CFD-DEM and experimental study of straw-impurity separation using novel double-helix separator

• DNA-inspired double-helix separator was designed and it improved straw impurity removal. • CFD-DEM with flexible straw model enhanced simulation reliability. • The effects of structure, rotational speed, and feed rate on performance were investigated. • Gas-solid interactions and the mechanisms of straw-impurity separation were revealed. The accurate measurement of straw mulch mass is essential for evaluating conservation tillage practices and advancing sustainable agriculture. However, impurities, including dust, were inevitably mixed in during the straw collection prior to measurement. This resulted in an overestimation of the mulch mass, thereby compromising the accuracy of field detection. A novel double-helix separator was developed to address this challenge. The separator, inspired by the structure of Deoxyribonucleic Acid (DNA), was integrated into the machine’s impurity-removal device. The coupling model of Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) incorporating a flexible straw representation was developed. To investigate gas–solid flow, particle motion, and separation efficiency. Four double-helix structures, as well as single-helix and no-helix designs, were assessed under varying rotational speeds and feed rates. The model underwent validation via bench experiments. The findings indicated that double-helix separators exhibited superior performance compared to single-helix and no-helix designs in low-feed conditions, attributed to the enhanced detachment of impurities from straw facilitated by tangential and radial airflow. However, enhanced gas–solid interactions resulted in a 2–6 % increase in straw loss, indicating a trade-off between separation efficiency and straw preservation. The uniform-diameter (UD) separator exhibited minimal sensitivity to variations in rotational speed. However, single-tapered (ST), double-tapered (DT), and inverse double-tapered (IDT) separators were more sensitive to speed. Moderate speeds produced vortices and velocity gradients that promoted separation. From the perspective of entrainment, smaller impurities exhibited a higher tendency for local entrainment but were eliminated more efficiently than larger impurities. A moderate pressure drop, as opposed to the maximum, facilitated effective separation by maintaining uniform velocity gradients and promoting favorable airflow distribution without destabilizing straw transport. The effect became particularly pronounced with an increase in feed rate, as airflow dynamics affected the performance of separators in managing larger volumes of straw and impurities. At elevated feed rates, all separators showed reduced impurity removal and increased straw loss, yet ST maintained high efficiency despite larger straw loss. This study demonstrated the mechanisms by which structural design, operating speed, and feed rate jointly influence separation efficiency and straw preservation. The findings provided theoretical guidance for the optimization of straw impurity-removal devices in conservation tillage applications.