The flows of rigid polyvinyl chloride （R-PVC） in co-rotating twin screw extruders with screw mixing elements and regular screw elements were simulated by using the finite element method. The three-dimensional, non-i...The flows of rigid polyvinyl chloride （R-PVC） in co-rotating twin screw extruders with screw mixing elements and regular screw elements were simulated by using the finite element method. The three-dimensional, non-isothermal flow fields of R-PVC in the two kinds of screw elements were calculated. The mixing performance of each type of element was studied by the particle tracking analysis method. The results show that the temperature distribution and shear-rate distribution are more uniform in the flow channel with screw mixing elements than in the flow channel with regular screw elements. Screw mixing elements provide better distributive and dispersive mixing performance but worse conveying capacity than regular screw elements.展开更多
Because the deployable structures are complex multi-loop structures and methods of derivation which lead to simpler kinematic and dynamic equations of motion are the subject of research effort, the kinematics and dyna...Because the deployable structures are complex multi-loop structures and methods of derivation which lead to simpler kinematic and dynamic equations of motion are the subject of research effort, the kinematics and dynamics of deployable structures with scissor-like-elements are presented based on screw theory and the principle of virtual work respectively. According to the geometric characteristic of the deployable structure examined, the basic structural unit is the common scissor-like-element（SLE）. First, a spatial deployable structure, comprised of three SLEs, is defined, and the constraint topology graph is obtained. The equations of motion are then derived based on screw theory and the geometric nature of scissor elements. Second, to develop the dynamics of the whole deployable structure, the local coordinates of the SLEs and the Jacobian matrices of the center of mass of the deployable structure are derived. Then, the equivalent forces are assembled and added in the equations of motion based on the principle of virtual work. Finally, dynamic behavior and unfolded process of the deployable structure are simulated. Its figures of velocity, acceleration and input torque are obtained based on the simulate results. Screw theory not only provides an efficient solution formulation and theory guidance for complex multi-closed loop deployable structures, but also extends the method to solve dynamics of deployable structures. As an efficient mathematical tool, the simper equations of motion are derived based on screw theory.展开更多
Adjacent vertebral fractures are common in patients with osteoporotic vertebral compression fractures（OVCFs） after kyphoplasty.This finite element study was to examine whether short segment pedicle screw fixation（...Adjacent vertebral fractures are common in patients with osteoporotic vertebral compression fractures（OVCFs） after kyphoplasty.This finite element study was to examine whether short segment pedicle screw fixation（PSF） with kyphoplasty may decrease the fracture risk of the treated and adjacent non-treated vertebrae after kyphoplasty for OVCFs.By simulating cement augmentation with or without short segment pedicle screw fixation（PSF）,two tridimensional,anatomically detailed finite element models of the T10–L2 functional spinal junction were developed.The insertion of pedicle screws into the intact vertebra apparently decreased the stress distribution of the treated vertebra in vertical compression and other load situations.The stress distribution in the bone structures of the intact vertebra adjacent to the intact-screwed vertebra was much less than that in the one adjacent to the treated vertebra.The insertion of pedicle screws into the intact vertebra greatly decreased the maximum displacement of the cortical bones and cancellous bones of the vertebrae.Our results indicated that short segment PSF with kyphoplasty may decrease the fracture risk of the treated and adjacent non-treated vertebrae in the management of OVCFs.展开更多
【目的】为提高螺旋输送机的输送效率,降低输送机的功耗与磨损,探究在不同进料速率、螺旋轴转速与几何体摩擦系数下,超细碳酸钙在水平变径变距螺旋输送机内的颗粒流动状态、出口质量流量、输送机功耗与磨损分布。【方法】使用计算流体...【目的】为提高螺旋输送机的输送效率,降低输送机的功耗与磨损,探究在不同进料速率、螺旋轴转速与几何体摩擦系数下,超细碳酸钙在水平变径变距螺旋输送机内的颗粒流动状态、出口质量流量、输送机功耗与磨损分布。【方法】使用计算流体动力学(computational fluid dynamics,CFD)与离散单元法(discrete element method,DEM)双向耦合数值模拟的方法,对螺旋输送机在不同转速下的质量流率进行分析对比,验证数值模型的正确性。【结果】摩擦系数对颗粒的运动有较大影响,颗粒流的轴向速度峰值和质量流率峰值随着摩擦系数的增加先增大再减小;随着下料速度和摩擦系数的增大,输送机功率明显增大,且摩擦系数在高进料速度与低转速的情况下对功耗的影响相对于低进料速度和高转速更加明显;磨损较严重的区域集中在下料口处的螺旋轴与螺旋叶片的边缘处。【结论】简单增大或减小摩擦系数并不能提高颗粒的轴向速度和质量流量,而是存在一个局部最优参数组合;适当地提高转速能够减小颗粒密实度与颗粒停留时间,从而减小输送机的功耗与几何体磨损。展开更多
文摘The flows of rigid polyvinyl chloride （R-PVC） in co-rotating twin screw extruders with screw mixing elements and regular screw elements were simulated by using the finite element method. The three-dimensional, non-isothermal flow fields of R-PVC in the two kinds of screw elements were calculated. The mixing performance of each type of element was studied by the particle tracking analysis method. The results show that the temperature distribution and shear-rate distribution are more uniform in the flow channel with screw mixing elements than in the flow channel with regular screw elements. Screw mixing elements provide better distributive and dispersive mixing performance but worse conveying capacity than regular screw elements.
基金Supported by National Natural Science Foundation of China(Grant No.51175422)
文摘Because the deployable structures are complex multi-loop structures and methods of derivation which lead to simpler kinematic and dynamic equations of motion are the subject of research effort, the kinematics and dynamics of deployable structures with scissor-like-elements are presented based on screw theory and the principle of virtual work respectively. According to the geometric characteristic of the deployable structure examined, the basic structural unit is the common scissor-like-element（SLE）. First, a spatial deployable structure, comprised of three SLEs, is defined, and the constraint topology graph is obtained. The equations of motion are then derived based on screw theory and the geometric nature of scissor elements. Second, to develop the dynamics of the whole deployable structure, the local coordinates of the SLEs and the Jacobian matrices of the center of mass of the deployable structure are derived. Then, the equivalent forces are assembled and added in the equations of motion based on the principle of virtual work. Finally, dynamic behavior and unfolded process of the deployable structure are simulated. Its figures of velocity, acceleration and input torque are obtained based on the simulate results. Screw theory not only provides an efficient solution formulation and theory guidance for complex multi-closed loop deployable structures, but also extends the method to solve dynamics of deployable structures. As an efficient mathematical tool, the simper equations of motion are derived based on screw theory.
基金supported by the National High Technology Research and Development Program("863"Program)of China(No.SS2012AA022811)the Science and Technology Program of Guangzhou(No.201508020253)the Special Project on the Integration of Industry,Education and Research of Guangzhou(No.158100062)
文摘Adjacent vertebral fractures are common in patients with osteoporotic vertebral compression fractures（OVCFs） after kyphoplasty.This finite element study was to examine whether short segment pedicle screw fixation（PSF） with kyphoplasty may decrease the fracture risk of the treated and adjacent non-treated vertebrae after kyphoplasty for OVCFs.By simulating cement augmentation with or without short segment pedicle screw fixation（PSF）,two tridimensional,anatomically detailed finite element models of the T10–L2 functional spinal junction were developed.The insertion of pedicle screws into the intact vertebra apparently decreased the stress distribution of the treated vertebra in vertical compression and other load situations.The stress distribution in the bone structures of the intact vertebra adjacent to the intact-screwed vertebra was much less than that in the one adjacent to the treated vertebra.The insertion of pedicle screws into the intact vertebra greatly decreased the maximum displacement of the cortical bones and cancellous bones of the vertebrae.Our results indicated that short segment PSF with kyphoplasty may decrease the fracture risk of the treated and adjacent non-treated vertebrae in the management of OVCFs.
文摘【目的】为提高螺旋输送机的输送效率,降低输送机的功耗与磨损,探究在不同进料速率、螺旋轴转速与几何体摩擦系数下,超细碳酸钙在水平变径变距螺旋输送机内的颗粒流动状态、出口质量流量、输送机功耗与磨损分布。【方法】使用计算流体动力学(computational fluid dynamics,CFD)与离散单元法(discrete element method,DEM)双向耦合数值模拟的方法,对螺旋输送机在不同转速下的质量流率进行分析对比,验证数值模型的正确性。【结果】摩擦系数对颗粒的运动有较大影响,颗粒流的轴向速度峰值和质量流率峰值随着摩擦系数的增加先增大再减小;随着下料速度和摩擦系数的增大,输送机功率明显增大,且摩擦系数在高进料速度与低转速的情况下对功耗的影响相对于低进料速度和高转速更加明显;磨损较严重的区域集中在下料口处的螺旋轴与螺旋叶片的边缘处。【结论】简单增大或减小摩擦系数并不能提高颗粒的轴向速度和质量流量,而是存在一个局部最优参数组合;适当地提高转速能够减小颗粒密实度与颗粒停留时间,从而减小输送机的功耗与几何体磨损。
