Multi-scale Modeling Of Multiphase And Inhomogeneous Transmission Mechanisms In Continuous Casting Mold

Continuous casting mold is the last metallurgical reactor for controlling the cleanliness of liquid steel,and called the heart of continuous casting equipment.In addition,the multi-scale transport phenomena(multi-scale turbulence,multi-scale phase change)inside the mold have a great influence on the quality of the casting billet.Multi-scale flow in the mold can couple heat transfer,mass transfer,phase transformation,multiphase flow,and many other processes,forming very complex inhomogeneous multi-physical fields.Study on the hydrodynamic behaviors of multi-scale and multi-physical field in the mold can provide guidance in optimizing the operating process and designing a novel mold with high performance,which is very important to promote the quality improvement of steel products and the improvement of production.In this work,the complex hydrodynamic behaviors of multi-scale and multi-physical field in the mold were systematically studied by both experimental measurement and numerical simulation.And the plant measurements and physical experiment data were used to verify the reliability of many proposed mathematical models.The main achievements are as follows:Ultrasonic flaw detection defect characteristics of plant steel plates were arranged and classified.Ultrasonically tested defect is one of the main defects of continuous casting heavy slabs.By collating abundant ultrasonic flaw detection maps of steel plates and the corresponding component analysis of defects,the typical defect characteristics of steel plates were summed up:the asymmetric quarter bands punctate defects which locate away from quarter thickness and the center bands clumps defects which locate on the center of slabs.Hydrodynamic behaviors in the mold were systematically studied using the water model experiments.One vertical-curved mold system was set up,and the effects of different operating parameters on the periodic asymmetrical flow and slag entrapment behavior induced by vortex were investigated.Another water model of the slab continuous-casting mold was applied to investigate the distribution and size of bubbles by injecting air through a circumferential inlet chamber which was made of the specially-coated samples of mullite porous brick,which is used for the actual upper nozzle.The transient bubble distributions in the mold were captured by a laser light sheet and a high-speed video camera.Then,the location and size of bubbles were measured using the image analysis software of ImageJ.The effects of different operating parameters on the bubble size distribution were analysised.A large eddy simulation(LES)mathematical model has been developed to analyze the time dependent periodic flow in a mold.The wall function was used to improve the prediction precision of traditional LES on the eddy viscosity close to the wall.The periodic behavior of transient fluid flow in the mold is identified and characterized.The evolution of asymmetric flow can be divided in two categories:backflow in the vertical section and asymmetrical flow in the curved section.Various instantaneous vortex forms were obtained by the LES model when the SEN and mold are perfectly symmetric in geometry.The vortexing flow was resulted from asymmetric flow in the liquid pool,and the vortices were located at the low-velocity side adjacent to the SEN.Finally,based on the transient flow field,the Lagrange model is applied to investigate the transport and entrapment of argon bubbles in the mold.It is difficult for small bubbles to float upward to the top surface once they have moved downward to the curved section.They would be either trapped by the solidifying shell of the inner curved section or they flow out of the domain,leading to defects in the slabs.A coupled LES computational model was built to simulate the transient fluid flow,heat transfer,and solidification processes in a mold.The unsteady multi-physical fields coupling problem is solved well.The growth of solidification shell and transient turbulent flow of molten steel inside the liquid pool were studied using this model.The predicted complex instantaneous velocity field is composed of various small recirculation zones and multiple vortices.One particle motion model inside the liquid pool and criterion of particle entrapment in the solidified shell were developed using the user-defined functions.The basic characteristics of inclusion clusters was established based on the the fractal theory and mass conservation.And the transport and entrapment of bubble,nonmetallic inclusion and inclusion cluster inside the liquid pool is calculated using the Lagrangian approach based on the transient flow field.This research provides the theoretical fundament for further studying on the bubbles/inclusion defects mechanism in the solidified shell,avoiding and improving the heavy slab quality defect.Euler-Euler Large Eddy Simulation(EELES)scheme has been developed to simulate the transient two-phase flow of argon gas and molten steel in the mold.This model can capture the fluctuating pressure of liquid phase induced by gas injection which cannot be captured by the previous turbulence models,improve the prediction accuracy of the local turbulence,gas-liquid motion parameters.The EELES approach is able to predict qualitatively the transient asymmetrical two-phase flow in the mold.The bubble motion has a great influence on the flow behavior of molten steel in the mold.A lot of random distribution and multiple vortices were found inside the mold.Two typical transient flow structures,consisting of clockwise or counterclockwise rotational direction vortices,were found in the upper roll.A population balance model has been developed to investigate the bubble size distribution inside the slab continuous-casting mold,improving the predictive ability of mono-size two-fluid model on the polydispersed bubbly flow through improving the local bubble size accuracy.The bubble microscopic phenomena(coalescence and breakage)and macro properties(size and surface area)are coupled in this model.Bubble coalescence and breakup processes between all bubble size classes are taken into account by appropriate models.By incorporating the closure correlations and boundary conditions,this model can make a good prediction of bubble size distribution in a wider range of working conditions.The dynamic behavior on the space and time of gas-liquid two phase flow in the mold and the change of bubble size distribution were obtained using this model.This research not only contributes to expand the applicable scope of the two-fluid model,but also provides the theoretical fundament and mathematical framework for studying the polydispersed bubbly flow.A generalized inhomogeneous MUltiple SIze Group(MUSIG)model based on the Eulerian framework was developed to describe the polydispersed bubbly flow inside the continuous-casting mold.This model was improved according to the numerical research on the bubble size distribution by various turbulence models and interphase forces models were carried out under numerous water flow rates and gas flow rates.Comparison with experiment data shows that the turbulence modification and interphase forces have notable influence on the hydrodynamics behavior in the gas-liquid system.Finally,the MUSIG model is then used to estimate the argon bubble diameter in the molten steel of the mold.The improved model has good versatility and prediction ability,can provide theoretical guidance for the optimization of operating parameters.