AIRFLOW RESISTANCE OF SEEDS AT DIFFERENT BULK DENSITIES USING ERGUN’S EQUATION

Abstract

Airflow resistance of grains and oilseeds has been extensively studied. Traditionally the data has been presented using Shedd’s curves. However, this assumes that airflow resistance is independent of grain depth. Grain undergoes compaction during storage that changes the bulk density, porosity, and therefore the airflow resistance. Ergun’s equation is a function of particle size and porosity of the granular material. Airflow resistance by Ergun’s equation was used to predict the pressure drop across a column of corn, soft white winter wheat, soft red winter wheat, and soybeans at three moisture content levels and two bulk densities. The maximum root mean square error when predicting airflow resistance using Ergun’s equation was less than 23 Pa/m when the pressure drop was less than 500 Pa/m. If all data was included up to a pressure drop of 1800 Pa/m, the average root mean square error for calculating airflow resistance was 76 Pa/m. The effect of grain orientation that would be typical in storage bins was negligible, less than a 10% increase in airflow resistance over a range of kernel orientations that varied between -10°, +10°, and 20° from the angle of repose. However, the fill method and resulting bulk density increased the airflow resistance by an order of magnitude. Ergun’s equation, with an appropriate model of porosity variation within a storage bin, could be utilized for the design and analysis of grain aeration systems.