Fluid Flow Control In The Continuous Casting Tundish Using Ladle Shrouds:Basics And Practices

Mathematical modelling,physical modelling and plant trials were carried out to study the effects of different ladle shrouds on fluid flow and steel cleanness in steelmaking tundish.Firstly,several popular turbulent models were evaluated,including two kinds of Reynolds-averaged Navier-Stokes models(RANS,SKE and RMS)and Large Eddy Simulation(LES).The results showed that LES was found to be robust to capture instantaneous turbulent structure and small-scale vortices when the meshing and calculation procedure were well arranged.A water modelling was established and verified the simulation results of LES.LES was employed to compare the fluid flow in two types of industrialized ladle shrouds(1/3 scaled water model).One is the conventional ladle shroud(CLS)and the other is the trumpet-shaped ladle shroud(TLS).The TLS changed the flow in two ways.The trumpet-shaped section encouraged the formation of vortices and turbulent dissipation,and also enlarged the effective volume and outlet area of the ladle shroud.These brought about a lower outlet velocity and swinging outflow streamlines.The outflow of the ladle shroud heavily influences the flow inside the tundish.The simulation showed that the outflow of the CLS was larger and impacted the tundish wall with larger pressure compared with that with the TLS.The outflow velocity of the TLS was relatively slow and was able to mix with larger volume of melt in the tundish.The turbulent energy got dissipated and gave rise to a mild impact with tundish wall and a calm liquid surface.Physical simulation confirmed the conclusios of mathematical modelling.The liquid surface was relatively calm with little slag entrapment using the TLS.The fundamental study revealed the mechanisms of the TLS to improve production efficiency and steel quality.The LES model was further extended from scaled-down ladle shrouds to industrial-scale ones.The effect of geometrical designs on fluid flow was investigated for two types of TLSs.The flow structure is found to be sensitive to the ratio(H/D)of the trumpet length(H)to outlet diameter(D).The H/D is suggested to be equal to or larger than four in the current cases to obtain a well-developed flow pattern and avoid reverse flow at the outlet of the TLS.This value applies to the two kinds of popular TLSs in practice.The flow characteristics are also influenced by casting parameters.When the slide gate is half open,many vortices are generated around the entrance stream.The trajectory of the streamline is arc-shaped with outflow biased to the direction of opened gate,which means the outflow is not symmetric.Plant trials were carried out to compare fluid flow and steel cleanness using a CLS and a TLS.Steel splashing,smoking and flames were recorded to evaluate the flow in the molten bath during both transient and steady casting.Samples were taken to study steel cleanness.The results showed that the TLS outperformed the CLS in terms of relieving steel splashing during ladle opening and descending,which showed potential in reducing secondary oxidation and realizing better safety operations.A relatively quiet liquid bath was achieved inside the tundish with less exposed liquid steel using the TLS during steady casting.When the TLS was utilized,average T.O.and[N]were 23.5 ppm and 33.0 ppm respectively in the tundish,which were 7.8%and 8.3%lower than that using the CLS.The TLS contributed to a 6.1%higher[Al]content(averaged at 365 ppm)than the value using the CLS.Comparison was made with other flow control devices to enhance steel cleanness,which showed the potential of the TLS for practical benefits.A new ladle shroud design,dissipative ladle shroud(DLS),was explored as well.Velocity distribution,vorticity islands and strain rate were analyzed in the DLS respectively,compared with that of a TLS(H/D=1).The results showed that the three chambers of the DLS gave rise to velocity differences,fluctuating strain rates and vortices,and promoted an increase on turbulence dissipation rate.The outflow swung intensively at a relatively low speed.These contribute to the mixing inside the tundish and reduce the dead volume.However,the practical application of the DLS is limited by manufacturing and serving issues.