Research On Characteristics Of Fluid Flow Based On Hot Metal Pretreatment Desulphurization

The requirements for the technology level of hot metal pretreatment are further increaseed with the increasing demand for the low sulfur and ultra-low sulfur steel.The mechanical stirring method and the injection method are the most commonly used method for the hot metal desulphurization.An important means to increase the desulfurization efficiency and to reduce the desulfurizer waste is to improve the dynamic conditions of hot metal,to reduce the bubbles size,and to disperse them into the molten device evenly in the process of deep desulphurization using the injection method.In the process of deep desulfurization using the mechanical stirring method,fast involving the solid desulfurizer into the hot metal surface and distributing them uniformly not only can improve the desulfurization efficiency and reduce the desulfurizer waste but also can reduce the temperature drop.Therefore,the research on the dynamics behavior in the process of hot metal deep desulfurization is very important.In this dissertation,the characteristics of fluid flow in the desulphurization process have been studied deeply based on the physical process of hot metal deep desulfurization using the physical simulation and mathematical simulation methods.Based on the stirring-injection method,an improved impeller type is developed,and a multiple size bubble model is also developed to describe the bubble behavior of breaking and dispersing in the molten device.The bubble behaviors of disintegration and dispersion in the stirring-injection device are discussed.Based on the mechanical stirring method,a method of variable-velocity mechanical stirring is proposed,which is suitable for the hot metal desulfurization.The behaviors of desulfurizer involving and mixing in the hot metal are studied numerically,and the operating parameters are optimized.The main research contents and results are as follows.1.The fluid flow structure and the bubble behaviors of breaking and dispersing in the molten device using the new type of stirring-injection method1)A water model experiment is carried out,and the bubble process of breaking and dispersing in the molten device is observed to investigate the effects of stirring modes on the bubble behavior of breaking and dispersing.Results indicate that the follows.(1)The bubbles are thrown out of the air-pocket on the back of blade and broken by the enough speed.When the speed of stirring impeller is more than 400 rpm,bubbles can be broken fully.Large vortices appear on the liquid surface and result in the bubble entrainment,which is not useful for the full use of desulfurizer.(2)Variable-speed stirring modes can produce chaotic gas-liquid flow,which has strong power to throw bubbles out of the air-pocket and breaks them.In the process of bubble rising,the turbulent flow field further breaks bubbles and disperses them in the molten device.(3)Comparing with the clockwise or anticlockwise variable-speed rotation modes,the clockwise-anticlockwise rotation mode(C-ARM)has better ability to break bubbles and disperse bubbles under the same operating conditions,and the operating parameters of variable-speed rotation modes affect the bubble size and its distribution.2)A mathematical model of single size bubble in the gas-liquid two-phase flows is established to investigate the gas-liquid flow characteristics,and the flow field structure under different stirring modes is analyzed.The conclusions are as follows,(1)Comparing with the standard k-? model and the improved k-? model,the RNG k-? model is more suitable to describe the turbulent behavior in the stirring-injecton device.(2)The impeller type has influence on the gas-liquid flow structure.The disc impeller is helpful for preventing bubbles from escaping rapidly and reducing vortex produced on the liquid surface.(3)Vortex in different locations appear in different time under variable-velocity stirring modes,and the flow field structure is changed,which is favorable for the bubble breaking and bubble dispersing.3)The mechanism of bubble breaking and coalescence are analyzed in the stirring-injection device,and a mathematical model to describe the bubble breakup and dispersion is developed.The bubbles population balance model is applied to the gas-liquid system using the stirring-injection method for the first time to predict the bubble size and its distribution.Based on the process analysis to the breakup and coalescence and the comprehensive test to the mechanism of breakup rate and coalescence rate,a breakup rate and coalescence rate model is developed,which is more suitable for adopting when the stirring-injection method is used.The bubble size and its distribution are investigated under the C-ARM,and the model accuracy for the prediction of bubble size is studied as well.Results show that the bubbles population balance model can be used to describe the bubble breaking and bubble dispersing when the stirring-injection method is used.4)Bubble size distribution in the stirring-injection device is predicted by using the multiple size group model,and results show the follows,(1)The effect of bubble breaking and bubble dispersion varies greatly using different stirring modes.The bubble size is uniform and small under the C-ARM.(2)Increasing the rotating speed can reduce the bubble size within a certain range under variable speed stirring mode.Further increasing the speed under C-ARM,the decreasing trend of bubble size gets slowly.The small speed span under the clockwise variable speed rotation mode(C-VRM)can lead to the appearance of vortex around the axis,which results in the nonuniform distribution of bubble sizes.(3)Increasing the injection rate can raise bubble volume fraction under the variable speed stirring mode,but it is harmful to the bubble breakage and dispersion.2.The behavior of involvement and dispersion for desulfurizer particles based on the variable-velocity stirring methodThe mixing behavior of desulfurizer in the molten iron is studied under the variable-velocity stirring mode using the Euler-Granular model.The factors of desulfurizer involvement and dispersion efficiency are analyzed,and the distribution and its range are investigated when the rotation speed and the variable-velocity period are varied.The operation parameters are optimized as well.Following conclusions are drawn,(1)The variable-velocity stirring mode can improve the flow field structure when the desulfurizer is involved in flow field and eliminate isolated mixing regions in the device,which can increase the desulfurizer dispersion effect and improve the desulfurization efficiency and the utilization efficiency of desulfurizer.(2)Increasing the rotation speed or decreasing the period of variable velocity can increase the uniformity of desulfurizer dispersion and decrease the mixing time.The period of variable velocity affects greatly the mixing time and mixing uniformity,and the variable velocity span affects the desulfurizer volume fraction.(3)Under the operating condition of 50-90 rpm for 8 s switch time,the desulfurizer distribution is most uniform and the mixing time is shortest in our research range using the variable-velocity stirring mode.