Effect of Draw Blank Velocity on Steel Fluid Flow Field in Blank Continuous Caster Crystallizer

As the heart of continuous caster, crystallizer is the cradle of most surface deficiencies and inside quality problems in steel blank. Steel blank surface quality, nonmetal impurity content and relevant distribution rely on the steel fluid solidification behavior namely steel fluid flow field distribution on great extent. For the high temperature steel fluid has big kinetic energy, so, the immixture dregs, solidification heat conduction, temperature field distribution in crystallizer, solidification blank shell thickness distribution and continuous caster blank quality were influenced by steel fluid flow. The numerical simulation analysis on flow field and temperature field in crystallizer were conducted in this paper. Three dimensions turbulent flow model was adopted to computate flow field. The heat conduction was ignored on draw blank direction in temperature field. The conjugate heat conduction model of ANSYS CFX was adopted to analyze temperature field, which can consider heat conduction in solid layer and convection heat conduction between solid shell face and fluid simultaneity. The draw blank velocity was found by setting crystallizer water gap insertion depth and crystallizer water gap angle, which can obtain reasonable flow field in blank crystallizer.


Basic parameters and theory
The basic parameters of blank continuous caster crystallizer.The basic parameters of blank continuous caster crystallizer were shown in table 1. The linking research of flow and heat conduction phenomenon was conducted by ANSYS CFX. The both side hole and downward obliquity immersion water gap was adopted, which is shown in Fig.1 [2] .

Basic hypothesis of model
The solidification shell in crystallizer was ignored. Steel fluid was treated as homogeneous phase medium. The influence of curved face fluctuation was ignored. The vibration influence on flow was ignored. The steel fluid gravity influence was ignored. Steel fluid flow was driven by initial velocity on water gap exit [3] .

Governing equation
The governing equation of steel fluid flow may be shown as: (1) Mass conservation equation: (1) (2) Momentum equation:

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(3)Turbulency kinetic energy equation: (4) Turbulency kinetic energy dissipation equation: (6) Among above equation, ui are velocity on X-axis, Y-axis and Z-axis separately namely uX, uY, and uZ, in which i take 1,2 and 3. K is turbulency kinetic energy. ε is turbulency kinetic energy dissipation ratio. Correlation coefficient were chosen according to recommended data. C1 is

Boundary conditions treatment
Relevant boundary conditions must be required in solving Eq.1 to Eq.6. Considering the characteristic of continuous caster crystallizer, in-built model computation boundary conditions of CFX were given as follows: (1) On crystallizer free fluid surface, Eq.7 must be satisfied.
(3) On insertion water gap exit section, Eq.10 must be satisfied.
(4) Boundary conditions treatment on crystallizer inner wall surface Adopting wall surface function method contains three main parts. Inside nodes adjacent to first wall surface were laid in turbulence area. Any nodes were laid in viscosity surface layer. High Reynolds number k ε − model was adopted on turbulence flow core.

Confirmation of draw blank velocity
Draw blank velocity is on the premise of obtaining good metallurgy quality, safe continuous casting process and high productive capacity continuous caster. Follow factors should be considered on confirming draw blank velocity.

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The computation of theory draw blank velocity maximum is shown in Eq.11 .
Among above equation, δ is crystallizer exit blank shell thickness, whose unit is millimeter. m k is solidification coefficient in crystallizer. m L is crystallizer effective length.

Numerical simulation of steel fluid flow in crystallizer Basic hypothesis of linking model and simplified model
Basic hypothesis of linking model were as follows: (1) Continuous casting process was regarded as steady state process.
(2)Steel fluid flow was regarded as incompressible Newtonian fluid flow. High Reynolds number k ε − model was adopted on turbulence effect simulation.
(4) Fluid state density and solid state density were regarded as constant.The simplified steel fluid model in crystallizer is shown in Fig.2. Fig.2 The simplified steel fluid model in crystallizer

Influence factors analysis of steel fluid flow field in crystallizer
ANSYS CFX can well simulate turbulence flow in fusion area by adopting ozeny-Carman equation and considering the special situation of turbulence attenuation in fusion area. [4] The turbulence characteristic curve under reasonable flow field was shown in fig.3. Among fig.3, Red line is residual error curve of steel fluid flow. Green line is turbulence curve. Fig.3 shows that turbulence formation curve is unsteady at initial teeming period. While, turbulence formation curve tends to steady along with time addition. Fig.3 The steel fluid turbulence characteristic curve The draw blank velocity was influenced by types of steel , blank section, middle steel bag capability, fluid surface height and steel fluid temperature [3] .For adjustment convenience, draw blank velocity sets to 0.  fig.5 show that the influence of draw blank velocity change on upper back flow field is small. But the influence of draw blank velocity on under back flow field can not be ignored. This phenomenon is harmful to blank shell formation on crystallizer bottom, which should cause draw blank leaking steel accident easily. The difference of flow field distribution is small between draw blank velocity under 1.6 m/min and 1.8 m/min. But the flow field distribution under 1.8 m/min is more reasonable. Under this draw blank velocity, dregs immixture can be avoided, impurity is easy float upward [5] . The increasement of draw blank velocity can also cause the increasement of steel scouring velocity on narrow surface. It can be seen from flow field distribution that draw blank velocity under 1.8 m/min can increase productivity and ensure production safety.

Conclusions
(1) It can be seen from turbulence model curve that steel fluid flow tends to steady gradually in crystallizer along with teeming time addition, which is benefit to uniform blank shell formation.
(2) By simulation computation under different draw blank velocity, it was found that draw blank velocity affects on crystallization quality and production efficiency. When immersion water gap insertion depth and incline angle is fixed, the flow field in crystallizer is the best under draw blank velocity 1.8 m/min. The numerical simulation results are coincide with practical production data. So, through this research, blank produce can be raised effectively under the base of assurance production safety.