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As the grain-cleaning process in a combine harvester relies on pneumatic separation of the grain and chaff, the aerodynamic forces created by the airflow have a large impact on the cleaning performance. Therefore, to optimize a multi-duct cleaning device in a rice combine harvester，First, the equivalent resistances for the different fan ducts were quantified with a resistance model of fluidised grain and an airflow resistance model for the cleaning sieve.Perforated plates with different opening ratios were designed to simulate working loads. A computational fluid dynamics (CFD) model was validated by comparing simulation results with hot-wire anemometer measurements. The effects of working loads on airflow velocity and volume distribution at the ducts, and total pressure distribution inside the fan were studied in simulation and experiments. After integraton of the designed multi-duct fan into the cleaning system, airflow measurements inside the cleaning shoe were made and cleaning performance tests were carried out on a test bench. The ideal airflow velocity within the cleaning shoe was investigated by correlating airflow velocities at different measuring points and analysing the cleaning performance. An evaluation of this multi-duct cleaning system in the field showed large improvements in terms of the grain sieve losses ratio and grain impurity ratio compared to a combine harvester with the single-duct cleaning system. The corresponding grain sieve loss decreased from 2.46% to 0.08%, while the grain impurity ratio decreased from 4.78% to 0.511%.

Rice is one of the most important grain crops in China and most of these fields are harvested by combine harvesters. With increasing rice grain yields and feed rates, the cleaning systems rice combine harvesters have to deal with larger volumes of threshed output. Field experiments have shown that the standard single-duct cleaning systems used in most rice combine harvesters has become a limiting factor, resulting in high grain losses and producing high grain impurity ratios. To investigate the reasons for poor cleaning performance, the terminal velocity of the different categories of threshed outputs were quantified and turbine flow meter measurements were performed to obtain insights into the airflow distribution inside the cleaning shoe. Based on these insights, a multi-duct cleaning device with return pan was proposed for tangential-longitudinal-flow rice combine harvesters. The new cleaning system was benchmarked against a commercial system during parallel field tests and it was concluded that the design adaptations improved the sieve losses by 85% and the grain impurity ration by 73%.