| With the increasing quality requirements for multi-component polymers,the mixing performance of single/twin screw extruders can not meet all application scenarios.In order to improve the mixing performance of single/twin screw extruder,many scholars have made beneficial exploration.It was found that introducing eccentric structure can produce certain tensile flow and improve the mixing performance.Wavy screw element has obvious eccentric structure.In order to explore its flow field and mixing characteristics,several groups of wavy screw element were optimized and designed in this paper.Numerical simulation of wavy single screw element and wavy twin screw element was carried out with the help of fluid dynamics software POLYFLOW,which can clearly and intuitively display the flow and mixing process of materials.At the same time,a variety of mixing parameters were used to evaluate the mixing performance,and the influence of the main structural parameters and process conditions on the mixing performance of the wavy screw elements was studied.The main contents are as follows:(1)Study on flow field and mixing performance of wavy single screw element:Four kinds of undulating single screw elements with different eccentricities were optimized and numerically simulated by using POLYFLOW software.The flow field and mixing performance of the four groups of screw elements were compared and analyzed,and the influences of eccentricity and screw speed on mixing performance and transport efficiency of undulating single screw elements were explored.The results show that the eccentricity is beneficial to adjust the flow pattern of the melt.With the increase of eccentricity,the proportion of tensile flow experienced by the melt increases,the mixing performance is constantly enhanced,and the transportation performance is slightly decreased.Screw rotational speed is beneficial to control residence time,shear and stress.With the increase of rotational speed,the overall mixing performance decreases and the conveying performance increases significantly.(2)Study on flow field and mixing performance of wavy twin screw element:A group of same/opposite rotating wavy twin-screw elements and the same specifications of the same/opposite rotating conventional twin-screw elements were optimized,and the numerical simulation was carried out to compare and analyze the flow field characteristics and mixing performance.The results show that the conventional twin-screw element has better material transport and pressure building capacity,which is beneficial to improve the yield,but the melt mainly flows in shear,and the residence time in the flow channel is shorter,which is not conducive to the full mixing of materials.The wavy twin-screw element has the effect of periodic tensile and compression,the melt experiences a higher proportion of tensile flow,and the level of multiple mixing characteristics is higher,which has better mixing performance.(3)Influence of structural parameters and process conditions on mixing performance of co-rotating wavy twin screw elements:The effects of structural conditions and process parameters such as eccentricity,screw pitch,screw speed and feed amount on mixing performance and conveying efficiency of the coincident undulating twin screw element were further investigated.The results show that the proportion of tensile flow and residence time increase with the increase of eccentricity,which is beneficial to improve the mixing performance but not to improve the conveying performance.Increasing pitch and feed amount is beneficial to improving mixing efficiency and conveying performance,but unfavorable to improving mixing performance.Increasing the rotational speed is beneficial to improve the dispersive mixing performance and production efficiency,but unfavorable to improve the mixing efficiency and distributed mixing performance.Under the premise of satisfying the production efficiency,increasing the eccentricity appropriately and decreasing the screw pitch,rotating speed and feeding quantity appropriately according to the basic screw parameters are beneficial to improving the mixing performance of the co-rotating wavy twin screw elements. |
