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Design and Experiment of a Centralized Pneumatic Metering Device

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Tutor: ZhangDongXing
School: China Agricultural University
Course: Agricultural Mechanization Engineering
Keywords: Precision planting,pneumatic metering device,centralized metering,seeds state,co
CLC: S223.2
Type: PhD thesis
Year:  2014
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With the fast development of seed quality and cultivation techniques, precision planting becomes a problem urgently to be solved in China. Pneumatic precision metering device, which is flexible to seed shape and adapts to high-speed operation, has been the development trend. As affording only one row¡¯s sowing and cannot being separated from feeler mechanism, conventional pneumatic precision metering device has many shortcomings such as excessive vibration, layout difficulty and tedious manual adjustment. Besides, pneumatic metering device has poor performance when planting corn seeds which have hard fluidity. To solve these problems, the reasonable state of corn seeds were analyzed, a new airflow structure which could effectively reduce the moving resistances was presented. Based on the airflow structure, a centralized pneumatic metering device (CP) was designed.By the airflow structure of CP, the direction of the airflow is changed to blow the seeds in "boiling" movement which can reduce the seed-filling resistances, getting good seed-filling performance. CP makes a seeder more concise in structure as it distributes seeds to several sowing rows through a centre drum and a positive pressure system. A pneumatic conveying system is applied to throwing seeds, which enables the separation of the metering device from the placement system and reduces the vibration of the metering device obviously.In this paper, mechanical model of the new air flow structure was built to analyze the influence mechanism of seeds state to seed-filling. Main influence factors and their influence rules were acquired through the analysis of filling process in radial direction of the drum under positive pressure. Occupied area coefficient was presented, the process of seed-cleaning was analyzed to detect the affecting factors and the relation between them. Based on the theoretical analysis, the key structural parameters were confirmed.Numerical simulation of the fluid field characteristic of CP was carried out in Fluent to investigate the affect of the structure of CP. The number of holes was confirmed by the simulation of fluid model under different hole diameters and numbers. The whole flow field simulation showed that there is no apparent difference between different sowing rows and filling area has higher pressure, and pressure and velocity were high in filling area. The structure conforms to design requirements.The performance of CP was investigated under laboratory. The experiments of pressure and rotation speed under4kinds of hole diameters during4to5.5mm showed that:hole diameter of4.5mm had the best performance under low operating speed, and required slightly higher pressure, and miss rate was very high under high operating speed. Hole diameter of5.5mm was the best at seed-filling and flexibility of high-speed operation. Its required pressure was the lowest. But it had higher multiple rate under low operating speed. As precision planting of corn has a low tolerance for miss-seeding, hole diameter of5.5mm was selected. The experiments for the optimization of seed-cleaning device were conducted to solve the problem of multiple-seeding of the hole diameter of5.5mm, and the cleaning-distance was confirmed as0.25mm. The experiments of the optimized structure showed that:after the optimization, the single rate increased obviously under low operating speed and had no big change under high operating speed. The comparison with other3metering devices showed that CP had the highest single rate. CP has good seed-filling performance and adapts to the high-speed operation..
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