颗粒尺寸对螺旋加料机定量加料性能的影响

Effect of particle size on precision dosing of screw feeder

为提高螺旋加料机定量加料性能，开展颗粒尺寸对加料量稳定性和准确性的影响研究。以粒径3～5 mm的球形谷物颗粒单圈加料约40 g螺旋加料机为研究对象，基于软球模型，通过对螺旋加料过程颗粒物料力链及颗粒群运动分析，建立具有分料装置的螺旋加料机离散元仿真模型；应用 PFC3D（particle flow code in 3 dimensions）软件对粒径3、4及5 mm颗粒物料进行螺旋加料过程仿真分析，并以粒径3、4及5mm的球形散珠为试验对象，进行颗粒运动和定量加料试验。综合分析得到：随着球形颗粒粒径的逐步减小，最大接触力逐渐减小，力链分布愈均匀浓密，颗粒间运动的一致性得到提高，物料混合运动趋势逐步减弱，定量加料的稳定性和准确性逐步提高。粒径3 mm球形散珠螺旋加料转动60＆#176;，平均加料量为6.477 g，加料量标准差为0.537，加料量最大相对误差为＆#177;0.153%。

Screw feeder is a traditional device for particulate materials feeding, and associated with many advantages such as simple structure, environmentally sealed case, controllable output and ease of regulation. But, its application in precision dose feeding is restricted by the cyclical fluctuations and unstable phenomenon commonly found during the feeding. Structure and operation parameters are assumed to be two important factors affecting the feeding stability and accuracy of screw feeders. The possible influence of these two factors on precision dosing has been confirmed by many researchers using numerical and experimental method. However, most of the researches focus on a specified particle size, and do not take into account of the impacts of particle size on precision feeding. In this study, the influence of different particle sizes on feeding was investigated by analyzing feeding stability and accuracy. To achieve this objective, common spherical cereal （millet, soybeans, etc.） in diameter of 3-5 mm was selected, and a screw feeder designed for 3-5 mm particles with lap feeding quantity of about 40 g were utilized. A numerical simulation model was established by discrete element method based on the soft ball with a constant stiffness coefficient and linear elastic constitutive relations. The model had an inner diameter of 40 mm, thread pitch of 40 mm and inlet size of 40 mm ＆#215; 60 mm. The feeding processes of different cereal sizes （3, 4 and 5 mm） were simulated, and force chains, particle group motions and feeding＆amp;nbsp;quantity were determined. Furthermore, an experiment of particle group motions and feeding was carried out to verify the effects of particle sizes on feeding stability and accuracy. The PFC3D （Particle Flow Code in 3 Dimensions） software simulation and feeding experiment showed that： 1） When feeding at a fixed speed of 60 rpm and an angle of 60o, the packed number and the average contacts number of 3 mm particles were larger than those of 4 mm and 5 mm particles. As a result, the force chain network in 3 mm particles was distributed more evenly than that in 4-5 mm particles;2） As the particle sizes decreased, the distribution of force chain was more uniform, the movement between 3 mm particles was more consistent, and the trend of the mixed movement was weaker. Accordingly, the 3 mm particles could keep the shape better than the 4 mm and 5 mm particles during transportation, while the stability in 5 mm particles was worst and the mixing phenomenon in 5 mm particles was most obvious;and 3） The average feeding quantity of 3 mm particles was higher than that of 4 mm and 5 mm particles. With decreasing particle sizes, the standard deviation and the maximum relative error of feeding quantity decreased and the stability and accuracy of feeding were improved. When feeding at angle of 60o, the average feeding quantity, the standard deviation and maximum relative error of 3 mm particles were 6.477 g, 0.537 and ＆#177;0.153%, respectively.