Fundamental Study On Behaviours Of Supersonic Jets And Their Interaction With Liquids Bath In BOF

The impingement of a high-speed gas jet onto a deformable liquid surface is commonly encountered in many industrial engineering applications,such as BOF and EAF steelmaking,VOD,Copper smelting converter,internal combustion engine,etc.In oxygen steelmaking process,the high-pressure oxygen is transformed into the supersonic oxygen jets and then ejected into the molten bath,during which the behaviors of supersonic jets play an extremely significant role in supplying oxygen,slagging and so on,and dominate the performance of the reactor.The interaction of the jets with the molten bath is also the root of many complex physical phenomena occurred in the furnace,for instance splashing,stirring,oscillating,etc,influencing the mass,momentum and heat transfers during the refining process.However,attributed to the harsh environment of process involved with high temperature and high speed characteristic,an insight into the behaviors of jets and their interaction with the molten bath is lacking.In this study,the behaviors of jets and their interaction with the molten bath were studied by computational fluid dynamic(CFD)technology and physical modeling method based on the principle of similarity.Flow characteristics of an oxygen jet under off-designed operation pressures were investigated and shock waves of jet along with their shapes were indicated by a developed 2D axisymmetric CFD model of compressible oxygen jet combined with the standard k?? model.The quantitative relationship between the length of supersonic region of jet and operating pressure was obtained.Hydrodynamics behaviors of multiple supersonic jets,especially their coalescence mechanism were studied by a developed and validated compressible,non-isothermal and three-dimensional CFD model,and the effects of ambient temperature,nozzle design and operating parameters were discussed.What's more,the evolution of the energy of the jets along their axial travel at different operating parameters was analyzed,and the attenuation law of an individual jet centerline velocity was characterized by normalization.It is found that:(1)The axial velocity and dynamic pressure attenuation of the jets is delayed with the increase of ambient temperature or operation pressure,the decrease of nozzle inclination angle or number.(2)The jets prefer to deviate from their nozzle axes and tend to coalescence with the decrease of ambient temperature or inclination angle,or the increase of nozzle number,but it is not affected by operation pressure.The critical inclination angle of being free from interaction among jets is 25° for the present 4-nozzle lance.(3)Combining the dynamic parameters distribution of the jets with the jets trajectory can estimate the cavity shape and splashing modes in BOF steelmaking.(4)The kinetic energy for the jets suffering from shock waves likely cause more intensive damping compared to the jets with expansion waves.A great part of the impinging kinetic energy of the jets is dissipated during their flow,and(5)the flow rate of turbulence kinetic energy and turbulence dissipation rate are observed to firstly increase and then decrease.Based on these results,the optimum theoretical lance height was identified for a rapid slagging and splashing slag operation:1.1?11.2 m(0.8P0),1.2?1.3 m(P0),1.4?1,5 m(1.2P0).Water model experiments were conducted with a 1/10 scale-down top-blown convertor.The cavity dimensions,shape and oscillation of the bath with respect to operating parameters,nozzle angle and slag volume were studied.The resulting erosion characteristics of the furnace wall were analyzed according to the oscillation characteristcs of the bath,and it is found that the localized erosion of furnace wall is the most serious at the early stage of blowing.Furthermore,the importance of the influence of liquid surface tension on the cavity depth was theoretically identified.It was found that the commonly used theoretical model overestimates the crater depth by about 12%under the present experimental conditions because it neglects the influence of liquid surface tension,and thereby,significant inaccuracies can be produced if the results from water model experiments are used for predicting those in the real BOF.An improved theoretical model was hence proposed and validated using the experimental data obtained from both the single-or two-layer liquid(s)baths.The new model includes not only the explicit consideration of the liquid surface tension but also that of the energy utilization efficiency of the jets impinging kinetic energy contributed to the cratering process.A compressible,non-isothermal and three-dimensional CFD model was developed to study the physical phenomena occurred during the interaction of the jets with the molten bath in a real 150 ton steelmaking BOF.Efforts were especially made to study the change of gas-slag-metal interface during blowing process,to reveal the formation mechanism of splashing droplet and to analyze the energy exchange between the jets and the molten bath.The simulation results show that:(1)The blowing process has an evidently unstable characteristic,and the cavities are random.The gas-slag-metal interface remain unstable as a result of the propagation of surface waves,which,likely as a major factor,governs the generation of metal droplets and their initial spatiotemporal distribution.The instability of gas-slag-metal interface is intensified with the lowering of lance height or increase of operation pressure,but presents relatively weak effect by physical properties of fluids.(2)The splashing occurrence of droplet has two mechanisms:one is the tearing of metal sheet at the cavity edge,and the other is the direct ejectment of single droplet at the cavity edge.The two mechanisms simultaneously occur and attribute to the splashing during converter practice.The splashing rate is in the range of 6.1?34.4 kg·Nm-3 at the present varying lance height of 1.2-1.8 m.The lowering of lance height or the increase of operation pressure leads to more intensive splashing,but the effect of physical properties of melts on splashing is insignificant.(3)The "dead zones" occur near the bottom and side wall of furnace,as well as the central bulge of the depression,and they are expanded with the increasing of lance height or decreasing of operation pressure,and(4)the utilization efficiency of the jets kinetic energy attributed to the stirring of the molten bath is very low.Decreasing lance height or increasing operation pressure promotes the efficiency of the momentum transfer from the jets to the molten bath but lowers the efficiency of the kinetic energy transfer for the stirring of the molten bath.