Mathematical And Physical Modeling Of Fluid Flow Phenomena In An AOD Converter Bath With Side And Top Combined Blowing

The fundamentals and developments of argon-oxygen decarburization(AOD) refining process of stainless steel have briefly been introduced.The available studies in the literature on physical and mathematical modeling of the refining process have been analyzed and reviewed.Taken a 120 t side and top combined blowing AOD converter in the Stainless Steel Branch,Baoshan Iron & Steel Co.,Ltd.as a prototype,a water model with a geometric similarity ratio of 1:4 to its prototype has been designed and established. With the blowing process of the main decarburization period of the side and top blowing refining of 304 type stainless steel in the AOD converter as an object,the friction factors of the main tuyeres and sub-tuyeres used for the prototype and its model to gas stream have been determined;and the theoretical calculations of the parameters of the gas streams in the annular side tuyeres as well as the top Laval top lance have been carried out;then,the gas flow rates of the side and top blowing used for the model have more reasonably been evaluated;thus,fuller kinematic similarity between the model and its prototype has been ensured.Water modeling has been employed to investigate the features of gas jet stirring, the fluid flow patterns,the stability of liquid surface,the characteristics of liquid flow and mixing,and the "back-attack" phenomenon of the gas jets through side tuyeres and its effects on the erosion and wear of the refractory lining in the bath of an AOD converter during the side and top combined blowing process.The influence of the gas flow rate for side blowing and the side tuyere number,the angle included between each tuyere and the gas flow rate of top blowing and others has been examined.It was shown that the liquid in the bath underwent vigorous stirring and circulatory motion during the blowing,and there was no obvious dead zone in the bath,leading to excellent mixing and a short mixing time.The gas flow rates of side tuyeres, particularly those of the main tuyeres,had a key role on the fluid flow and mixing characteristics.However,the gas jets of the sub-tuyere could give a physical shielding effect on the gas jets of the main tuyeres,and the mixing efficiency could be improved by a suitable increase in the gas blowing rate of the sub-tuyere.The gas jet from the top lance could change the agitation and fluid flow pattern caused by the gas jets from the side tuyeres in the bath,making the turbulent intensity increase and the mixing efficiency decrease,thus,the mixing time prolong.The larger the gas flow rate of the top lance,the more obvious these conditions.With a given number and flow rate of the side tuyeres,a larger angular separation between each tuyere would be benefit for improving the stirring efficiency of gas streams in the bath,so the mixing time would be shortened.Under a given angle included between each tuyere and flow rate of the side tuyeres,increasing the number of side tuyere would not necessarily enable to reach a similar effectiveness,but with a decreased gas flow rate of each side tuyere, thus reducing the horizontal penetration distance of gas jets from side tuyere and making the high temperature zone move towards and closer to the vessel wall,leading to decreasing the life of the refractory lining.As far as only the mixing in the bath is concerned,comprehensively considering the influence of the gas flow rate of side blowing,the side tuyere number,the angle between each tuyere and the gas flow rate of top blowing,using 6 side tuyeres with an angle of 27°included between each tuyere would all bring about a roughly equivalent mixing effectiveness,corresponding to an oxygen flow rate of 6600 Nm~3/h for the top blowing specified by the technology. For an angle of 18°separation between each tuyere utilized in the existing practice, using 7,6 or 5 side tuyeres would all be able to provide a good mixing result.Compared to the situation in a simple side blowing or bottom blowing,the back-attack phenomenon of the submerged horizontal gas jets under the conditions of the side and top combined blowing process had its own features.At the conditions,the gas streams of the main tuyere plaid a dominant role on the back-attack phenomenon of the gas jets horizontally submerged blowing,and the sub-tuyere streams showed an obvious alleviation and prevention effect to it.The top stream would make the back-attack action of the side blowing streams more uniform and the action intensity (pressure) enhance.With a side and top combined blowing,the circulation motion of the liquid in the bath would also be another important reason for causing the back-attack;the buoyancy has a considerable influence,it is not only able to increase the back-attack intensity of a horizontal gas jet,but also to enlarge the locally eroded and worn zone of the refractory lining.Compared with the situation of a simple side blowing,the buoyancy in a side and top combined blowing could lower the increased amplitude of the back-attack intensity of a horizontally submerged blowing jet,but an interaction with the impinge force from the top blowing jet would make the acting area of the back-attack force,i.e.the eroded and worn zone of the refractory lining larger.With the given side tuyere number and flow rates of the top and side blowing in the present work,a proper increase of the angle included between each tuyere in the range of 18°-27°would be beneficial to alleviating the back-attack action of the side blowing stream.Relatively,at the given side and top gas flow rates,the combination of 7 side tuyeres with an angle separation of 22.5°,or 6 side tuyeres with an angle of 27°for both simple side blowing and side and top combined blowing would all be able to cause a less erosion extent of the refractory lining than other arrangement schemes.Based on the two-fluid(Eulerian-Eulerian) model for a gas-liquid two-phase flow and the modified k-εmodel for turbulent flow,a three-dimensional mathematical model for the flow of molten steel in the bath of an AOD converter under the conditions of a simple side blowing has been proposed and developed.The related parameters of the model have been determined.With considering a heating and friction flow of gas and on the basis of the theoretical calculations of the parameters of the gas streams in the annular side tuyeres and the estimations for heat transfer between the gas jets and the molten steel,the inlet boundary conditions used for the mathematical model were more reasonably predicted.The fluid flow fields in the 120 t side and top combined blowing AOD converter and its water model unit with a linear scale ratio of 1/4 linear scale ratio have been computed by using of this model, respectively.The influences of the side tuyere number,and the angle included between each tuyere were examined.The results indicated that the flow pattern of molten steel in the bath of AOD converter during a simple side blowing process could be well modeled by the model.The liquid in the bath underwent vigorous stirring and circulation motion during the blowing,and there was no obvious dead zone in the bath. Changing the number of side tuyere could not alter the essential characteristics of gas stirring and fluid flow,neither the distributions of turbulent kinetic energy and gas holdups.However,with a given angle separation between each tuyere and gas flow rate of side tuyere,decreasing the number of side tuyere could make the agitating intensity increase,thus bringing about the local variation of the flow field,and the distributions of turbulent kinetic energy and gas holdups were more uniform by using the arrangement scheme of 6 side tuyeres with an angle of 27°between each tuyere than by using 7 side tuyeres with 18°or 22.5°.At a given tyuere number and side gas flow rate,a larger angular separation between each tyuere could also not change the basic characteristics of gas agitation and fluid flow,but the flow field would have some evident differences from each other.In the range of 18°-27°,the larger the angle included between each tuyere,the large the total sector area between the tuyeres,and the larger the area occupied by the plume at the surface of the bath,thus making the interaction among the gas jets weaken,and the related energy dissipation reduce,so the gas agitation would be more homogeneous.Compared with the situation in a simple side blowing AOD converter,the flow field under the conditions of the side and top blowing had the different features,which would be attributed to a combined effect of the both blowing processes.On this consideration,a three-dimensional mathematical model for the flow of molten steel in the AOD converter bath during the simple top blowing process had been proposed and developed.And the flow fields and turbulent kinetic energy of liquid phases in the 120 t AOD converter and its water model unit under the conditions of simple top blowing had been computed using this model.Then,the flow field of side and top combined blowing could be obtained from the superposition of flow fields of the simple side blowing and simple top one.The results demonstrated that the liquid flow in the bath of the AOD converter with the side and top blowing was in combining resulted from the side blowing streams under the influence of the top blowing jet.The liquid in the bath underwent vigorous stirring and circulatory motion during the combined blowing, and there was also no obvious dead zone in the bath.The top lance injection did not change the essential features of the liquid agitation and fluid flow,compared with the situations in a simple side blowing AOD converter,but evidently changed the local flow field in the bath.Similarly,under the conditions of the combined blowing,the change of the number of side tuyeres did not alter the basic features of the gas stirring, fluid flow and the distribution pattern of turbulent kinetic energy.And with a given angle between each tuyere and side tuyere blowing rate,the agitating intensity would be increased with decreasing of the side tuyere number,thus the flow field and the distribution of turbulent kinetic energy would be locally changed.Using 6 side tuyeres with an angle of 27°would give more effective stirring than using 7 side tuyeres with an angle of 18°or 22.5°;this is the same as that of the simple side blowing.And similarly,for a given number and gas flow rate of side tyueres,a larger angular separation between each tyuere would not change the characteristic patterns of the gas stirring and fluid flow,but change obviously the characteristics of gas agitation and flow field,but the flow field would have some more obvious discrepancies.In the range of 18°-27°,the larger the angle between each tuyere,the larger the total sector area between tuyeres,and the larger the area occupied by the plume at the surface of the bath,thus making the interaction between gas jets decrease,and the relevant energy dissipation reduce,the gas stirring would be moreAccording to the results mentioned above,the shell of the 120 t AOD converter has technically been reformed and the industrial experiments have been carried out. The conclusions given above have been confirmed to be entirely reliable,believable and correct.Under the conditions of the industrial experiments,using 7 side tuyeres with 22.5°to replace the arrangement scheme of 7 tuyeres with 18°could make the life of the refractory lining of the converter obviously increase,and the economic and technical indications of the refining process raise with larger amplitude.