Modeling And Simulation Of Transport Phenomena In Twin-Roll Continuous Casting Process

It is certainly that the real commercialization of twin-roll ontinuous casting process for steel strips will profound changes in the steel industry.However,though one century and a half have passed,this process is still a dream to the steel engineers,one of the most possible reasons is that we don’t know clearly the details of the transport phenomena in the pool region.Apparatenly,it’s impossible for us to investigate the transport phenomena directly or indirectly by the hot model experiment,and there are significant difference using water modeling experiments because that solidification is not considered.This reaearch based on the mathematical models achieved great success in conventional continuous casting process,toghether with the water modeling experiments to predict the transport phenomena in twin-roll continuous casting process.The research contents and innovative achievements of this paper are as follows:(1)A three-dimensional enthalpy-porosity mixture solidification model is employed to describe the fluid flow,heat transfer and solidification in a twin-roll strip continuous casting machine equipped with a multi-port trough shaped feeding system.The results show that,the fluid flow field in the pool region is quite different with/without the consideration of rotating rolls when the solidification is ignored,which indicates the importance of the consideration of rotating rolls in the water modeling equipment design;there are three aspects including molten steel jet impingement,roll rotating and backward flow of the mushy zone,which strongly influence the temperature distribution in the pool region;the solidification process in the pool region could be divided into three stages:at the first stage,there exists a fast solidification region which obeys the square root law,and an approximate linear growth region could be found at the second stage,while a parabolic growth region could be found at the last stage.(2)A two-dimensional mathematical model is employed to study the fluid flow,heat transfer and solidification during the twin-roll casting for both steel and aluminum.In order to facilitate the comparison and analysis,a new transformed coordinate system(r,φ)is established.The results show that,the twin-roll casting is a roll-rotating-driven process.The variation of the thickness of the momentum boundary layer can be divided into three stages and its thickness is very uniform at the last stage;near the roll surface,there exists a wedge-shaped zone induced by the near-roll-surface shear flow washes the mushy zone front,which increases the depth of the liquid pool and decreases the length of the rolling region;in the transformed coordinate system(r,φ),the effect of the centrifugal force induced by the rotating roll is presented using the velocity component in the r direction and the pressure gradient in the r direction.The rotating roll gives rise to the stirring effect to the pool region and the metal is moving away from the roll surface in the positive radial velocity region and the effect of the centrifugal force becomes weak in the lower part of the pool;at the solidification front,the non-dimensional effective heat transfer coefficient distribution in steel twin-roll casting is larger than that in aluminum twin-roll casting.Considering that the turbulence level is determined by the flow pattern in the pool region,which demonstrates the importance of the geometry of the feeding system in steel twin-roll casting;the evaluation of the solidified shell in aluminum twin-roll casting is a parabolic growth,while in steel twin-roll casting,the parabolic growth only occurs in the lower part of the pool.(3)The transient turbulent fluid flow in the wedge-shaped pool region of a full-scale twin-roll continuous casting machine is investigated by the large eddy simulation(LES).The results show that,the roller rotation decreases the meniscus instantaneous fluctuation range,but increases the horizontal velocity;the motion of the roller can significantly change the turbulence levels in the pool region;the vortex shedding induced by the Karman Vortex Street from the submerged entry nozzle(SEN)causes the"velocity magnitude wave" and strongly influences the boundary layer stability and the backflow stability.There are strong interactions between the boundary layer and the backflow.The boundary layer fluctuations are weakened by the buffer effect of the backflow,however,decrease the backflow stability,then influence the flow pattern in the lower part of the pool region;in steel twin-roll process,the vortex shedding phenomenon should be attached importance in the design of the SEN structure and the setting of the operating parameters.(4)Based on the previous studies,two principles to the SEN design are presented for the first time:in order to get a more stable process,the SEN should have the function to suppress the development of the momentum boundary layer to avoid the deep wedge-shaped zone and increase the length of the rolling region;in order to obtain a more even solidification and get a more stable process,the SEN should suppress the vortex shedding phenomenon to avoid the uneven solidification and to stabilize the backflow.The numerical simulation results and the water modeling results show that the new designed SEN structure could be a big progress for twin-roll process.