Evaluating grain silo aeration of maize using CFD simulation and scaling

Abstract

Concrete grain silos are used in South Africa to store maize to ensure year-round availability. Aeration is used to manage grain temperature that affects storage safety and quality. At a large South African grain management company, aeration management is primarily guided by experience-based practice, and limited quantitative information is available to predict the effect of aeration on bulk temperature and moisture content, or the time required for these effects to become observable. Full-scale modelling and controlled validation are additionally constrained by the size of the silo, continuous plant operation, and limited instrumentation. This study, therefore, develops and validates a small-scale modelling and experimental framework to quantify aeration outcomes and support more consistent operational decision-making. A transient three-dimensional CFD model of an aerated maize bulk has been developed, modelling the packed bed as a porous medium with coupled heat and moisture transfer. In addition, a numerical calculation model has been implemented to estimate dwell temperature and moisture content for specified inlet air temperature and relative humidity conditions. A scaling approach has been formulated using the temperature-front concept, relating front propagation to air velocity, grain temperature, and bulk geometry. The CFD model has been validated against measurements from a purpose-built small-scale silo testbench. The simulation reproduced the measured temperature and front speed trends, with moderate overprediction of initial cooling; this behaviour is attributed to the literature-based drying constant correlation used in the model. The validated model has been used to quantify temperature front speed ratios (front speed normalised by air velocity) over a range of initial grain temperatures and aeration scenarios, highlighting that cooling effectiveness is highest at aeration onset and decreases with continued operation. For initial grain temperatures ranging between 20 and 35 [℃], the temperature front speed ratios ranged between 4.86 and 15.27 [mm/s]/[m/s]. A secondary comparison has been performed using available full-scale silo data; however, due to non-ideal comparability of operating conditions and uncertainty in full-scale airflow and initial bulk state, the scaling relationship could not be definitively validated from the available dataset. Finally, practical guidelines have been proposed to support silo managers in predicting aeration outcomes and time-to-effect under defined inlet conditions.