Analysis and experiment on the aerodynamic characteristics of walnut shell and kernel material based on suspension velocity interval estimation

Walnut has been one of the most popular fruits in the Asia areas. The walnut shell and kernel can be firstly separated to obtain the fruit seeds during deep processing. Wind separation has been widely used for walnut shell-kernel separation, due to its stable performance, high efficiency, and small damage to the kernel. It is a high demand to clarify the aerodynamic characteristics of walnut shell-kernel materials during wind separation. In this study, the interval estimation of suspension velocity was employed to explore the aerodynamic properties of walnut shells and kernel materials. The irregular shapes of walnut shell and kernel materials were then selected to accurately measure the key parameters of the aerodynamic characteristics. A series of experiments were also conducted to verify the simulation. Initially, the flow field was constructed using the CFD-DEM (Computational fluid dynamics, Discrete element method) platform and the finite element software of ANSYS Fluent. The discrete element model of the shell kernel material was established to fill the small particles in the discrete element software of EDEM. The movement of the shell kernel material was then simulated in the airflow. According to the motion trajectories of the materials and the velocity distribution of the flow field, the range of suspension velocity was determined for the six types of materials, such as one-half shells, one-quarter shells, one-eighth shells, one-half kernels, one-quarter kernels, and one-eighth kernels. Subsequently, an experimental bench was constructed for the bench tests after simulation. High-speed photography was then utilized to record the motion of the shell and kernel materials. A statistical analysis was performed on the distribution of materials in the different ranges of wind speed. It was found that the six types of materials followed a normal pattern of position distribution in the airflow within the cavity after K-S testing. Finally, Gaussian fitting was performed on the origin data using processing software, according to the statistical data of various types of materials. The specific range of the levitation speeds was statistically derived for the various types of materials using the interval estimation method: 7.25-9.50 m/s for one-half shells, 6.90-8.95 m/s for one-quarter shells, 6.20-7.75 m/s for one-eighth shells, 9.20-14.85 m/s for one-half kernels, 9.60-14.55 m/s for one-quarter kernels and 9.30-11.45 m/s for one-eighth kernels. The optimal range of wind selection speed was then determined to be 9.0-9.4 m/s for the shell and kernel in the air separation equipment. Additionally, the theoretical formula of suspension velocity was corrected using experimental data. The coefficients of shape correction applicable to Yunnan Yangbi walnuts were obtained as follows: 3.82 for the one-half shell, 3.06 for the one-quarter shell, 2.80 for the one-eighth shell, 1.49 for the one-half kernel, 1.12 for the one-quarter kernel, and 1.17 for the one-eighth kernel. Furthermore, a systematic investigation was implemented to explore the causes of shell and kernel stacking and their potential impact on the efficiency of wind selection separation. The rotational motion of shell and kernel materials in the airflow was used to reduce the frequency of stacking. The separation efficiency of mixed shell and kernel materials was improved using the suspension velocity interval estimation. The finding can also provide a strong reference for the aerodynamic characteristics and key parameters of the irregular agricultural materials.