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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%. • Terminal velocity of different components of threshing outputs were quantified. • Distribution of the threshing outputs in the cleaning shoe were studied. • The proposed multi-duct cleaning device improved cleaning performance greatly. [ABSTRACT FROM AUTHOR]