Large Eddy Simultion On The Multiphase Flow,Heat Transfer,Solidification,and Entrapment Of Inclusions During Steel Continuous Casting

In the current study,a three-dimensional computational model,combining the Large eddy simulation(LES)turbulent model,VOF multiphase model,and Discrete Phase Model(DPM),was established to investigate the multiphase flow and the distribution of bubbles in the continuous casting(CC)mold.The net slag entrainment rate,the probability of slag entrainment at the meniscus,and the diameter of slag droplets were calculated using the user-defined function(UDF),which provide initial conditions for the subsequent prediction of the distribution of slag inclusions.The drag coefficient model proposed by Kolve[1],the lift coefficient model proposed by Tomiyama[2],and the 0.5 virtual mass coefficient were recommended to accurately simulate the fluid flow and bubble distribution in the CC mold by comparing the predicted fluid flow with the measured results one using Particle Image Velocimetry(PIV).The drag and the buoyancy were the dominant ones.The drag force,lift force,pressure gradient force,and virtual mass force had approximately the same magnitude in locations where bubbles and liquid interacted violently.The magnitude of the lift force,pressure gradient force,and virtual mass force was reduced to 1%-10%of the drag force and buoyancy force during the floating of large bubbles.The effect of the lift force could ignore for small bubbles.The effect of each interphase force needed to be considered when the bubble diameter larger was than 0.5 mm.A typical double roll flow was formed with the current 1.8 m/min casting speed,6 NL/min argon flow rate,150 mm SEN immersion depth,and 1000 mm×230 mm CC mold.The level fluctuation gradually decreased from the narrow surface to the submerged entry nozzle(SEN),and the net slag entrainment rate at 0.13 m below the initial steel slag interface was 0.05456 kg/s under the current double roll flow.The slag entrainment mainly occurred at the 1/4 width of the mold and between the SEN and the wide face under the current casting state.Most of slag droplets had an average diameter of 2-5 mm,and a fitting formula for the diameter and percentage of slag droplets was propesed.The level fluctuation and slag entrapment rate increased with the increase of the argon flow rate and casting speed.Nextly,the fluid flow,temperature.solute concentration,and shell thickness was successfully predicted using the coupled LES turbulent model,heat transfer and solidification model,species transport model.The fluid flow in the mold presented a transient asymmetric periodic change.The period of the transient velocity near the narrow surface of the mold outlet was about 43 s.The asymmetric flow field caused the asymmetry distribution of the temperature and solute concentration.The uneven growth of the shell and asymmetry distribution of the inclusion was also induced by the asymmetric flow field.The three-dimensional morphology of the solidification end presentd a "W" shape.The thickness of the solidified shell was relatively large with the effect of the carbon and sulfur content,which was closer to the measured value.The cooling rate and the solidification rate near the top surface were relatively large,which can reach about 35-75 K/s and 2.2 mm/s,respectively.The cooling rate and the solidification rate gradually decreased along the casting direction,and remained at 1-4 K/s and 0.1 mm/s after exiting the mold.The primary dendrite spacing(PDAS)also decreased correspondingly along the casting direction,and the minimum value was above 60 ?m.Finally,the number,size,and spatial distribution of non-metallic inclusions(NMIs)on the entire cross section of the CC slab was successfully predicted using the coupled full solidification model,DPM,and capture criterion at the solidification front.The improved capture criterion that including the PDAS,different forces acting on inclusions,and the critical capture speed,successfully eliminated the banded distribution characteristics of NMIs under the simple capture criterion.The prediction results were in good agreement with the detection results.Four accumulation zones of NMIs along the thickness of the CC slab,including the 1/4 thickness and 3/4 thickness from the loose side,and the layer beneath the surface of the CC slab.Two accumulation zones near the layer beneath the surface of the CC slab were issued from the double roll flow pattern in the CC strand.The probability of inclusions contacting the solidification front in the upper recirculation zone increased,resulting in an increase in the entrapment rate.Accumulation zones near the 1/4 thickness and 3/4 thickness from the loose side werer corresponded to the the peak value of the Marangoni force on inclusions at the 1/4 thickness and 3/4 thickness from the loose side.The Marangoni force increased the probability of inclusions being pushed to the solidification front.In addition,the accumulation near the 1/4 thickness from the loose side became more severe as the diameter of the NMIs increased.In addition,NMIs tend to be captured on the loose side due to buoyancy.