Study On Effects Of Wet Cohesive Surface On Fluidization And Accumulation Movement

The fluidization and accumulation movement of wet cohesive particles are widely applied in industrial processes,such as coating of drugs,wet granulation,coal storage and transportation,food drying,petroleum cracking catalysis and so on.In wet systems,the viscous liquid on the surface of particles affects the characteristics of fluidization and accumulation movement,arising from the viscous force and the interaction force between particles.At present,aiming at wet cohesive particles,the research of the fluidization characteristics and the bubble behavior is very scattered and superficial.The movement of wet cohesive particles is not fully understood.The flow pattern and the mixing rule of wet powder particles are reported rarely.The traditional mathematical models are not available in the researches,which limits the deep investigations in this area.The present work is devoted in the fluidization of Geldart-D wet cohesive particles and accumulation movement of Geldart-A wet cohesive particles,which is researched systematically with the method of experiments and numerical simulations.To study the fluidization characteristics,a three-dimensional visualized fluidized bed platform,a pressure signal acquisition system and an image recording system are established.Combing the method of time-domain analysis and frequency-domain analysis,the transition of fluidization status reflected by the pressure signals is obtained.The results dipict that multi-peaks and wide frequency brand exist in pressure signal power spectral density during normal fluidization while peaks of low frequency are observed during slugging and channeling.The critical velocity when normal fludization transists to defluidization is obtained.With the increasing moisture content,the multip-bubbles fluidization transists to slugging and channeling.A new bubble identification algorithm is proposed to describe the properties and shape of bubbles,which connects the bubble behavior with the global fluidization.Five typical types of bubbles are characterized(rectangular bubble,oval bubble,elongated bubble,irregular and tiny bubble).With the increasing moisture content,the gas holding capacity of emulsion phase increases,resulting in the decreasing bubble fraction.Particles cohereing at both sides of the bed contribute to the decreasing average equivalent diameter of bubbles and the decreasing equivalent diameter of bubbles at different heights of the bed.Bubbles of particles with the larger moisture content have more irregular shape,and yet the shape turns to be regualr as the particle size increases.When the moisture content increases,the horizontal movement of bubbles intensifies,and the vertical coalescence of bubbles weakens.The size of bubbles decreases in the vertical direction while the shape of bubbles becomes more uneven.To study the characteristics of accumulation movement,a bulk density measurement system,a repose angle measurement system,an ash and liquid saturation measurement system and an image acquisition system are established.The experimental data indicates that the flowability of easy flow transits into cohesive when the liquid saturation is between 9% to 10%,describing the flowability quantitatively.The increasing liquid saturation results in the changing shape of the coal bulk and the relationship is described using a correlation formula.A two-dimensional visualized coal silo system,a three-dimensional system with one outlet,a three-dimensional system with two outlets,discharge rate measurement system,an ash and liquid saturation measurement system and an image acquisition system are established.The results show that the liquid bridge force increases the reverse pressure gradient at the outlet,increases the resistance at the outlet,and decreases the discharge rate.With the increase of moisture content and static bed height,the discharge flow rate presents a downward trend.With the increasing moisture content,the creep of flow pattern is not uniform and the mixing is more complete.When moisture content is greater than 11%,the angle of collapsing section achieves 100° to 125°.A solid phase shear stress model is proposed,which is suitable for the calculation of the accumulation and movement of wet cohesive coal particles,overcoming the shortcomings of the traditional models.The model is verified in detail from the discharge rate,flow pattern and mixing characteristics with a tiny error below 8% between the numerical calculation results and the experimental data,which has high accuracy.The 3d simulation of the discharge of coal particles in the silo is successfully realized.The velocity difference between partciles in the central axis and near the side wall increases.Using this numerical simulation method,it is found that the flow pattern of coal particles will change from the mass flow direction to funnel flow when the half tip Angle of silo increases from 15° to 60°,the discharge rate decreases from 11.46kg/s to 2.86kg/s,and the moisture content increases from 6.91% to 14.68%.The numerical prediction results are compared with the experimental data in a four-port test bin,which proves that the numerical method has reliable and accurate guiding value for industrial production.