Research On Three-screw Straw Extruder

The emergence of straw expander solves the waste of straw resources to some extent.At present,there are problems of serious abrasion and low expansion efficiency for the widely used single-screw and twin-screw straw extruders.In order to solve the shortcomings,this thesis puts forward the concept of three-screw straw expander.The spatial geometry theory of the three-screw straw extruder is studied,and the finite element method is used to analyze the internal velocity field,pressure field,and fluid-solid coupling field of the three-screw straw extruder,which provides a reference for the selection of process conditions and screw structure design of the subsequent three-screw straw extruder.Firstly,this thesis analies the spatial geometry of the three-screw straw expander.Compared with the three arrangement modes of the three screws,the inverted triangle arrangement has three meshing areas and an unique central area in space.The relationship between the center distance and the screw top angle is derived by calculating the center distance and the screw groove depth range at different number of screw heads.According to the kinematic principle of the co-directional meshing three-screw,the multi-thread profile equation and the three-screw profile equation are derived respectively.Limited by the coordination between the screws,the three-screw element is selected in this thesis.Secondly,the flow field of the three-screw straw extruder is simulated and analyzed.The internal flow field model of the three-screw straw expander is established,and the pretreatment setting is completed by dividing the fluid domain grid,compiling the UDF function and defining the dynamic grid motion interval.In the speed field,the high-shear zone mainly exists in the meshing zone of the three screws,and the low-shear zone is mainly distributed between the screw and the inner wall of the barrel,and high shear velocity zone accounts for 36.1%of the overall velocity field.When the screw speed is set at 350 rpm,the circulation effect of the material in the center area has the least influence on the outlet flow.ANSYS/workbench software is used to analyze the influence of fluid-solid coupling on the force and deformation of the screw of the three-screw straw expander.By comparing the total deformation of the screw under the combined action of pure flow field and applied flow field pressure and screw torque,the total deformation difference is only 5.2×10^-5mm,and the main factor affecting the force and deformation of the screw is the flow field pressure load.Finally,under the same screw parameters and initial conditions,the flow channel area and flow channel volume of the three-screw straw extruder are 1.1 times and 1.6 times that of the twin-screw straw extruder,respectively.The average material velocity in the flow field of the three-screw straw extruder is 2.5 times that of the twin-screw,which shows that the shear capacity of the three-screw straw extruder is better than that of the twin-screw.The pressure at the outlet of the three-screw straw extruder is 0.3759 MPa and the pressure at the outlet of the twin-screw is0.0157 MPa,which indicates that the pressure-building capacity of the three-screw straw extruder is better than that of the twin-screw.Comparing the fluid-solid coupling field of the straw extruder,the maximum variable of the three-screw is 2.6×10^-3 mm,and the twin-screw is 2.7×10^-2mm,which shows that the wear degree of the screw in the three-screw straw extruder is less than twin-screw.