TURBULENT INSTABILITIES IN A THIN SLAB MOLD

Rodolfo D. Morales 1)

, Saul G. Hernandez2)

.

1) Professor at the Department of Metallurgy and Materials Engineering, National Polytechnic

Institute-ESIQIE, Apdo. Postal 75-874, México D.F. rmorales@ipn.mx,

ketechnologies@prodigy.net.mx

2) Graduate Students at the Department of Metallurgy and Materials Engineering, National

Polytechnic Institute-ESIQIE, Apdo. Postal 75-874, México, D.F. iq_sagahz@hotmail.com

Key words: funnel thin slab mold, meniscus oscillation, dynamic distortion.

Abstract

A water-physical model of a funnel-type thin slab mold fed by a 2 ports SEN was employed to

characterize the flow of liquid steel using dye tracer, PIV and video recording experiments. A

cyclic and energetic flow distortion of short live inducing high meniscus oscillation was

identified. Its intensity grows with high casting speeds of 7 m/min and at a shallow immersion

position of a SEN. This distortion is originated by the apparent existence of vortex flows located

below the two discharging jets which are formed by the existence of shear stresses in their ends

and acting on the surrounding fluid. It is inferred, under the ground of experimental results, that

this flow distortion is originated by an instantaneous unbalance of the turbulent kinetic energy in

the discharging jets. Negative production of kinetic energy is ascribed as the source of this

unbalance which is compensated by higher contribution of the turbulent kinetic energy by mean

convection and turbulent transport mechanisms manifested through higher velocities. After the

restoration of the energy balance the system yields a stable meniscus to repeat the cycle.

Introduction

Design of submerged entry nozzles (SEN) is critical for controlling steel flow turbulence in

continuous casting molds and especially in confined spaces such as funnel-type thin slab molds

which operate at higher casting speeds than conventional thick molds. Controlling turbulence is

very important to avoid flux particles entrainment which transforms into slivers in the final

product [1,2]

, to avoid meniscus instability [3,4]

, distribute evenly heat transfer aiming at uniform

shell growth [5]

and attainment of steady flow conditions. Some thin slab and conventional mold

SEN designs induce strong meniscus instability and high amplitude oscillations of liquid in the

mold [6]

. Takatani et al. [7]

found that the horizontal velocity under the meniscus fluctuates

strongly through the width of a conventional slab mold. Using a physical model Yoshida et al. [8]

identified a meniscus descending flow along the outer surface of the SEN due to the existence of

differential pressure in this region. According to those authors driven force for the descending

flow is the instantaneous meniscus velocity difference between metal and liquid flux provided by

bath oscillations. Morales et al. [9]

reported that in a four ports SEN strong backflows, from the

upper roll flow, are responsible for the existence of strong bath oscillations indicating that full

port utilization (FPU) is a key factor for SEN design. A second key factor for SEN design is to

avoid high free shear strain rates induced by long discharging jets, especially when the casting

speed increases as Torres-Alonso et al. [10]

indicated. In other work the same author reported also

the existence of energetic vortexes formed very close to the SEN due to the existence of biased

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upper roll flows in a thin slab mold [11]

. Other authors claim that bath oscillations, specifically in

thin slab molds, have their origin in an intermittent-confined cross-flow which passes through the

gap located between the outer SEN wall and the wide mold wall [12,13]

during its travel from one

of the upper corner narrow walls side to the other. However, it is a fact that being the fluid flow

in a continuous casting mold highly turbulent the flow history plays the most important role on

flow behavior. Indeed, that confined cross-flow is a consequence of wall shear and strain rates

generated by walls of the SEN discharging ports shaping the history of the flow downstream the

mold. In the present work the authors focus their effort in emphasizing and explaining the role

that wall and free shear stresses have on strong bath oscillations in funnel-type thin slab molds,

although, the same principles find application for designing SEN´s for conventional molds. In

this research several techniques including water modeling, video recording of meniscus

oscillations, red dye tracer experiments and PIV measurements are employed in order to

characterize fluid flow dynamics in a thin slab mold.

Experimental Description of Water Model

A full scale plastic model of a thin slab mold and a two ports-SEN were built, with the

dimensions shown in Figure 1 and 2, respectively. Geometry of this mold is commonly called as

funnel-type mold due to the enlargement of the thickness in the upper central part in order to

allow the positioning of the SEN. The thickness of the plastic sheets employed to build the mold

was 20 mm and the SEN was built by pouring a semitransparent resin in a previous treated

surface wood mold. The plastic mold is fitted into the open chamber which receives the water

flow exiting the mold. Two casting speeds were considered: a typical 5 m/min and a higher one

of 7 m/min. Two SEN depths were studied: 200 mm (shallow position) and 340 mm (deep

position) measured from the meniscus to the tip of the SEN, and a fixed mold width of 1370 mm.

Freeboard between the mold top and the static bath level was 60 mm. During a given experiment

a flow rate of water, video recordings of the meniscus (interface water-air) were taken during 2

Figure 2. Geometry and dimensions of the full-

scale model of two-port SEN used in the

present study: a) Frontal view, b) Bottom view

and c) Details of discharging ports.

Figure 1. Geometry and dimensions of

a full-scale model of funnel type thin

slab mold: a) Top view, b) Side view

and c) 3-D view.

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minutes at three critical areas. These areas include the mold corner, area 1, the funnel expansion,

area 2 and the zone close to the SEN shaft, area 3. After finishing meniscus recordings a pulse

injection of red dye was performed and the mixing patterns were also recorded over a field that

included the whole mold. Video photos at every second of recording time were processed with

image analysis software. Using photos of the video recordings and an image analyzer the time

changing shapes of the meniscus were quantified taking as arbitrarily reference the static level of

water in the mold. Fluid flow was also studied using Particle Image Velocimetry (PIV)

equipment from DANTEC. In this system, Coupled Charged Devices (CCD) capture images of

seeding particles interacting with a laser sheet and the signals are processed through Fast Fourier

Transforms to obtain velocity fields as it is described by Sanchez-Perez et al. [14]

.

Results and Discussion

Meniscus Oscillations

Visual observations with the SEN in the shallow position at 7 m/min reported a stable meniscus

which lasted for a relatively long period of 90-100 seconds, see Figure 3a. During this time,

meniscus oscillation reported a high measured frequency of 5-6 Hz. After this period the

meniscus reported a growing instability with formation of intermittent waves close to the SEN as

is seen in Figure 3b. With forthcoming times a depression forms in the proximities of the funnel

and a meniscus elevation close to the narrow wall forms due to the approaching of a high

momentum stream provided by the upper flow roll coming toward the SEN as is seen in Figures

3c-3e. Finally, after this energetic instability, the meniscus attains again the stability, see Figure

3f. This condition lasts again during other 90-100 seconds to repeat the cycle of instability.

Figure 4. Formation of the meniscus

dynamic disturb for 7 m/min with the SEN

at shallow position, area 3: a) Arbitrary time

of 0 s, b) After 2 s, c) After 4 s, d) After 9 s,

e) After 11 s, and f) After 13 s.

Figure 3. Formation of the meniscus

dynamic disturb for 7 m/min with the SEN

at shallow position: a) Arbitrary time of 0 s,

b) After 2 s, c) After 4 s, d) After 9 s, e)

After 11 s, and f) After 13 s.

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Figures 4a-4e correspond to zoom views close to the SEN for approximately same times reported

in Figure 3. At the beginning the meniscus reports the unexpected appearing of the instability in

Figure 4a and rapidly a vortex is formed as can be seen in Figure 4b. The origin of this vortex is

the high momentum streams of the upper flow roll which, once in or close to the funnel region,

loses momentum by shearing the liquid in that region forming a rotational motion. After the

vortex impacts the SEN the standing wave decreases its momentum as is seen in Figures 4c and

4d, but the meniscus instability remains, see Figure 4e. After the disturbing period the meniscus

is about to attain a stable condition as is seen in Figure 4f. Figures 5a-5f report the meniscus

instability in the funnel region corresponding to approximately the same times of Figures 3 and

4, respectively. It is worthy to mention the dramatic level changes originated by this instability

particularly recorded in Figures 5c and 5d. At the time when the standing wave sends back the

vortex toward the direction of the narrow mold wall the fluid energy is dissipated through a

strong surface instability folding layers of water one over the other and entraining air to form

bubbles as is seen in Figures 5d and 5e. For the casting speed of 5 m/min with the SEN at the

same shallow position very similar meniscus instabilities, as those described above, were

observed without any apparent difference but a relative lower intensity.

Further observations for a casting speed of 7 m/min with the SEN at the deep position indicated

also the formation of meniscus instabilities, although, their life and their frequency were slightly

larger. When the mold was operated at a casting speed of 5 m/min at the deep position there was

not any meniscus instability and the flow behaved quite calmed. This is a condition where this

particular SEN design operates safely without the danger of air or flux entrainment into the steel

bulk. As these instabilities evidently depend on the fluid flow dynamics of the liquid inside the

Figure 6. Life time of Dynamic Distortions

for the four cases, measured as a function of

the time of meniscus deformation.

Figure 5. Formation of the meniscus

dynamic disturb for 7 m/min with the SEN

at shallow position, area 2: a) Arbitrary time

of 0 s, b) After 2 s, c) After 4 s, d) After 9 s,

e) After 11 s, and f) After 13 s.

90

mold they will be called hereinafter simply as dynamic distortions (DD) of the meniscus. Figure

6 summarizes the life time of these DD for the four cases analyzed in this work. Therefore, as is

seen, a casting speed of 7 m/min with a deep position of the SEN increases the life time of the

DD. Table I reports quantitative results for the four experimental conditions for the three regions

mentioned. It is also important to notice that the funnel region is the one with the highest

standard deviation due to the existence of complex flows formed by the high momentum stream

toward the SEN which suffers an expansion in the funnel and shears the fluid existing there

forming intermittent vortexes. This table indicates that definitively this SEN design is not

recommendable to operate at high casting speeds such as 7 m/min for any position or even at 5

m/min at the shallow position due to the high amplitude meniscus level oscillations.

Table I. Quantitative results of meniscus oscillations for the four experimental conditions.

Casting

speed

(m/min)

SEN

immersion

(mm)

Area*

Area of

meniscus

distortion

(mm2)

Max

oscillation

(mm)

Mean

oscillation

(mm)

7

200

1 929 17 6.5

2 2234 - 37 17

3 737 ±10 5

340

1 616 12 4

2 1340 -20 9

3 609 10 4

5

200

1 626 10 4

2 1638 -24 11

3 617 ±5 4

340

1 259 3 1.7

2 267 4 1.7

3 241 3 1.6

* Area 1 is mold corner, area 2 is funnel expansion and area 3 is close to the SEN.

Tracer Mixing Patterns

Dye tracer was visualized through image analysis software allowing high contrast among region

with different dye concentrations. As seen in Figure 7a, for a casting speed of 7 m/min at the

shallow position, both jets provide a double upper roll flow and leaves a stagnant zone below the

nozzle tip leaving deep depressions on the meniscus surface in the funnel region particularly

during the generation of the DD. At the deep position, Figure 7b, both jets induce also a double

upper roll flow but the stagnant region below the SEN tip is partially eliminated and the

meniscus depressions remain during DD development time. Mixing dye patterns at the beginning

of the DD for the same case of Figure 7a are presented in Figures 8. Figure 8a shows the entry of

the tracer into the mold just 10 seconds after its injection; the flow is approximately symmetric

with meniscus depressions. After 12 seconds of the tracer injection, the right jet bends close to

the mold end and the flow ascends with a relatively high velocity as is indicated by number “1”,

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see Figure 8b. Meanwhile, the left jet impacts the left narrow face at approximately half the mold

length and part of the fluid descends downstream and another one ascends toward the meniscus

to form part of the upper flow roll as it is indicated by number “2” in Figure 8c, just 15 seconds

after the injection of the tracer. At the same time, number “3” in Figure 8c, part of the right jet

flows further downstream along the right narrow mold face. Other part of the flow ascends

toward the meniscus with relatively high velocities, number “4”. Close to the end of this flow

distortion, the flow starts becoming more symmetric and the depression depth decreases as is

observed in Figure 8d. After all these flow phenomena have occurred, a relatively long period of

flat meniscus conditions comes approaching the image shown in Figures 3a. These flow pattern

descriptions apply also for the casting speed of 5 m/min with the SEN at the shallow position;

only for 7 m/min at the deep position the flow distortions have apparently smaller intensities.

The deformation of the discharging jets, which induce the meniscus dynamic distortion through

increases of the momentum transfer to the upper flow rolls, is apparently originated by vortex-

shearing flows travelling from the lower mold zone to the SEN tip following a cascade effect.

This effect is apparent as is indicated by the numbers 1 and 1’ in Figures 7a and 7b, respectively.

Initial vortex flows probably exist permanently even during the off-time of the DD, but

eventually acquire strengths high enough to transfer a higher momentum. This cascade-upward-

driven flow mechanism transfers momentum and opens and deforms the legs formed by both

discharging jets as can be seen in Figures 8b-8d. The final effect is an increment of fluid

velocities to the upper roll flows giving place to the formation of the meniscus dynamic

distortion described above.

Figure 8. Mixing patterns of dye tracer for 7 m/min

with the SEN at the shallow position, seconds after the

tracer injection: a) 10 s, b) 12 s, c) 15 s, and d) 18 s.

Figure 7. Tracer mixing patterns

for 7 m/min: a) SEN at shallow

position, and b) SEN at deep

position.

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PIV Measurements

PIV measurements reported here correspond to the casting speed of 7 m/min with the SEN at the

shallow position. Figure 9 shows fluid velocities measured through the PIV technique during the

period of flat meniscus (DD off-time). Figure 9a corresponds to an arbitrary time assumed as

zero, which separates by periods of 250 milliseconds from each one the other figures. This figure

shows a recirculating eye in the middle of the flow field indicated by number 1. Fluid close to

the narrow mold face ascends toward the bath surface reaching the mold corner, number 2, and

turns toward the SEN position to be finally entrained by the entering jet. Number 3 indicates how

the jet trajectory bends toward the narrow face. Fluid close to the SEN, indicated by number 4, is

almost stagnant. Figure 9b, shows the recirculating eye expands and ascends toward the

meniscus as is indicated by number 1’ and the amount of fluid flowing along the narrow mold

wall increases. The recirculating eye in the mold corners, number 2’, becomes smaller than in the

previous flow field and the entering jet, marked by number 3’, is contracted toward the left side

of the flow field. In Figure 9c the flow suffers again changes as can be seen by comparing with

Figure 9a, the entering jet contracts more toward the left side, number 3’’, indicating that its

trajectory is straightened downwards. At the longest time, Figure 9d, there is not a clear

recirculating eye and the fluid entrained by the jet from the upper bath surface falls down

vertically increasing its straightening effects. These radical changes of the flow field are

consequences of the high turbulent conditions in the mold at this high casting speed.

Conclusions

Fluid flow dynamics of steel in a funnel-type thin slab mold was studied through modeling

experiments using a full scale water physical model and commercial two-ports SEN aided with

Figure 9. Velocity fields measured through PIV technique: a) At a time arbitrarily

designated as zero, b) After 250 ms, c) After 500 ms, and d) After 750 ms.

93

video recording, tracer injection techniques and PIV measurements and the following

conclusions can be drawn from the results:

1. At a casting speed of 5 m/min with the SEN in the deep position a steady flow with a flat

meniscus condition remain.

2. At casting speeds of 5 and 7 m/min with the SEN at the shallow position an energetic DD of

the fluid flow was observed. This DD generates vortexes and standing waves of high

amplitude in the meniscus and lasts for periods of about 10-15 seconds. This SEN is not

recommended for casting under those operating conditions.

3. At a casting speed of 7 m/min with the SEN at the deep position this DD was also observed.

4. Experimental measurements indicated that the DD originates in the lower part of the mold

through the probable formation of small vortexes between both discharging jets shearing the

jet boundaries enhancing the momentum transfer of the upper roll flow.

5. Timescale analysis and PIV measurements indicated that long time effects are possible

especially in long scale flows as is the present case.

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