Characterization of pore structure and airflow distribution in bulk grains
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Date
2018-12-10
Authors
Nwaizu, Charles
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Abstract
Airflow behavior in stored bulk grains is a function of the geometry and topology of pore structure within the grain bed. An image analysis technique was used for reconstruction of the complex 3D pore structure within bulk grain from 2D thin section images. The 3D model was developed by aligning successive 2D thin section images obtained from colored-wax solidified soybean grain beds. The results showed that the proposed method of 3D image reconstruction had potential of quantifying critical pore structure parameters such as porosity, tortuosity and pore connectivity in bulk grains. Compaction caused by gain pressure (depth) and vibration affected both porosity and tortuosity significantly. Specifically, porosity increased quickly at the low grain pressure and then gradually approached a minimum value. Porosity decreased from 0.42 to 0.34, or 19%, when the grain mass was subjected to a compaction pressure of 14.2kPa (equivalent to 2.2 m of grain depth). Tortuosity increased with the compaction pressure from 1.15 at 0 kPa to 1.58 at 14.2kPa, or by 37.4%. The effects of vibration on pore structure characteristics (porosity and tortuosity) were evident. At lower vibration intensity (<1g), porosity and tortuosity changed little. Porosity reached a minimum value at critical vibration intensity around2g, and further increase in vibration intensity loosened the grain bulk, causing porosity to increase. At the critical vibration intensity (2g), porosity decreased from its initial value by 21%, whereas tortuosity increased by 15%.
To further investigate the significance of grain compaction in designing grain aeration and drying systems, its overall effect on airflow distribution and pressure drop within stored bulk grains was simulated with CFD (computational fluid dynamics) models by the integration of the porosity and tortuosity models to account for variations in pore structure within the grain bed. The relative differences in pressure drop between simulation based on variable pore structure and experimental values reported to be below 10%, whereas the differences were around 40% when constant pore structure was used in the CFD simulations.
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Keywords
Grain bulks, CFD, Pore structure, Porosity, Tortuosity, Image analysis, Anisotropic, Pore-scale model, Resistance to airflow