located in Q volumetric flow rate
the opposite side of the outlets, removes the dead zones very t time
efficiently, which would form downstream otherwise. tcalc mean calculated time
These fluid flow characteristics are observed neither in VDead dead volume fraction
the BT nor in the tundish with a BWIP arrangement; these VPlug plug volume fractiondevices were not as effective as the TI&D arrangement. VMixed mixed volume fraction
v+ as defined in Eq. [17]
u, v, w velocity vectorsV. CONCLUSIONS
y+ as defined in Eq. [18]
Water and mathematical modeling techniques have been
Greek Symbolsapplied to study the flow of liquid steel in a multiple-strand
« dissipation rate of the turbulent kinetic energytundish of a bloom caster and the conclusions derived from
m fluid viscositythe results are as follows.
meff effective fluid viscosity
1. The employment of an arrangement consisting of a TI mt turbulent fluid viscosity
and a pair of dams is more effective to increase the plug r