Penetration Of Liquid Steel And Transport Of Particle-Gas Flow In The Device For Ladle Bottom Powder Injection

Ladle Bottom Powder Injection(L-BPI),invented by Northeastern University,is a new secondary refining technology,and its application will reform the production process of clean steelmaking.In present thesis,the key device of this new technique viz.injection plug was systematically studied by laboratory and industrial experiments,as well as numerical simulations.The penetration mechanism of liquid steel and transport behavior of gas-particle flow in the injection plug were focused on,while the design theory of the plug was proposed based on the results.Consequently,suitable injection plugs were developed to obtain high wear resistance and high temperature corrosion resistance.In the present work,the key issues on the plugs for ladle bottom power injection,including liquid steel penetration(security),powder injection clogging(stability),service life(reliability),etc.were successfully solved,and the achievement would build a foundation for industrial application of L-BPI.The main contents and conclusions are listed as follows:(1)The performance of the injection plug on the resistance of liquid steel penetration was investigated.In order to solve the problem of liquid steel penetration in L-BPI process,a theoretical model was developed to predict the relationship between the safe width of injection plug slot and the depth of molten bath.The model was further improved by considering slot geographical parameters,the contact of interfaces,the roughness and macroscopic morphology of refractory surfaces.Based on the momentum conservation and energy conservation,a mathematic model was also established to describe the variation of penetration velocity and depth with time.Furthermore,a simple approach was presented to detect penetration behavior by the measurement of electrical signals.The corresponding experiment was successfully performed to verify the penetration models by Wood’s metal on the basis of similar principles.It is found that the present model can accurately predict the variation of the safe slot width with the molten bath depth.When molten steel penetrates into a slot,a stable value of the penetration depth will be rapidly obtained.Penetration process can be divided into three stages,namely unsteady penetration,main penetration,and terminal penetration.It can efficiently prevent penetration by some approaches,such as the increase of slot uniformity,and the control of refractory composition and surface macroscopic morphology,etc.(2)The behavior of the gas-particle flow in the bottom injection plug was studied.The motion characteristics of a single particle were studied.On this basis,an Euler model was employed to accurately simulate the gas-particle two-phase flow in the injection plug,and the effects of interphase forces and wall boundary conditions were taken into consideration.Finally,the model parameters were also determined.It is found that the trajectory of an injected particle is a curve rather than a straight line along the flow direction,so particles will also generate a displacement in horizontal direction.The motion characteristics were affected by both the physical parameters and motion parameters of particles,such as diameter,density,inlet velocity etc.Calculated results show that smaller particles with lower density and higher inlet velocity would be easier to be injected into the refining ladle through a slot plug.During the processes of bottom powder injection,there are typical raceways on both sides of the main stream in the chamber,where the volume concentration of particles is very low.The two phase flow in the injection plug presents evident transient behavior,asymmetry,and irregular patterns.The chamber angle,gas flow rate and particle volume loading all significantly affect the gas-particle flow behavior.In detail,chamber angle,too large or too small,would lead to serious bias flow in the slots;high gas flow rate could not greatly increase the particle concentration in the slots,but it would result in particle accumulation at the bottom of the chamber;high particle volume loading would easily cause clogging of injection plug,so it should be controlled below 15%at the inlet.(3)The behavior of particle-rough wall collision in a plug slot was investigated.Traditional models usually predict many grazing particles during the simulation of particle-wall collision dominating flow,which is far from reality.In order to solve this problem,a virtual-wall-group concept was proposed,and a particle history effect model(PHEM)was developed to describe the effect of wall roughness on particle-wall collision processes.It consists of two aspects,namely a new probability function of roughness angle,and a refined multi-collision process.The former was completed by using a virtual testing method,and latter was divided in detail according to the real rebound angle of particles.Combined with the coordinate rotation operation,the algorithm of the new model was presented,and the corresponding parameters were solved according to the measurements.It is found that PHEM model can accurately predict the distribution of particle rebound angles,and the effect of wall roughness decreases with the increase of impact angle.Wall roughness not only increases the collision frequency of particle-wall,leading to the loss of particle kinetic energy,but also causes the particle phase to be dispersedly distributed with high fluctuating velocity in the slot.(4)The wall erosions of the injection plug and its affiliated bend from the gas-particle flow were studied.Based on the stochastic approach,the effect of wear pocket on particle-gas flows and wall erosion was firstly considered,and a wear-pocket effect model(WPEM)was established with the consideration of characteristic angle and fluctuation term.A CFD-WPEM approach was developed based on the new model.It was employed to simulate the erosion of injection plug and the affiliated bend.This new model was verified by the measurements of bend erosion that it could accurately predict the eroded locations and depth.The simulation results show that there are three typical eroded belts close to the slot inlet.However,the erosion on the downstream wall shows dotted distribution.The parameters of both the flow and slot would affect the erosion rate.For instance,higher velocity gas-particle flow or higher particle mass loading would lead to more serious erosion,while a wider slot would result in less wear.Certainly,the slot width has to be strictly limited to prevent the penetration of molten steel.(5)The injection device used for L-BPI was examined by experiment.By using the self-designed bottom powder injection system,the injection device was tested under cold air atmosphere.It is found that the new injection device shows a good performance for powder injection.During the whole injection process,the gas flow rate could be easily adjusted,and the powder injection rate can also be continuously controlled.Furthermore,no clogging occurs.Bottom powder injection was also conducted in a 1.5t induction furnace by using the injection device.A good refining result was achieved,and no liquid steel penetration and clogging were found in the plug.Meanwhile,the material of the injection plug has a strong resistance against molten slag or steel.So the injection device developed in present work is able to satisfy the technical requirements of ladle bottom powder injection.