flow optimization in the tundish with the different ...

FLOW OPTIMIZATION IN THE TUNDISH WITH THE DIFFERENT COMBINATION OF FLOW MODIFIERS

Sujata Devi, R. K. Singh, A. Paul, T. K. Pratihar &

s. K. Srivastava

ABSTRACT The efficiency and optimization of continuous casting operation require a close control of the molten steel flow characteristics within the tundish. Therefore, to improve the flow characteristics in the tundish, different flow modifiers such as dams, weirs, baffles, turbostop etc. are used. The present study consists of mathematical simulations of 1/4th scaled down tundish of Rourkela Steel Plant (RSP) with different flow modifiers. RSP tundish of 30T capacity is presently having striker pad and baffle as flow modifiers. Fluid flow has been simulated using k- turbulent model with species transport for residence time distribution (RTD) analysis and discrete phase model to investigate inclusion floatation. With the existing condition of the plant tundish, results in a recirculatory motion with intense mixing to the region below the inlet at the inlet region, due to presence of baffle, which leads more dead zone, higher turbulence on the free surface of the melt and higher shear stress on the walls of the tundish. Use of turbostop with existing condition of tundish hampers tundish function resulting in higher dead volume and less mean residence time. Similar types of results are found when turbostop is used with the baffle, whereas tundish with turbostop, gives better performance with respect to higher mean residence time, less dead zone, higher dispersed plug volume and mixed volume. The turbostop also provides more surface directed flow with lesser turbulence on the free surface of the melt which improves percentage of inclusions removal and also generates less shear stress on the walls of the tundish which increases tundish life, Keywords: CFD, Tundish, Fluid flow, RTD analysis INTRODUCTION

flow characteristics. In this method of transfer of molten steel to the mold, molten steel remains in the

Earlier, the tundish in a continuous casting operation

tundish for the time;

was used as a reservoir and distributor of molten steel only but now its role has considerably expanded to

1:

deliver metal of desired cleanliness and composition. The efficiency and

optimization

Tundish Volume

= ---------

(1)

Volumetric Flowrate

of continuous

require a close control of the

This time is known as theoretical average residence

molten steel flow characteristics, inclusion floatation

time'. With the availability of the residence time,

and separation,

adjustment

tundish provides an excellent opportunity to perform

within the tundish. If the flow of metal in the tundish

some metallurgical treatments like inclusion separation

is not properly controlled, it may even deteriorate the

and floatation, alloy trimming of steel, calcium

'quality' of steel produced

in the ladle. Thus,

induced inclusion modification etc., during the process

tundishes, in terms of their shape and use of the flow

of continuous casting. The ideal flow requirements of

control devices (dams, weirs, baffles, striker pads,

steel melt depend

turbostop, etc.) are designed to provide optimum

treatment. For example, plug flow is the best condition

casting operation

and the composition

Research & Development STEEL INDIA

on the type of metallurgical

Centre for Iron & Steel, Steel Authority of India Ltd., Ranchi-834002, 109

India VOL. 34 NO. 2 September

2011

Flow

110

Optimization

for the separation of non-metallic inclusions but is unfavourable

for carrying out dissolution of the

additive and the distribution of the dissolved products into the tundish. Therefore, a detailed knowledge of the behaviour of the steel melt flowing in the tundish is necessary so that the conventional tundish can be made

amenable

to perform

the

metallurgical

treatments during the process of continuous casting. To assess the effectiveness of a given tundish design,

in

the Tundish with the Different

MATHEMATICAL

Mathematical tundish

Combination

MODELlNG

modelling

metallurgy

of Flow Modifiers

STUDIES

of various

has

been

aspects

carried

of out.

Considerable efforts have been made through these studies

to design

a turbostop

to increase

the

efficiency of the continuous casting tundish system. For the convenience

of the present

discussion,

mathematical model studies have been summarised in th is section under three main headings namely;

researchers have simulated the metal flow either mathematically or physically, before actually using the design in actual industrial production. Mathematical

Governing

Equation

Fluid Flow:

modelling has been used by many researchers for flow The flow profile of the tundish is calculated using

predictions inside the tundish.

standard

turbulent

equations

of the k-s model

SCOPE OF WORK

expressed in their three-dimensional

The present work deals with the study of steel flow in

standard wall functions.

the

tundish

under

isothermal

condition.

version with

The (2)

mathematical simulations have been carried out for 1I4th scaled down tundish of Rourkela Steel Plant (RSP) with different flow modifiers. RSP tundish of

(3)

30T capacity presently having striker pad and baffle as flow modifiers. Fluid flow has been simulated using k- turbulent model with species transport for residence discrete

time distribution phase

model

to

(RTD) analysis investigate

inclusion

floatation. In this study different combination of flow modifiers

(striker pad, baffle, and turbostop)

evaluated

and

optimum

combination

modifiers has been determined

Residence

Time Distribution

Residence

Time Distribution

and

is

of flow

for RSP tundish.

Material properties of the liquid steel and modelling parameters used in the mathematical simulation are

representation

(RTD) is statistical

of the time spent by an arbitrary

volume of the fluid in the tundish. To theoretically evaluate the process performance

of continuous

casting tundish systems, residence time distributions have been

predicted

under

a wide variety of

conditions by mathematically simulating the pulse injection of an inert tracer in to a tundish. To this end,

given in the Table-I.

the variation of mass fraction or concentration of an Table-L: Physical Properties of liquid steel and Modelling Parameter

function

Property

Value

Density

7000

Unit Kg/m3

Molecular Viscosity

0.00555

Kgm

3500

Kg/rrJl

Inclusion density Parameters Mass flow rate in actual tundish Mass flow rate in model tundish

VOL. 34 NO. 2 September

2011

injected tracer, i, within the tundish is estimated as a

40.28 1.26

dimensional,

convection

three

+ diffusion equation, as

given below

.15.1

a(rm)

a(pum) a(pum) a(pum)

---+----+----+---at ax ay

Kg/s Kg/s

of time by solving a transient,

=

a (r a;;:

(r ayam;) +az-a (r azam,)

am;) +a-y a

effax

dz

eff

eff

(4)

STEEL INDIA

111

Sujata Devi, R. K. Singh, A. Paul, T K. Pratihar & S. K. Srivastava

Inclusion Transport and Separation

formulation utilized segregated steady state solver

The movement of inclusion particles in the liquid

with implicit formulation.

melt is tracked using the Discrete Phase Model. The

standard

Discrete Phase Model in essence is a combined

pressure-velocity

Lagrangian-Eulerian

The

SIMPLE algorithm. The solution is started with

equation for the inclusion transport can be written as:

default under-relaxation parameters, which are later

calculation

procedure.

discretization

For the pressure term, scheme

was used.

coupling is achieved

The

by using

reduced to get the converged solution. Convergence dU/ dt

=-~~C 4

P/

d

criteria of le-06 are fixed for all equations. Solution is

Re (Up -U.) 2 b

d

b ~

I

+

I

'-----v---------' Drag

rr dU

i

_

PP

Pr essure

~2..L(dU,' 2 Pg

dt "----..r---'

and w velocity reached

dU,

dt

dt

)+ (1- ~)g

'--y---

force

Virtual

considered converged when the residuals for u, v,

force

p

~~ mass

force

Bouyancy

below the convergence

criteria. For inclusion transport

600 particles of

different sizes are injected into the tundish from inlet I

P force

and allowed to reflect on the walls. For the residence time distribution model, the solution is considered converged when the residuals for the mass fraction of

Initial and Boundary Conditions

tracer

is below

le-12.

Time step of Is with

The boundary conditions for momentum transfer are

approximately 100 iterations in each time step for a

those of no slip at the solid surfaces, and zero normal

total duration of 2000 s is utilized to generate RTD

velocity gradients on the free surface of the liquid. In

curves.

a similar way, both k and turbulent

are assigned through

intensity (5%) and hydraulic diameter

Design Configuration

correlations2. The initial guess values of velocities in

In the proposed tundish configurations, the location

three dimensions of tundish are estimated from the

of the existing metal feeding and discharge points

inlet velocity.

have been kept unchanged.

For inclusion floatation simulation,

A turbostop has been

particles are allowed to reflect from all the walls of the

designed, and the detailed design of the turbostop

tundish and are allowed to escape from the outlet.

used in the present study is shown in Fig. 1(a-d). The

However, once they touch the free surface, they are

turbostop,

trapped. For residence time distribution, all walls,

shroud. Alldimensions shown in the figure is in mm.

if present, is placed exactly below the

and free surface are considered to have zero diffusive Ut~jGn 01 Turhl)Sll)p

flux. This means, that the tracer is allowed to be injected only at the inlet and can escape only through

I---'~ ---j I

T-='

r-'~---i

~:,;;-r

the outlet.

rzr

Numerical

Solution

of

the

Governing

Equations

~'h--r

.

I T

",,-, i.