Effects of stored grain bulk properties on airflow distribution

Project Description

This project is designed to develop models that can help to understand airflow, and airflow distribution inside grain bulks. The main focus is on the potential impact of zones of different permeability (as affected by loading methods, layering, amount of dockage, etc.) on airflow.

This research builds on the project funded by SWDC on impact of bin filling method on bulk density, and the current study will generate information to understand the sensitivity of airflow to variations in properties inside the stored grain; to understand the practical relationships between airflow distribution, bin size, variable grain properties; and the best management practices for aeration and drying. This information will support producers management decisions and contribute to preserve grain quality.

Research Results

Understanding the impact of variable permeability on airflow in stored grain was a problem for producers managing the risk of grain spoilage in grain bins. Variable permeability can result from factors such as loading method, distribution of dockage, and layering effects; these factors can be much more relevant as storage structures increase in size, and as operational or environmental pressures result in nonoptimal harvest conditions.

In order to address this gap in knowledge, the following research activities were completed:

1)      A prototype in-bin sensor for measuring very low in-grain airspeeds associated with aeration (less than 0.1 cfm/bu) was developed and assessed.

2)      Along with other equipment, the sensor was used to measure in-grain resistance to airflow for wheat with varying bulk densities.

3)      Data was then used to develop and validate a computer model that could predict air velocity and pressure patterns (± 6%) for grain stored in an 18’ diameter hopper bottom bin with varying permeability scenarios.

The following conclusions were made based on the simulation results:

• Generally, the most consistent airflow through grain will be achieved if the top is flat.
  • Leaving a peak on the grain bulk resulted in reduced airflow at the top of the peak.
  • Emptying the bin to create a funnel-shaped top reduced the air velocity toward the bin wall and detrimentally “short-circuited” air through the center of the bin.
• Compared to the rest of the grain, the velocity through a low-permeability core was reduced by 20% to 38% at an aeration rate of 1.0 cfm/bu.
  • Somewhat unexpectedly, a smaller diameter core experienced a greater reduction in velocity.
• The severity of airflow reduction depended on the shape of the low-permeability region.
  • Narrow, tall veins of dockage created a region where air flowed around rather than through the dockage. Therefore, if dockage has to enter a bin, it’s better to spread it out rather than allowing it to concentrate in one spot.
  • Typically, the impact of low-permeability regions remained local; that is, the air velocity was reduced only within the low-permeability regions. However, less consistent airflow could result in areas susceptible to storage stability issues.
• Discrete regions of low permeability on the top surface of the bin (e.g., due to crusted grain typically caused by moisture condensation) experienced low air flow and amplified the effect of a peaked grain pile.

From these conclusions, the following beneficial management practices were compiled:

• Level off a peaked grain surface to flatten the profile.
• Use a spreader or remove a small amount of grain after filling to avoid creating a low-permeability core.
• Use a spreader to also help distribute dockage from concentrating in a one spot.
• Water-proof bin rooves and lids to prevent moisture infiltration that could cause surface spoilage.
• Avoid placing tough grain in the hopper bottom if aeration air cannot be introduced below it.

The in-grain, low airspeed sensor was identified as a valuable tool for measuring in-grain air velocity and monitoring grain storage. However, it was found that the local orientation of wheat kernels had a large impact on measurement repeatability; as such, the design of an appropriate, robust housing for the airspeed sensor would be an important next step for continuation of this research application.

These simulations predicted the steady-state airflow patterns in a bin; however, over longer storage times, stored grain is a dynamic system which is impacted by heat and mass (moisture) transfer. Inclusion of these mechanisms in future work is critical for more accurate predictions of storage conditions and for understanding how energy is utilized within a storage and drying system.