Research On Nonlinear Dynamic Characteristics And Vibration Screening Behaviors Of Flip-flow Screen

Coal preparation is the foundation and prerequisite for achieving clean utilization,energy conservation,emission reduction,and sustainable development.Dry deep screening of moist fine coal is a key technology that urgently needs to be solved in China’s coal processing industry.The main reason for the poor performance of dry deep screening is the adhesion and agglomeration of moist fine coal particles.At present,the flip-flow screen is proved to be the most effective equipment in the dry deep screening field of moist fine coal.With flexure motions of screen panels,the flip-flow screen can provide a projection acceleration up to 30-50 g to screening materials.As a result,it become more easier for material particles loosening.However,the flip-flow screen undergoes the forced vibration under the excitation force,screen panels periodically flex and exposure to the alternating stress in a long term.Thus,screen panels are prone to damage,and screen frames are prone to crack.Therefore,reducing the working load of key components while ensuring screening efficiency is of great significance to the flip-flow screen achieving both well screening performance and long serving life.Aiming to the above research objectives,a solution that is proposed.The solution is reducing the working load of key components and ensuring high screening efficiency of the flip-flow screen.Specific measures include,on the one hand,reducing the excitation force of the screen frame and the boundary excitation of the screen panel,on the other hand,improving the collision depolymerization degree of agglomerates as well as optimizing the screening behavior of material groups.In this paper,theoretical analysis,numerical simulation,and experimental testing methods are used to conduct an in-depth research on dynamic characteristics of the flip-flow screen,the collision depolymerization mechanism of moist fine coal agglomerates,and the vibration screening behavior of material groups.Firstly,a linear dynamic model of the screen frame system considering damping effects of rubber springs is proposed,as well as a nonlinear dynamic model of the screen frame system considering the amplitude correlation and frequency correlation of shear springs dynamic characteristics.Dynamic characteristics influences of rubber springs on the vibration stability of the screen frame system under a certain excitation force are clarified.Then,to clarify parameters effects on the flexure dynamics of flexible screen panels under certain boundary excitations,the Catenary model of the flip-flow screen panel in statically indeterminate structure is proposed and applied to the nonlinear finite element analysis.Moreover,a collision depolymerization model of moist fine coal agglomerate on flexible screen panel is established and the collision depolymerization mode is revealed.Besides,the evaluation method of collision energy and degree are given.To further improve the screening efficiency of material groups under a certain projection acceleration,a screen panel with the concave-convex surface morphology is proposed,which leads to a lower migration speed and longer residence time of the material group on screen panel,promotes the collision depolymerization of agglomerates as well as the contact and penetration of material particles.As a result,a better screening performance can be obtained under a certain ejection acceleration condition.Rubber springs(including shear springs and vibration isolation springs)are important components in the flip-flow screen frame system,and their dynamic characteristics have a direct impact on vibration responses of screen frames.To clarify the influence mechanism of rubber spring dynamic characteristics on the screen frame system,firstly,damping parameters are introduced into the vibration equation of the screen frame system,and the analytical expressions involves damping parameters for screen frames vibration displacements are derived.Influences of damping ratio and relative damping on the fixed screen frame and the floating screen frames motion are analyzed.Secondly,considering the amplitude and frequency correlations of shear springs dynamic characteristics,an improved generalized Maxwell shear spring model is proposed and applied to the nonlinear dynamic modeling of the screen frame system.Thus,coupling effects between shear springs and screen frames are considered.Finally,the influence mechanism of screen frames masses,shear springs parameters,and the excitation force on the vibration stability of the screen frame system is explored.The flexible screen panel is the key component involved in screening operations,and its flexure motion plays a crucial role in achieving effective dry deep screening of moist fine coal.In order to explore the influence of installation parameters,material parameters and structural parameters on the flexure dynamic characteristics of flexible screen panels under certain boundary excitation,firstly,considering two ends constraint modes of the screen panel,a statically indeterminate Catenary model of screen panels is proposed,and the deflection curve expression of the screen panel is derived,which expands the application scope of the screen panel theory and improve the accuracy of the geometric nonlinear characteristics solving of the screen panel in the flexure process.Secondly,based on the above expression,a nonlinear finite element model of the flexible screen panel with initial deflection is established,and influences of hardness,thickness,and screen aperture distribution on the dynamic distribution and fluctuation of the screen panel are explored.In addition,the coupling mechanism of geometric nonlinearity and material nonlinearity of the screen panel on its dynamic response is also analyzed.The collision depolymerization of agglomerates is of great significance for improving the deep screening efficiency of the vibrating flip-flow screen.In response to unclear conditions for the collision and depolymerization of moist fine-grained coal aggregates under the excitation of flexible screen panel ejection,Since the collision depolymerization condition of the moist fine coal agglomerate on flexible screen panel is not clear,first,a ‘one to multiple wall model’ is proposed for the moist fine coal agglomerate,and the agglomerate collision depolymerization energy is calculated based on the liquid bridge theory.Then,a coupled simulation analysis is conducted on the collision depolymerization between the moist fine coal agglomerates and the flexible screen panel.According to the simulation,the liquid bridge and the agglomerate microstructure changes during the collision depolymerization process are explored.Based on this,the damage ratio expression of the agglomerate collision depolymerization is derived,which reveals the essential influence factors of the collision depolymerization degree.Moreover,the average vertical stress,the expansion characteristics and the microstructure parameters change of moist fine coal agglomerate under different collision and liquid bridge conditions are comparative studied.In order to further improve the screening efficiency of material groups under the certain acceleration condition,a screen panel with concave-convex surface morphology is proposed.Through theoretical calculation and numerical simulation,ejection dynamics of a single particle and collision depolymerization characteristics of agglomerates impact on the screen panel with and without concave-convex surface morphology are compared.Experiment and simulation studies on vibration screening behaviors of the material group on screen panels are conducted.By comparing screening behaviors characteristics,such as the particle motion trajectory,the migration speed,the number of collisions,the screening efficiency,and the percentage of liquid bridge fracture,on screen panels with and without surface morphology,the feasibility of using screen panel with concave-convex surface morphology to improve the screening efficiency of flip-flow screen is demonstrated.