Simulation and experiment of air flow field in the cleaning device of ratooning rice combine harvesters

Abstract: Ratooning rice is a kind of double cropping in rice, where only the upper 2/3 part of the rice plant is cut off after the first season of mature rice, leaving the lower third of the plant and root system to let it grow another season of rice. The combine harvester can crush the rice piles, when working in the field, thereby reducing the yield of the second season of ratooning rice. A wide header is widely used in the combine harvester for the ratooning rice, in order to reduce the number of round trips of the combine harvester in the field, and thus reduce the rolling rate, while, leave enough stalks to meet the growth requirements of the second season of ratooning rice. The ear part is cut off in harvesting the first season rice. Therefore, the ratooning rice extract has the characteristics of large material volume, high grain content and high water content. Long grass and leaves are easy to block on the screen surface, and a large number of grains can not be screened efficiently, which greatly increases the screening load. In this study, a cleaning device was developed in combine harvesters for ratooning rice, in order to efficiently complete the cleaning of ratooning rice extracts. A six-blade centrifugal fan was used as a cleaning fan to ensure suffcient wind speed and flow. The shutter screen was used as the upper of vibrating screen, indicating excellent wind guiding.With the help of vibration, the materials on the screen surface can be quickly blown away and layered. Meanwhile, the grains can be screened efficiently, due to the screen plate was flat and unobstructed. The working parameters were obtained, when the distribution of air flow field was optimal, via simulating the internal flow field in a cleaning device with three different screen openings. The CFD software was used to perform numerical simulation and comparative analysis on the internal airflow field inside the cleaning device in combine harvester for the ratooning rice under different screen opening scales. The diagram of gas streamline was obtained in the cleaning room, whereas, the diagram of wind speed cloud was obtained at different height above the vibrating screen. When the fan speed was 1 050 r / min, it was found that the screen had the best wind guiding effect, where the screen opening was 20 mm, indicating that the optimal distribution of wind speed was above the screen surface. The wind speed in most areas can meet the separation requirements of ratooning rice extract. The wind speed was 3.67-4.29 m/s in the 1/2 place in front of screen, indicating that there was enough to separate the light impurities in the mixture. The wind speed increased above 11.02 m/s in the last 1/2 places, showing evenly distribution suitable for cleaning. The wind speed was measured at a total of 75 measurement points above the screen surface, where the fan speed was 1050 r/min and the screen opening was 25 mm , using the TES1340 precision hot wire anemometer. The measured wind speed was generally lower than the simulated wind speed, and the gap between the two values was lager in the high wind speed area than that in the low wind speed area. The reason for this phenomenon is that the numerical simulation was completed under ideal conditions, where the airflow attenuation was slower than the actual situation. The grain impurity rate can reach the lowest, when the screen opening was 20 mm in the field test. A variance analysis of data showed that there was no main effect on the cleaning loss rate of shutter screen, indicating that the cleaning effect was the best, when the screen opening is 20 mm. These values are consistent with the results that obtained from numerical simulation of air flow field. When the rotation speed of fan was 1 050 r/min, under the working condition of the feeding amount of 5.36 kg/s, the preferred combination of working parameter can be: the opening scale of shutter screen was 20 mm, and the vibration frequency of the cleaning sieve was 6 Hz. Specifically , the impurity rate of the ratooning rice grain was 1.52%, while the loss rate was 1.11%. The feasibility of developed cleaning device was verified for the cleaning of regenerated rice materials.