Aerodynamic characteristics of lotus seed mixtures and test on pneumatic separating device for lotus seed kernel and contaminants

Abstract: Lotus seed hulling is the first process in rough processing, which is removing the tough shell that covers lotus seed kernel. Generally, it can utilize the mechanical processing method. After processing, the mixtures of lotus seed kernel, lotus seed shell, kernel debris and a small amount of imperfectly shelled lotus seed can be yielded. Because the lotus seed shell, kernel debris and other contaminants are easy to block the entrance of next working position (kernel coring process), a separating process for shell and kernel after the hulling process would be necessary and crucial. Pneumatic separating method is commonly used in separating equipment in agriculture products. However, there is limited academic research on pneumatic separating technology for lotus seed mixtures. In this paper, a negative pressure, pneumatic separating method was proposed to separate lotus seed kernel and shell. Firstly, the density parameter, the moisture content and the shape parameter of lotus seed mixture constituent were measured. The test results showed that the density parameters of lotus seed kernel, lotus seed, shell and kernel debris were 1 080, 1 042, 1 210 and 1 163 kg/m3 respectively, the moisture content of lotus seed kernel, unhulled lotus seed (with imperfectly shelled lotus seed), shell and kernel debris were 7.35%, 3.91%, 12.36% and 12.45% respectively. The shape of lotus seed kernel, lotus seed and the imperfectly shelled lotus seed could be abstracted as the spherical, and the grain diameter could be represented by maximum cross section. Secondly, the theoretical analysis and aerodynamics characteristics test of lotus seed mixtures were carried out. The results showed that the drag coefficient of lotus seed mixtures was within the Newton area, and the drag coefficient was 0.44. The ranges of theoretical suspension velocity of lotus seed kernel, lotus seed and imperfectly shelled lotus seed calculated were 14.134-16.115, 14.745-17.327 and 15.763-17.623 m/s respectively. The ranges of actual suspension velocity of lotus seed kernel, lotus seed shell, kernel debris, lotus seed and lotus seed which is shelled imperfectly were 11.230-14.680, 2.511-4.891, 6.505-7.865, 12.573-16.289 and 12.533-16.017 m/s respectively. The range of suitable air flow velocity for separation was confirmed as 7.865-11.23 m/s. There was no overlaps between the ranges of suspension velocity of lotus seed kernel and contaminants such as kernel debris and shell. Therefore, it's suitable to use pneumatic separating method to separate lotus seed kernel and contaminants. According to the comparison of test results and theoretical calculation results, the shape correction coefficient of products which was used in theoretical calculation was corrected. The ranges of shape correction coefficient of lotus seed kernel, lotus seed and lotus seed which was shelled imperfectly were 1.46-1.76, 1.29-1.66, 1.65-1.81 respectively. Thirdly, the negative pressure, pneumatic separating device for lotus seed kernel and shell, which matched the lotus seed huller with multiple working positions, had been designed and manufactured. This device consists of centrifugal fan, separation channel, cyclone separator, frame and inverter. Single factor test and orthogonal test were carried out. The effect law of factors on test indices was studied through the single factor test, and the influential factor and level were identified. The key parameters of separating processing of the device, which was identified and corrected by the orthogonal test, were as follows: inclination angel of inlet tube was 60°, inclination angle of separation channel was 35°, air flow velocity of separation channel was 7.881 m/s, length of inlet tube was 220 mm, and feed rate was 6 grains every time. Verification test on the optimum scheme was carried out and the result showed that the separating ratio could reach 98.283%. The result of this study in this paper can be taken as theoretical basis and reference for the research and development on separating equipment for lotus seed kernel and contaminants.